Heterogeneous Pt and MoOx Co-Loaded TiO2 Catalysts for Low-Temperature CO2 Hydrogenation To Form CH3OH

Toward Methanol Production by CO2 Hydrogenation beyond Formic Acid Formation

ConspectusThe Paradigm shift in considering CO2 as an alternative carbon feedstock as opposed to a waste product has recently prompted intense research activities. The implementation of CO2 utilization may be achieved by designing highly efficient catalysts, exploring processes that minimize energy consumption and simplifying product purification and separation. Among possible target products derived from CO2, methanol is highly valuable because it can be used in various chemical feedstocks and as a fuel. Although it is currently produced on a plant scale by heterogeneous catalysis using a Cu/ZnO-based catalyst, a limited theoretical conversion ratio at high reaction temperatures remains an issue. In addition, a catalytic system that can be adjusted to accommodate a variable renewable energy source for the synthesis of methanol is more desirable than current continuous-operation systems, which require a reliable energy supply. Recently, significant progress has been made in the field of homogeneous catalysis, which primarily relies on an indirect route to synthesize methanol via the hydrogenation of carbonate or formate derivatives in the presence of additives and solvents. However, homogeneous catalysis is inappropriate for industrial-scale methanol production because of the inefficient separation and purification processes involved.In this Account, we demonstrate a novel approach for methanol production under mild reaction conditions by CO2 hydrogenation catalyzed by multinuclear iridium complexes under heterogeneous gas-solid phase conditions without any additives and solvents. One of the aims of this Account provides insights for overcoming the barriers for efficient CO2 hydrogenation by focusing on catalyst design, specifically by incorporating varying functionalities into the ligand. The fundamental strategy entails activating hydrogen molecule and enhancing the hydricity of the resulting metal-hydride species, which is based on the following two concepts of catalyst design: (i) Activating a metal-hydride by electronic effects; and (ii) accelerating H2 heterolysis. We have elucidated the mechanism for accelerating H2 heterolysis using a state-of-the-art catalyst that contains an actor-ligand that responds to or participates in catalysis as opposed to a classical spectator-ligand.We have also demonstrated a novel heterogeneous catalysis using a molecular catalyst as a key step for the hydrogenation of CO2 to methanol beyond formic acid formation. The dehydrogenation of formic acid as a reverse reaction of formic acid hydrogenation is strongly favored in acidic aqueous solution. To circumvent the equilibrium limitation, we have envisioned an alternative route that both prevents the liberation of formic acid into the reaction medium, and develops a multinuclear complex to facilitate the transfer of multiple reactive hydrides. The unconventional gas-solid phase catalysis is capable of preventing the liberation of formate species and promoting further hydrogenation of formic acid through multihydride transfer.This novel catalytic system, which is the fusion of a molecular catalyst in heterogeneous catalysis, provides high performance for methanol synthesis through a sophisticated catalyst design and straightforward separation processes. A detailed mechanistic analysis of molecular catalysts in the gas phase would lead to significant progress in the field of Surface Organometallic Chemistry (SOMC).

Syngas Production by Chemical Looping Dry Reforming of Methane over Ni-modified MoO3/ZrO2

We investigated supported-MoO3 materials effective for the chemical looping dry reforming of methane (CL-DRM) to decrease the reaction temperature. Ni-modified molybdenum zirconia (Ni/MoO3/ZrO2) showed CL-DRM activity under isothermal reaction conditions of 650 °C, which was 100-200 °C lower than the previously reported oxide-based materials. Ni/MoO3/ZrO2 activity strongly depends on the MoO3 loading amount. The optimal loading amount was 9.0 wt.% (Ni/MoO3(9.0)/ZrO2), wherein two-dimensional polymolybdate species were dominantly formed. Increasing the loading amount to more than 12.0 wt.% resulted in a loss of activity owing to the formation of bulk Zr(MoO4)2 and/or MoO3. In situ Mo K-edge XANES studies revealed that the surface polymolybdate species serve as oxygen storage sites. The Mo6+ species were reduced to Mo4+ species by CH4 to produce CO and H2. The reduced Mo species reoxidized by CO2 with the concomitant formation of CO. The developed Ni/MoO3(9.0)/ZrO2 was applied to the long-term CL-DRM under high concentration conditions (20 % CH4 and 20 % CO2) at 650 °C, with two pathways possible for converting CH4 and CO2 to CO and H2 via the redox reaction of the Mo species and coke formation.

Designing Dual-Site Catalysts for Selectively Converting CO2 into Methanol

The variability of CO2 hydrogenation reaction demands new potential strategies to regulate the fine structure of the catalysts for optimizing the reaction pathways. Herein, we report a dual-site strategy to boost the catalytic efficiency of CO2-to-methanol conversion. A new descriptor, τ, was initially established for screening the promising candidates with low-temperature activation capability of CO2, and sequentially a high-performance catalyst was fabricated centred with oxophilic Mo single atoms, who was further decorated with Pt nanoparticles. In CO2 hydrogenation, the obtained dual-site catalysts possess a remarkably-improved methanol generation rate (0.27 mmol gcat. -1 h-1). For comparison, the singe-site Mo and Pt-based catalysts can only produce ethanol and formate acid at a relatively low reaction rate (0.11 mmol gcat. -1 h-1 for ethanol and 0.034 mmol gcat. -1 h-1 for formate acid), respectively. Mechanism studies indicate that the introduction of Pt species could create an active hydrogen-rich environment, leading to the alterations of the adsorption configuration and conversion pathways of the *OCH2 intermediates on Mo sites. As a result, the catalytic selectivity was successfully switched.

Reactivity Switch of Platinum with Gallium: From Reverse Water Gas Shift to Methanol Synthesis

The development of efficient catalysts for the hydrogenation of CO2 to methanol using "green" H2 is foreseen to be a key step to close the carbon cycle. In this study, we show that small and narrowly distributed alloyed PtGa nanoparticles supported on silica, prepared via a surface organometallic chemistry (SOMC) approach, display notable activity for the hydrogenation of CO2 to methanol, reaching a 7.2 molCH3OH h-1 molPt-1 methanol formation rate with a 54% intrinsic CH3OH selectivity. This reactivity sharply contrasts with what is expected for Pt, which favors the reverse water gas shift reaction, albeit with poor activity (2.6 molCO2 h-1 molPt-1). In situ XAS studies indicate that ca. 50% of Ga is reduced to Ga0 yielding alloyed PtGa nanoparticles, while the remaining 50% persist as isolated GaIII sites. The PtGa catalyst slightly dealloys under CO2 hydrogenation conditions and displays redox dynamics with PtGa-GaOx interfaces responsible for promoting both the CO2 hydrogenation activity and methanol selectivity. Further tailoring the catalyst interface by using a carbon support in place of silica enables to improve the methanol formation rate by a factor of ∼5.

Low-Temperature Hydrogenation of CO2 to Methanol in Water on ZnO-Supported CuAu Nanoalloys

Optimizing processes and materials for the valorization of CO2 to hydrogen carriers or platform chemicals is a key step for mitigating global warming and for the sustainable use of renewables. We report here on the hydrogenation of CO2 in water on ZnO-supported CuAu nanoalloys, based on ≤7 mol % Au. Cux Auy /ZnO catalysts were characterized using 197 Au Mössbauer, in situ X-ray absorption (Au LIII - and Cu K-edges), and ambient pressure X-ray photoelectron (APXP) spectroscopic methods together with X-ray diffraction and high-resolution electron microscopy. At 200 °C, the conversion of CO2 showed a significant increase by 34 times (from 0.1 to 3.4 %) upon increasing Cu93 Au7 loading from 1 to 10 wt %, while maintaining methanol selectivity at 100 %. Limited CO selectivity (4-6 %) was observed upon increasing temperature up to 240 °C but associated with a ≈3-fold increase in CO2 conversion. Based on APXPS during CO2 hydrogenation in an H2 O-rich mixture, Cu segregates preferentially to the surface in a mainly metallic state, while slightly charged Au submerges deeper into the subsurface region. These results and detailed structural analyses are topics of the present contribution.

Recent Advances of Indium Oxide-Based Catalysts for CO2 Hydrogenation to Methanol: Experimental and Theoretical

Methanol synthesis from the hydrogenation of carbon dioxide (CO₂) with green H₂ has been proven as a promising method for CO₂ utilization. Among the various catalysts, indium oxide (In₂O₃)-based catalysts received tremendous research interest due to the excellent methanol selectivity with appreciable CO₂ conversion. Herein, the recent experimental and theoretical studies on In₂O₃-based catalysts for thermochemical CO₂ hydrogenation to methanol were systematically reviewed. It can be found that a variety of steps, such as the synthesis method and pretreatment conditions, were taken to promote the formation of oxygen vacancies on the In₂O₃ surface, which can inhibit side reactions to ensure the highly selective conversion of CO₂ into methanol. The catalytic mechanism involving the formate pathway or carboxyl pathway over In₂O₃ was comprehensively explored by kinetic studies, in situ and ex situ characterizations, and density functional theory calculations, mostly demonstrating that the formate pathway was extremely significant for methanol production. Additionally, based on the cognition of the In₂O₃ active site and the reaction path of CO₂ hydrogenation over In₂O₃, strategies were adopted to improve the catalytic performance, including (i) metal doping to enhance the adsorption and dissociation of hydrogen, improve the ability of hydrogen spillover, and form a special metal-In₂O₃ interface, and (ii) hybrid with other metal oxides to improve the dispersion of In₂O₃, enhance CO₂ adsorption capacity, and stabilize the key intermediates. Lastly, some suggestions in future research were proposed to enhance the catalytic activity of In₂O₃-based catalysts for methanol production. The present review is helpful for researchers to have an explicit version of the research status of In₂O₃-based catalysts for CO₂ hydrogenation to methanol and the design direction of next-generation catalysts.

Hydrogenation of Carboxylic Acids, Esters, and Related Compounds over Heterogeneous Catalysts: A Step toward Sustainable and Carbon-Neutral Processes

The catalytic hydrogenation of esters and carboxylic acids represents a fundamental and important class of organic transformations, which is widely applied in energy, environmental, agricultural, and pharmaceutical industries. Due to the low reactivity of the carbonyl group in carboxylic acids and esters, this type of reaction is, however, rather challenging. Hence, specifically active catalysts are required to achieve a satisfactory yield. Nevertheless, in recent years, remarkable progress has been made on the development of catalysts for this type of reaction, especially heterogeneous catalysts, which are generally dominating in industry. Here in this review, we discuss the recent breakthroughs as well as milestone achievements for the hydrogenation of industrially important carboxylic acids and esters utilizing heterogeneous catalysts. In addition, related catalytic hydrogenations that are considered of importance for the development of cleaner energy technologies and a circular chemical industry will be discussed in detail. Special attention is paid to the insights into the structure-activity relationship, which will help the readers to develop rational design strategies for the synthesis of more efficient heterogeneous catalysts.

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.

Ionic Conjugated Polymers as Heterogeneous Catalysts for the Cycloaddition of Carbon Dioxide to Epoxides to Form Carbonates under Solvent- and Cocatalyst-Free Conditions

The generation of cyclic carbonates by the cycloaddition of CO₂ with epoxides is attractive in the industry, by which CO₂ is efficiently used as C1 source. Herein, a series of catalysts were developed to efficient mediate the cycloaddition of CO₂ with epoxides to generate carbonates. The catalysts were easily synthesized via the amine-formaldehyde condensation of ethidium bromide with a variety of linkers. The newly prepared heterogeneous catalysts have high thermal stability and degradation temperatures. The surface of the catalysts is smooth and spherical in shape. The effect of temperature, pressure, reaction time and catalyst dosage on the cycloaddition of CO₂ with epoxide were investigated. The results show that the catalyst with 1,3,5-tris(4-formylphenyl)benzene as the linker can achieve 97.4 % conversion efficiency at the conditions of 100 °C, reaction time of 12 h, and the reaction pressure of 1.2 MPa in a solvent-free environment. Notably, the polymers serve as homogeneous catalysts during the reaction (reaction temperature above T_g ) and can be separated and recovered easily as homogeneous catalysts at room temperature. In addition, the catalyst is not only suitable for a wide range of epoxide substrates, but also can be recycled many times. Furthermore, DFT calculations show that the coordination between the electrophilic center of the catalyst and the epoxide reduces the energy barrier, and the reaction mechanism is proposed based on the reaction kinetic studies and DFT calculations.

Development of defective molybdenum oxides for photocatalysis, thermal catalysis, and photothermal catalysis

The localized surface plasmon resonance (LSPR) of noble metals has been investigated for decades for applications in various catalysis reactions and optical research studies, but its development has been hampered by inefficient light absorption and high costs. In comparison, the creation of less expensive semiconductors (metal oxides) with strong plasmonic absorption is an appealing option, particularly defective molybdenum oxide (H_xMoO_3-y) has received considerable attention and investigation as a promising plasmonic material for a variety of catalytic reactions (photocatalysis, thermocatalysis, photothermal catalysis, etc.).The LSPR effect of H_xMoO_3-y can be tuned throughout a broad spectrum range from visible to near-infrared (NIR) by altering the doping amount by electrochemical control, chemical reduction, or photochemical control. Notably, defects (oxygen vacancies) in H_xMoO_3-y arise in conjunction with the LSPR effect, resulting in the formation of unique and useful active sites in a range of catalytic processes. In this review, we explore the formation mechanism of H_xMoO_3-y with plasmonic features and discuss its applications in photocatalysis, thermocatalysis, and photothermal catalysis.

Hydrogenation of n-octanoic acid over the MoPt alloy of Mo-Pt/SiO2 catalyst under solvent-free conditions

SiO₂-Supported bimetallic Mo and Pt worked as an effective heterogeneous catalyst for hydrogenation of n-octanoic acid at a low temperature of 373 K in the absence of solvent, providing 78% yield of hydrogenated products (62% yield of 1-octanol and 32% yield of octyl octanoate). Based on the catalyst characterization, MoPt alloy (Mo/Pt = 1) was formed by high-temperature reduction and was proposed to be the active site for the reaction.

Selective hydroconversion of coconut oil-derived lauric acid to alcohol and aliphatic alkane over MoO x -modified Ru catalysts under mild conditions

Molybdenum oxide-modified ruthenium on titanium oxide (Ru-(y)MoO _x /TiO₂; y is the loading amount of Mo) catalysts show high activity for the hydroconversion of carboxylic acids to the corresponding alcohols (fatty alcohols) and aliphatic alkanes (biofuels) in 2-propanol/water (4.0/1.0 v/v) solvent in a batch reactor under mild reaction conditions. Among the Ru-(y)MoO _x /TiO₂ catalysts tested, the Ru-(0.026)MoO _x /TiO₂ (Mo loading amount of 0.026 mmol g^-1) catalyst shows the highest yield of aliphatic n-alkanes from hydroconversion of coconut oil derived lauric acid and various aliphatic fatty acid C6-C18 precursors at 170-230 °C, 30-40 bar for 7-20 h. Over Ru-(0.026)MoO _x /TiO₂, as the best catalyst, the hydroconversion of lauric acid at lower reaction temperatures (130 ≥ T ≤ 150 °C) produced dodecane-1-ol and dodecyl dodecanoate as the result of further esterification of lauric acid and the corresponding alcohols. An increase in reaction temperature up to 230 °C significantly enhanced the degree of hydrodeoxygenation of lauric acid and produced n-dodecane with maximum yield (up to 80%) at 230 °C, H₂ 40 bar for 7 h. Notably, the reusability of the Ru-(0.026)MoO _x /TiO₂ catalyst is slightly limited by the aggregation of Ru nanoparticles and the collapse of the catalyst structure.

The biopolymer-assisted synthesis of assembled g-C3N4 open frameworks with electron delocalization channels for prompt H2 production

Three-dimensional (3D) photocatalysts can overcome the serious aggregation exhibited by two-dimensional (2D) nanosheets caused by unavoidable van der Waals forces.

A quasi-stable molybdenum sub-oxide with abundant oxygen vacancies that promotes CO2 hydrogenation to methanol

Production of methanol from anthropogenic carbon dioxide (CO₂) is a promising chemical process that can alleviate both the environmental burden and the dependence on fossil fuels. In catalytic CO₂ hydrogenation to methanol, reduction of CO₂ to intermediate species is generally considered to be a crucial step. It is of great significance to design and develop advanced heterogeneous catalysts and to engineer the surface structures to promote CO₂-to-methanol conversion. We herein report an oxygen-defective molybdenum sub-oxide coupled with Pt nanoparticles (Pt/H_xMoO_3−y) which affords high methanol yield with a methanol formation rate of 1.53 mmol g_-cat⁻¹ h⁻¹ in liquid-phase CO₂ hydrogenation under relatively mild reaction conditions (total 4.0 MPa, 200 °C), outperforming other oxide-supported Pt catalysts in terms of both the yield and selectivity for methanol. Experiments and comprehensive analyses including in situ X-ray absorption fine structure (XAFS), in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and density functional theory (DFT) calculations reveal that both abundant surface oxygen vacancies (V_O) and the redox ability of Mo species in quasi-stable H_xMoO_3−y confer the catalyst with enhanced adsorption and activation capability to subsequently transform CO₂ to methanol. Moreover, the Pt NPs act as H₂ dissociation sites to regenerate oxygen vacancies and as hydrogenation sites for the CO intermediate to finally afford methanol. Based on the experimental and computational studies, an oxygen-vacancy-mediated “reverse Mars–van Krevelen (M–vK)” mechanism is proposed. This study affords a new strategy for the design and development of an efficient heterogeneous catalyst for CO₂ conversion.

Oxygen-defective molybdenum sub-oxide coupled with Pt nanoparticles affords high methanol yield in liquid-phase CO₂ hydrogenation via reverse Mars–van Krevelen mechanism.

Oxygenated Transport Fuels from Carbon Dioxide : Driving towards Net Zero

The restructuring of the economy post-COVID-19 coupled to the drive towards Net Zero carbon dioxide emissions means we must rethink the way we use transport fuels. Fossil-carbon based fuels are ubiquitous in the transport sector, however there are alternative synthetic fuels that could be used as drop-in or replacement fuels. The main hurdles to achieving a transition to synthetic fuels are the limited availability of low-cost carbon dioxide at an appropriate purity, the availability of renewable hydrogen and, in the case of hydrocarbons, catalysts that are selective for small and particular chain lengths. In this paper we will consider some of the alternative fuels and methods that could reduce cost, both economically and environmentally. We recommend that increased effort in the rapid development of these fuels should be a priority in order to accelerate the possibility of achieving Net Zero without costly infrastructure changes. As ground transportation offers a more straightforward approach legislatively, we will look at oxygenated organic fuels as an alternative drop-in replacement for hydrocarbons.

Homogeneous and heterogeneous catalysts for hydrogenation of CO2 to methanol under mild conditions

This review summarizes the concepts, mechanisms, drawbacks and challenges of the state-of-the-art catalysis for CO₂ to MeOH under mild conditions. Thoughtful guidelines and principles for future research are presented and discussed.

Reverse water-gas shift reaction over Pt/MoOx/TiO2: reverse Mars–van Krevelen mechanism via redox of supported MoOx

Pt/MoO_x/TiO₂ shows excellent catalytic performance for the reverse water-gas shift reaction at 250 °C via reverse Mars–van Krevelen mechanism.

In-situ preparation of molybdenum trioxide-silver composites for the improved photothermal catalytic performance of cyclohexane oxidation

The selective catalytic oxidation of cyclohexane has important theoretical and practical application value. However, high conversion rate and high selectivity are difficult to achieve simultaneously by conventional catalytic system. In this work, blue molybdenum trioxide (MoO₃) nanorods with oxygen vacancies were prepared by hydrothermal method using hydrated molybdic acid as a precursor under the reduction of formic acid, and in-situ produced MoO₃-silver (MoO₃-Ag) composites were further used in the photothermal catalytic oxidation of cyclohexane with high conversion and high selectivity using dry air as oxidant. The results showed that the best conversion rate of cyclohexanone and cyclohexanol (KA oil) could reach 8.6% with the selectivity of 99.0%. The excellent catalytic performance of MoO₃-Ag composites can be attributed to the significantly increased visible and near-infrared light absorption caused by the plasma resonance effect of Ag nanoparticles and oxygen vacancies, and the prevented charge recombination by MoO₃-Ag Schottky heterojunction. This work provides new reference solutions for the design and preparation of high-performance photothermal catalysts for the selective oxidation of hydrocarbons.

TiO2-Supported catalysts with ZnO and ZrO2 for non-oxidative dehydrogenation of propane: mechanistic analysis and application potential

Non-oxidative dehydrogenation of propane is one of the most promising technologies for propene production in terms of environmental impact and sustainability.

State of the art and perspectives in heterogeneous catalysis of CO2 hydrogenation to methanol

The ever-increasing amount of anthropogenic carbon dioxide (CO₂) emissions has resulted in great environmental impacts, the heterogeneous catalysis of CO₂ hydrogenation to methanol is of great significance.

CO2 hydrogenation to high-value products via heterogeneous catalysis

Recently, carbon dioxide capture and conversion, along with hydrogen from renewable resources, provide an alternative approach to synthesis of useful fuels and chemicals. People are increasingly interested in developing innovative carbon dioxide hydrogenation catalysts, and the pace of progress in this area is accelerating. Accordingly, this perspective presents current state of the art and outlook in synthesis of light olefins, dimethyl ether, liquid fuels, and alcohols through two leading hydrogenation mechanisms: methanol reaction and Fischer-Tropsch based carbon dioxide hydrogenation. The future research directions for developing new heterogeneous catalysts with transformational technologies, including 3D printing and artificial intelligence, are provided.

Carbon dioxide (CO₂) capture and conversion provide an alternative approach to synthesis of useful fuels and chemicals. Here, Ye et al. give a comprehensive perspective on the current state of the art and outlook of CO₂ catalytic hydrogenation to the synthesis of light olefins, dimethyl ether, liquid fuels, and alcohols.