8170 Phys. Chem. Chem. Phys., 2012, 14, 8170–8178 This journal is c the Owner Societies 2012 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 8170–8178 Preparation, microstructure characterization and catalytic performance of Cu/ZnO and ZnO/Cu composite nanoparticles for liquid phase methanol synthesisw Mahmoud A. Sliem, a Stuart Turner, b Denise Heeskens, c Suresh Babu Kalidindi, a Gustaaf Van Tendeloo, b Martin Muhler c and Roland A. Fischer* a Received 16th February 2012, Accepted 16th April 2012 DOI: 10.1039/c2cp40482f Stearate@Cu/ZnO nanocomposite particles with molar ratios of ZnO : Cu = 2 and 5 are synthesized by reduction of the metal–organic Cu precursor [Cu{(OCH(CH 3 )CH 2 N(CH 3 ) 2 )} 2 ] in the presence of stearate@ZnO nanoparticles. In the case of ZnO : Cu = 5, high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) combined with electron- energy-loss-spectroscopy (EELS) as well as attenuated total reflection Fourier transform infrared (ATR-IR) spectroscopy are used to localize the small amount of Cu deposited on the surface of 3–5 nm sized stearate@ZnO particles. For ZnO : Cu = 2, the microstructure of the nanocomposites after catalytic activity testing is characterized by HAADF-STEM techniques. This reveals the construction of large Cu nanoparticles (20–50 nm) decorated by small ZnO nanoparticles (3–5 nm). The catalytic activity of both composites for the synthesis of methanol from syn gas is evaluated. 1. Introduction Binary Cu/ZnO and ternary Cu/ZnO/Al 2 O 3 solid state nano- composites, which are usually fabricated by co-precipitation/ calcination/reduction processes, are considered as typical examples of supported heterogeneous copper catalysts in two-phase gas/solid systems and have been utilized and optimized for industrial production of methanol from synthesis gas (CO, CO 2 and H 2 O) for a long time. These systems show the so-called strong metal support interaction (SMSI) that depends upon the interface between the Cu and the ZnO phase, which is also important for related catalytic reactions such as methanol oxidation and methanol steam reforming. 1–4 The same type of catalyst for methanol synthesis can also be used in a three-phase system. An example is the use of finely ground solid catalyst powder, dispersed in a liquid slurry phase, in a pressurized batch or continuously operated reactor. 5 Quasi homogeneous, colloidal catalysts in non-aqueous media featuring Cu and ZnO nano- particles in interfacial contact have also been developed by Fischer et al. 6 and Schu¨th et al. 7 Sophisticated organometallic precursor chemistry was employed in order to take advantage of both colloidal and nanoparticle chemistry in terms of control of composition, size and shape of the dispersed catalyst particles and modulation of the Cu/ZnO interface. Kimura et al. 9 and Muhler et al. 8 reported on related colloidal systems which were prepared by the reduction of low-cost and easily available air-stable metal stearates, with H 2 . In particular, highly active and selective colloidal Cu/ZnO catalysts for liquid phase methanol synthesis were obtained by this stearate precursor route and the mechanism and dynamic properties of Cu and ZnO particle formation and aggregation were studied. 8 Microstructure–activity relationships for ZnO supported Cu catalysts have been studied extensively and conclusions were drawn about the specific copper surface area 10 and the microstrain in the copper particles 11 as important parameters affecting the activity of this type of catalyst. Industrial Cu/ZnO/Al 2 O 3 catalysts are commonly prepared by precipitating copper–zinc hydroxycarbonates from metal nitrate solutions, and it is established that this precipitation process greatly influences the activity of the resulting Cu/ZnO catalysts for methanol synthesis through precipitate ageing and reduction, i.e. the kinetics of the nanocomposite formation. 2b,12 Schlo¨gl et al. showed that the activity of Cu/ZnO catalysts obtained from precipitates aged for more than 30 min correlates with an a Lehrstuhl fu ¨r Anorganische Chemie II, Ruhr Universita ¨t Bochum, Universita ¨tstrasse 150, 44780 Bochum, Germany. E-mail: roland.fischer@rub.de; Fax: +49 234 3214174; Tel: +49 234 3224174 b Electron Microscopy for Materials Science (EMAT), University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium. E-mail: staf.vantendeloo@ua.ac.be; Fax: +32 3 265 32 57; Tel: +32 3 265 32 62 c Lehrstuhl fu ¨r Technische Chemie, Ruhr-Universita ¨t Bochum, Universita ¨tstrasse 150, 44801 Bochum, Germany. E-mail: muhler@techem.rub.de; Fax: +49(0)234 32-14115; Tel: +49(0)234 32-28754 w Electronic supplementary information (ESI) available: Cu/ZnO and ZnO/Cu colloids were characterised by means of photoluminescence, FTIR, HRTEM, EDX and high resolution HAADF-STEM (Fig. S1–S10). See DOI: 10.1039/c2cp40482f PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by UNIVERSITEIT ANTWERPEN on 04 June 2012 Published on 16 April 2012 on http://pubs.rsc.org | doi:10.1039/C2CP40482F View Online / Journal Homepage / Table of Contents for this issue