Assignment of Vibrational Bands of Critical Surface Species Containing Nitrogen in the Selective Catalytic Reduction of NO by NH 3 .

The selective catalytic reduction (SCR) of NO by NH 3 on metal oxides plays a key role in minimizing NO x emissions. Electronic structure calculations at the density functional theory level have been performed to predict the vibrational modes of NH 3 /NH 4 + bound to validated cluster models of vanadium oxide bound to a TiO 2 surface. Excellent agreement of the scaled calculated values with the observed bands attributed to surface-bound species is found. The presence of NH 3 bound to Lewis acid sites and NH 4 + bound to Brønsted acid sites when VOH groups are present is supported by our predictions. NH 4 + is expected to dominate the spectra even at low concentrations, with predicted intensities 5 to 30 times greater than those predicted for surface-bound NH 3 . This is particularly evident in the lowest-energy N-H stretches of surface NH 4 + due to partial proton transfer interactions with the vanadium oxide surface model. The current work is consistent with experimental vibrational spectroscopy results and does not support the presence of a significant amount of NH 2 on the catalyst surface for the SCR reaction on VO x /TiO 2 . The combined experimental and computational results support the presence of both NH 3 - and NH 4 + -type species bound to the surface.