Elsevier

Journal of Catalysis

Volume 383, March 2020, Pages 283-296
Journal of Catalysis

A combined experimental and DFT study of H2O effect on In2O3/ZrO2 catalyst for CO2 hydrogenation to methanol

https://doi.org/10.1016/j.jcat.2020.01.014Get rights and content

Highlights

  • Adding 0.1 mol% H2O in the feed increases CH3OH formation rate by 20% over In2O3/ZrO2.

  • H2O-induced oxygen vacancies improve CO2 adsorption capacity.

  • DFT reveals the correlation of InOOH and CH3OH formation with H2O addition.

  • Excess H2O leads to aggregation of catalyst and negatively affects H2 dissociation.

  • Excess H2O causes surface variations of InOOH species and oxygen vacancies.

Abstract

CO2 hydrogenation with renewable energy is one of the promising approaches to mitigate CO2 emissions and produce sustainable chemicals and fuels. The effect of adding H2O in the feed gas on the activity and selectivity of In2O3/ZrO2 catalysts for CO2 hydrogenation to methanol was studied using combined experimentatal and computational efforts. Notably, adding an appropriate amount of H2O (0.1 mol%) in the feed gas significantly enhanced the CH3OH formation (ca. 20%) with improved selectivity. Characterization with STEM/EDS and CO2-TPD confirmed the preservation of In-Zr strong interaction in the presence of additional H2O and H2O-induced oxygen vacancies, which significantly improved CO2 adsorption capacity. XPS analysis revealed the formation of InOOH species due to H2O addition, which appeared to correlate to H2O-dependant enhancement of CH3OH formation. Density functional theory calculations rationalized the effect of surface H2O on InOOH formation and its correlation to CH3OH synthesis activity. Adding H2O was found to facilitate surface InOOH formation, suppress CO formation through COOH* intermediate, and promote CH3OH formation via HCOO* intermediate. However, excess H2O addition resulted in aggregation of In species and reduction of surface In0 for H2 dissociation.

Introduction

CO2 capture, storage, and utilization are of great significance in terms of reducing the CO2 emission and mitigating the dependence on non-renewable fossil fuels [1]. Recently, the utilization of CO2 as carbon source for synthesizing chemical feedstocks and transportation fuels has attracted great attention [1], [2], [3], [4], [5], [6], [7], [8]. Such new carbon-based sustainable recycle will be more energy-efficient and environmentally friendly if in conjunction with renewable energy input [1], [8]. The CO2 conversion is energy demanding because CO2 is a highly stable molecule. By introducing another substance with higher Gibbs energy as co-reactant, such as hydrogen, the conversion will become thermodynamically more facile [1]. Hence, CO2 hydrogenation to clean fuels and chemicals is one of the promising approaches to recycle the carbon source in CO2 in a sustainable manner [9], [10], [11], [12], [13], [14], [15], [16], [17].

Methanol (CH3OH) is an important chemical feedstock, as well as an energy storage medium [18]. Three reactions are possibly involved in the CO2 hydrogenation to CH3OH, as listed below.Over the past two decades, significant efforts have been devoted to modifying Cu-Zn-based catalysts for CO2 hydrogenation to CH3OH [19], [20], [21], [22], [23]. Noble metal-based catalysts, such as Pd-based series, gained attention due to their activity in CO hydrogenation to methanol at relatively milder conditions, where Cu-ZnO based catalysts do not work well [24], [25], [26], [27]. The Pd-based catalysts were indeed active for CH3OH synthesis from CO2/H2 [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]. Most recently, Ye et al. applied density functional theory (DFT) calculations to investigate the adsorption and hydrogenation of CO2 for methanol synthesis on the surface of In2O3 [42]. Computational results demonstrated that the undesired CO production was energetically hindered, thereof making the selective production of CH3OH possible. Remarkably, their prediction was confirmed by Perez-Ramirez, et al. in 2016, wherein the In2O3 and ZrO2-supported In2O3 exhibited high selectivity of CH3OH and displayed prominent stability within 1000 h TOS in comparison to Cu/ZnO/Al2O3 catalyst [43]. Similar to other ZrO2-supported catalysts [44], there exists a synergy between In2O3 and ZrO2. The roles of ZrO2 include (i) determining catalytic properties by electronic interactions with In2O3 and (ii) preventing sintering [43]. A recent study reveals that a pronounced lattice mismatching occurs on monoclinic ZrO2-supported In2O3 catalysts, resulting in the formation of more oxygen vacancies that are beneficial to the activation of both reactants [45].

As reported in the literature, the surface oxygen sites of stoichiometric In2O3(1 1 0) surface were responsible for CO2 adsorption to form carbonate species; the H2 dissociation took place on both oxygen site and In site, forming hydroxyl and hydride, respectively [42]. The following surface reactions proceeded through protonation of CO2 with the hydrogen from hydroxyl to carboxyl species (COOH) and hydrogenation with the hydrogen from hydride to formate species (HCOO), respectively. Instead of forming CO and H2O, the reaction between COOH and surface hydroxyl groups to form CO2 and H2O was energetically more favorable, indicating that the COOH* pathway was not favored in the presence of surface In-OH species. Therefore, a selective production of CH3OH was expected through the formation of HCOO* intermediate. Their continued work on defective In2O3(1 1 0) surface with different oxygen vacancy sites demonstrated that the location and stability of vacancy sites played a role in impacting CO2 conversion chemistry and its hydrogenation to HCOO* on the less stable D4 surface (with an OV4 vacancy site) was energetically more favorable [46]. The rate-limiting step for methanol synthesis was the hydrogenation of CH2O* to CH3O* species over oxygen defective In2O3. The recent work by Perez-Ramirez, et al. confirmed the crucial role of oxygen vacancies as active sites in methanol synthesis from CO2 hydrogenation over In2O3-ZrO2 catalysts [43]. The (1 1 1) facet of In2O3 was considered as the active surface for the enhanced methanol formation in their studies [47], [48].

In our previous work for methanol synthesis from CO2 hydrogenation over Pd-Cu alloy catalysts, an increase of 52.4% of the methanol selectivity was achieved by adding a trace amount of H2O (0.03 mol%) [38]. This promotional effect was attributed to the participation of H2O in H-transfer steps involved in CO2 conversion which can significantly reduce the kinetic barriers and increase the TOF by approximately 3000 times, as evidenced from our DFT calculations [38]. Yang and co-workers also found a remarkable CH3OH promotion in the presence of in the presence of a small amount of water in low-temperature conversion of CO/H2 and CO2/H2 mixture to methanol over Cu catalysts [49]. As the previous DFT work on stoichiometric In2O3(1 1 0) surface showed that H2O molecules can be dissociatively adsorbed on the surface, and the produced InOOH species played a role in the transformation of COOH to form CO2 and H2O [42]. Thus, it would be of great interest to investigate the impact of InOOH and H2O on methanol synthesis over In2O3 catalyst. It was hypothesized that there would be potential relationships between the concentration of H2O and InOOH species as well as the two with CH3OH synthesis, because the produced InOOH species from H2O dissociation could alter CO2 hydrogenation pathways and impact the formation of CH3OH. Therefore, the objectives of the present work are (i) to clarify the effect of H2O concentration on the activity and selectivity of CH3OH formation from CO2 hydrogenation, (ii) to develop a fundamental understanding of the roles of H2O in the promoting and/or impeding effect. For these purposes, the ZrO2-supported In2O3 catalyst was prepared using wet impregnation method, and evaluated in CO2 hydrogenation at 523 K and 5 MPa. The roles of H2O were investigated and rationalized by the integration of activity test, characterization, and DFT calculations.

Section snippets

Catalyst preparation

The preparation method is similar to the method reported elsewhere [43]. The commercially available carrier, ZrO2 (monoclinic phase, extrudates, SZ 31164, NORPRO, radius = 3.15 mm, length = 5 ± 1 mm), was used as support material, and was crushed and treated at 573 K prior to use (size: 100–200 µm). The preparation of ZrO2-supported In2O3 catalysts was prepared by wet impregnation method. Briefly, a precursor solution, comprising of In(NO3)3·xH2O (1.52 g, Alfa Aesar, 99.99%) in a mixture of

Effect of H2O concentration

Fig. 1 illustrates the correlation of product STYs with the H2O concentration on In2O3/ZrO2 and In2O3 catalysts, as well as that of product selectivity and conversion. In the absence of H2O, the catalyst selectively yields CH3OH with STY and selectivity at 2.75 mol kg−1 h−1 and 66.5 C-mol%, respectively. With the addition of a little amount of H2O, namely 0.1 mol%, both STYs of CH3OH and CO are markedly improved to 3.42 and 1.54 mol kg−1 h−1, respectively, whereas the enhancement of CH3OH (ca.

Conclusions

In summary, the effect of adding H2O in the feed gas on the activity and selectivity of In2O3/ZrO2 catalysts for CO2 hydrogenation to methanol has been investigated. It is found that an appropriate (trace) amount of H2O (0.1 mol%) added to feed gas can significantly improve CH3OH formation rate by 20% with higher methanol selectivity. The In species distributes surrounding ZrO2 on the surface, and such unique distribution is well preserved at H2O conc. = 0.1 mol%. However, excess H2O in the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Work by XJ was supported by the Department of Energy’s Office of Energy Efficient and Renewable Energy’s Advanced Manufacturing Office under Award Number(s) DE-EE0007888. Work by KSW and CMM was supported by the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center funded by U.S. Department of Energy (US DoE), Office of Science, Basic Energy Sciences (BES) under Award Number DE-SC0012577. This work was also

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    X. Jiang and X. Nie are co-first authors who contributed equally to this work.

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