High-Performance MaZrOx (Ma = Cd, Ga) Solid-Solution Catalysts for CO2 Hydrogenation to Methanol

Generation of Surface Ga-H Hydride and Reactivity toward CO2

Hydrogen adsorption on gallium oxide was investigated by in situ infrared (IR) spectroscopy over a temperature range of 30-450 °C. Both hydroxyl groups and Ga-H hydrides with a pair of characteristic bands at 3685 (3532) cm-1 and 2011 (1988) cm-1 were detected on the surface gallium oxide. The formation and stability of surface Ga-H hydrides were found to be highly dependent on the temperature of H2 dissociation. Through a combination of density functional theory (DFT) calculations and isotopic experiments, a mechanism involving both heterolytic and homolytic pathways for hydrogen dissociation was proposed for the generation of Ga-H hydrides. Furthermore, the reactivity of surface Ga-H hydrides was explored by their interactions with various probe molecules such as carbon dioxide, oxygen, and nitrogen. A potential reaction mechanism involving the attraction between nucleophilic hydrogen and positively charged intermediates was suggested during those hydrogenations.

Engineering high production of fatty alcohols from methanol by constructing coordinated dual biosynthetic pathways

Microbial cell factories provide an efficient approach for the green manufacturing of chemicals. However, the excessive use of sugars increases the potential risk of food crisis. Methanol, an abundant feedstock, holds promise in facilitating low-carbon production processes. However, the current methanol bioconversion is hindered by limited regulatory strategies and relatively low conversion efficiency. Here, a yeast biocatalyst was extensively engineered for efficient biosynthesis of fatty alcohols through reinforcement of precursor supply and methanol assimilation in Pichia pastoris. Furthermore, the dual cytoplasmic and peroxisomal biosynthetic pathways were constructed by mating and exhibited robust production of 5.6 g/L fatty alcohols by using methanol as the sole carbon source. This study provides a heterozygous diploid P. pastoris strain with dual cytoplasmic and peroxisomal biosynthetic pathways, which achieved the highest fatty alcohol production from one-carbon feedstocks to date.

Hydrogenation of CO2 into Value-added Chemicals Using Solid-Supported Catalysts

Reducing CO2 emissions is an urgent global priority. In this context, several mitigation strategies, including CO2 tax and stringent legislation, have been adopted to halt the deterioration of the natural environment. Also, carbon recycling procedures undoubtedly help reduce net emissions into the atmosphere, enhancing sustainability. Utilizing Earth's abundant CO2 to produce high-potential green chemicals and light fuels opens new avenues for the chemical industry. In this context, many attempts have been devoted to converting CO2 as a feedstock into various value-added chemicals, such as CH4, lower methanol, light olefins, gasoline, and higher hydrocarbons, for numerous applications involving various catalytic reactions. Although several CO2-conversion methods have been used, including electrochemical, photochemical, and biological approaches, the hydrogenation method allows the reaction to be tuned to produce the targeted compound without significantly altering infrastructure. This review discusses the numerous hydrogenation routes and their challenges, such as catalyst design, operation, and the combined art of structure-activity relationships for the various product formations.

CO2 Hydrogenation to CH3OH on Metal-Doped TiO2(110): Mechanisms, Strain Effect and a New Thermodynamic-Kinetic Relation

Surface strain and linear thermodynamic-kinetic relation are interesting topics in catalysis. Development of low temperature methanol catalysts of high activity and selectivity is of particularly importance for conversion of CO2 to methanol. In the present paper CO2 hydrogenation to methanol on Znx@TiO2(110) (x=0-2) was explored using density functional calculations and microkinetic simulations. The reaction mechanisms on the three model systems were determined and it is shown that Zn2@TiO2(110) is the most active. The most favorable pathway on Zn2@TiO2(110) is identified and CO2+H to HCOO is found to be the rate-controlling step. It is demonstrated that there is a linear relation (named AEB relation) between the adsorption energies of the initial states and the barriers for the controlling step on the 18 systems studied. Calculations on strained surfaces show that the AEB relation exists within ±1 % strain. Sr2@TiO2(110) and -1 % strained CaZn and ZnCu doped TiO2(110) are potential good low temperature catalysts and deserve experimental testing.

Difference in reaction mechanism between ZnZrOx and InZrOx for CO2 hydrogenation

Oxide solid-solution catalysts, such as Zn-doped ZrO2 (ZnZrOx) and In-doped ZrO2 (InZrOx), exhibit distinctive catalytic capabilities for CH3OH synthesis via CO2 hydrogenation. We investigated the active site structures of these catalysts and their associated reaction mechanisms using both experimental and computational approaches. Electron microscopy and X-ray absorption spectroscopy reveal that the primary active sites are isolated cations, such as Zn2+ and In3+, dissolved in tetragonal ZrO2. Notably, for Zn2+, decomposition of the methoxy group, which is an essential intermediate in CH4 synthesis, is partially suppressed because of the relatively high stability of the methoxy group. Conversely, the methyl group strongly adsorbs on In3+, facilitating the conversion of the methoxy species into methyl groups. The decomposition of CH3OH is also suggested to contribute to CH4 synthesis. These results highlight the generation of CH4 as a byproduct of the InZrOx catalyst. Understanding the active site structure and elucidating the reaction mechanism at the atomic level are anticipated to contribute significantly to the future development of oxide solid-solution catalysts.

Rare-earth Element-based Electrocatalysts Designed for CO2 Electro-reduction

Electrochemical CO2 reduction presents a promising approach for synthesizing fuels and chemical feedstocks using renewable energy sources. Although significant advancements have been made in the design of catalysts for CO2 reduction reaction (CO2RR) in recent years, the linear scaling relationship of key intermediates, selectivity, stability, and economical efficiency are still required to be improved. Rare earth (RE) elements, recognized as pivotal components in various industrial applications, have been widely used in catalysis due to their unique properties such as redox characteristics, orbital structure, oxygen affinity, large ion radius, and electronic configuration. Furthermore, RE elements could effectively modulate the adsorption strength of intermediates and provide abundant metal active sites for CO2RR. Despite their potential, there is still a shortage of comprehensive and systematic analysis of RE elements employed in the design of electrocatalysts of CO2RR. Therefore, the current approaches for the design of RE element-based electrocatalysts and their applications in CO2RR are thoroughly summarized in this review. The review starts by outlining the characteristics of CO2RR and RE elements, followed by a summary of design strategies and synthetic methods for RE element-based electrocatalysts. Finally, an overview of current limitations in research and an outline of the prospects for future investigations are proposed.

Mitigating the Poisoning Effect of Formate during CO2 Hydrogenation to Methanol over Co-Containing Dual-Atom Oxide Catalysts

During the hydrogenation of CO2 to methanol over mixed-oxide catalysts, the strong adsorption of CO2 and formate poses a barrier for H2 dissociation, limiting methanol selectivity and productivity. Here we show that by using Co-containing dual-atom oxide catalysts, the poisoning effect can be countered by separating the site for H2 dissociation and the adsorption of intermediates. We synthesized a Co- and In-doped ZrO2 catalyst (Co-In-ZrO2) containing atomically dispersed Co and In species. Catalyst characterization showed that Co and In atoms were atomically dispersed and were in proximity to each other owing to a random distribution. During the CO2 hydrogenation reaction, the Co atom was responsible for the adsorption of CO2 and formate species, while the nearby In atoms promoted the hydrogenation of adsorbed intermediates. The cooperative effect increased the methanol selectivity to 86% over the dual-atom catalyst, and methanol productivity increased 2-fold in comparison to single-atom catalysts. This cooperative effect was extended to Co-Zn and Co-Ga doped ZrO2 catalysts. This work presents a different approach to designing mixed-oxide catalysts for CO2 hydrogenation based on the preferential adsorption of substrates and intermediates instead of promoting H2 dissociation to mitigate the poisonous effects of substrates and intermediates.

Hydrogenation of CO2 to Light Olefins over ZnZrOx /SSZ-13

Converting CO2 to olefins is an ideal route to achieve carbon neutrality. However, selective hydrogenation to light olefins, especially single-component olefin, while reducing CH4 formation remains a great challenge. Herein, we developed ZnZrOx /SSZ-13 tandem catalyst for the highly selective hydrogenation of CO2 to light olefins. This catalyst shows C2 = -C4 = and propylene selectivity up to 89.4 % and 52 %, respectively, while CH4 is suppressed down to 2 %, and there is no obvious deactivation. It is demonstrated that the isolated moderate Brønsted acid sites (BAS) of SSZ-13 promotes the rapid conversion of intermediate species derived from ZnZrOx , thereby enhancing the kinetic coupling of the reactions and inhibit the formation of alkanes and improve the light olefins selectivity. Besides, the weaker BAS of SSZ-13 promote the conversion of intermediates into aromatics with 4-6 methyl groups, which is conducive to the aromatics cycle. Accordingly, more propene can be obtained by elevating the Si/Al ratio of SSZ-13. This provides an efficient strategy for CO2 hydrogenation to light olefins with high selectivity.

First-principles study of CO2 hydrogenation on Cd-doped ZrO2: Insights into the heterolytic dissociation of H2

Cd-doped ZrO2 catalyst has been found to have high selectivity and activity for CO2 hydrogenation to methanol. In this work, density functional theory calculations were carried out to investigate the microscopic mechanism of the reaction. The results show that Cd doping effectively promotes the generation of oxygen vacancies, which significantly activate the CO2 with stable adsorption configurations. Compared with CO2, gaseous H2 adsorption is more difficult, and it is mainly dissociated and adsorbed on the surface as [HCd-HO]* or [HZr-HO]* compact ion pairs, with [HCd-HO]* having the lower energy barrier. The reaction pathways of CO2 to methanol has been investigated, revealing the formate path as the dominated pathway via HCOO* to H2COO* and to H3CO*. The hydrogen anions, HCd* and HZr*, significantly reduce the energy barriers of the reaction.

The Oxygenate-Mediated Conversion of COx to Hydrocarbons─On the Role of Zeolites in Tandem Catalysis

Decentralized chemical plants close to circular carbon sources will play an important role in shaping the postfossil society. This scenario calls for carbon technologies which valorize CO2 and CO with renewable H2 and utilize process intensification approaches. The single-reactor tandem reaction approach to convert COx to hydrocarbons via oxygenate intermediates offers clear benefits in terms of improved thermodynamics and energy efficiency. Simultaneously, challenges and complexity in terms of catalyst material and mechanism, reactor, and process gaps have to be addressed. While the separate processes, namely methanol synthesis and methanol to hydrocarbons, are commercialized and extensively discussed, this review focuses on the zeolite/zeotype function in the oxygenate-mediated conversion of COx to hydrocarbons. Use of shape-selective zeolite/zeotype catalysts enables the selective production of fuel components as well as key intermediates for the chemical industry, such as BTX, gasoline, light olefins, and C3+ alkanes. In contrast to the separate processes which use methanol as a platform, this review examines the potential of methanol, dimethyl ether, and ketene as possible oxygenate intermediates in separate chapters. We explore the connection between literature on the individual reactions for converting oxygenates and the tandem reaction, so as to identify transferable knowledge from the individual processes which could drive progress in the intensification of the tandem process. This encompasses a multiscale approach, from molecule (mechanism, oxygenate molecule), to catalyst, to reactor configuration, and finally to process level. Finally, we present our perspectives on related emerging technologies, outstanding challenges, and potential directions for future research.

Strained few-layer MoS2 with atomic copper and selectively exposed in-plane sulfur vacancies for CO2 hydrogenation to methanol

In-plane sulfur vacancies (Sv) in molybdenum disulfide (MoS2) were newly unveiled for CO2 hydrogenation to methanol, whereas edge Sv were found to facilitate methane formation. Thus, selective exposure and activation of basal plane is crucial for methanol synthesis. Here, we report a mesoporous silica-encapsulated MoS2 catalysts with fullerene-like structure and atomic copper (Cu/MoS2@SiO2). The main approach is based on a physically constrained topologic conversion of molybdenum dioxide (MoO2) to MoS2 within silica. The spherical curvature enables the generation of strain and Sv in inert basal plane. More importantly, fullerene-like structure of few-layer MoS2 can selectively expose in-plane Sv and reduce the exposure of edge Sv. After promotion by atomic copper, the resultant Cu/MoS2@SiO2 exhibits stable specific methanol yield of 6.11 molMeOH molMo-1 h-1 with methanol selectivity of 72.5% at 260 °C, much superior to its counterparts lacking the fullerene-like structure and copper decoration. The reaction mechanism and promoting role of copper are investigated by in-situ DRIFTS and in-situ XAS. Theoretical calculations demonstrate that the compressive strain facilitates Sv formation and CO2 hydrogenation, while tensile strain accelerates the regeneration of active sites, rationalizing the critical role of strain.

Dual active sites over Cu-ZnO-ZrO2 catalysts for carbon dioxide hydrogenation to methanol

CO₂ hydrogenation to methanol is a significant approach to tackle the problem of global warming and simultaneously meet the demand for the portable fuel. Cu-ZnO catalysts with various kinds of promoters have received wide attention. However, the role of promoter and the form of active sites in CO₂ hydrogenation are still in debate. Here, various molar ratios of ZrO₂ were added into the Cu-ZnO catalysts to tune the distributions of Cu⁰ and Cu⁺ species. A volcano-like trend between the ratio of Cu⁺/ (Cu⁺ + Cu⁰) and the amount of ZrO₂ is presented, among which the CuZn10Zr (the molar ratio of ZrO₂ is 10%) catalyst reaches the highest value. Correspondingly, the maximum value of space-time yield to methanol with 0.65 g_MeOH/(g_cat·hr) is obtained on CuZn10Zr at reaction conditions of 220°C and 3 MPa. Detailed characterizations demonstrate that dual active sites are proposed during CO₂ hydrogenation over CuZn10Zr catalyst. The exposed Cu⁰ takes participate in the activation of H₂, while on the Cu⁺ species, the intermediate of formate from the co-adsorption of CO₂ and H₂ prefers to be further hydrogenated to CH₃OH than decomposing into the by-product of CO, yielding a high selectivity of methanol.

Copper-doped ZnO-ZrO2 solid solution catalysts for promoting methanol synthesis from CO2 hydrogenation

Copper-doped ZnO-ZrO2 solid solution catalysts were synthesized via co-precipitation for promoting CH3OH synthesis via hydrogenation of CO2. Various testing methods were applied to investigate the effect of various copper contents on the catalysts. The catalytic performance was evaluated by a fixed bed reactor. XRD, HRTEM and Raman spectra collectively indicated that a ZnO-ZrO2 solid solution catalyst with 3% Cu had a higher Cu dispersion, while the H2-TPR results confirmed that a catalyst with 3% Cu had more Cu active sites under low temperature H2 pretreatment. When the copper content increased to 5% and 10%, the catalyst showed a better Cu crystallinity and a worse Cu dispersion, which could have a negative effect. Therefore, the CO2 conversion and methanol yield with a 3% CuZnO-ZrO2 catalyst at 5 MPa, 250°C and 12 000 ml/(g h) increased by 8.6% and 7.6%, respectively. Moreover, the CH3OH selectivity and catalytic stability of the solid solution catalyst were better than those of the traditional CZA catalyst.

Engineering nanoscale H supply chain to accelerate methanol synthesis on ZnZrOx

Metal promotion is the most widely adopted strategy for enhancing the hydrogenation functionality of an oxide catalyst. Typically, metal nanoparticles or dopants are located directly on the catalyst surface to create interfacial synergy with active sites on the oxide, but the enhancement effect may be compromised by insufficient hydrogen delivery to these sites. Here, we introduce a strategy to promote a ZnZrO_x methanol synthesis catalyst by incorporating hydrogen activation and delivery functions through optimized integration of ZnZrO_x and Pd supported on carbon nanotube (Pd/CNT). The CNT in the Pd/CNT + ZnZrO_x system delivers hydrogen activated on Pd to a broad area on the ZnZrO_x surface, with an enhancement factor of 10 compared to the conventional Pd-promoted ZnZrO_x catalyst, which only transfers hydrogen to Pd-adjacent sites. In CO₂ hydrogenation to methanol, Pd/CNT + ZnZrO_x exhibits drastically boosted activity-the highest among reported ZnZrO_x-based catalysts-and excellent stability over 600 h on stream test, showing potential for practical implementation.

Effects of manganese on the catalytic performance of CuCo catalysts for direct conversion of CO/CO2 to higher alcohols

The catalytic conversion of CO or CO/CO₂ mixtures to higher alcohols (HAs) using hydrogenation reactions remains challenging in C1 chemistry and also one of the most promising reactions for the utilization of non-petroleum resources. Here, the experiment and characterization tests of CuCoMn/Al₂O₃ show that copper is much more dispersed on γ-Al₂O₃ than cobalt, and the interaction between cobalt and Mn metals is stronger. And, mixed cobalt-manganese oxides are formed in the calcined catalyst, promoting the formation of higher alcohols. Under the optimum conditions, the catalyst demonstrated a total alcohol selectivity of 44.6%, and the fraction of higher alcohols reached up to 85.3% among the total alcohol products, which is superior to the classical modified CuCo-based catalysts. And in the gas mixture reaction with a CO : CO₂ ratio of 8 : 2, the conversion rate of the catalyst to CO and CO₂ reached 34.8% and 27.3%, respectively, and the selectivity (C1+ slate 1-alcohol) was 53.2%.

A Review on Green Hydrogen Valorization by Heterogeneous Catalytic Hydrogenation of Captured CO2 into Value-Added Products

The catalytic hydrogenation of captured CO2 by different industrial processes allows obtaining liquid biofuels and some chemical products that not only present the interest of being obtained from a very low-cost raw material (CO2) that indeed constitutes an environmental pollution problem but also constitute an energy vector, which can facilitate the storage and transport of very diverse renewable energies. Thus, the combined use of green H2 and captured CO2 to obtain chemical products and biofuels has become attractive for different processes such as power-to-liquids (P2L) and power-to-gas (P2G), which use any renewable power to convert carbon dioxide and water into value-added, synthetic renewable E-fuels and renewable platform molecules, also contributing in an important way to CO2 mitigation. In this regard, there has been an extraordinary increase in the study of supported metal catalysts capable of converting CO2 into synthetic natural gas, according to the Sabatier reaction, or in dimethyl ether, as in power-to-gas processes, as well as in liquid hydrocarbons by the Fischer-Tropsch process, and especially in producing methanol by P2L processes. As a result, the current review aims to provide an overall picture of the most recent research, focusing on the last five years, when research in this field has increased dramatically.

Exploring the Effects of the Interaction of Carbon and MoS2 Catalyst on CO2 Hydrogenation to Methanol

Hydrogenation of CO2 to form methanol utilizing green hydrogen is a promising route to realizing carbon neutrality. However, the development of catalyst with high activity and selectivity to methanol from the CO2 hydrogenation is still a challenge due to the chemical inertness of CO2 and its characteristics of multi-path conversion. Herein, a series of highly active carbon-confining molybdenum sulfide (MoS2@C) catalysts were prepared by the in-situ pyrolysis method. In comparison with the bulk MoS2 and MoS2/C, the stronger interaction between MoS2 and the carbon layer was clearly generated. Under the optimized reaction conditions, MoS2@C showed better catalytic performance and long-term stability. The MoS2@C catalyst could sustain around 32.4% conversion of CO2 with 94.8% selectivity of MeOH for at least 150 h.

Atomically Dispersed Zn-Stabilized Niδ+ Enabling Tunable Selectivity for CO2 Hydrogenation

For a heterogeneous catalytic process, the performance of catalysts could be improved by modifying the active metal with a second element. Determining the enhanced mechanism of the second element is essential to the rational design of catalysts. In this work, Zn was introduced as a second element into Ni/ZrO₂ for CO₂ hydrogenation. In contrast to Ni/ZrO₂ , the selectivity of NiZn/ZrO₂ is observed to shift from CH₄ to CO. A series of structural characterization results reveals that Zn is atomically dispersed in the NiO and ZrO₂ phases as NiZnO_x and ZnZrO_x , respectively during CO₂ hydrogenation, stabilizing a higher valence state of Ni (Ni^δ+ ) under a hydrogenation atmosphere over Ni-O-Zn site and thus promoting the generation of CO. These findings shed light on the O-mediated bimetallic effect of NiZn/ZrO₂ and bring new insight into the rational design of more efficient heterogeneous catalysts.

Catalytic Hydrogenation of CO2 to Methanol: A Review

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.

Tribology Dependence of Annealed a-C:H Films in Dry Air and Methanol Environments

In the present study, we obtained a-C:H films with different nanostructures by annealing from room temperature to 400 °C. The influence of the nanostructures on tribological performance in dry air and methanol environments was investigated by a reciprocating tribometer. Our results show that the tribological performance of a-C:H films follows different rules in the two environments. In dry air, tribological properties are controlled by the transfer film and mechanical properties. In methanol, friction and wear are determined by the hydrogen bonding structure, where friction coefficients relate on saturation effects of sp3-CH, and the wear properties depend on the "collapse effect" that the more the sp3-CH3 and sp3-CH2, the easier the wear out of bulk a-C:H films. Our work provides guidance for the application of a-C:H films in the methanol environment.

Dependence of copper particle size and interface on methanol and CO formation in CO2 hydrogenation over Cu@ZnO catalysts

The copper particle size and the interface of Cu and ZnO showed strong impacts on the formation of methanol and CO in CO2 hydrogenation over Cu@ZnO catalysts.

Hydrogenation of CO2 into aromatics over ZnZrO–Zn/HZSM-5 composite catalysts derived from ZIF-8

CO2 hydrogenation to aromatics over a ZnZr8O(350)–Zn/Z5 composite catalyst.

Ru-Catalyzed Reverse Water Gas Shift Reaction with Near-Unity Selectivity and Superior Stability

Cascade catalysis of reverse water gas shift (RWGS) and well-established CO hydrogenation holds promise for the conversion of greenhouse gas CO₂ and renewable H₂ into liquid hydrocarbons and methanol under mild conditions. However, it remains a big challenge to develop low-temperature RWGS catalysts with high activity, selectivity, and stability. Here, we report the design of an efficient RWGS catalyst by encapsulating ruthenium clusters with the size of 1 nm inside hollow silica shells. The spatially confined structure prevents the sintering of Ru clusters while the permeable silica layer allows the diffusion of gaseous reactants and products. This catalyst with reduced particle sizes not only inherits the excellent activity of Ru in CO₂ hydrogenation reactions but also exhibits nearly 100% CO selectivity and superior stability at 200-500 °C. The ability to selectively produce CO from CO₂ at relatively low temperatures paves the way for the production of value-added fuels from CO₂ and renewable H₂.

Highlights and challenges in the selective reduction of carbon dioxide to methanol

Carbon dioxide (CO₂) is the iconic greenhouse gas and the major factor driving present global climate change, incentivizing its capture and recycling into valuable products and fuels. The 6H⁺/6e^- reduction of CO₂ affords CH₃OH, a key compound that is a fuel and a platform molecule. In this Review, we compare different routes for CO₂ reduction to CH₃OH, namely, heterogeneous and homogeneous catalytic hydrogenation, as well as enzymatic catalysis, photocatalysis and electrocatalysis. We describe the leading catalysts and the conditions under which they operate, and then consider their advantages and drawbacks in terms of selectivity, productivity, stability, operating conditions, cost and technical readiness. At present, heterogeneous hydrogenation catalysis and electrocatalysis have the greatest promise for large-scale CO₂ reduction to CH₃OH. The availability and price of sustainable electricity appear to be essential prerequisites for efficient CH₃OH synthesis.