Bimetallic AgCu/SBA-15 System: The Effect of Metal Loading and Treatment of Catalyst on Surface Properties

Monometallic (Ag, Cu) and bimetallic (Ag + Cu) catalysts were prepared by metal loading (Ag:Cu = 0.3 and 1.8) on 3-aminopropyl-trimethoxysilane-grafted SBA-15 and calcination at 773 K and either reduction by NaBH₄ before calcination or activation in inert gas after calcination. The catalysts treated in this way were fully characterized. Cu/SBA-15 samples contained CuO and oligonuclear [Cu^δ+···O^δ−···Cu^δ+]_<i>n</i> clusters irrespective of the catalyst treatment. Silver–SBA-15 contained cationic silver in the form of Ag₂O which was transformed to metallic Au⁰ by the reduction with NaBH₄ and was not reoxidized during calcination. In bimetallic catalysts, different species were identified depending mainly on the Ag:Cu atomic ratio and the post modification treatment. When the excess of copper was applied the core (Ag₂O)–shell (CuO) structure of the bimetallic phase was formed. If the reduction with NaBH₄ was used prior to calcination, the same core–shell structure was present but with higher dispersion of CuO, manifested as a higher basicity of the catalysts revealed as a higher selectivity to acetone in 2-propanol dehydrogenation and to CO₂ in methanol oxidation. The use of silver in excess led to the presence of both cationic silver and copper species in calcined AgCu(1)/S­(C) material. In the sample reduced with NaBH₄ and then calcined (AgCu(1)/S­(RC)), metallic copper was partially surrounded by metallic silver. In bimetallic samples Cu–Ag interaction led to the electron transfer from copper to silver species enhancing their redox properties and causing the superior activity in the low-temperature total oxidation of methanol to CO<sub>2.</sub

Size of Au-Nanoparticles Supported on Mesostructural Cellular Foams Studied by the Pair Distribution Function Technique

Mesostructural cellular foam (MCF) materials that were modified by Zr, Nb, and Mo incorporation, followed by APTMS (3-aminopropyl-trimethoxysilane) grafting and gold loading were studied using the pair distribution function (PDF) technique. Measurements were focused on changes in gold crystallite sizes and on local geometry changes in the supports. Initially, ex situ prepared samples were investigated at different stages of synthesis and after catalytic oxidation of carbon monoxide. The crystallization and agglomeration of gold species as well as carbon monoxide oxidation were then tracked by in situ high energy diffraction measurements. The influence of metal type (Nb or Mo) and incorporation method in the MCF material on the agglomeration of metallic gold particles during increasing calcination temperature was determined. The structure of MCF materials was preserved during calcination and oxidation of CO and local symmetry of gold particles is not changed under CO oxidation conditions. In samples oxidized in the laboratory flow reactor, the interaction of gold particles with the reagents (CO and O₂) leads to slight decrease in gold particle size

Zeolite MCM-22 Modified with Au and Cu for Catalytic Total Oxidation of Methanol and Carbon Monoxide

The goal of this work was to use MCM-22 zeolites for preparation of monometallic (Cu or Au) and bimetallic (Cu and Au) catalysts for oxidation reactions. The focus was on precise determination of the nature of gold and copper species and their activity in the oxidation processes. For that purpose several characterization techniques were applied (XRD, N₂ adsorption/desorption, TEM, SEM, UV–vis, H₂-TPR, ²⁷Al MAS NMR, FT-IR with the adsorption of pyridine, NO, and CO, ESR spectroscopy). They allowed us to define the following species formed on MCM-22 surface: metallic gold particles (XRD, UV–vis), isolated Cu²⁺ with octahedral coordination (UV–vis, ESR), square planar Cu²⁺ cations (ESR, IR), Cu⁺ species (ESR+NO, FTIR+CO, and FTIR+NO), and oligonuclear clusters (UV–vis) as well as CuO-like species (H₂-TPR). The presence of gold on the MCM-22 surface modified further by copper species caused the interaction between two modifiers leading to much easier reduction of CuO-like species and higher mobility of oxygen-promoting oxidative properties. The bimetallic catalyst was highly active in total oxidation of methanol and CO in the temperature range 523–623 K. Cu/Au-MCM-22 zeolite appeared useful for simultaneous removal of CO and methanol (by total oxidation) from gases emitted from automotive devices and during a variety of industrial process operations