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Titania-supported tungsten and molybdenum SCR catalysts were investigated before and after poisoning with arsenic(III) oxide. Diffuse reflectance and transmission infrared spectroscopy, including the adsorption of carbon monoxide and ammonia as probe molecules, were applied. The surface of the unpoisoned catalysts is characterized by hydroxyl groups of the support and by surface polytungstate and polymolybdate species carrying terminal metal oxygen double bonds. Carbon monoxide adsorbs on coordinatively unsaturated (cus) metal cations, representing Lewis acid sites, and via H-bonding on hydroxyl groups, representing Brønsted acid sites. From the carbon monoxide adsorption data it is concluded, that the cus cations on tungsten containing samples are stronger Lewis acid sites than those on molybdenum containing samples. Ammonia is adsorbed on Lewis acid sites on both catalysts. Poisoning of the samples by arsenic(III) oxide leads to replacement of the hydroxyl groups by new species that are assigned to arsenic-hydroxyls of a well distributed surface arsenate species. Interaction of the arsenate with the polyoxostructures is shown by the perturbation of the metal-oxygen stretching band of the active phase after poisoning. Important with regard to catalysis may be the fact that neither carbon monoxide nor ammonia can be adsorbed on Lewis surface sites on catalysts poisoned with sufficient amounts of arsenic(III) oxide.
