Pretreatment of TiO2-supported Fe, Co and Ru catalysts: an in situ powder diffraction study.

Abstract

There has been increasing interest in in situ or in operando studies of heterogeneous catalysts in the past two decades. This drive has seen the invention and modification of analytical instruments that allow for the investigation of certain catalyst properties under realistic reaction conditions. Powder X-ray diffraction has been used to investigate the structural properties of heterogeneous catalysts for many years and has not escaped the in situ revolution. Reaction cells have been designed, tested and used, both on laboratory-based instruments and at neutron and synchrotron facilities. The experiments performed at these institutions have yielded information regarding the active solid-state phases of heterogeneous catalysts used in a multitude of different reactions, often with surprising results. Little attention has, however, been given to the use of the technique during catalyst preparation and its importance in such investigations is highlighted in this work in conjunction with whole-pattern Rietveld refinement techniques. This study focused on the use of in situ PXRD as a means of monitoring the structural properties of TiO2-supported metal catalysts during heat treatment, or calcination, and reduction, or activation. The TiO2 supports included Degussa P25, Sigma-Aldrich anatase and nanosized anatase prepared using a sol gel method. The metals of interest were those shown to be active in the Fischer-Tropsch reaction, namely Fe, Co and Ru. In situ heat treatment of the supports was used to measure thermal stability and to monitor the anatase to rutile phase transition in terms of concentration and particle size. We were able to show that the Sigma anatase was the most stable of the three supports, while Degussa P25 was the least stable. By calculating the rates of change of concentration between each collection, a reasonably accurate determination of the phase transition temperature in each support was possible. Our results confirmed the growth of anatase particles before the phase transition and the rapid agglomeration of rutile particles after the transition, both of which had been discussed elsewhere. During the course of the heat treatment of the supports, mathematical corrections were developed to eliminate the effects of sample penetration that affected the quantitative phase analysis. In situ heat treatment experiments were employed to determine the ideal calcination temperature of each catalyst in terms of maximum metal oxide concentration and particle size. By using an internal standard (spike), we were able to monitor the crystallization of the amorphous metal precursors to form supported metal oxides. The addition of the metal oxide phases was shown to affect the anatase to rutile phase transition in various ways, depending on the support and metal oxide species. The effect of excessive heat treatment was also investigated and provided insight into the formation of metal titanates. Using the rates of change of concentration between collections, we were able to determine if anatase and/or rutile took part in the formation of the metal titanates. In situ activation experiments were conducted in order to determine the ideal reduction procedure for each catalyst. The reduction results confirmed that the concentration of H2 in H2/N2 gas mixtures had an effect on the reduction profile of a Degussa-supported Fe catalyst. We were able to monitor the formation of amorphous phases, which form during the second stages of reduction of both hematite and cobalt(II) oxide. It is thought that these amorphous materials represent the interfacial region between the support and the metal particles. This is the first time that the quantification of this interface material has been performed and its presence has been used to explain the poor degrees of reducibility observed for these types of catalysts. We found that the Ru and Co particles were less sensitive to elevated reduction temperatures than the Fe particles, which demonstrated a massive degree of sintering even at low temperatures.