Copper–zinc oxide catalysts. Activity in relation to precursor structure and morphology

Abstract

Cu-Zn hydroxycarbonates have been studied as precursors of Cu-ZnO catalysts, with particular reference to the effect on catalyst activity of ageing the precursor prior to decomposition and reduction. The precursor obtained by precipitation from mixed nitrate solution ( C u / Z n molar ratio 2 : 1 ) at 333 K and pH 7.0 consisted of zincian malachite ( C u / Z n = 8.5: 1.5) and aurichalcite. The precursor was aged in the mother liquor at 333 K for various times. Characterisation by XRD, i.r., DTA, electron microscopy, EDAX and XPS showed that ageing led to loss of the aurichalcite and production of a more finely divided copper-enriched ( C u / Z n = 2 : 1) malachite phase. The unaged precursor yielded a catalyst of low activity for both methanol synthesis (studied at 50 bar and at 1 bar) and the reverse water-gas shift reaction. The aged precursor gave catalysts of much higher activity for both reactions. Increased ageing did not change the selectivity ratio for methanol synthesis cs. reverse shift in the CO,+ H, reaction at normal pressure.

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“…For reference, a coprecipitated CuẐ nO catalyst was prepared in the same atomic ratio as indicated above, according to ref. [4].…”

“…The consensus reached in the literature is that a good intimate mixture of Cu and ZnO (or Cu^Zn^Al/Cr) must be obtained during catalyst preparation in order to achieve high activity [1,4,5]. For the commercial type Cu^Zn catalyst this is achieved by coprecipitation of Cu(NO 3 ) 2 and Zn(NO 3 ) 2 solutions with subsequent ageing and calcination [4,5]. In this paper high intensity mechanical mixing of copper powder and zinc oxide without high temperature treatment is studied as a novel approach to obtain such well-mixed sytems.…”

“…From a scientific point of view, copper catalysis is actively studied because of the controversy that still exists about the active site for methanol synthesis [1^3]. The consensus reached in the literature is that a good intimate mixture of Cu and ZnO (or Cu^Zn^Al/Cr) must be obtained during catalyst preparation in order to achieve high activity [1,4,5]. For the commercial type Cu^Zn catalyst this is achieved by coprecipitation of Cu(NO 3 ) 2 and Zn(NO 3 ) 2 solutions with subsequent ageing and calcination [4,5].…”

Coprecipitation: An excellent tool for the synthesis of supported metal catalysts – From the understanding of the well known recipes to new materials

“…In case of the methanol synthesis catalyst, a multistep synthesis route towards Cu/ZnO catalysts introduced by ICI in the 1960s achieves this in a very efficient manner [4]. It is schematically shown in Figure 1 and it comprises co-precipitation [24] and ageing [25][26] of a mixed Cu,Zn,(Al) hydroxy-carbonate precursor material [27], thermal decomposition yielding an intimate mixture of the oxides and finally activation of the catalyst by reduction of the Cu component [28].…”

“…A prominent example is the co-precipitation of mixed Cu,Zn,Al hydroxycarbonates as precursor material for Cu/ZnO/Al 2 O 3 catalysts which are employed for the industrial synthesis of methanol [4][5][6]. Herein, the recent progress that has been made in analyzing and understanding the well-documented industrial synthesis of this important catalyst is briefly reviewed.…”