Controllable synthesis of mesoporous alumina as support for palladium catalysts and reconstruction of active sites during methane combustion

Uniform mesoporous alumina (MA) nanospheres with excellent thermal stability and controllable pore size distribution were facilely obtained through ultrasonic assisted sol-gel method then taken as carriers for synthesizing a series of supported palladium catalysts. Results evidenced that the pore size distribution of MA significantly affected the catalytic performance of as-prepared catalysts. More importantly, the methane pretreatment tuned valence state of palladium species and distribution of surface oxygen species, along with promoting the adsorption of CH₄ on catalysts and the generation of CH₃1 group to form methane-derived adsorbates. Moreover, the synergistic effect of PdO and surface adsorbed oxygen effectively activated CH₄ and invoked the oxidation of methane-derived adsorbates, hence boosting the catalytic activity with the 90% CH₄ conversion temperature (T₉₀) decreased by 40 °C relative to the unpretreated catalyst. Noticeably, the T₉₀ of Pd/MA(CH₄) catalyst could be attained at 380 °C after subsequent long term and cyclic stability test.     

Kinetics of catalytic oxidation of methane,ethane and propane over palladium oxide

Catalytic oxidation of methane, ethane and propane over a palladium oxide (PdO) surface was investigated experimentally by wire microcalorimetry. The oxidation rate was determined for each reactant at atmospheric pressure in the temperature range of 600–800 K. The apparent kinetic parameters were extracted from the experimental measurements. It is shown that the oxidation of these hydrocarbons over the PdO surface proceeds with a similar mechanism: they undergo dissociative adsorption followed by the conversion of surface fragments to final products. A detailed surface reaction model is proposed, and the kinetic parameters of the crucial reactions are deduced from the present experimental observations. The catalytic oxidation rates are found to increase in the order of methane, ethane and propane. This observation is consistent with density functional theory calculations and may be correlated with the C–H bond energies of the corresponding surface intermediates.     

Barium promoted manganese oxide catalysts in low-temperature methane catalytic combustion

In this study, a series of BaO-MnO_x mixed oxide catalysts were synthesized by the mechanochemical method and employed in lean methane catalytic combustion (MCC) at low temperatures. The synthesized catalysts were characterized by XRD, BET, TGA, FT-IR, H₂-TPR, O₂-TPD, and FESEM analyses. The results indicated that the 10 wt% BaO-MnO_x catalyst with a BET surface area of 25 m² g^?1 possessed the best catalytic performance. The higher activity of the 10 wt% BaO-MnO_x catalyst was due to the higher ability to supply oxygen through the components during the MCC process. The light-off temperature corresponding to 50% of the methane conversion was about 330 °C, which was about 50 °C lower than the pure MnO_x. Moreover, for the BaO(10)-MnO_x catalyst, the 10 and 90% of methane conversion temperatures were about 305 and 427 °C, respectively. Also, the 10 wt% BaO-MnO_x catalyst exhibited high catalytic stability under dry feed condition at 450 °C for 50 h. Furthermore, the influence of various parameters such as calcination temperature, feed ratio, GHSV, pretreatment condition, and presence of water vapor in the feedstock was studied on the catalytic performance.     

Ni and Ni3C catalysts supported on mesoporous silica for dry reforming of methane

Ni and Ni₃C catalysts supported on nano-sized and mesoporous silica were used to study the dry reforming reaction of biogas. Ni catalysts were deposited over either nano-sized silica or a novel mesoporous silica (450 m²/g), which were the main supports used in this study. Subsequently, a secondary active phase (Pt) and support (MgO) were added. In addition, mesoporous-supported Ni was also subjected to a carburization process with CH₄. Size effects, preparation techniques and chemical nature of co-supports were studied. Catalytical, microstructural, chemical and molecular characterization of fresh and spent materials were carried out using BET, H₂-TPR, XRD, HRTEM, WDXRF, and Raman spectroscopy. The reaction was undertaken at 700 °C and 1 atm. Results evidence that catalyst supported on both mesoporous silica and also nickel carbide catalyst presented high stability and slow deactivation overall in spite the high content of carbon. The addition of Pt did not increase stability of the catalysts.     

Ordered mesoporous alumina supported nickel based catalysts for carbon dioxide reforming of methane

Ordered mesoporous alumina facilely synthesized via improved evaporation-induced self-assembly (EISA) strategy was provided with large specific surface area, big pore volume, uniform pore size and excellent thermal stability. The obtained mesoporous material was used as the carrier of the Ni based catalysts for carbon dioxide reforming of methane. These mesoporous catalysts performed high catalytic activity and long stability. Typically, the catalytic conversions of the CH₄ and CO₂ were greatly close to the equilibrium conversion and no deactivation was observed during the 100 h long lifetime test. The advantageous structural properties of ordered mesoporous alumina contributed to high dispersion of the Ni particles among the mesoporous framework, which further accounted for the good catalytic activity due to more “accessible” Ni active sites for the reactants. The “confinement effect” of the mesopores could effectively prevent the thermal sintering of the Ni nanoparticles to some extent, committed to its long-term catalytic stability. Besides, the mesoporous catalysts possessed enhanced ability to withstand coke, although not any modifiers had been added. Properties of the coke over the mesoporous catalyst were also carefully investigated. Therefore, the ordered mesoporous alumina was a promising catalyst support for the carbon dioxide reforming with methane.     

La-doped cobalt supported on mesoporous alumina catalysts for improved methane dry reforming and coke mitigation

The promotional La₂O₃ effect on the physicochemical features of mesoporous alumina (MA) supported cobalt catalyst and its catalytic performance for methane dry reforming (MDR) was examined at varied temperature and stoichiometry feedstock. The Co₃O₄ nanoparticles were evidently scattered on fibrous mesoporous alumina with small crystal size of 8–10 nm. The promotion behavior of La₂O₃ facilitated H₂-reduction by providing higher electron density and enhanced oxygen vacancy in 10%Co/MA. The addition of La₂O₃ could reduce the apparent activation energy of CH₄ consumption; hence, increasing CH₄ conversion up to 93.7% at 1073 K. The enhancement of catalytic activity with La₂O₃ addition was also due to smaller crystallite size, alleviated H₂-reduction and the basic character of La₂O₃. Lanthanum dioxycarbonate transitional phase formed in situ during MDR was accountable for mitigating deposited carbon via redox cycle for 17–30% relying on reaction temperature. Additionally, the oxygen vacancy degree increased to 73.3% with La₂O₃ promotion. The variation of H₂/CO ratios within 0.63–0.99 was preferred for downstream generation of long-chain olefinic hydrocarbons.     

Improved methane oxidation activity of P-doped γ-Al2O3 supported palladium catalysts by tailoring the oxygen mobility and electronic properties

Micro-mesoporous P-doped γ-Al₂O₃ with cluster morphology was obtained via an efficient ultrasound-assisted sol-gel process and taken as carrier to construct palladium catalysts for methane oxidation. It was revealed that the structure and properties of catalysts were significantly influenced by the phosphorus precursors with diverse valence and acidity. Dissimilar reducibility of surface hydroxyl and oxygen species is observed in the catalysts derived from different phosphorus sources, indicating the difference in the oxygen mobility and the capacity of the catalysts to convert intermediate CO. The behavior of charge-transfer transition and d-d transition, the transfer ability of electrons from palladium particles into the antibonding 2π* orbitals of CO, together with the surface acidity and electronic density of palladium species was likewise tailored, which demonstrated the metal-support interaction could be tuned, making palladium species behave with diverse status and electronic structures. The optimized properties cooperatively provided an enhancement in catalytic performance of P-containing catalysts.     

Perovskite catalysts for methane combustion: applications,design, effects for reactivity and partial oxidation

This review underlines the importance of the developments in perovskite catalysts for methane combustion from the past up to the present. In this review, after a general and brief introduction to perovskites, the mechanisms of catalytic combustion of methane have been included. Moreover, current studies on perovskites have been summarized including the effects of substitutions, doped perovskites, perovskite preparation methods, and the effect of sulfur presence on perovskite catalysts. Besides, recent studies on perovskite oxides and phenomenon of oxygen (O₂) deficiency, porous perovskite oxides, and nanostructured perovskites have been conducted. In addition, partial oxidation of methane (POM) has been reviewed. The loss of active component during the POM reaction can take place in the nickel catalyst, in particular. Since nickel has a lower melting point than noble metals and other active components, such as Co and Fe, in general, to deactivate nickel is easier. Compared with conventional structure, the porous structure with the unique morphology significantly enhances the catalytic activity through a much larger surface area (SA) and greater reactivity of the active sites. Furthermore, the monolithic nanoarrayed perovskite presents very good results in well‐defined faceted catalysts and takes part in porous channel hydrocarbon combustion. This review study is prepared as a guide to cover the profound knowledge of perovskite oxides catalysts, considering the methane combustion reaction mechanisms, and addresses prospective studies in this field for researchers.     

The recent areas of applicability of palladium based membrane technologies for hydrogen production from methane and natural gas: A review

In the wake of the devastating consequences of climate change, many countries are searching for alternative renewable energy. Hydrogen, the most abundant element on earth, is an alternative clean and non-toxic energy source. Palladium-based membranes and their alloys are categorized as inorganic metallic membranes with the highest selectivity and permeation rate for hydrogen production. Pd-based membranes have great potential for resolving environmental concerns and adverse side effects of greenhouse gases resulting from industrial processes. This paper analyses Pd-based membranes and their industrial applications while focusing on natural gas and methane as non-renewable feedstocks for hydrogen production. Steam reforming of natural gas and methane, partial oxidation reaction, auto thermal reforming, dry reforming, and gas to liquid process are among the processes that take place in a Pd-based membrane reactor and are discussed in this paper. Finally, all the ongoing research and development on both laboratory and industrial scales are reviewed.     

Low metal content Co and Ni alumina supported catalysts for the CO2 reforming of methane

Low metal content Co and Ni alumina supported catalysts (4.0, 2.5 and 1.0 wt% nominal metal content) have been prepared, characterized (by ICP-OES, TEM, TPR-H₂ and TPO) and tested for the CO₂ reforming of methane. The objective is to optimize the metal loading in order to have a more efficient system. The selected reaction temperature is 973 K, although some tests at higher reaction temperature have been also performed. The results show that the amount of deposited carbon is noticeably lower than that obtained with the Co and Ni reference catalysts (9 wt%), but the CH₄ and CO₂ conversions are also lower. Among the catalysts tested, the Co(1) catalyst (the value in brackets corresponds to the nominal wt% loading) is deactivated during the first minutes of reaction because CoAl₂O₄ is formed, while Ni(1) and Co(2.5) catalysts show a high specific activity for methane conversion, a high stability and a very low carbon deposition.     

Lean methane catalytic combustion over the mesoporous MnOx-Ni/MgAl2O4 catalysts: Effects of Mn loading

In the current investigation, the textural features and catalytic efficiency of the Mn-promoted Ni/MgAl₂O₄ catalysts were evaluated in the catalytic combustion of lean methane. The results demonstrated that adding manganese oxide up to 5 wt.% to the catalyst improved the structural features and light-off temperatures due to the increase of the reduction degree and oxygen vacancies. The role of the T_calcination and processing factors on the methane conversion of the 5%Mn-20%Ni/MgAl₂O₄ catalyst was also studied. The results indicated that the CH₄ conversion decreased with the increment of O₂/CH₄ molar ratio and GHSV value. The stability test revealed that the prepared sample exhibited high stability during 10 h time on stream. Furthermore, the obtained results indicated that the BET area, reduction degree, oxygen mobility, and catalytic performance increased with decreasing the calcination temperature from 700 to 500 °C.     

Ammonia chemistry in oxy-fuel combustion of methane

The oxidation of NH₃ during oxy-fuel combustion of methane, i.e., at high [CO₂], has been studied in a flow reactor. The experiments covered stoichiometries ranging from fuel rich to very fuel lean and temperatures from 973 to 1773 K. The results have been interpreted in terms of an updated detailed chemical kinetic model. A high CO₂ level enhanced formation of NO under reducing conditions while it inhibited NO under stoichiometric and lean conditions. The detailed chemical kinetic model captured fairly well all the experimental trends. According to the present study, the enhanced CO concentrations and alteration in the amount and partitioning of O/H radicals, rather than direct reactions between N-radicals and CO₂, are responsible for the effect of a high CO₂ concentration on ammonia conversion. When CO₂ is present as a bulk gas, formation of NO is facilitated by the increased OH/H ratio. Besides, the high CO levels enhance HNCO formation through NH₂+CO. However, reactions NH₂+O to form HNO and NH₂+H to form NH are inhibited due to the reduced concentration of O and H radicals. Instead reactions of NH₂ with species from the hydrocarbon/methylamine pool preserve reactive nitrogen as reduced species. These reactions reduce the NH₂ availability to form NO by other pathways like via HNO or NH and increase the probability of forming N₂ instead of NO.     

Hydrogen production by methane decomposition: A comparative study of supported and bulk ex-hydrotalcite mixed oxide catalysts with Ni,Mg and Al

A catalytic comparative study of CO_x-free hydrogen production by methane decomposition was carried out. Catalytic performances of bulk Ni-mixed oxides derived from Ni/Mg/Al-hydrotalcites (ex-HTs-Ni) were compared with those obtained with Ni supported on mixed oxides derived from Mg/Al-hydrotalcites (Ni/ex-HTs), or on commercial supports (γ-Al₂O₃, MgO and MgO-modified γ-Al₂O₃). Catalyst characterization and their catalytic performance showed both ex-HTs-Ni and Ni/ex-HTs appear to be a similar regardless of their method of preparation. Ni/γ-Al₂O₃ was the best supported catalyst, although the catalytic performances of the ex-HTs catalysts were better. Higher NiMg interaction in ex-HTs provides higher resistance to deactivation. Characterization by TG, Raman spectroscopy and TEM of spent catalysts in the reaction suggest the degree of ordering of the graphitic layers of the carbon deposit onto the catalyst surface is the key factor in the catalyst deactivation. The higher degree of ordering or graphitization of the carbon produced with the higher concentration of sp2 carbons on the surface of the Ni/γ-Al₂O₃ favours its faster deactivation by Ni-coverage than the bulk catalyst (ex-HT-Ni), in which the MWNT type carbon is mainly obtained.     

Promoting effect of ruthenium,platinum and palladium on alumina supported cobalt catalysts for ultimate generation of hydrogen from hydrazine

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High temperature iron-based catalysts for hydrogen and nanostructured carbon production by methane decomposition

The production of hydrogen and filamentous carbon by means of methane decomposition was investigated in a fixed-bed reactor using iron-based catalysts. The effect of the textural promoter and the addition of Mo as a dopant affects the catalysts performance substantially: iron catalyst prepared with Al₂O₃ showed slightly higher catalytic performance as compared to those prepared with MgO; Mo addition was found to improve the catalytic performance of the catalyst prepared with MgO, whereas in the catalyst prepared with Al₂O₃ displayed similar or slightly poorer results. Additionally, the influence of the catalyst reduction temperature, the reaction temperature and the space velocity on the hydrogen yield was thoroughly investigated. The study reveals that iron catalysts allow achieving high methane conversions at operating temperatures higher than 800 °C, yielding simultaneously carbon nanofilaments with interesting properties. Thus, at 900 °C reaction temperature and 1 l g⁻¹_cat h⁻¹ space velocity, ca. 93 vol% hydrogen concentration was obtained, which corresponds to a methane conversion of 87%. Additionally, it was found that at temperatures higher than 700 °C, carbon co-product is deposited mainly as multi walled carbon nanotubes. The textural and structural properties of the carbonaceous structures obtained are also presented.     

Influence of the preparation method in the metal-support interaction and reducibility of Ni-Mg-Al based catalysts for methane steam reforming

Ni-Mg-Al based catalysts were prepared using different preparation methods (impregnation, impregnation-coprecipitation and coprecipitation) and tested in steam reforming of methane. The differences observed in catalytic activity were directly correlated to the physicochemical properties and the different degree of Ni-Mg-Al interaction. The reducibility results showed that the catalyst prepared by the impregnation-coprecipitation method presented the most optimal metal-support interaction to reduce the NiO preserving the Ni⁰ particles highly dispersed on the support surface. These results demonstrate that the structure and catalytic performance of Ni-Mg-Al based catalysts can be tuned by controlling the metal-support interaction through of the preparation method.     

Preparation of supported Ni catalysts with a core/shell structure and their catalytic tests of partial oxidation of methane

Synthesis of supported Ni catalysts with a core/shell structure at the multibubble sonoluminescence (MBSL) condition and their catalytic tests for methane decomposition by partial oxidation were performed in this study. The catalysts prepared were analyzed by XRD, TEM and XPS. Without doping the third components, the supported catalyst of core/shell structure made with 10% Ni loading on Al₂O₃ yields 96% conversion efficiency of methane at reaction temperature of 800 °C and shows excellent thermal stability for the first 40 h. It turns out that coexistence of NiO and NiOx species on the surface of the catalysts play a very important role in the partial oxidation of methane. In addition, the uniform layer of Ni particles on the surface of support material hindered coke formation and sintering process, which enhances thermal stability for the catalysts.     

Experimental investigation of premixed combustion limits of hydrogen and methane additives in ammonia

In this study, a specially designed premixed combustion chamber system for ammonia-hydrogen and methane-air laminar premixed flames is introduced and the combustion limits of ammonia-hydrogen and methane-air flames are explored. The measurements obtained the blow-out limits (mixed methane: 400–700 mL/min, mixed hydrogen: 200–700 mL/min), mixing gas lean limit characteristics (mixed methane: 0–82%, mixed hydrogen: 0–37%) and lean/rich combustion characteristics (mixed methane: ? = 0.6–1.9, mixed hydrogen: ? = 0.9–3.2) of the flames. The results show that the ammonia-hydrogen-air flame has a smaller lower blow-out limit, mixing gas ratio, lean combustion limit and higher rich combustion limit, thereby proving the advantages of hydrogen as an effective additive in the combustion performance of ammonia fuel. In addition, the experiments show that increasing the initial temperature of the premixed gas can expand the lean/rich combustion limits of both the ammonia-hydrogen and ammonia-methane flames.     

Catalytic performance and characterization of Neodymium-containing mesoporous silica supported nickel catalysts for methane reforming to syngas

Ordered mesoporous silica materials based on nickel and other elements have been extensively studied because controlling the size of metal nanoparticles is an effective method to tune the superficial physicochemical process. Neodymium (Nd)-promoted mesoporous silica xNdMS (x: molar ratio of Nd/Si = 0.01, 0.02, 0.04, 0.06) were prepared through a sol–gel strategy. Nickel-based catalysts with high dispersion by using xNdMS as supports were investigated for methane reforming with carbon dioxide and/or oxygen to produce syngas. xNdMS supports and nickel catalysts were examined by combining textural, structural, local and surface information. The characterization results showed that Nd was successfully incorporated into the mesoporous framework of MS and Nd was beneficial to improve the metal dispersion. All Nd-promoted Ni/MS catalysts were effective for the methane reforming reaction. Ni/0.04NdMS catalyst exhibited the highest initial catalytic activity during 12 h time on stream, which was attributed to its high metal dispersion, more basic sites and the strengthened nickel-support interaction. The readily deactivation and poorest catalytic activity of Ni/MS catalyst were due to the serious oxidation of metallic nickel under reaction medium.