High-pressure in situ X-ray absorption fine structure measurements for hydrogenation of CO₂ to methanol over Zn-doped ZrO₂

Shohei Tada*^a, Kazumasa Oshima^b, Tastuya Joutsuka^c, Masahiko Nishijima^d, Ryuji Kikuchi^a and Tetsuo Honma*^e
aDivision of Applied Chemistry, Hokkaido University, N13 W8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan. E-mail: shohei.tada.st@eng.hokudai.ac.jp
^bDepartment of Chemical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
^cDepartment of Materials Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
^dFlexible 3D System Integration Laboratory, Osaka University, 8-1 Mihogaoka, Ibaraki-Shi, Osaka 567-0047, Japan
^eJapan Synchrotron Radiation Research Institute, 468-1, Aramaki Aza Aoba, Aobaku, Sendai, Miyagi 980-8572, Japan. E-mail: honma@jasri.jp

First published on 20th August 2024

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Introduction

The development of new solid catalysts specific to high-temperature (higher than 200 °C) and high-pressure (higher than 10 bars) reactions, including CO₂ hydrogenation reactions^1–3 and ammonia synthesis,⁴ is crucial. This study specifically focuses on the hydrogenation of CO₂ to methanol using Zn-doped ZrO₂, a type of metal oxide solid solution.⁵ Methanol synthesis via CO₂ hydrogenation is related to the following reactions:

CO₂-to-methanol hydrogenation

CO2(g) + 3H2(g) → CH3OH(g) + H2O(g)	(1)

CO-to-methanol hydrogenation

CO(g) + 2H2(g) → CH3OH(g)	(2)

Reverse water gas shift reaction

CO2(g) + H2(g) → CO(g) + H2O(g)	(3)

In general, Cu- and Pd-based catalysts have been used for methanol synthesis at mild temperatures (200–250 °C).^2,3 Owing to the well-established effectiveness of these catalysts, other potential catalysts have received little attention. Pérez-Ramírez et al. reported an In₂O₃/m-ZrO₂ (m-: monoclinic) catalyst for CO₂-to-methanol hydrogenation.⁶ Since then, research on metal oxide solid-solution catalysts for this reaction has become increasingly active.^7–11 For example, Li et al. reported Zn-doped ZrO₂ in 2017¹⁰ and Ga- and Cd-doped ZrO₂ in 2019.⁹ Compared to Cu- and Pd-based catalysts, these metal oxide solid solution catalysts operate at higher reaction temperatures of 300–350 °C. Consequently, they have attracted a significant attention for new and unprecedented applications. The research group of Fukuoka and Shrotri has successfully enhanced the H₂ dissociation ability by co-doping Co species into Zn-, In-, and Ga-doped ZrO₂, achieving a high methanol selectivity of 86%.⁸ Recently, the co-doping of Zn and Cu into ZrO₂ was reported to facilitate H₂ dissociation during CO₂-to-methanol hydrogenation.¹²

We have also reported the catalytic performances of Zn-^13–15 and In-doped ZrO₂¹⁵ in CO₂-to-methanol hydrogenation. Experimental and computational analyses in our previous studies revealed that Zn species in the as-prepared Zn-doped ZrO₂ are [ZnO_a] isolated clusters. During the CO₂ hydrogenation, these clusters are expected to transform into Zn–H species.¹³ Regardless of whether they form [ZnO_a] clusters or Zn–H species, the Zn species are considered divalent cations. Simultaneously, CO₂ adsorbs onto the Zr⁴⁺ sites in the proximity of Zn–H species, and the reaction between these sites produce of methanol. The formation of Zn–H species has also been reported by several studies.^9,10,12 Redekop et al. used the Auger parameter to show that subjecting Zn-doped ZrO₂ to H₂ flow partially reduced the Zn species in the catalyst.¹⁶ Li et al. showed that in Zn-doped ZrO₂, electron transfer occurs from Zr to Zn, leading to electronic interactions between Zr and Zn.⁷ This promotes the dissociation of H₂ to form more Zn–H species.

In high-pressure reactions, such as CO₂-to-methanol hydrogenation, the reactor is generally made of stainless steel; hence, the state of the catalyst during the reaction can only be speculated. For example, in high-temperature reactions at 300–400 °C, it is challenging to confirm whether H₂ molecules are adsorbed via heterolytic cleavage near the Zn species, resulting in the formation of Zn–H species.

In this study, we aimed to elucidate subtle variations in the Zn coordination structure during CO₂-to-methanol hydrogenation. This was achieved through high-pressure in situ X-ray absorption fine structure (XAFS) measurements and density functional theory (DFT) calculations for a metal oxide solid solution of Zn-doped ZrO₂. Although Cu-added ZnZrO_x catalysts have been evaluated using high-pressure in situ XAFS, little structural changes have been observed in the Zn species.¹² Hence, capturing structural changes in the stable divalent cation Zn²⁺ species is inherently challenging. Our current research demonstrates that the fine structure of Zn species undergoes modification upon CO₂ adsorption onto the Zn–O–Zr sites.

Experimental

Catalyst preparation

Zn-doped ZrO₂ was prepared using the incipient wetness impregnation method. Amorphous ZrO₂ (NND provided by Daiichi Kigenso Kagaku Kogyo) was impregnated with an aqueous solution of Zn nitrate hydrate (Fujifilm Wako). The obtained powder was dried at 110 °C for 12 h, followed by calcination at 500 °C for 3 h.

Characterisation

We conducted X-ray diffraction (XRD) measurements using a Rigaku Ultima IV diffractometer with Cu-Kα radiation, operating at 40 kV and 20 mA. The catalyst composition was measured using X-ray fluorescence (Malvern Panalytical, Epsilon 1, Ag radiation). Omnian software (Malvern Panalytical) was used to analyse the raw data.

Scanning transmission electron microscopy (STEM) was used to observe the catalyst powder directly (JEOL, JEM-ARM200F). The samples were dispersed in ethanol, dropped onto Cu microgrids (Ohkenshoji, NP-C15), and dried. Elemental maps of Zr, Zn and O were obtained using energy-dispersive X-ray spectroscopy.

We also performed high-pressure in situ XAFS measurements at the Zn K-edge (9.6 keV) on the Zn-doped ZrO₂ catalyst (Fig. 1). The resulting catalyst powder was packed into a stainless-steel tube, which was then placed in a high-pressure in situ cell. The cell window was composed of polyether ether ketone with a thickness of 0.5 mm. The temperature of the cell was increased to 400 °C (measured by an electric furnace) at a rate of 10 °C min⁻¹ under a N₂ flow (30 sccm). The sample temperature was measured to be approximately 50 °C lower than the temperature of the electric furnace. Subsequently, the gas was switched to H₂ (30 sccm), and the pressure was increased from 1 to 9 bar. This experiment is referred to as Test-1. Additionally, a similar test was conducted using a reaction gas mixture (CO₂/H₂/N₂ = 1/3/1, 30 sccm) instead of H₂ to observe the structural changes in the Zn species under different reaction conditions. This experiment is referred to as Test-2. Measurements were performed using a Si(111) double-crystal monochromator at BL14B2 with a quick scan measurement time of approximately 180 s per spectrum. XAFS data analysis was performed using the Athena and Artemis software.¹⁷ The composition of the outlet gas was analysed using a mass spectrometer (Pfeiffer Vacuum, OmniStar GSD301 O1).

Calculation

We conducted DFT calculations using the Perdew–Burke–Ernzerhof (PBE)¹⁸ exchange and correlation functional, along with a D3 dispersion correction.¹⁹ The cp2k program package was used for the calculations.²⁰ The Goedecker–Teter–Hutter pseudopotentials^21,22 and a double-ζ valence Gaussian basis set²³ for the orbitals were used. The cutoff associated with mapping Gaussians onto a multi-grid was set to 60 Ry, and the plane-wave cutoff for the finest level of the multi-grid was 400 Ry. A 3 × 3 × 1 Monkhorst–Pack²⁴ k-point mesh was used. The (101) surface of tetragonal Zn_xZr_1−xO_2−x, with x = 0.125 (ZnZr₁₉O₃₉), and the adsorption structure of H₂ were obtained from our previous work.¹³ Additionally, CO₂ + H₂, formate (HCOO) + 3H, and methoxy (CH₃O) + 3H, which are possible intermediates in the formate pathway for CO₂ hydrogenation to methanol,¹⁰ were also introduced to the Zn_xZr_1−xO_2−x surface and their geometry was optimised. On this surface, Zr and O atoms were replaced by Zn atoms and oxygen vacancies, respectively, to maintain the neutrality of the system. The surface consisted of five ZrO₂ layers, and the dimensions of the simulation cell were L_x × L_y × L_z = 6.361 × 7.227 × 35.000 ų. Hirshfeld analysis²⁵ was performed to examine the partial atomic charges.

Results and discussion

As-prepared catalyst

Fig. 2 shows the XRD patterns of as-prepared Zn-doped ZrO₂. No peaks corresponding to Zn species were observed, indicating that the Zn species were either amorphous, smaller than the XRD detection limit (<5 nm), or dissolved in the ZrO₂ lattice. The peaks indicated by the filled circles are assigned to tetragonal (t-) ZrO₂, whereas those indicated by the cross marks are assigned to m-ZrO₂. As described in the experimental part, the catalyst was pre-calcined at 500 °C. Considering the thermal stability of pure ZrO₂, m-ZrO₂ is expected to appear at this temperature.²⁶ However, the predominant presence of t-ZrO₂ suggests that guest cations (such as Zn²⁺) were doped into the ZrO₂ lattice, thereby stabilising the t-ZrO₂ phase. This stabilisation effect has been clearly demonstrated in our previous study.^13,27 On the other hand, the presence of m-ZrO₂ indicates that a small portion of the prepared catalyst was pure ZrO₂, which existed in the monoclinic phase. The Zn/(Zn + Zr) atomic ratio was 9%, as measured by X-ray fluorescence. In our previous studies, we showed that Zn-doped ZrO₂ with a ratio of less than 20% exhibited a high methanol production rate per unit Zn content.¹³

Fig. 3a shows the Zn K-edge XANES spectra of the Zn-doped ZrO₂, ZnO, and Zn foil. The shape of the spectrum of Zn-doped ZrO₂ was similar to that of ZnO, indicating that the Zn species in Zn-doped ZrO₂ were similar to the Zn²⁺ species in ZnO. Fig. 3b shows the Zn K-edge radial structure functions (RSFs) of Zn-doped ZrO₂ and ZnO. In ZnO, the RSF plot displays peaks corresponding to Zn–O bonds at 1.5 Å and Zn–Zn interactions at 3 Å. On the other hand, the RSF plot for Zn-doped ZrO₂ only shows a peak corresponding to Zn–O bonds. According to the EXAFS fitting (Table 1), the Zn species in Zn-doped ZrO₂ prepared in this study formed isolated [ZnO_a] clusters, as previously reported.¹³

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Fig. 4 shows a high-angle annular dark field STEM (HAADF-STEM) image of Zn-doped ZrO₂. Particles with sizes ranging from 10 to 20 nm were observed. The elemental maps for each element overlapped with each other. This image also supports that a portion of the Zn species was dissolved in the ZrO₂. A closer examination of the HAADF-STEM image reveals areas with varying contrast. Additionally, the Zn map shows that the locations of the Zn species do not completely coincide with the particles observed in the HAADF-STEM image. Therefore, it is concluded that the doped Zn species were unevenly distributed on the ZrO₂.¹³

High-temperature and high-pressure H₂ treatment

Fig. 5 shows the DFT-calculated structures of the Zn_xZr_1−xO_2−x surface (x = 0.125) and the adsorption structure of H₂. The Hirshfeld charges in Fig. 5 show that a H₂ molecule dissociates into a hydride (H⁻) on the Zn atom and a proton (H⁺) on the O atom when adsorbed on the Zn_xZr_1−xO_2−x surface. The partial charge of the Zn atom in the catalyst surface decreases from 0.66 to 0.43 after H₂ adsorption, indicating the reduction of Zn atom. In other words, by discussing the valence state of the Zn species during the high-temperature and high-pressure H₂ treatments, it is possible to consider the formation of Zn–H species. The DFT calculations reported by Wang et al.¹⁰ have suggested that CO₂ hydrogenation over Zn-doped ZrO₂ involves the dissociative adsorption of H₂; therefore, they revealed that Zn atoms are reduced during CO₂ hydrogenation. They were in line with the experimental findings that the Zn species are slightly reduced during heat treatment under H₂ flow. However, in their study, the experimental evaluation of H₂ adsorption was limited to ambient pressure.

We carried out in situ XAFS measurements to understand H₂ adsorption on Zn species under high-pressure conditions. The coordination structure change of the Zn species in Zn-doped ZrO₂ under a N₂ or H₂ flow was examined in Test-1. The experimental procedure is summarised in Fig. 6a. Fig. 6b shows the Zn K-edge XANES spectra of Zn-doped ZrO₂ during heat treatment from room temperature to 400 °C under a N₂ flow. With the increase in the time on the stream from 0 to 69 min, the peak intensity at 9667 eV gradually decreased, and the slope near 9662 eV shifted slightly to a lower energy region. This could be attributed to the Debye–Waller factor caused by the sample temperature change. The XANES spectra remained stable between 69 and 82 min, indicating that the sample temperature was stable and unaffected by the Debye–Waller factor. After 82 min, the gas stream was switched from N₂ to H₂. Fig. 6c shows the Zn K-edge XANES spectra during heat treatment at 400 °C under a H₂ flow. Following the gas exchange, no significant differences were observed in the XANES spectra between 82 and 142 min. After 150 min, the pressure of the H₂ gas increased from 1 bar to 9 bar, but little change was observed in the XANES spectrum. Therefore, even after the high-temperature and high-pressure H₂ treatment, most of the Zn species remained as [ZnO_a] isolated clusters, and the formation of Zn–H, if present at all, was very limited.

As shown in Fig. 6a, at 195 min, both the cell temperature and H₂ gas pressure started to decrease. After 270 min, the temperature was returned to room temperature, and the pressure was decreased to 1 bar. Fig. 6d compares the XANES spectra at the start (0 min) and end (270 min) of Test-1. After 270 min, the slope at 9662 eV shifted to lower energies, indicating that the Zn species in Zn-doped ZrO₂ were slightly reduced by the high-pressure H₂ treatment. Additionally, Zn–H species were formed. Considering that there was almost no change in the XANES spectrum after the high-temperature and high-pressure H₂ treatment, the formation of Zn–H species likely resulted from H₂ chemisorption at the Zn–O binding sites near room temperature. This phenomenon has been well-documented.²⁹

Fig. 6e shows the results of Q-mass analysis of the outlet gas during this experiment. At 82 min, the gas was switched from N₂ to H₂, slightly increasing the mass intensity to 31. This increase was not due to the production of methanol (m/z = 31), but was likely caused by a baseline shift due to the change in the gas type. This result is important for the subsequent Test-2 experiment.

High-temperature and high-pressure CO₂ hydrogenation condition

Before presenting the XAFS measurement results under reaction conditions, we summarize our results on the reaction performance of ZrO₂, ZnO, and Zn-doped ZrO₂ at 1.0 MPa using a CO₂ hydrogenation model gas (CO₂/H₂/N₂ = 1/3/1, 1.0 MPa). All experiments were carried out using a stainless-steel fixed-bed tubular reactor. When using commercial ZrO₂ (NND) in the temperature range between 190 and 250 °C, no changes were observed (not shown, W/F = 17 g min L⁻¹). Additionally, when using commercial ZnO (Fujifilm Wako), CO₂ was converted, but most of it became CO (325 °C, W/F = 7 g min L⁻¹).¹⁵ In other words, methanol production was very low. We have evaluated the performance of Zn-doped ZrO₂ by varying the amount of Zn doping (300 °C, W/F = 17 g min L⁻¹).¹³ As the Zn/(Zn + Zr) atomic ratio was increased from 7% to 25%, the space–time yield of methanol slightly increased from 1.5 to 1.9 mmol g_cat⁻¹. Conversely, within this ratio range, it was found that the space–time yield remained stable between 1.5 and 1.9 mmol g_cat⁻¹. Even from the reaction tests alone, it is evident that the Zn species in Zn-doped ZrO₂ play a crucial role in the methanol synthesis from CO₂ hydrogenation.

In addition, to understand CO₂ adsorption property, we introduce our previously reported CO₂-TPD (temperature programmed desorption of CO₂) data for ZrO₂ (NND) and Zn-doped ZrO₂ (Fig. 7).²⁷ To briefly explain the procedure, 200 mg of the sample was preheated at 300 °C for 3 h in a N₂ flow. After that, CO₂ was adsorbed at 50 °C, followed by a switch to N₂ gas and heating up to 800 °C. Comparing the CO₂ desorption profiles per unit surface area, both catalysts exhibited similar shapes. There was a peak top at 130 °C, which gradually weakened up to about 400 °C. From these results, it can be conclusively stated that CO₂ adsorbs near Zr⁴⁺ sites.

Under the CO₂ hydrogenation reaction conditions, the Zn species in the Zn-doped ZrO₂ catalyst were observed using in situ XAFS measurements (Test-2). A simplified procedure for this experiment is shown in Fig. 8a. Fig. 8b shows the changes in the XANES spectra when heated under a N₂ flow. The results are similar to those of Test-1 (Fig. 6b), with the spectra at 55 and 72 min being almost identical. This was likely due to the stabilisation of cell heating after 55 min, resulting in no further temperature changes in the cell. Fig. 8c shows the XANES spectra obtained when a CO₂/H₂/N₂ mixed gas was introduced. For CO₂/H₂/N₂ supplied at 1 bar, the peak at 9667 eV increased. To examine this change in detail, the region of peak rise between 9660 and 9663 eV was enlarged, as shown in Fig. 8d. In the 9661–9663 eV range, the spectrum shifted to the lower energy side, whereas it shifted to higher energy in the 9660–9661 eV range. Next, the CO₂/H₂/N₂ gas was pressurised from 1 bar to 9 bar. The intensity of the peak at 9667 eV further increased (Fig. 8c). In Fig. 8d, the peak increased slightly and its position shifted to a lower energy region. As shown in Fig. 8e, in the 9661–9663 eV range, the spectrum shifted further to the lower energy side, whereas in the 9660–9661 eV range, it was shifted further to the higher energy side. Hence, the spectral changes observed after the introduction of CO₂/H₂/N₂ gas suggest a change in the coordination structure of Zn. Furthermore, upon increasing the pressure, the structural changes became even more pronounced.

Fig. 9 shows the results of the Q-mass analysis of the outlet gas composition during the in situ XAFS measurements. At atmospheric pressure, when the gas was switched from N₂ to CO₂/H₂/N₂, the signal for mass intensity 28 decreased to one-third of its original value. This was consistent with the expected decrease in the concentration of N₂ (mass 28) in the outlet gas from 100% to 20%. Although the concentration of CO₂ (the mass 28 fragments) increased from 0% to 20%, its contribution to the mass signal at 28 was considered negligible. This is because the fragment signal of mass 28 from CO₂ is approximately 10% of the signal for mass 44 (around 3 × 10⁻⁹), as explained in the ref. 30. When examining the signal for m/z = 31, a slight increase upon introduction of CO₂/H₂/N₂ was observed. As shown in Fig. 6e, this was likely due to the baseline shift that occurred when the gas type was changed. Notably, when pressurisation began (150 min), the signal for mass 31 increased significantly, and the signal for mass 18 became stronger. The former is attributed to the generation of methanol, whereas the latter is attributed to the generation of water. These results indicate that supplying CO₂/H₂/N₂ at 9 bar successfully promotes CO₂-to-methanol hydrogenation. Integrating the results from the high-pressure in situ XAFS study, the [ZnO_a] isolated clusters in the as-prepared Zn-doped ZrO₂ underwent a change in the coordination structure under the reaction conditions. The altered Zn species are important active sites for methanol synthesis.

At this stage, the nature and coordination structure of the distorted Zn species remain unclear. However, it is evident that the introduction of CO₂ induces this distortion, as demonstrated by a comparison with the results of the high-pressure in situ XAFS study. CO₂ selectively adsorbs onto the Zr sites of catalysts, as confirmed by adsorption experiments and DFT calculations.¹³ To investigate this, DFT simulations were conducted to model the structure of Zn-doped ZrO₂ with reaction intermediates such as CO₂, formate (HCOO), and methoxy (CH₃O) on the Zr sites (Fig. 10). The introduction of CO₂ transformed the tricoordinated Zn species (Fig. 10a) into tetracoordinated clusters (Fig. 10b). Additionally, when the adsorbed species were converted to formate (Fig. 10c), the coordination structures of the Zn species differed from those of the Zn species in Fig. 10a. For methoxy adsorption (Fig. 10d), the coordination environment of the Zn species did not seem to differ significantly from that of the initial H₂ adsorption state. The Zn–H distances in Fig. 10a–d are 1.58, 1.91, 1.56, and 1.61 Å, respectively, indicating CO₂ species can strongly distort the [ZnO_a] cluster. According to high-pressure in situ XAFS measurements and DFT calculations, we can conclude that the presence of CO₂ and CO₂-derived intermediate species on the Zr sites induces distortions in the coordination structure of the Zn species. Because other possible structures beyond the calculated ones may exist, further investigation of this phenomenon will be pursued.

The results of the high-pressure in situ XAFS measurements indicate that the amount of Zn–H formed during the reaction is very limited. Considering this, it seems quite natural that H atoms do not remain on Zn²⁺ at temperatures above 300 °C. We believe that this weak Zn–H bond enables mild reduction reactions such as CO₂-to-methanol hydrogenation. However, the scarcity of Zn–H bonds likely makes it challenging to improve CO₂ conversion rates. As a design principle for metal oxide catalysts like Zn-doped ZrO₂, the key will likely be how to increase the adsorbed H species on the catalyst surface. Indeed, the development of catalysts that utilize spillover H species is crucial and cannot be overlooked.³¹

Conclusion

The coordination structure changes of the Zn species in Zn-doped ZrO₂, exhibiting remarkable performance in CO₂-to-methanol hydrogenation, were investigated using high-pressure in situ XAFS measurements. During hydrogen reduction under pressurised conditions, structural changes in the Zn species were minimal. Even after the high-temperature and high-pressure H₂ treatment, the majority of the Zn species remained as isolated [ZnO_a] clusters, and the formation of Zn–H bonds, if present at all, was very limited. In contrast, during the CO₂ hydrogenation reaction under pressurised conditions, slight distortions were observed in the structure of the Zn species. Our DFT calculations revealed that the adsorption of CO₂ at Zr⁴⁺ sites near Zn species can induce these structural distortions.

Data availability

The data that support the findings of this study are available from the corresponding authors, ST and TH, upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was financially supported by the Feasibility Study Program on Energy and New Environmental Technology (Grant Number 22100386-0) of the New Energy and Industrial Technology Development Organization of Japan, JSPS KAKENHI (Grant Number 24K01240) and the Environment Research and Technology Development Fund (Grant Number JPMEERF20243RA3) of the Environmental Restoration and Conservation Agency provided by the Ministry of the Environment of Japan. The synchrotron radiation experiments were performed at BL14B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, Proposal Number 2023B1576 and 2024A1517). Calculations were performed using supercomputers at ACCMS, Kyoto University. We thank the Advanced Research Infrastructure for Materials and Nanotechnology of Japan (ARIM) program (Proposal Number JPMXP1222OS0026) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan, at the Research Center for Ultra-High Voltage Electron Microscopy (Nanotechnology Open Facilities), Osaka University.

Notes and references

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