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Gallium oxide‐based catalysts are promising candidates for CO2 hydrogenation to methanol, whilst mechanistic understanding of the catalytic systems is not yet fully understood. Here, transient in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies of CO2 hydrogenation over the unsupported CuGa and CuGaN (with N doping) catalysts were conducted. The findings show that N doping could possibly promote the formation of surface Cu+ sites on the interface between Cu and Ga2O3, and hence increasing the selectivity to methanol. However, N doping prohibited the reduction of Ga2O3 to oxygen‐deficient Ga2O3‐x, which is unfavorable for CO2 conversion. Additionally, during DRIFTS the steady‐state isotope transient kinetic analysis (SSITKA) was performed (under H2/D2 isotopic switching conditions). The SSITKA results confirm that the Cu+‐bound formates were likely the key intermediates for methanol synthesis over the catalyst, and the N doping could also weak the interaction between CO* species and the catalyst surface, and thus facilitating the selectivity towards methanol rather than CO. Findings of the work show that the partial nitridation of the metal oxides could be the strategy to promote methanol synthesis selectivity. [ABSTRACT FROM AUTHOR]

The experimental data over CuO/ZnO/ZrO2 catalyst for a wide range of operating conditions were used to develop the kinetics of the reaction CO2 hydrogenation to methanol. Three kinetic models such as the power law model, the Graaf kinetic model, and the Park kinetic model were tested against the experimental data. Both the mechanistic models have been developed based on the Langmuir‐Hinshelwood‐Hougen‐Watson approach and are specific only to the methanol synthesis from CO/CO2 hydrogenation. In an attempt to reduce the number of parameters in the two models, the abridged forms of these models were also tried. Overall, 25 kinetic rate equations were tested and the best‐fit kinetic rate expression with optimized parameters was worked out. Both the integral and differential methods of kinetic analysis were employed and their efficacy in finding the best‐fit expression was compared. The MATLAB built‐in function fminsearch was employed to perform the regression of the data. The Graaf model in its parent form, but with the new optimized values of the parameters, was found to be the best‐fit rate model. The Graaf kinetics, with re‐estimated parameters, could be helpful in designing and simulating a methanol synthesis reactor operating on CO2 and H2 feed and utilizing a CuO/ZnO/ZrO2 catalyst. [ABSTRACT FROM AUTHOR]

Catalytic conversion of CO2 to methanol with H2 from renewable energy has attracted increasing interest as a promising strategy for reducing excessive CO2 emissions. However, the performance of reported various catalysts still suffers from low methanol yield with a passable CO2 conversion. In this work, Cu–ZnO–La2O3 interfaces are constructed with various La2O3 mole ratio. Compared to Cu/ZnO, the optimized catalyst (i.e., Cu0.6Zn0.2La0.4) exhibits a much higher mass-specific methanol formation rate (159.3 gMeOH/kgcat/h) at 240 °C and 3 MPa. A series of ex-situ and in-situ characterizations, such as X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), N2O titration measurements, and in-situ diffuse reflectance Fourier transform infrared spectroscopy (in-situ DRIFTS) study, are used to investigate its structure and mechanism study. The dispersion of Cu over Cu/ZnO/La2O3 catalyst is significantly enhanced, forming more Cu–ZnO–La2O3 interfaces. LaOx species favor CO2 activation and generate more carbon intermediates species for CO2 hydrogenation. Furthermore, more Cu+ is bonded, which stabilizes the key intermediate, inhibits its desorption, and facilitates its further hydrogenation to methanol. This work is expected to offer an effective strategy to develop new catalysts with high performance for CO2 hydrogenation to methanol. [ABSTRACT FROM AUTHOR]

The synthesis of methanol from CO2 hydrogenation is an effective measure to deal with global climate change and an important route for the chemical fixation of CO2. In this work, carbon-confined MoSe2 (MoSe2@C) catalysts were prepared by in situ pyrolysis using glucose as a carbon source. The physico-chemical properties and catalytic performance of CO2 hydrogenation to yield methanol were compared with MoSe2 and MoSe2/C. The results of the structure characterization showed MoSe2 displayed few layers and a small particle size. Owing to the synergistic effect of the Mo2C-MoSe2 heterojunction and in situ carbon doping, MoSe2@C with a suitable C/Mo mole ratio in the precursor showed excellent catalytic performance in the synthesis of methanol from CO2 hydrogenation. Under the optimal catalyst MoSe2@C-55, the selectivity of methanol reached 93.7% at a 9.7% conversion of CO2 under optimized reaction conditions, and its catalytic performance was maintained without deactivation during a continuous reaction of 100 h. In situ diffuse infrared Fourier transform spectroscopy studies suggested that formate and CO were the key intermediates in CO2 hydrogenation to methanol.

The quest for efficient catalysts based on abundant elements that can promote the selective CO2 hydrogenation to green methanol still continues. Most of the reported catalysts are based on Cu/ZnO supported in inorganic oxides, with not much progress with respect to the benchmark Cu/ZnO/Al2O3 catalyst. The use of carbon supports for Cu/ZnO particles is much less explored in spite of the favorable strong metal support interaction that these doped carbons can establish. This manuscript reports the preparation of a series of Cu-ZnO@(N)C samples consisting of Cu/ZnO particles embedded within a N-doped graphitic carbon with a wide range of Cu/Zn atomic ratio. The preparation procedure relies on the transformation of chitosan, a biomass waste, into N-doped graphitic carbon by pyrolysis, which establishes a strong interaction with Cu nanoparticles (NPs) formed simultaneously by Cu2+ salt reduction during the graphitization. Zn2+ ions are subsequently added to the Cu–graphene material by impregnation. All the Cu/ZnO@(N)C samples promote methanol formation in the CO2 hydrogenation at temperatures from 200 to 300 °C, with the temperature increasing CO2 conversion and decreasing methanol selectivity. The best performing Cu-ZnO@(N)C sample achieves at 300 °C a CO2 conversion of 23% and a methanol selectivity of 21% that is among the highest reported, particularly for a carbon-based support. DFT calculations indicate the role of pyridinic N doping atoms stabilizing the Cu/ZnO NPs and supporting the formate pathway as the most likely reaction mechanism.

This article explores the influence of the Mg/Al molar ratio on the performance of CuMgAl-x catalysts for CO2 hydrogenation to methanol. The study found that developing catalysts with highly dispersed Cu nanoparticles and efficient active interfaces is crucial for promoting CO2 hydrogenation to methanol. The results showed that the CuMgAl-3 catalyst exhibited the smallest Cu particle size and good dispersion, leading to the highest CO2 conversion and methanol selectivity. The study suggests that the Mg/Al molar ratio and Cu dispersion play important roles in the catalytic activity of CuMgAl-x catalysts. [Extracted from the article]

One of the most significant industrial processes is the catalytic methanol synthesis from carbon dioxide because methanol is a future energy carrier for producing fuels and high-value-added commodities, the so-called “methanol economy” is carbon neutral. As a solution to climate change, the widespread belief that carbon dioxide can be recycled by hydrogenation into methanol has motivated the development of more efficient and selective catalysts. Efficient 2 wt% Pd/CeO2 catalysts for thermochemical CO2 hydrogenation have recently been investigated. However, the rationale behind the low Pd loading (2 wt%) in CeO2 needs to be clarified, and comprehensive research into Pd tuning is lacking. In this article, we describe the synthesis ofvarious palladium contents (0.5, 1, 2, 4, and 6 wt%) supported on ceria nanorods (Pd/CeO2) for selective hydrogenation of CO2 to methanol under vapor-phase. The impact of Pd on the physicochemical properties of CeO2 was examined using various characterization techniques. The enhanced catalytic activity was caused by the 2 wt% Pd/CeO2 catalyst's most significant level of metallic Pd species, strong interactions between Pd and CeO2, uniform Pd dispersion on CeO2, increased reducibility, oxygen mobility, and weak basic sites. This study reveals that changing the percentage of metal in the catalyst supports a valuable technique for designing efficient oxides-supported metal-based catalysts for CO2 conversions.

Palladium (Pd)-based catalysts are widely used for CO2 hydrogenation reactions due to the capability to activate H2, which enhances the overall CO2 conversion. However, most of the current Pd catalysts reported show high CO2 conversion only at higher temperature, and the exploration of efficient catalyst enabling low temperature conversion (180–300 °C) is of importance. In this study, a series of Pd-B/ZnAl2O4 catalysts were synthesized via in-situ doping, resulting in the Pd crystal transformation induced from crystal to amorphous. The catalytic hydrogenation activity of the amorphous Pd-B catalyst reaches its zenith at a Pd-B mole ratio of 1:2, accompanied by the superior catalysis stability in CO2 hydrogenation. Various characterization techniques, including XRD, CO-DRIFTS, H2-TPR, XPS and TEM, were used to investigate the properties of the Pd-B/ZnAl2O4 catalyst. The results showed that the Pd-B/ZnAl2O4 catalyst exhibited higher dispersion and better CO2 conversion efficiency compared to unmodified Pd catalyst. Additionally, the methanol space–time yields are nearly doubled when the B decoration is employed. At 280 °C, CO2 conversion was improved and methanol selectivity remained at ~ 65%. This study provides valuable insights into the design and optimization of Pd-B based catalysts for CO2 hydrogenation at low temperatures. [ABSTRACT FROM AUTHOR]

The tunability of structural properties such as oxygen vacancies, Cu surface composition, reducibility of CuO, and basicity by solution combustion synthesised (SCS) Cu‐CeO2 catalysts was investigated. Five different fuels, viz. urea (U), tartaric acid (TA), citric acid (CA), ethylene glycol (EG) and oxalyl dihydrazide (ODH), were employed in the synthesis of the catalysts. Raman spectroscopy, O2‐TPD and XPS revealed that the urea synthesised catalyst had the highest concentration of oxygen vacancies, the amount of which was controlled by the fuel used. The oxygen vacancies were the catalytic sites for CO2 adsorption and promoted electron donation to Cu at the Cu‐CeO2 junction, thus improving H2 adsorption and spillover, which was beneficial for methanol synthesis. The better catalytic performance of the urea synthesised Cu‐CeO2 catalyst with a methanol STY of 163 gCH3OH kg−1cat h−1 compared to the other solution combustion synthesis fuels, was attributable to higher oxygen vacancies, more facile Cu reduction, larger Cu surface area and higher Cu surface composition. Based on the absence of the Cu+ species capable of directing the methanol synthesis reaction through the CO intermediate based on XPS data, we surmised that the formate route was the preferred pathway for the studied Cu‐CeO2 catalysts. [ABSTRACT FROM AUTHOR]

Cu/ZnO/CeO2 nanocomposite was supported on metal organic framework (MOF-5) to enhance active sites dispersion and control the nanoparticles agglomeration during synthesis through strong metal-support interactions. The incorporation of MOF-5 alleviated the obstacle facing the commercial ternary Cu/ZnO/Al2O3 regarding low surface area due to nanoparticles agglomeration. In addition, Cu/ZnO/CeO2@MOF-5 gave higher methanol selectivity than the commercial catalyst which can be accounted for by the interfacial sites generated between MOF-5 and Cu/ZnO which favour methanol synthesis over carbon monoxide through regulating the intermediates bonding energies. CeO2 as support for Cu/ZnO nanoparticles was also compared with commercial support and showed to have led to smaller particle size and superior dispersion of Cu active sites as well. Cu/ZnO/CeO2@MOF-5 resulted in methanol STY of 23.3 mg gcat h−1 and selectivity of 79% at mild reaction temperature (260 °C) and pressure (10 bar). Two different MOFs including cerium based MOF and ZIF-8 demonstrated inferior performance compared to MOF-5. [ABSTRACT FROM AUTHOR]