Bing Hu,Yazhi Yin,Zixin Zhong,Dengdeng Wu,Guoliang Liu,Xinlin Hong
Maximizing the Cu–ZnO interface is regarded as the best strategy to promote methanol synthesis activity over CuZn catalysts. Reducing the ZnO particle size can enhance the Cu–ZnO interaction, but its synthesis with a sub-5 nm size still remains a challenge. Moreover, the fabrication of an active powder catalyst with an inverse ZnO/Cu configuration is still very rare. In this work, we report a facile synthesis of inverse ZnO/Cu catalysts by direct calcination of Cu@ZIF-8 (zeolitic imidazolate framework-8). The particle size of ZnO and Cu can be tuned by changing the Cu loading as well as the calcination temperature. Importantly, inverse ZnO/Cu catalysts with sub-5 nm ZnO were obtained under gentle calcination at 623 K. In the CO2 hydrogenation reaction, the highest methanol synthesis activity reached 933 g kgcat−1 h−1 at 533 K and 4.5 MPa, with a high turnover frequency (TOF) of 0.013 s−1, over an optimized C2Z-623 catalyst, which exceeds the commercial CuZnAl catalyst (HiFuel R120, Johnson Matthey) and has ranked the top reported value over advanced CuZn catalysts under comparable conditions. Such excellent activity is attributed to the inverse structure of Cu particles covered by sub-5 nm ZnO, which promotes the intimate Cu–ZnO interface with enhanced SMSI. Moreover, we find that the methanol turnover frequency (TOF) monotonically increases with decreasing the particle size of ZnO as well as Cu. It is proposed that reducing the ZnO size favors the spillover of reducible ZnOx species onto the Cu surface while lowering the Cu size is responsible for the enhanced amount of exposed Cu steps for accommodating Zn species, both of which could facilitate the formation of Zn covered Cu steps as active sites for methanol synthesis.
Regulation of abundant Cu-ZnO interfaces in Cu/ZnO catalysts is critical to the thermal hydrogenation of CO 2 to methanol. Herein, Cu-ZnO interfaces were precisely regulated by depositing thin ZnO films on copper phyllosilicates (CuSiO 3 ) through atomic layer deposition (ALD) to prepare Cu/ZnO/SiO 2 composite catalysts (denoted as x ZnO/CuSiO 3 , x is the number of ALD cycles) for CO 2 hydrogenation to methanol. Interestingly, 5ZnO/CuSiO 3 exhibited much higher methanol selectivity of 68.1 % and methanol space-time yield (STY MeOH ) of 0.39 g MeOH h −1 g cat −1 toward CO 2 hydrogenation at 275 ℃ and 3 MPa, as compared to the pristine CuSiO 3 (38.1 % and 0.24 g MeOH h −1 g cat −1 ), suggesting the significant role of Cu-ZnO interfaces. Compared to ZnO/CuSiO 3 (IM) prepared by the conventional impregnation method, 5ZnO/CuSiO 3 possessed much higher CO 2 conversion (13.8 % vs. 10.1 %) and methanol selectivity (68.1 % vs. 56.2 %). Furthermore, the catalytic activities of the prepared Cu/ZnO catalysts can be effectively regulated by changing the thickness of ZnO-ALD films, in which 5ZnO/CuSiO 3 showed the best catalytic activity. Various characterization methods were applied to demonstrate that ALD-ZnO was evenly laid flat on the surface of CuSiO 3 to construct abundant Cu-ZnO interfaces during the H 2 reduction treatment, which facilitated the dissociated activation of CO 2 and formation of CH 3 O* reaction intermediate and simultaneously inhibited the generation of CO byproduct. In addition, the density functional theory (DFT) calculations revealed the stronger interaction between Cu and ZnO over 5ZnO/CuSiO 3 , which implied more Cu-ZnO interfaces were generated for methanol production. Accordingly, this study verifies the significance of Cu-ZnO interfaces for CO 2 hydrogenation to methanol and provides a facile method to regulate Cu-ZnO interfaces by depositing ALD-ZnO films on the copper phyllosilicate precursor.
CO2hydrogenation to methanol is one of the main and valuable catalytic reactions applied on Cu/ZnO-based catalysts; the interface formed through Zn migration from ZnO support to the surface of Cu nanoparticle (ZnOx-Cu NP-ZnO) has been reported to account for methanol synthesis from CO2hydrogenation. However, the accompanied reverse water gas shift (RWGS) reaction significantly decreases methanol selectivity and deactivates catalysts soon. Inhibition of RWGS is thus of great importance to afford high yield of methanol. The clear understanding of the reactivity of RWGS reaction on both the direct contact Cu-ZnO interface and ZnOx-Cu NP-ZnO interface is essential to reveal the low methanol selectivity in CO2hydrogenation to methanol and look for efficient catalysts for RWGS reaction. Cu doped plate ZnO (ZnO:XCu) model catalysts were prepared through a hydrothermal method to simulate direct contact Cu-ZnO interface and plate ZnO supported 1 wt % Cu (1Cu/ZnO) catalyst was prepared by wet impregnation for comparison in RWGS reaction. Electron paramagnetic resonance (EPR), XRD, SEM, Raman, hydrogen temperature-programmed reduction (H2-TPR) and CO2temperature-programmed desorption (CO2-TPD) were employed to characterize these catalysts. The characterization results confirmed that Cu incorporated into ZnO lattice and finally formed direct contact Cu-ZnO interface after H2reduction. The catalytic performance revealed that direct contact Cu-ZnO interface displays inferior RWGS reaction reactivity at reaction temperature lower than 500 °C, compared with the ZnOx-Cu NP-ZnO interface; however, it is more stable at reaction temperature higher than 500 °C, enables ZnO:XCu model catalysts superior catalytic activity to that of 1Cu/ZnO. This finding will facilitate the designing of robust and efficient catalysts for both CO2hydrogenation to methanol and RWGS reactions.Keywords:reverse water–gas shift reaction;CO2hydrogenation to methanol;Cu doping;direct contact Cu-ZnO interface;ZnOx-Cu NP-ZnO interface
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