NO 2 inhibits the catalytic reaction of NO and O 2 over Pt
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NO2 inhibits the catalytic reaction of NO and O2 over Pt S.S. Mullaa, N. Chena, W.N. Delgassa, W.S. Eplinga,b and F.H. Ribeiroa,* a
School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907-2100 b Cummins, Inc., 1900 McKinley Ave, Columbus, 47201 IN
Received 20 October 2004; accepted 14 December 2004
The rate equation for the overall reaction of NO and O2 over Pt/Al2O3 was determined to be r ¼ kf ½NO1:050:08 ½O2 1:030:08 ½NO2 0:920:07 ð1 bÞ, with kf as the forward rate constant, b ¼ ð½NO2 =K½NO½O2 1=2 Þ, and K as the equilibrium constant for the overall reaction. An apparent activation energy of 82 kJ mol)1 ± 9 kJ mol)1 was observed. The inhibition by the product NO2 makes it imperative to include the influence of NO2 concentration in any analysis of the kinetics of this reaction. The reaction mechanism that fits our observed orders consists of the equilibrated dissociation of NO2 to produce a surface mostly covered by oxygen, thereby inhibiting the equilibrium adsorption of NO, and the non-dissociative adsorption of O2, which is the proposed rate determining step. KEY WORDS: NO oxidation on Pt; NO2 inhibition on NO oxidation; reaction orders; apparent activation energy.
1. Introduction The oxidation of NO to NO2 over a supported noble metal component is an important step involved in NOx abatement techniques, e.g., selective catalytic reduction (SCR) [1–3] and NOx storage/reduction (NSR) process [4] being developed for lean-burn diesel engines. These processes are a result of new regulations that limit the emissions of NOx, CO and hydrocarbon (HC) from diesel engines [5]. Pt has been the oxidation catalyst of choice due to its high red–ox activity [3,6] and its presence as a major component in the current automobile exhaust (or three-way) catalysts. We studied the kinetics of this reaction over a Pt/Al2O3 catalyst and found that the forward rate is inhibited by the product NO2. Previous work on the reaction kinetics of NO oxidation over Pt catalysts [1,2,7,8] did not explicitly consider the inhibitory effect of NO2. This new finding is important because NO2 affects the rate of reaction significantly and, thus, it needs to be included in any kinetic model for NO oxidation. 2. Experimental methods The Pt/Al2O3 catalyst used in this study was supplied by EmeraChem in monolithic form. Based on a known liquid uptake of the bare monolith, the appropriate amount of c-Al2O3 was mixed as an aqueous slurry. The bare monolith was dipped into this slurry, drained, dried and then calcined at 500 C for 1 h. The monolith was then dipped into the Pt-containing aqueous solution such that a final Pt loading of approximately 50 g/ft3 of * To whom correspondence should be addressed. E-mail: [email protected]
monolith was attained. The Pt was put into solution as an amine-based precursor. The monolith had a length of 1 inch, a cross-section of 60 channels with a cell density of 200 channels/in2. The percentage-of metal exposed (PME) measured by H2–O2 titration [9] was 42%. The total
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