Highly efficient MOF-templated Ni catalyst towards CO selective methanation in hydrogen-rich reformate gases

Highly efficient and non-noble metal-based Ni/ZrO₂ catalyst templated with Ni/UiO-66 precursor was successfully prepared and applied to CO selective methanation in H₂-rich gases. This catalyst showed excellent activity and selectivity in an extremely wide temperature window of 215–350 °C, and it also had high stability with no deactivation during a long-term stability test (120 h). The increased specific surface area, smaller crystallite size (3.5 nm) and higher dispersion (15.3%) of Ni nanoparticles, and the enhanced chemisorption capability for CO might contribute to its excellent performance.     

Promising hydrothermal technique for efficient CO2 methanation over Ni/SBA-15

The comparative study of different hydrothermal treatment techniques (Reflux (R) and Teflon (T)) and without hydrothermal technique (W) towards efficient CO₂ methanation over Ni/SBA-15 was discussed. X-ray diffraction (XRD), inductive coupling plasma-atomic emission spectroscopy (ICP-AES), N₂ adsorption-desorption isotherms (BET), Fourier transform infrared (FTIR) spectroscopy, UV-vis diffuse reflectance spectroscopy (UV-Vis DRS), scanning electron microscope – energy dispersion x-ray (SEM-EDX), and transmission electron microscope (TEM) analysis showed that Ni/SBA-15(R) possessed fascinating catalytic properties owing to the highest surface area (814 m²/g) and pore diameter (5.49 nm) of SBA-15(R), finest metal particles (17.92 nm), strongest metal-support interaction and highest concentration of basic sites. The efficacy of Ni/SBA-15 towards CO₂ methanation was descending as Ni/SBA-15(R) > Ni/SBA-15(T) > Ni/SBA-15(W), implying the outstanding performance of Ni/SBA-15(R) which in parallel with the characterization results. The lowest performance of Ni/SBA-15(W) was due to the poorest properties of support; lowest surface area and pore diameter, largest Ni sizes, weakest metal-support interaction and lowest concentration of basic sites. This study successfully developed fascinating Ni/SBA-15 through the reflux hydrothermal treatment technique for CO₂ methanation.     

CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity

Ni based catalysts are usually used for catalyzing the CO₂ methanation to produce synthetic natural gas due to their low cost, though their catalytic activities cannot be comparable with the noble metal counterparts. In order to address this challenge, a series of rare earth (La, Ce, Sm, and Pr) doped Ni based mesoporous materials had been facilely fabricated by the one-pot evaporation induced self-assembly (EISA) strategy and directly employed as the catalysts for CO₂ methanation. These mesoporous catalysts had been systematically characterized by means of X-ray diffraction, N₂ physisorption, transmission electron microscope, X-ray photoelectron spectroscopy, H₂ temperature programmed reduction, CO₂ temperature programmed desorption, and so on. It was found that the Ni species were highly dispersed among the mesoporous framework and the strong metal-framework interaction had been formed. Thus, the thermal sintering of the metallic Ni nanoparticles could be effectively suppressed under CO₂ methanation conditions, promising these mesoporous catalysts with 50 h excellent catalytic stabilities without evident deactivation. Besides, the rare earth dopants could greatly increase the surface basicity of the catalysts and intensify the chemisorption the CO₂. Further, the rare earth elements were also functioned as the electron modifiers, which was also helpful in activating the CO₂ molecule. The apparent activation energies of CO₂ could be obviously decreased by rare earth dopants. As a result, their low-temperature catalytic activity had been greatly intensified over these rare earth elements promoted catalysts.     

Optimal Ni loading towards efficient CH4 production from H2 and CO2 over Ni supported onto fibrous SBA-15

The transformation of SBA-15 into fibrous type SBA-15 (F-SBA-15) as well as the influence of Ni loadings (1, 3, 5, and 10 wt%) towards an efficient CH₄ production from H₂ and CO₂ were explored. The synthesized catalysts were characterized using XRD, BET, ICP-MS, FTIR, FESEM-EDX, TEM, and in-situ FTIR adsorbed pyrrole. Increasing Ni loadings onto F-SBA-15 support promoted excellent performance towards CO₂ methanation. The efficacy in CO₂ methanation over Ni/F-SBA-15 increased with a sequence of 1%Ni/F-SBA-15 < 3%Ni/F-SBA-15 < 5%Ni/F-SBA-15 ≈ 10%Ni/F-SBA-15, indicating the superior performance and stability of 5%Ni/F-SBA-15. The increasing trend was due to the fibrous morphology of support which enhanced the quantity of SiONi bond, triggered better Ni dispersion, strengthen metal-support interaction, and increased the basicity. However, higher Ni loadings (10 wt%) onto F-SBA-15 slightly declined the performance and stability of CO₂ methanation due to the limited spaces for substitution of Ni species with the silanol groups of F-SBA-15 upon the bulk Ni phase, poorer Ni dispersion, weaker metal-support interaction, and lower basicity. The new finding of combination between fibrous SBA-15 (F-SBA-15) with an optimum Ni loading contributed towards an outstanding performance and thus could be applied in various applications.     

Effect of Ni ratio on mesoporous Ni/MgO nanocatalyst synthesized by one-step hydrothermal method for thermal catalytic decomposition of CH4 to H2

The Ni/MgO catalysts were synthesized by hydrothermal method and the effect of changes in nickel ratio on the physicochemical properties and catalytic performance were investigated for TCD of CH₄ to H₂. In all catalysts, the only NiO–MgO phase as solid solution was formed, and crystals size were in the nanoparticles range. The results confirmed that the catalysts with mesoporous structure and high porosity were successfully synthesized by one step in the absence of surfactants. The increasing nickel ratio from 10 to 40 wt% increases the methane conversion from 28.3 to 48.6% and hydrogen yield from 33.2 to 53.2%, respectively. Also, the stability of catalysts depends on the amount and size of nickel particles in the structure of the catalysts. Among the catalysts, 40 and 30 wt% showed the highest initial activity (48.6% methane conversion) and the highest stability (above 45% hydrogen yield for 180 min), respectively.     

Combustion-impregnation preparation of Ni/SiO2 catalyst with improved low-temperature activity for CO2 methanation

Exploiting Ni-based catalysts with excellent low-temperature activity is significant for CO₂ methanation, which is a promising route to CO₂ utilization. In this work, a facile combustion-impregnation method was developed to prepare the SiO₂ supported Ni catalysts. Small Ni particles (around 6 nm) and massive Ni–SiO₂ interface could be obtained due to the “combustion” process. The H₂-temperature programmed desorption (H₂-TPD) revealed the existence of Ni–SiO₂ interface and confirmed the high Ni dispersion obtained by this method, which were vital for the activation of reactant. Moreover, more medium basic sites which were beneficial for the CO₂ activation could also be created. In comparison with the reference Ni/SiO₂ catalyst prepared by the conventional impregnation method, much higher CO₂ conversion (66.9%) and more superior selectivity to CH₄ (94.1%) were achieved with the Ni/SiO₂-Gly catalyst at 350 °C. Additionally, it was also found that glucose, citric acid and glycine were all effective fuels for this combustion-impregnation method, and the as-prepared catalysts all exhibited greatly improved low-temperature activity. Therefore, this work represents an important step toward developing Ni-based catalysts for CO₂ methanation by a promising wide-used method.     

Dual application of Ti-catalyzed Li-RHC composite for H2 purification and CO methanation

2LiH + MgB₂ composite doped with TiO₂ (Li-RHC-Ti) is employed with a two-fold purpose: hydrogen purification under a H₂–CO (0.1 mol%) mixture and CO methanation. Upon dynamic cycling under CO–H₂ mixture, hydrogen release curves display a quite stable amount of pure hydrogen of about 10 wt%, short release times of around 60 min, and minor degradation. Gas analysis by Fourier transform infrared spectroscopy (FTIR) after a thermal dehydrogenation process of MgH₂ and LiBH₄ under CO evidence the conversion of CO to CH₄. Li-RHC-Ti dehydrogenated under CO shows the simultaneous formation of CH₄, CH₃OH, and B(CH₃)₃ in the gas phase. X-ray powder diffraction (XRPD) and FTIR characterizations of the solid phases of Li-RHC-Ti after both H₂–CO mixture and CO interactions demonstrate the formation of MgO, LiBO_2, and HCOO⁻ species. Li-RHC-Ti acts as a hydrogen source and promoter for the CO conversion. Reaction pathways are proposed based on experimental results and equilibrium composition calculations.     

Ordered mesoporous Ni/Silica-carbon as an efficient and stable catalyst for CO2 reforming of methane

Ordered mesoporous silica-carbon (MSC) were used as supports of Ni based catalysts for dry reforming of methane (DRM) reaction. The effects of preparation method and precipitant on the catalysts are investigated. The physical and chemical properties are discussed based on the H₂-TPR, FTIR, XRD, TEM, H₂-TPD and N₂ adsorption/desorption characterization. It is found that the preparation method and choice of precipitants affect the catalysts significantly in terms of the properties and catalytic performance in DRM reaction. In detail, the catalysts prepared by the precipitation method show more highly dispersed Ni particles and further better catalytic activity than the impregnated catalyst. That is attributed to the forming Ni₃Si₂O₅(OH)₄ nanoflakes in the catalyst precursors with the existence of alkaline precipitants. And this Ni₃Si₂O₅(OH)₄ species bind the support more tightly than NiO in the impregnated Ni/MSC catalyst. Moreover, the choice of precipitants also influences the form of Ni₃Si₂O₅(OH)₄ species in the catalysts. Specially, the strong electrolytic capacity of NaOH gives the most Ni₃Si₂O₅(OH)₄ nanoflakes formed in Ni-MSC-1 catalyst, which results in the most highly Ni dispersity and further highest catalytic activity. Besides, the strong interaction between the Ni₃Si₂O₅(OH)₄ species and support are also advantageous to the resist sintering and formation of carbon deposition, that is related to the good catalytic stability of catalysts.     

CO2 methanation in the presence of methane: Catalysts design and effect of methane concentration in the reaction mixture

The work in this paper evidences the viability of producing synthetic natural gas (SNG) via the methanation reaction tackling two fundamental challenges on methanation catalysis (i) the development of advanced catalysts able to achieve high CO₂ conversion and high methane yields and (ii) the unexplored effect of residual methane on the methanation stream. Both challenges have been successfully addressed using Ni/CeO₂-ZrO₂ catalysts promoted with Mn and Co. Mn does not seem to be a good promoter while Co prevents carbon deposition and secondary reactions. In fact, our Co-doped sample reached high levels of CO₂ conversion and CH₄ selectivity, especially at low reaction temperatures. In addition, this catalyst exhibits excellent catalytic behaviour when methane is introduced into the gas mixture, indicating its feasibility for further study to be conducted in realistic flue gases environments and methanation units with recycling loops. Furthermore, when methane is introduced in the reactant mixture, the Ni-Co/CeO₂-ZrO₂ sample is very stable maintaining high levels of conversion and selectivity. Overall our Co-doped catalyst can deliver high purity synthetic natural gas for long-term runs, promising results for gas-phase CO₂ conversion units.     

CO2 methanation over Ni and Rh based catalysts: Process optimization at moderate temperature

H₂ was produced from aluminum/water reaction and reacted with CO₂ over Ni and Rh based catalysts to optimize the process conditions for CO₂ methanation at moderate temperature. Monometallic catalysts were prepared by incorporating Ni and Rh using nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) and rhodium(III) chloride trihydrate (RhCl₃·3H₂O)as a precursor chemical. The preliminary study of the catalysts revealed higher activity and CH₄ selectivity for Rh based catalyst compared to that of Ni based catalyst. Further, Rh based catalyst was investigated using response surface methodology (RSM) involving central composite design. The quadratic model was employed to correlate the effects of variable parameters including methanation temperature, %humidity, and catalyst weight with the %CO₂ conversion, %CH₄ selectivity, and CH₄ production capacity. Analysis of variance revealed that methanation temperature and humidity play an important role in CO₂ methanation. Higher response values of CO₂ conversion (54.4%), CH₄ selectivity (73.5%) and CH₄ production capacity (8.4 μmol g^?1 min^?1) were noted at optimum conditions of 206.7°C of methanation temperature, 12.5% humidity and 100 mg of the catalyst. The results demonstrated the ability of Rh catalyst supported on palm shell activated carbon (PSAC) for CO₂ methanation at low temperature and atmospheric pressure.     

Highly stable Ni-based catalysts derived from LDHs supported on zeolite for CO2 methanation

CO₂ methanation can effectively reduce the concentration of CO₂ in the air and decrease environmental pollution. Therefore, it is essential to synthesize catalysts with high carbon deposition resistance and stability. Herein, the highly stabilized Ni-based catalysts derived from hydrotalcites are prepared by the self-sacrificial template method and used for CO₂ methanation. The prepared Ni-based catalysts maintain the morphology of the hydrotalcites precursors and the Ni particles are embedded in the AlO_x substrate. The catalyst show high performance at 350 °C, 0.1 MPa and a high space velocity of 30,000 mL g⁻¹ h⁻¹, with the conversion rate of CO₂ and selectivity of CH₄ reaching 87.5% and 100%, respectively. More importantly, the activity of catalyst does not decrease after continuous reaction for 200 h at 350 °C with different space velocities (30,000 and 60,000 mL g⁻¹ h⁻¹) owing to the confinement of the AlO_x substrate, which suppress the undesirable agglomeration and sintering of the Ni particles. This unique mosaic structure has certain reference significance for studying materials with excellent stability at high temperatures.     

Effect of carbon nanotubes purification in the performance of a negative electrode of a Ni/MH battery

Multiwall carbon nanotubes (MWCNT) with a diameter of 30–50 nm was added to the negative electrode of a Ni–MH battery in order to study the effect of its purification treatment on the electrochemical performance. Three different reflux assisted digestion methods were analyzed. MWCNT were structural characterized before and after purifications by means of different techniques such as HRTEM, EDS, SEM, XRD and FTIR. Subsequently they were incorporated into the working electrode to evaluate the electrochemical performance by charge/discharge cycling and rate capability.     

A strategy to regenerate coked and sintered Ni/Al2O3 catalyst for methanation reaction

Supported Ni/Al₂O₃ catalysts are widely used in chemical industries. Regeneration of the deactivated Ni catalysts caused by sintering of Ni nanoparticles and carbon deposition after long-term operation is significant but still very challenging. In this work, a feasible strategy via solid-phase reaction between NiO and Al₂O₃ followed by a controlled reduction is developed which can burn out the deposited carbon and re-disperse the Ni nanoparticles well, thus regenerating the deactivated Ni catalysts. To demonstrate the feasibility of this method, Ni catalyst supported on α-Al₂O₃ (Ni/Al₂O₃) for CO methanation reaction was selected as a model system. The structure and composition of the fresh, deactivated and regenerated Ni/Al₂O₃ catalysts were comprehensively characterized by various techniques. The reduction and redistribution of Ni species as well as the interfacial interaction between Ni nanoparticles and Al₂O₃ support were investigated in detail. It is found that calcining the deactivated Ni/Al₂O₃ in air at high temperature can burn out the coke, while the sintered Ni species can combine with superficial Al₂O₃ to form a surface NiAl₂O₄ spinel phase through the solid-phase reaction. After the controlled reduction of the NiAl₂O₄ spinel, highly dispersed Ni nanoparticles on Al₂O₃ support are re-generated, thus achieving the regeneration of the deactivated Ni/Al₂O₃. Interestingly, compared with the fresh Ni/Al₂O₃ catalyst, the sizes of Ni nanoparticles became even smaller in the regenerated ones. The regenerated Ni/Al₂O₃ showed much enhanced catalytic activity in CO methanation and became more resistant to carbon deposition, due to the better dispersed Ni nanoparticles and strengthened interaction between Ni and Al₂O₃ support. Our work not only addresses the long existing catalyst regeneration issue, but also provides effective and renewable Ni-based catalysts for CO methanation.     

Effect of noble metal addition over active Ru/TiO2 catalyst for CO selective methanation from H2 rich-streams

Selective CO methanation from H₂-rich stream has been regarded as a promising route for deep removal of low CO concentration and catalytic hydrogen purification processes. This work is focused on the development of more efficient catalysts applied in practical conditions. For this purpose, we prepared a series of catalysts based on Ru supported over titania and promoted with small amounts of Rh and Pt. Characterization details revealed that Rh and Pt modify the electronic properties of Ru. The results of catalytic activity showed that Pt has a negative effect since it promotes the reverse water gas shift reaction decreasing the selectivity of methanation but Rh increases remarkably the activity and selectivity of CO methanation. The obtained results suggest that RuRh-based catalyst could become important for the treatment of industrial-volume streams.     

Enhanced performance of Ni catalysts supported on ZrO2 nanosheets for CO2 methanation: Effects of support morphology and chelating ligands

A series of Ni/ZrO₂ catalysts was prepared by the impregnation method with modification of the morphology of ZrO₂ support as well as the impregnation procedure and tested for CO₂ methanation. The catalysts supported on the ZrO₂ nanosheets displayed superior catalytic performance as compared with that on ZrO₂ nanoparticles, which could be mainly attributed to the abundant oxygen vacancies promoting the adsorption and dissociation of CO₂ molecules as well as the high dispersion of Ni species. With the introduction of ethylenediamine (En) in the impregnation procedure, the resulting Ni-15En/ZrO₂-1.5 catalyst showed the optimal activity with CO₂ conversion of 86% significantly higher than Ni/ZrO₂-0 of 44% and Ni/ZrO₂-1.5 of 79% at 0.5 MPa and 300 °C. The excellent performance was attributed to increased moderately basic sites for CO₂ adsorption in ZrO₂ nanosheets, as well as the enhanced dispersion of nickel caused by the complexation of Ni ions with En, which inhibited the aggregation of nickel particles in the subsequent thermal treatments. In conclusion, the synergistic effects of the morphology of ZrO₂ nanosheets as well as the chelating behavior of En contributed to the enhanced performance of Ni-15En/ZrO₂-1.5 in the CO₂ methanation reaction. The strategy shows good prospects for controlling the size of active metals, especially those that were dispersed on the surface of the two-dimensional (2D) metal oxide materials.     

Promotion effect by Mg on the catalytic behavior of MgNi/WO3 in the CO methanation

In this study, tungsten oxide with a high specific surface area was fabricated using a nanocasting technique and used to prepare support for nickel catalysts for CO methanation. Additionally, Mg was further introduced as a promoter for tuning the catalytic performance. The 25Ni/WO₃ catalyst demonstrated a relatively high CO conversion, but a poor CH₄ selectivity; however, with the addition of 7 wt% Mg to the catalyst, the CH₄ selectivity reached 92% at a temperature of 440 °C. The improved CH₄ selectivity can be attributed to the enhanced CO dissociation, which was related to the reduced Ni particle size, as well as the enhanced Ni electron cloud density. The role of a physical barrier and electron transfer of MgO induces an enhancement of the metal–support interactions, which are conducive to decreasing the Ni particle size. Meanwhile, the electron transfer performance of MgO constitutes a crucial factor in enhancing the Ni electron cloud density. Furthermore, with benefit from the inhibition of agglomeration of the Ni particles by the MgO promoter, a significantly better catalytic stability was also observed on 7Mg25Ni/WO₃ than with the 25Ni/WO₃ catalyst.     

Impacts of metal loading in Ni/attapulgite on distribution of the alkalinity sites and reaction intermediates in CO2 methanation reaction

Both metal sites and alkaline sites are essential parameters for a catalyst used in methanation of CO₂. This study investigated the impacts of the relative abundance of metal sites and alkaline sites on the catalytic performances of nickel-based catalyst with attapulgite, a natural mineral, as the support. The results showed that the increase of nickel loading to attapulgite significantly decreased the abundance of alkaline sites, remarkably enhanced the catalytic activity, and suppressed the formation of CO. The in situ DRIFTS characterization of the CO₂ methanation indicated that the alkaline sites favored formation of the oxygen-containing reaction intermediates such as CO1, –OH, 1CO₂, formate, carbonate and bicarbonate species. In comparison, metallic nickel species promoted their further hydrogenation to form CH₄. Besides the absorption/activation of 1CO₂ was more preferable on surface of metallic nickel, but not on the alkaline sites. The availability of the alkaline sites was not as important as the metallic nickel species for preparation of an efficient catalyst for CO₂ methanation.     

Low-temperature CO2 reforming of methane over Ni supported on ZnAl mixed metal oxides

Ni catalysts supported on mixed ZnOAl₂O₃ and on pure ZnO and Al₂O₃ were prepared, characterized by XRD, TPR, and XPS, and tested in long-term methane dry reforming at low temperature (400 °C). Depending on Zn/Al ratio in the supports, the catalysts varied in their physico-chemical properties and exhibited different trends in their on-stream catalytic activity. Catalysts with high alumina content consist of a mixture of alumina and zinc aluminate phases with metallic Ni particles on their surface. These samples show medium activity for reforming and high on-stream stability. The catalysts on mixed Zn-rich supports were more active than those on Al-rich supports and exhibited maxima in their activity after 30–40 h on stream, while Ni on pure ZnO possessed very low activity. Such contrast in performance of Zn-rich catalysts was explained by detected transformation of initially formed NiZn alloy to a mixture of Ni and Ni₃ZnC_0.7 particles that are assumed to have higher activity for reforming. Moreover, the size of Ni-containing particles on Zn-rich supports decreased under reaction conditions resulting in higher Ni dispersion.     

Interfacing CdS particles on Ni foam as a three-dimensional monolithic photocatalyst for efficient visible-light-driven H2 evolution

Solar photocatalytic water splitting using particulate semiconductors has been valued as a potentially scalable way for the production of clean H₂ energy, yet the performances of the powder-suspension systems are constrained by insufficient utilization of light energy and tedious recycling of photocatalyst particles. Here, we present a high-performance photocatalytic H₂ evolution using a visible-light-driven CdS-based monolithic photocatalyst with three-dimensional (3D) heterostructure. The monolithic photocatalyst is fabricated by firmly growing CdS microspheres on a Ni(OH)₂ nanosheet-modified Ni foam (NF) (denoted as CdS-NiS_x/NF) via a simple hydrothermal process. The structure and component synergy endows the monolithic CdS-NiS_x/NF photocatalyst advantageous features including high-density CdS microspheres for visible light harvesting, multiple heterojunction interfaces for efficient electron-hole separation, and abundant interfacial NiS_x active sites for efficient H₂ evolution reaction (HER). Upon visible light irradiation, the monolithic CdS-NiS_x/NF photocatalyst exhibits an outstanding photocatalytic H₂ evolution activity with an enhanced rate of 6.2 mmol·h⁻¹ g⁻¹_CdS, which is 6 times higher than that of the suspended CdS powder. In addition, the structural integrity of the CdS-NiS_x/NF enables a good stability for H₂ evolution over a 30 h reaction. This monolithic photocatalyst is scalable in preparation and compatible for device fabrication, which offers great potentials for applications in solar cells, photoelectrocatalysis, and electrocatalysis.     

A comprehensive DFT study of CO2 catalytic conversion by H2 over Pt-doped Ni catalysts

PtNi bimetallic catalysts show superior performance for CO₂ catalytic conversion by hydrogen, but the underlying mechanism and the key elementary steps in controlling the activity and selectivity of CO₂ hydrogenation remain unclear. In present work, the complete reaction network for CO₂ hydrogenation has been investigated systematically over Pt/Ni (111) surface based on periodic density functional theory, and active sites and reaction mechanism have been determined. It is found that HCOOH is mainly produced by undergoing the HCOO pathways while synthesis of CH₃OH and CH₄ via RWGS+CO hydrogenation is the dominant reaction pathway, and their selectivity are determined by the competitive reaction between hydrogenation and CO bond scission of H₂COH species. The dissociation of COOH is regarded as the rate-determining step as it has the highest barrier (2.07 eV) in RWGS+CO hydrogenation. Moreover, it is observed that the doping of Pt on Ni surface can promote the transformation of CO₂ into chemisorbed CO₂^δ− and reduce the barrier in H₂ dissociation, which further facilitate the activation and hydrogenation of CO₂. More importantly, the doped Pt atom could promote H_xCO hydrogenation to H_xCOH, meanwhile, suppress H_xCOH dissociation into CH_x. Especially, the activation barrier and reaction energy for C formation is markedly enhanced, and the ability for C hydrogenation is promoted over Pt/Ni (111) surface, which could lower the possibility of coke formation. These results provide helpful information in understanding the process of CO₂ hydrogenation at atomic scale, and could benefit for the synthesis of Ni-based bimetallic catalysts.