Catalytic performance of Ni-Al layered double hydroxides in CO purification processes

Ni-Al layered double hydroxides with Ni²⁺/Al³⁺ molar ratios of 1.5 and 3.0 have been synthesized by co-precipitation and studied as catalyst precursors for purification of CO-containing gas-mixtures by means of CO oxidation to CO₂ and conversion of CO by water vapor (water-gas shift reaction). The influence of the alkali additives (K⁺ ions) on the water-gas shift activity has been also examined. It was established that the catalytic activity of both reactions increases with the temperature and the nickel content. Hypothetic schemes are proposed about activation of the catalysts in the WGSR and CO oxidation including redox Ni²⁺ ? Ni³⁺ transition on the catalyst surface. The activity in WGSR is positively affected by the presence of potassium promoter, depending on its amount. The sample with higher nickel loading is the most effective catalyst as for CO oxidation as well as for WGSR at intermediate temperatures after potassium promotion.     

助剂对NiMgAl催化剂的结构和甲烷二氧化碳重整反应性能的影响(英文)

向担载镍基催化剂NiMgAl中添加助剂(Co,Ir或Pt)制备了三种助剂促进型催化剂,通过氢气程序升温还原(H2-TPR),CO2/CH4程序升温表面反应(CO2/CH4-TPSR)和CO2程序升温脱附(CO2-TPD)等方法对催化剂进行表征.助剂对催化剂性能的影响通过甲烷干重整实验进行评价.添加少量的Pt或Ir助剂可以降低Ni活性组分的还原温度和提高反应性能.添加助剂的样品与原始NiMgAl催化剂相比能够降低反应的活化能,添加Co或Ir助剂的催化剂与NiMgAl催化剂相比活化能有了明显的降低.NiMgAl催化剂的活化能为51.8 kJ·mol-1,添加Pt助剂的NiPtMgAl催化剂活化能降至26.4 kJ·mol-1.NiMgAl催化剂中添加Pt助剂制备的催化剂具有较好的催化活性和较低的活化能.CH4-TPSR和CO2-TPSR结果表明添加Pt助剂可以在更低的温度下(与NiMgAl催化剂相比)提高CH4的活化能力,并在催化剂表面形成更多的碳物种.CO2-TPD结果显示,添加助剂的催化剂与NiMgAl样品相比在反应温度区间内增加了CO2的吸附/脱附量.     

Methane conversion by various metal, metal oxide and metal carbide catalysts

The progress in the field of methane conversion into higher hydrocarbons including aromatics and oxygenated compounds in the recent five years will be reviewed shortly, together with a new type of the methane conversion reaction with carbon monoxide at lower temperatures (600–700 K) by supported group VIII metal catalysts. Benzene was formed selectively among hydrocarbons in the CH₄–CO reaction over silica-supported Rh, Ru, Pd and Os catalysts under atmospheric pressure. Both CH₄ and CO were required for benzene formation, and only ethane and ethylene were formed besides benzene. The amount of C₃–C₅ hydrocarbons was negligible, which suggests that a completely different mechanism from the CO–H₂ reaction may be operating over these catalysts despite of the similarity in the reaction conditions with the CO–H₂ reaction. The mechanism of benzene formation was studied deeply by means of kinetical investigation as well as infrared spectroscopy and isotopic tracer method in connection with that of CO hydrogenation.     

甲烷催化部分氧化制合成气的反应机理

借助脉冲反应、质谱-程序升温表面反应（MS-TPSR）等技术研究了Ni／α-Al2O3催化剂上甲烷催化部分氧化制合成气（POM）的反应机理．结果表明,NiO上CH4不能解离产生H2只有当NiO被CH4还原为Ni0后,CH4才能解高产生H2,Ni0是CH4活化和POM反应的活性相;POM反应机理遵循直接氧化机理,CH4和O2均在Ni0上活化,活化过程形成的Ni…C和Niδ…Oδ物种是反应历程中的关键物种,Niδ …Oδ物种高选择性地与CH4解离产生的碳物种Ni…C反应生成CO．     

Activation of a single-chamber solid oxide fuel cell by a simple catalyst-assisted in-situ process

Initialization is a critical processing step that has thus far limited the application of the single-chamber solid oxide fuel cell (SC-SOFC). In-situ initialization of a SC-SOFC with a nickel-based anode by methane–air mixtures was investigated. Porous Ru–CeO₂ was used as a catalyst layer over a Ni-ScSZ cermet anode. Catalytic testing demonstrated Ru–CeO₂ had high activity for methane oxidation. The Ru in the catalyst layer catalyzed the formation of syngas, which successfully reduced the nickel oxide to metallic nickel in the anode. Single cells with a La_0.8Sr_0.2MnO₃ (LSM) cathode, initialized by this in-situ reduction method, delivered peak power densities of 205 and 327 mW cm⁻² at 800 °C and 850 °C, respectively. Such performances were better than those of the cell without the Ru–CeO₂ catalyst layer that was initialized by an ex-situ reduction method were.     

Selective oxidation of CO in excess hydrogen on copper—cerium-containing catalysts

Selective oxidation of CO that is in mixtures enriched in H₂ was studied to investigate catalytic properties of the 0.5—80% CuO/Ce_0.7Zr_0.3O₂ system. The catalysts were prepared by the combined decomposition of copper, cerium, and zirconyl nitrates at 300 °C. The systems studied are active and stable under mild conditions of the process (80—160 °C) and at high space velocities (to 100000 h^–1) of the reaction mixture (2% CO, 1% O₂, 40—50% H₂). With an increase in the CuO content in the catalysts up to 20%, the degree of CO removal achieves 60% (120 °C and V = 35000 h^–1) and further does not change appreciably. The contribution of oxygen participation into CO oxidation is virtually independent of the copper concentration in the sample and ranges from 65 to 75%. The dependences of the Arrhenius equation parameters for CO and H₂ oxidation on the catalyst composition were determined, which makes it possible to calculate the conversion of reactants and selectivity of CO conversion under the specified conditions of the process. The addition of CO₂ and H₂O (12—15%) to the reaction mixture decreases the catalyst activity and simultaneously increases the selectivity of CO oxidation to 100%. It is shown by the TPR and X-ray diffraction methods that the combined decomposition of the starting Cu²⁺, Ce³⁺, and ZrO²⁺ nitrates produces solid solutions of oxides with a high content of CuO. The reductive pre-treatment of fresh samples of the studied catalysts results in the destruction of the solid solution and formation of highly dispersed Cu particles on the surface of Ce—Zr—O. These particles are active in CO oxidation.     

Robust nickel silicate catalysts with high Ni loading for CO2 methanation

CO₂ is the main component of greenhouse gases and also an important carbon source. The hydrogenation of CO₂ to methane using Ni-based catalysts can not only alleviate CO₂ emissions but also obtain useful fuels. However, Ni-based catalysts face one major problem of the sintering of Ni nanoparticles in the process of CO₂ methanation. Thus, this work has synthesized a series of efficient and robust nickel silicate catalysts (NiPS−X) with different nickel content derived from nickel phyllosilicate by the hydrothermal method. It was found that the Ni loading plays a critical role in the structure and catalytic performance of the NiPS−X catalysts. The catalytic performance gradually increases with the increase of Ni loading. In particular, the highly dispersed NiPS-1.6 catalyst with a high Ni loading of 34.3 wt% could obtain the CO₂ conversion greater than 80%, and the methane selectivity was close to 100% for 48 h at 330 °C and the GHSV of 40,000 mL g⁻¹ h⁻¹. The excellent catalytic property can be assigned to the high dispersion of Ni nanoparticles and the strong interaction between the active component and the carrier, which is derived from a unique layered silicate structure with lots of nickel phyllosilicate and a large number of Lewis acid sites.     

Hydrogenation of Carbon Dioxide on Iron Manganese Catalysts

The synthesis of hydrocarbons from hydrogenation of carbon dioxide has been studied on a series of coprecipitated iron-manganese catalysts. Kinetic measurements, X-ray diffraction, Mössbauer spectroscopy, and temperature-programmed reaction of adsorbed species were used for activity tests and catalyst characterizations. The results showed that the yields of low-carbon olefins decrease, whereas the amount of methane increases with increasing manganese content in catalysts. The conversion to hydrocarbons is suppressed by the reverse water-gas shift (RWGS) reaction equilibrium. Mössbauer spectra and XRD patterns of catalysts after reaction indicate that catalysts are severely oxidized; it is speculated that the olefin producing surface structure in CO hydrogenation may be destroyed by this oxidation. A pulse-reactor study of the Boudouard reaction showed that manganese has the effect of suppressing CO dissociation and thus decreasing carbon content on catalysts. For CO₂ hydrogenation, the affinity to carbon on catalysts is important; therefore manganese is not a good promoter. Among all catalysts tested, pure iron has the best selectivity to olefinic and long-chain hydrocarbons.     

Studies on Carbon Deposition on Hexaaluminate LaNiAl11O19 Catalysts during CO2 Reforming of Methane

The amount of carbon deposited on hexaaluminate LaNiAl₁₁O₁₉ catalyst in CH₄ decomposition and CO₂ reforming of methane was determined by means of thermogravimetric analysis (TGA). The properties of carbon formed on the catalysts were characterized by X-ray photoelectron spectroscopy (XPS), temperature-programmed CO₂ reaction (TPR-CO₂), and temperature-programmed oxidation (TPO) techniques. The experimental results showed that hexaaluminate LaNiAl₁₁O₁₉ catalyst possessed high resistance to carbon deposition in CO₂ reforming of methane to synthesis gas at high temperatures, and CO₂ played an important role in eliminating carbon during the reaction. At least two forms of the deposited carbon, graphite and carbide, were produced during methane reforming with CO₂.     

Combustion of Carbon Black Catalyzed by Transition Metal-Promoted Y2O3–CeO2–ZrO2 Solid Solutions1

The activity of M-free and M-loaded 10YO_1.5–10CeO₂–80ZrO₂ solid solution (M = Cu, V, or W) towards carbon black combustion was studied using TG/DTA and TPO techniques. It was demonstrated that all studied catalysts lower the temperature of carbon black combustion. The selectivity of the catalytic reaction in CO₂ formation was 100%. It was evidenced that the fast oxidation of carbon at lower temperatures, observed only in the TG/DTA apparatus, was due to heat- and mass-transfer limitations, resulting in a runaway reaction. Using TPR technique, it was shown that, in the temperature range of DTA curve, oxygen on the catalyst surface was rather reactive (and, therefore, it could be easily released by support for the oxidation of carbon), whereas the reactivity of bulk oxygen was negligible. The activity of the metal-loaded 10YO_1.5–10CeO₂–80ZrO₂ (Y-10) samples varied according to the following sequence: Cu/Y-10 > V/Y-10 > W/Y-10. For Cu- and V-containing catalysts, a contribution of a surface redox mechanism in reaction was proposed by comparing EPR spectra of pure catalysts with those of the samples (catalysts mixed with carbon black) after catalysis.     

Selective methanation of CO in the presence of CO<Subscript>2</Subscript> in hydrogen-containing mixtures on nickel catalysts

The screening of commercial nickel catalysts for methanation and a series of nickel catalysts supported on CeO₂, γ-Al₂O₃, and ZrO₂ in the reaction of selective CO methanation in the presence of CO₂ in hydrogen-containing mixtures (1.5 vol % CO, 20 vol % CO₂, 10 vol % H₂O, and the balance H₂) was performed at the flow rate WHSV = 26000 cm³ (g Cat)⁻¹ h⁻¹. It was found that commercial catalytic systems like NKM-2A and NKM-4A (NIAP-07-02) were insufficiently effective for the selective removal of CO to a level of <100 ppm. The most promising catalyst is 2 wt % Ni/CeO₂. This catalyst decreased the concentration of CO from 1.5 vol % to 100 ppm in the presence of 20 vol % CO₂ in the temperature range of 280–360°C at a selectivity of >40%, and it retained its activity even after contact with air. The minimum outlet CO concentration of 10 ppm at 80% selectivity on a 2 wt % Ni/CeO₂ catalyst was reached at a temperature of 300°C.     

一氧化碳低温催化氧化

一氧化碳 (CO) 催化氧化反应因在实际生活中应用广泛而受到人们普遍关注,如激光器中微量CO的消除、封闭体系中CO的消除、汽车尾气净化以及质子交换膜燃料电池中少量CO的消除等。本文总结了近年来CO低温催化氧化研究进展,包括催化剂及其制备方法、CO氧化反应机理以及不同环境气氛对催化剂CO低温氧化性能的影响。催化剂的制备方法主要包括传统浸渍法、共沉淀法、沉积-沉淀法、溶胶-凝胶法、离子交换法、化学气相沉积法、溶剂化金属原子浸渍法等。催化剂可分为贵金属催化剂、非贵金属催化剂、以分子筛为载体的催化剂和合金催化剂等。CO氧化反应机理方面的相关报道较多,人们针对不同催化剂体系提出了各种假设。不同环境气氛对催化剂CO低温氧化性能的影响主要分为H₂O、CO₂、H₂和其它气氛等4部分进行描述。最后对该领域的发展前景进行了展望。     

Production of Hydrogen from Methane by a CO2 Emission-Suppressed Process: Methane Decomposition and Gasification of Carbon Nanofibers

Catalytic methane decomposition into hydrogen and carbon nanofibers and the oxidations of carbon nanofibers with CO₂, H₂O and O₂ were overviewed. Supported Ni catalysts (Ni/SiO₂, Ni/TiO₂ and Ni/carbon nanofiber) were effective for the methane decomposition. The activity and life of the supported Ni catalysts for methane decomposition strongly depended on the particle size of Ni metal on the catalysts. The modification of the catalysts with Pd enhanced the catalytic activity and life for methane decomposition. In particular, the supported Ni catalysts modified with Pd showed high turnover number of hydrogen formation at temperatures higher than 973 K with a high one-pass methane conversion (>70%). However, sooner or later, every catalyst completely lost their catalytic activities due to the carbon layer formation on active metal surfaces. In order to utilize a large quantity of the carbon nanofibers formed during methane decomposition as a chemical feedstock or a powdered fuel for heat generation, they were oxidized with CO₂, H₂O and O₂ into CO, synthesis gas and CO₂, respectively. In every case, the conversion of carbon was greater than 95%. These oxidations of carbon nanofibers recovered or enhanced the initial activities of the supported Ni catalysts for methane decomposition.     

Atomically Dispersed Nickel(I) on an Alloy-Encapsulated Nitrogen-Doped Carbon Nanotube Array for High-Performance Electrochemical CO2 Reduction Reaction

Single-atom catalysts (SACs) show great promise for electrochemical CO₂ reduction reaction (CRR), but the low density of active sites and the poor electrical conduction and mass transport of the single-atom electrode greatly limit their performance. Herein, we prepared a nickel single-atom electrode consisting of isolated, high-density and low-valent nickel(I) sites anchored on a self-standing N-doped carbon nanotube array with nickel–copper alloy encapsulation on a carbon-fiber paper. The combination of single-atom nickel(I) sites and self-standing array structure gives rise to an excellent electrocatalytic CO₂ reduction performance. The introduction of copper tunes the d-band electron configuration and enhances the adsorption of hydrogen, which impedes the hydrogen evolution reaction. The single-nickel-atom electrode exhibits a specific current density of −32.87 mA cm⁻² and turnover frequency of 1962 h⁻¹ at a mild overpotential of 620 mV for CO formation with 97 % Faradic efficiency.     

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.     

Effect of Interactions between Components in Nickel–Silica Catalysts on the Yield of Carbon in Methane Decomposition

A number of 90% Ni–10% SiO₂ catalysts for methane decomposition were studied at different stages of preparation and operation in the reaction using X-ray diffraction analysis, differential dissolution, temperature-programmed reduction, IR spectroscopy, and high-resolution electron microscopy. It was found that an increase in the interaction between components in the catalytic system decreased the ability of nickel to accumulate carbon in the decomposition of methane.     

Effects of Ethanol Impregnation on the Catalytic Properties of Silica-Supported Cobalt Catalysts

Silca-supported Co₃O₄ (6 wt% as Co) catalysts were prepared by pore volume impregnation of ethanol or aqueous cobalt nitrate solutions, and calcined in vacuo to 300 °C. The catalytic performances of these catalysts for oxidation and hydrogenation of CO were examined. All Co₃O₄/SiO₂ catalysts were found to be very active in catalyzing oxidation of CO to CO₂ as compared to a commercial 1 wt% Pt/Al₂O₃. The ethanol-prepared catalysts exhibited higher activity than those of the aqua-prepared catalysts. Pre-calcination of the ethanol-prepared catalysts in oxygen at 600 °C resulted in a dramatic decrease in the activity. Temperature programmed oxidation indicated the presence of carbon deposits on the surface of used catalysts. Infrared spectra showed the continuous generation of CO₂ when these catalysts were exposed to CO. These indicate the primary role of CO disproportionation in catalytic oxidation of CO on Co₃O₄ at low temperature and explain the sharp decrease in activity in the initial period. After reduction at 400 °C, the ethanol-prepared catalysts were also found to be more active in catalyzing hydrogenation of CO, and produced less methane and olefin (C2-C4) fraction. Higher turnover frequencies were observed after high temperature reduction (600 °C) as well, at which ethoxyl groups were removed from silica surface. In both reactions, the enhanced activity for the ethanol-prepared catalysts can not be fully accounted for by the increase in the dispersion of Co₃O₄ or CO metal. This suggests that the surface structures of Co₃O₄ or CO were further modified by the carbonaceous species derived from ethanol.     

Atomically Dispersed Nickel(I) on an Alloy‐Encapsulated Nitrogen‐Doped Carbon Nanotube Array for High‐Performance Electrochemical CO2 Reduction Reaction

Single‐atom catalysts (SACs) show great promise for electrochemical CO₂ reduction reaction (CRR), but the low density of active sites and the poor electrical conduction and mass transport of the single‐atom electrode greatly limit their performance. Herein, we prepared a nickel single‐atom electrode consisting of isolated, high‐density and low‐valent nickel(I) sites anchored on a self‐standing N‐doped carbon nanotube array with nickel–copper alloy encapsulation on a carbon‐fiber paper. The combination of single‐atom nickel(I) sites and self‐standing array structure gives rise to an excellent electrocatalytic CO₂ reduction performance. The introduction of copper tunes the d‐band electron configuration and enhances the adsorption of hydrogen, which impedes the hydrogen evolution reaction. The single‐nickel‐atom electrode exhibits a specific current density of ?32.87 mA cm^?2 and turnover frequency of 1962 h^?1 at a mild overpotential of 620 mV for CO formation with 97 % Faradic efficiency.     

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.     

Catalytic properties of aluminum/nickel-, copper-containing oxide film compositions

The possibility of the single-step formation of nickel- and copper-containing thin-film oxide systems on aluminum by plasma electrolytic oxidation was demonstrated. The resulting structures were found to be active in the reaction of CO oxidation to CO₂ in the temperature region 300–500°C. However, the resulting structures exhibited stable catalytic activity only in the simultaneous presence of nickel and copper compounds. The films were studied using X-ray diffraction, X-ray spectroscopic analysis, X-ray photoelectron spectroscopy, and electron microscopy. The resulting films exhibited an essentially inhomogeneous composition through the thickness. Electrolyte elements such as nickel, copper, sodium, and phosphorus were concentrated at the surface. Nickel occurred as Ni²⁺, and copper occurred as Cu⁺ and Cu²⁺. The surface contained carbon in detectable amounts.