Evidence for structure sensitivity in the high pressure CO+NO reaction over Pd(111) and Pd(100)

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Abstract
The chemisorption of coadsorbed CO and NO has been studied over Pd(111) and Pd(100) at 1×10−6 Torr using infrared reflection absorption spectroscopy. At a pressure ratio of PNO/PCO= 1, the ratio of adsorbed NO to adsorbed CO, [NOa]/[COa], increased with temperature over both single crystal surfaces. Furthermore, the [NO]a/[CO]a was significantly higher on Pd(111) than on Pd(100) in the temperature range of 100–550 K, with NO being the predominant surface species above 400 K. The reaction of CO+NO to form CO2, N2O and N2 was followed with infrared spectroscopy by monitoring the evolution of gas phase CO2 and N2O. At temperatures from 525–650 K, partial pressure ratios, PNO/PCO, from 16:1 to 1:16, and total pressures of 2–17 Torr, Pd(111) always showed higher activity than Pd(100) for both CO2 and N2O production. The Pd(100) surface, however, gave a higher branching ratio for N2 versus N2O production than Pd(111). The apparent activation energies of the reaction indicate a positive order in PNO and a negative order in PCO over both single crystal surfaces, with a zero order total pressure dependence between 2 and 20 Torr. The higher N2/N2O branching ratio observed on Pd(100) is likely due to the higher stability and surface coverage of dissociated nitrogen atoms on Pd(100) versus Pd(111), thus promoting the reaction 2 Na→N2g. Both the higher activity and the higher selectivity for N2O versus N2 over Pd(111) are strongly correlated with the higher surface coverage of NOa. In contrast, the lower activity of the Pd(100) surface relative to Pd(111) is likely due to enhanced poisoning of NO adsorption on this surface by dissociated nitrogen or oxygen atoms, or by adsorbed CO.

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