Correlation between CO surface coverage and selectivity/kinetics for the preferential CO oxidation over Pt/gamma-Al2O3 and Au/alpha-Fe2O3: an in-situDRIFTS study

We present in-situ IR (DRIFTS) measurements on CO adsorption and preferenti al CO oxidation (PROX) in H-2-rich gas on Pt/gamma-Al2O3 and Au/alpha-Fe2O3 catalysts at their envisaged operating temperatures of 200 degrees C and 8 0 degrees C, respectively, which in combination with kinetic data show that the underlying reason for the very different PROX reaction kinetics on the se two catalysts is the difference in steady-state CO coverage. Whereas on the platinum catalyst this is always near saturation under reaction conditi ons, causing a negative reaction order (- 0.4) and a p(CO)-independent sele ctivity, the amount of adsorbed CO on the gold particles (indicated by an I R band at similar to 2110 cm(-1)) strongly depends on the CO partial pressu re. From the position of the IR band of CO adsorbed on Au/alpha-Fe2O3, the steady-state coverages on the Au surface are shown to be significantly belo w saturation, with an upper limit of approximately theta(CO) = 0.2. Low rea ctant surface concentrations on Au explain the positive reaction order with respect to p(CO) (+0.55 at 80 degrees C) as well as the observed decouplin g of the CO and H-2 oxidation rates, which results in a loss of selectivity with decreasing p(CO). (C) 1999 Elsevier Science S.A. All rights reserved.