Tuning product selectivity in CO2 hydrogenation over metal-based catalysts

Conversion of CO₂ into chemicals is a promising strategy for CO₂ utilization, but its intricate transformation pathways and insufficient product selectivity still pose challenges. Exploiting new catalysts for tuning product selectivity in CO₂ hydrogenation is important to improve the viability of this technology, where reverse water-gas shift (RWGS) and methanation as competitive reactions play key roles in controlling product selectivity in CO₂ hydrogenation. So far, a series of metal-based catalysts with adjustable strong metal–support interactions, metal surface structure, and local environment of active sites have been developed, significantly tuning the product selectivity in CO₂ hydrogenation. Herein, we describe the recent advances in the fundamental understanding of the two reactions in CO₂ hydrogenation, in terms of emerging new catalysts which regulate the catalytic structure and switch reaction pathways, where the strong metal–support interactions, metal surface structure, and local environment of the active sites are particularly discussed. They are expected to enable efficient catalyst design for minimizing the deep hydrogenation and controlling the reaction towards the RWGS reaction. Finally, the potential utilization of these strategies for improving the performance of industrial catalysts is examined.

A series of metal oxide, phosphate, alloy, and carbide-based catalysts for selective CO₂ hydrogenation are summarized, showing their abilities to switch CO₂ methanation to RWGS.     

Methanol Synthesis at a Wide Range of H2/CO2 Ratios over a Rh-In Bimetallic Catalyst

There is increasing interest in capturing H₂ generated from renewables with CO₂ to produce methanol. However, renewable hydrogen production is expensive and in limited quantity compared to CO₂. Excess CO₂ and limited H₂ in the feedstock gas is not favorable for CO₂ hydrogenation to methanol, causing low activity and poor methanol selectivity. Now, a class of Rh-In catalysts with optimal adsorption properties to the intermediates of methanol production is presented. The Rh-In catalyst can effectively catalyze methanol synthesis but inhibit the reverse water-gas shift reaction under H₂-deficient gas flow and shows the best competitive methanol productivity under industrially applicable conditions in comparison with reported values. This work demonstrates a strong potential of Rh-In bimetallic composition, from which a convenient methanol synthesis based on flexible feedstock compositions (such as H₂/CO₂ from biomass derivatives) with lower energy cost can be established.     

Methanol Synthesis at a Wide Range of H2/CO2 Ratios over a Rh‐In Bimetallic Catalyst

There is increasing interest in capturing H₂ generated from renewables with CO₂ to produce methanol. However, renewable hydrogen production is expensive and in limited quantity compared to CO₂. Excess CO₂ and limited H₂ in the feedstock gas is not favorable for CO₂ hydrogenation to methanol, causing low activity and poor methanol selectivity. Now, a class of Rh‐In catalysts with optimal adsorption properties to the intermediates of methanol production is presented. The Rh‐In catalyst can effectively catalyze methanol synthesis but inhibit the reverse water‐gas shift reaction under H₂‐deficient gas flow and shows the best competitive methanol productivity under industrially applicable conditions in comparison with reported values. This work demonstrates a strong potential of Rh‐In bimetallic composition, from which a convenient methanol synthesis based on flexible feedstock compositions (such as H₂/CO₂ from biomass derivatives) with lower energy cost can be established.     

Carbon Dynamics on the Molybdenum Carbide Surface during Catalytic Propane Dehydrogenation

The effect of the gas‐phase chemical potential on surface chemistry and reactivity of molybdenum carbide has been investigated in catalytic reactions of propane in oxidizing and reducing reactant mixtures by adding H₂, O₂, H₂O, and CO₂ to a C₃H₈/N₂ feed. The balance between surface oxidation state, phase stability, carbon deposition, and the complex reaction network involving dehydrogenation reactions, hydrogenolysis, metathesis, water‐gas shift reaction, hydrogenation, and steam reforming is discussed. Raman spectroscopy and a surface‐sensitive study by means of in situ X‐ray photoelectron spectroscopy evidence that the dynamic formation of surface carbon species under a reducing atmosphere strongly shifts the product spectrum to the C₃‐alkene at the expense of hydrogenolysis products. A similar response of selectivity, which is accompanied by a boost of activity, is observed by tuning the oxidation state of Mo in the presence of mild oxidants, such as H₂O and CO₂, in the feed as well as by V doping. The results obtained allow us to draw a picture of the active catalyst surface and to propose a structure–activity correlation as a map for catalyst optimization.     

Highly Active Hydrogen-rich Photothermal Reverse Water Gas Shift Reaction on Ni/LaInO3 Perovskite Catalysts with Near-unity Selectivity

Photo-assisted reverse water gas shift (RWGS) reaction is regarded green and promising in controlling the reaction gas ratio in Fischer Tropsch synthesis. But it is inclined to produce more byproducts in high H₂ concentration condition. Herein, LaInO₃ loaded with Ni-nanoparticles (Ni NPs) was designed to obtain an efficient photothermal RWGS reaction rate, where LaInO₃ was enriched with oxygen vacancies to roundly adsorbing CO₂ and the strong interaction with Ni NPs endowed the catalysts with powerful H₂ activity. The optimized catalyst performed a large CO yield rate (1314 mmol g_Ni⁻¹ h⁻¹) and ≈100 % selectivity. In situ characterizations demonstrated a COOH* pathway of the reaction and photoinduced charge transfer process for reducing the RWGS reaction active energy. Our work provides valuable insights on the construction of catalysts concerning products selectivity and photoelectronic activating mechanism on CO₂ hydrogenation.     

多孔SiO_2·xH_2O负载RuB纳米粒子催化喹啉加氢反应

通过水解,聚乙烯吡咯烷酮(PVP)保护,NaOH刻蚀等方法制备了多孔及富含表面羟基的SiO2·xH2O负载的RuB催化剂RuB/SiO2·xH2O,并用X射线衍射(XRD)、X射线光电子能谱(XPS)、透射电子显微镜(TEM)、傅里叶变换红外(FT-IR)光谱和BET(Brunauer-Emmett-Teller)等手段对该催化剂进行了表征.结果表明该催化剂具有良好的抗中毒能力,在3.0MPa的H2压力和80℃的温和反应条件下,喹啉的转化率高于95%,生成1,2,3,4-四氢喹啉的选择性高于97%.并系统研究了表面羟基和溶剂对催化剂性能的影响,发现以水为溶剂时,RuB/SiO2·xH2O对喹啉加氢反应展示出较高的活性和对1,2,3,4-四氢喹啉较高的选择性,催化剂能够多次循环使用.这一体系的优异催化性能归属于载体表面羟基和水的协同作用.     

The catalytic performance of different promoted iron catalysts on combined supports Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for carbon dioxide hydrogenation

Global warming, fossil fuel depletion and fuel price increases have motivated scientists to search for methods for the storage and reduction of the amount of greenhouse gases, especially CO₂. The hydrogenation process has been introduced as an emerging method of CO₂ capture and convertion into value-added products. In this study, new types of catalysts are introduced for CO₂ hydrogenation and are compared based on catalytic activity and product selectivity. The physical properties of the samples are specified using BET. Iron catalysts supported on γ-Al₂O₃ with different metal promoters (X = Ni, K, Mn, Cu) are prepared through the impregnation method. Moreover, Fe–Ni catalysts supported on HZSM5-Al₂O₃ and Ce–Al₂O₃ are synthesized. Samples are reduced by pure H₂ and involved in hydrogenation reaction in a fixed bed reactor (H₂/CO₂ = 3, total pressure = 10 MPa, temperature = 523 K, GHSV = 2000, 1250 nml/min). All catalysts provide high conversion in hydrogenation reactions and the results illustrate that the selectivity of light hydrocarbons is higher than that of methane and CO. It is found that Ni has a promoting effect on the conversion fluctuations throughout the reaction with 66.13% conversion. Using combined supported catalysts leads to enhancing catalytic performance. When Fe–Ni/γ–Al₂O₃—HZSM5 is utilized, CO₂ conversion is 81.66% and the stability of the Fe–Ni catalyst supported on Al₂O₃ and Ce–Al₂O₃ furthey improves.     

In-Situ-Formed Potassium-Modified Nickel-Zinc Carbide Boosts Production of Higher Alcohols beyond CH4 in CO2 Hydrogenation

Ni-based catalysts have been widely studied in the hydrogenation of CO₂ to CH₄, but selective and efficient synthesis of higher alcohols (C₂₊OH) from CO₂ hydrogenation over Ni-based catalyst is still challenging due to successive hydrogenation of C1 intermediates leading to methanation. Herein, we report an unprecedented synthesis of C₂₊OH from CO₂ hydrogenation over K-modified Ni−Zn bimetal catalyst with promising activity and selectivity. Systematic experiments (including XRD, in situ spectroscopic characterization) and computational studies reveal the in situ generation of an active K-modified Ni−Zn carbide (K-Ni₃Zn₁C_0.7) by carburization of Zn-incorporated Ni⁰, which can significantly enhance CO₂ adsorption and the surface coverage of alkyl intermediates, and boost the C−C coupling to C₂₊OH rather than conventional CH₄. This work opens a new catalytic avenue toward CO₂ hydrogenation to C₂₊OH, and also provides an insightful example for the rational design of selective and efficient Ni-based catalysts for CO₂ hydrogenation to multiple carbon products.     

Selective hydrogenation of citral over Au-based bimetallic catalysts in supercritical carbon dioxide

Selective hydrogenation of citral was investigated over Au-based bimetallic catalysts in the environmentally benign supercritical carbon dioxide (scCO₂) medium. The catalytic performances were different in citral hydrogenation when Pd or Ru was mixed (physically and chemically) with Au. Compared with the corresponding monometallic catalyst, the total conversion and the selectivity to citronellal (CAL) were significantly enhanced over TiO₂ supported Pd and Au bimetallic catalysts (physically and chemically mixed); however, the conversion and selectivity did not change when Ru was physically mixed with Au catalyst compared to the monometallic Ru/TiO₂, and the chemically mixed Ru-Au/TiO₂ catalyst did not show any activity. The effect of CO₂ pressure on the conversion of citral and product selectivity was significantly different over the Au/TiO₂, Pd-Au/TiO₂, and Pd/TiO₂ catalysts. It was assumed to be ascribed to the difference in the interactions between Au, Pd nanoparticles and CO₂ under different CO₂ pressures.     

The Effect of Chemical Reducing Agents in the Synthesis of Sol-Gel Ru-Sn Catalysts: Selective Hydrogenation of Cinnamaldehyde

The influence of chemical reduction on the properties of bimetallic Ru-Sn/SiO₂ catalyst was studied. Prepared sol-gel catalysts were reduced by NaBH₄, KBH₄, H₂, and CH₂O. Physical and catalytic properties of reduced catalysts were compared to non-reduced ones in the liquid-phase hydrogenation of cinnamaldehyde. The influence of boron impregnation was investigated. The observed hydrogenation activity and selectivity towards cinnamylalcohol were effected by reduction pretreatment. The NaBH₄ reduced catalyst exhibited the highest activity and selectivity. Reduction pretreatment influenced the pore sizes, active metal surface as well as content of carbon impurities. Traces of the reduction agents were detected in the catalysts. Boron impregnation presumably improved the catalyst activity, but had a minor effect on the selectivity.     

Efficient Ni-based catalysts for low-temperature reverse water-gas shift (RWGS) reaction

CO₂ hydrogenation for syngas can alleviate the pressure of un-controlled emissions of CO₂ and bring enormous economic benefits. Advantageous Ni-catalysts have good CO₂ hydrogenation activity and high CO selectivity merely over 700 °C. Herein, we introduced Cu into Ni catalysts, which were evaluated by H₂-TPR, XRD, BET, in-situ XPS and CO₂-TPD, and their CO₂ hydrogenation activity and CO selectivity were significantly affected by the Ni/Cu ratios, which was rationalized by the synergistic effect of bimetallic catalysts. In addition, the reduction temperatures of studied catalysts apparently affected the CO₂ hydrogenation, which were caused by the number and dispersion of the active species. It's found that the Ni₁Cu₁-400 had good stability, high CO selectivity (up to 90%), and fast formation rate (1.81×10⁻⁵ mol/g_cat/s) at 400 °C, which demonstrated a good potential as a superior catalyst for reverse water-gas shift (RWGS) reaction.     

非负载型铁催化剂上二氧化碳加氢制低碳烯烃

研究了非负载型铁催化剂上CO₂加氢制低碳烯烃反应.结果显示,添加碱金属可显著提高铁催化剂上的CO₂转化率和烯烃选择性.在经K和Rb修饰的Fe催化剂上,CO₂转化率可达约40%,烯烃选择性达到50%以上,其中C₂~C₄烯烃收率超过10%.催化剂表征结果表明,碱金属促进了催化剂中碳化铁的生成,这可能是催化剂性能提高的一个关键原因.随着K含量由1 wt%增加至5 wt%,CO₂转化率及烯烃选择性均升高.但K含量过高时,催化剂活性降低.这可能是由于催化剂比表面积和CO₂化学吸附量降低所致.当K含量为5%~10%时,K-Fe催化剂上烯烃收率较高; 进一步添加适量的硼可进一步提高烯烃选择性,且CO₂转化率下降不大.     

Combined transformations of carbon dioxide and ethanol in the presence of the intermetallic compound TiFe0.95Zr0.03Mo0.02, Pd/SiO2, and Al2O3

The influence of the composition of catalytic systems and the method for H₂ feed into the reaction area on the degree of conversion of CO₂ during its joint transformations with ethanol and on the selectivity of formation of liquid organic products (ethyl acetate, acetaldehyde, and hydrocarbons) was studied atp=15 atm andT=573 K. A noticeable conversion of CO₂ and ethanol into ethyl acetate and acetaldehyde was observed in the presence of only the intermetallic compound, its composition with a palladium-containing catalyst, and the whole ternary catalytic system. The selectivity of the reaction changed when the binary catalytic composition consisting of the intermetallic and γ-Al₂O₃ was used. In this case, the fraction of C₉–C₁₄ alkenes and alkenes with normal and iso structures was mostly formed; its content was as high as 40%. The degree of conversion of CO₂ reached 30–36% and the selectivity to liquid products was 70–80% only when the hydrogen desorbed from the intermetallic was used. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1360–1364, July, 1998.     

Combined hydrogenation of carbon oxides on catalysts bearing iron and nickel nanoparticles

The reaction of the hydrogenation of a mixture of carbon oxides on ultradisperse powder (UDP) catalysts containing Fe and Ni nanoparticles and their bimetallic mechanical mixtures was investigated. It was established that the main reaction product on UDP Ni is methane, while the main products on the bimetallic systems are methane and ethylene. A synergetic effect was observed on the bimetallic catalyst under investigation. It was revealed that the hydrogenation of a mixture of carbon oxides proceeds through the stage of dissociative adsorption of both components, CO and CO₂. The olefin selectivity of the process was explained by the participation of different forms of adsorbed hydrogen (H_I: H_II) at the catalyst surface. It is assumed that the hydrogenation of carbon oxides on iron-nickel catalysts proceeds either through the jumpover effect or via hydrogen spillover.     

Correlating Oxidation State and Surface Ligand Motifs with the Selectivity of CO2 Photoreduction to C2 Products

The design for non-Cu-based catalysts with the function of producing C₂₊ products requires systematic knowledge of the intrinsic connection between the surface state as well as the catalytic activity and selectivity. In this work, photochemical in situ spectral surface characterization techniques combined with the first principle calculations (DFT) were applied to investigate the relationships between the composition of surface states, coordinated motifs, and catalytic selectivity of a titanium oxynitride catalyst. When the catalyst mediates CO₂ photoreduction, C₂ product selectivity is positively correlated with the surface Ti²⁺/Ti³⁺ ratio and the surface oxidation state is regulated and controlled by coordinated motifs of N−Ti-O/V[O], which can reduce the potential dimerization energy barriers of *CO−CO* and promote spontaneous formation of the subsequent *CO−CH₂* intermediate. This phenomenon provides a new perspective for the design of heterogeneous catalysts for photoreduction of CO₂ into useful products.     

Selective Hydrogenation of CO2 to Ethanol over Cobalt Catalysts

Methods for the hydrogenation of CO₂ into valuable chemicals are in great demand but their development is still challenging. Herein, we report the selective hydrogenation of CO₂ into ethanol over non‐noble cobalt catalysts (CoAlO_x), presenting a significant advance for the conversion of CO₂ into ethanol as the major product. By adjusting the composition of the catalysts through the use of different prereduction temperatures, the efficiency of CO₂ to ethanol hydrogenation was optimized; the catalyst reduced at 600 ° gave an ethanol selectivity of 92.1 % at 140 °C with an ethanol time yield of 0.444 mmol g^?1 h^?1. Operando FT‐IR spectroscopy revealed that the high ethanol selectivity over the CoAlO_x catalyst might be due to the formation of acetate from formate by insertion of *CH_x, a key intermediate in the production of ethanol by CO₂ hydrogenation.     

CO2 hydrogenation to methanol and dimethyl ether at atmospheric pressure using Cu-Ho-Ga/γ–Al2O3 and Cu-Ho-Ga/ZSM-5: Experimental study and thermodynamic analysis

CO₂ valorization through chemical reactions attracts significant attention due to the mitigation of greenhouse gas effects. This article covers the catalytic hydrogenation of CO₂ to methanol and dimethyl ether using Cu-Ho-Ga containing ZSM-5 and g-Al₂O₃ at atmospheric pressure and at temperatures of 210 °C and 260 °C using a CO₂:H₂ feed ratio of 1:3 and 1:9. In addition, the thermodynamic limitations of methanol and DME formation from CO₂ was investigated at a temperature range of 100–400 °C. Cu-Ho-Ga/g-Al₂O₃ catalyst shows the highest formation rate of methanol (90.3 µmol_CH3OH/g_cat/h ) and DME (13.2 µmol_DME/g_cat/h) as well as the highest selectivity towards methanol and DME (39.9 %) at 210 °C using a CO₂:H₂ 1:9 feed ratio. In both the thermodynamic analysis and reaction results, the higher concentration of H₂ in the feed and lower reaction temperature resulted in higher DME selectivity and lower CO production rates.     

非负载纳米多孔钯催化喹啉及其衍生物的化学选择性氢化反应:H_2分子异裂（英文）

纳米多孔金属是近十年发展起来的一类具有三维通孔结构的新型功能材料,其由纳米尺度的细孔和韧带构成,具有极大的比表面积;它还是一种无毒无载体的宏观材料,并且易制备、易回收和重复利用,因此作为高效的非均相催化剂已逐渐引起人们的重视.1,2,3,4-四氢喹啉是许多医药、农药、染料和天然产物的重要骨架.通过喹啉及其衍生物的选择性加氢反应制备1,2,3,4-四氢喹啉,具有原子利用率高和原料易得等优点.在过去,已经开发了许多类型的均相和非均相催化体系,并成功地用于催化喹啉及其衍生物的选择性加氢反应.尽管非均相催化体系具有诸多优点,但仍存在H_2压力(10–50 atm)和反应温度(60–150℃)相对较高的缺点.因此,开发更加温和条件下的喹啉及其衍生物的选择性加氢反应具有重要意义.此外,在喹啉及其衍生物的加氢反应过程中,H_2分子在非均相催化剂表面的裂解模式,即均裂还是异裂尚不清楚.因此,本文采用新型非均相催化剂纳米多孔钯,研究了喹啉及其衍生物的选择性加氢反应,在相对较低的H_2压力(2–5 atm)和温度(室温–50℃)下实现了目标反应,高收率、高选择性地得到1,2,3,4-四氢喹啉化合物.在最佳反应条件下,对底物的适用范围进行了考察.结果表明,各种含喹啉结构单元的化合物均能顺利发生反应,产物收率在62%–95%.而且该反应对甲基、甲氧基、羟基、酯基、醛基、酰胺基、卤素(F,Cl和Br)等官能团具有较好的兼容性.苯环上取代基的电子效应对反应有一定的影响,吸电子基有利于目标反应的进行.反应完成后,纳米多孔钯催化剂很容易回收,且循环使用多次后,仍未见催化活性降低.扫描电镜和透射电镜结果发现,循环使用后的纳米多孔钯催化剂结构没有发生明显改变,表明其结构稳定.浸出实验结果证明,没有钯原子浸出到反应液中,表明该纳米多孔钯催化反应属于多相催化过程.喹啉的选择性氢化反应被放大到克级的规模时,目标产物的收率仅略有降低,说明该方法具有很好的实用性.通过动力学实验发现,随着反应的进行,反应速率不断加快,表明反应过程中生成的乙胺和1,2,3,4-四氢喹啉同样扮演着路易斯碱性添加剂的角色,促进了反应的进行.通过反应机理研究,揭示了H–H键在纳米多孔钯表面发生了异裂,原位形成的Pd–H物种作为弱亲核试剂,对目标反应的选择性控制起到了至关重要的作用.     

二氧化碳加氢制甲醇催化剂研究进展

随着全球能源消耗的不断增长和环境污染问题的日益严重,寻找清洁、高效的CO₂利用路径成为研究热点。甲醇由于用途广泛,既是重要的化工原料,也是一种新型清洁能源。CO₂催化加氢制甲醇过程不仅实现CO₂减排,还是碳资源循环利用的有效途径之一,对解决能源紧缺和环境问题具有重要意义。高活性、高选择性和高稳定性的CO₂加氢制甲醇催化剂的开发一直是该过程的核心技术。本文综述了二氧化碳加氢制甲醇的研究进展,主要介绍了反应机理和催化剂,并以Cu基催化剂重点总结了活性位、载体和助剂对催化性能的影响,最后对二氧化碳加氢制甲醇的应用前景进行了展望。     

H2 Production via the Steam Reforming of Methanol Over NiAl-layered Double Hydroxide Derived Catalysts

This article reviews our recent results on the steam reforming of methanol over a series of NiAl-layered double hydroxide catalysts prepared by the co-precipitation method. The influence of calcination temperature, reaction temperature, pretreatment temperature and atmospheres, inorganic salt ions and steam to methanol ratio on the catalytic performance was studied. The major products for many of the catalysts were H₂, CO, CO₂ and CH₄. However, the product composition varies significantly with the experimental parameters and high selectivity for CO₂ and H₂ was observed under various conditions, showing the potential of Ni based catalysts for the production of highly pure hydrogen.