Negative Charging of Au Nanoparticles during Methanol Synthesis from CO2/H2 on a Au/ZnO Catalyst: Insights from Operando Infrared and Near-ambient Pressure XPS and XAS

Abstract

Kinetic measurements in combination with time resolved operando infrared (DRIFTS), in situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and X-ray absorption near edge spectroscopy (XANES) measurements at the O K-edge together with high resolution electron microscopy were applied to evaluate the electronic and structural properties of Au/ZnO under industrial and idealized methanol synthesis conditions. CO adsorption during the reaction revealed the presence of negatively charged Au nanoparticles / Au sites under reaction conditions, which are formed during the initial phase of the reaction. Near ambient pressure XPS and XANES demonstrate the build-up of O-vacancies during the reaction, which goes along with a substantial increase in the methanol formation rate. The results are discussed in comparison with previous findings for Cu/ZnO and Au/ZnO catalysts. Supported Au catalysts have attracted considerable interest due to their high activity at rather mild conditions in a number of reactions, including the hydrogenation of CO2 to methanol. For the latter reaction, supported gold catalysts, in particular Au/ZnO, were found to be at least as active as commercial Cu/ZnO catalysts. In addition, they showed a higher selectivity for methanol formation relative to the competing reverse water gas shift (RWGS) reaction at pressures up to 50 bar. To better understand the origin of the high activity and selectivity of these catalysts, a detailed knowledge of their electronic and geometric structure under reaction conditions is required. First of all this requires reliable information on the oxidation state of the active Au species and of the oxide support during reaction. We had recently shown for Au/TiO2 catalysts in the CO oxidation reaction that the number of surface oxygen vacancies on the TiO2 support depends sensitively on the composition of the gas phase, specifically on the CO : O2 ratio. In the same way one might expect that a reductive reaction atmosphere, as encountered for a CO2 / H2 methanol synthesis mixture, will lead to a partial reduction of the ZnO support in the Au/ZnO catalysts. Similar reduction effects have been debated controversially for Cu/ZnO catalysts. Some of those authors had suggested that the Cu crystallites are overgrown by a metastable layer of partially reduced ZnOx during reaction. It can also be envisaged that the electronic properties of the metal nanoparticles (NPs) are modified by such changes in the ZnO support. Therefore, scrutinizing the electronic properties of the Au NPs and of the ZnO support and their mutual interactions under reaction conditions is a prerequisite for the detailed understanding of their catalytic performance. In this work, we employed kinetic measurements as well as time resolved operando diffuse reflectance FTIR spectroscopy (DRIFTS) in the pressure range of up to industrial methanol synthesis conditions (up to 50 bar, 240°C), together with in situ near-ambient pressure X-ray photoelectron spectroscopy (NAPXPS) and X-ray absorption near edge spectroscopy (XANES) to i) probe the changes in the electronic state of Au during methanol synthesis and to ii) inspect how the changes in the (defect) structure of ZnO can induce or be related to changes in the electronic state of the Au NPs. In addition, we make use of information on the Au particle size, obtained from transmission electron microscopy (TEM), and of results on the atomic Au:Zn surface ratios derived from quasi in situ laboratory XPS measurements. Based on these data we will discuss changes in the electronic / geometric structure of the Au/ZnO catalyst during time on stream and their effect on the activity for methanol formation. The trends mapped out for Au/ZnO are expected to provide insight also for the more complex Cu/ZnO system. First, we monitored possible changes in the electronic state of the Au NPs during reaction, following the vibrational properties of CO adsorbed on the Au/ZnO catalyst by operando DRIFTS measurements at 240°C and pressures from 1 to 50 bar. Here, we make use of the ongoing CO formation during methanol synthesis via the competing RWGS reaction. Representative spectra, recorded under steady-state conditions at different pressures, are displayed in Figure 1a. Under these conditions, there is essentially no CO adsorption detectable at 1 bar. A COad related band centered at 2078 cm appears at 5 bar and increases in intensity with increasing pressure. Starting at 20 bar, a broad band evolves, ranging from 2050 – 2130 cm, and at 40 50 bar, distinct maxima appear at 2109 cm as well as at 2018, 2068, and 2094 cm, respectively. Bands in these ranges are characteristic for CO adsorption on small metallic Au NPs (Au, [a] Dr. A.M. Abdel-Mageed, M.Sc. A. Rezvani, Prof. Dr. R.J. Behm, Institute of Surface Chemistry and Catalysis, Ulm University, D89069 Ulm (Germany) E-mail:[email protected]. [b] Dr. A. Klyushin, Dr. A. Knop-Gericke, Prof. Dr. R. Schlögl, Fritz-Haber-Institute, Dept. Inorganic Chemistry, Faradayweg 4-6 D-14195 Berlin, Germany, and Max Planck Institute for Chemical Energy Conversion, Heterogeneous Reactions, Stiftstrasse 34-36, D-45470 Mülheim, Germany [c] Dr. A. Klyushin, Helmholtz-Zentrum Berlin für Materialien und Energie, BESSY II, Albert-Einstein-Straße 15, 12489 Berlin, Germany [#] Dr. A.M. Abdel-Mageed, Permanent address: Department of Chemistry, Faculty of Science, Cairo University, Giza 12613, Egypt Supporting information for this article is given via a link at the end of the document. 10.1002/anie.201900150 A cc ep te d M an us cr ip t Angewandte Chemie International Edition This article is protected by copyright. All rights reserved.