Comparative study of Cu/ZnO catalysts derived from different precursors as a function of aging E.N. Muhamad a , R. Irmawati a, * , Y.H. Taufiq-Yap a , A.H. Abdullah a , B.L. Kniep b , F. Girgsdies b , T. Ressler b a Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia b Department of Inorganic Chemistry, Fritz-Haber-Institute der MPG, Faradayweg 4-6, 14915 Berlin, Germany Available online 26 November 2007 Abstract Structural modifications of Cu/ZnO catalysts for methanol steam reforming (MSR) as a function of precipitate aging in catalysts preparation process has been investigated comparatively. Freshly precipitated Cu,Zn-hydroxycarbonate (HC) and Cu,Zn-hydroxynitrate (HN) were aged in their mother liquor for a period of 120 min followed by washing, drying, calcination and reduction. Pronounced effect of aging was found for aged HC precipitates while no significant effect of aging was observed for aged HN solids. The bulk structure of the Cu/ZnO catalysts was investigated by means of TG/MS, in situ XRD and 63 Cu NMR. The increase in the activity of the catalysts prepared by HC aging did not correlate linearly with the specific Cu surface area but coincides with an increase in the microstrain in the copper clusters presumably because of the improved interface between Cu and ZnO. Meanwhile, aging of HN precipitates results in large, separated and less strained Cu and ZnO particles with an inferior catalytic activity. Finally, both aged Cu/ZnO catalysts revealed smaller copper crystallite size compared to unaged samples. # 2007 Published by Elsevier B.V. Keywords: Precipitating agent; Aging; Copper; Zinc oxide; Microstrain 1. Introduction Cu/ZnO-based catalysts are well known industrial catalysts for the low temperature-pressure methanol synthesis [1,2], water-gas shift reaction [3] and hydrogenation of CO x [4–7]. Recently, a lot of researches discussed its application for the production of hydrogen from methanol steam reforming and/or partial oxidation reaction especially for fuel cell application [5,6]. In spite of that many works have been carried out on in the applied and fundamental studies on Cu/ZnO catalysts, the focus has been mainly on improving the catalytic activity of Cu/ZnO- based catalysts by various modification techniques, i.e. addition of suitable promoter/support [8–10], combination with effec- tive component [8,11,12] and implementation of new preparative method [13–15]. Nowadays however, many researchers have put their enthusiasm in tailoring the microstructural properties of Cu/ZnO catalysts without modifying the simple copper/zinc binary system. One of the attempts is to modify the bulk structure and induce structural disorder by aging process. From literature survey, it is reported that in the production of commercial Cu/ZnO catalysts, a period of aging is essential [16]. During the aging process, the initial precipitate remains in contact with the precipitating agents, and the particles can partially redissolve or increase in size due to Ostwald ripening [17]. This particle growing phenomenon concerns only crystal of the same composition and structure (i.e. crystals of the same phase) which is in contrast with phase transition. In addition, phase composition is often reportedly changed during aging time [6,17]. The process of aging is found very often promoted by maintaining the precipitate and precipitation medium together at elevated temperatures for a period of time. Study by Li and Inui [18] on the effect of temperature and pH during precipitation of Cu/Zn/Al with sodium carbonate concluded that the optimum condition for the initial co-precipitation is at 343 K and at constant pH of 7. In their work they observed that despite the composition of the precursors obtained at pH 7 and temperature lower than 323 K was the same as those catalysts, which was obtained at pH 7 and temperature of 343 K, the catalytic activity of the former catalyst in methanol synthesis is www.elsevier.com/locate/cattod Available online at www.sciencedirect.com Catalysis Today 131 (2008) 118–124 * Corresponding author. Tel.: +603 8946 6786; fax: +603 8946 6758. E-mail address: irmawati@science.upm.edu.my (R. Irmawati). 0920-5861/$ – see front matter # 2007 Published by Elsevier B.V. doi:10.1016/j.cattod.2007.10.010