Correlation between CO surface coverage and selectivity/kinetics for the preferential CO oxidation over Pt/γ-Al2O3 and Au/α-Fe2O3: an in-situ DRIFTS study

We present in-situ IR (DRIFTS) measurements on CO adsorption and preferential CO oxidation (PROX) in H2-rich gas on Pt/γ-Al2O3 and Au/α-Fe2O3 catalysts at their envisaged operating temperatures of 200°C and 80°C, respectively, which in combination with kinetic data show that the underlying reason for the very different PROX reaction kinetics on these two catalysts is the difference in steady-state CO coverage. Whereas on the platinum catalyst this is always near saturation under reaction conditions, causing a negative reaction order (−0.4) and a pCO-independent selectivity, the amount of adsorbed CO on the gold particles (indicated by an IR band at 2110 cm−1) strongly depends on the CO partial pressure. From the position of the IR band of CO adsorbed on Au/α-Fe2O3, the steady-state coverages on the Au surface are shown to be significantly below saturation, with an upper limit of approximately θCO=0.2. Low reactant surface concentrations on Au explain the positive reaction order with respect to pCO (+0.55 at 80°C) as well as the observed decoupling of the CO and H2 oxidation rates, which results in a loss of selectivity with decreasing pCO.