Adsorbed Species and Reaction Rates for NO–CO over Rh(111)

Abstract

We have studied the reaction of NO with CO over a Rh(111) catalyst by monitoring the infrared (IR) intensity of surface CO and NO at various partial pressures of NO (PNO), CO (PCO), and sample temperatures (T). Reaction rates for the products (CO2, N2O, and N2) were previously measured at the same conditions in our apparatus [J. Phys. Chem.99, 16344 (1995)]. Surface coverages were observed both atPNO=8 Torr, where the reaction yields mostly N2O and the selectivity is insensitive to eitherPCO(1 to 40 Torr) orT(<673 K), and atPNO=0.8 Torr, where the major product changes from N2O below 635 K to N2above 635 K. Changes in the surface coverages of NO and CO correlate well with the observed changes in N2O selectivity. Below 635 K, where N2O formation is favored, NO dominates the surface. Above 635 K, where N2formation is preferred, CO is the majority surface species. Our IR data support a model in which product N2O and N2are formed as adsorbed N reacts with either adsorbed NO or N, respectively. In an additional kinetic experiment, we used isotopically labeled N2O to show that gas phase N2O is not an intermediate to N2formation at 648 K—this helps to rule out an alternative model. In our IR experiments, two types of spectra were obtained with a Fourier transform IR spectrometer. Polarization spectra were obtained using two fixed polarizers, one before the sample, oriented to pass approximately equal amounts ofs- andp-polarized light, and one after the sample that selecteds- orp-polarized light. Spectra obtained withs- andp-polarized light were ratioed. We also obtained conventional reflectance absorbance IR spectra. In a separate calibration experiment with NO on Rh(111) in ultrahigh vacuum we observed five IR bands at 1448, 1530, 1590, 1643, and 1693 cm−1. We attribute all of these to bridging NO, most likely at threefold hollow sites. As NO coverage increases up to saturation, the distribution of population among these bands (and their individual frequencies) gradually changes—the average vibrational frequency of the NO increases from ∼1440 to ∼1650cm−1. From the linear dependence of the integrated IR absorption on NO coverage we find that the vibrational polarizability of the adsorbed NO is 0.20±0.02 Å3, a factor 6.5 larger than for free NO. This increase, proportionally larger than for CO on transition metal surfaces, is explained by the facile charge transfer that accompanies donor–acceptor bonding between NO and Rh(111).