Role of lattice strain and defects in copper particles on the activity of Cu/ZnO/Al(2)O(3) catalysts for methanol synthesis.

Copper-based catalysts are industrially applied in various reactions including water-gas shift, synthesis of fatty alcohols from fatty acid methyl esters, and methanol synthesis. Today, methanol is produced at low pressures (35-55 bar) and 200-300°C over Cu/ZnO/Al2O3 catalysts. Due to the great commercial relevance, Cu/ZnO-based catalysts have been extensively studied and many different models have been proposed regarding the nature of active sites and the valence of copper under conditions of methanol formation, such as Cu dispersed in ZnO, metallic copper supported on ZnO, dynamic surface and bulk alloy formation depending on the reduction potential of the synthesis gas, Cu at the so-called Schottky junction between metallic Cu and the semiconductor ZnO, and ZnO segregated on Cu. The catalytic activity of the binary catalyst has been reported to be several orders of magnitude greater than that of metallic Cu or pure ZnO, respectively, indicating a synergetic interaction of the two components. ZnO is regarded either as provider of atomic spillover hydrogen for further hydrogenation of adsorbed reaction intermediates on Cu sites, or as a structure directing support controlling dispersion, morphology, and specific activity of the metal particles. Strong interaction between the metal and the support, especially in the case of large lattice mismatch, is known to cause strain in the metal particles, to which an increase in catalytic performance has been attributed. On the other hand, 1-ML-high and thicker Cu islands epitaxially grown on the ZnO (000⎯1) surface were experimentally found to be strain-free. In most of the earlier studies model catalysts with low Cu loadings (Cu/Zn << 1) containing large ZnO single crystals have been investigated, although, usually, in commercial catalysts copper represents the main component (Cu/Zn > 1) and the ZnO particles, acting rather as a spacer than as a support, are comparable in size, or even smaller than the Cu particles. In this paper we report the results of TEM and in situ XRD characterization of a series of Cu/ZnO/Al2O3 catalysts exhibiting different catalytic activities. The molar ratio Cu:Zn:Al = 60:30:10 is characteristic of commercial catalysts. The microstructural features of the materials prepared by coprecipitation with sodium carbonate from metal nitrate solution are analyzed after calcination in air at 330°C and subsequent reduction in hydrogen at 250°C. A quantitative estimation of imperfections in metal particles determined by combination of independent TEM and in situ XRD investigations is established. The implications of strain in Cu crystallites and the defect frequency associated therewith on the catalytic activity of Cu/ZnO/Al2O3 catalysts in methanol synthesis are discussed. The TEM and HRTEM images shown in Figure 1 illustrate the microstructure typical of the catalysts studied. Generally, 10 to 15 clusters similar to the one shown in Figure 1a, which had the size that varied from 100 nm to several micrometers, were analyzed in each of the catalysts with Energy-Dispersive X-Ray spectroscopy (EDX) to determine the concentrations of Al, Cu, and Zn. The average value (Al: 10.6 ±4.5 at.%; Cu: 62.7 ±7.2 at.%; Zn: 26.7 ±4.2 at.%) is close to the nominal composition. Figure 2 that contains data of all catalysts illustrates the scattering due to local inhomogeneities. The particles of Cu and ZnO form an irregular framework (Figure 1b). The particles of ZnO, which are comparable in size or smaller than the Cu particles, serve as spacers between the latter, preventing them from sintering.