The Potential of Microstructural Optimization in Metal/Oxide Catalysts: Higher Intrinsic Activity of Copper by Partial Embedding of Copper Nanoparticles

Optimal dispersion of the active metal usually is the main goal in preparation of supported metal catalysts. Whereas the oxide support often can be considered as inert, in many systems it is known to chemically affect the catalytic material under reaction conditions exceeding the simple role as a physical carrier of metal particles. Strong metal support interaction (SMSI) is an example, which was first detected for Pt/TiO2 [1] and was also found in Cu/ZnO. Cu/ZnO/(Al2O3) catalysts are active in industrial methanol synthesis and a Cu/ZnO synergy is discussed in literature, the nature of which is still under debate. However, it seems clear that synergetic effects should have their origin in the Cu–ZnO interface region. The catalytic properties are, thus, not only determined by the amount of Cu surface area (SCu) accessible to gas but also by the nature of the Cu–ZnO interface contacts. As a result of the latter effect, deviations from linearity of SCu–activity correlations can be observed. Usually, the value of SCu is regarded as the dominating effect and a catalyst of considerably lower SCu would be estimated to exhibit inferior activity compared to a high SCu sample of the same Cu loading. The potential of intrinsic or synergistic effects, which are responsible for different specific activities of Cu, is not easily determined because surface and interface area are interrelated by morphology and microstructure, which are difficult to control during preparation of applied Cu/ZnO/(Al2O3) systems. Today, the Cu/ZnO/Al2O3 system has been industrially employed for more than 40 years. During this period of time the preparation of successful catalysts has been extensively studied in academic and industrial laboratories and a considerable optimization regarding activity and stability could be achieved. The resulting technically relevant synthesis route is based on preparation of Cu, Zn basic carbonate precursors by coprecipitation, for example, from metal nitrate solutions and sodium carbonate as precipitating agent. The precipitate is aged, recovered, and calcined, which results in an intimate mixture of the oxides. Finally, the CuO component is reduced to obtain active catalysts with the typical atomic Cu/Zn ratio near 70:30 and a molar Al content of 10–20 %. Catalysts prepared according to this procedure usually possess high values of SCu and are highly active due to their efficient mesoand nanostructuring during synthesis. Whereas the optimization of Cu dispersion can be regarded as widely advanced in these systems, optimization of the Cu–oxide contact interface, that is, the microstructural arrangement of the components, presents further unexplored room for improvement. Herein, we present a novel type of Cu/Zn/Al2O3 catalyst that was prepared according to the technically applied synthesis route described above with two important modifications: 1) Instead of pure sodium carbonate solution and Cu, Zn, Al–nitrate solution, as used in conventional catalyst preparation, a combination of sodium aluminate and carbonate solution was used as precipitating agent for a Cu, Zn–nitrate solution during coprecipitation; 2) the ageing period was suppressed by realizing a continuous process with direct spray-drying of the fresh precipitate suspension. Structural and catalytic characteristics of this sample, denoted catalyst A, are compared to a conventionally prepared catalyst, referred to as catalyst B. The latter sample showed an activity similar to a commercial methanol synthesis catalyst, provided by S d-Chemie AG. As revealed by X-ray diffraction (XRD), the washed precipitate consisted of crystalline hydroxy carbonate phases for catalyst B, whereas the precursor of catalyst A was completely X-ray amorphous. The precursors were subjected to the same treatments during washing, calcination, and reduction. It is important to note that both samples exhibit exactly the same industrially relevant metal composition of Cu/Zn/Al =60:25:15 and, thus, experimental differences can be related to a different microstructure. Investigation of the microstructure of the reduced catalysts was carried out by means of high resolution transmission electron microscopy (HRTEM). Recently, a comprehensive characterization of the microstructure of a series of industrially relevant Cu/ZnO/Al2O3 samples was presented by some of the authors. These bulk catalysts were quite different from classical supported systems and Cu particles of an average diameter(<10 nm) were detected to be separated by small ZnO particles, which prevented them from sintering, and forming a porous framework of individual particles. This type of morphology was also found in catalyst B and is represented in Figure 1, right. Unlike catalyst B, individual separated oxide particles are hardly detected and, consequently, the porous Cu/ZnO particle arrangement is absent in the novel material. The Cu particles are partly embedded in the oxide matrix re[a] Dr. M. Behrens, Dr. A. Furche, Dr. I. Kasatkin, Dr. A. Trunschke, Prof. Dr. R. Schlçgl Fritz Haber Institute of the Max Planck Society Department of Inorganic Chemistry, Faradayweg 4–6, 14195 Berlin (Germany) Fax: (+ 49) 30-8413-4405 E-mail : behrens@fhi-berlin.mpg. de [b] Dr. W. Busser, Prof. Dr. M. Muhler Ruhr University Bochum, Industrial Chemistry Universit tsstraße 150, 44801 Bochum (Germany) [c] Dr. B. Kniep, Dr. R. Fischer S d-Chemie AG, Research & Development Catalysts Waldheimer Straße 13, 83052 Bruckm hl (Germany)