Mechanism and site requirements for NO oxidation on Pd catalysts

• Published in: Journal of catalysis Vol. 272; no. 1; pp. 74 - 81
• Main Authors: Weiss, Brian M., Iglesia, Enrique
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 25.05.2010; Elsevier; Elsevier BV
• Subjects: Catalysis; Catalysts; Chemistry; Cluster size effects; Exact sciences and technology; General and physical chemistry; Lean NO x traps; Nitric oxide; NO x storage; Oxidation; Oxidation catalysis; Palladium; Surface physical chemistry; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Lean NO x traps; Palladium; Cluster size effects; Oxidation catalysis; Nitric oxide; NO x storage; Water; NO; Activation; Combustion; Supported catalyst; Mechanism; Size effect; Oxidation; Catalysis; Dissociation; Inhibition; Adsorbent; Platinoid; Binding; Nitrogen oxide; Oxygen; Catalytic reaction; Vacancy; Lean NO; Kinetic method; Transition metal; Trap; Steady state; Heterogeneous catalysis; traps; Storage; Adsorption; Chemical potential; Kinetics; Catalyst; Effluent
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2010.03.010

O 2 activation on scarce vacancy sites in PdO clusters limits NO oxidation rates. The kinetic relevance of these vacancies causes turnover rates to increase as oxygen binding weakens with increasing cluster size and as adsorbents deplete NO 2 near active sites. Kinetic and isotopic methods were used to establish the identity and kinetic relevance of elementary steps and the effects of cluster size for NO oxidation on supported Pd catalysts. O 2 activation on vacancies within nearly saturated oxygen adlayers is the sole kinetically relevant step. Water, present in combustion effluents, inhibits NO oxidation via reversible adsorption on vacancies to form unreactive OH species. The coverage and chemical potential of oxygen are determined by the NO 2/NO ratios prevalent during catalysis; they are rigorously described by O 2 virtual pressures accessible to measurement. At a given oxygen chemical potential, O 2 activation during 16O 2– 18O 2 exchange and NO oxidation occurs at similar rates, indicating that NO-assisted O 2 activation is not required for NO oxidation turnovers. These findings confirm the rigor and relevance of O 2 virtual pressure as a measurable surrogate for the oxygen chemical potential during catalysis, as well as the role of mobile oxygen species, which allow O 2 dissociation to occur on isolated vacancies. Oxygen chemical potentials prevalent during NO oxidation cause active clusters to exist as PdO at all relevant conditions, consistent with steady-state turnover rates that are insensitive to reductive or oxidative treatments. NO oxidation turnover rates decrease as PdO clusters become smaller because of stronger oxygen binding and lower vacancy concentrations in small clusters. The catalytic consequences of size are similar on PdO and Pt clusters, despite their different oxidation state, consistent with a common requirement for surface vacancies in O 2 activation steps on both catalysts. Such requirements lead to strong NO 2 inhibition effects and to a marked increase in NO oxidation rates when NO 2 adsorbents, present as physical mixtures, decrease NO 2 concentrations near active sites.