CO2 hydrogenation to methanol over La-Mn-Cu-Zn-O based catalysts derived from perovskite precursors

A series of La-Mn-Cu-Zn-O samples with different molar ratio Cu:Zn derived from perovskite precursors were prepared and characterized. LaMnO₃ and La₂CuO₄ perovskite structures were obtained after calcination and ‘Cu on oxide’ was formed after reduction. Both Cu⁺ and Cu⁰ existed in the reduced samples with highly dispersion. When doping Zn to La-Mn-Cu-O system, the reducibility of part Cu species was improved, and the reduction of part of Cu species which interacted with Zn strongly became difficult. The strong interaction between Cu and Zn is benefit for CO₂ hydrogenation to methanol. It is found that the trend of the CO₂ conversion and Cu surface area is same, and methanol selectivity increases when the proportion of moderate basic sites increases.     

Promotional effect of acetic acid on simultaneous NO and Hg0 oxidation over LaCoO3 perovskite

Acid etching can effectively alter the mineralization properties of perovskite. In this study, LaCoO₃ perovskites were etched via acetic acid (1–3 M) and were used to simultaneously catalyze the oxidation of NO and Hg⁰ in coal-fired flue gas. The results demonstrated the ability of the modified acetic acid to improve the low-temperature activity of LaCoO₃ perovskite. The acetic acid could erode or dissolve the La–O surface termination and emerge more Co–O surface termination, changing the pore structure, surface morphology, redox capacity, metal oxidation state, and acid-site strength. These changes on the LaCoO₃ perovskite allowed for the low-temperature to simultaneously improve the NO and Hg⁰ oxidation performances, and the NO reaction temperature was reduced by 25 °C. Moreover, when the concentration of acetic acid exceeded 1 M, part of the active Co ions in the perovskite structure were further dissolved, thereby reducing the NO and Hg⁰ simultaneous oxidation activity.     

Structural and electrochemical property evolutions of perovskite SOFC anodes: Role of fuel atmosphere in (La0.4Sr0.6)1-xCo0.2Fe0.7Nb0.1O3-δ

The in-situ precipitated nano metal particle-perovskite anodes show promising application prospects in Solid Oxide Fuel Cells. Previously reports mainly show new exsolved material, but barely focus on their practical application concerns. Herein, the structural and electrochemical properties evolution of (La_0.4Sr_0.6)_1-xCo_0.2Fe_0.7Nb_0.1O_3-δ (LSCFN, x = 0, 0.05, 0.1) versus varied anode fuel atmospheres are presented. Results show that perovskite-type LSCFN anodes are fully reduced to K₂NiF₄ structure in dry H₂ at 850 °C, but maintain to be ABO₃ structure with minor structure change under water containing H₂. The exsolved Co_1-xFe_x phases are observed under both conditions, but with much higher Fe content under dry H₂. The Electrochemical Impedance Spectra shows that LSCFN anodes are more sensitive to $p_{H_2}$ change under humidified fuels than dry conditions. Single cells with (La_0.4Sr_0.6)_0.9Co_0.2Fe_0.7Nb_0.1O_3-δ anode show promising performance with maximum power densities of 1.2 and 0.95 W/cm² under dry and wet H₂ at 850 °C, respectively.     

Resistance of Ni/perovskite catalysts to H2S in toluene steam reforming for H2 production

H₂S is a kind of common impurity produced during the gasification of biomass, and it will poison the catalysts used in biomass tar steam reforming, leading to a rapid degradation on the catalytic performance. The purpose of this work is to investigate the characteristics of biomass tar steam reforming using Ni/perovskite catalysts with the presence of H₂S. Results show that H₂S could significantly reduce catalytic activity due to the adsorption of sulfur on Ni surface, and Ni/perovskite catalysts are less susceptible to this poisoning in comparison to the Ni-catalyst loaded on γ-Al₂O₃. To understand the mechanism, fresh and spent catalysts were characterized using various techniques of XRD, SEM-EDS, XPS and TPO. It is proved that the lattice oxygen in perovskite could transform into surface species, inhibit the adsorption of sulfur and thus benefit to the reactivity of catalysts during biomass tar steam reforming.     

Renewable syngas & hydrogen synthesis via steam reforming of glycerol over ceria-mediated exsolved metal nano catalysts

Currently, catalyst design and development has drawn much attention as results of its strategic importance in the area of energy applications particular those involving biomass conversion. This work tailored exsolution of metal catalysts through the use of ceria for enhanced structural and catalytic behaviour in steam reforming of glycerol. Aside the understanding that defects due to A-site deficiency facilitates formation of vacant sites and exsolution of metal catalysts on the B-site of perovskite systems, this work has exemplified that metals such as ceria significantly influences the exsolution and general morphological surface architecture and catalytic behaviour. The exsolved nickel catalysts anchored and socketed on a titanate support and the ceria's basic surface properties and oxygen storage-release behaviour has modified the perovskite surface chemistry and enhanced catalytic behaviour particularly deactivation due to carbon deposition and reusability. Other exsolvable dopant metal species such as Fe and Co forms alloys with nickel on the surface and the synergy between the dopant metals in the alloy yielded better results. Furthermore, in one of the catalyst systems, the most commonly observed tolerable A-site deficiency and doping limit of 2% known for SrTiO₃ perovskite was overstretched by 0.5% (2.5%) thereby increasing the defect chemistry. The catalyst system with such formulation has shown a dramatic exsolution phenomenon and catalytic behaviour and robust suppression of coke deposition. CO selectivity >60% and H₂ selectivity >40% was recorded with all the catalyst systems. The catalysts used in this work are useful for applications in energy and production of value added chemicals.     

Modeling and optimization of hydrogenation of CO2: Estimation of kinetic parameters via Artificial Bee Colony (ABC) and Differential Evolution (DE) algorithms

Global warming, climate change, fossil fuel depletion and steep hikes in the price of environmentally friendly hydrocarbons motivate researchers to investigate CO₂ hydrogenation for hydrocarbons production. However, due to the reaction complexities and varieties of produced species, the process mechanism and subsequently estimation of the kinetic parameters have been controversial yet. Therefore, estimating the kinetic parameters using Artificial Bee Colony (ABC) and Differential Evolution (DE) optimization algorithms based on Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism is proposed as a possible remedy to fulfil the requirements. To this end, a one-dimensional heterogeneous model comprising detailed reaction rates of reverse water gas shift (RWGS), Fisher-Tropsch (FT) reactions and direct hydrogenation (DH) of CO₂ is developed. It is observed that ABC exhibiting 6.3% error in predicting total hydrocarbons selectivity is superior to DE algorithm with 32.9% error. Therefore, the model employed the estimated kinetic parameters obtained via ABC algorithm, is exploited for products distribution analysis. Results reveal that maximum 73.21% hydrocarbons (C₁C₄) selectivity can be achieved at 573 K and 1 MPa with 0.85% error compared to the experimental value of 72.59%. Accordingly, the proposed model can be exploited as a powerful tool for evaluating and predicting the performance of CO₂ hydrogenation to hydrocarbons process.     

Coal char supported Ni catalysts prepared for CO2 methanation by hydrogenation

Catalytic CO₂ methanation is a potential solution for conversion of CO₂ into valuable products, and the catalyst plays a crucial role on the CO₂ conversion and CH₄ selectivity. However, some details involved in the CO₂ methanation over the carbon supported Ni catalysts are not yet fully understood. In this work, commercial coal char (CC) supported Ni catalysts were designed and prepared by two different methods (impregnation-thermal treatment method and thermal treatment-impregnation method) for CO₂ methanation. Effects of the preparation conditions (including the thermal treatment temperature and time, the mass ratio of CC:Ni and the preparation method), as well as the reaction temperature of CO₂ methanation, were investigated on the catalyst morphology, reducibility, structure and catalytic performance. Fibrous Ni-CC catalyst is achieved and shows high CO₂ conversion (72.9%–100%) and CH₄ selectivity (>99.0%) during the 600-min methanation process. Adverse changes of the catalyst surface and textural properties, reducibility, particle size and morphology are the potential factors leading to the catalyst deactivation, and possible solutions resistant to the deactivation were analyzed and discussed. The CO₂ methanation mechanism with the CO route was proposed based on the oxidation-reduction cycle of Ni in this work.     

Alumina-supported cobalt catalysts in the hydrogenation of CO2 at atmospheric pressure

20 wt.% cobalt catalysts supported on pure and 5 wt.% silica-containing alumina have been prepared and characterized by X-Ray Diffraction, IR and DR-UV-vis-NIR spectroscopies and Field-Emission Scanning Electron Microscopy (FE-SEM). The presence of a cobalt-containing surface spinel phase Co_3-xAl_xO₄ and, for the silica-containing sample, of a segregated Co₃O₄ phase is evident. These catalysts have been tested in CO₂ hydrogenation at atmospheric pressure in steady state conditions in the temperature range 523–773 K. Both catalysts are active in CO₂ hydrogenation to methane (methanation) and to CO (reverse Water Gas Shift, rWGS). CO₂ hydrogenation activity is higher on freshly pre-reduced silica-free Co/Al₂O₃, while selectivity to methane is slightly higher on the silica-containing sample. Spent catalysts contain clustered or amorphous cobalt metal centers as active sites for methanation. The silica-containing catalyst shows slow deactivation in CO₂ hydrogenation upon 13 h experiments, with quite stable or even slightly increasing rWGS activity and decreasing CH₄ selectivity. This confirms previous data suggesting that, over cobalt catalysts, sites for methanation are metal centers prone to deactivation by carbon deposition. However, in contrast with what happens with unsupported and silica-supported cobalt catalysts, where deactivation is very fast, over these alumina-based catalysts carbon deposition and deactivation occur much more slowly. Sites for rWGS are unreduced cobalt centers which do not undergo such a deactivation phenomenon.     

CO2 hydrogenation on Ru/Ce based catalysts dispersed on highly ordered micro-channelled 3YSZ monoliths fabricated by freeze-casting

The activity of Ru–Ce/3 mol% Y₂O₃ doped ZrO₂ (3YSZ) catalysts was studied for the CO₂ hydrogenation to form CH₄ using catalyst-loaded freeze-cast ceramic monolith as possible substitute to conventional support fixed bed catalysts. Two set of three catalysts were elaborated by varying the Ru loading at 2.5, 5 and 7.5 wt% and by using as support (i) 3YSZ powder and (ii) 3YSZ hierarchically ordered and highly porous freeze-cast monolith. Freeze-cast monoliths present an interesting feature sought for catalytic devices, i.e., large and organized porosity boosting gas transport and minimizing pressure loss through it. The catalytic results reveal the adequate catalytic activity of both catalyst configurations and detail the beneficial effect of a monolith as support on CH₄ selectivity and CO production. As expected thermodynamically, an increase in the H₂ content in the reactor feed shifts the intermediate Reverse Water Gas Shift (RWGS) reaction to the equilibrium and thus improves CH₄ yield. This improvement can be explained by the difference of packing between both configuration and at a similar GHSV, the monolith favors the RWGS reaction further in the monolith length which apparently does not occur in the same proportion in the conventional fixed-bed. Finally a study of the pressure drop through all the catalysts did not reveal any significant increase of pressure due to carbon deposition or high inlet flow.     

On the effect of yttrium promotion on Ni-layered double hydroxides-derived catalysts for hydrogenation of CO2 to methane

Ni-containing mixed oxides derived from layered double hydroxides with various amounts of yttrium were synthesized by a co-precipitation method at constant pH and then obtained by thermal decomposition. The characterization techniques of XRD, elemental analysis, low-temperature N₂ sorption, H₂-TPR, CO₂-TPD, TGA and TPO were used on the studied catalysts. The catalytic activity of the catalysts was evaluated in the CO₂ methanation reaction performed at atmospheric pressure. The obtained results confirmed the formation of nano-sized mixed oxides after the thermal decomposition of hydrotalcites. The introduction of yttrium to Ni/Mg/Al layered double hydroxides led to a stronger interaction between nickel species and the matrix support and decreased nickel particle size as compared to the yttrium-free catalyst. The modification with Y (0.4 and 2 wt%) had a positive effect on the catalytic performance in the moderate temperature region (250–300 °C), with CO₂ conversion increasing from 16% for MO-0Y to 81% and 40% for MO-0.4Y and MO-2.0Y at 250 °C, respectively. The improved activity may be correlated with the increase of percentage of medium-strength basic sites, the stronger metal-support interaction, as well as decreased crystallite size of metallic nickel. High selectivity towards methane of 99% formation at 250 °C was registered for all the catalysts.     

Properties of BaYO3 perovskite and hydrogen storage properties of BaYO3Hx

In this study, Density Functional Theory (DFT) calculations have been performed for BaYO₃ perovskite with the generalized gradient approximation (GGA) as implemented in Vienna Ab-initio Simulation Package (VASP). The structural optimization of BaYO₃ perovskite have been studied for the five possible phases: cubic, tetragonal, hexagonal, orthorhombic and rhombohedral to determine the most stable phase of BaYO₃ perovskite. It has been found that the cubic phase is the most stable one and electronic and mechanical properties of this phase have been investigated. Moreover, the elastic anisotropy has been visualized in detail by plotting the directional dependence of compressibility, Poisson ratio, Young's and Shear moduli for cubic phase. Then, hydrogen bonding to BaYO₃ perovskite has been conducted and hydrogen storage properties of BaYO₃H_x (x = 3 and 9) such as: formation energy, cohesive energy and gravimetric hydrogen storage capacity have been analyzed. Having no study about BaYO₃ perovskite and hydrogen bonding in the literature makes this study the first considerations of BaYO₃ perovskite. Hence, this work could enlighten the possible future studies.     

Effect of pressure on photocatalytic water splitting performance of Z-scheme RP/CH3NH3PbI3 perovskite heterostructure

Energy is an extremely important strategic material basis for socio-economic development. Here, we have investigated the structural, electronic, optical, charge transfer and photocatalytic mechanism of red phosphorus RP/CH₃NH₃PbI₃ heterostructure under hydrostatic pressure by density functional theory (DFT) calculations. Ab initio molecular dynamics (AIMD) prove RP/CH₃NH₃PbI₃ is thermodynamically stable. Interestingly, direct band gap of RP/CH₃NH₃PbI₃ decreases linearly with increase of pressure. The band gap is 1.43 eV at 1.5 GPa, which can enhance the absorption of visible light and generate redshift in absorption edge. Interface charge redistribution makes valence/conduction band edge positions of RP monolayer and CH₃NH₃PbI₃ (010) surface vary with Fermi level, forming an internal electric field have ability to separate electron-hole pairs. Typical Z-scheme heterostructure is formed and supposed to have resulted strong redox ability. The theoretical results hope to provide a possibility for the future experimental preparation of CH₃NH₃PbI₃-based photocatalysts.     

Significant improvement in the dehydriding properties of perovskite hydrides,NaMgH3, by doping with K2TiF6

In this study, perovskite hydrides (NaMgH₃) were synthesized in the presence and absence of a dopant, e.g., K₂TiF₆, by high-energy ball milling. Clear lattice distortion was observed in K₂TiF₆-doped NaMgH₃ as compared to pristine NaMgH₃. The doped sample exhibited better dehydriding properties, and the onset desorption temperature significantly decreased from 580 K to 328 K. The maximum amount of desorbed hydrogen was approximately 3.8 wt% at 638 K, and approximately 90% of hydrogen was desorbed within 20 min. Differential scanning calorimetry results indicated that the activation energy for the two steps in the decomposition of NaMgH₃ decreases by doping with K₂TiF₆. This result suggested that the lattice distortion and low decomposition energy of the NaMgH₃ phase, caused by the introduction of the K₂TiF₆ dopant, result in improved dehydriding properties for NaMgH₃.     

Phase transition of doped LaFeO3 anode in reducing atmosphere and their power generation property in intermediate temperature solid oxide fuel cell

In general, transition metal-doped La_0.6Sr_0.4FeO₃ (LSF) has been used as a cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) because of its high mixed electronic−ionic conductivity and catalytic properties. Recently, some research groups have been investigating the doped LSF as an anode material. In this study, we evaluated the influence of dopant in LSF on anodic properties of LSF in SOFCs. Whereas Mn-doped LSF showed typical perovskite oxide structure even after reduction in hydrogen at high temperature, the LSF and Co-doped LSF exhibited phase transition partially to LaSrFeO₄ and exsolution of metal particles after reduction. The phase transition and metal exsolution occurred at temperature higher than 1008 K in a reducing atmosphere. Despite the partial phase transition, the cell using Co-doped LSF anode exhibited fairly high power density of 1.33 W/cm² at 1173 K with the lowest polarization resistance. These results may originate from the high oxygen-ion conductivity of LaSrFeO₄–La(Sr)Fe(Co)O₃ and the high hydrogen oxidation property of the Co–Fe particles on ceramic anode surface.     

Activity of perovskite La1−xSrxMnO3 catalysts towards oxygen reduction in alkaline electrolytes

The behaviour of the perovskite-based series of compounds La_1−xSr_xMnO₃ (where x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) towards oxygen reduction in an ambient temperature alkaline 1 M KOH electrolyte is presented. Within this series, the intermediate compound La_0.4Sr_0.6MnO₃ exhibits the greatest catalytic activity, approaching that of the considerably more expensive fuel cell grade Pt-black examined under the same conditions. The origin of this activity is discussed in terms of material structure and morphology, which exists in the structural transition region between cubic LaMnO₃ and hexagonal SrMnO₃. The small crystallite size and relatively large BET surface area of this material reflect this high level of structural disorder. Furthermore, these features enable this compound to exhibit the greatest proportion of direct four-electron oxygen reduction (preferred) compared to the less efficient two-electron reduction to peroxide.     

Catalytic hydrogenation of CO2 to light olefins by using K-doped FeCx catalysts derived from the Fe-chitosan complex

K-doped FeC_x catalysts derived from the carbothermal reduction of the Fe-chitosan complex were investigated and tested in the hydrogenation of CO₂ to light olefins (C_2-6⁼). Catalyst characterization, including bulk composition, porosity, crystallinity, Fe coordination, morphology, surface property, and basicity, was conducted. The physicochemical properties of tested catalysts were correlated with their performances. The mechanistic study discovered that bidentate carbonates, monodentate carbonate, bicarbonate, and formate species were intermediates involved in C_2-6⁼ synthesis. The yield of C_2-6⁼ was found to be dependent on the composition of the carbide phases of K-doped FeC_x catalysts. The most active catalyst, i.e., FeK@CS-(0.5)-py, showed the most promising space-time yield of C_2-6⁼ (13.7 μmol_C2-6=/g_Fe/s).     

Electrocatalytic evolution of hydrogen on a novel SrPdO3 perovskite electrode

SrPdO₃ was prepared for the first time by the citrate method. XRD, SEM and TGA characterizations were carried out. The catalytic activity toward hydrogen evolution reaction (HER) was investigated, the activation energy, and reaction order and reaction mechanism have been determined using Tafel polarization and impedance techniques. The modified surface showed up to 100 times more efficiency towards electrocatalytic production of hydrogen. Adsorption of hydrogen on the catalyst was the rate-determining step and the reaction order at the surface of the catalyst is 0.86. The molar magnetic susceptibility was measured using Faraday's method and anti-ferromagnetic character was observed.     

Nickel-based perovskite catalysts with iron-doping via self-combustion for hydrogen production in auto-thermal reforming of Ethanol

Iron-doped LaNiO₃ catalysts with a perovskite structure were prepared via self-combustion and tested in auto-thermal reforming (ATR) of ethanol. Characterizations of temperature-programmed surface reaction (TPSR), X-ray diffraction (XRD), physical N₂ adsorption, and temperature-programmed reduction (TPR) were carried out. The results indicate that LaNiO₃ perovskite structure was successfully formed via self-combustion. With iron-doping in LaNiO₃, the perovskite structure still remains, in the form of solid solution La(Ni, Fe)O₃, where iron is reducible and the nickel-iron alloy forms after the reduction. In addition, the surface area of the iron-doped samples increased. The TPSR results indicate that with iron-doping, the activity for adsorbed ethanol species is modified and a higher activity for methane transformation is achieved. As a result, an LNF10 sample (LaNi_0.90Fe_0.10O₃) with both nickel and nickel-iron alloy shows better performance in ATR: the ethanol conversion is near 100%, while the selectivity to by-products, such as ethylene, ethane, acetaldehyde and methane, is decreased, and CO₂ is the main carbon-containing product; consequently, a hydrogen yield near 3.0 mol H₂/mol EtOH is obtained and remains stable in the 30-h test of ATR.     

CO2 hydrogenation over differently morphological CeO2‐supported Cu‐Ni catalysts

Differently morphological CeO₂‐supported Cu‐Ni catalysts utilized for carbon dioxide hydrogenation to methanol were prepared by the method of impregnation. The 100‐ to 300‐nm CeO₂ nanorod‐supported catalyst dominantly exposed low‐energy (100) and (110) facets, and the Cu‐Ni supported on 10‐ to 20‐nm CeO₂ nanospheres and on irregular CeO₂ nanoparticles were both enclosed by (111) facets owning high energy. Besides, all CeO₂‐supported Cu‐Ni catalysts possess oxygen vacancies, which can active and absorb CO₂ and is further beneficial for the reaction. Most oxygen vacancies were generated from the Ce⁴⁺ reduction to Ce³⁺ with the ceria lattice cell expansion, and small amount of oxygen vacancies resulted from the Ce⁴⁺ replacement by Cu or/and Ni atom. Because of the exposed (100) and (110) facets and numerous oxygen vacancies, well‐defined CeO₂ nanorod‐supported Cu‐Ni alloy showed more superior catalytic performance than on CeO₂ nanospheres and nanoparticles.     

LaCoO3钙钛矿型催化剂对柴油机NOx净化性能研究

采用柠檬酸络合法制备了LaCoO3钙钛矿型催化剂。对其理化特性及NH3-选择性催化还原催化性能的研究结果表明：纯LaCoO3颗粒具有一定的NOx催化还原能力,在250～450℃活性较高;但该催化剂对NH3具有较高的氧化活性,且催化活性随反应温度的升高而提高;在SCR反应中,在400℃以下时,该催化剂显示出一定的NOx净化能力,但当温度超过400℃以后,还原剂的加入反而恶化了NOx排放。不管反应气组成如何,LaCoO3钙钛矿型催化剂对HC和CO都具有良好的催化性能。