Catalysts For CO2 Hydrogenation Research Articles

Enhancing methanol selectivity of commercial Cu/ZnO/Al2O3 catalyst in CO2 hydrogenation by surface silylation

Suppressing reverse water-gas-shift (RWGS) reaction is high desirable but challenging and underdeveloped for Cu/ZnO catalysts, particularly for commercial Cu/ZnO/Al2O3 catalysts. Different from the current methodologies to reduce RWGS reaction, we report a simply surface silylation method for efficiently minimizing RWGS reaction over a commercial Cu/ZnO/Al2O3 catalyst. This method suppresses STYCO (Space-time yield) from 97.4 to 0.7 gCO·kgcat−1·h−1, improving STYMeOH from 20.2 to 39.9 gMeOH·kgcat−1·h−1 and methanol selectivity from 15.1 to 92.9 mol%. The combination of characterization methods and density functional theory calculations provide insight into the suppressing mechanism of surface silylation on catalyst. A hydroxyl (on ZnO)-promoted RWGS reaction cycle is discovered, which can be efficiently inhibited by the consuming of hydroxyls via surface silyation. Our results provide a way to regulate RWGS reaction on Cu/ZnO-based catalysts and are expected to the further use of silylation strategy to tune the interconversion of CO and CO2 via RWGS/WGS reaction on hydrogenation catalysts.

Optimal mixing method of ZnZrOx and MOR-type zeolite to prepare a bifunctional catalyst for CO2 hydrogenation to lower olefins

In recent years, the issue of global warming caused by CO2 emissions has become a critical problem and poses a worldwide challenge. To address this issue, we have developed a bifunctional catalyst that can convert CO2 to lower olefins in a single stage. Our bifunctional catalysts are a combination of two catalysts, a CO2-to-methanol hydrogenation catalyst (Zn-doped ZrO2, named ZnZrOx) and a methanol-to-olefin catalyst (MOR-type zeolite, named MOR104). In this research, we examined the impact of various mixing modes of the two catalysts on product distribution. We tested four different mixing modes using a down-flow fixed bed reactor: (a) ZnZrOx in the upper layer and MOR104 in the lower layer, (b) catalysts were mixed randomly after being pelletized separately, (c) MOR104 in the upper layer and ZnZrOx in the lower layer, and (d) granulated catalyst produced by physically mixing both catalyst powders. Our findings indicated that the best performance was achieved with catalyst (d), where the two catalysts were mixed in close proximity. This proximity resulted in efficient supply of methanol produced on ZnZrOx to MOR104. In other words, the MTO reaction in MOR104 efficiently consumed methanol molecules produced via equilibrium-limited CO2-to-methanol hydrogenation. When ZnZrOx and MOR104 were thoroughly mixed, the conversion of CO2 to methanol shifted towards the product side, resulting in a greater overall utilization of CO2. Furthermore, the bifunctional catalyst we developed was stable for six hours. Since there have been few studies of bifunctional catalysts containing zeolites other than ZSM-5 and SAPO-34, this study opens up new opportunities for bifunctional catalysts specialized for one-pass hydrocarbon synthesis through CO2 hydrogenation.

Highly Selective Conversion of Carbon Dioxide to Methanol through a Cu−ZnO−Al2O3−ZrO2/Cu−MOR Tandem Catalyst

AbstractMethanol formation from CO2 hydrogenation attracts great attention in view of utilization of carbon resources. However, CO2 transformation to methanol is challenging because of the thermodynamic equilibrium restriction and water‐caused catalyst deactivation. It is desired, therefore, to develop highly active, selective and stable catalysts for CO2 hydrogenation to methanol. Herein, we propose a novel tandem catalyst composed of Cu−ZnO−Al2O3−ZrO2 (CZAZ) and Cu−MOR for highly selective conversion of CO2 to methanol. During CO2 hydrogenation by the CZAZ catalyst, the by‐product methane is continuously transformed to methanol through reaction with water via the Cu−MOR catalyst, thus enhancing CO2 conversion and methanol selectivity. Under mild reaction conditions (200 °C and 3.0 MPa), high CO2 conversion (40.7 %) and methanol selectivity (97.6 %) are achieved, outperforming state‐of‐the‐art CO2 hydrogenation catalysts. Further, water‐caused deactivation of the catalyst through aggregation and densification is suppressed owing to water consumption via methane oxidation to methanol, validating a high CZAZ/Cu−MOR tandem catalyst stability.

A principal component analysis assisted machine learning modeling and validation of methanol formation over Cu-based catalysts in direct CO2 hydrogenation

With the advent of powerful machine learning algorithms strongly supporting complex non-linear regression modeling, catalyst design features for a wide and customized set of catalysts used for a specific reaction have been made easy. Herein, we use these techniques in the methanol synthesis by CO2 reduction over Cu-based binary and ternary catalysts with the help of three machine learning algorithms: artificial neural network, support vector machine regression, and gaussian process regression. 227 catalytic performance dataset points from existing literature on CO2 hydrogenation to methanol were compiled and initially accessed by Principal Component Analysis (PCA) for training and preliminary evaluation of the algorithms, which was further guided using a 10-fold cross-validation method. The predictive model and its insights were validated experimentally over 30 datasets derived from experimental runs of this reaction over a ternary Cu/ZnO/ZrO2 laboratory-synthesized catalyst at varying conditions of temperature, pressure, and space velocity in a continuous mode fixed-bed plug-flow reactor. The assessment of the space of input and laboratory data was aided by Principal Component Analysis (PCA), scores, and loadings plot. This work shows how experimentalists can predict typical heterogeneous catalytic reaction outputs (R2 greater than 0.9 for three variables) with fair accuracy using a combination of machine learning and PCA.

The role of Na for efficient CO2 hydrogenation to higher hydrocarbons over Fe-based catalysts under externally forced dynamic conditions

Considering the on-going developments of approaches for CO2 capture, storage, and utilization, CO2 hydrogenation with renewable H2 can become sustainable technology to produce value-added chemicals/fuels in future feedstock scenarios. Herein, we investigate temporal behaviors of Fe-based catalysts in CO2 hydrogenation under externally forced dynamics of reactor operation from the respective of composition and performance. Different catalyst preparation methods were applied to modulate the surface concentration of Na promoter but keeping the bulk concentration constant. The crucial promoter role in reserving catalyst performance and (un)steady-state catalyst composition during dynamic reactor operation was investigated by means of sophisticated catalytic tests combined with in situ and ex situ catalyst characterizations. The surface Na/Fe ratio of spent catalysts was identified as the decisive descriptor for production of the desired products under dynamic CO2 hydrogenation conditions. The derived knowledge could be useful for redesigning catalysts and optimizing reaction processes when employing fluctuating energy supply.

Recent Progress in Nickel and Silica Containing Catalysts for CO2 Hydrogenation to CH4

The recent unusual weather changes occurring in different parts of the world are caused by global warming, a consequence of the release of extreme amounts of greenhouse gases into the atmosphere. Carbon dioxide (CO2) is one of these greenhouse gasses, which can be captured and reused to generate fuel through the methanation process. Nickel- and silica-based catalysts have been recognized as promising catalysts due to their efficiency, availability, and low prices. However, these catalysts suffer from metal sintering at high temperatures. Researchers have achieved remarkable improvements through altering conventional synthesis methods, supports, metal loading amounts, and promoters. The modified routes have enhanced stability and activity while the supports offer large surface areas, dispersion, and strong metal–support interactions. Nickel loading affects the formed structure and catalytic activity, whereas doping causes CO2 conversion at low temperatures and forms basic sites. This review aims to discuss the CO2 methanation process over Ni- and SiO2-based catalysts, in particular the silica-supported Ni metal in previously reported research works and point out directions for potential future work.

Exploiting the latency of carbon as catalyst in CO2 hydrogenation

The versatility of carbon has made it propitious for catalyst fabrication across various sectors of heterogeneous catalysis. Herein, a fundamental insight into the effects of carbon-supports in CO2 hydrogenation is established by facilely preparing and investigating diversely configured carbon supported Fe catalysts. Although the active component remains Fe, unique synergies attained over the Fe-carbon interfaces created an ambience disparate among the catalysts for enhanced activity and selective distribution. Generally, the carbons facilitated C–C coupling but more C5 + hydrocarbons are observed in the products when there is a stronger interaction between the carbon support and Fe phases. The selectivity for C5+ hydrocarbons reached the highest of ∼ 40 % in total hydrocarbon products when Fe and Na were encapsulated in carbon. Carbide formation was eased as a result of enhanced CO2 adsorption by the carbon supports whereas some surface functional groups attached to carbon surfaces improved C–C coupling to olefins and heavy hydrocarbons. The results presented maintains carbon as a promising catalytic material while the emphasis on structure, configuration, and surface properties reveal essentials in designing carbon-based catalysts in heterogeneous catalysis.

The influence of the compositions and structures of Cu-ZrO2 catalysts on the catalytic performance of CO2 hydrogenation to CH3OH

In this paper, Cu-ZrO2 catalysts were prepared and used for CH3OH synthesis through CO2 hydrogenation to clarify the influence of their compositions and structures on the catalytic performance. We mainly address the problem that how the oxygen vacancy and Cu-ZrO2 interface affect the catalytic performance jointly, because the previous articles only pay attention to the influence of the interface on CH3OH synthesis. Combined with experiments and various structural characterizations, we propose that oxygen vacancies play an essential role in CO2 activation instead of the interface; CH3OH selectivity is determined by the low-valence Cu species (Cu0 and Cu+) and ZrO2 interface (the main factor), and the pore size. The in-situ DRIFTS demonstrates that the conversion of *HCOO active intermediates to *CH3O on the optimal catalyst (CuZr-1) is faster. DFT calculations also shed light on the importance of oxygen vacancy in CO2 adsorption and the formate pathway. We believe this work provides useful insight for researchers to design highly efficient Cu-based catalysts for CO2 hydrogenation to CH3OH.

Improved copper-based catalysts for alcohol-assisted CO2 hydrogenation: A comparative study between novel nano-stabilized and deposition-precipitation method

A growing awareness of the need to control CO2 concentration in atmosphere spurs the demand for novel technologies to convert CO2 to useful chemicals. The conversion of CO2 into high-quality liquid fuel such as methanol is a primary focus, and using active catalysts is the key to achieving successful methanol synthesis from CO2 hydrogenation under mild conditions. A new way of synthesizing catalysts with highly dispersed nano copper particles, as well as the characteristics of the catalysts are reported in this study. This is the first instance of methanol catalysts being prepared using a combination of copolymer of polyethylene glycol and maleic acid (PEG-MA) as a stabilizer and layered hydrotalcite (HTC) as a support material. This resulted in a significant decrease in particle size and exhibited efficient catalytic performance in a 1-butanol promoted CO2 hydrogenation reaction at 170 °C, 3 MPa. The work also confirms the status of Zn as an influential factor for methanol synthesis. The application of ion-exchange process and stabilizers in catalyst preparation process effectively improves the CO2 conversion rate and methanol selectivity, which also inspires the development of catalysts in various fields.

Spatially resolved analysis of CO2 hydrogenation to higher hydrocarbons over alkali-metal promoted well-defined FexOyCz

A series of catalysts for CO2 hydrogenation to higher (C2+) hydrocarbons were prepared through controlled decomposition of iron (II) oxalate dihydrate impregnated with Li, Na, K, Rb or Cs carbonate. They consist of Fe3O4, Fe5C2, Fe3C and Fe. Their steady-state spatial phase distribution is, however, affected by the presence and the kind of the promoter. These structural characteristics determine catalyst activity and product selectivity. Fe3C seems to favor CH4 formation, while C2+-hydrocarbons are formed over Fe5C2. A positive relationship between the rate of C2+-hydrocarbons formation and the rate constant of dissociation of adsorbed CO2 was established, while an opposite dependence is valid for the rate of CO2 methanation. The rate constant of this reaction increases in the presence of alkali metal and with an increase in its atomic number. Therefore, CH4 is mainly formed over Rb- or Cs-doped catalysts through CO hydrogenation, while CO2 methanation prevails over the remaining catalysts.

Surface Redox Dynamics in Gold-Zinc CO2 Hydrogenation Catalysts.

Au-Zn catalysts have previously been shown to promote the hydrogenation of CO2 to methanol, but their active state is poorly understood. Here, silica-supported bimetallic Au-Zn alloys, prepared by surface organometallic chemistry (SOMC), are shown to be proficient catalysts for hydrogenation of CO2 to methanol. In situ X-ray absorption spectroscopy (XAS), in conjunction with gas-switching experiments, is used to amplify subtle changes occurring at the surface of this tailored catalyst during reaction. Consequently, an Au-Zn alloy is identified and is shown to undergo subsequent reversible redox changes under reaction conditions according to multivariate curve resolution alternating least-squares (MCR-ALS) analysis. These results highlight the role of alloying and dealloying in Au-based CO2 hydrogenation catalysts and illustrate the role of these reversible processes in driving reactivity.

Highly dispersed Fe/EG catalysts assisted by ammonium citrate and their application in CO2 hydrogenation to olefins

This study employed ammonium citrate as a small molecule additive to modulate the structure of the expanded graphite-loaded iron-based catalyst and its performance in CO2 hydrogenation to olefins. Through catalytic activity, the optimal performance was determined for a 1:1 M ratio of ammonium citrate to Fe salt precursor. The addition of ammonium citrate facilitated the dispersion of Fe particles by increasing the steric hindrance between Fe3+ and anchoring the Fe particles with amorphous carbon generated during calcination, resulting in a decrease in the particle size of iron particles from 20 to 45 nm to 12–27 nm. Furthermore, the nitrogen doping of the support was increased from 0.56% to 2.57%, which provided a strong electron-donating effect and facilitated the reduction and carbonization of iron species. The space time yield (STY) of light olefins was raised from 432.6 mg·gFe-1·h−1 to 604.1 mg·gFe-1·h−1, with a maximum O/P of 5.7. These results demonstrate the effectiveness of using ammonium citrate as an additive for improving the performance of expanded graphite-loaded iron-based catalysts in CO2 hydrogenation to olefins.

First-principles and microkinetic simulation studies of CO2 hydrogenation mechanism and active site on MoS2 catalyst

Molybdenum disulphide (MoS2) has attracted much attention as a promising non-precious metal catalyst for CO2 hydrogenation to energy-rich commodities. However, the exact reaction mechanism and active site on the MoS2 catalyst are still a matter of debate. Using density functional theory (DFT) calculations and microkinetic modeling, we study the competitive pathways leading to the formation of CO, methane and methanol at the S edge site in MoS2 catalyst, and the results are compared to those on the Mo edge and MoS2(001) in our previous work. It is found that CO is exclusively produced through redox mechanism on the S edge at 580–780 K, 1 bar and H2/CO2 ratio of 1, with the rate determining step of C-O bond scission in CO2. The rate of CO formation follows the order of Mo edge > S edge > MoS2(001), suggesting the Mo edge as the likely active site on the MoS2 catalyst. This work offers a mechanistic understanding toward CO2 hydrogenation on the MoS2 catalyst at the atomic level, and the insights gained can be used to design improved catalysts for the CO2 and CO hydrogenation and other important reactions of technological interest.

Tuning the content of S vacancies in MoS2 by Cu doping for enhancing catalytic hydrogenation of CO2 to methanol

Hydrogenation of CO2 to methanol is one of effective ways to promote "carbon neutrality". The activity of MoS2 as an efficient catalyst for the catalytic hydrogenation of CO2 to methanol is influenced by the content of in-plane S vacancies. Theoretically, the higher the density of in-plane S vacancies is, the better the performance of the catalyst is. In this work, a series of MoS2 nanosheets with different copper (Cu) dopant concentrations were prepared via hydrothermal method. It was found that the highest selectivity of MoS2-catalyzed CO2 hydrogenation to methanol along with the spatial-temporal yield was achieved at the Cu doping of 5%. Compared to the undoped MoS2 catalyst, the increase in methanol selectivity and spatial-temporal yield after doping was 13.37% and 2.27 times, respectively. This was due to the fact that Cu doping multiplied the number of S vacancies on MoS2, which altered the microstructure and chemical properties of the catalyst. However, a further increase in Cu dopant concentration reduced the S vacancy content on the MoS2 substrate, hindering the hydrogenation of the decomposed CO and thus decreasing the methanol yield. Therefore, a new technique to enhance MoS2-catalyzed CO2 hydrogenation to methanol was proposed.

Role of the structure and morphology of zirconia in ZnO/ZrO2 catalyst for CO2 hydrogenation to methanol

The global carbon dioxide (CO2) emission problem is getting increasingly severe nowadays. Hence a suitable CO2 hydrogenation technology to obtain methanol is an efficient way to achieve carbon neutrality. Zinc oxide (ZnO) nanoparticles were loaded on zirconium dioxide (ZrO2) with different morphologies via the impregnation method. This was followed by the investigation of the effect of zirconia structure and morphology on the interfacial synergy manifested in the catalysts and the catalytic performance in CO2 hydrogenation. The results showed that the CO2 conversion of the ZnO/ZrO2 catalyst possessing a hollow nanoframe-like morphology was significantly enhanced. This catalyst also exhibited the highest methanol space-time yield, which was 1.78 times higher than that of the ZnO/ZrO2 catalyst having an aggregate shape of irregular particles. The unique morphology effectively inhibited the transition of the metastable tetragonal ZrO2 (t-ZrO2) to the monoclinic ZrO2 (m-ZrO2) while forming a special interfacial structure with the loaded ZnO to strengthen the interfacial interaction. This enhanced the hydrogen activation capacity and generated more oxygen vacancies favorable to the activation of CO2.

Crucial Role of Surface FeOx Components on Supported Fe-Based Nanocatalysts for CO2 Hydrogenation to Light Olefins

Currently, Fe-based catalysts show excellent catalytic performance in the hydrogenation of CO2 to produce light olefins. Despite numerous studies, the identification of the role of catalytically active components over Fe-based catalysts for CO2 hydrogenation is not quite clear. In this work, a series of ZrO2-supported cobalt-doped Fe-based catalysts were fabricated by a microliquid film reactor-assisted coprecipitation method. It was demonstrated that appropriately strong interactions between Fe-containing species and the ZrO2 support in catalyst precursors could facilitate the generation of a defective FeOx component upon reduction treatment and inhibit the carburization of metallic iron during the reaction. Among all Fe-based catalysts, the catalyst bearing a (Co + Fe)/ZrO2 mass ratio of 3:7 and a Co/Fe molar ratio of 1:9 achieved a highest light olefins selectivity of about 38% at 49% CO2 conversion under reaction conditions (i.e., 320 °C, 2.0 MPa, and 4800 mL·gcat–1·h–1), along with a high Fe time yield to light olefins of 9.2 μmolCO2·gFe–1·s–1 and a low CO selectivity of 4.7%. Comprehensive structural characterization and catalytic CO2 hydrogenation experiments clarified that the surface-dominant defect-rich FeOx component as catalytically active species substantially played a crucial role in governing the hydrogenation of CO2, mainly owing to their enhanced adsorption and binding ability for CO2 and the CO intermediate. This study offers a new strategy to design Fe-based catalysts and provides deep insight into the role of iron oxide species for CO2 hydrogenation to produce light olefins.

CO2 hydrogenation to light olefins using In2O3 and SSZ-13 catalyst − Understanding the role of zeolite acidity in olefin production

With the aim to explore the effect of acidic properties of zeolites in tandem catalysts on their performance for CO2 hydrogenation, two types of SSZ-13 zeolites with similar bulk composition, but different arrangements of framework Al, were prepared. Their morphology, pore structure, distribution of framework Al, surface acid strength and density, were explored. The results showed that SSZ-13 zeolites with isolated aluminum distribution could be successfully synthesized, however, they contained structural defects. During calcination, the framework underwent dealumination, resulting in weaker Brønsted acidity and lower crystallinity. The morphologies were, however, well preserved. Compared with the SSZ-13 zeolites, synthesized conventionally, these low acidity SSZ-13 zeolites with isolated aluminum were good zeolite components in bifunctional catalysts for CO2 hydrogenation to light olefins. By combining with In2O3, they exhibited better catalytic performance for light olefin production during CO2 hydrogenation at low temperatures. Na+ cation exchange was used to adjust the Brønsted acid site (BAS) density with only minor changes to the cavity structure. Comparative experiments established that the BAS density of the zeolite, rather than the framework Al distribution (BAS distribution), overwhelmingly affected catalyst stability and product selectivity. A higher acid density reduced the selectivity for light olefins, while lower acid density tended to form inert coke species leading to rapid deactivation. The ideal amount of BAS density in the bifunctional catalyst was approximately 0.25 mmol/g, which exhibited 70% selectivity for light olefins among hydrocarbons, and 74% selectivity for CO without deactivation, after 12 h reaction at 325 ℃ and 10 bar.

Coverage effect of surface oxygen vacancy on In2O3-catalyzed CO2 hydrogenation revealed by first principles-based microkinetic simulations

Density functional theory (DFT) calculations and microkinetic simulations were performed to study the effect of the surface oxygen vacancy (OV) coverage on the In2O3 catalyst for methanol synthesis from CO2 hydrogenation, focusing on the In2O3(110) surface with a high OV coverage of 1 ML (In2O3(110)-allov) and compared with that with a low OV coverage of < 0.1 ML (In2O3(110)-vac). We find that the In2O3(110)-allov surface is less favorable for CO2 and H2 adsorptions than the In2O3(110)-vac surface, but the former has lower energy barriers for both the HCOO and COOH routes than the latter. Furthermore, the high OV coverage for the In2O3(110)-allov surface results in a significant lower CO2 reactivity, but gives surprisingly higher CH3OH selectivity. Our simulations may explain the experimentally observed deactivation of the In2O3 catalyst when over-reduced towards metallic In. Physical insights from this work should contribute to the rational design of efficient oxide-based CO2 hydrogenation catalysts.

Cu2In alloy-embedded ZrO2 catalysts for efficient CO2 hydrogenation to methanol: promotion of plasma modification.

Cu1In2Zr4-O-C catalysts with Cu2In alloy structure were prepared by using the sol-gel method. Cu1In2Zr4-O-PC and Cu1In2Zr4-O-CP catalysts were obtained from plasma-modified Cu1In2Zr4-O-C before and after calcination, respectively. Under the conditions of reaction temperature 270°C, reaction pressure 2MPa, CO2/H2 = 1/3, and GHSV = 12,000mL/(g h), Cu1In2Zr4-O-PC catalyst has a high CO2 conversion of 13.3%, methanol selectivity of 74.3%, and CH3OH space-time yield of 3.26mmol/gcat/h. The characterization results of X-ray diffraction (XRD), scanning electron microscopy (SEM), and temperature-programmed reduction chemisorption (H2-TPR) showed that the plasma-modified catalyst had a low crystallinity, small particle size, good dispersion, and excellent reduction performance, leading to a better activity and selectivity. Through plasma modification, the strong interaction between Cu and In in Cu1In2Zr4-O-CP catalyst, the shift of Cu 2p orbital binding energy to a lower position, and the decrease in reduction temperature all indicate that the reduction ability of Cu1In2Zr4-O-CP catalyst is enhanced, and the CO2 hydrogenation activity is improved.

Hydrogenation of CO2 to formic acid over efficient heterogeneous ruthenium oxide supported on cubic phase zirconium oxide catalyst

The development of active, cost-effective heterogeneous catalysts for the hydrogenation of CO2 to chemicals is currently progressing rapidly. In this study, we have demonstrated a significantly lower weight percentage (wt.%) RuO2/ZrO2 oxide catalyst that is effective for CO2 hydrogenation to formic acid (FA) synthesis. RuO2/ZrO2 was synthesized using a co-precipitation followed by a hydrothermal method. The X-ray diffraction pattern exhibits the cubic phase of ZrO2 with an average crystal size of 80.7 nm and tetragonal Ru oxide. Scanning electron microscopy and transmission electron microscopy confirmed that RuO2 was uniformly dispersed on the ZrO2 surface, with fringes measuring 0.22 and 0.25 nm corresponding to tetragonal Ru, which were incorporated in the ZrO2 oxide. XPS analysis showed that the Ru3/4+ chemical state strongly interacts with the ZrO2 facet, as confirmed via the Ru (110) and (211) planes. The outstanding performance of the 2.7 wt% RuO2/ZrO2 catalyst for CO2 hydrogenation yielded 40 mmol of FA with 39.7 TON. Based on the above investigations, we proposed a plausible mechanistic pathway for the hydrogenation of CO2 to FA over a RuO2/ZrO2 catalyst.