Density functional theory (DFT) study of the interaction of ammonia with pure and tungsten-doped ceria

We report here our theoretical investigations of the interaction of ammonia (NH3) with pure and tungsten-doped ceria surfaces. The objective of the work was to understand the modification of surface acidity via the use of dopants, at the fundamental level. Surface acidity is an important property that is frequently used to characterize the reactivity of surfaces. Interest in ammonia stems from its use as a probe molecule in experimentally determining the acidity of surfaces, while ceria and tungsten oxide are examples of metal oxides widely used in catalytic systems. Density functional theory calculations were performed using the Vienna ab-initio simulation package (VASP 4.6.2). The surfaces were modeled using finite number of layers, cleaved from the relaxed bulk structures. The geometries were optimized and resulting binding energies of ammonia were used to correlate to acid site strength. Our calculations show that ammonia binds to ceria at the Lewis and Bronsted acid sites, however this binding is very weak (<5 kcal/mol). In the presence of tungsten, the binding to the Lewis acid sites continues to be weak, while the binding to the Bronsted acid sites is increased significantly (to similar to 10 kcal/mol). We have used the H-terminated surface to study the Bronsted acidity and as a corollary. we also report the significantly different binding of hydrogen to the pure and tungsten-doped ceria surfaces. (C) 2009 Elsevier B.V. All rights reserved.