Pd/ZnO catalysts for direct CO2 hydrogenation to methanol

Hydrogenation of CO2 for sustainable fuel and chemical production

Catalytic carbon dioxide (CO2) hydrogenation is a potential route for producing sustainable fuels and chemicals, but existing catalysts need improvement. In particular, identifying active sites and understanding the interaction between components and the dynamic behavior of the participant species remain unclear. This fundamental knowledge is essential for the design of more efficient and stable catalysts. Because the nature of the active site (metal, oxide, carbide) is the main factor that determines the catalytic activity of the catalysts, this Review focuses on various types of heterogeneous catalysts that have been recently reported in the literature as efficient for CO2 conversion to C1 [carbon monoxide (CO), methanol (CH3OH), methane (CH4)], and higher hydrocarbons. We focus on establishing key connections between active-site structures and selectivity, regardless of catalyst composition.

Rules Describing CO2 Activation on Single-Atom Alloys from DFT-Meta-GGA Calculations and Artificial Intelligence

Single-atom alloys (SAAs) arise as a promising concept for the design of improved CO2 hydrogenation catalysts. However, from the immense number of possible SAA compositions and structures, only a few might display the properties required to be useful catalysts. Thus, the direct, high-throughput screening of materials is inefficient. Here, we use artificial intelligence to derive rules describing surface sites of SAAs that provide an effective CO2 activation, a crucial initial step to convert the molecule into valuable products. We start by modeling the CO2 interaction with 780 sites of flat and stepped surfaces of SAAs composed by Cu, Zn, and Pd hosts via high-quality DFT-mBEEF calculations. Then, we apply subgroup discovery to determine constraints on key physicochemical properties, out of 24 offered candidate descriptive parameters, characterizing subgroups (SGs) of surface sites where chemisorbed CO2 displays large elongations of its C-O bonds. The key identified parameters are free-atom properties of the elements constituting the surface sites, such as their electron affinity, electronegativity, and radii of the d-orbitals. Additionally, the generalized coordination number is selected as a key geometrical parameter. The SG rules are applied to identify promising surface sites from a candidate space of over 1500 possible ones in different single-atom and dual-atom alloys. Some of the promising alloys predicted by the SG rules were explicitly tested by additional DFT-mBEEF calculations and confirmed to provide a significant CO2 activation.

Highly Dispersed ZnO Sites in a ZnO/ZrO2 Catalyst Promote Carbon Dioxide-to-Methanol Conversion

ZnO/ZrO2 catalysts have shown better activity in the CO2 hydrogenation to methanol compared with single component counterparts, but the interaction between ZnO and ZrO2 is still poorly understood. In particular, the effect of the ZrO2 support phase (tetragonal vs. monoclinic) was not systematically explored. Here, we have synthesized ZnO/ZrO2 catalysts supported on tetragonal ZrO2 (ZnO/ZrO2-t) and monoclinic ZrO2 (ZnO/ZrO2-m), which resulted in the formation of different ZnOx species, consisting of sub-nanometer ZnO moieties and large-sized ZnO particles, respectively. ZnO/ZrO2-t exhibited a higher methanol selectivity (81 vs. 39 %) and methanol yield (1.25 vs. 0.67 mmol g-1 h-1) compared with ZnO/ZrO2-m. The difference in performance was attributed to the redox state and degree of dispersion of Zn, based on spectroscopy and microscopy results. ZnO/ZrO2-t had a high density of ZnOx-ZrOy sites, which favored the formation of active HCOO* species and enhanced the yield and selectivity of methanol along the formate pathway. Such ZnO clusters were further dispersed on ZrO2-t during catalysis, while larger ZnO particles on ZnO/ZrO2-m remained stable throughout the reaction. This study shows that the phase of ZrO2 supports can be used to control the dispersion of ZnO and the catalyst surface chemistry, and lead to enhanced catalytic performance.

Efficient amine-assisted CO2 hydrogenation to methanol co-catalyzed by metallic and oxidized sites within ruthenium clusters

Amine-assisted two-step CO2 hydrogenation is an efficient route for methanol production. To maximize the overall catalytic performance, both the N-formylation of amine with CO2 (i.e., first step) and the subsequent amide hydrogenation (i.e., second step) are required to be optimized. Herein, a class of Al2O3-supported Ru catalysts, featuring multiple activated Ru species (i.e., metallic and oxidized Ru), are rationally fabricated. Density functional theory calculations suggest that metallic Ru forms are preferred for N-formylation step, whereas oxidized Ru species demonstrate enhanced amide hydrogenation activity. Thus, the optimal catalyst, containing unique Ru clusters with coexisting metallic and oxidized Ru species, efficiently synergize the conversion of CO2 into methanol with exceptional selectivity (>95%) in a one-pot two-step process. This work not only presents an advanced catalyst for CO2-based methanol production but also highlights the strategic design of catalysts with multiple active species for optimizing the catalytic performances of multistep reactions in the future.

C-C bond coupling with sp3 C-H bond via active intermediates from CO2 hydrogenation

Compared to the sluggish kinetics observed in methanol-mediated side-chain alkylation of methyl groups with sp3 C-H bonds, CO2 hydrogenation emerges as a sustainable alternative strategy, yet it remains a challenge. Here, as far as we know, it is first reported that using CO2 hydrogenation replacing methanol can conduct the side-chain alkylation of 4-methylpyridine (MEPY) over a binary metal oxide-zeolite Zn40Zr60O/CsX tandem catalyst (ZZO/CsX). This ZZO/CsX catalyst can achieve 19.6% MEPY single-pass conversion and 82% 4-ethylpyridine (ETPY) selectivity by using CO2 hydrogenation, which is 6.5 times more active than methanol as an alkylation agent. The excellent catalytic performance is realized on the basis of the dual functions of the tandem catalyst: hydrogenation of CO2 on the ZZO and activation of sp3 C-H bond and C-C bond coupling on the CsX zeolite. The thermodynamic and kinetic coupling between the tandem reactions enables the highly efficient CO2 hydrogenation and C-C bond coupling. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations suggest that the CHxO* (CH2O*) species, rather than methanol produced from CO2 hydrogenation, is the key intermediate to achieve the C-C bond coupling.

Development of supported intermetallic compounds: advancing the Frontiers of heterogeneous catalysis

Intermetallic compound (IMC) catalysts have garnered significant attention due to their unique surface and electronic properties, which can lead to enhanced catalytic performance compared to traditional monometallic catalysts. However, developing IMC materials as high-performance catalysts has been hindered by the inherent complexity of synthesizing nanoparticles with well-defined bulk and surface compositions. Achieving precise control over the composition of supported bimetallic IMC catalysts, especially those with high surface area and stability, has proven challenging. This review provides a comprehensive overview of the recent progress in developing supported IMC catalysts. We first examine the various synthetic approaches that have been explored to prepare supported IMC nanoparticles with phase-pure bulk structures and tailored surface compositions. Key factors influencing the formation kinetics and compositional control of these materials are discussed in detail. Then the strategies for manipulating the surface composition of supported IMCs are delved into. Applications of high-performance supported IMCs in important reactions such as selective hydrogenation, reforming, dehydrogenation, and deoxygenation are comprehensively reviewed, showcasing the unique advantages offered by these materials. Finally, the prevailing research challenges associated with supported IMCs are identified, including the need for a better understanding of the composition-property relationships and the development of scalable synthesis methods. The prospects for the practical implementation of these versatile catalysts in industrial processes are also highlighted, underscoring the importance of continued research in this field.

Hydrothermal synthesis of ZnZrO x catalysts for CO2 hydrogenation to methanol: the effect of pH on structure and activity

With the growing necessity of achieving carbon neutrality in the industrial sector, the catalytic hydrogenation of carbon dioxide into methanol has been widely considered one of the key strategies for the utilization of captured CO2. For this reason, the development of alternative catalysts such as ZnZrO x has attracted considerable interest, given its superior stability and versatility in comparison to the conventional Cu-based materials. In this work, ZnZrO x has been produced by a hydrothermal synthesis method at varied synthesis pH between 7 and 10 and a positive association between pH and catalytic CO2 conversion is observed. At 2.0 MPa and 250 °C, ZnZrO x produced at pH 10 shows a methanol selectivity of 95% at a CO2 conversion of 3.4%. According to characterization, basic pH conditions enable the formation of abundant t-ZrO2 and the subsequent incorporation of Zn2+ into this phase, although the content of surface Zn does not increase between pH 8 and 10. Nevertheless, synthesis pH values can be correlated with surface oxygen content and CO2 adsorption capacity, which could be important contributors to the higher catalytic activity observed as a result of higher synthesis pH values. However, upon synthesis at pH 10, an inferior selectivity to methanol is observed above 250 °C, as a possible result of the excessive formation of ZnO. Interestingly, this secondary phase can be prevented and the selectivity can be slightly improved by utilizing NH4OH instead of NaOH in the hydrothermal method.

Highly Dispersed Pd@ZIF-8 for Photo-Assisted Cross-Couplings and CO2 to Methanol: Activity and Selectivity Insights

Our study unveils a pioneering methodology that effectively distributes Pd species within a zeolitic imidazolate framework-8 (ZIF-8). We demonstrate that Pd can be encapsulated within ZIF-8 as atomically dispersed Pd species that function as an excited-state transition metal catalyst for promoting carbon-carbon (C-C) cross-couplings at room temperature using visible light as the driving force. Furthermore, the same material can be reduced at 250 °C, forming Pd metal nanoparticles encapsulated in ZIF-8. This catalyst shows high rates and selectivity for carbon dioxide hydrogenation to methanol under industrially relevant conditions (250 °C, 50 bar): 7.46 molmethanol molmetal -1 h-1 and >99 %. Our results demonstrate the correlations of the catalyst structure with the performances at experimental and theoretical levels.

Bimetallic CuPd nanoparticles supported on ZnO or graphene for CO2 and CO conversion to methane and methanol

Carbon dioxide (CO2) and carbon monoxide (CO) hydrogenation to methane (CH4) or methanol (MeOH) is a promising pathway to reduce CO2 emissions and to mitigate dependence on rapidly depleting fossil fuels. Along these lines, a series of catalysts comprising copper (Cu) or palladium (Pd) nanoparticles (NPs) supported on zinc oxide (ZnO) as well as bimetallic CuPd NPs supported on ZnO or graphene were synthesized via various methodologies. The prepared catalysts underwent comprehensive characterization via high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX) mapping, electron energy loss spectroscopy (EELS), X-ray diffraction (XRD), hydrogen temperature-programmed reduction and desorption (H2-TPR and H2-TPD), and deuterium temperature-programmed desorption (D2O-TPD). In the CO2 hydrogenation process carried out at 20 bar and elevated temperatures (300 to 500 °C), Cu, Pd, and CuPd NPs (<5 wt% loading) supported on ZnO or graphene predominantly yielded CH4 as the primary product, with CO generated as a byproduct via the reverse water gas shift (RWGS) reaction. For CO hydrogenation between 400 and 500 °C, the CO conversion was at least 40% higher than the CO2 conversion, with CH4 and CO2 identified as the main products, the latter from water gas shift. Employing 90 wt% Cu on ZnO led to an enhanced CO conversion of 14%, with the MeOH yield reaching 10% and the CO2 yield reaching 4.3% at 230 °C. Overall, the results demonstrate that lower Cu/Pd loading (<5 wt%) supported on ZnO/graphene favored CH4 production, while higher Cu content (90 wt%) promoted MeOH production, for both CO2 and CO hydrogenation at high pressure.

Kinetic Insights into Methanol Synthesis from CO2 Hydrogenation at Atmospheric Pressure over Intermetallic Pd2Ga Catalyst

This study presents a single-site microkinetic model for methanol synthesis by CO2 hydrogenation over intermetallic Pd2Ga/SiO2. A reaction path analysis (RPA) combining theoretical results and realistic catalyst surface reaction data is established to elucidate the reaction mechanism and kinetic models of CO2 hydrogenation to methanol and CO. The RPA leads to the derivation of rate expressions for both reactions without presumptions about the most abundant reactive intermediate (MARI) and rate-determining step (rds). The formation of H2COOH* is found to be the rds (step 19) for methanol synthesis via the formate pathway, with CO2 and H-atoms adsorbed on intermetallic sites as the MARIs. The derived kinetic model is corroborated with experimental data acquired under different reaction conditions, using a lab-scale fixed-bed reactor and Pd2Ga/SiO2 nanoparticles prepared by incipient wetness impregnation. The excellent agreement between the experimental data and the kinetic model (R 2 = 0.99) substantiates the proposed mechanism with an activation energy of 61.52 kJ mol-1 for methanol synthesis. The reported catalyst exhibits high selectivity to methanol (96%) at 1 bar, 150 °C, and H2/CO2 ratio of 3:1. These findings provide critical insights to optimize catalysts and processes targeting CO2 hydrogenation at atmospheric pressure and low temperatures for on-demand energy production.

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.

Micro-Structure Engineering in Pd-InOx Catalysts and Mechanism Studies for CO2 Hydrogenation to Methanol

Significant interest has emerged for the application of Pd-In2O3 catalysts as high-performance catalysts for CO2 hydrogenation to CH3OH. However, precise active site control in these catalysts and understanding their reaction mechanisms remain major challenges. In this investigation, a series of Pd-InOx catalysts were synthesized, revealing three distinct types of active sites: In-O, Pd-O(H)-In, and Pd2In3. Lower Pd loadings exhibited Pd-O(H)-In sites, while higher loadings resulted in Pd2In3 intermetallic compounds. These variations impacted catalytic performance, with Pd-O(H)-In catalysts showing heightened activity at lower temperatures due to the enhanced CO2 adsorption and H2 activation, and Pd2In3 catalysts performing better at elevated temperatures due to the further enhanced H2 activation. In situ DRIFTS studies revealed an alteration in key intermediates from *HCOO over In-O bonds to *COOH over Pd-O(H)-In and Pd2In3 sites, leading to a shift in the main reaction pathway transition and product distribution. Our findings underscore the importance of active site engineering for optimizing catalytic performance and offer valuable insights for the rational design of efficient CO2 conversion 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.

Low-nuclearity CuZn ensembles on ZnZrOx catalyze methanol synthesis from CO2

Metal promotion could unlock high performance in zinc-zirconium catalysts, ZnZrOx, for CO2 hydrogenation to methanol. Still, with most efforts devoted to costly palladium, the optimal metal choice and necessary atomic-level architecture remain unclear. Herein, we investigate the promotion of ZnZrOx catalysts with small amounts (0.5 mol%) of diverse hydrogenation metals (Re, Co, Au, Ni, Rh, Ag, Ir, Ru, Pt, Pd, and Cu) prepared via a standardized flame spray pyrolysis approach. Cu emerges as the most effective promoter, doubling methanol productivity. Operando X-ray absorption, infrared, and electron paramagnetic resonance spectroscopic analyses and density functional theory simulations reveal that Cu0 species form Zn-rich low-nuclearity CuZn clusters on the ZrO2 surface during reaction, which correlates with the generation of oxygen vacancies in their vicinity. Mechanistic studies demonstrate that this catalytic ensemble promotes the rapid hydrogenation of intermediate formate into methanol while effectively suppressing CO production, showcasing the potential of low-nuclearity metal ensembles in CO2-based methanol synthesis.

A microkinetic study of CO2 hydrogenation to methanol on Pd1-Cu(111) and Pd1-Ag(111) catalysts: a DFT analysis

The thermochemical conversion of CO2 into methanol, a process known for its selectivity, often encounters a significant obstacle: the reverse water gas reaction. This problem emerges due to the demanding high temperatures and pressures, causing instability in catalytic performance. Recent endeavours have focused on innovatively designing catalysts capable of withstanding such conditions. Given the costliness of experimental approaches, a theoretical framework has emerged as a promising avenue for addressing the challenges in methanol production. It has been reported that transition metals, especially Pd, provide ideal binding sites for CO2 molecules and hydrogen atoms, facilitating their interactions and subsequent conversion to methanol. In the geometric single-atom form, their surface enables precise control over the reaction pathways and enhances the selectivity towards methanol. In our study, we employed density functional theory (DFT) to explore the conversion of CO2 to CH3OH on Pd1-Cu(111) and Pd1-Ag(111) single-atom alloy (SAA) catalysts. Our investigation involved mapping out the complex reaction pathways of CO2 hydrogenation to CH3OH using microkinetic reaction modelling and mechanisms. We examined three distinct pathways: the COOH* formation pathway, the HCOO* formation pathway, and the dissociation of CO2* to CO* pathway. This comprehensive analysis encompassed the determination of adsorption energies for all reactants, transition states, and resultant products. Additionally, we investigated the thermodynamic and kinetic profiles of individual reaction steps. Our findings emphasised the essential role of the Pd single atom in enhancing the activation of CO2, highlighting the key mechanism underlying this catalytic process. The favoured route for methanol generation on the Pd1-Ag(111) single-atom alloy (SAA) surface unfolds as follows: CO2* progresses through a series of transformations, transitioning successively into HCOO*, HCOOH*, H2COOH*, CH2O*, and CH2OH*, terminating in the formation of CH3OH*, due to lower activation energies and higher rate constants.

Fabrication of nano-sized Pd catalyst supported on sodium carboxymethyl cellulose/gum Arabic/sodium alginate functionalized microspheres for catalytic reduction of nitro compounds, organic dyes, K3[Fe(CN)6], and chromium(VI) pollutants

The rapid development of industrialization and urbanization, along with the increasing human population, has led to serious water pollution. Among water pollutants, organic and inorganic pollutants cause serious problems for both the environment and human health due to their toxicity and carcinogenic properties. One of the best ways to eliminate these pollutants is to develop eco-friendly, efficient, and long-life catalysts. For this purpose, in this study, environmentally friendly microspheres containing sodium alginate (SA), sodium carboxymethyl cellulose (Na-CMC), and gum Arabic (GA) were fabricated as potential stabilizers (SA/Na-CMC/GA). Subsequently, newly heterogeneous catalyst system was designed by immobilizing Pd nanoparticles on them and characterized (Pd@SA/Na-CMC/GA). The catalytic reduction ability of Pd@SA/Na-CMC/GA was then investigated against the reduction of 4-nitroaniline (4-NA), 4-nitrophenol (4-NP), 2-nitroaniline (2-NA), 4-nitro-o-phenylenediamine (4-NPDA), methylene blue (MB), methyl orange (MO), Rodamin B (RhB), potassium hexacyanoferrate(III) (K3[Fe(CN)6]), and hexavalent chromium (Cr(VI)) using NaBH4. The Pd@SA/Na-CMC/GA effectively catalyzed these contaminants in a short period of time under mild reaction conditions. As a result of the performed kinetics studies, rate constants were found to be 0.009 s-1, 0.016 s-1, 0.027 s-1, 0.018 s-1, 0.043 s-1, 0.058 s-1, 0.038 s-1 and 0.041 s-1 for the reduction of 4-NP, 2-NA, 4-NA, 4-NPDA, MO, RhB, K3[Fe(CN)6], and Cr(VI), respectively. Additionally, MO was immediately reduced by Pd@SA/Na-CMC/GA. The microsphere nature of Pd@SA/Na-CMC/GA allowed for easy recovery through simple filtration and successful reuse for up to six cycles.

Catalytic removal of harmful volatile organic compounds by reutilizing zinc rods waste from spent batteries as a palladium catalyst support

The emission of volatile organic compounds (VOCs) has led to significant deterioration in air quality, making it imperative to ensure that these compounds are removed from emission sources before they are released into the atmosphere. In this context, the present study recycled spent primary batteries to use their zinc rods waste (ZRW) as a palladium catalyst support for the removal of harmful VOCs. To this end, palladium supported on ZRW (Pd/ZRW) catalysts were prepared and tested for the catalytic oxidation of benzene, methylbenzene and 1,2-dimethylbenzene. The physicochemical properties of the Pd/ZRW catalysts were carefully characterized by ICP-OES, BET, SEM, XRD, FE-TEM, XPS, and H2-TPR analyses. The main component of ZRW was identified as ZnO. Consistent with expectations, increases in the loading of Pd from 0.1 to 1.0 wt% in the Pd/ZRW catalysts resulted in enhanced VOCs removal efficiency. The reaction temperature required for the complete oxidation (100% removal efficiency) of methylbenzene and 1,2-dimethylbenzene on the 1.0 wt% Pd/ZRW catalyst was below 340 °C at a gas hourly space velocity of 50,000 h-1. TEM, XPS, and H2-TPR results implied that the enhancement of catalytic activity with the addition of Pd could be attributed to the readily movable surface lattice oxygen as well as the active component (Pd species). Ultimately, ZRW of spent primary batteries appear to show promise as a catalyst support for VOCs removal. This study has introduced a novel strategy for reducing air pollutants by utilizing waste, which promotes the disposal of hazardous solid waste and ensures clean air quality.

Controllable deposition of dispersed Pd nanoparticles on ZnO for Suzuki-Miyaura cross-coupling reactions

Palladium nanoparticles find extensive applications in catalysis in both homogeneously and heterogeneously catalyzed processes. Supporting metal nanoparticles enhances their stability as compared to their unsupported counterparts. The role of catalytic support is increasingly recognized as crucial in determining the behaviour of these materials. However, controlling the deposition and anchoring of palladium nanoparticles remains a significant challenge. This contribution discusses the preparation of straight lines of palladium particles on zinc oxide by wet impregnation. This phenomenon is attributed to the highly stepped morphology of the employed ZnO that created steric anchoring sites to stabilize the metal particles. Palladium-based catalysts were evaluated for the valuable Suzuki-Miyaura cross-coupling reaction. The dispersed Pd/ZnO catalyst achieved a conversion rate of 86% with 100% selectivity, remarkably superior to that of the Pd/Al2O3 and Pd/TiO2 counterparts.

Core-shell silica@CuxZnAl LDH catalysts for efficient CO2 hydrogenation to methanol

The efficient production of methanol by reduction of CO2 using green hydrogen is a promising strategy from both a green chemistry and a carbon net zero perspective. Herein, we report the synthesis of well-dispersed core-shell catalyst precursors using silica@CuxZnAl-LDHs that can convert CO2 to methanol. The catalyst precursors can be formed using either a commercially available silica (ES757) or a mesoporous silica (e.g. MCM-48). These hybrid materials show significantly enhanced catalytic performance compared to the equivalent unsupported CuxZnAl LDH precursor. Space-time yields of up to 0.7 gMeOH gcat-1 h-1 under mild operating conditions were observed.

Heterogeneous Catalytic Systems for Carbon Dioxide Hydrogenation to Value-Added Chemicals

Utilizing renewable energy to hydrogenate carbon dioxide into fuels eliminates massive CO2 emissions from the atmosphere and diminishes our need for using fossil fuels. This review presents the most recent developments for designing heterogeneous catalysts for the hydrogenation of CO2 to formate, methanol, and C2+ hydrocarbons. Thermodynamic challenges and mechanistic insights are discussed, providing a strong foundation to propose a suitable catalyst. The main body of this review focuses on nanostructured catalysts for constructing efficient heterogeneous systems. The most important factors affecting catalytic performance are highlighted, including active metals, supports and promoters that can potentially be used. The summary of the results and the outlook are presented in the final section. During the past few decades, heterogeneous CO2 hydrogenation has gained much attention and made tremendous progress. Thus, many highly efficient catalysts have been studied to discover their active sites and provide mechanistic insights. This paper summarizes recent advances in CO2 hydrogenation and its conversion into various hydrocarbons such as formate, methanol, and C2+ products. As for formate production, Au and Ru nanocatalysts show superior activity. However, considering the catalyst cost, Cu-based catalysts have an excellent prospect for methanol production, among other catalysts. Ultra-small nanoparticles and nanoclusters appear promising to provide highly active cost-effective catalysts. A growing number of researchers are investigating the possibility of directly synthesizing C2+ products through CO2 hydrogenation. The major challenge in producing heavy hydrocarbons is breaking the ASF limitations, which have been achieved over bifunctional catalysts using zeolites. Using suitable support and promoter can lead to a superior activity, ascribed to structural, electronic, and chemical promotional effects.

Zeolite-encaged mononuclear copper centers catalyze CO2 selective hydrogenation to methanol

The selective hydrogenation of CO2 to methanol by renewable hydrogen source represents an attractive route for CO2 recycling and is carbon neutral. Stable catalysts with high activity and methanol selectivity are being vigorously pursued, and current debates on the active site and reaction pathway need to be clarified. Here, we report a design of faujasite-encaged mononuclear Cu centers, namely Cu@FAU, for this challenging reaction. Stable methanol space-time-yield (STY) of 12.8 mmol gcat-1 h-1 and methanol selectivity of 89.5% are simultaneously achieved at a relatively low reaction temperature of 513 K, making Cu@FAU a potential methanol synthesis catalyst from CO2 hydrogenation. With zeolite-encaged mononuclear Cu centers as the destined active sites, the unique reaction pathway of stepwise CO2 hydrogenation over Cu@FAU is illustrated. This work provides a clear example of catalytic reaction with explicit structure-activity relationship and highlights the power of zeolite catalysis in complex chemical transformations.

Confinement Effects in Well-Defined Metal-Organic Frameworks (MOFs) for Selective CO2 Hydrogenation: A Review

Decarbonization has become an urgent affair to restrain global warming. CO₂ hydrogenation coupled with H₂ derived from water electrolysis is considered a promising route to mitigate the negative impact of carbon emission and also promote the application of hydrogen. It is of great significance to develop catalysts with excellent performance and large-scale implementation. In the past decades, metal-organic frameworks (MOFs) have been widely involved in the rational design of catalysts for CO₂ hydrogenation due to their high surface areas, tunable porosities, well-ordered pore structures, and diversities in metals and functional groups. Confinement effects in MOFs or MOF-derived materials have been reported to promote the stability of CO₂ hydrogenation catalysts, such as molecular complexes of immobilization effect, active sites in size effect, stabilization in the encapsulation effect, and electron transfer and interfacial catalysis in the synergistic effect. This review attempts to summarize the progress of MOF-based CO₂ hydrogenation catalysts up to now, and demonstrate the synthetic strategies, unique features, and enhancement mechanisms compared with traditionally supported catalysts. Great emphasis will be placed on various confinement effects in CO₂ hydrogenation. The challenges and opportunities in precise design, synthesis, and applications of MOF-confined catalysis for CO₂ hydrogenation are also summarized.

Methanol synthesis from CO2 and H2 using supported Pd alloy catalysts

A number of Pd based materials have been synthesised and evaluated as catalysts for the conversion of carbon dioxide and hydrogen to methanol, a useful platform chemical and hydrogen storage molecule. Monometallic Pd catalysts show poor methanol selectivity, but this is improved through the formation of Pd alloys, with both PdZn and PdGa alloys showing greatly enhanced methanol productivity compared with monometallic Pd/Al2O3 and Pd/TiO2 catalysts. Catalyst characterisation shows that the 1 : 1 β-PdZn alloy is present in all Zn containing post-reaction samples, including PdZn/Ga2O3, with the Pd2Ga alloy formed for the Pd/Ga2O3 sample. The heat of mixing was calculated for a variety of alloy compositions with high values determined for both PdZn and Pd2Ga alloys, at ca. -0.6 eV per atom and ca. -0.8 eV per atom, respectively. However, ZnO is more readily reduced than Ga2O3, providing a possible explanation for the preferential formation of the PdZn alloy, rather than PdGa, when in the presence of Ga2O3.

Cr-Zn/Ni-Containing Nanocomposites as Effective Magnetically Recoverable Catalysts for CO2 Hydrogenation to Methanol: The Role of Metal Doping and Polymer Co-Support

CO2 hydrogenation to methanol is an important process that could solve the problem of emitted CO2 that contributes to environmental concern. Here we developed Cr-, Cr-Zn-, and Cr-Ni-containing nanocomposites based on a solid support (SiO2 or Al2O3) with embedded magnetic nanoparticles (NPs) and covered by a cross-linked pyridylphenylene polymer layer. The decomposition of Cr, Zn, and Ni precursors in the presence of supports containing magnetic oxide led to formation of amorphous metal oxides evenly distributed over the support-polymer space, together with the partial diffusion of metal species into magnetic NPs. We demonstrated the catalytic activity of Cr2O3 in the hydrogenation reaction of CO2 to methanol, which was further increased by 50% and 204% by incorporation of Ni and Zn species, respectively. The fine intermixing of metal species ensures an enhanced methanol productivity. Careful adjustment of constituent elements, e.g., catalytic metal, type of support, presence of magnetic NPs, and deposition of hydrophobic polymer layer contributes to the synergetic promotional effect required for activation of CO2 molecules as well. The results of catalytic recycle experiments revealed excellent stability of the catalysts due to protective role of hydrophobic polymer.

Tailoring the Dispersion of Metals on ZnO with Preadsorbed Water

The dispersity of metal particles over oxide surfaces is generally critical for the applications of the metal/oxide hybridized systems. In this work, we have experimentally investigated the hydration effect of preadsorbed water species over the Cu and Pd particles deposited on the ZnO(10-10) surface. Using scanning tunneling microscopy (STM), we clearly saw that both Cu and Pd grow as three-dimensional particles on the clean ZnO(10-10) surface but disperse into single atoms and few-atom clusters on the water-covered surfaces. Moreover, X-ray photoelectron spectroscopy (XPS) measurements revealed that Cu is readily oxidized by interacting with the molecular water while Pd tends to bind the surface hydroxyls and keep neutral status. Our work has demonstrated the effective role of the surface water in tuning the morphologies as well as electronic states of the supported metals, which may bring new insights to a number of important surface processes with water in presence.

The Activity of Ultrafine Cu Clusters Encapsulated in Nano-Zeolite for Selective Hydrogenation of CO2 to Methanol

Narrowly dispersed ultrafine Cu clusters of sizes smaller than 2.0 nm have been encapsulated in nanosized silicalite-1 zeolite through direct crystallization in the presence of Cu(en)22+ complex ions as the metal precursor. The growing silicalite-1 crystals are rich in vacancy defects and connectivity defects on the grain boundaries, where the terminating silanols promote the decomposition of Cu(en)22+, thus the deposition of ultrafine Cu species. The obtained composite material as a model catalyst is active for CO2 activation and hydrogenation to methanol. The preliminary in situ FTIR study recognizes a series of surface-adsorbed carbonyl, formyl, carbonate, and formate species when the material is exposed to CO2 and H2. Among others, the adsorbed formate decays most rapidly upon cofeeding CO2 and H2, implying that the most probable pathway toward methanol formation over this material is via the formate-mediated mechanism.

Competition between reverse water gas shift reaction and methanol synthesis from CO2: influence of copper particle size

Converting CO₂ into value-added chemicals and fuels, such as methanol, is a promising approach to limit the environmental impact of human activities. Conventional methanol synthesis catalysts have shown limited efficiency and poor stability in a CO₂/H₂ mixture. To design improved catalysts, crucial for the effective utilization of CO₂, an in-depth understanding of the active sites and reaction mechanism is desired. The catalytic performance of a series of carbon-supported Cu catalysts, with Cu particle sizes in the range of 5 to 20 nm, was evaluated under industrially relevant temperature and pressure, i.e. 260 °C and 40 bar(g). The CO₂ hydrogenation reaction exhibited clear particle size effects up to 13 nm particles, with small nanoparticles having the lower activity, but higher methanol selectivity. MeOH and CO formation showed a different size-dependence. The TOF_CO increased from 1.9 × 10^-3 s^-1 to 9.4 × 10^-3 s^-1 with Cu size increasing from 5 nm to 20 nm, while the TOF_MeOH was size-independent (8.4 × 10^-4 s^-1 on average). The apparent activation energies for MeOH and CO formation were size-independent with values of 63 ± 7 kJ mol^-1 and 118 ± 6 kJ mol^-1, respectively. Hence the size dependence was ascribed to a decrease in the fraction of active sites suitable for CO formation with decreasing particle size. Theoretical models and DFT calculations showed that the origin of the particle size effect is most likely related to the differences in formate coverage for different Cu facets whose abundancy depends on particle size. Hence, the CO₂ hydrogenation reaction is intrinsically sensitive to the Cu particle size.

Hydrogenation of CO2 to methanol over In-doped m-ZrO2: a DFT investigation into the oxygen vacancy size-dependent reaction mechanism

Selective methanol synthesis via CO₂ hydrogenation has been thoroughly investigated over defective In-doped m-ZrO₂ using density functional theory (DFT). Three types of oxygen vacancies (Ovs) generated either at the top layer (O1_v and O4_v) or at the subsurface layer (O2_v) are chosen as surface models due to low Ov formation energy. Surface morphology reveals that O1_v has smaller oxygen vacancy size than O4_v. Compared with perfect In@m-ZrO₂, indium on both O1_v and O4_v is partially reduced, whereas the Bader charge of In on O2_v remains almost the same. Our calculations show that CO₂ is moderate in adsorption energy (∼-0.8 eV) for all investigated surface models, which facilitates the formate pathway for both O1_v and O4_v. O2_v is not directly involved in CO₂ methanolization but could readily transform into O1_v once CO₂/H₂ feed gas is introduced. Based on the results, the synthesis of methanol from CO₂ hydrogenation turns out to exhibit conspicuous vacancy size-dependency for both O1_v and O4_v. The reaction mechanism for small-sized O1_v is controlled by both the vacancy size effect and surface reducibility effect. Thus, H₂COO* favors direct C-O bond cleavage (c-mechanism) before further hydrogenation to methanol, which is similar to the defective In₂O₃. The vacancy size effect is more competitive than the surface reducibility effect for large-sized O4_v. Therefore, H₂COO* prefers protonation to H₂COOH before C-O bond cleavage (p-mechanism) which is similar to the ZnO-ZrO₂ solid solution. Furthermore, we also determined that stable-CH₃O*, which is too stable to be hydrogenated, originates from the O1_v surface. In contrast, CH₃O* with similar configuration is allowed to be further converted to methanol on O4_v. Overall, our findings offer a new perspective towards how reaction mechanisms are determined by the size of oxygen vacancies.

Reaction-driven selective CO2 hydrogenation to formic acid on Pd(111)

Conversion of CO₂ to useful fuels and chemicals has gained great attention in the past decades; yet the challenge persists due to the inert nature of CO₂ and the wide range of products formed. Pd-based catalysts are extensively studied to facilitate CO₂ hydrogenation to methanol via a reverse water gas shift (rWGS) pathway or formate pathway where formic acid may serve as an intermediate species. Here, we report the selective production of formic acid on the stable Pd(111) surface phase under CO₂ hydrogenation conditions, which is fully covered by chemisorbed hydrogen, using combined Density Functional Theory (DFT) and Kinetic Monte Carlo (KMC) simulations. The results show that with the full coverage of hydrogen, instead of producing methanol as reported for Pd(111), the CO₂ activation is highly selective to formic acid via a multi-step process involving the carboxyl intermediate. The high formic acid selectivity is associated with surface hydrogen species on Pd(111), which not only acts as a hydrogen reservoir to facilitate the hydrogenation steps, but also enables the formation of confined vacancy sites to facilitate the production and removal of formic acid. Our study highlights the importance of reactive environments, which can transform the surface structures and thus tune the activity/selectivity of catalysts.

In Situ Spectroscopic Characterization and Theoretical Calculations Identify Partially Reduced ZnO1-x /Cu Interfaces for Methanol Synthesis from CO2

The active site of the industrial Cu/ZnO/Al₂ O₃ catalyst used in CO₂ hydrogenation to methanol has been debated for decades. Grand challenges remain in the characterization of structure, composition, and chemical state, both microscopically and spectroscopically, and complete theoretical calculations are limited when it comes to describing the intrinsic activity of the catalyst over the diverse range of structures that emerge under realistic conditions. Here a series of inverse model catalysts of ZnO on copper hydroxide were prepared where the size of ZnO was precisely tuned from atomically dispersed species to nanoparticles using atomic layer deposition. ZnO decoration boosted methanol formation to a rate of 877 g_MeOH  kg_cat ^-1  h^-1 with ≈80 % selectivity at 493 K. High pressure in situ X-ray absorption spectroscopy demonstrated that the atomically dispersed ZnO species are prone to aggregate at oxygen-deficient ZnO ensembles instead of forming CuZn metal alloys. By modeling various potential active structures, density functional theory calculations and microkinetic simulations revealed that ZnO/Cu interfaces with oxygen vacancies, rather than stoichiometric interfaces, Cu and CuZn alloys were essential to catalytic activation.

The Critical Role of βPdZn Alloy in Pd/ZnO Catalysts for the Hydrogenation of Carbon Dioxide to Methanol

The rise in atmospheric CO₂ concentration and the concomitant rise in global surface temperature have prompted massive research effort in designing catalytic routes to utilize CO₂ as a feedstock. Prime among these is the hydrogenation of CO₂ to make methanol, which is a key commodity chemical intermediate, a hydrogen storage molecule, and a possible future fuel for transport sectors that cannot be electrified. Pd/ZnO has been identified as an effective candidate as a catalyst for this reaction, yet there has been no attempt to gain a fundamental understanding of how this catalyst works and more importantly to establish specific design criteria for CO₂ hydrogenation catalysts. Here, we show that Pd/ZnO catalysts have the same metal particle composition, irrespective of the different synthesis procedures and types of ZnO used here. We demonstrate that all of these Pd/ZnO catalysts exhibit the same activity trend. In all cases, the β-PdZn 1:1 alloy is produced and dictates the catalysis. This conclusion is further supported by the relationship between conversion and selectivity and their small variation with ZnO surface area in the range 6–80 m²g^–1. Without alloying with Zn, Pd is a reverse water-gas shift catalyst and when supported on alumina and silica is much less active for CO₂ conversion to methanol than on ZnO. Our approach is applicable to the discovery and design of improved catalysts for CO₂ hydrogenation and will aid future catalyst discovery.

A computational study of direct CO2 hydrogenation to methanol on Pd surfaces

The reaction mechanism of direct CO₂ hydrogenation to methanol is investigated in detail on Pd (111), (100) and (110) surfaces using density functional theory (DFT), supporting investigations into emergent Pd-based catalysts. Hydrogen adsorption and surface mobility are firstly considered, with high-coordination surface sites having the largest adsorption energy and being connected by diffusion channels with low energy barriers. Surface chemisorption of CO₂, forming a partially charged CO₂^δ-, is weakly endothermic on a Pd (111) whilst slightly exothermic on Pd (100) and (110), with adsorption enthalpies of 0.09, -0.09 and -0.19 eV, respectively; the low stability of CO₂^δ- on the Pd (111) surface is attributed to negative charge accumulating on the surface Pd atoms that interact directly with the CO₂^δ- adsorbate. Detailed consideration for sequential hydrogenation of the CO₂ shows that HCOOH hydrogenation to H₂COOH would be the rate determining step in the conversion to methanol, for all surfaces, with activation barriers of 1.41, 1.51, and 0.84 eV on Pd (111), (100) and (110) facets, respectively. The Pd (110) surface exhibits overall lower activation energies than the most studied Pd (111) and (100) surfaces, and therefore should be considered in more detail in future Pd catalytic studies.