Individual amounts of Lewis and Brønsted acid sites on metal oxides from NH3 adsorption equilibrium: Case of TiO2 based solids

Abstract

The present study is dedicated to the development of an original method for the measurement of the individual amounts QASi of the acid sites ASi (acidity of Lewis and Brønsted) present on three TiO2 based solids of increasing composition complexity: TiO2-P25, 6% WO3/TiO2-P25 and a sulfated 0.7 % V2O5/9% WO3/TiO2 NH3-SCR catalyst. The method is based on quantitative characterizations of the NH3 adsorption equilibrium (adsorption temperature Ta and pressure Pa) by the association of (a) the AEIR method providing the individual coverage θASi(Ta, Pa) of the adsorbed NH3 species on the ASi sites and (b) the total amount (in μmol/g) of the adsorbed NH3 species: QTNH3(Ta, Pa) by using a mass spectrometer. For a solid having n types of ASi sites, the QASi values are obtained from the numerical solution of linear equation systems (with at least n equations) obtained considering that QTNH3(Ta, Pa) is equal to the sum of the contribution of each adsorbed NH3 species: QASi θASi(Ta, Pa).This imposes the measurement of at least n QTNH3(Ta, Pa) in a Ta range preventing the contribution of parallel surface processes (i.e NH3 oxidation). On TiO2-P25, n = 3 (two Lewis and one weak Brønsted acid sites) and the QASi amounts are obtained from three QTNH3(Ta, Pa) values. The others solids having two Lewis (L1 and L2) and two Brønsted (B1 and B2) acid sites impose a series of m measurements of QTNH3(Ta, Pa) with m>> n: the QASi are obtained by optimization between theoretical and experimental QTNH3(Ta, Pa) curves such as 109, 202, 70 and 130 μmol/g for QL2, QL1, QB2 and QB1 respectively of 0.7 % V2O5/9% WO3/TiO2. It is shown that these four amounts of sites permit to conclude that the L2 Lewis acid site is the one forming the pivotal NH3ads-L2 species of the reaction by using nitrogen mass balances between the amounts of adsorbed NH3 species and the N2 production in the presence of NO.