1 
 
 

 

 

 

 

MXene-based nanostructures for electrochemical CO oxidation. 

 

BY 

Belal Salah Mohammed Hussien 

Student Number: 2435467 

 

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment 

of the requirements for the degree of Master of Science. 

 

Supervisors: Prof. Kenneth I. Ozoemena, Prof. Aboubakr M. Abdullah, and  

Dr. Kamel Eid. 

 

 

 

University of the Witwatersrand, Johannesburg, 2022 

 

 

 

 



2 
 
 

DECLARATION 

 

 

I declare that this dissertation is my own, unaided work. It is being submitted for the Degree of Master of 

Science at the University of the Witwatersrand, Johannesburg. It has not been submitted before for any degree 

or examination at any other University. 

 

 

 

 

 

           (Signature of candidate) 

 

 

 

 

 

 

  12 day of july 2023 at Johannesburg 

 

 

 

 

 

 

 

 

 

 

 

 

 



3 
 
 

ABSTRACT 

2-D MXene based nanostructure owns several unique physicochemical and catalytic properties. Herein, this 

research used MXene (Ti3C2Tx) for the electrochemical COoxid reaction experimentally for the first time. This work 

divided to two sections, the first is using mono metal decorated Ti3C2Tx and the second using binary metals 

decorated Ti3C2Tx to investigate and compered the electrochemical COoxid reaction activity of mono and binary 

metals with Ti3C2Tx. At first, Ti3C2Tx (TX=OH, O, and F) well-ordered and highly exfoliated 2-D nanosheets used 

as substrate for the NPs growth and prepared via the selective chemical etching and delamination of MAX phase 

(Ti3AlC2) with sonication assistance to form Ti3C2Tx nanosheets. After that, Mono and binary metals were prepared 

via using a facile method by in situ impregnation of Pd or Pt or both salts with Ti3C2Tx in aqueous medium under 

sonication without using reducing agent or surfactant.  

The as prepared Pt/Ti3AlC2, Pd/Ti3AlC2, and PtPd/Ti3AlC2 composition and structure were characterized by the 

scanning electron microscope (SEM), Energy Dispersive X-Ray Analyzer (EDX), the transmission electron 

microscope (TEM) equipped with high-angle annular dark-field scanning transmission electron microscopy 

(HAADF-SEM), energy dispersive spectrometer (EDS), The X-ray photoelectron spectroscopy (XPS) spectra and 

The X-ray diffraction patterns (XRD). 

The electrochemical COoxid activity were explored using The cyclic voltammogram (CVs), linear sweep 

voltammogram (LSV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) for all 

prepared of different samples using Gamry electrochemical analyzer using a three-electrode system contains a 

Pt wire (counter electrode), Ag/AgCl (reference electrode), and glassy carbon ((3mm) WE) in an aqueous solution 

saturated with CO of three electrolytes acidic (0.1 M HClO4), neutral (0.5 M NaHCO3) and basic (0.1M KOH) at 

different sweep rate. 

 The first study showed the electrochemical activity and effect of mono metals NPs of Pd/Ti3C2Tx compared with 

metal-free Ti3C2Tx in acidic electrolyte, Interestingly, Ti3C2Tx displayed poorer COoxid activity and the integrating 

of Pd NPs enhance the activity, ascribed to the combination between outstanding physical and chemical 

properties of Ti3C2Tx and the catalytic advantages of Pd. In contrast, the second study the electrochemical COoxid 

activity for binary compared with mono NPs decorated Ti3C2Tx. Results showed   PdPt/ Ti3C2Tx was substantially 

superior to Pd/ Ti3C2Tx, Pt/ Ti3C2Tx, and metal-free Ti3C2Tx in three electrolytes experimentally, owing to the 

electronic and synergetic effect of PdPt and physiochemical properties of Ti3C2Tx. This study may pave the way 

for the employment of Ti3C2Tx in electrochemical COoxid.   

 

 

 

 

 



4 
 
 

DEDICATION 

 

I dedicate my dissertation work to my whole family and friends. A special feeling of gratitude to my loving parents, 

and my wife whose words of encouragement and push for tenacity ring in my ears.  

I also dedicate this dissertation to my supervisors (Prof. Kenneth Ozoemena, Prof. Aboubakr Abdullah, and Dr. 

Kamel Eid.) who have supported me throughout the process. I will always appreciate all they have done, 

especially Dr. Kamel Eid for helping me continuously to develop my skills. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



5 
 
 

ACKNOWLEDGEMENTS 

 

I would like to express my gratitude and deep thanks to Professor Kenneth Ozoemena, Professor Aboubakr M. 

Abdullah, and Dr. Kamel Eid for the great chance to work on this interesting project. There are not enough words 

to thank you for your continuous support since the first day of my registration, patience, guidance all the time. 

I am thankful to Dr. Kamel Eid and Professor Aboubakr M. Abdullah for assisting me in using the chemistry lab 

for the electrochemical COoxid application at Qatar University and the different analysis techniques for the materials 

characterization.  

Many thanks to all members of the School of Chemistry who assisted me on my MSc journey. Special thanks to 

Dr. Adewale of the Ozoemena group for helping me get settled and finding my way around Wits University and 

my project. 

I would also love to express my sincere appreciation for financial support by (i) the Qatar National Research Fund 

(QNRF, a member of the Qatar Foundation) through the National Priority Research Program Grant (NPRP) 

NPRP12S-0227-190168, (ii) International Research Collaboration Co-Fund grant, IRCC-2021-015 and (iii) the 

South African National Research Foundation (NRF). And the Open Access funding is provided by the Qatar 

National Library (QNL) 

My heartfelt gratitude to the Ozoemena family for supporting and to my family and friends for good wishes. 

 

 

 

 

 

 

 

 

 

 

 

 



6 
 
 

RESEARCH OUTPUT ARISING FROM THIS WORK 

 

PUBLICATION 

 

1. Salah, B., Eid, K., Abdelgwad, A. M., Ibrahim, Y., Abdullah, A. M., Hassan, M. K., & Ozoemena, K. I. (2022). 

Titanium Carbide (Ti3C2Tx) MXene Ornamented with Palladium Nanoparticles for Electrochemical CO 

oxidation. Electroanalysis, 34(4), 677-683. (doi.org/10.1002/elan.202100269). 

2. Ahmed Abdelgawad, Belal Salah, Kamel Eid, Aboubakr M. Abdullah, Rashid S. Al-Hajri, Mohammed Al-Abri, 

Mohammad K. Hassan, Leena A. Al-Sulaiti, Doniyorbek Ahmadaliev, andKenneth I. Ozoemena. Pt-Based 

Nanostructures for Electrochemical Oxidation of CO: Unveiling the Effect of Shapes and Electrolytes. Int. J. Mol. 

Sci. 2022, 23(23), 15034; (doi.org/10.3390/ijms232315034) 

 

MANUSCRIPTS FOR PUBLICATION 

 

1. Belal Salah, Kamel Eid, Abobakr Abdullah, and Kenneth I. Ozoemena. PdPt Nanoparticles Supported Ti3C2Tx 

MXene for Electrochemical CO oxidation Over a Wide pH Range.  (answer question and resubmitted to Energy 

& Environmental Science RSC publisher impact factor= 38.5) 

 

CONFERENCE PRESENTATIONS 

 

1. Belal Salah, Kamel Eid, Abobakr Abdullah, and Kenneth I. Ozoemena. Controlled fabrication of Titanium Carbide 

(Ti3C2Tx) MXene Decorated with Palladium Nanoparticles for Electrochemical CO oxidation. 2021 international 

conference on materials science and engineering Brisbane, Australia. 

2. Belal Salah, Kamel Eid, Abobakr Abdullah, and Kenneth I. Ozoemena. Controlled fabrication of Titanium Carbide 

(Ti3C2Tx) MXene Decorated with Palladium Nanoparticles for Electrochemical CO oxidation. Qatar University 

Annual Research Forum and Exhibition October 3-4, 2022, Qatar. 

 

 

 

 

 



7 
 
 

Table of Contents 

DECLARATION ................................................................................................ 2 

ABSTRACT ....................................................................................................... 3 

DEDICATION .................................................................................................... 4 

ACKNOWLEDGEMENTS ................................................................................. 5 

RESEARCH OUTPUT ARISING FROM THIS WORK ...................................... 6 

PUBLICATION ............................................................................................... 6 

MANUSCRIPTS FOR PUBLICATION ........................................................... 6 

CONFERENCE PRESENTATIONS .............................................................. 6 

LIST OF FIGURES ......................................................................................... 10 

LIST OF SCHEMES ........................................................................................ 13 

LIST OF TABLES ............................................................................................ 13 

LIST OF ABBREVIATIONS AND NOMENCLATURE ..................................... 13 

Chapter 1 ........................................................................................................ 17 

INTRODUCTION ......................................................................................... 17 

Motivation and Rationale .......................................................................... 17 

Statement of Research Problem............................................................... 17 

Research Aims and Objectives................................................................. 18 

Structure of the Dissertation ..................................................................... 18 

Refrences ...................................................... Error! Bookmark not defined. 

Chapter 2 ........................................................................................................ 20 

Introduction .................................................................................................. 20 

Carbon monoxide sources ........................................................................... 20 

Effects of Carbon monoxide ........................................................................ 21 

Effects on the human health ..................................................................... 21 

Effects on the plant ................................................................................... 22 

Effects on the environment ....................................................................... 22 



8 
 
 

Carbon monoxide oxidation (reactions mechanisms) .................................. 23 

Carbon monoxide oxidation methods .......................................................... 23 

Thermal CO oxidation............................................................................... 24 

Photocatalytic CO oxidation ..................................................................... 26 

Electrochemical CO oxidation .................................................................. 28 

Parameters effect on the CO oxidation activity ............................................ 30 

Carbon-based materials .............................................................................. 33 

References .................................................................................................. 38 

CHAPTER 3 .................................................................................................... 43 

Abstract ....................................................................................................... 43 

Introduction .................................................................................................. 43 

Experimental ................................................................................................ 44 

Chemicals and materials .......................................................................... 44 

Synthesis of Ti3C2Tx nanosheets ............................................................ 44 

Synthesis of Pd/Ti3C2Tx .......................................................................... 45 

Materials characterization ............................................................................ 45 

Electrochemical CO oxidation reaction ........................................................ 45 

Results and discussion ................................................................................ 45 

Conclusion ................................................................................................... 51 

References .................................................................................................. 51 

CHAPTER 4 .................................................................................................... 54 

Abstract ....................................................................................................... 54 

Graphical Abstract ....................................................................................... 54 

Introduction .................................................................................................. 54 

Results and discussion ............................................................................. 56 

Electrocatalytic CO oxidation activity in acidic media ............................... 62 

CO oxidation activity in neutral electrolyte ................................................ 65 



9 
 
 

CO oxidation activity in alkaline electrolyte............................................... 66 

Conclusions ................................................................................................. 68 

Experimental methods ................................................................................. 68 

Chemicals and materials .......................................................................... 68 

Synthesis of Ti3C2Tx nanosheets .............................................................. 69 

Synthesis of PdPt/Ti3C2Tx ........................................................................ 69 

Materials characterization......................................................................... 69 

Supporting information................................................................................. 69 

Electrochemical CO oxidation reaction ..................................................... 69 

References .................................................................................................. 74 

Chapter 5 ........................................................................................................ 77 

Conclusion ................................................................................................... 77 

Chapter 1 ..................................................................................................... 77 

Chapter 2 ..................................................................................................... 77 

Chapter 3 ..................................................................................................... 77 

Chapter 4 ..................................................................................................... 78 

Future Prospects ......................................................................................... 79 
 

 

 

 

 

 

 

 

 

 

 



10 
 
 

LIST OF FIGURES 

 

Figure 2.  1 : The anthropogenic sources of CO emissions. Data is obtained from ref [19] 

 

Figure 2.  2 : The effect of CO gas on human health. 

 

Figure 2.3: (a) SEM image and (b) TEM image of Pd/Cu/gC3N4NTs and (c) CO conversion efficiency related to 

the temperature. copyright permission Elsevier [10] (d) SEM image, and (e) TEM of the as-prepared CeO2-ZnO. 

(c) the XRD, and (f) Thermal COoxid for the prepared catalysts. Copyright permesiion elsevir [28]. 

 

Figure 2.4: (a) SEM image Cu2O/g-C3N4-4 (b) TEM and (c) Catalytic activities of Cu2O and Cu2O/g-C3N4. 

Copyright permission RSC [32]. 

 

Figure 2.5: (a) Photocurrent–voltage relations of Pt/TiO2 catalysts: a1, b1, c1 denote Pt content 0.5, 1, 2 wt.% 

stabilized under N2 respectively. a2, b2, c2 denote Pt content 0.5, 1, 2 wt.% annealed in air respectively. (b) 

Pt/TiO2 was annealed at 423 K for 2 h under N2 and then annealed at 673 K under air for 2 h, copyright permission 

from Elsevier [41]. 

 

Figure 2.6: (a) TEM of RP-AH, (b) Transient photocurrent response of prepared catalysts at a bias of10 mV vs 

SCE under UV irradiation, (c) A- Photocatalytic COoxid for the prepared catalyst, B- The stabilty recycling test of 

RP-AH. Copyright permission ACS 2016 [42]. 

 

Figure 2.7: (a) TEM image of multicubic PtNi, and (b) CO-stripping in (1 M) KOH electrolyte at a scan rate of 50 

mVs−1, Copyright permission RSC [48]. (c) TEM image of PtPdRu NDs, and (d) CO-stripping CV of different 

catalysts measured in 1 M NaOH CO-saturated at rate of 50 mVs -1 and Copyright permission RSC [51]. 

 

Figure 2.8: (a) SEM images, (b) TEM image of PtPd/gC3N4 nanorods. CVs in a CO-saturated aqueous solution 

of 0.1 M KOH at rate of 50 mVs−1 (c) in dark, and (d) under the light. Copyright permission RSC [6]. 

 

Figure 2.9: (a) COoxid conversation versus temperature over supported noble metal. Copyright permission 2010, 

Elsevier [54]. (b)TEM of the prepared Pd/COP-4 in DMF and water, and (C)the COoxid of Pd/COP-4 prepared by 

different solvents. Copyright permission 2013, RSC [55]. 

 

Figure 2.10: (a) Top and side views of the charge density difference of Pd/OV–Mo2CO2 (b) Reaction mechanism 

of COoxid via TER-mechanism, and (c) Adsorption configurations and adsorption energies. copyright permission 

RSC [60].  



11 
 
 

 

Figure 2.11: (a) The diagram for Cu3 active site of Cu3/d-Mo2CO2 works as an electron reservoir to facilitate the 

COoxid conversation rate. (b) COoxid pathways reactions (side views) over Cu3/d-Mo2CO2 by diffrent mechanisms. 

Copyright 2018, American Chemical Society [61]. 

 

Figure 2.12: Periodic tables showing compositions of MXenes and MAX phases. (a) Elements of MXenes 

chemical structure. (b) Elements of different MAX phases, Copyright 2019 American Chemical Society [67]. 

 

Figure 2.13: (a) the chemical structure for MAX and MXene phase (b) Schematic illustration of the development 

path of MXenes synthesis. Copyright 2022 Elsevier [69]. 

 

Figure 3.1: (a-b) SEM image of Ti3C2Tx MXene. SEM image (c) and TEM images (d-e) of Pd/Ti3C2Tx, HRTEM (f) 

of the marked area in (e). 

 
Figure 3.2: (a) SAED and elemental mapping images of (b) Ti, (c) C, (d) O, (e) F, and (f) Pd of Pd/Ti3C2Tx. 

 
Figure 3.3: XRD analysis of Pd/Ti3C2Tx and Ti3C2Tx. 

 
Figure 3.4: (a) XPS survey of Pd/Ti3C2Tx and Ti3C2Tx. High-resolution XPS spectra of (b) Ti, (c) C, (d) O, (e) F, 

and (f) Pd. 

 
Figure 3.5: (a) CVs measured in a N2 saturated 0.1 M NaHCO3 of Pd/Ti3C2Tx and Ti3C2Tx. (b) CVs and (c) LSV 

of Pd/Ti3C2Tx and Ti3C2Tx. (d) CVs measured at different scan rates in a CO-saturated an aqueous solution of 0.1 

M HClO4 at 50 mV s-1 and (e) the corresponding plots of forwarding peak currents against the square root of the 

scan rates. (f) EIS of Pd/Ti3C2Tx and Ti3C2Tx measured in a frequency range from 100 kHz to 5 Hz with an AC 

voltage amplitude of 0.8 V at an open circuit potential in 0.1 M HClO4. 

 
Figure 3.6: (a) The I-T tests measured for 2000 s at 0.8 V in a CO-saturated 0.1 M HClO4, (b) TEM image of 

Pd/Ti3C2Tx after COoxid stability test.  

 

Figure 4.1: images of SEM for (Ti3AlC2 MXene, (b) Pd/Ti3C2Tx, (c) Pt/Ti3C2Tx, (d-e) PdPt/Ti3C2Tx. (f-g) 

TEM images of PdPt/Ti3C2Tx and (i) HRTEM of PdPt NPs and (j) SAED of PdPt NPs The fabrication 

process of PdPt/Ti3C2Tx.    

 

Figure 4.2: Elemental mapping of elements for PdPt/Ti3C2Tx: (a) HAADF/STEM, (b) Ti, (c) C, (d) O, (e) 

F, (f) Pt, (g) Pd. (h) EDS, and (i) EDX. 

 

Figure 4.3: (a) XRD and (b) XPS surveys of the as-prepared catalyst. High-resolution XPS spectra of (c) 

Ti, (d) C, (e) O, (f) F, and (g) Pd, and (h) Pt.). 



12 
 
 

 

Figure 4.4: (a) CVs, and (b) LSVs of the as-prepared catalysts. CVs measured under different scan rates 

in saturated 0.1 M HClO4 aqueous solution by CO at 25 oC at 50 mV s-1 of (c) PdPt/Ti3C2Tx and (e) 

Pd/Ti3C2Tx with the corresponding plots of the forward peaks currents against the square root of the 

scan rates of (d) PdPt/Ti3C2Tx and (f) Pd/Ti3C2Tx. 

 

Figure 4.5: (a) CVs and (b) LSV of the as-prepared materials. CVs measured under different scan rates 

in saturated 0.5 M NaHCO3 aqueous solution by CO at 25 oC at 50 mVs-1 of (c) of PdPt/Ti3C2Tx (e) 

Pd/Ti3C2Tx with the corresponding plots of the forward peaks currents against the square root of the 

scan rates (d) of PdPt/Ti3C2Tx, and (f) of Pd/Ti3C2Tx. 

 

Figure 4.6: (a) CVs and (b) LSV of the as-prepared catalyst. CVs measured under different scan rates 

in saturated 0.1 M KOH aqueous solution by CO at 25 oC at 50 mV s-1 of (c) of PdPt/Ti3C2Tx (e) of 

Pd/Ti3C2Tx with the corresponding plots of the forward peaks currents vs the square root of the scan 

rates of (d) PdPt/Ti3C2Tx, and (f) Pd/Ti3C2Tx. 

 

Figure 4.S1: cyclic voltammetry measured in N2 saturated 0.1 M HClO4 at 50 mV s-1 of the as-prepared 

catalyst. 

 

Figure 4.S2: (a) The I-T tests measured for 2000 s in CO-saturated 0.1 M HClO4  of  PdPt/Ti3C2Tx at 0.71 

V and  Pd/Ti3C2Tx at 0.84V. CVs before and after stability tests (b) of PdPt/Ti3C2Tx, and (d) of Pd/Ti3C2Tx. 

LSV before and after stability tests (c) of PdPt/Ti3C2Tx, and (e) of Pd/Ti3C2Tx tested in 0.1 M HClO4 

aqueous solution saturated by CO at 50 mV s-1 and (f) EIS of the as-prepared catalyst (frequency, 100 

kHz - 5 Hz) of 0.78V in 0.1 M HClO4. 

 

Figure 4.S3: (a)TEM, and (b) EDX of PdPt/Ti3C2Tx after COoxid stability test in 0.1 M HClO4. 

 

Figure 4.S4: (a) The I-T tests recorded for 2000 s in CO-saturated 0.5 M NaHCO3 of PdPt/ Ti3C2Tx at 

0.23 V and Pd/ Ti3C2Tx at 0.39V. CVs before and after stability tests (b) of PdPt/ Ti3C2Tx, and (d) of 

Pd/Ti3C2Tx. LSV before and after stability tests (c) of PdPt/ Ti3C2Tx, and (e) of Pd/ Ti3C2Tx tested in 0.5 

M NaHCO3 aqueous solution saturated by CO of at 50 mV s-1 and (f) ) EIS of the as-prepared catalyst 

(frequency, 100 kHz - 5 Hz) of 0.3V in 0.5 M NaHCO3. 

 

Figure 4.S5: (a) The I-T tests measured for 2000 s in CO-saturated 0.1 M KOH of PdPt/Ti3C2Tx at -0.19 

V and Pd/Ti3C2Tx at 0.02 V. CVs before and after stability tests (b) of PdPt/Ti3C2Tx, and (d) of Pd/Ti3C2Tx. 

LSV before and after stability tests (c) of PdPt/ Ti3C2Tx, and (e) of Pd/T Ti3C2Tx tested in 0.1 M KOH 



13 
 
 

aqueous solution saturated by CO at 50 mV s-1 and (f) EIS of the as-prepared catalyst (frequency, 100 

kHz - 5 Hz) of 0.15V in 0.1 M KOH. 

 

 

LIST OF SCHEMES 

 

Scheme 4.1: The fabrication process of PdPt/ Ti3C2Tx N. 

 

Scheme 4.2: The proposed COoxid mechanism on PdPt/ Ti3C2Tx in HClO4 electrolyte  

 

LIST OF TABLES 

 

Table 2.1: The COoxid performance for different catalysts and (T100) the temperature required for 

complete COoxid to CO2. 

Table 2.1:  The electrochemical COoxid performance for different catalysts in different mediums. 

Table 4.1: The CO adsorption capacity on thus obtained materials in different electrolytes obtained 

from the integration of the anodic peak current area. 

 
Table 4.S1 Detailed XPS binding energies of Ti, C, F, O, d, and Pt in as-synthesized PdPt/ Ti3C2Tx 

 

LIST OF ABBREVIATIONS AND NOMENCLATURE 

 

 

Abbreviation Description 

2-D 

Ag|AgCl 

two-dimensional 

Silver/Silver Chloride 

BET Branauer-Emmett-Teller 

CA Chronoamperometry 



14 
 
 

CE Counter electrode 

CH3CH2OH Ethanol 

cm Centimeters 

CPE Constant Phase Element 

CV 

COoxid 

Cyclic Voltammetry 

CO oxidation 

D.C. Direct Current 

DLC Double Layer Capacitance 

E Ideal potential of the cell 

ECSA Electrochemically Active Surface Area 

EDS Energy Dispersive X-ray Spectroscopy 

EEC Electronic Equivalent Circuit 

EIS Electrochemical Impedance 

Spectroscopy 

F Faraday’s constant 

FC Fuel Cell 

FWHM Full Width at Half Maximum 

GCE Glassy carbon electrode 

GDL Gas diffusion layers 

HRTEM High resolution transmission electron 

microscopy 

I Current 

J Current density 

jo,s Exchange current density 

jo,m Mass activity 



15 
 
 

LSV Linear Sweep Voltammetry 

mA 

mm 

Milliamperes 

Millimeter  

mg Milligram 

min Minutes 

mL Millilitre 

N2 Nitrogen 

nm Nanometer 

O2 Oxygen 

Pd Palladium  

PEIS Potentiostatic Electrochemical 

Impedance Spectroscopy 

Pt Platinum 

PXRD Powder X-ray diffraction 

R Resistance 

Rct Capacitive resistance 

RE Reference electrode 

rpm Revolution per minute 

Rs 

RT 

Series connection resistance 

Room temperature 

s Seconds 

SEM Scanning electron microscopy 

TEM Transmission electron microscopy 

um Micrometer 

WE WE 



16 
 
 

wt % Weight percentage 

XAS X-Ray Absorption Spectroscopy 

XPS X-Ray Photoelectron Spectroscopy 

XRD X-Ray Diffraction 

Z’ Real impedance 

Z” Imaginary impedance 

θ Scattering angle 

λ Wavelength 

ω Frequency 

  

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 



17 
 
 

Chapter 1 

 

INTRODUCTION 

This chapter summarizes the motivation and objectives of this research project, as well as the 

structure of the dissertation. 

Motivation and Rationale 

The need for traditional energy alternatives such as coal and petroleum is vital to protect and 

enhance the surrounding environment from the effect of emitted toxic gases like CO2, and CO 

that led to climate change and negatively affected living organisms [1-3]. So, clean and renewable 

energy is one of the most important topics in the scientific arena like solar cell energy and wind. 

In contrast, Proton exchange membrane fuel cells are sustainable, environmentally green, and 

efficient energy conversion technologies. however, poisoning by CO reduces their catalytic 

activity [4, 5]. Therefore, the COoxid reaction is of particular interest in industrial, fuel cells, and 

environmental applications. Engineering new low cost with higher activity compared to the 

commercial catalyst Pt/C is crucial for electrochemical COoxid [6-10]. This study paves a new 

avenue for the facile preparation of new catalysts via the combination of unique physicochemical 

properties of MXene and the superior catalytic properties of Pt and Pd mono and binary NPs. 

MXene is used here for the first time experimentally in electrochemical COoxid and will pave the 

way to use it in different electrochemical applications. 

Statement of Research Problem 

Electrochemical COoxid reaction is highly attractive in environmental applications due to the higher 

toxicity towards the living organism with low ppm concentration, and also an as undesirable 

product in the industry. In contrast, the CO poisoning in the proton exchange membrane fuel cells 

which is highly efficient energy weakens their performance [11-13]. The need for low-cost 

catalysts with high performance is highly important for the industry. Also, Pt and Pd are the most 

promising catalysts for COoxid but their high cost, stability, and self-poisonings are critical issues 

that faced the commercialization process. So the need for designing new low-cost catalysts for 

electrochemical COoxid is important with higher activity and stability [14-16]. Interestingly, the 

unique physicochemical properties of MXene like , abundant active sites, electron density great 

defects, abundant surface functionalities (i.e., O2, OH, and F), and high electrical conductivity, 

which are significant merits in various catalytic applications, [17-24] when combined with the 

unique catalytic properties of Pt and Pd as mono or binary NPs with less than 15nm size and low 

amount > 3wt% found promising for electrochemical COoxid.  This study used MXene for the first 



18 
 
 

time experimentally in COoxid, and will pave the way for applying it in different application.    

Research Aims and Objectives 

The aim of this research is to explore the feasibility of combination between the low cost with 

outstanding physicochemical properties of Ti3C2Tx nanosheets and low wt% (<3) of the catalytic 

merits of mono and binary Pd and Pt NPs experimentally for the first time in the electrochemical 

COoxid application in wide PH range. 

This study involves synthesizing different catalysts of mono palladium decorated Ti3C2Tx in the 

first study and PtPd decorated Ti3C2Tx in the second study, and the evolution of their physical 

characteristics – structural and morphological properties besides studying the catalytic effect by 

the combination between Ti3C2Tx and metals NPs and catalysts effect on the electrochemical 

COoxid reaction properties. The electrochemical COoxid was evaluated using cyclic voltammetry 

(CV), linear sweep voltammetry (LSV), chronoamperometry (CA), and electrochemical 

impedance spectroscopy (EIS). 

The microelectrochemical COoxid is designed, fabricated, and tested to prove the COoxid concept. 

Structure of the Dissertation 

Chapter one: discusses the rationale behind this research, as well as the aims and objectives. 

Chapter two gives a brief overview of the electrochemical COoxid related to my work. 

Chapter three presents the first experimental study of mono metals nanoparticles (Pt and pd) 

decorated MXene for the electrochemical COoxid (published in an electro analysis journal). 

Chapter four the experimental studying and comparison of electrochemical COoxid activity for 

mono or binary metals Nanoparticles in wide PH rang (Answer questions and resubmitted to the 

Energy & Environmental Science) 

Finally, Chapter five concludes the dissertation and gives suggestions for future directions. 

 

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19 
 
 

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Doped Carbon Nanotubes/Nitrogen‐Doped Reduced Graphene Oxide Hybrid for Efficient Oxygen 
Reduction. 2017. 23(3): p. 637-643. 

14. Liu, X., et al., Superior catalytic performance of atomically dispersed palladium on graphene in 
COoxid. 2020. 10(5): p. 3084-3093. 

15. Xie, S., et al., Highly Active and Stable Palladium Catalysts on Novel Ceria–Alumina Supports for 
Efficient Oxidation of Carbon Monoxide and Hydrocarbons. 2021. 55(11): p. 7624-7633. 

16. Salama, R.S., et al., Palladium supported on mixed-metal–organic framework (Co–Mn-MOF-74) 
for efficient catalytic oxidation of CO. 2021. 11(8): p. 4318-4326. 

17. Li, X., et al., Applications of MXene (Ti 3 C 2 T x) in photocatalysis: A review. 2021. 2(5): p. 1570-
1594. 

18. Tahini, H.A., X. Tan, and S.C.J.C. Smith, Facile CO oxidation on Oxygen‐functionalized MXenes 

via the Mars‐van Krevelen Mechanism. 2020. 12(4): p. 1007-1012. 
19. Chen, Z., et al., Grafted MXene/polymer electrolyte for high performance solid zinc batteries with 

enhanced shelf life at low/high temperatures. 2021. 14(6): p. 3492-3501. 
20. You, Z., et al., State-of-the-art recent progress in MXene-based photocatalysts: a comprehensive 

review. 2021. 13(21): p. 9463-9504. 
21. Zhao, R., et al., Self-assembled Ti 3 C 2 MXene and N-rich porous carbon hybrids as superior 

anodes for high-performance potassium-ion batteries. 2020. 13(1): p. 246-257. 
22. Gao, Q., et al., Tracking ion intercalation into layered Ti 3 C 2 MXene films across length scales. 

2020. 13(8): p. 2549-2558. 
23. Peng, Y.-Y., et al., All-MXene (2D titanium carbide) solid-state microsupercapacitors for on-chip 

energy storage. 2016. 9(9): p. 2847-2854. 
24. Zhao, D., et al., Alkali-induced 3D crinkled porous Ti 3 C 2 MXene architectures coupled with 

NiCoP bimetallic phosphide nanoparticles as anodes for high-performance sodium-ion batteries. 
2019. 12(8): p. 2422-2432. 

 
 
 
 
 
 
 
 
 
 
 
 
 



20 
 
 

Chapter 2 

(LITERATURE REVIEW) 

Introduction 

COoxid is considered a hot research topic due to the high toxicity of CO with severe threats to the 

environment and human health. CO is a colorless, tasteless, and odorless poisonous gas, even in small 

traces. It’s therefore important to purify the air and enclosed atmospheres from the CO by controlling the 

main sources either by using catalysts to prevent the formation CO as a byproduct or via catalytic oxidation 

of the produced CO directly to CO2; or indirectly to valuable products.  This requires designing new low-

cost catalysts with high catalytic activity and durability. 

Interestingly, CO can be involved in a wide range of catalytic reactions, such as mixing CO and 

hydrogen (syngas) and convert thermally to light olefins via (Fischer-Tropsch) synthesis [1-3]. Additionally, 

CO can be electrochemically oxidized to CO2 for electric power production [4-6]. Likewise, it can be 

catalytically oxidized thermally or photo-oxidation to CO2 [7, 8]. The electrochemical or 

photoelectrochemical of CO2 produce fuels like formic acid, methanol, and methane in addition to electric 

power production [9-12]. Pt-based catalyst is a highly efficient catalyst but the high cost and the poisoning 

effect are the main problems [6, 13]. So the need for designing a new low-cost and efficient catalyst for 

COoxid is an important issue, where carbon-based materials are considered a good option to use as a 

catalyst for COoxid related to the unique catalytic properties as well as the low cost and earth-abundant.  

The research presented here focuses on designing new catalysts based on using MXene as the carbon-

based catalyst for the first time in experimental COoxid. 

Carbon monoxide sources 

 CO is released via several natural and anthropogenic sources. Naturally, methane 

oxidation that derived from the photochemical reactions in the soil, which is the main source of 

CO emissions in the environment [14]. Additionally, CO can be naturally released from the 

molten rocks of volcanos and also from the photochemical oxidation of the organic materials in 

wastewater and solid waste. It is estimated that 60% of the emitted CO come from 

anthropogenic sources and can be classified into three different categories [19]: wood burning, 

industry, and transportation which is the largest anthropogenic source of CO production. 

Figure (1) shows the contribution of different pollutants to CO emissions [19]. It can be seen 

that the CO released from transportation including on-road mobile sources and non-road 

mobile sources accounts for 71% of total CO emissions.  Another report suggests that 90% of 

anthropogenic CO emissions come from the transportation sector [20]. The incomplete 

combustion that occurs inside the engine of the vehicle is responsible for the CO production; 

the mount of which depends on the engine efficiency. The amount of CO released from the 

engine is increased in cold climates, during the cold start of mobile and while vehicles are 



21 
 
 

moving at low speeds [20].  

 

Figure 1: The anthropogenic sources of CO emissions. Data is obtained from ref.[19] 

Effects of Carbon monoxide 

CO is a highly toxic, flammable, colorless, tasteless, and odorless gas with a lower 

density than air. The effect of CO on human health, plants, and the surrounding environment 

is discussed in detail below. 

 Effects on the human health  

All living organisms are affected by CO gas toxicity even at low concentrations in air. 

CO can cause headache or dizziness at concentrations lower than 50 ppm as seen in Figure 2 

[14, 19]. At slightly higher concentratons (50-1200ppm), CO leads to more dangerous 

symptoms such as dyspnoea, loss of vision, and coma. At higher concentrations up to 1200 

ppm, CO can lead to a serious health effects such as comma or seizures.  At concentrations 

higher than 1900, exposure to CO can be lethal. This lethal effect is related to the faster 

interaction of the CO molecules with the hemoglobin of red blood cells at a rate (200 times) 

faster than the O2-producing carboxyhemoglobin (CoHb) which disrupts the normal O2 level in 

blood. 



22 
 
 

 

Fig 2. The effect of CO gas on human health.  

Effects on the plant 

  Most plants have the ability to oxidize the CO to CO2 and used it with water under 

sunlight to produce energy and oxygen which is known as the photosynthesis process. 

Although, continuous exposure to CO is harmful to the plant with time. CO can affect the 

nitrogen and protein that are responsible for the growth process and damage the germination 

system which leads to a negative effect on food quality.[14, 19] in addition to the ability to 

damage the cellular respiratory system of plants and reduce water adsorption.  

Effects on the environment 

The negative effect of CO also extends to the environment and is considered one of 

the green gas groups responsible for global warming. Indirectly, CO can increase the global 

warming potential of green gases at a higher level in the atmosphere and can increase the 

depletion of the ozone layer and global warming where CO has the ability to react with OH 

radicals decreasing its level in the atmosphere.[19] Moreover, the higher concentration of CO 

in the atmosphere leads to the form of toxic ozone (tropospheric greenhouse gas) which 

affects the respiratory system. The amount of CO in the atmosphere varies widely all over the 



23 
 
 

world and over the year.[19] Remarkably, there was a significant drop in CO level in the 

atmosphere after spreading the catalytic converters in vehicles all over the world which 

convert it to CO2. 

 

Carbon monoxide oxidation (reactions mechanisms)  

There are two mechanisms commonly used for COoxid:  Langmuir-Hinshelwood (LH) and Mars-van 

Krevelen (MvK)) mechanisms. In LH mechanism, both CO and O2 gases are adsorbed (COads and Oads) 

while the metal oxide interface and the utilized support boost the catalytic activity of the oxides of the 

transition metal group as a promotor [14, 15]. The COoxid reaction happens over the catalyst surface via 3 

steps as described in the following equations: - 

𝐶𝑂 (𝑔) → 𝐶𝑂 𝑎𝑑𝑠                                   

𝑂2(𝑔) → 2𝑂 𝑎𝑑𝑠                                    

𝐶𝑂 𝑎𝑑𝑠 + 𝑂 𝑎𝑑𝑠 → 𝐶𝑂2(𝑔)                  

In the case of MvK mechanism, the CO and O2 reactants molecules are adsorbed on the surface 

of the catalyst. This is followed by the O2 splitting into two oxygen atoms and having the ability to spread 

over the whole metal surface to combine with CO to give CO2 [14, 16]. Lastly, the formed CO2  is desorbed 

leaving the catalyst surface into the gas phase. The rate of CO reaction is controlled by the adsorption of 

CO and O2 dissociation, which in general happen at higher temperatures. The CO molecules in the gas 

phase are directly oxidized to CO2 in presence of O species but the reaction rate of CO2 formation is 

directly proportional to the catalyst surface coverage with Oads and COads as described in the following 

equations: - 

𝑂2 + 2 ∗  → 2𝑂 𝑎𝑑𝑠                              

𝐶𝑂 𝑎𝑑𝑠 + 𝑂 𝑎𝑑𝑠 → 𝐶𝑂2 + 2 ∗            

𝐶𝑂 +  ∗  → 𝐶𝑂 𝑎𝑑𝑠                               

In the oxidation step, the oxygen of the metal oxide surface is responsible for the oxidation of CO 

which leaves oxygen vacancies behind that reduce the oxidation state of metal ions to a lower value. The 

metal ions get oxidized again by consuming the available oxygen gas phase.  

Another mechanism knows as Eley-Rideal (ER) mechanism suggests that the CO and adsorbed 

O2 reactants directly reacted to form CO2 [17, 18]. 

𝐶𝑂 (𝑔) + 𝑂2 𝑎𝑑𝑠 → 𝐶𝑂2 + 𝑂 𝑎𝑑𝑠  

     

Carbon monoxide oxidation methods 

CO can be catalytically converted into CO2 gas by thermal, photocatalytic, electrochemical or 



24 
 
 

photo-electrochemical means. In thermal catalytic CO conversion, CO gas reacts with O2 at a higher 

temperature and catalysts are used to decrease the required temperature to save energy. In the case of 

the photocatalytic CO conversion, CO converts to CO2 by using photo catalysts where the catalyst surface 

splits O2 into two Oxygen atoms with light photons' assistance and then combined with CO to form CO2. 

In the case of the electrochemical CO conversion, the oxidation reaction can occur in different electrolytes 

over a wide range of pH under applied potential. The methods are described below in detail with examples. 

Thermal CO oxidation 

In the thermal COoxid method CO gas reacts with O2 gas in the presence of an assistance heat 

source and the CO conversion proportional to the reaction temperature and is function of gas flow rate, 

catalyst type and loading amount. To satisfy the standers of fuel efficiency with low emission of toxic gases, 

especially CO. In addition, complete CO conversion to CO2 at a low temperature lower than 100°C. Many 

efforts have been made to design different catalysts for COoxid at low temperatures (T100) as revealed in 

table (1) [21-25]. Nobel metals like (Pt, Pd, Au, and Ag) as mono, binary, and ternary metals-based 

catalysts, and their mixtures are most commonly used for thermal COoxid due to their high catalytic 

properties and work at lower temperatures even it can work atRTor 0oC. [62]. Where Au-based catalyst is 

the best for thermal COoxid even can work atRTand (Pt, Pd) have good thermally catalytic conversion but 

not lower than 100oC. But the rarity in nature, high cost, and also the poisoning effect of CO barred to use 

of it directly in catalytic CO converters. So is reinforced by using different supporters like metal oxides, and 

carbon materials which can rise the catalytic activity and stability besides to be cost-effective to be 

applicable. For example, in figure (3a-c) the COoxid activity for porous gC3N4 nanotubes doped with Pd and 

Cu (Pd/Cu/gC3N4NTs) was superior complete conversion temperature (T100) 154 °C related to the 

synergatic effect throug the combination between Pd and Cu and the outstanding catalytic propertirs of 

gC3N4 as suborter [10]. Pd-impeded 3D porous graphene with nanohole structrue has a complete 

conversion temperature (T100)  at 190°C[26]. The binary metals AuPd NPs decorated TiO2 catalyst gave 

a full CO conversion at (T100) 190 °C [27].  Fan et al. have synthesized Pt/CeO2 nanorodes with a complete  



25 
 
 

 

Fig.3  (a) SEM image and (b) TEM image of Pd/Cu/gC3N4NTs and (c) CO conversion efficiency 

as a function of temperature. copyright permission Elsevier [10] (d) SEM image, and (e) TEM of 

the as-prepared CeO2-ZnO. (c) the XRD, and (f) Thermal COoxid for the prepared catalysts. 

Copyright permesiion elsevir [28].  

CO converted to CO2 at 80 °C when Ce-BTC was used, where the MOF-derived CeO2 is superior to 

synthesis CeO2 [29]. Transition metal oxides or mixed are greatly active towards the thermal COoxid like 

(Fe2O3, CeO2, MnO2, NiO, Co3O4, CuO, Cr2O3, etc). low price high performance and different preparation 

methods with high yield make it easily applicable commercially. The cobalt-based catalysts are highly 

active catalysts in CO conversion and have an outstanding thermal resistance.[30, 31] Fig (3d-f) showed 

a mixed oxide of hierarchically CeO2-ZnO nanostructure with higher surface area (100 m2/g), which was 

used for thermal COoxid [28]. The meso-/macroporous structure contains wurtzite ZnO and cubic 

CeO2 phases confirmed by the XRD. The hierarchical structure was found to be a significant way to 

increase the COoxid activity. The COoxid activity for Ce60Zn40 is better compared to the other prepared 

catalysts with complete CO conversation at 250oC [28]. Additionally, combination  the merties catalytic 

properties of oxides and outstanding physicochemical advantages of carbon materials is good in CO 

conversion. For example, figure 4 shows Cu2O/g-C3N4 with amass ratio 4:10 showed a good catalytic 

conversation at (T100) ∼200 °C due to the synergetic effects act between the 2D lamellar g-C3N4 and Cu2O 

besides the stable shape and structure [32]. 

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26 
 
 

 

Fig.4 (a) SEM image Cu2O/g-C3N4-4 (b) TEM and (c) Catalytic effeciency of Cu2O and Cu2O/g-C3N4 

with different mass ratios. Copyright permission RSC [32]. 

Furthermore, there are different materials like chalcogenides (MoS, CuS, and FeS)  and 

perovskites which have the potential for CO conversion [33-35].  A theoretical study investigated that Al-

doped MOS2 is a promised single atom catalyst in COoxid [35].  Macroporous structure La0.8Sr0.2CoO3 

perovskites with oxygen vacancy-rich have a superior activity in COoxid with a complete CO conversion 

lower than 200oC.[36] 

Table 1. The COoxid performance for different catalysts and (T100) the temperature required for complete 

COoxid to CO2. 

Catalyst T100 (°C) Ref. 

Au/Pd/gC3N4NFs 144  [24] 

Pd/Cu/gC3N4NTs 154  [10] 

Pd-impeded nanohole structured 

3D porous graphene 

190 [26] 

AuPd/TiO2 190  [27] 

Pt/CeO2 80 [29] 

Cu2O/C3N4 200 [32] 

AuPd/SiO2 182  [37] 

AuPd@Al2O3 200 [38] 

Pd/La-doped -alumina 175  [25] 

MnOx 310  [22] 

Cu1/Mn1 180  [21] 

Co3O4/mesoporous g-C3N4 160  [23] 

 

Photocatalytic CO oxidation 

Herein, the Photocatalytic COoxid method uses photocatalysts which work depending on the e- and 

h+ that are generated by photo power assist as redox reactants. Where the COoxid rate depends on 

generated photocurrent velocity [39, 40]. Nobel metals are known as the best catalysts with superior the 

photo-induced electron transfer rate when added to photocatalytic active materials like TiO2, CeO2, and 



27 
 
 

carbon materials [40]. Furthermore, the superior photocurrent means more charge transfer which 

accelerates the chemisorption dissociation of O2 on the catalyst surface. TiO2 is known as one of the most 

photocatalytic activities. For instance, fig 5 shows Pt/TiO2 catalyst that is treated under air (673 K) has 

superior photocatalytic COoxid compared to only the treated under N2 and increases with increasing the Pt 

content which increases the kinetic reaction and also proved by measuring the photoelectrochemical 

current [41].  

 

Fig 5 (a) Photocurrent–voltage relations of Pt/TiO2 catalysts: a1, b1, c1 denote Pt content 0.5, 1, 2 wt.% 

stabilized under N2 respectively. a2, b2, c2 denote Pt content 0.5, 1, 2 wt.% annealed in air respectively. 

(b) Pt/TiO2 was annealed at 423 K for 2 h under N2 and then annealed at 673 K under air for 2 h, copyright 

permission from Elsevier [41]. 

The RP-AH (p-type) catalyst shows a superb photocatalytic activity compared to RP, RP-A, and RP-H 

(Ru/TiO2/Pt) in COoxid for 1000 ppm CO under UV irradiation and completely oxidized after 2hr. the higher 

hole mobility assets of RP-AH benefit the holes reactivity in converting CO into CO2 with O− [42]. The RP-

AH has a stable reactivity after 4 cycles of 100% that is related to the synergetic and the alloying effect of 

both Pt and Ru metals as well as combined with TiO2 figure 6. 

 

Fig 6 (a) TEM of RP-AH, (b) Transient photocurrent response of prepared catalysts at a bias of10 mV vs 

SCE under UV irradiation, (c) A- Photocatalytic COoxid for the prepared catalyst, B- The stabilty recycling 

test of RP-AH. Copyright permission ACS 2016 [42].  

In contrast, the electrochemical COoxid can be enhanced when the catalyst is exposed to a light source like  

PtPd/gC3N4 nanorods was found to be 1.48 fold current density higher than in dark in KOH electrolyte 



28 
 
 

related to the unique photocatalytic properties of gC3N4 fig(8d) [6].  

 

Electrochemical CO oxidation 

The electrochemical COoxid conversion is greatly important to convert the dissolved CO in different 

electrolytes to CO2 under applied voltage between the WE loaded with the catalyst and the counter. The 

technique can be also used to convert CO indirectly to valuable fuels, in addition, to examining the 

catalyst's surface stability towards CO poisoning through the electrochemical CO stripping measurements 

that affect the catalytic activity in different electrochemical reactions like MOR. [43-47]. This method 

converts the dissolved CO in electrolytes to CO2 under applied potential in a wide PH range (table 2). 

Nobel metals have superior activity in COoxid related to their outstanding catalytic properties [48, 49]. For 

instance, The electrochemical COoxid for Pt dendrimer-encapsulated nanoparticles provided a current 

density of 0.2 mAcm−2 in 1M HClO4 and a sweep rate of 50 mV/s vs reference electrode (Hg/Hg2SO4) at 

0.3 V [49]. The Pt/SnOx catalysts shows a good  COoxid current density of about 0.87 mAcm−2 with a lower 

onset potential of 0.7 V compared to commercial Pt/C Pt/SnOx with a sweeping rate of 20 mV s−1 in 1 M 

HClO4 vs RHE [50]. Binary PtNi multi cubes with particle sizes of about 40nm were conducted with the CO 

stripping after adsorption of CO electrochemically in 1M KOH at 0.1V for 900s followed by flushing in N2 

gas (Fig 7a-b). The CVs display a higher current density of (0.58 mA cm−2) at 0.65 V at a rate of 50 mV s–

1 compared to commercial Pt/C. This shows the multi-cubic structure of PtNi has a superior activity for 

COoxid [48]. Fig 7c shows a ternary porous nanodendrites PtPdRu (NDs) with highly porosity degree which 

displays a superior anti CO-poisoning compared with PtPdRu nanoflowers, PtPd NDs, and commercial 

catalyst (Pt/C) fig 7d [34]. 

 



29 
 
 

Fig.7  (a) TEM image of multicubic PtNi, and (b) CO-stripping in (1 M) KOH electrolyte at a scan rate of 50 

mVs−1, Copyright permission RSC [48]. (c) TEM image of PtPdRu NDs, and (d) CO-stripping CV of different 

catalysts measured in 1 M NaOH CO-saturated at rate of 50 mVs -1 and Copyright permission RSC [51]. 

  

The CO stripping for PtRu/C gave a higher current density in 0.1 M H2SO4 at a scan rate of 20 mV/s of 5 

mA cm−2 versus the WE RHE at 0.6 V [52]. In another study, the COoxid activity for PtPd/gC3N4 nanorods 

with 1.5 wt.% metals was greater than commercial Pt/C in KOH electrolyte [6]. Which gave a higher current 

density of 14.75 mA/cm2 at a lower voltage for PtPd/gC3N4 nanorods 2 times more than Pt/C as shown in 

fig (8a-c). This is owing to the superior electrochemical surface area for the prepared nanorods 75 m2 /g 

in addition to the synergetic effect electronic of PtPd together. As shown below, the integration between 

the unique catalytic properties of noble metals and the physicochemical merits of carbon-based materials 

gave a good activity and stability with low cost and high production yield. 

 

 

Fig.8 (a) SEM images, (b) TEM image of PtPd/gC3N4 nanorods. CVs in a CO-saturated aqueous solution 

of 0.1 M KOH at rate of 50 mVs−1 (c) in dark, and (d) under the light. Copyright permission RSC [6]. 

 

Table 2. The electrochemical CO oxidation performance for different catalysts in different mediums. 

Catalyst Medium / Scan 

rate / reference 

electrode 

Maximum 

Current 

(mA/cm-2) / 

Ref 



30 
 
 

Voltage (V) 

60 wt % Pt/C 0.5 H2SO4 

10 mVs−1 

 SHE 

0.2 & 0.64  [43] 

Well-ordered 

Pt(111) 

0.1 M NaOH 

50 mV/s 

RHE 

0.5 & 0.8  

 

[44] 

PtNi multicubes 1 M KOH 

50 mV/s 

RHE 

0.58 & 0.65  [48] 

PtPd 

Nanodendrites 

1 M KOH 

50 mV/s 

Ag/AgCl 

5.1 & -0.15  [51] 

Pt dendrimer-

encapsulated 

nanoparticles 

0.1m HClO4 

50 mV/s 

Hg/Hg2SO4 

0.2 & 0.3  [49] 

Pt(110)–Ru 0.5 M H2SO4 

100 mV/s 

RHE 

0.025 & 0.5  [45] 

Pt/SnOx 1 M HClO4 

20 mV s−1 

RHE 

0.87 & 0.7  [50] 

Pt-NbOx 0.5 M H2SO4  

20 mV/s 

RHE 

0.5 & 0.75  [46] 

Pt(FAM) 0.1 M H2SO4  

50 mV/s 

RHE 

0.32 & 0.72  [47] 

PtRu/C 0.1 M H2SO4  

20 mV/s 

RHE 

5 & 0.6  [52] 

 

 

Parameters effect on the CO oxidation activity 

CO conversion activity and durability to CO2 for catalysts may be influenced by various parameters like 

the catalyst type and its composition as (Nobel metals, transition metals oxides, carbon materials, 



31 
 
 

halloysite, and chalcogenides), morphology (shape structure, size, and surface area), preparation 

methods, supporter type and loading amount in addition to the CO conversion method and the parameters 

of the pretreatment [53]. These parameters can predict and determines the catalytic activity, catalyst 

stability, and durability towards COoxid. Noble metals are among the most active catalysts towards COoxid, 

ascribed to the outstanding catalytic activity and electronic behavior [44]. Moreover, noble metals give a 

superior current density in electrochemical COoxid and can significantly promote the photocatalytic CO 

conversion [51]. However, the high cost, rarity, and CO poisoning hinder makes them non-ideal 

candidates. Various efforts were dedicated to developing low-cost, efficient catalysts for COoxid and 

preventing CO poisoning in fuel cells and came to fruition in tailoring. Transition metals, carbon materials, 

and other developed materials show a good catalytic activity toward COoxid, and the activity is enhanced 

when combined with Nobel metals due to their unique catalytic properties. In addition to the parameters 

mentioned above, other parameters can improve the catalytic activity as well as the stability. For instance, 

The preparation method can affect the COoxid efficiency, Santos et al. found the metallic dispersion of 

different noble metals (Pd, Rh, Pt, Au, and Ir) over TiO2 as a support affected by the synthesis method, for 

instance, the Pd NPs synthesized by incipient wetness impregnation (IMP) was 8 nm and by liquid-phase 

reduction deposition (LPRD) was 3nm with higher dispersion, as shown in fig 9a using LPRD method is 

preferred to prepare the catalyst that gave a complete CO conversation at a lower temperature compared 

with the applied IMP method [54]. Binary metals AuPd NPs decorated TiO2 catalyst gave a complete CO 

conversion at (T100) 190 °C when thermally treated in reducing (H2), but gave a lower (T100) 240 °C if treated 

under O2 (oxidizing). This is related to morphological changes [27]. Also, the surface area and the pore 

size for AuPd@Al2O3 have been affected by the calcination temperature which increased by increasing the 

temperature where the catalyst calcined at 473K gave the highest activity with complete CO conversion at 

423K  [38]. The solvent type that used in the preparation of catalysts 

 

Fig. 9 (a) COoxid conversation versus temperature over supported noble metal. Copyright permission 2010, 



32 
 
 

Elsevier [54]. (b)TEM of the prepared Pd/COP-4 in DMF and water, and (C)the COoxid of Pd/COP-4 

prepared by different solvents  Copyright permission 2013, RSC [55]. 

can affect its catalytic activity towards the COoxid. For example, when the water was used as a solvent for 

the preparation of Pd/COP-4, the Pd size outside pores of COP-4 was found about 2.4 nm, and in the case 

using the DMF as solvent was found 1.1 nm inside the pores due to the DMF contains Pd ions can easily 

enter the pores as well as reduced inside the pores while water cannot [55]. The COoxid activity for Pd/COP-

4 prepared by the DMF solvent displays an excellent COoxid activity compared with the water fig 9b-c. The 

shape and size can enhance the catalytic COoxid activity by increas the active surface area. where, The 

CO conversion for Ru nanoworms decorated TiO2 gave a complete CO conversion at a lower temperature 

(150 °C) compared with using Ru nanoparticles, which attributed to the unique structure of 1-D with a low 

diameter of 1.6 nm and 13.6 nm in length also the high dispersity and high active surface area. The catalyst 

morphology has an effect on the catalytic activity, the wormlike shape of Ru NWs on TiO2 supporters was 

found to have a complete CO conversion lower than using Ru NPs by 50 oC [56]. Due to the catalytic 

properties of shape and sub-nanosize effect rushes the kinetic reaction. The COoxid activity for AuPd/SiO2 

was found to depend on the preparation method, Au: Pd weight ratio in catalyst, and also the ratio effect 

on the PdAu particle size. The alloying prevents oxidation compared to the monometallic Pd which 

negatively affects the activity [37]. The addition of additives can affect the catalytic activity, Tanikawa et al, 

this group prepared two different heterogeneous mixtures (Pd/Ba/Al) and (Pd/Ba/CZ) and investigated the 

outcome of Ba loading on the COoxid activity. They found the increase of Ba loading in Pd/Ba/Al catalyst, 

the COoxid activity enhanced. Contrary to for Pd/Ba/CZ, the activity decreased. They suggested that the 

increase of Ba when combined with CZ supporter it avoided the direct contact between CZ and Pd so the 

supply rate of oxygen from CZ to Pd will decrease and the COoxid rate will reduce [57].  Pt-based catalysts 

like Pt/CeO2/I3-Al2O3, and Pt/CeO2 have a great activity toward the COoxid conversation [58, 59]. 

Surprisingly, the electrochemical COoxid for the prepared one-dimensional doped graphitic carbon nitride 

(1-D PtPd/gCNs) shows a superior catalytic activity and gave current density of 14.75 mA/cm2 at a lower 

voltage -0.19V in aqueous 0.1 M KOH that is two times lower than the commercial (Pt/C), that’s due to the 

integration between the catalytic and synergetic properties of co-atomic doping of Pt and Pd with the 

outstanding physicochemical properties of 1-D g-CNs [6]. As well as, when comparing the electrochemical 

method with the photoelectrochemical method, the current density for the COoxid increased 1.4 times photo 

electrochemically with UV-vis light compared to in the dark. lower complete conversion temperature (T100) 

with fast COoxid kinetics was observed when binary Pd and Cu doped gC3N4NTs (154 °C) figure (3a-c) [10]. 

When compared with Pd doped gC3N4NTs and Cu doped gC3N4NTs 210 °C and 250 °C respectively fig 

(x), because of the combination between electronic and synergetic effect of Pd and Cu besides to the 

physicochemical properties of 1-D poures gC3N4. Interstingely the of  catalytic activity of Pd/Cu/gC3N4NTs 

was significantly greater compered to Pd-impeded 3D porous graphene with nanohole structrue complete 

conversion temperature (T100)  at 190°C [26]. This shows the designed shape and size (1D,2D, or 3D), 

physicochemical properties of suporter, and the loding amount with the type of metals as dopent or NPs 

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are the mainely effect on the COoxid activity. 

Carbon-based materials 

The type of catalysts and their composition plays a vital role in the COoxid activity according to 

their own physicochemical properties. There are need to design new low-cost catalysts, high yield, 

good catalytic activity, stability, and also stability toward CO poisoning. Carbon-based materials 

like (graphene, carbon dot, carbon nanotube, carbon nitride, fullerenes, Metal-organic 

frameworks (MOF), and MXene) are one of the most promising catalysis for COoxid related to their 

superior catalytic activity in different catalytic reactions owing to their physicochemical properties 

like high electrical conductivity and surface area, corrosion resistance, surface functionalities, 

tunable porosity, thermal and chemical stability [26]. In addition to easy preparation with a high 

yield from various synthetic or natural abundant materials, low price, and friendly to the 

environment. As described above some examples of carbon-based materials showed good 

catalytic activity and also stability toward the COoxid reactions. 

 

Novel carbon-based catalyst (MXenes) 

Herein, among carbon-based materials, this work focuses on using 2D-MXenes nanomaterials 

as a support for mono and binary Pd and Pt nanoparticles for the electrochemical COoxid due to 

the sample preparation of MXene with high yield, low cost, earth-abundant, and its significant 

physicochemical properties of high active surface area, high electron mobility/density, abundant 

catalytic active sites, surface hydrophilicity with rich functional groups, outstanding mechanical 

strength, and good thermal and electrical conductivity. But it's rarely reported only theoretically 

for COoxid. For instance, cheng et al. found the Mo2CO2 MXene act as a superior supporter of the 

single atomic catalyst (SACs) (fig 10), where The COoxid activity for Pd as SAC occupied the 

defective sites of Mo2CO2 monolayer and oxygen vacancy was better than the Pd(111) according 

to the TER mechanism with low energy [60]. 



34 
 
 

 

Fig. 10 (a) Top and side views of the charge density difference of Pd/OV–Mo2CO2 (b) Reaction 

mechanism of COoxid via TER-mechanism, and (c) Adsorption configurations and adsorption 

energies. copyright permission RSC [60].  

 

 

Fig 11 showed that Mo2CO2 supported by the Cu3 clusters gave a good catalytic COoxid activity 

and good stability [61]. Where, the Cu3 cluster operated as a reservoir of the electrons to control 

the gained or loosed electrons to promote the COoxid reaction and found the rate-limiting energy 

barrier 0.72 eV lower than Pt and Pd [62, 63]   which showed a high catalytic COoxid activity. 

 

 

 

Fig. 11 (a) The diagram for Cu3 active site of Cu3/d-Mo2CO2 works as an electron reservoir to 



35 
 
 

facilitate the COoxid conversation rate. (b) COoxid pathways reactions (side views) over Cu3/d-

Mo2CO2 by diffrent mechanisms. Copyright 2018, American Chemical Society [61]. 

 

In another study, Zn-doped Mo2CO2
-δ  [64] was found a promising catalyst for COoxid compared 

to other different catalysts via the ER mechanism with barrier energy of 0.15 eV that is lower than 

Pt and Pd catalysts (1.05 and 0.93 eV) [62, 63]. Furthermore, the Ag monolayer supported on 

Mo2C MXene exhibited excellent CO poisoning resistance [65], and also the Ti anchored Ti2CO2 

MXene  [66] own a good catalytic activity towards the COoxid theoretically. So, combining the 

outstanding catalytic properties of Pt and Pd and the superior physicochemical properties of 

MXene was superior for the electrochemical COoxid experimental reactions in a wide pH range. 

 

Fig. 12 Periodic tables showing compositions of MXenes and MAX phases. (a) Elements of 

MXenes chemical structure. (b) Elements of different MAX phases, Copyright 2019 

American Chemical Society [67].  

History and structure  



36 
 
 

MXenes belong to 2D transition carbonitrides, metal carbides, and nitrides which discovered 

at Drexel University by researchers (Yury Gogotsi and Michel Barsoum) in 2011  [68]. More 

than 30 MXene structures successively prepared since 2011 after selectively etching MAX 

phases with some different chemical compositions as shown in (Fig 12a). The formula of 

MXenes is Mn+1XnTX, where n = 1, 2, or 3, demonstrating three common structures. M means 

a transition metal, for instance, V, Ti, and Mo, X refers to C/N, while TX represents various 

surface functionalization by F−, OH−, and/or oxygen O-2 as shown in fig (12b). 

 

Fig. 13 (a) the chemical structure for MAX and MXene phase (b) Schematic illustration of 

the development path of MXenes synthesis. Copyright 2022 Elsevier [69]. 

preparation methods of MXenes 

The development path of MXenes synthesis and its chemical structures until now shows in 

fig 13 [69], where there are two mainly preparation methods called direct and indirect HF 

etching via using a mixture of (LiF+HCl+NH2HF2) [70]. in the direct method, MXene was 

delaminated only to 2D layers by sonication. while in the indirect method, sonication is only 

required at low concertation of a mixture of (HCl/Lif and NH4HF2) [71]. There are other 

etching methods to overcome the hazardous effect of HF like LiF/HCl, NH4HF2, mechanical, 



37 
 
 

hydrothermal, molten, salt templating, and gas process etching methods [72-76]. 

Direct HF method 

The HF etching method is the most popular synthesis method for 2D MXenes.  The first 

experimental preparation was reported by Naguib et al.  The process includes etching of the 

MAX phase (Ti3AlC2) in 50% pure HF for 2hr at RT [77]. At first, Al is etched by HF under 

sonication giving parallel Ti-C layers that are neighboring. (H2O, urea, DMSO, TBAOH, 

hydrazine, and isopropyl amine) are used to add ions by intercalation with a large radius (–

OH and H2O) into the framework of Ti-C mechanically or chemically through formation bonds 

of hydrogen, and thereafter enlarges the spacing between Ti3AlC2 layers by sonication 

assisting. HF has high etching power which etches all Al layers and also etch some 

neighboring Ti and C atoms causing single vacancies which able to reduce and 

accommodate positions for several metal ions [78, 79]. 

This method depends on the HF concentration and the time of etching, so numerous of 

MXenes like Mo2CTx, Nb4/3CTx, W4/3CTx, V2CTx, Hf3C2Tx, Ti2NTx, V4C3Tx, Mo2TiC2Tx, 

Nb2CTx, Zr3C2Tx, Mo4/3CTx, Ti2CTx, V2CTx, Ti3C2Tx, and Nb4C3Tx were prepared using 

concentrations of 1, 5, 10, 35, 48, and 50 % [80-82]. The required time for etching ranged 

from 2 h to 165 h, related to the concentration of HF. The etching time is directly proportional 

to the HF concentration. 50 % HF is commonly used to allow etching of Al completely with 

uniformed 2D MXenes sheets.  MXenes layers conserve the hexagonal close packing (hcp) 

like MAX phase structure after etching but with increased d-spacing lattice and decreasing 

the basal planes [80].  

 Indirect HF etching 

This method is used to prevent using hazardous HF etching solutions for the synthesis of 

MXenes by using moderate and eco-friendly etchings such as NH4HF2 or acids like HCl and 

H2SO4 mixed with metals or its fluorides as LiF, FeF3, NaF, CaF2 KF, and CsF [83]. This 

permits the successive etching of A elements in conjunction with the interaction of cations 

(NH4+ and Li+) with H2O into the formed 2D sheets surface of MXenes to form highly 

exfoliated MXene with a higher interlayer spacing, higher oxygen content, larger defects, 

great electronegativity of fluorine, more attached hydroxyl, and higher stability than direct 

etched MXenes[84, 85]. Also, this method is controlled by time of etching and etchant 

concertation as the main effecting factors. The etching solution contains 6–12 M of HCl and 



38 
 
 

0.6–5 g of LiF formed different MXenes like Mo2CTx, Cr2TiC2Tx, Ti3C2Tx, V2CTx, and Ti3CNTx. 

for complete etching of (A) element of MAX phase via using LiF/HCl needs 12-384hr, 1M 

NH4HF2 needs only 12 hr to form Ti3C2Tx [86]. 

both direct and indirect HF etching procedures, the sonication procedure such as water-bath 

and probe sonicating can govern the MXene interlayer spacing and sheets/flake sizes. Due 

to the higher irradiation power of the probe sonicator, it forms a more smaller sheets/flake 

size during the delamination of MXene than sonication in water-bath [53] as the probe 

sonicator carries the ultrasonic waves straight through the circular cross-section of the 

sonicator tip to MXene, as mentioned in detail in the review [87]. To prevent the formed 

active MXene from oxidation, it is protected by an inert environment like Ar or N2 gas in an 

ice bath during the sonication process. 

The preferred method used in this study 

In this study, the direct etching method was employed via HF etching solution followed by 

the delamination process and the intercalation occurred under sonication using organic 

dimethyl sulfoxide (DMSO) solvent in an ice bath under continuous N2 flow. This method is 

used due to: - 

-  Simple process with a high yield of 2D-MXene. 

- High yield and low cost using abundant materials. 

-  Uniform 2D-MXene sheets formed and stably dispersed as large DMSO molecules 

inserted between flaks and also as dispersants at the same time. 

- High HF concentration gave preferred 2D-MXene morphology with abundant defects and 

surface functionalities to synthesize different catalysts such as Pt/Ti3C2Tx, Pd/ Ti3C2Tx, 

and PtPd/Ti3C2Tx. where the highly functionalized surface with abundant defects made 

MXene act as a reducing agent for the metal salts to form a uniform dispersed spherical 

NPs staked to the 2D-sheets without using any surfactant at RT with complete metal ions 

reduction under sonication.    

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10843-10857. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



43 
 
 

CHAPTER 3 

 

Titanium Carbide (Ti3C2Tx) MXene Ornamented with Pallidum 

Nanoparticles for Electrochemical CO oxidation 

Abstract 

Titanium carbide (Ti3C2Tx) MXene possesses various unique physicochemical and catalytic 

properties. However, the electrochemical COoxid performance is not yet addressed experimentally. 

Herein, Ti3C2Tx (TX=OH, O, and F) ordered and exfoliated 2-D nanosheets ornamented with semi-

spherical pallidum nanoparticles (2.5 Wt. %) with an average diameter of (10 ±1 nm) (denoted as 

Pd/Ti3C2Tx) is rationally designed for the electrochemical COoxid. The fabrication process is based 

on the selective chemical etching of Ti3AlC2 and delamination under sonication to form Ti3C2Tx 

nanosheets that are used as a substrate and reducing agent for supporting in situ growth of Pd 

nanoparticles via impregnation with Pd salt. Interestingly, Pd-free Ti3C2Tx displayed inferior COoxid 

activity, while Pd/Ti3C2Tx enhanced the COoxid activity substantially. This is attributed to the 

combination of outstanding physicochemical properties of Ti3C2Tx and the catalytic merits of Pd 

nanoparticles.  

 

Introduction  

Proton exchange membrane fuel cells are promising green, sustainable, and efficient energy 

conversion technologies; however, the CO poisoning diminishes their performance [1-3] . Therefore, 

the COoxid reaction is of particular interest in industrial, fuel cells, and environmental applications. [4, 

5] Wide varieties of noble metals [6], transition metals [7, 8], and carbon nitride [9-14] were used for 

catalytic COoxid. Pd nanoparticles are among the most effective catalysts for the electrochemical 

COoxid, but their high cost and self-poisonings are critical issues precluding large-scale applications. 

These obstacles could be defeated by controlling the shape [15] and composition [16, 17] of Pd 

nanoparticle as well as using a metal-oxide support [18, 19].  

Distinct from other supports, Ti3C2Tx MXene possesses various unique physicochemical properties 

and catalytic properties such as high surface area, rich electron density, abundant active sites, great 

defects, and high electrical conductivity endowing their utilization in different energy and 

environmental applications [20-28]. Additionally, the surface of the multi-layered 2D structure can 

be terminated with abundant surface functionalities (i.e., O2, OH, and F), which provides accessible 

catalytic sites, accelerates charge mobility, promotes chemisorption of reactants, and tolerates the 

https://www.sciencedirect.com/topics/chemistry/chemisorption


44 
 
 

binding energies of both COoxid intermediates and products [25, 28-30].  Notably, MXene could be 

easily prepared in a high yield from earth-abundant and inexpensive resources that are highly 

required merits in the practical applications [31]. Despite the significant achievements in the 

controlling design of Ti3C2Tx for various applications, their electrocatalytic COoxid performance is not 

yet addressed experimentally. To the best of our knowledge, the catalytic oxidation of CO on Ti3C2Tx 

has been addressed only theoretically [29, 32, 33]. For example, Mo2CS2 with non-noble metal 

single-atom catalyst showed a promising COoxid activity, owing to the charge transfer from the 

surface to the adsorbed CO and O2, allowing their activation over the catalyst surface [34]. The 

COoxid activity of Pd single-atom on Mo2CO2 (Pd/Ov-Mo2CO2) was predicted to be superior to Pd 

(111) and was close to the activity of Pt (111) at low CO coverage [35]. Ti anchored on Nobel-metal 

free Ti2CO2 exhibited a great COoxid activity and could be achieved experimentally [33]. 

In this work, Ti3C2Tx decorated with pallidum nanoparticles (Pd/Ti3C2Tx) were synthesized and used 

for the electrocatalytic COoxid. Ti3AlC2 was initially etched by hydrofluoric acid and then delaminated 

under sonication to form Ti3C2Tx. The formed Ti3C2Tx nanosheets were subsequently impregnated 

with Pd salt to form Pd/Ti3C2Tx. The fabrication process was simple, reducing agent-free, and 

enabled the formation of uniform exfoliated 2D nanosheets decorated with well-dispersed semi-

spherical Pd nanoparticles with an average diameter of (10 ±1 nm). The electrochemical COoxid 

activity of Pd/Ti3C2Tx was investigated and compared with Pd-free Ti3C2Tx in perchloric acid at RE.   

  

Experimental 

Chemicals and materials 

Sodium tetrachloropalladate (II) ((Na2PdCl4), 99%), acetone ( ≥ 99.9 %), hydrofluoric acid (48 Wt. 

%), and dimethyl sulfoxide anhydrous ((DMSO), ≥ 99.9 %) were purchased from Sigma-Aldrich 

Chemie GmbH (Munich, Germany). Aluminum titanium carbide powder (Ti3AlC2, 99.9 %, 40 μm 

particle size) was purchased from Carbon-Ukraine ltd (Kiev, Ukraine).  

Synthesis of Ti3C2Tx nanosheets 

Ti3C2Tx nanosheets were synthesized by etching of Ti3AlC2 (2 g) powder by HF (40 ml, 50 Wt. %)  

under magnetic stirring atRTovernight, then centrifuged at 3500 rpm and washed with double 

deionized H2O (D-H2O) till the pH ≥ 5. The wet sediment 2 g was dispersed in 30 mL DMSO under 

magnetic stirring overnight atRTand then centrifuged at 3500 rpm for 4 min and washed with D-H2O 

three times before being sonicated under argon. The final product was purified by centrifugation at 

4000 rpm for 1 hr, and the supernatant was finally dried overnight under vacuum at 50 oC.  



45 
 
 

Synthesis of Pd/Ti3C2Tx  

An aqueous solution of Ti3C2Tx (20 mL of 12 mg /mL) was slowly mixed with Na2PdCl4 (2 mL of 20 

mM) under sonication atRTfor 15 min followed by addition of acetone (10 mL) and kept for 2 h. [36] 

The solution was centrifuged for 10 min three times at 7000 rpm and washed with D-H2O. The as-

obtained sediment was vacuum dried at 50 oC overnight and then kept for further characterization 

and usage. 

Materials characterization 

The shape of thus obtained materials was analyzed by scanning electron microscope ((SEM), 

Hitachi S-4800, Hitachi, Tokyo, Japan) and transmission electron microscope (TEM, TecnaiG220, 

FEI, Hillsboro, OR, USA) equipped with selected area diffraction pattern (SAED). The X-ray 

photoelectron spectroscopy (XPS) spectra were recorded on a Kratos Axis (Ultra DLD XPS Kratos, 

Manchester, UK). The X-ray diffraction patterns (XRD) were measured on an X-ray diffractometer 

(X’Pert-Pro MPD, PANalytical Co., Almelo, Netherlands).  

Electrochemical CO oxidation reaction 

The electrochemical COoxid tests were conducted using on Gamry work-station (reference 3000, 

Gamry Co., USA) using the traditional three-electrode cell including Pt wire (counter electrode), leak-

free Ag/AgCl (KCl, 3 M)) (reference electrode), and glassy carbon electrode ((GCE) (WE). The 

catalyst inks were prepared by mixing 2 mg of each catalyst in 1 mL of deionized H2O/ethanol (3/1 

v/v ratio) contains Nafion solution (30 μL, 0.05 Wt %) under sonication until getting a homogeneous 

solution. The catalyst inks (10 µg of each catalyst) were cast onto GC electrodes and then left to dry 

under vacuum at 50 oC.  

Results and discussion 

Figure 1a shows the SEM image of Ti3C2Tx formed in a multilayered 2-D nanosheets morphology 

with an average thickness of nearly 1.9 nm. The nanosheets are exfoliated with an interlayer space 

of 50 ± 10 nm (Figure 1b). The as-obtained Ti3C2Tx nanosheets were used as a substrate and 

reducing agent for supporting the growth of Pd nanoparticles. The formation of Pd nanoparticles 

over Ti3C2Tx nanosheets enhanced the exfoliation of the obtained nanosheets (Figure 1c).  Figure 

1d shows the TEM image of Pd/ Ti3C2Tx that reveals the formation of 2-D thin nanosheets decorated 

with Pd nanoparticles. 



46 
 
 

 

Figure 3.1 (a-b) SEM image of Ti3C2Tx MXene. SEM image (c) and TEM images (d-e) of 

Pd/Ti3C2Tx, HRTEM (f) of the marked area in (e)  

      Interestingly, Pd nanoparticles were highly dispersed over Ti3C2Tx and had a semispherical 

shape with an average size of (10 ±1 nm) (Figure 1e). The high-resolution TEM image of Pd 

nanoparticles depicts uniform lattice fringes arranged in one direction without any defects or lattice 

distortion, implying the absence of any undesired microscopic phases (Figure 1f). The interlayer d-

spacing of the Pd lattice is estimated to be 2.29 A, attributed to the (111) facet of face-centered 

cubic (fcc) Pd.  The selected area electron diffraction (SAED) pattern of randomly selected Pd 

nanoparticles shows the typical rings associated with the diffraction of (111), (200), and (220) facets 

of the fcc Pd structure (Figure 2a).  



47 
 
 

 

Figure 3.2 (a) SAED and elemental mapping images of (b) Ti, (c) C, (d) O, (e) F, and (f) Pd of 

Pd/Ti3C2Tx. 

 

     The elemental mapping analysis of thus formed Pd/Ti3C2Tx reveals the presence of titanium (Ti) 

(Figure 2b), carbon (C) (Figure 2c), oxygen (O) (Figure 2d), fluoride (F) (Figure 2e), and palladium 

(Figure 2f) with atomic contents of 50.09, 30.79, 10.2, 6.7, 2.5 %, respectively. The presence of -F 

surface termination in Ti3C2Tx is due to HF etching, while the detected -O surface termination is 

attributed to the intercalation by DMSO. The surface terminations Tx are plausibly important for 

enhancing the hydrophilicity of Pd/Ti3C2Tx, as well as promoting the adsorption and activation of gas 

molecules (CO and O2) reactants during the COoxid [29, 32, 33].  

 

Figure 3.3 XRD analysis of Pd/Ti3C2Tx and Ti3C2Tx 



48 
 
 

 

        The crystalline structures of thus formed Pd/Ti3C2Tx and Ti3C2Tx were investigated by the XRD 

analysis (Figure 3). Pd-free Ti3C2Tx exhibits the XRD diffraction patterns assigned to the (002), (004) 

,(006), (008), (0012), (0014), facets of Ti3C2Tx in line with reports elsewhere [11]. The absence of 

any peaks for TiO2 nanoparticles reflects the uniformity of the formed Ti3C2Tx MXene. Pd/Ti3C2Tx 

display the diffraction peaks of Ti3C2Tx in addition to (111), (200), and (220) facets of fcc Pd-metal 

[37]. The electronic structure and valence of Pd/Ti3C2Tx relative to Ti3C2Tx were investigated by the 

XPS analysis. Figure 4a discloses the XPS surveys for Pd/Ti3C2TxTx and Ti3C2TxTx, which both 

display the valence band of Ti 2p, C 1s, O 1s, and F 1s, but Pd/Ti3C2TxTx shows an additional peak 

for Pd 3d. The high-resolution spectra of Ti 2p spectra display Ti-C [2p3/2 (455.24 eV) and 2p1/2 

(461.13 eV)], Ti2+ [2p3/2 (456.25 eV), 2p1/2 (462.3 eV)], and Ti3+ [2p3/2 (458 eV), 2p1/2 (463.38 eV) 

(Figure 4b). The C 1s spectra reveal C-C at (285 eV), C-OH at (286.95 eV), and C-O=C at (288.97 

eV) (Figure 4c). The O 1s spectra exhibit O bonded to Ti at (530.02 eV) and OH at (531.03 eV) 

besides C-O at 532.8 eV (Figure 4d). The Fs1 spectra show Ti-F at (685 eV), and C bonded F (C-

F) at (687.2 eV) (Figure 4e).  

 

Figure 3.4 (a) XPS survey of Pd/Ti3C2Tx and Ti3C2Tx. High-resolution XPS spectra of (b) Ti, (c) C, 

(d) O, (e) F, and (f) Pd 

 

   The Pd 3d spectra show Pd0 (3d3/2 at 339.79 eV and 3d5/2 at 334.6 eV), as a major phase and both 

Pd2+ (3d3/2 at 341.75 eV and 3d5/2 at 336.34 eV) and Pd4+ (3d3/2 at 344.08 eV and 3d5/2 at 337.72 eV) 



49 
 
 

as minor phases (Figure 4f). The presence of Pd0 metallic state as a major phase indicates the great 

reduction power of Ti3C2Tx; meanwhile, the presence of Pd2+ and Pd4+ is plausibly attributed to the 

great content of O in Ti3C2Tx, which may facilitate partial oxidation of Pd.  

The fabrication of Ti3C2Tx with or without metal nanoparticles (i.e., Pd, Co, Au, and Ag) has attracted 

much great attention for various catalytic applications, but their utilization in the electrochemical 

COoxid reaction is not yet reported [38, 39].  In our study, the electrochemical COoxid reaction of 

Pd/Ti3C2Tx as compared to Ti3C2Tx was assessed by various techniques including the cyclic 

voltammogram (CVs), linear sweep voltammogram (LSV), electrochemical impedance spectroscopy 

(EIS), and chronoamperometry (I-T). 

 

 

Figure 3.5 (a) CVs measured in a N2 saturated 0.1 M NaHCO3 of Pd/Ti3C2Tx and Ti3C2Tx. (b) CVs 

and (c) LSV of Pd/Ti3C2Tx and Ti3C2Tx. (d) CVs measured at different scan rates in a CO-saturated 

an aqueous solution of 0.1 M HClO4 at 50 mV s-1 and (e) the corresponding plots of forwarding peak 

currents against the square root of the scan rates. (f) EIS of Pd/Ti3C2Tx and Ti3C2Tx measured in a 

frequency range from 100 kHz to 5 Hz with an AC voltage amplitude of 0.8 V at an open circuit 

potential in 0.1 M HClO4. 

 

     Figure 5a shows the CVs measured in a nitrogen-saturated an aqueous solution of HClO4 (0.1 

M) at 50 mV s-1 at RE. It should be noted that initial CV were conducted at 200 mVs-1 for 50 segments 

to remove any impurities from the GCE surface.  The CV curve of Ti3C2Tx demonstrates quasi-



50 
 
 

rectangular shape originating from the pseudo-capacitance effect, which is common in MXens or 

carbon-based materials. Pd/Ti3C2Tx shows its typical voltammogram characteristics, including the 

double-layered Hads/des and Pd-O/Pd-H. The absence of any peak for TiO2 or PdO in Pd/Ti3C2Tx 

indicates its stability against oxidation or leaching in HClO4 media. Figure 5b depicts the CVs curves 

of Pd/Ti3C2Tx, and Ti3C2Tx measured in a CO-saturated an aqueous solution of HClO4 (0.1 M) at RE. 

Pd-free Ti3C2Tx voltammogram shows typical features of capacitive effect without any noticed activity 

for COoxid. Contrarily, Pd/Ti3C2Tx has a prevalent COoxid activity with the typical voltammogram 

characteristics of CO-oxidation reaction, including a sharp anodic oxidation peak in the positive 

direction (0.82-1.01 V) and a small cathodic peak in the negative direction (0.37-0.51V).  Figure 5c 

shows the LSV of Pd/Ti3C2Tx tested in a CO-saturated HCLO4 (0.1 M), which reveals the typical 

anodic oxidation features with a COoxid current density of (0.318 mA cm-2), oxidation potential of (0.9 

V) and onset potential around (0.75 V). The LSV of Ti3C2Tx showed an inferior COoxid activity (Figure 

5c). This is evident of the effect of Pd nanoparticles on boosting the COoxid activity of Ti3C2Tx. 

 

Figure 3.6 (a) The I-T tests measured for 2000 s at 0.8 V in a CO-saturated 0.1 M HClO4, (b) TEM 

image of Pd/Ti3C2Tx after COoxid stability test.  

 

Figure 5d shows the CVs curves of Pd/Ti3C2Tx measured at different sweeping rates of 25, 50, 100, 

150, and 200 mV s-1 in a CO-saturated HClO4 (0.5M), Incremental increases in the COoxid currents 

with increasing the sweeping rate can be seen. The maximum current density (0.544 mAcm-2) was 

obtained at (200 mV s-1) and the lowest current (0.2 mAcm-2) was observed at (25 mV s-1). Randles-

Sevcik equation was used to demonstrate the effect of scan rate (v) over the COoxid current density 

as well as to determine the diffusion coefficient of the electroactive species (Figure 5e). Plotting 

current density vs. ν1/2 showed a linear relationship, which is evident of the diffusion-controlled 



51 
 
 

process for the electroactive species. The lower slope of the plot (0.04 mV dec-1) indicates the fast 

reaction kinetics on Pd/Ti3C2Tx surface. This is further seen in the EIS measurements, which display 

the lower charge transfer resistance of Pd/Ti3C2Tx than that of Ti3C2Tx (Figure 5f), which infers a better 

electrolyte-electrode interaction and quick charge transfer on Pd/Ti3C2Tx, owing to the presence of Pd 

nanoparticles. Figure 6a shows the I-T stability test measured on Pd/Ti3C2Tx for 2000 s at 0.9 V in a CO-

saturated 0.1 M HClO4. Pd/Ti3C2Tx showed good stability with a slight loss in the current density over time. 

The I-T test shows that Pd/Ti3C2Tx maintains around 85 % of its initial current density (Figure 6a). The TEM 

image after stability tests shows the structural stability of Pd/Ti3C2Tx without any significant leaching or 

aggregation for Pd nanoparticles (Figure 6b). These results warrants that coupling Pd nanoparticles with 

Ti3C2Tx allows the slight enhancement in the COoxid activity. This is attributed to the combination of 

outstanding merits of Ti3C2Tx (i.e., great surface area, rich electron density, accessible active sites, 

surface functionalities, hydrophilicity, and electrical conductivity) and unique catalytic properties of 

Pd (i.e., electronic effect, synergism, and ad