Elsevier

Catalysis Today

Volume 63, Issues 2–4, 25 December 2000, Pages 447-452
Catalysis Today

Active sites in Cu/ZnO/ZrO2 catalysts for methanol synthesis from CO/H2

https://doi.org/10.1016/S0920-5861(00)00490-9Get rights and content

Abstract

Among various Cu/ZnO/ZrO2 catalysts with the Cu/Zn ratio of 3/7, the one with 15 wt.% of ZrO2 obtains the best activity for methanol synthesis by hydrogenation of CO. The TPR, TPO and XPS analyses reveal that a new copper oxide phase is formed in the calcined Cu/ZnO/ZrO2 catalysts by the dissolution of zirconium ions in copper oxide. In addition, the Cu/ZnO/ZrO2 catalyst with 15 wt.% of ZrO2 turns out to contain the largest amount of the new copper oxide phase. When the Cu/ZnO/ZrO2 catalysts is reduced, the Cu2+ species present in the ZrO2 lattice is transformed to Cu+ species. This leads to the speculation that the addition of ZrO2 to Cu/ZnO catalysts gives rise to the formation of Cu+ species, which is related to the methanol synthesis activity of Cu/ZnO/ZrO2 catalyst in addition to Cu metal particles. Consequently, the ratio of Cu+/Cu0 is an important factor for the specific activity of Cu/ZnO/ZrO2 catalyst for methanol synthesis.

Introduction

Effective synthesis of methanol from H2 and CO has become all the more important in industry, since methanol is considered to be one of the materials that can overcome the forthcoming shortage of petroleum. Industrially, all the methanol is now produced catalytically from synthesis gas containing carbon monoxide, carbon dioxide and hydrogen which is usually produced by steam reforming of natural gas or other hydrocarbons. The copper-containing catalysts such as Cu/ZnO/Al2O3 and Cu/ZnO/Cr2O3 have been used at low pressure and temperature.

Also, efforts have been undertaken to enhance the catalytic activity of supported Cu catalysts. In most instances, however, the effect is different depending on the composition of the feed (CO/H2 vs. CO2/H2). Among various metal oxides, zirconia has attracted much attention because it shows a good activity for both CO/H2 and CO2/H2 reactions [1], [2], [3], [4], [5], [6], [7]. However, attempts to explain how zirconia may influence the methanol synthesis activity of Cu are further clouded by the differences in catalyst preparation and reaction conditions used by different authors. Thus, it remains unclear whether zirconia acts as a textural promoter, a chemical promoter, or a bifunctional catalyst [2].

Despite a number of mechanistic studies, there are still controversies concerning the active species of Cu. On the basis of their opinions on active species of Cu, one may divide the authors into two groups. One group has insisted that the methanol synthesis activity is related linearly to the copper surface area and metallic copper is the active species [8], [9], [10]. The other group has claimed that copper metal alone is inactive for carbon monoxide hydrogenation and the amount of Cu+ is related to the methanol synthesis activity over the surface of alkali-doped unsupported Cu catalyst [11], [12], [13]. Apart from these two opinions, it is reported that both Cu+ and Cu0 species are essential for methanol formation and the ratio of Cu+/Cu0 determines the specific activity [14], [15], [16], [17], [18], [19]. Okamoto et al. [20] have extensively studied the active species of CuO–ZnO catalysts by means of X-ray photoelectron spectroscopy (XPS) and suggested that the two-dimensional Cu0–Cu+ layer forms active sites for the methanol synthesis at low temperature and pressure.

In the present work, we have prepared a variety of Cu/ZnO/ZrO2 catalysts and examined the catalytic behavior in the hydrogenation of CO to methanol. The XPS characterization was carried out to study the changes of surface copper species induced by the addition of ZrO2. This work was undertaken to provide further insight into the nature of active sites of Cu/ZnO/ZrO2 catalysts.

Section snippets

Sample preparation

Catalyst precursors with various ZrO2 contents were prepared by the coprecipitation method [21]. A volume of 50 ml of 1 M mixed solution containing copper, zinc, and zirconium nitrates (Cu:Zn molar ratio=3:7) was added dropwise to 100 ml of 1.1 M solution of NaHCO3 at 343 K over a period of 20 min under vigorous stirring, followed by aging for 90 min at the same temperature. The precipitates were filtered and thoroughly washed 10 times with distilled water to remove the residual sodium [22]. The

Results and discussion

Methanol and carbon dioxide were the only carbon-containing products found in this study, except for trace amounts of methane and higher hydrocarbons. The preliminary experiments showed that the Cu/ZnO catalyst with a Cu/Zn ratio of 3/7 has better methanol synthesis activity for CO hydrogenation than the catalyst with any other ratio. This is in accordance with the previous result reported in the literature [21]. Therefore, all the Cu/ZnO/ZrO2 catalysts in this study were prepared with a Cu/Zn

Conclusions

Cu/ZnO/ZrO2 catalysts are found to show a good activity for the methanol synthesis at low temperature and pressure. Among Cu/ZnO/ZrO2 catalysts with a Cu/Zn molar ratio of 3/7, the highest activity was obtained with the one containing 15 wt.% of ZrO2. Bulk characterization of catalysts revealed the presence of a new copper oxide phase that was formed by the dissolution of zirconium ions in the copper oxide phase. The result of XPS analysis of the calcined catalysts confirms that the copper oxide

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