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 f or the measurement of the individual amounts QASi of the acid sites ASi (acidity of Le wis and Brønsted) present on three TiO2 based solids of increasing composition compl exity: 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 adsor ption equilibrium (adsorption temperature Ta and pressure Pa) by the association of (a) the AEIR method providing the individual coverage qASi(Ta, Pa) of the adsorbed NH3 species on the ASi sites and (b) the total amount (in µmol/g) of the adsorbed NH3 spec ies: QTNH3(Ta, Pa) by using a mass spectrometer. For a solid having n types of ASi si tes, the QASi values are obtained from the numerical solution of linear equation system s (with at least n equation) obtained considering that QTNH3(Ta, Pa) is equal to the su m of the contribution of each adsorbed NH3 species: QASi qASi(Ta, Pa).This imposes the measurement of at least n QTNH3(Ta, Pa) in a Ta range preventing the contributio n 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 ob tained from three QTNH3(Ta, Pa) values. The others solids having two Lewis (L1 and L 2) and two Brønsted (B1 and B2) acid sites impose a series of m measurements of QT NH3(Ta, Pa) with m>> n: the QASi are obtained by optimization between theoretical an d 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 fo ur 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 a dsorbed NH3 species and the N2 production in the presence of NO.

Single Mo atoms paired with neighbouring Ti atoms catalytically decompose ammonium bisulfate formed in low-temperature SCR

“…3b, only a peak at ∼1600 cm −1 assigned to the adsorbed gas NH 3 on Lewis sites appears in the spectrum of TiO 2 . 29 However, when single Mo atoms were anchored on TiO 2 , significant signals at ∼1670 and ∼1455 cm −1 appeared, manifesting the increased NH 3 coordination of Brønsted sites in the form of NH 4 + , 29,38 i.e. , Mo–O–H structures were formed on the surface of Mo 1 /TiO 2 , which are totally different from that in bulk MoO 3 where no relevant peak was observed.…”

Assessment of Lewis‐Acidic Surface Sites Using Tetrahydrofuran as a Suitable and Smart Probe Molecule

“…Some of these attempts failed, while others yielded acceptable and successful results [1,2] . The majority of the published articles employed the IR‐spectroscopic methods to characterise the surface acidity of solid catalysts such as: 12‐membered ring zeolites probed with 2,4,6‐tri‐ tert ‐butylpyridine, [1] TiO 2 ‐based solid catalysts by adsorption of NH 3 , [2] commercial samples of Faujasite‐type zeolite probed with six molecules as follows: pyridine, 2,6‐di‐ tert ‐butylpyridine, 2,4 dimethylquinoline, tributylamine, trihexylamine, and 2,4,6‐tri‐ tert ‐butylpyridine that with different basicities and critical diameters [3] . Others demonstrated the NH 3 ‐TPD (Temperature‐programmed desorption) profiles of Fe 2 O 3 nanoparticles supported on γ‐χ‐Al 2 O 3 , [4] and promoted Mg−Al mixed oxides with transition metals [5] .…”

“…The above‐mentioned successful attempts,[ 1 , 2 , 3 , 4 , 5 , 6 , 7 ] which used different basic molecules, encourage us to investigate tetrahydrofuran (THF) as a probe molecule to determine acidic surface sites of our samples. Our research team has published numerous articles over the last two decades, including TPD studies of pyridine and NH 3 as probe molecules, as well as IR‐spectroscopy of pyridine[ 12 , 13 , 14 , 15 , 16 ] for the determination of the acidic sites of some catalysts.…”

“…In the last seven decades, great progress has been made in the acidity measurements of solid catalysts. Many researchers have estimated qualitatively and/or quantitatively‐semi‐quantitatively the acidic surface sites, particularly the external Brönsted acid sites, over a wide range of solid catalysts with high surface area and large pore volumes such as zeolites, alumina and SiO 2 [1,2] by using some compounds rather than the most common molecules, that is, NH 3 and pyridine. Some of these attempts failed, while others yielded acceptable and successful results [1,2] .…”

“…Many researchers have estimated qualitatively and/or quantitatively‐semi‐quantitatively the acidic surface sites, particularly the external Brönsted acid sites, over a wide range of solid catalysts with high surface area and large pore volumes such as zeolites, alumina and SiO 2 [1,2] by using some compounds rather than the most common molecules, that is, NH 3 and pyridine. Some of these attempts failed, while others yielded acceptable and successful results [1,2] . The majority of the published articles employed the IR‐spectroscopic methods to characterise the surface acidity of solid catalysts such as: 12‐membered ring zeolites probed with 2,4,6‐tri‐ tert ‐butylpyridine, [1] TiO 2 ‐based solid catalysts by adsorption of NH 3 , [2] commercial samples of Faujasite‐type zeolite probed with six molecules as follows: pyridine, 2,6‐di‐ tert ‐butylpyridine, 2,4 dimethylquinoline, tributylamine, trihexylamine, and 2,4,6‐tri‐ tert ‐butylpyridine that with different basicities and critical diameters [3] .…”