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

The present study is dedicated to the development of an original method for the measurement of the individual amounts Q(ASi) of the acid sites A(Si) (acidity of Lewis and Bronsted) 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 T-a and pressure P-a) by the association of (a) the AEIR method providing the individual coverage theta(ASi)(T-a, P-a) of the adsorbed NH3 species on the AS(i) sites and (b) the total amount (in mu mol/g) of the adsorbed NH3 species: QTNH(3)(T-a, P-a) by using a mass spectrometer. For a solid having n types of AS(i) sites, the Q(ASi) values are obtained from the numerical solution of linear equation systems (with at least n equations) obtained considering that QTNH(3)(T-a, P-a) is equal to the sum of the contribution of each adsorbed NH3 species: Q(ASi) theta(ASi)(T-a, P-a).This imposes the measurement of at least n QTNH(3)(T-a, P-a) 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 Bronsted acid sites) and the Q(ASi) amounts are obtained from three QTNH(3)(T-a, P-a) values. The others solids having two Lewis (L1 and L2) and two Bronsted (B1 and B2) acid sites impose a series of m measurements of QTNH(3)(T-a, P-a) with m>> n: the Q(ASi) are obtained by optimization between theoretical and experimental QTNH(3)(T-a, P-a) curves such as 109, 202, 70 and 130 mu mol/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 N-2 production in the presence of NO.

Automated summary

The study focuses on developing an original method for measuring individual amounts of acid sites on TiO2 based solids. The method involves quantitative characterizations of NH3 adsorption equilibrium and uses mass spectrometer analysis. The results show that the L2 Lewis acid site plays a pivotal role in the NH3ads-L2 species formation during reaction.