Surface study of the hydrogen-free or preferential oxidation of CO: Iridium vs. platinum

CO oxidation in the absence or presence of hydrogen has been investigated in the 0.4–15 kPa range over Ir(1 1 1), Ir(1 1 2), Pt(1 1 1) and Pt(1 1 2) single-crystal surfaces. Iridium appears more active than platinum in CO oxidation, and more active and selective than platinum in preferential CO oxidation in the presence of hydrogen (PROX), both under stoichiometric and oxygen-rich CO:O2 conditions. Stepped surfaces are more active than flat surfaces. These results are not consistent with a CO-blocking model if only adsorption energies of pure CO are taken into account. A beneficial effect of a small amount of hydrogen on CO oxidation over Pt and Ir surfaces has been evidenced under specific conditions. Experiments on Pt(1 1 1) show that it is not related to water formation and does not originate from an irreversible H2-induced surface restructuring.

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► Ir is intrinsically more active than Pt in CO oxidation and more selective in PROX.
► Surface steps are beneficial to CO oxidation activity.
► Unlike water, hydrogen promotes CO oxidation under certain conditions.