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.     

Lowering risk of methanation of carbon support in Ru/carbon catalysts for CO methanation by adding lanthanum

Two series of Ru/C catalysts doped with lanthanum ions are prepared and studied in CO methanation in the H₂-rich gas. The samples are characterized by N₂ physisorption, TG-MS studies, XRD, XPS, TEM/STEM and CO chemisorption. Two graphitized carbons differing in surface area (115 and 80.6 m²/g) are used as supports. The average sizes of ruthenium crystallites deposited on their surfaces are 4.33 and 5.95 nm, respectively. The addition of the proper amount of La to the Ru/carbon catalysts leads to an above 20% increase in the catalytic activity along with stable CH₄ selectivity higher than 99% at all temperatures. Simultaneously, lanthanum acts as the inhibitor of methanation of the carbon support under conditions of high temperature and hydrogen atmosphere. Such positive effects are achieved at a very low concentration of La in the prepared samples, a maximum 0.04 La/Ru (molar ratio). 0.01 mmol La introduced to the Ru/C system leads to 98% CO conversion at 270 °C.     

Influence of the supports ZrO2 on selective methanation of CO over the nickel supported catalysts

It is attempted to optimize preparation of ZrO₂ as support of the nickel catalysts for selective methanation of CO in H₂-rich gas (CO-SMET). Therefore, the supports ZrO₂ were prepared at first by thermal decomposition method from zirconium oxynitrate and zirconium oxychloride at the calcination temperature of 400 °C and 800 °C, respectively. It is illustrated that the salt kind and calcination temperature affected phase state (tetragonal, monoclinic), crystallite size and specific surface area (SSA) of the supports. The difference in property of the supports influenced catalytic performance of the catalysts Ni/ZrO₂ for CO-SMET reaction. Especially, the chlorine ion residues in the support ZrO₂ prepared from zirconium oxychloride was beneficial for CO removal selectively. Furthermore, a precipitation method was adopted to prepare ZrO₂ for comparison with the thermal decomposition method with use of the zirconium oxychloride as starting material. It is found that the supports ZrO₂ prepared by the precipitation method induced a better dispersion of metallic Ni on its surface. The catalyst Ni/ZrO₂ with use of the support ZrO₂ prepared by the precipitation method and calcination at 400 °C exhibited a good performance at the reaction temperature of 220 °C in the 100 h durability test, where CO outlet concentration was kept below 10 ppm and the selectivity remained constant at 100%. Relation of Ni crystallite size and chlorine ion residues with the catalytic performance was discussed.     

High temperature methanation over Ni catalysts supported on high surface area ZnxMg1-xAl2O4: Influence on Zn loading

A series of 10 wt % Ni based catalysts supported on Zn_xMg_1-xAl₂O₄ were prepared using a co-precipitation and impregnation method for high temperature syngas methanation. The effect of Zn loading on catalysts’ textural property and catalytic performance was investigated by BET, XRD, TEM, H₂-TPR, XPS and CO-TPD analysis. It was found that a modest addition of Zn significantly increased the surface area of the catalysts, which moderated the strong interaction between NiO and the support. This effect enhanced the reduction of the Ni, thereby improving the dispersion of the active metal on the support and intensifying the adsorption of CO. In addition, surface Ni⁰ concentration was improved by the Zn substitution. Among the various catalysts tested, Ni/Zn_0.7Mg_0.3Al exhibited the best catalytic performance at 500 °C, 2.0 MPa and 30, 000 ml g⁻¹·min⁻¹, with a CO conversion, CO₂ conversion and CH₄ selectivity of 99.7, 53.1 and 98.7%, respectively. Furthermore, the Ni/Zn_0.7Mg_0.3Al catalysts also maintained excellent stability during a 120 h life test.     

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.     

The influence of CO,CO2 and H2O on selective CO methanation over Ni(Cl)/CeO2 catalyst: On the way to formic acid derived CO-free hydrogen

For the first time the influence of CO, CO₂ and H₂O content on the performance of chlorinated NiCeO₂ catalyst in selective or preferential CO methanation was studied systematically. It was shown that the rate of CO methanation over Ni(Cl)/CeO₂ increases with the increasing H₂ concentration, is independent of CO₂ concentration and decreases with increasing CO and H₂O concentrations; the rate of CO₂ methanation is weakly sensitive to H₂ and CO₂ concentrations and decreases with increasing CO and H₂O concentrations. High catalyst selectivity was attributed to Ni surface blockage by strongly adsorbed CO molecules and ceria surface blockage by Cl, which both inhibit CO₂ hydrogenation.For the first time, selective CO methanation over Ni(Cl)/CeO₂ was studied for deep CO removal from formic acid derived hydrogen-rich gases characterized by high CO₂ (40–50 vol%), low CO (30–1000 ppm) content and trace amounts of water. Composite Ni(Cl)/CeO₂-η-Al₂O₃/FeCrAl wire mesh catalyst was demonstrated to be effective for this process at temperatures of 180–220°С, selectivity 30–70%, WHSV up to 200 L (STP)/(g∙h). The catalyst provides high process productivity, low pressure drop, uniform temperature distribution, and appears highly promising for the development of a compact CO cleanup reactor. Selective CO methanation was concluded to be a convenient way to CO-free hydrogen produced by formic acid decomposition.     

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.     

Mesoporous zirconia-modified clays supported nickel catalysts for CO and CO2 methanation

The Ni catalysts supported on a new structure with zirconia nanoparticles highly dispersed on the partly damaged clay layers has been prepared by the incipient wetness impregnation method and the new structure of the support has been prepared in one pot by the hydrothermal treatment of the mixture of the clay suspension and the ZrO(NO₃)₂ solution. The catalytic performances for the CO and CO₂ methanation on the catalysts have been investigated at a temperature range from 300 °C to 500 °C at atmospheric pressure. The catalysts and supports have been characterized by X-ray diffraction (XRD), transmittance electron microscopy (TEM), H₂ temperature-programmed reduction (H₂-TPR), nitrogen adsorption–desorption, and thermogravimetry and differential thermal analysis (TG-DTA). It is found that the zirconia-modified clays have the typical bimodal pore size distribution. Most of the pores with the sizes smaller than 10 nm are resulted from the zirconia pillared clays and the mesopores with the sizes larger than 10 nm and the macropores with the sizes larger than 50 nm are resulted from the partly damaged clay layers. The bimodal pore structure is beneficial to the dispersion of Ni on the layers of the zirconia-modified clays and the increase in Ni loading. The zirconia nanoparticles are highly dispersed on the partly damaged clay layers. Nickel oxide in cubic phase is the only Ni species that can be detected by XRD. The nickel oxide nanoparticles with the sizes of 12 nanometers or more are well dispersed on the zirconia-modified clay layers, which are observed to be buried in the stack layers of zirconia. The presence of nickel oxide in six different forms could be perceived on the new structure. Five of them except the Ni species that forms the spinel phase with Al in clays can be reduced to the active Ni species for the CO and CO₂ methanation. But the activity of the Ni species is different, which is associated with the chemical environment at which the Ni species is located. The catalyst with the higher zirconia content, which also has the larger specific surface area and pore volume, exhibits the better catalytic performance for the CO or CO₂ methanation. Zirconia in the catalyst is responsible for the dispersion of the Ni species, and it prevents the metallic Ni nanoparticles from sintering during the process of the reaction. In addition, it is also responsible for the reduction of the inactive carbon deposition. The catalyst with 15 wt.% zirconia content has the highest CO conversion of about 100% and the highest methane selectivity of about 93% at 450 °C for CO methanation, and the catalyst with 20% zirconia content has the CO₂ conversion of about 80% and the highest methane selectivity of about 99% for CO₂ methanation at 350 °C. The catalyst with 15 wt.% zirconia possesses promising stability and no distinct deactivation could be perceived after reaction for 40 h. This new catalyst has great potential to be used in the conversion of the blast furnace gas (BFG) and the coke oven gas (COG) to methane.     

Combined steam and CO2 reforming of methane over one-pot prepared Ni/La-Si catalysts

The highly dispersed mNi/xLa−Si catalysts with varied weight percentages of Ni and La were synthesized via one-pot sol-gel process and subsequently applied to combined carbon dioxide and steam reforming of methane (CSDRM) for syngas production. The addition of La improved the catalytic activity and stability as well as the coke resistance of the mNi/xLa−Si catalysts. The effects of preparation routes, Ni contents and CO₂/steam (C/S) ratios on the performances of the Ni/LaSi catalysts were studied in detail for the CSDRM. The 17.5Ni/3.0LaSi catalyst synthesized with the assistance of poly (ethylene glycol) and ethylene glycol exhibited the most excellent catalytic activity, stability and coke resistance. In addition, the H₂/CO ratios in the product gas could be tuned by changing the C/S ratios in the feed. When the C/S ratio was 0.5, the H₂/CO ratio of about 2 was achieved for the 17.5Ni/3.0LaSi catalyst.     

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.     

Enhanced low-temperature catalytic performance in CO2 hydrogenation over Mn-promoted NiMgAl catalysts derived from quaternary hydrotalcite-like compounds

Mn-promoted NiMgAl mixed-oxide (NiMnx-LDO, x = 0, 5, 10, 15) catalysts derived from hydrotalcite were synthesized using co-precipitation for CO₂ hydrogenation to synthetic natural gas. By regulating Mn contents, NiMn5-LDO delivered the most excellent catalytic performance, being about 2 times higher than that of undoped NiMn0-LDO catalyst (TOF of NiMn5-LDO and NiMn0-LDO: 0.61 s⁻¹ vs 0.31 s⁻¹ @ 240 °C). Through extensive characterization, it was found that Mn dopants promoted the reduction of bulk NiO through tuning the interaction between Ni and Mg(Mn)AlOx support. A high surface ratio of Ni⁰/Ni²⁺ was achieved over NiMn5-LDO. Furthermore, the surface basicity strength was tailored by Mn dopants. With 5 wt% of Mn, NiMn5-LDO catalyst showed a stronger medium-strength basicity and higher capacity of CO₂ adsorption than others. Particularly, TOF indicates a good correlation with medium-strength basicity over NiMnx-LDO catalysts. The strong metal-support interaction originated from the hydrotalcite structure kept nickel uniformly dispersed, endowing to the improved catalytic performance.     

Highly efficient Ru/TiO2‐NiAl mixed oxide catalysts for CO selective methanation in hydrogen‐rich gas

Selective CO methanation (CO‐SMET) is viewed as an effective H₂‐rich gas purification technique for proton exchange membrane fuel cells. In this work, improved composite‐supported Ru catalysts were developed for the CO‐SMET process. Mixed metal oxides (MMOs) obtained by calcination of layered double hydroxides precursor were used as an effective catalyst supports. After incorporation of TiO₂, the resulting TiO₂‐MMO composites were expected to have an enhanced catalytic performance. Therefore, a series of TiO₂‐NiAl layered double hydroxides was successfully prepared via 1‐pot deposition method. After calcination, the derived TiO₂‐NiAl MMO‐supported Ru catalysts obtained by impregnation method showed excellent catalytic performance for CO‐SMET reaction. The catalyst could deeply remove the CO outlet concentration (<10 ppm) with a high selectivity (>50%) over the wide low‐temperature window (175‐260°C). Furthermore, the catalyst also showed high stability with no deactivation during a long‐term durability test (120 h). Based on X‐ray diffraction, Fourier transform infrared, Raman, thermogravimetric differential scanning calorimetry, N₂ adsorption‐desorption, temperature‐programmed reduction, scanning electron microscopy, and transmission electron microscopy analyses, the enhanced catalytic performance of the TiO2‐NiAl MMO‐supported Ru catalyst was found to be related to the higher dispersion of Ru nanoparticles, partially reduced NiO species, and the increased specific surface area and structural stability of the support. The facile synthesis strategy proposed herein may open a new window for the efficient production of high‐quality H₂.     

Nickelcobalt catalyst supported on TiO2-coated SiO2 spheres for CO2 methanation in a fluidized bed

Carbon dioxide (CO₂) methanation, which is the reduction of carbon dioxide to methane by hydrogen generated from renewable energy, is a promising process for carbon recycling. Towards large-scale implementation, (i) fluidized beds, which have excellent heat transfer, are promising to perform the highly exothermic reaction; and (ii) catalysts suitable for long-term use in fluidized beds are needed. In this study, a novel NiCo bimetal catalyst supported on TiO₂-coated SiO₂ spheres (NiCo/TiO₂@SiO₂) was rationally designed and evaluated for CO₂ methanation in fluidized bed reactor. The results demonstrate that NiCo/TiO₂@SiO₂ exhibited high CO₂ conversion with CH₄ selectivity of greater than 95%. Moreover, the superior performance was sustained for more than 100 h in the fluidized bed reactor, affirming the long-term stability of the catalyst. Comprehensive characterizations were conducted to understand the relationship between structure and performance. This study is expected to be valuable for the potential implementation of the CO₂ methanation process in fluidized beds.     

Methanation of carbon dioxide over Ni/CeO2 catalysts: Effects of support CeO2 structure

The CeO₂, which were prepared by hard-template method, soft-template method, and precipitation method, were used as support to prepare Ni/CeO₂ catalysts (named as NCT, NCS, and NCP catalysts, respectively). The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Brunauer–Emmett–Teller (BET). Hydrogen temperature-programmed reduction (H₂-TPR) was also used to study the reducibility of the support nickel precursors. Moreover, CO₂ catalytic hydrogenation methanation was used to investigate the catalytic properties of the prepared NCT, NCS, and NCP catalysts. H₂-TPR and XRD results showed that the NiO can be reduced by H₂ to produce metal Ni species, and the surface oxygen species existing on the surface of the support CeO₂ can also be reduced by H₂ to form surface oxygen vacancies. Low-angle XRD, TEM, and BET results indicated that the NCT and NCS catalysts had developed mesoporous structure and high specific surface area of 104.7 m² g^?1 and 53.6 m² g^?1, respectively. The NCT catalyst had the highest CO₂ methanation activity among the studied NCT, NCS, and NCP catalysts. The CO₂ conversion and CH₄ selectivity of the NCT catalyst can reach 91.1% and 100% at 360 °C and atmospheric pressure. The NCP catalyst, which had low specific surface area and low porosity, performed less CO₂ conversion and higher CH₄ selectivity than the NCT and NCS catalysts till 400 °C.     

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.     

Computational study of CO2 methanation over two-dimensional molybdenum carbide catalysts

Two-dimensional molybdenum carbide (2D-Mo₂C) is thought to be promising for catalytic hydrogenation of CO₂ to CH₄, but little is known about its catalytic reaction mechanism. In this work, we investigate the hydrogenation of CO₂ to CH₄ on 2D-Mo₂C using density functional theory. Our calculations show that Mo on the surface can efficiently decompose CO₂ to CO and O, and also H₂ to H. The hydrogenation of CO produces CHO that is readily deoxygenated to CH, and CH is selectively hydrogenated to produce CH₄. Interestingly, the embedded Ir₁ on 2D-Mo₂C can act as a single-atom promoter to improve the performance of CO₂ methanation, while on the other hand maintaining its high selectivity for CH₄. This work provides insight into the mechanism of 2D-Mo₂C-catalyzed CO₂ methanation reactions and suggests a strategy to improve the performance of such catalysts through single-atom promoters.     

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.     

Methanation of CO on Ru/graphitized-carbon catalysts: Effects of the preparation method and the carbon support structure

Two kinds of Ru/C catalysts prepared by two different methods and supported on two graphitized carbons differing in their surface area were studied in CO methanation in the H₂-rich gas. The textural parameters of the support materials were characterized by means of N₂ physisorption. XRPD, XPS, TEM and CO- chemisorption studies indicate that the application of wet impregnation leads to more homogeneous composition of the Ru/carbon system and higher Ru dispersion than dry impregnation for both supports. The activity of the Ru/carbon samples in CO methanation in a H₂-rich gas stream depends on the structure and average size of the active phase crystallites. The combination of wet impregnation and the use of graphitized carbon of appropriate structure in the preparation of the Ru/C catalyst lead to a complete conversion of CO at 240 °C.     

Effect of Ni content on CO2 methanation performance with tubular-structured Ni-YSZ catalysts and optimization of catalytic activity for temperature management in the reactor

This paper presents high-performance Ni-YSZ tubular catalysts for CO₂ methanation prepared by the extrusion molding. We fabricated tubular-shaped Ni-YSZ catalysts with various Ni contents (25–100 wt% NiO) and investigated the effect of Ni content on CO₂ methanation performance under various temperatures and gas flow rates. Catalysts with Ni contents >75 wt% showed CH₄ yields >91% above 270 °C with high CH₄ selectivities (>99%). High CH₄ yields were also observed under high GHSVs at 300 °C: 93% and 92% at 8700 and 17,500 h⁻¹, respectively. Investigation of methanation with the catalysts revealed that CO₂ methanation was accelerated by a localized hotspot at the reactor inlet arising from the interaction between reaction kinetics and heat generation. Using a numerical simulation, we considered the optimum arrangement of catalytic activity in the reactor to avoid hotspot generation and realize a stable high CO₂ methanation performance. We can simultaneously achieve high CH₄ production and prevent hotspot formation by properly arranging catalysts with different activities.