Published: July 19, 2011 r2011 American Chemical Society 15487 dx.doi.org/10.1021/jp203992r | J. Phys. Chem. C 2011, 115, 15487–15495 ARTICLE pubs.acs.org/JPCC Kinetics of Nitric Oxide Adsorption on Pd(111) Surfaces through Molecular Beam Experiments: A Quantitative Study Sankaranarayanan Nagarajan, † Kandasamy Thirunavukkarasu, †,§ Chinnakonda S. Gopinath,* ,† and Sudarsan D. Prasad* ,‡ † Catalysis Division and ‡ Physical Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India 1. INTRODUCTION Invention of three-way automotive exhaust control catalysts has been one of the crowning achievements of modern day catalysis. Reduction of the NO to N 2 (often called deNO x catalysis) is an integral part of the catalyst spectrum. Often there is a trade-off as the air-rich fuel is necessary for complete oxidation of CO and volatile organic components, which is mandatory in all modern (Euro III low emission vehicles and ultralow emission vehicles) emission norms. 1 In sharp contrast to this, fuel-rich conditions favor the reduction of NO, which is an important constituent of photo- chemical smog. 24 Sophistication in the design of novel catalysts, especially effective under net oxidizing conditions, such as zeolite-based ones with a NO x storage function, is increasing. 5,6 Introduction of Pd is comparatively a recent development, nevertheless it is the focus of several studies. 712 Pd has several advantages over Rh in effectively reducing NO x to N 2 even at oxidizing conditions, including a cost advantage. There have been several papers dealing with the fundamental aspects of NO reduction with CO on Pd, including Pd(111), which is the most active plane for the NO + CO reaction among the Pd surfaces. 913 N 2 O decomposition that occurs as an intermediate step has also been pursued. It has been claimed that defect sites are particularly active for NO decomposition but nevertheless has been shown to occur even on atomically smooth Pd(111) planes. 9,11,12 There has been a paucity of studies in the fundamental aspects of NO adsorption, especially with techniques involving molecular beam instrument (MBI). 12,16,17 MBI can measure absolute sticking rates and coefficients with high accuracy, under ultrahigh vacuum (UHV) conditions needed for maintaining surface cleanliness. MBI has been shown to be an effective tool in unraveling the elementary steps in a catalytic reaction sequence. It has been suggested by experimental 10,14,15 and theoretical studies 18 that NO occupies face-centered cubic (fcc) hollow sites at low cover- age, a mixture of fcc and hexagonal close packed (hcp) sites at intermediate coverage and on-top sites in addition to the above sites at high coverages. NO adsorption kinetics on Pd(111) surfaces has been studied by very few groups. Schmick and Wassmuth 19 measured NO adsorption kinetics on stepped Pd- (111) surfaces and concluded that the initial sticking coefficient was neatly fitted by the precursor state model for just one temperature. In contrast to their work, we are using a defect-free Pd(111) surface as a model surface over a large range of temperatures. Recently we have undertaken a systematic study of the different aspects of NO adsorption and NO + CO reactions on Pd(111) surfaces. 1012 The present communication is a part of our ongoing efforts to understand the molecular level aspects of three-way catalytic converter reactions, with a view to designing better automotive exhaust control catalysts. 2023 The work is presented as follows: First, we develop detailed equations for the variation of the chamber partial pressure as a function of dose rate, pressure, and pumping rate for three kinetic models. Then we solve the two-coupled differential equations for the gas-phase concentration (partial pressure) and also the surface concentration (fractional surface coverage). The sticking coefficients will be estimated to show that the precursor state model is valid for low temperatures. The Langmuir model is perhaps more appropriate for the high temperature regime when desorption also starts becoming appreciable. Further, tempera- ture-programmed desorption (TPD) studies have been made to understand the state of NO adsorption on Pd(111), viz., Received: April 29, 2011 Revised: June 29, 2011 ABSTRACT: A detailed kinetic picture derived by molecular beam studies of the adsorptiondesorption of the NO/Pd(111) system is presented. Numerical simulations and detailed kinetic analysis show that the precursor state model of adsorption provides a valid picture of the sticking coefficient variation with surface coverage, especially at low temperatures. At higher temperatures, the precursor model gives way to the Langmuir molecular model of adsorption. All the parameters of the precursor state model have been quantified. Temperature programmed desorption (TPD) studies further show that there is a slight repulsive interaction between adsorbed NO molecules and there is only a negligible fraction of dissociated molecules on the surface for temperatures less than 500 K, as the Pd(111) surface is defect free. A BraggWilliams (BW) lattice gas model with repulsive interactions, within the framework of mean field approach (MFA), is shown to describe the TPD spectra reasonably well.