Gold supported on tungstated zirconia : synthesis, characterization and in situ FT-IR investigation of NO(formula) + CH(formula) surface reactions

Abstract

The potential of gold supported on tungstated zirconia as a catalyst for selective catalytic reduction of NOx with propene (C3H6-SCR) was investigated by in situ FT-IR spectroscopy. Samples of tungstated zirconia were prepared by both impregnation and coprecipitation methods using ammonium metatungstate (AMT) as a precursor. Gold was deposited on the supports via cationic adsorption from aqueous solution of [Au(en)2]Cl3 complex (en = ethylenediamine). The samples were characterized by XRD, XPS, BET, DRUV-vis, ICP-MS and FT-IR spectroscopy of adsorbed CO. The results show that the samples consist of tetragonal zirconia crystallites hosting uniform layer of polytungstate species. The gold particles occupy preferentially the WOx-free zirconia surface and the dispersion of gold depends on the amount of coodinatively unsaturated (cus) Zr4+ ions. The modification of zirconia by tungsten facilitates the gold uptake but at the same time causes decrease in the concentration of (cus) Zr4+ ions thus lowering the despersion of gold clusters. The interaction of gold supported on WOx-free (Au/ZrO2) and WOx-modified zirconia samples (Au/xWZ-I, where “I” denotes the incorporation of WO3 by impregnation and x = 5, 12, and 20 wt % of WO3) with NO+O2 gas mixture shows that the W-containing samples promote the formation of NO2 at room temperature. The FT-IR spectra obtained at room temperature during the contact of CO with gold samples containing pre-adsorbed NOx species reveal the formation of isocyanates (NCO) coordinated to gold sites. The generation of Au NCO species in the ad-NOx+CO reaction is confirmed by using 13CO and treatment of the adsorbed isocyanates with water vapor. The gold isocyanates display high thermal stability. However, they react readily with NO2 at room temperature. This finding suggests that gold supported on tungstated zirconia could be of interest as a low-temperature catalyst for COSCR of NOx. The FT-IR spectra recorded during the contact of C3H6 and O2 gas mixture with Au/ZrO2 and Au/xWZ-I samples lead to the conclusion that the WOx-free sample catalyzes the complete oxidation of propene. The Au-promoted tungstated samples, which contain redox (W=O groups) and Brønsted acid sites, favor the partial oxidation of the hydrocarbon. The results of a detailed mechanistic investigation show that the activation of propene in the presence of NOx species adsorbed on Au/xWZ-I samples takes place at room temperature producing surface isopropoxides. The interaction of the latter species with the surface nitrate complexes leads to the formation of nitroacetone [CH3C(O)CH2NO2]. It is proposed that at higher temperatures (e.g. 150oC) the nitroacetone coordinated to gold particles transforms through an internal redox process producing surface acetates and Au NCO species. The isocyanates react with the NO3 /NO2 surface complex formed by oxidation of NO yielding molecular nitrogen, N2O and COx as reaction products. The goldfree samples do not cause the formation of NCO species under the same experimental conditions. This experimental fact suggests that the Au particles play fundamental role in the formation of the NCO species. The amount of Au-NCO species produced is the highest on ZrO2-based catalyst containing 1.8 wt % of gold and 12 wt % of WO3 (Au/12WZ-I sample). This material combines better gold dispersion with sufficient amount of Brønsted acid sites necessary for the activation of propene to hydrocarbon oxygenates leading to the formation of nitroacetone. Based on the catalytic activity measurements, it is concluded that among the materials studied, the Au/12WZ-I catalyst could be promising in the C3H6-SCR of NOx.