Experimental Microkinetic Approach of De-NOx by NH3 on V2O5/WO3/TiO2 Catalysts. 1. Individual Heats of Adsorption of Adsorbed NH3 Species on a Sulfate-Free TiO2 Support Using Adsorption Isobars

Abstract

The present study is dedicated to the development of experimental procedures allowing the measurement of the individual heats of adsorption of adsorbed NH3 species on a sulfate-free TiO2 solid (P25 from Degussa). This solid has been selected because it is frequently used as support of V2O5– or/and WO3–TiO2 model catalysts for the understanding of the surface processes implicated in the selective catalytic reduction of NOx by NH3 (NH3-SCR). Two original analytical procedures denoted adsorption equilibrium infrared spectroscopy (AEIR) and temperature-programmed adsorption equilibrium (TPAE) (developed in previous works) were applied. These methods are based on (a) the experimental measurement of the change in the adsorption equilibrium coverage of the individual adsorbed species in isobar conditions and (b) the comparison of the experimental data to an adsorption model. It is shown that in the ranges of the ammonia partial pressures and reaction temperatures of the NH3-SCR process, only two adsorbed NH3 species on Lewis sites (Ti+δ) are detected on the solid dehydrated at 673 K. These species noted NH3ads-L1 and NH3ads-L2 are differentiated by their δs NH3 IR bands at 300 K (1149 and 1228 cm–1, respectively), whereas their δas IR bands are at the same position (1596 cm–1). The AEIR and TPAE methods indicate that the heats of adsorption of the NH3ads-L1 and NH3ads-L2 species (noted EL1(θ) and EL2(θ) accuracy ± 5 kJ/mol) vary linearly with their respective coverages θ from EL1(1) = 56 kJ/mol to EL1(0) = 105 kJ/mol and from EL2(1) = 105 kJ/mol to EL2(0) = 160 kJ/mol. These values are compared to (a) isosteric heat of adsorption provided by the Clausius–Clapeyron method and (b) literature data using temperature-programmed desorption, microcalorimetry, and DFT calculations. Forthcoming articles show that the simplicity of the analytical procedures allows studying the impact of the presence of sulfate and VOx/WOy depositions over TiO2 on the nature and heats of adsorption of adsorbed NH3 species.