Short communicationEffect of Ag loading on CO2-to-methanol hydrogenation over Ag/CuO/ZrO2
Graphical abstract
Introduction
The direct hydrogenation of CO2 into methanol has become a very active field of research because CO2 may be actively recycled, allowing mitigation of the greenhouse effect of CO2 [1]. If methanol is synthesized from renewable hydrogen, this synthesis will contribute to the creation of a sustainable society. The problem of this methanol synthesis is the high cost of H2 production without using fossil fuels [2]. To address the matter, we have tried to develop new catalysts selective towards the CO2-to-methanol hydrogenation. During the exothermic CO2-to-methanol hydrogenation reaction (Eq. (1)), the endothermic reverse water gas shift reaction (RWGS reaction, Eq. (2)) will compete in particular at high temperatures [3]. Therefore, the catalysts are required to suppress the RWGS reaction.
The specific catalysts which have been mainly investigated are Cu catalysts supported and/or promoted by ZnO [[4], [5], [6], [7], [8], [9], [10], [11]] and ZrO2 [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]]. We have investigated ZrO2-supported Cu catalysts and considered that ZrO2 has been particularly promising as a support, because it leads to a highly active, selective and stable catalyst [[17], [18], [19]]. In particular, we reported that Ag addition to CuO-ZrO2 catalysts improved the selectivity towards methanol [18].We considered that the selectivity will be related to the formation of CuAg alloy nanoparticles [18]. This phenomenon was also reported by Baiker et al. [20]. However, little attention has been given to the point concerning the likely synergy of the CuAg catalytic system. In this study, we investigated the effect of Ag loading (wt%) on the catalytic performance and redox properties of Ag/CuO/ZrO2 catalysts in order to elucidate possible relationship between the catalytic performance and CuAg synergy.
Section snippets
Catalysts preparation
The Catalysis Society of Japan provided the amorphous ZrO2 (JRC-ZRO-5). First, a CuO/ZrO2 catalyst was prepared using the incipient wetness impregnation (IWI) method. Amorphous ZrO2 was impregnated with an aqueous solution of Cu(NO3)2・3H2O (Wako Pure Chemical Industries, Ltd.). Next, the obtained material was dried at 110 °C overnight and then calcined at 350 °C for 5 h. The material was named CZ350. After that, CZ350 was calcined again at 500 °C for 5 h. The thus obtained material was named
Results and discussion
We examined the effect of Ag addition to CuO/ZrO2 catalysts on their catalytic performance for CO2-to-methanol hydrogenation. Fig. 1a shows methanol and CO production rates for the as-prepared catalysts at 230 °C and 10 bars. The methanol and CO production rates over CZ were 1.2 and 1.9 mmol h−1 gcat−1, respectively. After adding Ag to CZ, both of the rates were decreased. In particular, CO production rates over xACZ were dramatically decreased with increasing Ag loading. 5AZ produced less
Conclusions
We examined the effect of Ag loading to CuO/ZrO2 catalysts (Ag loading: 0–5 wt%, Cu loading: 10 wt%) on the CO2-to-methanol hydrogenation. With increasing Ag loading from 0 to 1 wt%, the turnover frequency of methanol production per exposed Cu sites (TOFmethanol) remained unchanged and the selectivity towards methanol increased. In the case of further increasing Ag loading, TOFmethanol decreased. Although Ag acted as a sintering aid for Cu particles, the reduction of TOFmethanol and enhancement
Acknowledgements
This work was supported by the Japan Society of the Promotion of Science (JSPS, NO. 15 J10157). A part of this work was conducted at the Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by the “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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Present address: Department of Chemical System Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.