Bimetallic NiPd/SBA-15 alloy as an effective catalyst for selective hydrogenation of CO2 to methane

It is very challenging but still promising to develop highly efficient heterogeneous catalysts for selective hydrogenation of CO₂ to methane. Supported bimetallic NiPd/SBA-15 alloy catalysts with a varied ratio of Ni/Pd were prepared by one-pot wet chemical and impregnation method. A series of techniques were employed to characterize the elemental composition and alloy structure of as-synthesized NiPd/SBA-15 catalysts. The alloyed bimetallic NiPd/SBA-15 catalysts showed relatively higher catalytic activity compared with monometallic Pd or Ni-supported SBA-15 and several other published catalysts. The bimetallic catalyst with Ni:Pd atom ratio of 3:1 was most active in the formation of CH₄ and yielded 0.93 mol CH₄ per mol CO₂ at 430 °C. This superior performance can be attributed to enhanced synergy between Ni and Pd with high dispersion of active sites.     

Recent developments in catalyst synthesis using DBD plasma for reforming applications

The catalyst has a significant role in gas processing applications such as reforming technologies for H₂ and syngas production. The stable catalyst is requisite for any industrial catalysis application to make it commercially viable. Several methods are employed to synthesize the catalysts. However, there is still a challenge to achieve a controlled morphology and pure catalyst which majorly influences the catalytic activity in reforming applications. The conventional methods are expansive, and the removal of the impurities are major challenges. Nevertheless, it is not straightforward to achieve the desired structure and stability. Therefore, significant interest has been developed on the advanced techniques to take control of the physicochemical properties of the catalyst through non-thermal plasma (NTP) techniques. In this review, the systematic evolution of the catalyst synthesis using NTP technique is elucidated. The emerging DBD plasma to synthesized and effective surface treatment is reviewed. DBD plasma synthesized catalyst performance in reforming application for H₂ and syngas production is summarised. Furthermore, the status of DBD plasma for catalyst synthesis and proposed future avenues to design environmentally suitable and cost-effective synthesis techniques are discussed.     

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.     

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.     

Ni-based catalysts with coke resistance enhance by radio frequency discharge plasma for CH4/CO2 reforming

Coke deposition has been considered to be one of the most important reasons hindering the stability of the catalyst during CH₄/CO₂ reforming. In this study, after the addition of P123 (PEG-PPG-PEG triblock copolymer), Ni²⁺ can be well-dispersed on the mesoporous molecular sieve MCM-41. And then, the catalysts were prepared by using N₂ radio frequency (RF) discharge plasma for different treatment times to reduce the size of Ni particles, improve the anti-coking performance, and thereby improve the stability of the catalyst. The results showed that the catalyst NM-P123-PN2h exhibits superior catalytic properties in the CH₄/CO₂ reforming. The initial conversions of CH₄ and CO₂ were 90.80% and 89.60% at 750 °C, respectively. The catalyst NM-P123-PN2h showed highly coke resistance with less carbon deposition (1.12%) at 750 °C after 10 h of continuous reaction, while the carbon deposition of the catalyst NM-C was 37.32%. Compared with the traditional calcination method, the catalyst prepared by plasma treatment has a smaller particle size and better dispersibility of nickel. In particular, the nickel particle size of the catalyst NM-C was 8.37 nm, however, that of the catalyst NM-P123-PN2h was only 1.70 nm, and the nickel particle size was reduced by 5 times. Therefore, it can be concluded that the catalyst prepared under the combined action of P123 and RF plasma-treated can effectively improve the coke resistance of the catalyst and the stability of the CH₄/CO₂ reforming.     

Steam reforming of toluene as model biomass tar to H2-rich syngas in a DBD plasma-catalytic system

Biomass tar is one of the most troublesome issues limiting the further development of biomass pyrolysis and gasification. In this study, a plasma enhanced catalytic steam reforming technology was applied for biomass tar removal. Toluene was selected as biomass tar surrogate. The nano-sized alumina-supported nickel and iron catalysts with different molar ratios of M/Al (M: Ni or Fe, 0:1, 1:3, 1:1, 3:1, 1:0) were prepared for catalytic steam reforming of toluene in a non-thermal plasma reactor featuring dielectric barrier discharge (DBD). The results showed that syngas was the dominant gas product of toluene decomposition. The conversion efficiency of toluene and energy efficiency using Ni-Al and Fe-Al catalysts both followed a sequence: M1Al3 > M1Al1 > M3Al1, which is in line with the BET surface area and pore volume. However, the selectivity of H₂ and CO catalysed by Ni-Al and Fe-Al catalysts follows the order of M1Al3 < M1Al1 < M3Al1. Presumably, toluene dissociation is a process composed of adsorption-reaction-desorption. The formation of syngas is supposed to proceed as a series of ionic and free radical reactions occurring preferably in the gas phase. Ni1Al3 catalyst shows the largest potential in converting biomass tar into H₂-rich syngas, with a maximum toluene conversion of 96% and a largest H₂ yield of 2.18 mol/mol-toluene. Besides, the results showed that this hybrid plasma-catalysis system was potential in anti-carbon deposition.     

Enhancement of plasma-assisted catalytic CO2 reforming of CH4 to syngas by avoiding outside air discharges from ground electrode

This work presents the effects of the insulation of ground electrode and operating parameters on CO₂ reforming of CH₄ to syngas in a coaxial-cylindrical dielectric barrier discharge (DBD) plasma reactor coupled with Ni/α-Al₂O₃ catalyst. For the conventional plasma reactor, abnormal outside discharge inevitably ignites and develops from the ground electrode, giving rise to the formation of harmful substances (e.g., NO_x) and the waste of energy. The power dissipation for the conventional reactor therefore includes both that used for the dry reforming reactions and the loss of energy due to the air discharge. The new finding of this work is that by covering the ground electrode with an insulating oil jacket, not only the NO_x formation is prevented but also the conversion rates, product selectivity and energy efficiency are largely enhanced by roughly 30, 10 and 100% at a specific energy input of about 47 kJ, respectively. The results are associated with the extinguishment of the discharge occurring outside the reactor, which is usually neglected when designing DBD reactors.     

CO2 reforming of CH4 by synergies of binode thermal plasma and catalysts

To reduce the energy consumption of the process of CO₂ reforming CH₄ by plasma, the experiments about synergies of thermal plasma and commercial Z107 Ni/Al₂O₃ catalysts are investigated in three elaborate modes: the binode plasma only, the combination of the plasma and catalysts (CPC), the CPC with part of feed gases introduced into plasma discharge region. The optimal specific energy of 193 kJ/mol and energy conversion efficiency of 66% are achieved under the conditions of CH₄/CO₂ of 4/6, input power at 14.4 kW, feed gases of 5 m³/h in mode 3, when the conversions of CH₄ and CO₂ are 77% and 62%, and the selectivities of H₂ and CO are 88% and 97%, respectively. The experimental results are very close to the industrial requirement compared with steam-reforming process. The excellent performance of this process is attributed to three different reaction mechanisms, which will be discussed in this paper.     

Enhancement of hydrogenation of CO2 to hydrocarbons via In-Situ water removal

Hydrogenation of CO₂ to hydrocarbons in fixed-bed and annular reactors (AR) can be limited via problems associated with high water production as the main by-product. Selective in-situ water removal using a hydrophilic membrane can be a promising solution for enhancing reactor performance. To this aim, a one-dimensional heterogeneous model comprising mass and heat transfers in the shell and tube of a membrane reactor (MR) is proposed. Firstly, the performance of different rector configurations exhibiting similar cross sectional areas and volumes are compared. Afterwards, influential factors affecting the MR performance such as shell/tube temperature, sweep ratio (θ) and pressure ratio (φ) are investigated thoroughly. Results show that increasing initial tube/shell temperature has positive effect on total hydrocarbons yield. However, sharp and sudden temperature elevation (hot spot) due to large extent of water removal, may have detrimental effects on catalyst performance. Moreover, it is observed that increasing θ and φ alter products distribution due to the equilibrium displacement and results in the lack of H₂ for further reactions. In addition, kinetic parameters corresponding to the inhibiting effect of water are indicated to have significant roles in hydrocarbons distribution. Therefore, water removal impose various changes, which cannot be considered independently in analyzing the MR performance.     

Effect of operating conditions and effectiveness factor on hydrogenation of CO2 to hydrocarbons

The development of an efficient reactor for hydrocarbons (C₂–C₄) production through hydrogenation of CO₂, requires a deep understanding of the operating conditions effects. Subsequently, a model is proposed to analyze the reaction rates and investigate the sensitivity of hydrocarbons yield and products distribution to the variations of temperature, pressure and space velocity (SV). Besides, Thiele modulus and effectiveness factor are calculated for all of the reactions considered in the model. Results reveal that simultaneous occurrence of both endothermic reverse water gas shift (RWGS) and exothermic Fischer-Tropsch (FT) reactions, may be the main reason of temperature and rate fluctuations at the fixed-bed reactor inlet. In addition, increasing temperature and pressure, and decreasing SV can shift the process to produce more light olefins. Finally, sensitivity analysis demonstrates that reactor behavior is independent of the changes in pressure and SV at high temperature, which is an indication of high temperature dependency of this process. These findings can be effectively employed to achieve a better insight about appropriate operating conditions of hydrocarbons production via hydrogenation of CO₂.     

Enhanced hydrogen evolution reaction of WS2–CoS2 heterostructure by synergistic effect

Transition metal dichalcogenides (TMDs) have attracted significant research interest due to its promising performance in hydrogen evolution reaction (HER). Synergistic effect between materials interface can improve the electrocatalytic properties. In this work, the WS₂–CoS₂ heterostructure supported on carbon paper (CP) was elaborately fabricated by a three-step method. Owing to the synergistic effect, WS₂–CoS₂ heterostructure exhibits an excellent electrocatalytic activity with a low overpotential of 245 mV at 100 mA/cm² and a small Tafel slope of 270 mV/dec toward HER. We demonstrate that the increased specific surface area and conductivity of the heterostructure play a key role in enhancing the overall catalytic efficiency. Moreover, the crystal lattice distortion in the heterostructure could induce charge redistribution and improve electron transfer efficiency, which may also benefit the whole HER activity.     

Preparation of Cu0.1-xNixCe0.9O2-y catalyst by ball milling and its CO catalytic oxidation performance

A series of Cu_0.1-xNi_xCe_0.9O_2-y catalysts with different Cu/Ni molar ratios were prepared by the ball milling method. The obtained catalytic materials were characterized by XRD, H₂-TPR, BET, XPS and Ramen and the effects of different Cu/Ni content on the structure, properties and CO catalytic oxidation performance of the catalysts were explored. The results evidenced the formation of Cu–Ni–Ce mixed oxide solid solution in all ternary catalysts. In addition, there is a synergistic interaction between Cu and Ni in ternary catalysts, resulting in more oxygen vacancies and improved reduction performance, and hence demonstrating better CO catalytic oxidation activity in the ternary catalysts than binary ones. Under a GHSV of 60000 mL·g_cat⁻¹·h⁻¹, the required reaction temperature for reaching less than 10 ppm CO is lowed from 160 °C with Cu_0·1Ce_0·9O_2-y to 130 °C with Cu_0·07Ni_0·03Ce_0·9O_2-y.     

Optimization of boron dispersion on fibrous-silica-nickel catalyst for enhanced CO2 hydrogenation to methane

There are numerous reports regarding boron-containing catalysts for hydrogen-related reactions from CO₂ including dry reforming of methane and methanation. Besides enhancing the productivity, boron also improved nickel activity and stability. However, the detailed mechanistic study, particularly in explaining the starring role of boron in the enhanced reactions, is still lacking. Thus, herein we loaded boron on fibrous-silica-nickel and investigated their physicochemical properties and mechanistic route by means of in-situ FTIR for enhanced CO₂ methanation. It was found that the appropriate dispersion of boron surrounds the nickel particles is an important factor to improve the adsorption of CO₂ before interacting with split hydrogen atom from the nickel sides to form intermediates which are subsequently dehydrated, and then serial hydrogenation gave the final product of methane. Boron also accelerated the methanation and restricted coke formation. A hybrid approach on optimization via a face-centered central composite design and a response surface methodology showed that reaction using H₂/CO₂ ratio of 6, GHSV of 10,500 mL g^?1 h^?1, at 500 °C gave the highest percentage of CH₄ of 84.3%. To indicate the error, the predicted values were compared to the experimental values, yielding an accurately minimal error ranging from 0 to 11%. As a result, the empirical models generated for CO₂ hydrogenation to methane were reasonably accurate, with all actual values for the confirmation runs fitting within the 94% prediction interval.     

Effect of nickel on combustion synthesized copper/fumed-SiO2 catalyst for selective reduction of CO2 to CO

In this study, we explore the effect of nickel incorporation in Cu/fumed-SiO₂ catalyst for CO₂ reduction reaction. Two catalysts, Cu and CuNi supported on fumed silica were synthesized using a novel surface restricted combustion synthesis technique, where the combustion reaction takes place on the surface of the inert fumed-SiO₂ support. An active solution consisting of a known amount of metal nitrate precursors and urea (fuel) was impregnated on fumed silica. The catalyst loading was limited to 1 wt% to ensure localized combustions on the surface of fumed-SiO₂ by restricting the combustion energy density. The synthesized catalysts were tested for CO₂ hydrogenation reaction using a tubular packed bed reactor between temperature 50°C and 650°C, where Cu/SiO₂ showed high CO₂ conversion to carbon monoxide, and the addition of Ni further improved the catalytic performance and showed some tendency for methane formation along with CO. Moreover, both the catalysts were highly stable under the reaction conditions and did not show any sign of deactivation for ~42 hours time on stream (TOS). The catalysts were characterized using X-ray diffractometer (XRD), scanning electron microscope/energy dispersive X-ray spectrometer (SEM/EDX), transmission electron microscope (TEM), and the Brunauer-Emmet-Teller (BET) surface area measurement technique to understand their structural properties and to assess the effect of CO₂ conversion reaction. In situ DRIFTS was also used to investigate the reaction pathway followed on the surface of the catalysts.     

Promotional Effect of ZrO2 on supported FeCoK Catalysts for Ethylene Synthesis from catalytic CO2 hydrogenation

This study is to convert renewable H₂ and increasingly concerned CO₂ to ethylene or C₂H₄ over Fe_3.33Co_1.67K₅/ZrO₂ and Fe_3.33Co_1.67K₅/Al₂O₃ catalysts. The ZrO₂ support provides amounts of surface oxygen vacancies (OVs) as well as stable and rich surface hydroxyl groups (-OH), which promotes the Fe_3.33Co_1.67K₅/ZrO₂ catalysts with 5% Fe and Co loadings to achieve C₂H₄ space time yield (STY) of 0.064 mmol_C2H4∙m⁻²_cat∙h⁻¹ at 290 °C and 2.0 MPa, while the Fe_3.33Co_1.67K₅/Al₂O₃ catalysts only reach C₂H₄ STY of 0.009 mmol_C2H4∙m⁻²_cat∙h⁻¹ at 330 °C and 2 MPa Fe_3.33Co_1.67K₅/ZrO₂ catalysts are very promising for converting the captured CO₂ and renewable H₂ to highly demanded C₂H₄. This work not only provides a guideline for developing efficient catalysts but also advances the mechanistic understanding of catalytic CO₂ hydrogenation.     

Enhanced CO hydrogenation performance via two-dimensional NiAl-layered double oxide decorated by SiO2 nanoparticles

The CO methanation reaction has been widely used in the fields of synthetic natural gas and ammonia (NH₃). This study improves the CO methanation performance using a two-dimensional NiAl-layered double oxide (2D NiAl-LDO) decorated by SiO₂ nanoparticles and reduced under hydrogen atmosphere. The as-obtained H–NiAl-LDO/SiO₂ exhibited a high specific surface area of 240.5 m²/g and high surface-adsorbed oxygen of 20.77%. Furthermore, it had an excellent CO conversion of 100% at 300 °C and 96.87% at 250 °C, which was much better than those of H–NiAl-LDO (84.03% at 300 °C and 0% at 250 °C). We believe that it provides an additional strategy to easily and effectively improve CO methanation performance and shows potential for the application of similar catalysts.     

Hydrogen production from partial oxidation of methane over dielectric barrier discharge plasma and NiO/γ-Al2O3 catalyst

Hydrogen production from partial oxidation of methane under the combination of dielectric barrier discharge (DBD) plasma and NiO/γ-Al₂O₃ catalyst with cordierite honeycomb monoliths as substrate was investigated. The results showed that obvious synergistic effect was generated between DBD plasma and catalyst. Compared with the DBD plasma reactor without catalyst, the CH₄ conversion and H₂ yield increased from 60.1% and 21.3% to 83.6% and 28.4%, respectively. When the discharge power is above 70 W, the combination of DBD plasma and NiO/γ-Al₂O₃ catalyst promotes partial oxidation of methane. The catalyst was characterized by X-ray diffraction (XRD). NiO on the surface of catalyst was reduced to Ni because of the introduction of DBD plasma. The activity of catalyst at low temperature was improved, and the generation of oiliness by-products was significantly reduced.     

Comprehensive review of Cu-based CO2 hydrogenation to CH3OH: Insights from experimental work and theoretical analysis

Developing the technology of CO₂ hydrogenation into methanol can not only alleviate environmental problems such as greenhouse effect, but also effectively promote the utilization of CO₂ resources. In general, Cu-based catalysts have been extensively studied due to its low cost and the effective synthesis of methanol. Thus, this review is to be reported based on Cu-based catalysts for methanol synthesis from CO₂ hydrogenation. The specific goal of this review is to provide some insights into the structural and surface properties of Cu-based catalysts and their functions on the reaction mechanisms, and further affecting on the catalytic selectivity, stability, and activity for the CO₂ hydrogenation to methanol. A vital issue discussed is the fundamental understanding of active sites, reaction mechanisms, and interactions (active metal-support, active metal-promoter, bimetal) in determining the catalytic performance. Through a comprehensive overview on Cu-based systems for CO₂ hydrogenation to methanol from both experimental and theoretical perspectives, it could provide some useful information to go into CO₂ hydrogenation to methanol for the outsider, and promote the design and synthesis of novel and efficient catalysts.     

Catalytic DBD plasma reactor for low temperature partial oxidation of methane: Maximization of synthesis gas and minimization of CO2

In this study, room temperature synthesis gas production from partial oxidation of methane through a dielectric barrier discharge plasma reactor was investigated experimentally. In this case, operating conditions including applied voltage and Ar flow rate were evaluated to obtain the best operating conditions for the purpose of enhancing CH₄ conversion and selective synthesis gas production. In addition, plasma alone system was compared with the catalytic system (NiO–CaO/Al₂O₃), revealing great improvement of synthesis gas selectivity through utilization of the catalyst. The maximum CH₄ conversion of 99.9% with the synthesis gas module of 1.84 and energy efficiency of 3.2% was gained in the catalytic system where the applied voltage was 10 kV. Besides, synthesis gas module of almost 2, which is in favor of commercial applications including methanol production, was achievable at the applied voltage of 8 kV in the catalytic system. Finally, the obtained results were compared with the similar previous studies.