CeO(2)/TiO(2) and CeO(2)-WO(3)/TiO(2) catalysts prepared by impregnation method assisted with ultrasonic energy were investigated on the selective catalytic reduction (SCR) of NO(x) (NO and NO(2)) by NH(3). The catalytic activity of 10% CeO(2)/TiO(2) (CeTi) was greatly enhanced by the addition of 6% WO(3) in the broad temperature range of 200-500 °C, the promotion mechanism was proposed on basis of the results of in situ diffuse reflectance infrared transform spectroscopy (DRIFT). When NH(3) was introduced into both catalysts preadsorbed with NO + O(2), SCR would not proceed except for the reaction between NO(2) and ammonia. For CeO(2)/TiO(2) catalysts, coordinated NH(3) linked to Lewis acid sites were the main adsorbed ammonia species. When NO + O(2) was introduced, all the ammonia species consumed rapidly, indicating that these species could react with NO(x) effectively. Two different reaction routes, L-H mechanism at low temperature (<200 °C) and E-R mechanism at high temperatures (>200 °C), were presented for SCR reaction over CeO(2)/TiO(2) catalyst. For CeO(2)-WO(3)/TiO(2) catalysts, the Lewis acid sites on Ce(4+) state could be converted to Brønsted acid sites due to the unsaturated coordination of Ce(n+) and W(n+) ions. When NO + O(2) was introduced, the reaction proceeded more quickly than that on CeO(2)/TiO(2). The reaction route mainly followed E-R mechanism in the temperature range investigated (150-350 °C) over CeO(2)-WO(3)/TiO(2) catalysts. Tungstation was beneficial for the formation of Ce(3+), which would influence the active sites of the catalyst and further change the mechanisms of SCR reaction. In this way, the cooperation of tungstation and the presence of Ce(3+) state resulted in the better activity of CeO(2)-WO(3)/TiO(2) compared to that of CeO(2)/TiO(2).

The structures of pure gold and silver clusters (Au k , Ag k , k ) 1-13) and neutral and anionic gold-silver binary clusters (Au m Ag n , 2 e k ) m + n e 7) have been investigated by using density functional theory (DFT) with generalized gradient approximation (GGA) and high level ab initio calculations including coupled cluster theory with relativistic ab initio pseudopotentials. Pure Au k clusters favor 2-D planar configurations, while pure Ag k clusters favor 3-D structures. In the case of Au, the valence orbital energies of 5d are close to that of 6s. This allows the hybridization of 6s and 5d orbitals in favor of planar structures of Au k clusters. Even 1-D linear structures show reasonable stability as local minima (or as global minima in a few small anionic clusters). This explains the ductility of gold. On the other hand, the Ag-4d orbital has a much lower energy than the 5s. This prevents hybridization, and so the coordination number (Nc) of Ag in Ag k tends to be large in s-like spherical 3-D coordination in contrast to that of Au in Au k which tends to be small in 1-D or 2-D coordination. This trend is critical in determining the cluster structures. The calculated electronic properties and dissociation energy of both pure and binary clusters are in good agreement with the available experimental data. Since the Ag-5s orbital is much higher in energy than the Au-6s orbital energy, the partial charge transfer from Au to Ag takes place in gold-silver binary clusters. Au atoms tend to be negatively charged, while Ag atoms tend to be positively charged. Combined with the trend that Au atoms favor the surface, edges, or vertices with smaller Nc, the outer part of the cluster tends to be negatively charged, while Ag atoms favor the inside with larger Nc, and so the inner part tends to be positively charged. The partial charge transfer in the binary system results in electrostatic energy gain for the binary Au m Ag n cluster over pure Au k and Ag k clusters, which is responsible for the formation of alloys. In a neutral alloy, the equivalent mixing is favored, and the even numbered k tends to be more stable due to the electron spin pairing, whereas in an anionic alloy the odd numbered k tends to be more stable.