MOLECULAR AND DISSOCIATIVE CHEMISORPTION OF NO ON PALLADIUM AND RHODIUM(100) AND RHODIUM(111) SURFACES - A DENSITY-FUNCTIONAL PERIODIC STUDY

The efforts to reduce NOx pollutants have stimulated a large interest in the understanding of the elementary processes for NO transformation on transition metal surfaces. Periodic density-functional calculation s have been performed for the molecular and dissociative chemisorption of NO on Pd and Rh(100) and (111) surfaces, with generalized gradient approximation exchange-correlation functionals. The periodic systems are modeled by two-dimensional palladium or rhodium slabs with frozen geometry, on which a NO, N, O, or (N+O) adlayer is set. On Pd and Rh(1 00) at a coverage of 0.5 monolayer (ML), the bridge site is the most s table one with respective binding energies of -1.54 and -2.18 eV. On t he (111) surfaces, at a coverage of 0.33 ML, the threefold hollow site s are favored with binding energies of -2.0eV for Pd(111) and -2.18 eV for Rh(111). For the dissociated structures, the mixed coadsorption o f N and O is favored in most cases compared to separated domains. The chemisorption of NO, N, or O is stronger on Ph surfaces than on Pd one s but the stability gain is larger for the atomic chemisorption. The a bsolute values of binding energies decrease with the coverage. The NO dissociation is exothermic only for Rh at low coverage, while it is en dothermic on Pd due to smaller atomic binding energies. This reaction becomes more endothermic when the coverage increases. (C) 1998 America n Institute of Physics. [S0021-9606(98)01515-3].