Molecular adsorption of NO on PdO(101)

Abstract

We investigated the adsorption of NO on the PdO(101) surface using temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations. From TPD, we find that NO reaches a saturation coverage of ~0.54 monolayer (ML) on PdO(101) at 90K, and that desorption from the saturated layer produces a complex TPD trace with five pronounced maxima between 100 and 550K. We observe negligible quantities of desorbing products other than NO during TPD, indicating that NO does not react on PdO(101) under the conditions studied. For low coverages, RAIR spectra reveal that NO adsorption at 90K produces both flat-lying and atop configurations of NO on the coordinatively unsaturated (cus) Pd atoms of PdO(101), but that these configurations are meta-stable and transform to bridge-NO species at higher temperature. RAIRS measurements further show that NO binding on atop-Pdcus sites becomes preferred with increasing coverage above ~25% of the Pdcus density, and provide evidence of a weakly-bound state at coverages above 100% (0.35 ML). Consistent with our experimental findings, DFT predicts that NO preferentially adsorbs on bridge-Pdcus sites at low coverage and that the site preference changes to atop-Pdcus sites with increasing NO coverage, resulting in mixtures of atop-bridge NO configurations as well as an all-atop configuration. DFT also predicts that NO can adopt a flat-lying configuration on the Pdcus row at low coverage, and can also achieve stable bonding on the fourfold Pd sites of PdO(101) at high NO coverages. Both the computational and experimental results show that NO binds on PdO(101) in multiple, local configurations even at low coverage.