Photoreduction of carbon dioxide by hydrogen and methane

Zirconium oxide is active for photoreduction of gaseous carbon dioxide to carbon monoxide with hydrogen. A stable surface species arises under the photoreduction of CO₂ on zirconium oxide, and it is identified as surface formate by infrared spectroscopy. Adsorbed CO₂ is converted to formate by photoreaction with hydrogen. The surface formate is a true reaction intermediate since CO is formed by the photoreaction of formate and CO₂; surface formate works as a reductant of carbon dioxide to yield carbon monoxide. The dependence on the wavelength of irradiation light shows that a bulk ZrO₂ is not a photoactive species. When ZrO₂ adsorbs CO₂ a new band appears in photoluminescence excitation spectrum. The photoactive species in the reaction that CO₂+H₂ yields HCOO⁻ is presumably formed by the adsorption of CO₂ on ZrO₂ surface. Hydrogen molecules play a role to supply an atomic hydrogen. Therefore, methane molecules can also be used as a reductant of carbon dioxide.     

Effect of sodium cation on the electrochemical reduction of CO<Subscript>2</Subscript> at a copper electrode in methanol

The electrochemical reduction of CO₂ with a Cu electrode in methanol was investigated with sodium hydroxide supporting salt. A divided H-type cell was employed; the supporting electrolytes were 80 mmol dm⁻³ sodium hydroxide in methanol (catholyte) and 300 mmol dm⁻³ potassium hydroxide in methanol (anolyte). The main products from CO₂ were methane, ethylene, carbon monoxide, and formic acid. The maximum current efficiency for hydrocarbons (methane and ethylene) was 80.6%, at −4.0 V vs Ag/AgCl, saturated KCl. The ratio of current efficiency for methane/ethylene, r _f(CH₄)/r _f(C₂H₄), was similar to those obtained in LiOH/methanol-based electrolyte and larger relative to those in methanol using KOH, RbOH, and CsOH supporting salts. In NaOH/methanol-based electrolyte, the efficiency of hydrogen formation, a competing reaction of CO₂ reduction, was suppressed to below 4%. The electrochemical CO₂ reduction to methane may be able to proceed efficiently in a hydrophilic environment near the electrode surface provided by sodium cation.     

CO2 reforming of methane over zirconia-supported nickel catalysts, II. Thermogravimetric analysis

Thermogravimetric analysis has been used to study carbon deposition during the CO₂ reforming of methane over Ni/ZrO₂ catalysts. The carbon deposits form on the reduced catalyst at a very fast rate during temperature-programmed surface reaction of reforming, and reach a steady state below 973 K. So, the amount of deposited carbon remains constant on the catalyst during the reaction at 973 K. A relationship between the amount of deposited carbon and the activity reveals that the initially formed carbon acts as a reaction intermediate and reacts with CO₂ to produce CO.     

单原子Ge助剂修饰Cu(111)晶面上CO2加氢制甲醇的机理研究

针对CO₂热催化转化制甲醇过程中CO₂吸附、活化较困难及副产物较多的问题,提出采用单原子Ge助剂修饰Cu(111)晶面的解决思路,通过密度泛函理论(DFT)计算研究了CO₂在Ge-Cu(111)晶面上加氢合成甲醇的反应机理。结果表明,单原子Ge助剂的电子调控增加了与其相邻的 Cu 原子的电子云密度,使 CO₂分子在含 Ge 活性界面上的吸附能力显著增强:CO₂在 Ge-Cu(111)晶面上的吸附能约为Cu(111)晶面的1.5倍,约为Pd改性Cu(111)晶面的2.4倍,进而使逆水煤气变换(RWGS)反应路径速控步骤的活化能降低了近 20 kJ·mol^-1,同时衍生出 3条生成甲醇的 RWGS新路径;此外,Ge-Cu(111)晶面上甲酸盐路径由于速控步骤活化能大幅上升而被禁阻,进而CO及烃类等副产物选择性大幅降低,Ge-Cu(111)晶面上CO₂加氢制甲醇选择性升高。     

单原子Ge助剂修饰Cu(111)晶面上CO2加氢制甲醇的机理研究

针对CO₂热催化转化制甲醇过程中CO₂吸附、活化较困难及副产物较多的问题,提出采用单原子Ge助剂修饰Cu (111)晶面的解决思路,通过密度泛函理论(DFT)计算研究了CO₂在Ge-Cu(111)晶面上加氢合成甲醇的反应机理。结果表明,单原子Ge助剂的电子调控增加了与其相邻的Cu原子的电子云密度,使CO₂分子在含Ge活性界面上的吸附能力显著增强：CO₂在Ge-Cu(111)晶面上的吸附能约为Cu (111)晶面的1.5倍,约为Pd改性Cu(111)晶面的2.4倍,进而使逆水煤气变换(RWGS)反应路径速控步骤的活化能降低了近20 kJ·mol^-1,同时衍生出3条生成甲醇的RWGS新路径;此外,Ge-Cu(111)晶面上甲酸盐路径由于速控步骤活化能大幅上升而被禁阻,进而CO及烃类等副产物选择性大幅降低,Ge-Cu(111)晶面上CO₂加氢制甲醇选择性升高。     

Effect of ceria loading on the carbon formation during low temperature methane steam reforming over a Ni/CeO2/ZrO2 catalyst

The characterization and catalytic activity of a Ni/CeO₂/ZrO₂ catalyst for methane steam reforming at 600°C were investigated. The addition of ceria increased the surface area and basicity of the catalysts. The redox reaction capability of the ceria increased the hydrogen yield and carbon monoxide selectivity, and inhibited carbon formation.     

Doped-ZrO2 Aerogels: Catalysts of Controlled Structure and Properties

Doped ZrO₂ aerogels (characterised by TEM, DTA and N₂ adsorption) have been prepared and catalytically tested in CO/CO₂ hydrogenation [1] and CH₄ oxidation [2]. The primary aerogels showed cross-linked clusters of (X-ray) amorphous particles smaller than 5 nm which led to well-developed mesoporous solids with an average pore size of about 10 nm and high surface area (up to 250 m²g⁻¹) [1]. Cu/ZrO₂ aerogels (known to be very active and selective towards methanol synthesis in CO hydrogenation without predominant formation of alkanes even at higher temperatures [1]) are now seen to show these effects even more clearly in CO₂ hydrogenation. In methane oxidation, both Rh/ZrO₂ and Y₂O₃/ZrO₂ were very active. Consideration is given to the nature of the active sites, the role of CO₂ and metal/oxide interfaces and how an understanding of this reactivity can lead to better dispersed ZrO₂.     

Effect of CO2/CO Addition in Dehydroaromatization of Methane on Zeolite-Supported Mo and Re Catalysts Towards Hydrogen, Benzene and Naphthalene

High activity and high formation selectivity for aromatics in the dehydrocondensation reaction of methane were realized only on selected catalysts. The requisites of a metal and a zeolite support as the selected catalyst were described. However, the catalytic activity steadily declined even on the selected catalysts with time on stream because of coke accumulation. A stable catalytic activity was obtained when CO₂ or CO was added into methane feed due to effective removal of coke from the catalyst surface by CO or CO₂. The route from methane to aromatics and the formation process of active phase of catalyst were discussed.     

CO2 Hydrogenation Catalyzed by Graphene-Based Materials

In the context of an increased interest in the abatement of CO₂ emissions generated by industrial activities, CO₂ hydrogenation processes show an important potential to be used for the production of valuable compounds (methane, methanol, formic acid, light olefins, aromatics, syngas and/or synthetic fuels), with important benefits for the decarbonization of the energy sector. However, in order to increase the efficiency of the CO₂ hydrogenation processes, the selection of active and selective catalysts is of utmost importance. In this context, the interest in graphene-based materials as catalysts for CO₂ hydrogenation has significantly increased in the last years. The aim of the present paper is to review and discuss the results published until now on graphene-based materials (graphene oxide, reduced graphene oxide, or N-dopped graphenes) used as metal-free catalysts or as catalytic support for the thermocatalytic hydrogenation of CO₂. The reactions discussed in this paper are CO₂ methanation, CO₂ hydrogenation to methanol, CO₂ transformation into formic acid, CO₂ hydrogenation to high hydrocarbons, and syngas production from CO₂. The discussions will focus on the effect of the support on the catalytic process, the involvement of the graphene-based support in the reaction mechanism, or the explanation of the graphene intervention in the hydrogenation process. Most of the papers emphasized the graphene’s role in dispersing and stabilizing the metal and/or oxide nanoparticles or in preventing the metal oxidation, but further investigations are needed to elucidate the actual role of graphenes and to propose reaction mechanisms.     

Syngas production in a novel perovskite membrane reactor with co-feed of CO2

Partial oxidation of methane（POM） co-fed with CO₂ to syngas in a novel catalytic BaCo_0.6Fe_0.2Ta_0.2O_3-δ oxygen permeable membrane reactor was successfully reported.Adding CO₂ to the partial oxidation of methane reaction not only alters the ratio of CO/H₂,but also increases the oxygen permeation flux and CH₄ conversion.Around 96%CH₄ conversion with more than 93%CO₂ conversion and 100%CO selectivity is achieved,which shows an excellent reaction performance.A steady oxygen permeation flux of 15 mL/（cm² min） is obtained during the 100-h operation,which shows good stability as well.     

红外光谱电化学研究金电极表面上CO2的还原

以碳酸丙烯酯(PrC)为溶剂,高氯酸四丁基胺(TBAP)为电解质,利用电化学及红外光谱电化学开展了金电极上二氧化碳的还原研究。运用现场红外光谱跟踪电化学还原过程反应物及产物的生成和消失。红外光谱电化学循环伏吸法表明,在消耗CO2的同时,金电极上有CO的产生,且伴随有碳酸根的形成。结合电化学和光谱电化学结果,提出了一种电还原机理:在非水介质中,CO2电还原过程中生成了中间体CO2.-,随后CO2.-分别以两个途径进行还原,其一是直接被还原成CO,其二是与CO2结合生成C2O4.-而后歧化成CO以及CO32-。两个反应同时进行,且第一个反应是可逆过程。     

Hydrogenation of CO2 Promoted by Silicon-Activated H2S: Origin and Implications

Unlike the usual method of CO_x (x = 1, 2) hydrogenation using H₂ directly, H₂S and HSiSH (silicon-activated H₂S) were selected as alternative hydrogen sources in this study for the CO_x hydrogenation reactions. Our results suggest that it is kinetically infeasible for hydrogen in the form of H₂S to transfer to CO_x at low temperatures. However, when HSiSH is employed instead, the title reaction can be achieved. For this approach, the activation of CO₂ is initiated by its interaction with the HSiSH molecule, a reactive species with both a hydridic H^δ− and protonic H^δ+. These active hydrogens are responsible for the successive C-end and O-end activations of CO₂ and hence the final product (HCOOH). This finding represents a good example of an indirect hydrogen source used in CO₂ hydrogenation through reactivity tuned by silicon incorporation, and thus the underlying mechanism will be valuable for the design of similar reactions.     

Characterization of Polypropylene Surface Treated in a CO2 Plasma

The polypropylene modification in CO₂ plasma mainly contributes to degradation, functionalization, and cross-linking. The degradation, whose rate is depending on CO₂ dissociation and oxygen atom formation, is a quite slow reaction and it is associated with surface topography alteration, especially of the amorphous phase of the polypropylene. The surface roughness increases with the treatment duration and the amorphous phase is more degraded than the crystallized part. The functionalization, corresponding to an increase of the surface energy (57.3 mJ m ^{– 2} in 30 s), and to an oxidation (23 oxygen at.%) with the appearance of alcohol, ketone, and acid functions is a much faster phenomenon. Cross-linking takes also place during this type of treatment and will reinforce the stability of the modified surface.     

Theoretical study on the reaction mechanism of (CH<Subscript>3</Subscript>)<Subscript>3</Subscript>CO<Superscript>.</Superscript> radical with NO

The reaction mechanism of (CH₃)₃CO^. radical with NO is theoretically investigated at the B3LYP/6-31G* level. The results show that the reaction is multi-channel in the single state and triplet state. The potential energy surfaces of reaction paths in the single state are lower than that in the triple state. The balance reaction: (CH₃)₃CONO⇔(CH₃)₃CO^.+NO, whose potential energy surface is the lowest in all the reaction paths, makes the probability of measuring (CH₃)₃CO^. radical increase. So NO may be considered as a stabilizing reagent for the (CH₃)₃CO^. radical.     

铬助剂对Cu/ZrO_2/CNTs-NH_2催化剂催化CO_2加氢合成甲醇性能的影响

采用并流共沉淀方法制备了一系列不同铬含量的Cu/ZrO2/CNTs-NH2催化剂,在固定床反应器上考察铬对催化剂催化CO2加氢合成甲醇反应性能的影响.当铬含量为1%(w),反应温度为260°C,压力为3.0MPa,原料气组成为V(H2):V(CO2):V(N2)=69:23:8,空速为3600 mL·h-1·g-1时,催化剂的促进效果最显著,甲醇收率达7.78%.氮吸附、粉末X射线衍射(XRD)、氢气程序升温脱附(H2-TPR)、X射线光电子能谱(XPS)、二氧化碳程序升温脱附(CO2-TPD)、差热分析(DTA)以及扫描电子显微镜(SEM)等表征结果表明,随着铬含量的增加,铜颗粒的粒径减小,催化剂的比表面积增大.铬的加入一方面提高了铜的分散性,抑制了ZrO2的相变和活性组分的烧结;另一方面提高了CO2的吸附量并促进CO2由弱吸附向强吸附转化,从而提高甲醇的收率;但是当铬含量大于1%时,催化剂表面Cu、Zr的总含量明显下降,降低了CO2的吸附量并且形成了超强CO2吸附物种,抑制了CO2及其中间产物的转化,从而降低了甲醇收率.     

Metal/zirconia catalysts for the synthesis of methanol: characterization by vibrational spectroscopy

The activation of carbon dioxide by catalytic hydrogenation has been studied as a route for methanol synthesis. Metal/zirconia catalysts suitable for this reaction have been prepared by (i) activation of amorphous metal alloys [1] or (ii) coprecipitation of amorphous zirconia and metal oxides [2]. Vibrational spectroscopy has been used to obtain information on the catalytic reaction mechanism, by the in situ identification of adsorbed species and intermediates under reaction conditions.The reverse water-gas shift reaction, producing CO from CO₂ and hydrogen, plays a crucial role in the reaction mechanism. This reduction is shown to proceed via surface formate, adsorbed close to the metal/zirconia interface. Over Pd/ZrO₂ and Ni/ZrO₂, formate is reduced to methane without further observable intermediates. Pivotal intermediates on the route to methanol, as observed on Cu/ZrO₂ catalysts, are -bound formaldehyde and surface methylate. Addition of silver as a promoter can result in enhanced selectivities and productivities for methanol formation. The synergy between the two metals becomes evident from the spectroscopic measurements; the most prominent feature of the silver-promoted catalysts is a high concentration of surface formaldehyde, which is either preferentially formed or stabilized by the silver component.     

CrOx/SiO2催化剂上丙烷在CO2气氛中脱氢反应的研究

采用XRD、UV-vis DRS、ESR和微分吸附量热等技术,考察了铬担载量分别为2.5、5和10wt%的CrO_x/SiO₂催化剂的结构、表面性质和氧化还原性能。结果表明,催化剂表面上存在多种Cr的氧化态和聚集形式。随着Cr担载量从2.5wt%到10wt%的逐渐增大,催化剂表面占主导地位的Cr物种由CrO₃单体转为多聚CrO₃和Cr₂O₃晶相。在CO₂气氛中催化剂对丙烷转化率和丙烯选择性的大小顺序为2.5wt%CrO_x/SiO₂>5wt%CrO_x/SiO₂>10wt%CrO_x/SiO₂,反应过程中的原位ESR和UV-visDRS测定结果表明,催化剂表面的反应活性中心为Cr⁵⁺,Cr⁵⁺可由催化剂预处理过程中Cr³⁺的氧化及丙烷反应过程中CrO₃单体的还原产生,在反应中CO₂可使Cr³⁺重新氧化为Cr⁵⁺.     

H2-induced CO adsorption and dissociation over Co/Al2O3 catalyst

The activation of adsorbed CO is an important step in CO hydrogenation. The results from TPSR of pre-adsorbed CO with H2 and syngas suggested that the presence of H2 increased the amount of CO adsorption and accelerated CO dissociation. The H2 was adsorbed first, and activated to form H＊ over metal sites, then reacted with carbonaceous species. The oxygen species for CO2 formation in the presence of hydrogen was mostly OH^＊, which reacted with adsorbed CO subsequently via CO^＊＋OH^＊ → CO2^＊＋H^＊; however, the direct CO dissociation was not excluded in CO hydrogenation. The dissociation of C-O bond in the presence of H2 proceeded by a concerted mechanism, which assisted the Boudourd reaction of adsorbed CO on the surface via CO^＊＋2H^＊ → CH^＊＋OH^＊. The formation of the surface species （CH） from adsorbed CO proceeded as indicated with the participation of surface hydrogen, was favored in the initial step of the Fischer-Tropsch synthesis.     

Methane generated during photocatalytic redox reaction of alcohols on TiO2 suspension in aqueous solutions

Studies on the photo-catalytic redox reaction of C1–C3 alcohols such as methanol, ethanol and 2-propanol were carried out in aqueous solution containing TiO₂ photocatalyst (0.1% w/v) as suspension using 350 nm light. Other hydrocarbons such as ethane and ethene in the case of ethanol, and propene in the case of 2-propanol with low yields were produced along with the major photolytic products methane and carbon dioxide. The yields of methane and CO₂ were found to be dependent on the light exposure time and ambient conditions. Methane yields were higher in 2-propanol and ethanol systems than in methanol system, showing their better hole-scavenging properties. In the aerated condition, methane was produced during photolysis of all alcohols in the presence of TiO₂ and the yield was comparable to those observed in the corresponding CO₂-saturated systems. The overall results reveal that the surface adsorbed, as well as in-situ-generated CO₂ from photo-oxidation of alcohols are equally responsible for methane formation through photo-reduction in presence of TiO₂. In the O₂-saturated system, the methane yield was lower as compared to that in aerated system, in contrast to the CO₂ yield. In N₂O-and N₂-purged systems, the yield of methane was observed to be low, inferring that the methane generation has not taken place through photodecomposition/photodissociation of alcohols. Again, photolysis of alcohols without TiO₂ did not generate any methane.     

Synthesis and CO2 capture evaluation of Li2−xKxZrO3 solid solutions and crystal structure of a new lithium-potassium zirconate phase

Solid solutions of lithium and potassium metazirconates Li_2−xK_xZrO₃ (where, 0?x?2) were prepared by coprecipitation. Samples were characterized by powder X-ray diffraction, scanning electron microscopy, and thermogravimetric analyses. Results showed that the solubility limits of potassium into Li₂ZrO₃ is x=0.2. Furthermore, at higher potassium concentrations, a new phase was synthesized, Li_2.27K_1.19Zr_2.16O_6.05. For structural studies of this new phase, XRD data were analyzed by Rietveld refinements. Additionally, at high potassium concentrations different phases of ZrO₂ were found, as potassium tends to sublimate. On the other hand, lithium-potassium metazirconate solid solutions, Li_2−xK_xZrO₃, were tested as CO₂ captors. Thermal analyses into a CO₂ flux showed that Li_2−xK_xZr₂O₃ solid solutions present a better CO₂ absorption than Li₂ZrO₃ pure. The differences observed in the CO₂ sorption processes were explained with thermodynamic data.