Catalysis Science & Technology PAPER Cite this: DOI: 10.1039/c4cy00848k Received 1st July 2014, Accepted 24th September 2014 DOI: 10.1039/c4cy00848k www.rsc.org/catalysis The role of copper particle size in low pressure methanol synthesis via CO 2 hydrogenation over Cu/ZnO catalysts† Alejandro Karelovic * ab and Patricio Ruiz a Cu/ZnO catalysts with different mean Cu particle sizes were prepared by wet impregnation of copper nitrate onto a zinc oxide support. Their performance was studied in methanol synthesis reaction from CO 2 and H 2 at temperatures between 160 and 225 °C and a pressure of 7 bar. Selective methanol formation is favored at lower temperatures due to the suppression of CO production. Activation energies were 8–11 kcal mol -1 for methanol formation and 29–31 kcal mol -1 for CO formation and were similar for all the catalysts. For catalysts with copper cluster sizes between 8.5 and 37.3 nm, the methanol formation rates normalized by surface copper atoms were independent of copper particle size. On the contrary, CO formation rates are enhanced over catalysts with smaller copper clusters. Higher selectivity to methanol is favored over catalysts possessing larger copper nanoparticles. Catalysts with copper loading ≥8 wt.% showed a strong sintering of copper nanoparticles and also a significant growth of ZnO support crystallites. These catalysts presented higher intrinsic rates for methanol formation (4 × 10 -3 s -1 at 180 °C) compared to catalysts with lower copper loading (0.9 × 10 -3 s -1 ). As the kinetic parameters were similar for all Cu/ZnO catalysts, it is proposed that catalysts with large copper and ZnO particles form new active sites that led ultimately to a very high methanol synthesis activity and selectivity. It is suggested that the important sintering of Cu particles modifies the structure of copper promoting the hydrogenation rate in methanol synthesis. 1. Introduction Methanol is currently produced in large quantities in the chemical industry. It is used as a feedstock for producing plastics, resins and paints. Besides its interesting properties as a chemical feedstock, methanol has been recently consid- ered as the fuel of the future owing to its interesting proper- ties. 1,2 Methanol is a liquid under normal conditions, which allows it to be easily stored in contrast to hydrogen. Further- more, methanol can be used directly as fuel or in methanol fuel cells. It can be easily converted into olefins such as ethyl- ene and propylene, which are presently obtained from oil and gas and are a very important feedstock in the petrochemical industry. The synthesis of methanol is an established technol- ogy. Methanol is produced industrially from synthesis gas mixtures (typically in CO/CO 2 /H 2 molar ratio = 10/10/80) at 230–250 °C and pressures around 50–100 bar. 3 A catalyst developed by Imperial Chemical Industries in the 1960s composed of copper, zinc oxide and alumina is the most used catalyst for this reaction. 4–6 There is a current need of decreasing carbon dioxide emissions in order to mitigate the global warming caused by the increasing concentration of this gas in the atmosphere. There is also an interest in adding value to the CO 2 effluents in the chemical industry, since this carbon-containing sub- stance is currently lost into the atmosphere. 7 One of the pos- sibilities is to hydrogenate CO 2 to methanol (eqn (1)) which, as has been exposed above, is a very prospective raw chemi- cal. Although Cu/ZnO catalysts are useful for both CO 2 - to-methanol and CO/CO 2 -to-methanol reactions, the condi- tions at which the future CO 2 -to-methanol processes will be performed are not necessarily those of the current and very drastic syngas-to-methanol technology. Low temperature and pressure are required for operation in small and decentralized units. 8 Additionally, selectivity becomes an important issue because CO can also be a product of the reaction besides methanol. CO can be produced by non-selective reverse water- gas shift (r-WGS) and methanol decomposition reactions Catal. Sci. Technol. This journal is © The Royal Society of Chemistry 2014 a Institute of Condensed Matter and Nanosciences (IMCN), Molecules, Solids and Reactivity (MOST), Université Catholique de Louvain, Croix du Sud 2/17, L7.05.15, 1348 Louvain-La-Neuve, Belgium b Departamento de Ingeniería Química, Universidad de Concepción, Barrio Universitario s/n, Concepción, Chile. E-mail: akarelov@udec.cl; Fax: +56 41 2203654 † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cy00848k Published on 25 September 2014. Downloaded by SCD Université Paris 7 on 17/10/2014 15:19:07. View Article Online View Journal