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

Journal Article · Mon May 01 00:00:00 EDT 1995 · Journal of Vacuum Science and Technology, A

DOI:https://doi.org/10.1116/1.579723· OSTI ID:64959

Vesecky, S M; Chen, P; Xu, X; Goodman, D W ^[1]

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1. Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255 (United States)

The chemisorption of coadsorbed CO and NO has been studied over Pd(111) and Pd(100) at 1{times}10{sup {minus}6} Torr using infrared reflection absorption spectroscopy. At a pressure ratio of {ital P}{sub NO}/{ital P}{sub CO}= 1, the ratio of adsorbed NO to adsorbed CO, [NO{sub a}]/[CO{sub a}], increased with temperature over both single crystal surfaces. Furthermore, the [NO]{sub a}/[CO]{sub 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 CO{sub 2}, N{sub 2}O and N{sub 2} was followed with infrared spectroscopy by monitoring the evolution of gas phase CO{sub 2} and N{sub 2}O. At temperatures from 525--650 K, partial pressure ratios, {ital P}{sub NO}/{ital P}{sub CO}, from 16:1 to 1:16, and total pressures of 2--17 Torr, Pd(111) always showed higher activity than Pd(100) for both CO{sub 2} and N{sub 2}O production. The Pd(100) surface, however, gave a higher branching ratio for N{sub 2} versus N{sub 2}O production than Pd(111). The apparent activation energies of the reaction indicate a positive order in {ital P}{sub NO} and a negative order in {ital P}{sub CO} over both single crystal surfaces, with a zero order total pressure dependence between 2 and 20 Torr. The higher N{sub 2}/N{sub 2}O 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 N{sub a}{r_arrow}N{sub 2g}. Both the higher activity and the higher selectivity for N{sub 2}O versus N{sub 2} over Pd(111) are strongly correlated with the higher surface coverage of NO{sub a}. 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. {copyright} {ital 1995} {ital American} {ital Vacuum} {ital Society}

• OSTI ID: 64959
• Journal Information: Journal of Vacuum Science and Technology, A, Journal Name: Journal of Vacuum Science and Technology, A Journal Issue: 3 Vol. 13; ISSN 0734-2101; ISSN JVTAD6
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
CARBON OXIDES
CATALYTIC EFFECTS
CHEMICAL REACTIONS
CHEMISORPTION
HIGH VACUUM
LOW PRESSURE
NITROGEN OXIDES
PALLADIUM
PARTIAL PRESSURE
PRESSURE DEPENDENCE
SORPTIVE PROPERTIES
TEMPERATURE DEPENDENCE
TEMPERATURE RANGE 0065-0273 K
TEMPERATURE RANGE 0273-0400 K
TEMPERATURE RANGE 0400-1000 K