INTERACTION OF ARSENIOUS OXIDE WITH DENOX-CATALYSTS - AN X-RAY ABSORPTION AND DIFFUSE REFLECTANCE INFRARED-SPECTROSCOPY STUDY

The interaction of arsenious oxide with deNOx-catalysts such as titania-supported oxides of molybdenum and tungsten has been studied by X-ray absorption spectroscopy (XANES and EXAFS) and by diffuse reflectance FT infrared spectroscopy (DRIFT). It is shown by XANES at the As K edge that the majority oxidation state of the arsenic surface species is As5+. EXAFS beyond the As K edge indicates a (3 + 1) oxygen coordination shell around the arsenic with As0 distances of 1.67 and 1.94 Å, respectively. A characteristic new infrared hydroxyl stretching band at 3610 cm-1 is formed and can be assigned as an AsO-H stretching mode. These experimental results can best be associated with an orthoarsenate(V) surface species which forms when arsenious oxide is deposited on the catalyst surfaces. XANES at the W L1 and W L3 edges suggest that the orthoarsenate interacts directly with tungsten sites, a condition which is supported by the strong perturbation of the new W=O stretching overtone on formation of the arsenate. It is tentatively suggested that the poisoning of titania-supported tungsten oxide deNOx- catalysts is due to blocking of active sites which involve coordinatively unsaturated tungsten centers by the orthoarsenate surface species. © 1991.