Surface Composition Changes of Redox Stabilized Bimetallic CoCu Nanoparticles Supported on Silica under H2 and O2 Atmospheres and During Reaction between CO2 and H2: In Situ X-ray Spectroscopic Characterization

Abstract

In this paper, we report the colloidal synthesis and detailed characterization of 11 nm bimetallic CoCu nanoparticle catalysts. Presently Co and Cu is an attractive combination because of their respective properties for industrially important Fischer–Tropsch and methanol synthesis reactions of CO (and CO2) with H2. We report the preparation of catalysts by deposition of bimetallic metal nanoparticles, both within mesoporous silica (MCF-17) and on the native oxide surface of a silicon wafer. Subsequent phase separation into phase-segregated (i.e., dimer) particles is found to occur upon redox treatment. These nanoparticle catalysts have then been investigated using an array of techniques including synchrotron-based ambient pressure X-ray photoelectron spectroscopy (APXPS) and in situ near edge and extended X-ray absorption fine structure (NEXAFS/EXAFS) spectroscopies. CO2 hydrogenation is used as a probe reaction. All three techniques combine to show that an oxygen atmosphere segregates copper to the surface. In doing so the oxygen produces oxides of both Co and Cu metals. Significant hydrogen pressure and temperature are required to fully rereduce both metals to a metallic state as demonstrated by NEXAFS spectroscopy. Under the conditions of the CO2/H2 reaction monitored in situ using NEXAFS spectroscopy, both metals exist in a fully reduced state at 2.7 bar, 1:3 CO2:H2, and 260 °C.