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The methanol synthesis via CO2 hydrogenation (MSCH) reaction is a useful CO2 utilization strategy, and this synthesis path has also been widely applied commercially for many years. In this work the performance of a MSCH reactor with the minimum entropy generation rate (EGR) as the objective function is optimized by using finite time thermodynamic and optimal control theory. The exterior wall temperature (EWR) is taken as the control variable, and the fixed methanol yield and conservation equations are taken as the constraints in the optimization problem. Compared with the reference reactor with a constant EWR, the total EGR of the optimal reactor decreases by 20.5%, and the EGR caused by the heat transfer decreases by 68.8%. In the optimal reactor, the total EGRs mainly distribute in the first 30% reactor length, and the EGRs caused by the chemical reaction accounts for more than 84% of the total EGRs. The selectivity of CH3OH can be enhanced by increasing the inlet molar flow rate of CO, and the CO2 conversion rate can be enhanced by removing H2O from the reaction system. The results obtained herein are in favor of optimal designs of practical tubular MSCH reactors.

The effect of oxide coating on the activity of a copper-zinc oxide–based catalyst for methanol synthesis via the hydrogenation of carbon dioxide was investigated. A commercial catalyst was coated with various oxides by a sol-gel method. The influence of the types of promoters used in the sol-gel reaction was investigated. Temperature-programmed reduction-thermogravimetric analysis revealed that the reduction peak assigned to the copper species in the oxide-coated catalysts prepared using ammonia shifts to lower temperatures than that of the pristine catalyst; in contrast, the reduction peak shifts to higher temperatures for the catalysts prepared using L(+)-arginine. These observations indicated that the copper species were weakly bonded with the oxide and were easily reduced by using ammonia. The catalysts prepared using ammonia show higher CO2 conversion than the catalysts prepared using L(+)-arginine. Among the catalysts prepared using ammonia, the silica-coated catalyst displayed a high activity at high temperatures, while the zirconia-coated catalyst and titania-coated catalyst had high activity at low temperatures. At high temperature the conversion over the silica-coated catalyst does not significantly change with reaction temperature, while the conversion over the zirconia-coated catalyst and titania-coated catalyst decreases with reaction time. From the results of FTIR, the durability depends on hydrophilicity of the oxides.

High-efficiency utilization of CO2 facilitates the reduction of CO2 concentration in the global atmosphere and hence the alleviation of the greenhouse effect. The catalytic hydrogenation of CO2 to produce value-added chemicals exhibits attractive prospects by potentially building energy recycling loops. Particularly, methanol is one of the practically important objective products, and the catalytic hydrogenation of CO2 to synthesize methanol has been extensively studied. In this review, we focus on some basic concepts on CO2 activation, the recent research advances in the catalytic hydrogenation of CO2 to methanol, the development of high-performance catalysts, and microscopic insight into the reaction mechanisms. Finally, some thinking on the present research and possible future trend is presented.

Electrochemical oxidation of CH₄ is known to be inefficient in aqueous electrolytes. The lower activity of methane oxidation reaction (MOR) is primarily attributed to the dominant oxygen evolution reaction (OER) and the higher barrier for CH₄ activation on transition metal oxides (TMOs). However, a satisfactory explanation for the origins of such lower activity of MOR on TMOs, along with the enabling strategies to partially oxidize CH₄ to CH₃OH, have not been developed yet. We report here the activation of CH₄ is governed by a previously unrecognized consequence of electrostatic (or Madelung) potential of metal atom in TMOs. The measured binding energies of CH₄ on 12 different TMOs scale linearly with the Madelung potentials of the metal in the TMOs. The MOR active TMOs are the ones with higher CH₄ binding energy and lower Madelung potential. Out of 12 TMOs studied here, only TiO₂, IrO₂, PbO₂, and PtO₂ are active for MOR, where the stable active site is the O on top of the metal in TMOs. The reaction pathway for MOR proceeds primarily through *CHₓ intermediates at lower potentials and through *CH₃OH intermediates at higher potentials. The key MOR intermediate *CH₃OH is identified on TiO₂ under operando conditions at higher potential using transient open-circuit potential measurement. To minimize the overoxidation of *CH₃OH, a bimetallic Cu₂O₃ on TiO₂ catalysts is developed, in which Cu reduces the barrier for the reaction of *CH₃ and *OH and facilitates the desorption of *CH₃OH. The highest faradaic efficiency of 6% is obtained using Cu-Ti bimetallic TMO.

Methanol synthesis is one of the most important industrially-viable approaches for carbon dioxide (CO 2 ) utilization, as the produced methanol can be used as a platform chemical for manufacturing green fuels and chemicals. The In 2 O 3 catalysts are ideal for sustainable methanol synthesis and have received considerable attention. Herein, Co-, Ni- and Cu-modified In 2 O 3 catalysts were fabricated with high dispersion and high stability to improve the hydrogenation performance. The Ni-promoted In 2 O 3 catalyst in the form of high dispersion possessed the largest amount of oxygen vacancies and the strongest ability for H 2 activation, leading to the highest CO 2 conversion and space time yield of methanol of 0.390 g MeOH g cat −1 h −1 with CH 3 OH selectivity of 68.7%. In addition, the catalyst exhibits very stable performance over 120 h on stream, which suggests the promising prospect for industrial applications. Further experimental and theoretical studies demonstrate that surface Ni doping promotes the formation of oxygen defects on the In 2 O 3 catalyst, although it also results in lower methanol selectivity. Surprisingly, subsurface Ni dopants are found to be more beneficial for methanol formation than surface Ni dopants, so the Ni promoted In 2 O 3 catalyst with a lower surface Ni content at the similar Ni loading can reach higher methanol selectivity and productivity. This work thus provides theoretical guidance for significantly improving the CO 2 reactivity of In 2 O 3 -based catalysts while maintaining high methanol selectivity.

CO 2 hydrogenation as a route for the chemical energy storage over a commercial Cu/ZnO/Al 2 O 3 catalyst has been studied. To check the optimal conditions for an efficient methanol production the influence of temperature and space velocity on the catalytic performance has been demonstrated. Time-on-stream measurements in the absence and the presence of benzene in the gas feed mixture were performed to investigate the possibility to use alternative carbon sources, which contain traces of aromatics. The catalyst can operate in a stable way without the presence of carbon monoxide in the feed, which means that increased water contents in the product gas cannot destroy the catalyst’s performance completely. The presence of benzene in the feed does not lead to a deactivation of the catalyst. With these findings methanol production starting from exhaust gases from steel mills seems to become an interesting alternative for sustainable methanol production. Graphical Abstract

Cu/ZnO/CeO 2 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/Al 2 O 3 regarding low surface area due to nanoparticles agglomeration. In addition, Cu/ZnO/CeO 2 @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. CeO 2 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/CeO 2 @MOF-5 resulted in methanol STY of 23.3 mg g cat 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. Graphical Abstract

The remarkable contribution of oxygen vacancies has been revealed by the operando spectroscopies for methanol synthesis from CO 2 hydrogeneration on the reducible metal oxide-supported copper catalysts. However, a challenge remains in the intrinsic understanding of the evolution and advantage of oxygen vacancies for methanol synthesis. Here we prepare the TiO 2− x /Cu with different oxygen vacancy densities by adjusting the ball-milling frequency. At the optimal condition, a TiO 2− x /Cu with more oxygen vacancies delivers an excellent methanol yield of 26.5 mmol g −1 h −1 at 300 °C with a selectivity of more than 70%. The combined analysis of experimental characterizations and theoretical calculations reveals that the mutual dispersions of TiO 2− x and Cu driven by mechanical energy induce the electron rearrangement in the d orbital of the Ti atom and relax the Ti–O binding at the interface, which facilitates the formation of oxygen vacancies that further reduce the barrier of CO 2 hydrogenation to *HCOO due to higher nucleophilicity of titanium ions.

A series of ZnO/ZrO 2 catalysts were synthesized by the precipitation method. The effect of different precipitation modes on the physicochemical properties of the ZnO/ZrO 2 catalysts for CO 2 hydrogenation to methanol was investigated. The catalysts were characterized by X-ray diffraction (XRD), N 2 adsorption–desorption, transmission electron microscopy (TEM), temperature programmed desorption of CO 2 (CO 2 -TPD), temperature programmed reduction of H 2 (H 2 -TPR), and X-ray photoelectron spectroscopy (XPS). The results showed that the catalyst of ZnO/ZrO 2 prepared by the concurrent precipitation method had more CO 2 adsorption sites, resulting in its higher CO 2 conversion. While the stepwise precipitation mode increased the surface oxygen content of ZnO/ZrO 2 , which enhanced the methanol selectivity. Overall, the ZnO/ZrO 2 prepared by the concurrent precipitation mode exhibited a highly competitive efficacy for CO 2 hydrogenation, giving a methanol selectivity of 81.4% with a CO 2 conversion of 5.1% at 280 ℃ and 3 MPa, and the methanol yield reached 4.2%. Graphical Abstract The effect of different precipitation modes on the physicochemical properties of the ZnO/ZrO 2 catalysts for CO 2 hydrogenation to methanol was investigated, and the ZnO/ZrO 2 prepared by the concurrent precipitation mode exhibited a highly competitive efficacy for CO 2 hydrogenation.

A modified two-step coprecipitation method has been formulated to prepare a series of model CuZnO/Al 2 O 3 catalysts for methanol synthesis from syngas. Evaluation at industrially relevant condition showed slightly higher activity and much higher stability for model catalysts than for a commercial catalyst. Due to the same CuZn-binary precursor, the model catalysts had similar structural properties with large specific surface area and pore volume, small CuO crystallites and good reducibility, resembling that of the commercial catalyst. However, the model catalysts showed much better uniformity of alumina distribution, which accounted for their better performance. Strong positive correlation between the deactivation rate and the coefficient of variation of Al/Zn indicates the uniformity of alumina distribution plays an important role in determining stability. Specifically, CuZnO/Al 2 O 3 catalysts with alumina distributed more uniformly had higher stability. This work demonstrated that the formulated modified two-step coprecipitation could provide excellent CuZnO/Al 2 O 3 catalysts for industrial applications. Additionally, it also affords new clues for the development of even better industrial catalysts. Graphical Abstract