Preferential CO oxidation in excess of hydrogen over Au/HMS catalysts modified by Ce, Fe and Ti oxides

Abstract

The effect of the modification of HMS (hexagonal mesoporous silica) support with Fe3+, Ce4+ or Ti4+ ions on the physico-chemical and catalytic properties of mesoporous Au/HMS catalyst was evaluated in the preferential oxidation (PROX) of CO, in hydrogen-rich stream (50vol.% H2) and in the total CO oxidation (TOX) reaction. The bare supports were prepared via neutral S0I0 templating route, employing one pot synthesis, whereas the supported gold catalysts were prepared by the deposition–precipitation method. The reduced and spent Au catalysts have been studied by different techniques such as N2 physisorption, oxygen storage capacity (OSC), XRD, HRTEM, hydrogen chemisorption (by TPD-H2), DRS UV–vis, Mössbauer and XPS spectroscopic techniques. Among the catalysts studied, Au/HMS–Fe recorded the highest activity and stability for total CO oxidation. The XPS and DRS UV–vis data suggest that the active-sites configuration of this catalyst during this reaction could be small gold particles and ionic gold species stabilized by the support. The Au/HMS–Fe catalyst was also able to oxidize CO selectively in a hydrogen-rich gas mixture (PROX reaction) at a relevant temperature to hydrogen fuel cell applications (T50=83°C). Additionally, the Au/HMS–Fe sample recorded the lowest hydrogen oxidation (undesired reaction) during the CO-PROX reaction in excess hydrogen. From the catalyst activity–structure correlation, the main factors influencing the superior catalytic behaviour of this sample in the CO-PROX reaction are (i) its largest Au species surface exposure, as determined by XPS for the spent catalysts; (ii) the largest population of small Au particles, as determined by HRTEM for the freshly reduced samples; (iii) the large oxygen and hydrogen storage capacities, and (iv) the lowest deactivation by the formation of surface coke species, as determined by coke burning followed by TGA technique.