NO2 inhibits the catalytic reaction of NO and O2 over Pt

Abstract

The rate equation for the overall reaction of NO and O 2 over Pt/Al 2 O 3 was determined to be r ¼ k f ½NO 1:05AE0:08 ½O 2 1:03AE0:08 ½NO 2 0:92AE0:07 ð1 À bÞ, with k f as the forward rate constant, b ¼ ð½NO 2 =K½NO½O 2 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 NO 2 makes it imperative to include the influence of NO 2 concentration in any analysis of the kinetics of this reaction. The reaction mechanism that fits our observed orders consists of the equilibrated dissociation of NO 2 to produce a surface mostly covered by oxygen, thereby inhibiting the equilibrium adsorption of NO, and the non-dissociative adsorption of O 2 , which is the proposed rate determining step.

NO Oxidation Catalysis on Pt Clusters: Elementary Steps, Structural Requirements, and Synergistic Effects of NO2Adsorption Sites

“…We also show that steady-state O* coverages during NO oxidation are lower than those that would exist in equilibrium with prevailing O 2 pressures and that the observed effects of Pt cluster size on turnover rates reflect the stronger binding of chemisorbed oxygen on low-coordination surface atoms that prevail on small clusters. These effects of size on turnover rates reflect the requirement for vacancies within chemisorbed oxygen adlayers in kinetically relevant steps, as also shown for CH 4 oxidation on Pt and PdO x clusters 10,11 and dimethyl either oxidation on Pt clusters. 12 Finally, we have measured NO oxidation rates on mixtures of Pt catalysts and BaCO 3 adsorbents to show that adsorbents increase NO oxidation rates by scavenging NO 2 molecules that inhibit NO oxidation.…”

Development of Nitric Oxide Oxidation Catalysts for the Fast SCR Reaction

“…From these plots the apparent activation energies (E a ) were determined to be 70.9 kJ/mol for Co 3 O 4 /SiO 2 , 40.6 kJ/mol for Pt/SiO 2 and 31.4 kJ/mol for Pt/CeO 2 ( Table 10). The activation energy determined for Pt/SiO 2 is similar to the value of 39 kJ/mol reported for Pt/Al 2 O 3 [37], but lower than that (57.7 kJ/mol) reported for Pt/SiO 2 by Marques et al [34].…”

“…Although several reports have recently appeared concerning the kinetics and mechanism of NO oxidation over Pt catalysts, and in particular Pt/Al 2 O 3 [37][38][39][40], there have been no reports concerning base metal catalysts such as Co 3 O 4 /SiO 2 . This fact, coupled with the desire to elucidate the mechanism of NO oxidation (as a possible means to assist catalyst optimization), prompted us to study the kinetics of NO oxidation over Co 3 O 4 /SiO 2 .…”

“…As detailed in the literature [37], the rate of NO oxidation expressed in a power rate form may be written as:…”

“…In the kinetic measurements the reaction was performed far from equilibrium (β <0.25) and so the term 1-β approximates to one. The presence of NO 2 in the above rate expression reflects the inhibiting effect of NO 2 on the NO oxidation rate, which can be attributed to the strong adsorption of NO 2 on catalyst surfaces (both oxidic [35] and metallic [37]). However, given that NO 2 was not present in the feed, and given the low NO conversion levels studied, the NO 2 term can be discounted to a first approximation.…”