Soft-templated NiO–CeO2 mixed oxides for biogas upgrading by direct CO2 methanation

The catalytic performance in the direct CO₂ methanation of a model biogas is investigated on NiO–CeO₂ nanostructured mixed oxides synthesized by the soft-template procedure with different Ni/Ce molar ratios. The samples are thoroughly characterized by means of ICP-AES, XRD, TEM and HR-TEM, N₂ physisorption at −196 °C, and H₂-TPR. They result to be constituted of CeO₂ rounded nanocrystals and of polycrystalline needle-like NiO particles. After a H₂-treatment at 400 °C for 1 h, the surface basic properties and the metal surface area are also assessed using CO₂ adsorption microcalorimetry and H₂-pulse chemisorption measurements, respectively. At increasing Ni content the Ni⁰ surface area increases, while the opposite occurs for the number of basic sites. Using a CO₂/CH₄/H₂ feed, at 11,000 cm³ h⁻¹ g_cat⁻¹, CO₂ conversions in the 83–89 mol% range and methane selectivities >99.5 mol% are reached at 275 °C and atmospheric pressure, highlighting the very good performances of the investigated catalysts.     

γ-MgH2 induced by high pressure for low temperature dehydrogenation

The formation of metastable γ-MgH₂ upon application of ultra-high pressure and its dehydrogenation properties were studied. Magnesium-nickel alloy (14 wt.% Ni) was hydrogenated and compressed at ultra-high pressures of 2.5 and 4 GPa. The phase composition and desorption properties of the products were investigated. Powder X-ray diffraction indicated that some α-MgH₂ converted to γ-MgH₂ during compression. This resulted in the onset of hydrogen desorption at 60 °C under vacuum. Our findings thus show that application of ultra-high pressure can facilitate the formation of γ-MgH₂, which has a lower dehydrogenation temperature (≤200 °C) than α-MgH₂, which desorbs at temperatures above 300 °C. The metastable phase possessed a high hydrogen storage capacity of at least 4.5 wt.%. These properties revealed the potential of γ-MgH₂ as a future hydrogen storage material.     

Synthesis of copper promoted high temperature water–gas shift catalysts by oxidation–precipitation

Copper promoted high temperature water–gas shift catalysts were directly synthesized in the active phase (Fe₃O₄) through an oxidation–precipitation (oxiprecipitation) method, avoiding the usual reduction step used to activate the catalyst in the conventional method based on coprecipitation. They were characterized using X-ray diffraction, X-ray fluorescence, infrared spectroscopy, adsorption–desorption of N₂ at 77 K, transmission electron microscopy and temperature programmed reduction. Their catalytic activity was also studied. The results indicated that the materials obtained through this new synthesis method exhibited higher catalytic activity than the commercialized catalysts. Furthermore, the addition of copper improved the catalytic performance of oxiprecipitated Fe–Cr materials, increasing the reducibility of the sample and its stability under reaction conditions.     

Preparation of NiO–YSZ composite powder by a combustion method and its application for cathode of SOEC

Nickel oxide and yttria-stabilized zirconia (NiO–YSZ) composite powders were synthesized by a new situ-combustion method in this paper. The adding amount of CO(NH₂)₂ was calculated by the combustion reaction equation. The products were characterized by X-ray diffraction, field emission scanning electronic microscope and electrochemical impedance spectra (EIS). The results showed that the products were well crystallized with NiO coating on YSZ particles. The optimized ratio of CO(NH₂)₂ to Ni(NO₃)₂ was 2:1. A single solid oxide electrolysis cell made from NiO–YSZ composite cathode with the powder prepared at optimized ratio 2:1 exhibited better performance than other samples with the electrolytic voltage of 0.98 V. The electrolytic cell was operated steadily at 900 °C for 2 h with the current of 0.33 A cm⁻² when the stream of 80% H₂O + 20% H₂ was input. EIS analysis indicated that H₂O adsorption and diffusion of the Ni–YSZ electrode were the limited step in the whole electrolysis reaction.     

Solid-state synthesis method for the preparation of cobalt doped Ni–Al2O3 mesoporous catalysts for CO2 methanation

In this study, a simple solid-state synthesis method was employed for the preparation of the Ni–Co–Al₂O₃ catalysts with various Co loadings, and the prepared catalysts were used in CO₂ methanation reaction. The results demonstrated that the incorporation of cobalt in nickel-based catalysts enhanced the activity of the catalyst. The results showed that the 15 wt%Ni-12.5 wt%Co–Al₂O₃ sample with a specific surface area of 129.96 m²/g possessed the highest catalytic performance in CO₂ methanation (76.2% CO₂ conversion and 96.39% CH₄ selectivity at 400 °C) and this catalyst presented high stability over 10 h time-on-stream. Also, CO methanation was investigated and the results showed a complete CO conversion at 300 °C.     

Promoted Ni–Co–Al2O3 nanostructured catalysts for CO2 methanation

15 wt.%Ni-12.5 wt.%Co–Al₂O₃ catalysts promoted with Fe, Mn, Cu, Zr, La, Ce, and Ba were prepared by a novel solid-state synthesis method and employed in CO₂ methanation reaction. BET, XRD, EDS, SEM, TPR, TGA, and FTIR analyses were conducted to identify the chemicophysical characteristics of the prepared samples. The addition of Fe, Mn, La, Ce, and Ba was effective to improve the catalytic performance of the 15 wt%Ni-12.5 wt%Co–Al₂O₃ due to the higher CO₂ adsorption capacity of the promoted catalysts. Among the studied promoters, the Fe-promoted catalyst possessed the highest catalytic activity (X_CO2 = 61.2% and S_CH4 = 98.87% at 300 °C). Also, the effect of calcination temperature, feed composition, and GHSV on the performance of the 15 wt%Ni-12.5 wt%Co-5wt%Fe–Al₂O₃ catalyst in CO₂ methanation reaction was assessed. The outcomes confirmed that the 15 wt%Ni-12.5 wt%Co-5wt%Fe–Al₂O₃ catalyst with the BET area of 122.4 m²/g and the highest pore volume and largest pore diameter had the highest catalytic activity. Also, the catalytic performance in the methanation of carbon monoxide was studied, and 100% conversion of carbon monoxide was observed at 250 °C.     

A review on the development of supported non-noble metal catalysts for the endothermic high temperature sulfuric acid decomposition step in the Iodine–Sulfur cycle for hydrogen production

The decomposition of sulfuric acid to sulfur dioxide is an important reaction section in both the thermochemical Iodine–Sulfur (IS) cycle and the Hybrid (Hybs) cycle for hydrogen production. This decomposition reaction is a high temperature, highly endothermic reaction whose activation barrier needs to be reduced by a highly active catalyst. The catalysts reported in this reaction are either exorbitantly expensive, synthesized using noble metals, or exhibiting low activity and stability. Hence, the development of non-noble active and stable catalysts in this reaction is a significant challenge. This review comprehensively discusses the recent developments and activity trends of supported non-noble catalysts including their synthesis, activity & stability trends, mechanistic and kinetic aspects. The catalytic activity of nano-catalysts in sulfuric acid decomposition is largely affected by the size of the metal nanoparticles, dispersion, oxygen vacancies, and metal-support interaction. Herein, we report an in-depth theoretical and experimental understanding of the catalytic challenges and solutions that leads to designing a cost-effective, efficient and stable catalyst in this reaction. The role of catalyst support modification for long run stability is also discussed. Further, literature considering reaction kinetics over various catalysts is also reviewed.     

Electrocatalytic activity of spinel-type oxides LiMn2−XCoXO4 with large specific surface areas for metal–air battery

A series of spinel-type oxides LiMn_2−XCo_XO₄ (X=1.0, 1.2, 1.4, 1.6) are synthesized by an improved amorphous citric precursor (ACP) method. Their specific surface areas are measured by the Brunauer-Emmet-Teller (BET) method, and their particle agglomerates are studied by scanning electron microscopy (SEM). Their electrocatalytic performances are characterized by the polarization curves of gas-diffusion electrodes that employ the oxides. The activity decreases with increasing X values.     

Enhanced activity of CO2 methanation over mesoporous nanocrystalline Ni–Al2O3 catalysts prepared by ultrasound-assisted co-precipitation method

The ultrasound-assisted co-precipitation method was employed for the synthesis of the Ni–Al₂O₃ catalysts with different metal loadings for the CO₂ methanation reaction. This study indicated that increasing the Ni loading up to 25 wt.% enhanced the surface area, decreased the crystallinity and improved the reducibility of the catalysts, while further raise in Ni loading adversely influenced the surface area. Improvements in catalytic performance were obtained with the raise in Ni content because of enhancing the BET area. The results confirmed that the 25Ni–Al₂O₃ catalyst with the highest BET area (188 m² g^?1) and dispersion of Ni has the highest catalytic activity in CO₂ methanation and reached to 74% CO₂ conversion and 99% CH₄ selectivity at 350 °C. In addition, this catalyst exhibited a great stability after 10 h time-on-stream.     

Comparative study of CuO–CeO2 catalysts prepared by wet impregnation and deposition–precipitation

Two different preparation methods are used to synthesize wt. 7% CuO–CeO₂ catalysts: a conventional wet impregnation method, and a deposition–precipitation (DP) method using Na₂CO₃ as precipitating agent. Both samples are characterized by a series of techniques. CuO–CeO₂ (Cu–Ce) prepared by DP shows a lower capacity to release the lattice oxygen to form CO₂. From CO-TPR results, it is demonstrated that this catalyst is not able to reduce copper clusters at low temperatures. Also, CO-TPD shows no CO₂ formation. The activity results confirm the worse performance of Cu–Ce prepared by DP especially when oxygen is not in excess (PROX reaction with stoichometric oxygen). A copper particle size which is too small could create a stronger metal-support interaction, with lower Cu–Ce interface to react.     

Dehydrogenation of Ammonia Borane with transition metal-doped Co–B alloy catalysts

The catalytic performance of transition metal-doped Co–B ternary alloys were tested for H₂ generation by hydrolysis of Ammonia Borane (AB). Chemical reduction method was used to dope Co–B catalyst with various transition metals, namely Cu, Cr, Mo, and W, using their corresponding metal salts. All transition metals induce significant promoting effects on the Co–B catalyst by increasing the H₂ generation rate by about 3–6 times as compared to the undoped catalyst. The effect of metal dopant concentration on overall catalyst structure, surface morphology, and catalytic efficiency were examined by varying the metal/(Co + metal) molar ratio. Characterizations such as XPS, XRD, SEM, BET surface area measurement, and particle size analysis were carried out to understand the promoting role of each dopant metal during AB hydrolysis. Dopant transition-metals, in either oxidized or/and metallic state, act as an atomic barrier to avoid Co–B particle agglomeration thus preserving the effective surface area. In addition, the oxidized species such as Cr³⁺, Mo⁴⁺, and W⁴⁺, act as Lewis acid sites to enhance the absorption of OH⁻ group to further assist the hydrolysis reaction over alloy catalysts. The promoting nature of transition metal dopants in Co–B alloy powders is demonstrated by the evaluated low activation energy of the rate limiting step and high H₂ generation rate (2460 ml H₂ min⁻¹ (g of catalyst)⁻¹ for Co–Mo–B) in the hydrolysis of AB.     

Role of chromium in Cr–Fe oxide catalysts for high temperature water-gas shift reaction – A DFT study

Carcinogenic hexavalent Cr in the current iron oxide-based catalysts of high temperature water-gas shift (HT-WGS) reaction is great environmental concern. Interpreting the role of the Cr in this important industrial catalyst system is required. In the present study, we investigated substitution of Cr atoms into the most stable termination of Fe₃O₄ (111) slab surface by spin-polarized periodic DFT approach to comprehend the role of Cr. We applied the projector augmented-wave (PAW) method within the Perdew-Wang 1991 (PW91) form of the generalized gradient approximation (GGA) on the Vienna Ab-initio Simulation Package (VASP). The calculations point out that Cr atoms choose being below the surface FeO₆ sites. Cr slightly affects the dissociative H₂O adsorption. There is no effect on the CO adsorption. Oxygen vacancy is favored to form on the topmost layer with less vacancy formation energy. Substitution of Cr into the structure increases the oxygen vacancy formation energy. This indicates that Cr does not act as a chemical promoter and does not affect the catalytic activity positively, which is experimentally confirmed by the previous studies.     

Phosphate-modified Al2O3–ZrO2 supported Ni catalysts for efficient selective methanation of CO in H2-rich reformate gas

CO selective methanation can remove the CO in H₂-rich reformate gas to prevent the poisoning of Pt anode electrode in proton exchange membrane fuel cell. However, the methanation of CO₂ in H₂-rich gas consumes a lot of hydrogen, which greatly reduces the energy efficiency. In order to inhibit CO₂ methanation, mesostructured Al₂O₃–ZrO₂ was modified by different amounts of phosphate, and then was as Ni support. The structures and surface properties of Ni/Al₂O₃–ZrO₂ catalyst modified by phosphate were studied to reveal the effect of phosphate-modification on CO conversion and selectivity for CO methanation. It was found that the phosphate-modification inhibited the adsorption of CO₂, which increased the selective for CO methanation. But the modification with excess phosphate lessened active sites of Ni and weakened the adsorption of H₂ and CO, which decreased the activity of CO methanation.     

Synthesis of mesoporous Co–Ce oxides catalysts by glycine-nitrate combustion approach for CO preferential oxidation reaction in excess H2

Preferential oxidation of CO (CO-PrOx) is an important step to meet the need of the proton exchange membrane (PEM) fuel cell without the Pt anion poison. A glycine-nitrate approach was used for the synthesis of Co/CeO₂ nanoparticle for preferential oxidation of CO, which a precursor solution was prepared by mixing glycine with an aqueous solution of blended nitrate in stoichiometric ratio. Then the glycine-mixed precursor solution was heated in a beaker for producing nanosized porous powders. Catalytic properties of the powders were investigated and results illustrate that the Co-loading of 30 wt.% catalysts exhibits excellent catalytic properties. Various characterization techniques like X-ray diffraction, SEM, BET, Raman and TPR were used to analyze the relationship between catalyst nature and catalytic performance. The X-ray diffraction patterns and SEM micrographs indicate that catalysts prepared by glycine-nitrate combustion own mesopore structure. The BET, Raman and TPR results showed that the high activity of the 30 wt.% Co-loading of Co/CeO₂ catalysts is related to the high BET surface and the strongly interaction between fine-dispersed Co species and CeO₂ support.     

Preparation of a high performance Pt–Co/C electrocatalyst for oxygen reduction in PEM fuel cell via a combined process of impregnation and seeding

The preparation of a Pt–Co/C electrocatalyst for the oxygen reduction reaction in PEM fuel cells was achieved via a combined process of impregnation and seeding. The effects of initial pH of the precursor solution and Pt loading were all found to have a significant effect on both the electrocatalyst morphology and the cell performance when tested in a single PEM fuel cell. The optimum condition found for preparing the Pt–Co/C electrocatalyst was from an initial precursor solution pH of 2 at the metal loading of 23.6–30.3% (w/w). The Pt–Co/C electrocatalysts, formed under these optimal conditions, tested in a single PEM fuel cell with the carbon sub-layer, gave a cell performance of 772 mA/cm² or 460 mW/cm² at 0.6 V in a H₂/O₂ system. An electron pathway of oxygen reduction on the prepared Pt–Co/C electrocatalyst was also determined using a rotating disk electrode.     

Preparation and evaluation of Ni/γ-Al2O3 catalysts promoted by alkaline earth metals in glycerol reforming with carbon dioxide

Glycerol is the main by-product in the biodiesel process and can be considered as a promising and renewable source for hydrogen generation through the reforming process. In this work, catalysts with 15 wt% Ni supported on 3 wt% M ? Al₂O₃ (M = MgO, CaO, SrO, and BaO) were prepared and employed in the glycerol dry reforming (GDR) reaction to produce hydrogen and carbon monoxide. The textural characteristics of the fresh and spent catalysts were determined using the ICP, BET, TPR, TPO, and SEM analyses. Based on the obtained results, the catalyst promoted by SrO had the highest catalytic activity. The results indicated that adding various alkaline-earth oxides into the catalyst support decreased the Ni crystalline size from 17.2 nm to 7.4–10.9 nm. Moreover, all promoted catalysts showed better catalytic performance and the promoted sample with 3 wt% SrO possessed higher stability than unpromoted catalyst during 20 h on stream.     

Studies on metal oxides suitable for enhancement of the O2-releasing step in water splitting by the MnFe2O4–Na2CO3 system

Reactivities of three metal oxides (Fe₂O₃, TiO₂ and MnO₂) with Na₂CO₃, and of their reaction products with CO₂, have been studied to enhance the O₂-releasing step in the two-step water splitting by the MnFe₂O₄–Na₂CO₃ system. X-ray diffraction analysis and thermogravimetric measurements showed that the reaction of α-Fe₂O₃ with Na₂CO₃ (mole ratio=1:1) completed within 10 min at 1073 K to produce NaFeO₂ (Fe₂O₃+Na₂CO₃→NaFeO₂+CO₂). Also, the regeneration of Fe₂O₃ and Na₂CO₃ proceeded readily by passing CO₂ gas through NaFeO₂ (71% yield). TiO₂ reacted with Na₂CO₃ (mole ratio=1:1) at 1073 K for 1 h to form Na₈Ti₅O₁₄ and Na₂TiO₃ (93% yield). However, in the reaction of the products with CO₂, the starting material (TiO₂) was not reproduced at the temperature range from 673 K to 1073 K, but Na₄Ti₅O₁₂ (having a lower Na content than the form Na₈Ti₅O₁₄) was formed. In the case of MnO₂ with Na₂CO₃ (mole ratio=2:1), Na_0.7MnO_2–2.05 and NaMnO₂ were produced at 1073 K for 1 h (80% yield), but the reaction between these products and CO₂ hardly proceeded.     

A proposed mechanism to form nanosized Mn oxides from the decomposition of β-cyclodextrin-Mn complex: Toward nanosized water-splitting catalysts with special morphology

Metal oxides are promising compounds for water-splitting systems toward hydrogen production. One of the most important methods to prepare nanosized metal oxides is the decomposition of metal complexes. Herein, different Mn oxides from the decomposition of β-cyclodextrin-Mn complex in different calcined temperatures were studied by scanning electron microscopy, thermal gravimetric analysis, X-ray diffraction and a few electrochemical methods. Using scanning electron microscopy, we propose a mechanism to answer to the question that how is Mn oxide formed by the decomposition of β-cyclodextrin-Mn complex.In the next step, usual chemometric methods, principal component analysis (PCA) and multivariate curve resolution-alternating least squares (MCR-ALS), were used to not only determine the number of components and data visualization, but also examine the square wave voltammograms of obtained Mn oxides from the decomposition of β-cyclodextrin-Mn complex. Water-oxidizing activities of the compounds under the presence of photo-produced Ru(bpy)₃³⁺ were also considered.     

Synthesis of high stability nanosized Rh/CeO2–ZrO2 three-way automotive catalysts by Rh chemical state regulation

The chemical state of precious metals plays an important role in redox reaction. In order to maintain rhodium (Rh) at metallic state, which is recognized as reactive center in three-way catalytic reactions, we synthesized Rh/CeO₂–ZrO₂ (Rh/CZ) catalyst by liquid phase reduction adsorption with glycerol as reductant (Rh/CZ-g). Detailed characterizations were conducted to investigate the chemical state of Rh species, and the CO-FTIR and XPS results indicated that Rh species mainly existed in metallic state for Rh/CZ-g. Through CO chemisorption and H₂-TPR, it’s proved that the liquid phase reduction treatment would weaken the interaction between Rh species and CeO₂–ZrO₂ support, avoiding the supply of oxygen species from CeO₂–ZrO₂ to Rh in some degree. Compared with Rh/CeO₂–ZrO₂ catalyst prepared by incipient wetness impregnation, Rh/CZ-g exactly had superior activity in three-way catalytic process. Combined with in situ DRIFTS, some important intermediate species, such as Rh–CN, Rh-(NO)₂, formates and acetates, preferred to adsorb and transform on Rh/CZ-g with larger proportion of metallic Rh, which determined Rh/CZ-g was more favorable for three-way catalytic reactions, even after aged at 950 °C for 5 h. The purpose of this work is to inspire researchers use simple and valid methods to synthesize highly efficient three-way catalysts.     

Preparation of Ni–Cu/Mg/Al catalysts from hydrotalcite-like compounds for hydrogen production by steam reforming of biomass tar

Ni–Cu/Mg/Al bimetallic catalysts were prepared by the calcination and reduction of hydrotalcite-like compounds containing Ni²⁺, Cu²⁺, Mg²⁺, and Al³⁺, and tested for the steam reforming of tar derived from the pyrolysis of biomass at low temperature. The characterizations with XRD, STEM-EDX, and H₂ chemisorption confirmed the formation of Ni–Cu alloy particles. The Ni–Cu/Mg/Al bimetallic catalyst with the optimum composition of Cu/Ni = 0.25 exhibited much higher catalytic performance than the corresponding monometallic Ni/Mg/Al and Cu/Mg/Al catalysts in the steam reforming of tar in terms of activity and coke resistance. The catalyst gave almost total conversion of tar even at temperature as low as 823 K. This high performance was related to the higher metal dispersion, larger amount of surface active sites, higher oxygen affinity, and surface modification caused by the formation of small Ni–Cu alloy particles. In addition, the Ni–Cu/Mg/Al catalyst showed better long-term stability than the Ni/Mg/Al catalyst. No obvious aggregation and structural change of the Ni–Cu alloy particles were observed. The coke deposition on the Ni–Cu/Mg/Al catalyst was approximately ten times smaller than that on the Ni/Mg/Al catalyst, indicating good coke-resistance of the Ni–Cu alloy particles.