Hydrogen spillover enabled active Cu sites for methanol synthesis from CO2 hydrogenation over Pd doped CuZn catalysts

Sustainable methanol production from carbon dioxide: advances, challenges, and future prospects

The urgent need to address global carbon emissions and promote sustainable energy solutions has led to a growing interest in carbon dioxide (CO2) conversion technologies. Among these, the transformation of CO2 into methanol (MeOH) has gained prominence as an effective mitigation strategy. This review paper provides a comprehensive exploration of recent advances and applications in the direct utilization of CO2 for the synthesis of MeOH, encompassing various aspects from catalysts to market analysis, environmental impact, and future prospects. We begin by introducing the current state of CO2 mitigation strategies, highlighting the significance of carbon recycling through MeOH production. The paper delves into the chemistry and technology behind the conversion of CO2 into MeOH, encompassing key themes such as feedstock selection, material and energy supply, and the various conversion processes, including chemical, electrochemical, photochemical, and photoelectrochemical pathways. An in-depth analysis of heterogeneous and homogeneous catalysts for MeOH synthesis is provided, shedding light on the advantages and drawbacks of each. Furthermore, we explore diverse routes for CO2 hydrogenation into MeOH, emphasizing the technological advances and production processes associated with this sustainable transformation. As MeOH holds a pivotal role in a wide range of chemical applications and emerges as a promising transportation fuel, the paper explores its various chemical uses, transportation, storage, and distribution, as well as the evolving MeOH market. The environmental and energy implications of CO2 conversion to MeOH are discussed, including a thermodynamic analysis of the process and cost and energy evaluations for large-scale catalytic hydrogenation.

Value-Added Upcycling of PET to 1,4-Cyclohexanedimethanol by a Hydrogenation/Hydrogenolysis Relay Catalysis

We present an innovative process for directly transforming poly(ethylene terephthalate) (PET), a polymer extensively used in food and beverage packaging, into trans-isomer-enriched 1,4-cyclohexanedimethanol (CHDM), a key ingredient in advanced specialty polymers. Our approach leverages a dual-catalyst system featuring palladium on reduced graphene oxide (Pd/r-GO) and oxalate-gel-derived copper-zinc oxide (og-CuZn), utilizing hydrogenation/hydrogenolysis relay catalysis. This method efficiently transforms PET into polyethylene-1,4-cyclohexanedicarboxylate (PECHD), which is then converted into CHDM with an impressive overall yield of 95 % in a two-stage process. Our process effectively handles various post-consumer PET plastics, converting them into CHDM with yields between 78 % and 89 % across different substrates. Additionally, we demonstrate the applicability and scalability of this approach through a temperature-programmed three-stage relay process on a 10-gram scale, which results in purified CHDM with an isolated yield of 87 % and a notably higher trans/cis ratio of up to 4.09/1, far exceeding that of commercially available CHDM. This research not only provides a viable route for repurposing PET waste but also enhances the control of selectivity patterns in multistage relay catalysis.

Boosting CO2 Hydrogenation to Methanol over Monolayer MoS2 Nanotubes by Creating More Strained Basal Planes

The controlled creation, selective exposure, and activation of more basal planes while simultaneously minimizing the generation and exposure of edge sites are crucial for accelerating methanol synthesis from CO2 hydrogenation over MoS2 catalysts but remain a bottleneck. Here, we report a facile method to fabricate heteronanotube catalysts with single-layer MoS2 coaxially encapsulating the carbon nanotubes (CNTs@MoS2) through host-guest chemistry. Inheriting the long tubular structure of CNTs, the grown MoS2 nanotubes exhibit significantly more basal planes than bulk MoS2 crystals. More importantly, the tubular curvature not only promotes strain and sulfur vacancy (Sv) generation but also preferentially exposes more in-plane Sv while limiting edge Sv exposure, which is conducive to methanol synthesis. Both the strain and layer number of MoS2 can be easily and finely adjusted by altering CNT diameter and quantity of precursors. Remarkably, CNTs@MoS2 with monolayer MoS2 and maximum strain displayed methanol selectivity of 78.1% and methanol space time yield of 1.6 g gMoS2-1 h-1 at 260 °C and GHSV of 24000 mL gcat.-1 h-1, representing the best results to date among Mo-based catalysts. This study provides prospects for novel catalyst design by synthesizing coaxial tubular heterostructure to create additional catalytic sites and ultimately enhance conversion and selectivity.

Metallic 1T-N-WS2 /WO3 Heterojunctions Featuring Interface-Engineered Cu-S Configuration for Selective Electrochemical CO2 Reduction Reaction

Electrocatalytic carbon-dioxide reduction reactions (ECO2 RR) are one of the most rational techniques to control one's carbon footprint. The desired product formation depends on deliberate reaction kinetics and a choice of electron-proton contribution. Herein the usage of novel CuS active centers decorated over stable 1T metallic N-WS2 /WO3 nanohybrids as an efficient selective formate conversion electrocatalyst with regard to ECO2 RR is reported. The preferred reaction pathway is identified as *OCHO, which is reduced (by gaining H+  + e- ) to HCOO- (HCOO- path) as the primary product. More significantly, at -1.3 V versus RHE yield of FEHCOO - is 55.6% ± 0.5 with a Jgeo of -125.05 mA cm-2 for CuS@1T-N-WS2 /WO3 nanohybrids. In addition, predominant catalytic activity, selectivity, and stability properties are observed; further post-mortem analysis demonstrates the choice of material importance. The present work describes an impressive approach to develop highly active electrocatalysts for selective ECO2 RR applications.

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.

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.

Plasma-Catalytic CO2 Hydrogenation over a Pd/ZnO Catalyst: In Situ Probing of Gas-Phase and Surface Reactions

Plasma-catalytic CO₂ hydrogenation is a complex chemical process combining plasma-assisted gas-phase and surface reactions. Herein, we investigated CO₂ hydrogenation over Pd/ZnO and ZnO in a tubular dielectric barrier discharge (DBD) reactor at ambient pressure. Compared to the CO₂ hydrogenation using Plasma Only or Plasma + ZnO, placing Pd/ZnO in the DBD almost doubled the conversion of CO₂ (36.7%) and CO yield (35.5%). The reaction pathways in the plasma-enhanced catalytic hydrogenation of CO₂ were investigated by in situ Fourier transform infrared (FTIR) spectroscopy using a novel integrated in situ DBD/FTIR gas cell reactor, combined with online mass spectrometry (MS) analysis, kinetic analysis, and emission spectroscopic measurements. In plasma CO₂ hydrogenation over Pd/ZnO, the hydrogenation of adsorbed surface CO₂ on Pd/ZnO is the dominant reaction route for the enhanced CO₂ conversion, which can be ascribed to the generation of a ZnO _x overlay as a result of the strong metal-support interactions (SMSI) at the Pd-ZnO interface and the presence of abundant H species at the surface of Pd/ZnO; however, this important surface reaction can be limited in the Plasma + ZnO system due to a lack of active H species present on the ZnO surface and the absence of the SMSI. Instead, CO₂ splitting to CO, both in the plasma gas phase and on the surface of ZnO, is believed to make an important contribution to the conversion of CO₂ in the Plasma + ZnO system.

Insights on hydrogen spillover on carbonaceous supports

Hydrogen spillover is important in solid-phase catalytic hydrogenation reactions, as well as in hydrogen storage and scavenging. The present study explores the nature of this phenomenon by examining the effects of hydrogen pressure and addition of carbonaceous additives, such as carbon nanotubes (CNT) and C₆₀ fullerene, on hydrogenation reaction kinetics and its products distribution. For these purposes, a solid-phase hydrogenation reaction was studied, where 1,4-bis-(phenyl-ethynyl)benzene (PEB) was used as a hydrogen acceptor. To the best of our knowledge, this is the first study in which both the reaction kinetics and products distribution of the solid-phase organic hydrogen acceptor were analyzed. A demonstration of hydrogen spillover phenomenon was provided on the basis of the combined interpretation of kinetics and hydrogenated organic products distribution, under different reaction conditions. The results were explained in terms of hydrogen active species availability, distribution and relative migration distance of these species through the carbonaceous media. The insights into the hydrogen spillover chemistry obtained in this research allow for a better understanding of this phenomenon and its implementation in the future hydrogen storage and transportation, and hydrogen-generating devices, including safety aspects of all these applications.

PdCu supported on dendritic mesoporous CexZr1-xO2 as superior catalysts to boost CO2 hydrogenation to methanol

A dendritic PdCu/Ce_0.3Zr_0.7O₂ (PdCu/CZ-3) catalyst with uniform spherical morphology was prepared for boosting the catalytic performance of CO₂ hydrogenation to methanol (MeOH). The open dendritic pore channels and small particle sizes could reduce not only the diffuse resistance of reactants and products but also increase the accessibility between the active sites (PdCu and oxygen vacancy) and the reactants (H₂ and CO₂). More spillover hydrogen could be generated due to the highly dispersed PdCu active metals over the PdCu/CZ-3 catalyst. PdCu/CZ-3 can stimulate the generation of more Ce³⁺ cations, which is beneficial to produce more oxygen vacancies on the surface of the CZ-3 composite. Spillover hydrogen and oxygen vacancy could promote the formate and methoxy routes over PdCu/CZ-3, the primary intermediates producing MeOH. PdCu/CZ-3 displayed the highest CO₂ conversions (25.5 %), highest MeOH yield (6.4 %), highest PdCu-TOF_MeOH (7.7 h^-1) and superior 100 h long-term stability than those of other PdCu/Ce_xZr_1-xO₂ analogs and the reference PdCu/CeO₂ and PdCu/ZrO₂ catalysts. Density functional theory (DFT) calculations and in situ DRIFTS were performed to investigate the CO₂ - MeOH hydrogenation mechanism.

Glycerol Hydrogenolysis to Produce 1,2-Propanediol in Absence of Molecular Hydrogen Using a Pd Promoted Cu/MgO/Al2O3 Catalyst

The catalytic process of glycerol hydrogenolysis to produce 1,2-propandiol (1,2-PD) in the absence of external hydrogen addition has been investigated. The methanol present in the crude glycerol from a biodiesel production process is used to provide in situ hydrogen produced via methanol steam reforming for the glycerol hydrogenolysis process. This process can reduce the additional cost for the transportation and storage of molecular hydrogen and also reduce the safety risks related to using high hydrogen pressure. It was found that the introduction of Pd onto a Cu/MgO/Al2O3 catalyst significantly improved the glycerol conversion and 1,2-PD selectivity. The pseudo-first-order kinetic results suggested that the promoting effect of Pd is primarily attributed to the enhanced activity for the hydrogenation of acetol, which is the intermediate formed via glycerol dehydration. A 27−3 fractional factorial design experiment was carried out to investigate the impacts of seven single factors and their binary effects on two responses, namely 1,2-PD selectivity and glycerol conversion. The results showed that the glycerol feed concentration has the most significant effect on the 1,2-PD selectivity, such that the 1,2-PD selectivity is lower if a more concentrated glycerol is used as the feedstock; stirring speed, inert gas pressure and water to methanol molar ratio have insignificant effects on the reaction system. The addition of Pd, higher temperature and higher catalyst loading are the essential factors in order to obtain a high selectivity of 1,2-PD and a high glycerol conversion.

Influence of Indium as a Promoter on the Stability and Selectivity of the Nanocrystalline Cu/CeO2 Catalyst for CO2 Hydrogenation to Methanol

Stable catalyst development for CO₂ hydrogenation to methanol is a challenge in catalysis. In this study, indium (In)-promoted Cu nanoparticles supported on nanocrystalline CeO₂ catalysts were prepared and explored for methanol production from CO₂. In-promoted Cu catalysts with ∼1 wt % In loading showed a methanol production rate of 0.016 mol g_Cu^-1 h^-1 with 95% methanol selectivity and no loss of activity for 100 h. It is found that the addition of indium remarkably increases Cu dispersion and decreases Cu particle size. In addition led to an increased metal-support interaction, which stabilizes Cu particles against sintering during the reaction, leading to high stability and activity. In addition, density functional theory calculations suggested that the reaction is proceeding via reverse water gas shift (RWGS) mechanism where the presence of In stabilized intermediate species and lowered CO₂ activation energy barriers.

Size-Controlled Synthesis of Pd Nanocatalysts on Defect-Engineered CeO2 for CO2 Hydrogenation

The size effects of metal catalysts have been widely investigated to optimize their catalytic activity and selectivity. However, the size-controllable synthesis of uniform supported metal nanoparticles without surfactants and/or additives remains a great challenge. Herein, we developed a green, surfactant-free, and universal strategy to tailor the sizes of uniform Pd nanoparticles on metal oxides by an electroless chemical deposition method via defect engineering of supports. The nucleation and growth mechanism suggest a strong electrostatic interaction between the Pd precursor and low-defective CeO₂ and a weak reducing capacity for low-defective CeO₂, resulting in small Pd nanoparticles. Conversely, large Pd nanoparticles were formed on a highly defective CeO₂ surface. Combined with various ex situ and in situ characterizations, a higher intrinsic activity of Pd for the CO₂-to-CO hydrogenation was found on large Pd nanoparticles with higher electron density owing to their stronger H₂ dissociation ability and H-spillover effects, as well as the larger number of oxygen vacancies generated in situ for CO₂ activation under hydrogenation conditions.

CO2 activation and dissociation on In2O3(110) supported PdnPt(4-n) (n = 0-4) catalysts: a density functional theory study

Converting CO2 into valuable chemicals via catalytic reactions can mitigate both the greenhouse effect and energy shortage problems, thus designing efficient catalysts have attracted considerable attention over the past decades. In this work, a density functional theory (DFT) calculation was carried out to investigate the CO2 activation and dissociation processes on various PdnPt(4-n)/In2O3 (n = 0-4) catalysts. The PdnPt(4-n)/In2O3 models were initially built, and the interface sites of PdnPt(4-n)/In2O3 for CO2 adsorption were confirmed among cluster sites and substrate sites. The CO2 adsorption geometries, charger transfer, and projected density of states (PDOS) were analyzed to study the CO2-PdnPt(4-n)/In2O3 interactions. From the adsorbed *CO2, the transition states (TSs) for CO2 dissociation to form *CO and *O were gained to reveal the characteristics of the activated CO2δ-. Overall, according to the adsorption energy Eads results, the bimetallic PdPt3/In2O3 and Pd3Pt/In2O3 catalysts showed the strongest and weakest CO2 adsorption stabilities, respectively, while the Pd element addition decreases the barriers for CO2 dissociation with the priority order of Pd4 > Pd3Pt > Pd2Pt2 > PdPt3 > Pt4. The Brønsted-Evans-Polanyi (BEP) relation between activation barriers (Eb) and reaction energies E was obtained for the CO2 dissociation mechanism on PdnPt(4-n)/In2O3 catalysts with the equation of E = 0.20Eb + 0.40. Finally, the optimal Pd2Pt2/In2O3 catalyst for CO2 activation and dissociation was proposed. This study provides useful information for CO2 activation and conversation procedures on bimetal-oxide catalysts, and helps to take the optimal design of PdPt/In2O3 catalysts for the CO2 reaction.

Crystallographic Orientation Dependence of Surface Segregation and Alloying on PdCu Catalysts for CO2 Hydrogenation

The influence of the crystallographic orientation on surface segregation and alloy formation in model PdCu methanol synthesis catalysts was investigated in situ using near-ambient pressure X-ray photoelectron spectroscopy under CO₂ hydrogenation conditions. Combined with scanning tunneling microscopy and density functional theory calculations, the study showed that submonolayers of Pd undergo spontaneous alloy formation on Cu(110) and Cu(100) surfaces in vacuum, whereas they do not form an alloy on Cu(111). Upon heating in H₂, inward diffusion of Pd into the Cu lattice is favored, facilitating alloying on all Cu surfaces. Under CO₂ hydrogenation reaction conditions, the alloying trend becomes stronger, promoted by the reaction intermediate HCOO*, especially on Pd/Cu(110). This work demonstrates that surface alloying may be a key factor in the enhancement of the catalytic activity of PdCu catalysts as compared to their monometallic counterparts. Furthermore, it sheds light on the hydrogen activation mechanism during catalytic hydrogenation on copper-based catalysts.

Structure-Performance Correlations over Cu/ZnO Interface for Low-Temperature Methanol Synthesis from Syngas Containing CO2

Cu/ZnO catalysts with varied Cu/(Cu + Zn) molar ratios were prepared by a facile solid-state method. The Cu/(Cu + Zn) molar ratio displayed a significant effect on the oxygen vacancy formation of the calcined catalysts, thereby influencing the CuO-ZnO interaction and the reducibility of CuO. The Cu/(Cu + Zn) molar ratio also exhibited a significant effect on Cu⁰ surface area, oxygen vacancy, the ratio of ZnO(002) plane to ZnO(100) plane, as well as the basicity and acidity of the reduced catalysts, thereby affecting the catalytic performance for low-temperature methanol synthesis from syngas containing CO₂. The correlations of methanol space time yield (STY) versus the physicochemical characteristics of Cu/ZnO catalysts were studied. The catalyst with equal amounts of Cu and Zn displayed the best catalytic activity owing to higher Cu⁰ surface area, more oxygen vacancy and ZnO(002) plane, as well as more moderately basic sites.

Glycerol Hydrogenolysis with In Situ Hydrogen Produced via Methanol Steam Reforming: The Promoting Effect of Pd on a Cu/ZnO/Al2O3 Catalyst

The glycerol hydrogenolysis to produce 1,2-propanediol without using externally supplied hydrogen was investigated using methanol present in crude glycerol to provide in situ hydrogen via its steam reforming reaction. This paper focuses on the promoting effect of Pd on the reactivity of a Cu/Zn/Al2O3 catalyst. Adding 2 wt% Pd onto a Cu/ZnO/Al2O3 catalyst significantly improved the selectivity to 1,2-propanediol from 63.0% to 82.4% and the glycerol conversion from 70.2% to 99.4%. This enhancement on the catalytic activity by Pd is mainly due to the improved hydrogenation of acetol, which is the intermediate formed during the glycerol dehydration. The rapid hydrogenation of acetol can shift the reaction equilibrium of glycerol dehydration forward resulting in a higher glycerol conversion. The improved reducibility of the catalyst by Pd allows the catalyst to be reduced in situ during the reaction preventing any loss of catalyst activity due to any potential oxidation of the catalyst. The catalyst was slightly deactivated when it was firstly recycled resulting in a 5.4% loss of glycerol conversion due to the aggregation of Cu and the deactivation became less noticeable upon further recycling.

How Size Matters: Electronic, Cooperative, and Geometric Effect in Perovskite-Supported Copper Catalysts for CO2 Reduction

In heterogeneous catalysis, bifunctional catalysts often outperform one-component catalysts. The activity is also strongly influenced by the morphology, size, and distribution of catalytic particles. For CO2 hydrogenation, the size of the active copper area on top of the SrTiO3 perovskite catalyst support can affect the activity, selectivity, and stability. Here, a detailed theoretical study of the effect of bifunctionality on an important chemical CO2 transformation reaction, the reverse water gas shift (RWGS) reaction, is presented. Using density functional theory computation results for the RWGS pathway on three surfaces, namely, Cu(111), SrTiO3, and the Cu/SrTiO3 interface between both solid phases, we construct the energy landscape of the reaction. The adsorbate-adsorbate lateral interactions are taken into account for catalytic surfaces, which show a sufficient intermediate coverage. The mechanism, combining all three surfaces, is used in mesoscale kinetic Monte Carlo simulations to study the turnover and yield for CO production as a function of particle size. It is shown that the reaction proceeds faster at the interface. However, including copper and the support sites in addition to the interface accelerates the conversion even further, showing that the bifunctionality of the catalyst manifests in a more complex interplay between the phases than just the interface effect, such as the hydrogen spillover. We identify three distinct effects, the electronic, cooperative, and geometric effects, and show that the surrounded smaller Cu features on the set of supporting SrTiO3 show a higher CO formation rate, resulting in a decreasing RWGS model trend with the increasing Cu island size. The findings are in parallel with experiments, showing that they explain the previously observed phenomena and confirming the size sensitivity for the catalytic RWGS reaction.

Thermal Modification Effect on Supported Cu-Based Activated Carbon Catalyst in Hydrogenolysis of Glycerol

Glycerol hydrogenolysis to 1,2-propanediol (1,2-PDO) was performed over activated carbon supported copper-based catalysts. The catalysts were prepared by impregnation using a pristine carbon support and thermally-treated carbon supports (450, 600, 750, and 1000 °C). The final hydrogen adsorption capacity, porous structure, and total acidity of the catalysts were found to be important descriptors to understand catalytic performance. Oxygen surface groups on the support controlled copper dispersion by modifying acidic and adsorption properties. The amount of oxygen species of thermally modified carbon supports was also found to be a function of its specific surface area. Carbon supports with high specific surface areas contained large amount of oxygen surface species, inducing homogeneous distribution of Cu species on the carbon support during impregnation. The oxygen surface groups likely acted as anchorage centers, whereby the more stable oxygen surface groups after the reduction treatment produced an increase in the interaction of the copper species with the carbon support, and determined catalytic performances.

The reaction pathway of the CO2RR to low-carbon alcohols: a theoretical study

Catalytic hydrogenation of CO₂ to chemicals is an important approach for reducing the concentration of CO₂ in the atmosphere.

Platinum-copper single atom alloy catalysts with high performance towards glycerol hydrogenolysis

Selective hydrogenolysis of biomass-derived glycerol to propanediol is an important reaction to produce high value-added chemicals but remains a big challenge. Herein we report a PtCu single atom alloy (SAA) catalyst with single Pt atom dispersed on Cu nanoclusters, which exhibits dramatically boosted catalytic performance (yield: 98.8%) towards glycerol hydrogenolysis to 1,2-propanediol. Remarkably, the turnover frequency reaches up to 2.6 × 10³ mol_glycerol·mol_PtCu-SAA^-1·h^-1, which is to our knowledge the largest value among reported heterogeneous metal catalysts. Both in situ experimental studies and theoretical calculations verify interface sites of PtCu-SAA serve as intrinsic active sites, in which the single Pt atom facilitates the breakage of central C-H bond whilst the terminal C-O bond undergoes dissociation adsorption on adjacent Cu atom. This interfacial synergistic catalysis based on PtCu-SAA changes the reaction pathway with a decreased activation energy, which can be extended to other noble metal alloy systems.

Atomic Layer Deposition ZnO Over-Coated Cu/SiO2 Catalysts for Methanol Synthesis from CO2 Hydrogenation

Cu-ZnO-based catalysts are of importance for CO2 utilization to synthesize methanol. However, the mechanisms of CO2 activation, the split of the C=O double bond, and the formation of C-H and O-H bonds are still debatable. To understand this mechanism and to improve the selectivity of methanol formation, the combination of strong electronic adsorption (SEA) and atomic layer deposition (ALD) was used to form catalysts with Cu nanoparticles surrounded by a non-uniform ZnO layer, uniform atomic layer of ZnO, or multiple layers of ZnO on porous SiO2. N2 adsorption, H2 temperature-programmed reduction (H2-TPR) X-ray diffraction (XRD), transmission electron microscope (TEM), energy-dispersive X-ray spectroscopy (EDX), CO-chemisorption, CO2 temperature-programmed desorption (CO2-TPD), X-ray adsorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) were used to characterize the catalysts. The catalyst activity was correlated to the number of metallic sites. The catalyst of 5 wt% Cu over-coated with a single atomic layer of ZnO exhibited higher methanol selectivity. This catalyst has comparatively more metallic sites (smaller Cu particles with good distribution) and basic site (uniform ZnO layer) formation, and a stronger interaction between them, which provided necessary synergy for the CO2 activation and hydrogenation to form methanol.

Hydrogenation of Carbon Dioxide to Value-Added Chemicals by Heterogeneous Catalysis and Plasma Catalysis

Due to the increasing emission of carbon dioxide (CO2), greenhouse effects are becoming more and more severe, causing global climate change. The conversion and utilization of CO2 is one of the possible solutions to reduce CO2 concentrations. This can be accomplished, among other methods, by direct hydrogenation of CO2, producing value-added products. In this review, the progress of mainly the last five years in direct hydrogenation of CO2 to value-added chemicals (e.g., CO, CH4, CH3OH, DME, olefins, and higher hydrocarbons) by heterogeneous catalysis and plasma catalysis is summarized, and research priorities for CO2 hydrogenation are proposed.