Role of Lattice Strain and Defects in Copper Particles on the Activity of Cu/ZnO/Al₂O₃ Catalysts for Methanol Synthesis

Abstract

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...

Meso- and nano-structuring of industrial Cu/ZnO/(Al2O3) catalysts

“…Both steps are prerequisite for the formation of porous aggregates with alternating arrangement of Cu and ZnO nanoparticles after activation, which can be seen as the target microstructure of commercial Cu/ZnO/Al 2 O 3 methanol synthesis catalysts [23] (Fig. 5d).…”

“…They observed reversible changes in Cu particle morphology and decoration with ZnO entities as an answer to the reductive potential of the gas atmosphere for Cu/ZnO model catalysts of low loading. For industrial samples -those prepared via the zincian malachite precursor -also the presence of defects and structural disorder is discussed to contribute to the specific activity of the Cu(0) surface [23].…”

The energies of formation and mobilities of Cu surface species on Cu and ZnO in methanol and water gas shift atmospheres studied by DFT

“…Particularly, the nature of the metalsupport synergy in the Cu/ZnO system has been much disputed [5][6][7][8][9][10][11][12] and the debate reflects several effects depending on the reaction conditions, preparation procedures, and complex catalyst nanostructures [13]. Another issue of major technical concern is the stability of the Cu/ZnO/Al 2 O 3 -based catalysts since approximately 1/3 of the initial activity is lost [14] during the first 1000 h of operation.…”

Knowledge-based development of a nitrate-free synthesis route for Cu/ZnO methanol synthesis catalysts via formate precursors

“…This microstructure is similar to that observed in industrial methanol synthesis catalysts. 17 In the tough comparison with such an optimized and promoted Cu/ZnO/Al 2 O 3 sample (provided by Su¨d-Chemie company), our novel catalyst showed a relative methanol synthesis activity of 66% (10 bar, 220 1C, for details see ESIw). The reason for the lower performance is most probably the lack of the Al 2 O 3 promoter, known to stabilize the Cu particles against sintering, 11 and the larger size of the particles due to the higher Cu : Zn ratio.…”