Carbon monoxide oxidation over modified titanium dioxide supported gold catalysts

Abstract

Highly dispersed gold nanoparticles on metal oxide surfaces have recently been reported to exhibit high catalytic activity for low-temperature carbon monoxide oxidation. Amongst the metal oxides, titanium dioxide, more often the commercial form Degussa P25, has been the most studied support for gold as a catalyst for CO oxidation because it yields some of the most active and stable catalysts. Physical and chemical modification of catalysts supports has been shown to affect their catalytic properties. In this research, modified gold supported catalysts have been prepared, characterized and tested for CO oxidation. Their properties have been compared with those of the unmodified catalysts. Catalysts containing1wt% Au supported on MxOy and TiO2/MxOy mixed oxide (M = Zn, Mg, Ni, Fe, Cr, Cu, Mn and Co; TiO2:MxOy mole ratio of 5:1; TiO2 = Degussa P25) were prepared by the single step borohydride reduction method and it was found that TiO2 gave the most superior activity as support for gold for CO oxidation, followed by TiO2/MxOy and the corresponding MxOy. The specific activities for CO oxidation of Au/TiO2 catalysts per unit of prepared in the range 0.05 to 1 wt% of Au indicates that for catalysts prepared by deposition precipitation, there is a significant decrease in specific activities with an increase with gold loading. For the single step borohydride reduction procedure, specific activities decrease less significantly with increasing gold content, implying that for economic and practical reasons, it would be advantageous to prepare gold catalysts with low gold loadings. Cyanide leaching of 1 wt% Au/TiO2 catalysts at different Au:CN- ratios, to selectively remove some of the gold in the catalysts, shows the activity per unit mass of gold to increases as more gold is removed from the catalyst. This is consistent with the idea that gold exists in more than one oxidation state in the systems and a significant fraction of the gold present in the catalysts do not contribute to catalytic activity. A number of anions and cations have been incorporated into TiO2 as support for gold catalysts and also into as-prepared Au/TiO2 catalysts at levels ranging from 0.05 mol% to 2.5 mol% with respect to the support. The activities of the catalysts for CO oxidation reveal that at the highest concentration levels of the ions, in all cases, a decrease in activity compared with unmodified Au/TiO2 is observed. However, addition of 0.05 to 0.4 mol% of the ions with respect to the support, prior to gold addition, in most cases, resulted in activity enhancement which increased with a decrease in the ion content. Similar addition of 0.05 to 0.4 mol% of the ions with respect to TiO2 to Au/TiO2 resulted in a decrease in activity. Attempts to understand the origins of these effects show that there is a degree of chemical interaction between the added ions and gold.