PNNL

Abstract

In this study we examine the interaction of TiO2 with TiO2 (110) in an effort to better understand the conversion of NOx species to N2 over TiO2-based catalysts. The TiO2 (110) surface was used as a model system because this material is commonly used as a support and because oxygen vacancies on this surface are perhaps the best available models for the role of electronic defects in catalysis. Annealing TiO2 (110) in vacuum at high temperature (above 800 K) generates oxygen vacancy sites that are associated with reduced surface cations (Ti3?sites) and that are easily quantified using temperature programmed desorption (TPD) of water. Using TPD, x-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), we found that the majority of N2O molecules adsorbed at 90 K on TiO2 (110) are weakly held, and desorb from the surface at 130 K. However, a small fraction of the N2O molecules exposed to TiO2 (110) at 90 K decompose to N2 via one of two channels, both of which are vacancy-mediated. One channel occurs at 90 K, and results in N2 ejection from the surface and vacancy oxidation. We propose that this channel involves N2O molecules bound at vacancies with the O-end of the molecule. The second channel results from an adsorbed state of N2O that decomposes at 170 K to liberate N2 in the gas phase and deposit oxygen adatoms at non-defect Ti4? sites. The presence of these O adatoms is clearly evident in subsequent water TPD measurements. We propose that this channel involves N2O molecules that are bound at vacancies with the N-end of the molecule, which permits the O-end of the molecule to interact with an adjacent Ti4? site. The partitioning between these two channels is roughly 1:1 for adsorption at 90 K, but neither is observed to occur for moderate N2O exposures at temperatures above 200 K. EELS data indicate that vacancies readily transfer charge to N2O at 90 K, and this charge transfer facilitates N2O decomposition. Based on this result, it appears that the decomposition of N2O to N2 requires trapping of the Molecule at vacancies and that the lifetime of the N2O-vacancy interaction may be key to the conversion of N2O to N2.