One of the main stumbling blocks in developing rational design strategies for heterogeneous catalysis is that the complexity of the catalysts impairs efforts to characterize their active sites. We show how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al(2)O(3) methanol synthesis catalyst by using a combination of experimental evidence from bulk, surface-sensitive, and imaging methods collected on real high-performance catalytic systems in combination with density functional theory calculations. The active site consists of Cu steps decorated with Zn atoms, all stabilized by a series of well-defined bulk defects and surface species that need to be present jointly for the system to work.

Addition of small amounts of promoters to solid catalysts can cause pronounced improvement in the catalytic properties. For the complex catalysts employed in industrial processes, the fate and mode of operation of promoters is often not well understood, which hinders a more rational optimization of these important materials. Herein we show for the example of the industrial Cu/ZnO/Al2O3 catalyst for methanol synthesis how structure-performance relationships can deliver such insights and shed light on the role of the Al promoter in this system. We were able to discriminate a structural effect and an electronic promoting effect, identify the relevant Al species as a dopant in ZnO, and determine the optimal Al content of improved Cu/ZnO:Al catalysts. By analogy to Ga- and Cr-promoted samples, we conclude that there is a general effect of promoter-induced defects in ZnO on the metal-support interactions and propose the relevance of this promotion mechanism for other metal/oxide catalysts also.

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/Al 2 O 3 catalysts. [1] 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 1+ dispersed in ZnO, [2,3] metallic copper supported on ZnO, [4] dynamic surface and bulk alloy formation depending on the reduction potential of the synthesis gas, [5,6] Cu -at the so-called Schottky junction between metallic Cu and the semiconductor ZnO, [7] and ZnO segregated on Cu 1+ . [8] 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. [9] ZnO is regarded either as provider of atomic spillover hydrogen for further hydrogenation of adsorbed reaction intermediates on Cu sites, [10,11] or as a structure directing support controlling dispersion, morphology, and specific activity of the metal particles. [12][13][14][15][16][17][18] 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. [19][20][21] 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. [22] 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/Al 2 O 3 catalysts exhibiting different catalytic activities. The molar ratio Cu:Zn:Al = 60:30:10 is characteristic of commercial catalysts. [1] 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/Al 2 O 3 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 Figur...

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/Al 2 O 3 catalysts. [1] 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 1+ dispersed in ZnO, [2,3] metallic copper supported on ZnO, [4] dynamic surface and bulk alloy formation depending on the reduction potential of the synthesis gas, [5,6] Cu -at the so-called Schottky junction between metallic Cu and the semiconductor ZnO, [7] and ZnO segregated on Cu 1+ .[8] 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. [9] ZnO is regarded either as provider of atomic spillover hydrogen for further hydrogenation of adsorbed reaction intermediates on Cu sites, [10,11] or as a structure directing support controlling dispersion, morphology, and specific activity of the metal particles. [12][13][14][15][16][17][18] 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. [19][20][21] 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. [22] 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/Al 2 O 3 catalysts exhibiting different catalytic activities. The molar ratio Cu:Zn:Al = 60:30:10 is characteristic of commercial catalysts.[1] 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/Al 2 O 3 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 Figur...

Microstructural characteristics of various real Cu/ZnO/Al 2 O 3 catalysts for methanol steam reforming (MSR) were investigated by in situ X-ray diffraction (XRD), in situ X-ray absorption spectroscopy (XAS), temperature programmed reduction (TPR) and electron microscopy (TEM). Structure-activity correlations of binary Cu/ZnO model catalysts were compared to microstructural properties of the ternary catalysts obtained from in situ experiments under MSR conditions. Similar to the binary system, in addition to a high specific copper surface area the catalytic activity of Cu/ZnO/Al 2 O 3 catalysts is determined by defects in the bulk structure. The presence of lattice strain in the copper particles as the result of an advanced Cu-ZnO interface was detected only for the most active Cu/ZnO/Al 2 O 3 catalyst in this study. Complementarily, a highly defect rich nature of both Cu and ZnO has been found in the short-range order structure (XAS). Conventional TPR and TEM investigations confirm a homogeneous microstructure of Cu and ZnO particles with a narrow particle size distribution. Conversely, a heterogeneous microstructure with large copper particles and a pronounced bimodal particle size distribution was identified for the less active catalysts. Apparently, lattice strain in the copper nanoparticles is an indicator for a homogeneous microstructure of superior Cu/ZnO/Al 2 O 3 catalyst for methanol chemistry.