High-performance ZnZrO_x-supported CuNi catalysts for CO₂ hydrogenation to methanol

Abstract

ZnZrO_x solid solutions have been extensively reported for methanol production from the hydrogenation of carbon dioxide (CO₂) on account of their high selectivity, prominent stability and sulfur tolerance. Herein, a series of ZnZrO_x supported-CuNi catalysts were fabricated using a liquid-phase reduction-deposition method and then utilized for CO₂ hydrogenation. The impact of the Cu : Ni molar ratio on the physicochemical properties of the catalysts and their CO₂ hydrogenation performance was systemically studied and discussed. The synchronous introduction of Cu and Ni into ZnZrO_x not only improved the BET-specific surface areas and the reducibility of the metallic species but also markedly increased the concentration of surface oxygen vacancies and the amount of CO₂ desorbed, thereby leading to excellent reactivity. Moreover, the CH₃OH space-time yield (STY) was positively correlated to the concentration of surface oxygen vacancies and the amount of desorbed CO₂. Because of the outstanding reducibility, high metal dispersion, superior CO₂ adsorption ability, sufficient surface oxygen vacancies, and the proper interaction between the metals and support, Cu₂Ni₁/ZnZrO_x achieved a CH₃OH selectivity close to 82% with a CO₂ conversion greater than 10% at 3.0 MPa, 15 000 mL g_cat⁻¹ h⁻¹ and 280 °C.

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1. Introduction

The unrestrained use of fossil energy releases excessive amounts of greenhouse gases, especially CO₂, into the environment, causing global warming and climate problems.¹ Therefore, CO₂ capture, utilization and storage (CCUS) has become a promising approach for realizing net zero CO₂ emissions. Especially, the highly efficient utilization of CO₂ originating from waste gas emissions is of great significance and value.^2–5 In recent years, converting CO₂ into liquid fuels or valuable chemicals has garnered widespread interest. Among the different strategies for CO₂ conversion and utilization, the hydrogenation of CO₂ to methanol (CH₃OH) using hydrogen from renewable energy sources has been regarded as an optimal method because methanol can be broadly applied as a fuel or as a key intermediate for producing many other high-value chemicals.^6–8 Nevertheless, it is still a huge challenge to achieve the efficient activation and conversion of CO₂ due to its high thermal stability and chemical inertness. Therefore, the keys to solving this problem are the exploration and preparation of high-performance catalysts that enhance CO₂ conversion and CH₃OH selectivity.^9–14

Currently, monometallic and bimetallic catalysts have been broadly reported for CH₃OH formation from CO₂ hydrogenation. Monometallic catalysts, mainly Cu-, Pd-, Au-, Ag- and Pt-based catalysts (especially Cu- and Pd-based catalysts), have been widely investigated and utilized due to their excellent physiochemical properties and reactivity. The results of these investigations have shown that the ZrO₂-loaded Cu sample exhibits greater advantages than the Cu/ZnO sample for CO₂ hydrogenation to CH₃OH due to its superior thermal stability and the high surface basicity imparted by ZrO₂.^15–25 Witoon et al.²⁵ compared the influence of the ZrO₂ polymorphs possessing monoclinic (m), tetragonal (t) and amorphous (a) phases of the crystal structure, morphology, adsorbability and interaction between metallic Cu and ZrO₂ in ZrO₂-supported Cu catalysts. Cu/a-ZrO₂ and Cu/t-ZrO₂ display smaller particle sizes and stronger metal–supports interaction than Cu/m-ZrO₂, suggesting that the Cu species in these two catalysts possess a greater interfacial area with ZrO₂, thus promoting the spillover of H atoms from the Cu surface to the ZrO₂ surface. Furthermore, the noble metal Pd can selectively convert CO₂ to CH₃OH.^26,27 Wang et al.²⁷ adopted an incipient wetness impregnation method for the precise incorporation of metallic Pd NPs inside and outside of carbon nanotubes (CNTs). The channel structure and electronic density difference between the inside and outside of the CNTs led to a relatively smaller size and a greater concentration of Pd^δ+ species on the surface of the palladium nanoparticles located inside of CNTs compared to those on the outside. Therefore, a relatively high conversion of CO₂ and selectivity of CH₃OH were obtained for the Pd/CNTs-in catalyst. Cai et al.²⁸ utilized MIL-68(In) nanorods as a morphological template for the preparation of hollow In₂O₃ nanotubes (h-In₂O₃) and in the in situ synthesis of palladium nanoparticles on both the inner and outer surfaces of h-In₂O₃ to form In₂O₃/Pd. Interestingly, h-In₂O₃/Pd possessed a suitable interaction caused by a fairly high percentage of Pd²⁺ situated on the In₂O₃ surface, which conferred both high catalytic performance and stability.

Although monometallic catalysts have proven effective for converting CO₂ to methanol, there are still a number of issues, including poor reaction performance and unsatisfactory stability. Bimetallic catalysts, intermetallic compounds (IMCs) and alloys, have been broadly reported for CH₃OH synthesis from CO₂ hydrogenation on account of their adjustable geometric, electronic, and chemical properties induced by a powerful synergistic effect. Jiang et al.²⁹ found that the combination of metallic copper and palladium resulted in a strong synergistic effect that promoted methanol synthesis, mainly because of the formation of two highly dispersed Pd–Cu alloy particles (PdCu and PdCu₃). In addition, detailed catalyst characterization and density functional theory (DFT) simulation have been utilized to further elucidate the correlation between the Pd–Cu alloy content and CO₂ hydrogenation performance.³⁰ The total quantity of the alloy phase increased with an increasing addition of Pd and reached a peak value at a Pd/(Pd + Cu) atomic ratio of 0.34, which was closely related to the coexistence of PdCu₃ and PdCu. Correspondingly, methanol production showed a volcano-shaped trend across the entire investigated range, characterized by a maximal CH₃OH yield at a Pd/(Pd + Cu) atomic ratio of 0.34. Studt et al.³¹ fabricated a sequence of Ni–Ga intermetallic compounds by tuning the molar ratio of Ni : Ga and discovered that various Ni–Ga intermetallic catalysts showed significantly different textural features, physicochemical properties and reaction activities. Cu–Ni bimetallic catalysts composed of two non-noble metals possess great research potential for the hydrogenation of CO₂ to methanol due to their intimate interaction and excellent synergistic effect.^32–35 Tan et al.³² compared catalytic performance and physiochemical properties over various shaped CeO₂-loaded, Cu–Ni bimetallic catalysts. The CeO₂ nanorod (CeO₂-NR)-loaded sample comprised mainly exposed low-energy (100) and (110) facets, but the Cu–Ni loaded onto the CeO₂ nanospheres (CeO₂-NS) and onto anomalous CeO₂ nanoparticles (CeO₂-NP) respectively were surrounded by a (111) facet possessing high energy. Furthermore, more oxygen vacancies were generated in CuNi/CeO₂-NR than in CuNi/CeO₂-NS and CuNi/CeO₂-NP, making CuNi/CeO₂-NR more conducive for CO₂ adsorption, activation and catalytic hydrogenation. Zhang et al.³⁵ used a chemical reduction method for preparing Cu–Ni/In₂O₃, which is a highly active catalyst for the hydrogenation of CO₂ to methanol. The strong metal-support interaction between the highly dispersed Cu–Ni and the In₂O₃ support effectively stabilized the metallic Cu species during CO₂ hydrogenation, suppressing the agglomeration of the Cu species and the formation of the Cu–In alloy.

According to previous studies, the structure, shape, adsorption properties, metal–support interactions and reactivity of supported catalysts are significantly influenced by the type of support. A metal-oxide ZnZrO_x solid-solution for methanol formation, which displayed high selectivity, excellent stability and sulfur tolerance, was first investigated by Wang et al.³⁶ In addition, Temvuttirojn et al.³⁷ explored the role of the calcination temperature of the ZrO₂ support on the crystalline phase and adsorption capacity of ZnO/ZrO₂ catalysts. The Zn/Zr catalyst with the ZrO₂ calcined at 600 °C achieved the largest selectivity for CH₃OH production (75.1%) at a reaction temperature of 300 °C, corresponding to the maximum amount of weak basic sites. Zhang et al.³⁸ used a metal–organic-framework (MOF) as a precursor for achieving Zn²⁺–O–Zr⁴⁺ sites anchored on the surface of ZrZnO_x. Mechanistic investigations revealed that Zn²⁺ was responsible for H₂ activation and the Zn²⁺–O–Zr⁴⁺ sites played a crucial role in adsorbing and converting CO₂. Schiff base-modified bimetallic UiO-66 frameworks were utilized for synthesizing Zn–Zr solid solutions. Notably, the Schiff base can bridge different metal ions into robust UiO-66 frameworks and the doping amount is adjustable. The resultant ZnO–ZrO₂ displayed the optimal CO₂ conversion (5.7%) and CH₃OH selectivity (70%) among the three different Zr-based solid solutions.³⁹ Tada et al.⁴⁰ examined the influence of the Zn component on the active-site structure and the catalytic performance of CO₂ hydrogenation over Zn_xZr_1−xO_2−x catalysts. When the Zn content increased from 0.19 to 0.53, the three categories of active sites, the Zn–O–Zr sites, ZnO surface sites, and interfacial sites between ZnO and Zn–O–Zr, could have all been generated by the incorporation of Zn-containing clusters into the tetragonal ZrO₂. Furthermore, the experiments and calculations suggested that the Zn species possessed H₂ splitting ability, and the Zr species were responsible for CO₂ activation. Above all, these metal–oxide solid-solutions catalysts exhibited superior CH₃OH selectivity and stability; thus, they possess high application potential.

Based on our summary and discussion of the relevant literature, we demonstrated that Cu–Ni and ZnZr solid-solutions are both effective candidate materials for the catalytic hydrogenation of CO₂. As far as we know, ZnZr solid-solution supported CuNi bimetallic catalysts have not yet been reported for CO₂ hydrogenation to methanol. Herein, the ZnZrO_x supported CuNi catalysts were fabricated utilizing a liquid-phase reduction-deposition method and then they were applied to CO₂ hydrogenation. The crystal structure, chemical states, Cu–Ni synergistic effect and metal–support interaction of the CuNi/ZnZrO_x samples were systematically investigated through various characterization technologies.

2. Experimental

2.1. Catalyst synthesis

The ZnZr solid-solution was synthesized through a co-precipitation method. Typically, 0.008 mol Zn(NO₃)₂·6H₂O and 0.054 mol Zr(NO₃)₄·5H₂O were dispersed into 400 mL deionized water. To this mixed solution, an aqueous solution of (NH₄)₂CO₃ (0.32 M) was slowly added at a drop rate of 3 mL min⁻¹ at 70 °C under vigorous stirring until the pH of the mixture reached 9.0. After precipitation, the resultant suspension was continuously stirred at the same temperature for 2 h. The precipitate was collected by centrifugation and thoroughly washed several times with deionized water, then dried at 110 °C for 10 h and calcined at 500 °C for 3 h. The resultant powder was designated as ZnZrO_x.

A series of ZnZrO_x supported CuNi catalysts were synthesized through a liquid- phase reduction-deposition method. Using the ZnZrO_x-supported CuNi catalyst with a Cu : Ni molar of 1 : 1 as an example, 2.0 g of ZnZrO_x were added into a 50 mL solution containing 0.33 g of Cu(CH₃COO)·H₂O and 0.417 g of Ni(CH₃COO)·4H₂O, resulting in a blue suspension. After the resultant slurry was stirred at an ambient temperature for 0.5 h, 80 mL of an aqueous solution containing 2.34 g of L-ascorbic acid was slowly injected into the mixed solution, and the resulting solution was heated at 80 °C for 3 h. Subsequently, the as-prepared products were collected by centrifugation and washed three times with acetone and then vacuum dried at 60 °C overnight; the resultant product was named Cu₁Ni₁/ZnZrO_x. The other synthesized catalysts were designated as ZnZrO_x, Cu/ZnZrO_x, Cu₂Ni₁/ZnZrO_x, Cu₁Ni₂/ZnZrO_x and Ni/ZnZrO_x, respectively.

2.2. Catalyst characterization

Elemental analysis of the fresh catalysts was carried out with a Varian 720-ES inductively coupled plasma-atomic emission spectrometer (ICP-AES). Powder X-ray diffraction (XRD) analysis was used for investigating the crystalline characteristics of the ZnZrO_x-based catalysts; analysis was achieved using a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation (λ = 1.542 Å) in the 2θ range of 10–80°. The crystal phases were determined using the diffraction database of the Joint Committee on Powder Diffraction Standards (JCPDS). The size of the crystals was calculated by the Scherrer equation, D_hkl = kλ/(βcos θ), where D_hkl is the size of crystal in a perpendicular direction to the plane [hkl], k is the Scherrer constant, which is usually 0.89, and λ is the incident X-ray wavelength, θ is the Bragg diffraction angle, and β is the half-height peak width of the diffraction peak. Nitrogen physisorption was performed in a Micromeritics 3 Flex apparatus at −196 °C. All samples of the catalyst were pre-degassed at 200 °C for 12 h under a high vacuum before the experiment. The shape and structure of the catalysts were obtained utilizing a FEI Nova Nano SEM 450 microscope with an accelerating voltage of 10.0 kV. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDX) were performed respectively using a JEM-2100F microscope at 200 kV. The catalysts were prepared by placing a drop of the nanoparticle ethanol slurry onto a copper grid and allowing the ethanol to vaporize. X-ray photoelectron spectroscopy (XPS) analyses were performed using a Thermo Fisher Scientific ESCALAB 250Xi spectrometer with an Al Kα irradiation (hν = 1486.6 eV). All the binding energies (BE) were calibrated against the peak of C 1s (284.6 eV) as a reference. The reducibility and adsorbability of the samples were measured using an AMI 300 chemisorption instrument. Under a flow of pure Ar gas at 200 °C, 50 mg of each powdered catalyst was purged for 30 min before the H₂ temperature programmed reduction (H₂-TPR) experiment. Shortly thereafter, the powder was cooled to 60 °C, and then, 10 vol% H₂/Ar was injected. The system was then heated to 800 °C at a heating rate of 10 °C min⁻¹. For the CO₂ temperature-programmed desorption (CO₂-TPD) experiment, 50 mg of each powdered catalyst was activated in a N₂ stream (50 mL min⁻¹) at 400 °C for 1 h and subsequently cooled to 50 °C; further purification of the catalyst's surface was achieved by purging with a helium stream (50 mL min⁻¹) for 0.5 h. At this temperature (50 °C), each sample was placed under a flow of CO₂ for 1 h and purged again by helium at a flowrate of 50 mL min⁻¹ for another 1 h. A spectrum was acquired from 50 °C to 700 °C at a heating rate of 5 °C min⁻¹.

2.3. Catalytic activity evaluation

Catalytic activity evaluation of the ZnZrO_x-based catalysts was performed in a fixed-bed tubular reactor outfitted with a gas chromatograph (GC). The powder samples (0.2 g) were diluted by quartz sand (0.5 g) with the same particle size as the powdered samples (20–40 mesh). Before hydrogenation, each sample was activated under N₂ gas at a flowrate of 50 mL min⁻¹ and 400 °C for 1 h. The catalyst bed was then cooled to 250 °C, and the mixed reactant gases (mass flow ratio of CO₂ : H₂ : N₂ = 1 : 3 : 1) were injected with a gas hourly space velocity (GHSV) of 15 000 mL g_cat⁻¹ h⁻¹. The whole reaction system was maintained at 3.0 MPa and the reaction temperature was controlled sequentially at 260 °C, 280 °C, 300 °C and 320 °C, respectively. The pressure of the exit gas was promptly decreased to ambient pressure, while the temperature was maintained at 150 °C, and the gas was transported to the GC-9790 II sampling valve, which was outfitted with dual detectors, namely, a thermal conductivity detector (TCD) and a flame ionization detector (FID). Porapak Q, 5A and GDX-502 molecular sieve packed columns were connected to TCD, whereas a PLOT Q capillary column was connected to FID. The CO₂ conversion (X_CO2), product selectivity (S_CH3OH, S_CH4 and S_CO) and CH₃OH space-time yield (STY_CH3OH) were calculated using the data obtained in 5 h after the reaction started and determined by the following equations:where n denoted the number of moles, V_CO2% was 1/5, and W_cat represented the sample weight. The catalytic performance of each catalyst was tested for at least 12 h under the same conditions, and the corresponding results were obtained by averaging the results from multiple runs to eliminate experimental errors. Hence, the error range of our experimental data was within the allowable range.

3. Results and discussion

3.1. Textural and crystalline properties of the catalysts

The metal composition of the ZnZrO_x-based catalysts was quantitatively tested through ICP-AES, and the corresponding results are summarized in Table 1. The measured metal composition in all six samples matched well with the theoretical data, verifying that the liquid-phase reduction-deposition method was a suitable approach for fabricating the supported catalysts.

Table 1 Composition and crystal size of the catalysts

Sample	Metal composition (wt%)				Crystal size (nm)
	Cu	Ni	Zn	Zr
ZnZrOx	—	—	9.81	90.19	10.7
Cu/ZnZrOx	12.72	—	8.43	78.85	11.2
Cu2Ni1/ZnZrOx	8.37	4.06	8.56	79.01	10.9
Cu1Ni1/ZnZrOx	6.21	5.99	8.47	79.23	11.5
Cu1Ni2/ZnZrOx	4.38	7.76	8.59	79.27	11.1
Ni/ZnZrOx	—	11.75	8.42	79.41	11.7

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The crystalline structures and phase compositions of the fresh and used ZnZrO_x-based catalysts are shown in Fig. 1. From Fig. 1a, five typical characteristic peaks can be clearly observed at 2θ = 30.2°, 35.2°, 50.2°, 60.1° and 62.8° corresponding to (101), (110), (112), (211) and (202) crystal planes of tetragonal zirconia (t-ZrO₂, JCPDS 50-1089), respectively. No diffraction peaks of the metals and metal oxides were found in all the samples, suggesting that the metal species were highly dispersed on the ZnZrO_x support. Additionally, the peaks related to ZnO were not observed mainly because Zn²⁺ was incorporated into the ZrO₂ lattice, forming the ZnZrO_x solid solution.^37,38,41 The crystal sizes of the fresh ZnZrO_x-based catalysts based on ZrO₂ species were calculated and listed in Table 1. The crystal sizes of ZrO₂ in all the catalysts exhibited no obvious change even after introduction of additional metal species. For the used catalysts, the main characteristic peaks ascribed to t-ZrO₂ remained unchanged compared with the fresh catalysts. Notably, after the Ni content increased, a small peak at 44.4° attributed to Ni (111) emerged in the used Cu₁Ni₂/ZnZrO_x and Ni/ZnZrO_x. This could be ascribed to the reduction of the Ni species to metallic Ni during CO₂ hydrogenation. Contrastingly, the characteristic peak of Ni was not observed in the other used CuNi-supported ZnZrO_x catalysts due to their lower nickel content and better metal dispersion.

Fig. 1 XRD patterns of the fresh (a) and used (b) ZnZrO_x-based catalysts.

The surface area and porous properties of all the ZnZrO_x-based catalysts were examined by N₂ adsorption/desorption. The N₂ adsorption–desorption isotherms and pore diameter distribution profiles are illustrated in Fig. 2, and the related data are listed in Table 2. As shown in Fig. 2a, all the isotherms of the ZnZrO_x-supported catalysts were categorized as type IV isotherms, possessing a remarkable H3-type hysteresis loop in the relative pressure range of 0.5 to 1.0. This result suggests that the these ZnZrO_x-supported catalysts were mesoporous. Upon closer examination of the isotherm of ZnZrO_x, the adsorption and desorption curves almost coincide with each other in most parts of the isotherm. Therefore, combined with the specific surface area, pore volume and pore size distribution, these results imply that ZnZrO_x had only a few pores or accumulated holes. The BET specific surface areas were 78.16 and 78.01 m² g⁻¹ for Cu/ZnZrO_x and Ni/ZnZrO_x, respectively, and the pore volumes were 0.161 and 0.139 cm³ g⁻¹ for Cu/ZnZrO_x and Ni/ZnZrO_x, respectively. After the second introduction of metals (Cu or Ni) into the ZnZrO_x-supported, monometallic catalysts, these two parameters improved, indicating that the second addition of metals was beneficial for enhancing the specific surface area and pore volume. Furthermore, as the Cu : Ni molar ratio increased, the specific surface area and pore volume increased monotonously. In the case of Cu₂Ni₁/ZnZrO_x, it possessed a maximum specific surface area and pore volume of 94.15 m² g⁻¹ and 0.184 cm³ g⁻¹, respectively.

Fig. 2 N₂ adsorption–desorption isotherms (a) and pore size distribution curves (b), of the ZnZrO_x-based catalysts. The isotherms of Cu/ZnZrO_x, Cu₂Ni₁/ZnZrO_x, Cu₁Ni₁/ZnZrO_x, Cu₁Ni₂/ZnZrO_x and Ni/ZnZrO_x, were upshifted by 10, 50, 90, 130, and 170 cm³ g⁻¹, respectively; their pore size distribution curves were upshifted by 0.1, 0.25, 0.4, 0.55, and 0.7 cm³ g⁻¹, respectively, for the sake of clarity.

Table 2 Structural properties, concentration of surface oxygen vacancies, and the total amount of desorbed CO₂ for the ZnZrO_x-supported CuNi catalysts

Sample	BET surface area (m^2 g^−1)	Pore volume (m^3 g^−1)	Average pore size (nm)	C Odefect (%)	Total amount of desorbed CO2 (μmol g^−1)
ZnZrOx	5.91	0.021	13.95	28.74	—
Cu/ZnZrOx	78.16	0.16	8.20	29.63	2185.7
Cu2Ni1/ZnZrOx	94.16	0.18	7.80	32.47	2438.5
Cu1Ni1/ZnZrOx	87.43	0.17	8.20	31.46	2020.6
Cu1Ni2/ZnZrOx	85.05	0.17	7.80	30.68	1874.8
Ni/ZnZrOx	78.01	0.14	7.12	29.24	1647.5

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The shape, particle size distribution and microstructure of ZnZrO_x and Cu₂Ni₁/ZnZrO_x were determined through TEM, and the resultant images are depicted in Fig. 3. The TEM photographs in Fig. 3a reveal the shape of ZnZrO_x, which is primarily characterized by block-like crystals with smooth surfaces and particle sizes of about 80–120 nm, although the support particles stacked and aggregated to form nanoclusters. From Fig. 3b, the fine powdered particles are uniformly and intimately distributed on the ZnZrO_x carrier surface. Furthermore, the morphology and size of the support remained unchanged after the incorporation of CuNi into ZnZrO_x, indicating that the ZnZrO_x-supported CuNi catalysts prepared using the liquid-phase reduction-deposition method possessed excellent structural stability and high dispersion. From the HRTEM images illustrated in Fig. 3a′ and b′, the two catalysts both show lattice fringes with a width of 0.181 and 0.291 nm, corresponding to the (112) and (101) planes of t-ZrO₂, respectively.³⁸ In Fig. 3b′, aside from the t-ZrO₂ species, the metallic Cu species with a lattice spacing of 0.209 nm appears in Cu₂Ni₁/ZnZrO_x, which was ascribed to the Cu (111) plane. In addition, the lattice fringes of metallic CuNi or Ni were not observed in Cu₂Ni₁/ZnZrO_x. The EDX elemental maps of the representative Cu₂Ni₁/ZnZrO_x catalyst (as displayed in Fig. 4) show that the metallic Cu and Ni nanoparticles are predominantly dispersed on the carrier surface of ZnZrO, reconfirming that CuNi interacted closely with the ZrO₂ support. This finding is in accordance with the results of XRD and the H₂-TPR characterization.

Fig. 3 TEM and corresponding HRTEM images of ZnZrO_x (a and a′) and Cu₂Ni₁/ZnZrO_x (b and b′).

Fig. 4 High-angle annular dark-field image and EDX elemental mappings of Cu₂Ni₁/ZnZrO_x.

3.2. Surface chemical properties of the catalysts

The chemical valence states, composition and electronic properties of the ZnZrO_x-based catalysts were studied and characterized using XPS analysis. As illustrated in Fig. 5a, double-binding energy peaks situated at around 932.5 and 952.5 eV are observed in Cu/ZnZrO_x; these peaks correspond to the Cu 2p_3/2 and Cu 2p_1/2 orbitals, respectively, both belonging to metallic Cu⁰. After the introduction of metallic Ni into Cu, the characteristic peaks of the copper species shifted toward lower binding energies, mainly due to the generation of CuNi compounds after the liquid-phase reduction.^42,43 Moreover, no satellite peaks appeared in the ranges of 940–945 eV and 960–965 eV, suggesting that Cu²⁺ was completely reduced to generate Cu⁰ on the catalyst's surface. From Fig. 5b, the two peaks at 852.5 and 869.8 eV were attributed to metallic Ni⁰. Furthermore, the two additional peaks at around 856.1 and 873.2 eV verified the presence of the Ni²⁺ species. This observation is probably linked to the following two reasons: the first reason is that the Ni²⁺ species in the catalytic system could not be entirely reduced to metallic Ni⁰ during the liquid-phase reduction, and the other reason is the reoxidation of a part of the metallic Ni⁰. In addition, a broad and satellite peak emerged at 860–865 eV, observable in all the Ni-containing samples, was attributed to charge transfer from oxygen to the metal.⁴⁴Fig. 5c presents the Zn 2p core level spectra of the ZnZrO_x-based catalysts; the characteristic peaks of the Zn species in all the ZnZrO_x supported-catalysts were shifted to a lower binding energy compared with that of ZnZrO_x, indicating the electron transfer from CuNi to the Zn species. In the Zr 3d XPS spectra (Fig. 5d), two peaks at 182.2 and 184.6 eV, which are separated by 2.4 eV, are observed for all the catalysts, associated with Zr 3d_5/2 and Zr 3d_3/2, illustrating that the valence state of Zr was +4. Fig. 5e shows that there are two peaks of O 1s: one peak at 529.8 eV, corresponding to lattice oxygen (O_lattice), and the other peak centered at 531.3 eV, ascribable to surface oxygen defects (O_defect). To ensure accuracy during deconvolution, the full width at half maximum of the O defect and O lattice peaks was kept consistent.^43,45,46 Furthermore, the surface oxygen defect content (C_Odefect) was computed from their peak areas (A) using the following formula: C_Odefect = A_Odefect/(A_Odefect + A_Olattice); the calculated data are summarized in Table 1 and show that ZnZrO_x had the lowest C_Odefect of 28.74%. After the introduction of metallic Cu or Ni into ZnZrO_x, C_Odefect increased with an increasing Cu : Ni molar ratio, attaining a peak value at the Cu : Ni molar ratio of 2 : 1 (32.47%). It is a widely held belief that oxygen vacancies are extremely important for boosting catalytic activity and a relatively higher oxygen vacancy content contributes to improved catalytic performance (this will be discussed and analyzed further herein).

Fig. 5 XPS spectra of Cu 2p (a), Ni 2p (b), Zn 2p (c), Zr 3d (d) and O 1s (e) in the ZnZrO_x-based catalysts.

3.3. Reduction behavior of the catalysts

H₂-TPR was employed for determining the reduction capability of the ZnZrO_x-based catalysts, and the results are presented in Fig. 6. For ZnZrO_x, two peaks in the temperature range of 350–550 °C and at a temperature higher than 600 °C, were observed, which were attributed to the reduction of ZnO and ZrO₂, respectively. A strong reduction peak in Cu/ZnZrO_x located between130–250 °C corresponded to the reduction of the surface-dispersed and bulk CuO species. The single broad peak that emerged at 250–400 °C in Ni/ZrO₂ was associated with the reduction of the NiO species. After the introduction of metallic Ni into Cu, forming the ZnZrO_x-supported CuNi bimetallic catalysts, the reduction temperatures of CuO and NiO simultaneously decreased compared to those of Cu/ZnZrO_x and Ni/ZnZrO_x. Notably, with a decrease in the Cu : Ni molar ratio, the reduction temperature of CuO initially decreased, attaining its lowest value at a Cu : Ni molar ratio of 1 : 1, and then slightly increased. This observation was also the same for NiO reduction in the ZnZrO_x-supported CuNi bimetallic catalysts. These results indicate that the Cu and Ni species in the CuNi/ZnZrO_x were easier to reduce than those in the ZnZrO_x-supported monometallic catalysts because the synergistic effect between Cu and Ni promoted metal species reduction.^47–50 The reduction temperatures of ZnO and ZrO₂ in the ZnZrO_x-supported catalysts all shifted to higher temperatures than in the unsupported sample, demonstrating that the strong interaction between the metals and the support made them harder to reduce.

Fig. 6 H₂-TPR plots of the ZnZrO_x-based catalysts.

3.4. Adsorption property of catalysts

The CO₂ adsorption capability and desorption temperatures of all the ZnZrO_x-supported catalysts were examined using CO₂-TPD measurements. Fig. 7 shows that all the catalysts exhibited three typical peaks caused by the desorption of CO₂ from weak, medium and strong basic sites, respectively. The prominent peak at a low temperature of 50–300 °C is assignable to the physically adsorbed CO₂ on the sample's surface and probably chemisorbed CO₂ on the support surface of ZnZrO_x. The second peak at a medium temperature range of 400–600 °C and the third peak at a high temperature above 600 °C were related to the desorption of moderately and strongly chemisorbed CO₂, respectively.^46,51,52 All three desorption peaks in Ni/ZnZrO_x possess the lowest temperature value among all the ZnZrO_x-supported catalysts. As the addition of Cu increased, the low-temperature desorption peak shifted towards a higher temperature relative to Ni/ZnZrO_x, but the medium- and high-temperature desorption peaks initially increased, attaining their peak value at a Cu : Ni molar of 1 : 1, after which the intensity of their peaks subsequently declined. This result indicates that the CO₂ adsorption strength exhibited a volcanic-shaped trend, associated with the incorporation of Cu into Ni. In addition, the total amount of CO₂ desorbed from the five ZnZrO_x-supported catalyst samples was calculated, and the resultant data are listed in Table 1. Ni/ZnZrO_x presented the lowest amount of CO₂ desorbed. Furthermore, the amount of CO₂ desorbed increased gradually following the introduction of Ni; hence, Cu₂Ni₁/ZnZrO_x possessed a maximum value of 2438.5 μmol g⁻¹. The sequence of the total amount of CO₂ desorbed is as follows: Cu₂Ni₁/ZnZrO_x > Cu/ZnZrO_x > Cu₁Ni₁/ZnZrO_x > Cu₁Ni₂/ZnZrO_x > Ni/ZnZrO_x.

Fig. 7 CO₂-TPD profiles of the ZnZrO_x-supported catalysts.

3.5. Catalytic performance of the catalysts

The effects of the Cu : Ni molar ratio and catalytic conditions on the reactivity of the fabricated catalysts are displayed in Fig. 8. CH₃OH and CO are the only two carbon-containing products tested with the catalysts under the operating conditions utilized in this study. Although these catalysts exhibited different catalytic performances, they all yielded a near 100% carbon balance during CO₂ hydrogenation. The six catalysts showed an increasing trend of CO₂ conversion as the temperature increased from 260 °C to 320 °C, illustrating that CO₂ activation and conversion were kinetically accelerated with an increasing reaction temperature. ZnZrO_x showed a minimum CO₂ conversion over the temperature range of 260–320 °C. A dramatic increase in CO₂ conversion accompanied the introduction of metallic Cu or Ni, especially for Cu/ZnZrO_x, which possessed an optimal CO₂ conversion among all the catalysts. It is clearly seen from Fig. 8a that the variation of the Cu : Ni molar ratio had an obvious impact on CO₂ conversion as evidenced by its monotonic decrease as the Cu : Ni molar ratio decreased. In Fig. 8b, all the ZnZrO_x-supported CuNi catalysts exhibited a high CH₃OH selectivity (>68%) over a wide temperature range (260–300 °C). Cu₂Ni₁/ZnZrO_x exhibited the highest CH₃OH selectivity, while Ni/ZnZrO_x exhibited the lowest CH₃OH selectivity. Notably, the CH₃OH selectivity in Ni/ZnZrO_x markedly decreased from 59.4% to 20.6% over the measured temperature range of 260–320 °C. Comparably, the selectivity of CH₃OH for Cu₂Ni₁/ZnZrO_x only declined slightly as the reaction temperature increased. CH₃OH STY, which is calculated based on the conversion of CO₂ and selectivity of CH₃OH, is an important parameter for assessing the catalytic efficiency. As shown in Fig. 8c, the CH₃OH STY of ZnZrO_x and Ni/ZnZrO_x was markedly inferior to that of the other ZnZrO_x-supported catalysts because of their relatively low CO₂ conversion and CH₃OH selectivity. The CH₃OH STY of Cu/ZnZrO_x, Cu₁Ni₁/ZnZrO_x and Cu₁Ni₂/ZnZrO_x decreased gradually as the temperature rose from 260 °C to 300 °C, but it decreased rapidly when the reaction temperature was higher than 300 °C. Comparably, Cu₂Ni₁/ZnZrO_x exhibited excellent stability for CH₃OH STY over the measured temperature range, suggesting this catalyst possessed superior thermal stability. Moreover, Cu₂Ni₁/ZnZrO_x displayed a relationship showing a volcanic-shaped trend between CH₃OH STY and the operating temperature, and obtained a maximum CH₃OH STY value of 11.86 mmol_CH3OH h⁻¹ g_cat⁻¹ at 280 °C. Most notably, Cu₂Ni₁/ZnZrO_x had the highest CH₃OH selectivity and CH₃OH STY among all the samples, so it has tremendous potential for CO₂ hydrogenation to CH₃OH.

Fig. 8 CO₂ conversion (a), CH₃OH selectivity (b), and CH₃OH STY (c) as a function of temperature for the different samples at a reaction pressure of 3.0 MPa and GHSV of 15 000 mL g_cat⁻¹ h⁻¹.

Given the practical application requirements of catalysts for industrial-scale CO₂ hydrogenation, a catalyst that is exceptionally resistant to long-term operating conditions appears to be extremely important. A durability test evaluating the time-on-stream behaviour of the Cu₂Ni₁/ZnZrO_x catalyst was carried out at 280 °C, 3.0 MPa and 15 000 mL g_cat⁻¹ h⁻¹ for 100 h, and the obtained results are presented in Fig. S1. It can be found from Fig. S1 that both CO₂ conversion and CH₃OH STY of Cu₂Ni₁/ZnZrO_x only slightly decreased over a continuous period of 100 h during the catalytic reaction. This result suggests that Cu₂Ni₁/ZnZrO_x exhibits impressive long-term stability.

To further evaluate the catalytic CO₂ hydrogenation activity of the ZnZrO_x-supported CuNi catalysts (using Cu₂Ni₁/ZnZrO_x as an example), several ZrO₂-based catalysts were selected for comparison and discussion using the corresponding data listed in Table S1.^{25,36,37,39–41,53} In terms of CH₃OH STY, Cu₂Ni₁/ZnZrO_x exhibited a more superior reaction activity compared to most of the ZrO₂-based catalysts. Although 20% ZnO–ZrO₂ (EISA) shows a slightly higher CH₃OH STY than Cu₂Ni₁/ZnZrO_x, its performance test proceeded at a considerably higher GHSV and reaction temperature. Apparently, a relatively higher GHSV usually indicates that more reactants are involved in CO₂ hydrogenation, leading to a relatively higher CH₃OH STY. Based on the above-detailed results and analysis, Cu₂Ni₁/ZnZrO_x holds great potential as a catalyst for methanol synthesis due to its excellent catalytic performance and outstanding stability.

3.6. Effects of physicochemical properties on the reaction activity

It is generally acknowledged that the physicochemical properties of catalysts play a decisive role in their catalytic activity. Oxygen vacancies and the CO₂ adsorption capacity are two such major influencing factors for CO₂ hydrogenation. Therefore, relationships factoring CH₃OH STY, C_Odefect, and the total amount of desorbed CO₂ were established. As depicted in Fig. 9a, within the reaction temperature range (260–320 °C), a significant linearity existed between CH₃OH STY and C_Odefec. We obtained this result after the data points were fitted, allowing us to acquire a relatively high determination coefficient (R²) value of 0.9129, suggesting that all the data points exhibited an ideal linear correlation. This linear relationship can be explained as follows: CO₂ became adsorbed and activated on the oxygen vacancies of ZnZrO_x, generating CO₂*, which then combined with H*, further generating carbon-containing intermediates and CH₃OH. A relatively higher C_Odefect implies that more CO₂ molecules were absorbed, activated, and converted, further indicating superior CH₃OH productivity. Furthermore, the correlation between CH₃OH STY and the amount of CO₂ desorbed was explored. For any temperature, it is clear from Fig. 9b that as CH₃OH STY increased, there was a linear trend with the amount of CO₂ desorbed. To sum up, Cu₂Ni₁/ZnZrO_x possesses a maximum C_Odefect value and the largest amount of CO₂ desorbed among the five ZnZrO_x-based catalysts, which indicates its optimal catalytic activity.

Fig. 9 The relationship between CH₃OH STY and C_Odefect (a), and the relationship between CH₃OH STY and the total amount of desorbed CO₂ (b).

4. Conclusion

The block-like ZnZrO_x support with a smooth surface and particle sizes of about 80–120 nm is seamlessly prepared through the co-precipitation method. After the incorporation of CuNi into ZnZrO_x using the liquid-phase reduction-deposition method, the structure and size of the block-like crystals are perfectly maintained and the CuNi metallic nanoparticles are evenly and intimately dispersed on the surface of the ZnZrO_x support. Our results indicate that the simultaneous introduction of the Cu and Ni species plays a significant role in improving the ZnZrO_x-supported catalytic system, which not only promotes the BET specific surface areas and metallic species reducibility but also markedly improves the concentration of the surface oxygen vacancies and the amount of CO₂ desorbed. These advantages facilitate the adsorption and catalytic activation of numerous CO₂ molecules and ensure that as many of them as possible are converted to CH₃OH. Especially, the CH₃OH STY is positively correlated with C_Odefec and the amount of CO₂ desorbed. Cu₂Ni₁/ZnZrO_x possesses favorable reducibility, good metal dispersion, a maximum C_Odefect value and the largest amount of total CO₂ desorbed among the five ZnZrO_x-based catalysts; therefore, it possesses the most remarkable catalytic activity.

Conflicts of interest

There are no conflicts to declare.

Data availability

Data are available within the article. Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5re00278h.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 22208032), the Anhui Postdoctoral Scientific Research Program Foundation (No. 2024B805), the Anhui Provincial Key Research and Development Project (No. 202304a05020029), the Anhui Provincial Science and Technology Innovation Tackling Plan Project (No. 202423i08050030), the Project of Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University (No. KJS2328), and the Innovation and Entrepreneurship Training Program for College Students (No. 202310363270).

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