Structure sensitive selectivity of the NO-CO reaction over rhodium single crystal catalysts

Conference · Mon May 01 04:00:00 EDT 1995

OSTI ID:86939

Peden, C H.F. ^[1]; Belton, D N; Schmieg, S J ^[2]

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1. Pacific Northwest Lab., Richland, WA (United States)
2. General Motors Research & Development Center, Warren, MI (United States)

The control of automotive emissions of nitrogen oxides (NO{sub x}) in passenger cars is accomplished by a heavy reliance on after-treatment of the engine exhaust using catalytic converters that contain a mixture of platinum (Pt), rhodium (Rh), and sometimes palladium (Pd). In this paper we examine the effect of surface structure on the NO-CO activity and selectivity by comparing the reactivity of Rh(110) and Rh(111) single crystal catalysts. Selectivity for the two possible nitrogen containing products from NO reduction, N{sub 2}O and N{sub 2}, is particularly interesting. Here we report that the selectivity of the NO-CO reaction is quite sensitive to the structure of the Rh catalyst metal surface. (A more complete description of these studies will be published elsewhere.) The more open Rh(110) surface tends to make significantly less N{sub 2}O than Rh(111) under virtually all conditions that we probed with these experiments. Furthermore, under the conditions used in this study, the NO-CO activity over Rh(110), as measured by the rate of NO loss, is somewhat faster than over Rh(111) with a lower apparent activation energy (Ea), 27.6 vs. 35.4 kcal/mol. We attribute these results to the greater tendency of the more open (110) surface to dissociate NO. Notably, more facile NO dissociation on Rh(110) would lead to greater steady-state concentrations of adsorbed N-atoms; thus, the (110) surface favors N-atom recombination over the surface reaction between adsorbed NO and N-atoms to make N{sub 2}O. In support of this, post-reaction surface analysis shows only NO on the Rh(111) surface while the Rh(110) surface contains predominantly N-atoms and much lower concentrations of adsorbed NO. NO dissociation on Rh(110) is more favorable than on Rh(111), in part, because it is less-severely poisoned by high surface concentrations of NO. In addition, the more-open (110) surface may be intrinsically more active for the elementary process of dissociating adsorbed NO.

• Research Organization: Pacific Northwest Lab., Richland, WA (United States)
• Sponsoring Organization: USDOE, Washington, DC (United States)
• DOE Contract Number: AC06-76RL01830
• OSTI ID: 86939
• Report Number(s): PNL-SA--24794; CONF-950561--1; ON: DE95014627
• Country of Publication: United States
• Language: English

Related Subjects

40 CHEMISTRY
54 ENVIRONMENTAL SCIENCES
AIR POLLUTION CONTROL
CARBON MONOXIDE
CATALYTIC EFFECTS
DISSOCIATION
EXHAUST GASES
NITRIC OXIDE
NITROUS OXIDE
RHODIUM
SPECIFICITY