Kinetics of the NO and CO reaction over platinum catalysts - I. Influence of the support

The kinetics of the CO + NO reaction over Pt-based catalysts was investigated using a fixed bed flow reactor at 300 degrees C, with CO and NO partial pressure ranges of 1.5 x 10(-3) to 9 x 10(-3) atm. Pt was deposited on various supports: gamma-Al2O3, Si3N4, and Cr3C2. XRD, XPS, and hydrogen chemisorption measurements seem to indicate that Pt dispersion. decreases as follows: Pt/Al2O3 > Pt/Si3N4 > Pt/Cr3C2, Kinetic data obtained on these Pt catalysts were interpreted on the basis of four mechanisms; as proposed in the literature; only one mechanism can correctly model the reactant partial pressure dependencies of the rate. This mechanism Involves nondissociative CO and NO competitive adsorptions followed by a dissociation of adsorbed NO which requires a vacant nearest-neighbor adsorption site, Clearly, the adsorption equilibrium constants, lambda(CO) and lambda(NO) of CO and NO, together with the rate constant of the NO dissociation step are strongly influenced by the support, particularly lambda(NO), which is consistently lower than lambda(CO) on Pt/Al2O3, increases notably for Pt/Si3N4, and increases even more for Pt/Cr3C2, It has been shown that Pt/Si3N4 may be the most suitable catalyst if the Pt dispersion on this support can be improved. The nature of the support also has a significant effect on the selectivity of the NO transformation into N-2 and N2O, Pt/Cr3C2 is the most selective catalyst for N-2 formation (much more than Pt/Si3N4 and particularly Pt/Al2O3). This seems to be related to the fad that the rate constant of the step 2N(ads)--> N-2 is much higher than that of the other step N-ads + NOads --> N2O. (C) 1998 Academic Press.