Preferential oxidation of CO in a H2-rich stream over multi-walled carbon nanotubes confined Ru catalysts

Multi-walled carbon nanotubes (MWNTs) confined Ru catalysts were prepared by a modified procedure using ultrasonication-aided capillarity action to deposit Ru nanoparticles onto MWNTs inner surface. The structure properties of MWNTs supports and Ru catalysts were extensively characterized by XRD, TGA, H₂-TPR, XPS, TEM, FTIR and Raman spectra. The catalytic performance in the preferential oxidation of CO in a H₂-rich stream was examined in detail with respect to the influences of Ru loading, MWNTs diameter, various pretreatment conditions, and the presence of CO₂ and H₂O in the feed stream. In contrast with Ru catalysts supported on MWNTs external surface and other carbon materials, the superior activity was observed for the MWNTs-confined Ru catalyst, which was discussed intensively in terms of the confinement effect of carbon nanotubes. The optimized catalyst of 5 wt.% Ru confined in MWNTs with diameter of 8–15 nm can achieve the complete CO conversion in the wider temperature range and the favorable stability at 80 °C under the simulated reformatted gas mixture, which proves a promising catalyst for preferential CO oxidation in H₂-rich stream.     

Catalytic CO2 reforming of CH4 over MgAl2O4 supported Ni-Co catalysts for the syngas production

Ni, Co and Ni–Co bimetallic catalysts of different ratios were synthesized by the Incipient Wetness Impregnation Method (IWI) over Magnesium Aluminate support, keeping the total metal loading 15 wt.%, characterized and tested for the reforming of methane with carbon dioxide at 873 K and 1 atm pressure. Magnesium Aluminate supported catalysts were also compared with Al₂O₃ supported Ni catalysts with similar metal loading. The results obtained revealed that MgAl₂O₄ exhibited excellent thermal stability as compared to Al₂O₃ as support at higher temperatures. Ni–Co catalyst, with an explicit Ni:Co (3:1) ratio for the 75Ni25Co/MgAl₂O₄ provided the highest CH₄ conversion and was about 1.82 times that of the 100Ni/MgAl₂O₄; CO₂ conversion also followed similar trends. Co-existence of Ni and Co with synergic effect in an explicit Ni:Co (3:1) ratio reduced the reduction temperature and increased the amount of metal in 75Ni25Co/MgAl₂O₄. CH₄ and CO₂ conversions, TOF_DRM, H₂: CO ratios and catalyst deactivations were related to the concentrations of the Ni–Co and particularly an explicit ratio of 3:1 for the Ni:Co in 75Ni25Co/MgAl₂O₄ catalyst provided the best initial & final conversions, TOF_DRM and H₂:CO ratio. Detail carbon analysis suggested that the type of coke deposited on 75Ni25Co/MgAl₂O₄ after the DRM reaction is of the same nature and are originating from the CH₄ cracking reaction and are of reactive type.     

Enhanced activity of Co catalysts supported on tungsten carbide-activated carbon for CO2 reforming of CH4 to produce syngas

Carbon materials are widely used as catalysts or supports due to their excellent properties. In this paper, the tungsten carbide-activated carbon (WC-AC) composite support was successfully prepared by in-situ carburizing on AC matrix, which is characterized by the covalent anchoring of WC on the AC support. The active metal Co was supported on WC-AC for dry reforming of methane (DRM). Samples were analyzed by N₂ physisorption measurements, XRD, XPS, H₂-TPR, H₂-TPD, CH₄&CO₂-TPSR, TG-DTG. The WC-AC stabilizes the disturbance of C in AC, alleviates the gasification effect of CO₂ and increases the active sites for CH₄ cracking. Moreover, WC provides a resistance-less bridge suitable for the Co³⁺ → Co²⁺, resulting in a high Co²⁺/Co³⁺ ratio on the catalyst surface. This enhances the interaction between the Co species and the WC-AC, thereby enhancing the CH₄ activation. In the process of WC-AC promoting Co³⁺→Co²⁺, the catalyst surface is accompanied by the generation of oxygen vacancies. This can enhance the dissociative adsorption of CO₂ on surface of the WC-cobalt oxide, and at the same time increase the relative proportion of adsorbed oxygen on the catalyst surface, thereby effectively inhibiting the formation of coke. However, the small amount of graphitic carbon generated due to the strong coupling of WC and Co is the main reason for deactivation of Co/WC-AC.     

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.     

Low-temperature catalytic conversion of greenhouse gases (CO2 and CH4) to syngas over ceria-magnesia mixed oxide supported nickel catalysts

Running dry reforming of methane (DRM) reaction at low-temperature is highly regarded to increase thermal efficiency. However, the process requires a robust catalyst that has a strong ability to activate both CH₄ and CO₂ as well as strong resistance against deactivation at the reaction conditions. Thus, this paper examines the prospect of DRM reaction at low temperature (400–600 °C) over CeO₂–MgO supported Nickel (Ni/CeO₂–MgO) catalysts. The catalysts were synthesized and characterized by XRD, N₂ adsorption/desorption, FE-SEM, H₂-TPR, and TPD-CO₂ methods. The results revealed that Ni/CeO₂–MgO catalysts possess suitable BET specific surface, pore volume, reducibility and basic sites, typical of heterogeneous catalysts required for DRM reaction. Remarkably, the activity of the catalysts at lower temperature reaction indicates the workability of the catalysts to activate both CH₄ and CO₂ at 400 °C. Increasing Ni loading and reaction temperature has gradually increased CH₄ conversion. 20 wt% Ni/CeO₂–MgO catalyst, CH₄ conversion reached 17% at 400 °C while at 900 °C it was 97.6% with considerable stability during the time on stream. Whereas, CO₂ conversions were 18.4% and 98.9% at 400 °C and 900 °C, respectively. Additionally, a higher CO₂ conversion was obtained over the catalysts with 15 wt% Ni content when the temperature was higher than 600 °C. This is because of the balance between a high number of Ni active sites and high basicity. The characterization of the used catalyst by TGA, FE-SEM and Raman Spectroscopy confirmed the presence of amorphous carbon at lower temperature reaction and carbon nanotubes at higher temperature.     

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.     

Ni/Al2O3 catalysts for CO2 methanation: Effect of silica and nickel loading

Samples containing from 1 to 33 wt.% of NiO on silica and alumina doped with silica (1 and 20 wt.% silica in the support) have been prepared and characterized by BET, XRD, FT-IR, UV–vis–NIR, FE-SEM, EDXS, and TPR techniques. Catalysts have been pre-reduced in situ before catalytic experiments and data have been compared with Ni/Al₂O₃ reference sample. Characterization results showed that SiO₂ support has a low Ni dispersion ability mainly producing segregated NiO particles and a small amount of dispersed Ni²⁺ in exchange sites. Instead, for the Si-doped alumina a “surface spinel monolayer phase” is formed by increasing Ni loading and, only when the support surface is completely covered by this layer, NiO is formed. Moreover, H₂-TPR results indicated that NiO particles are more easily reduced compared to Ni species. Low loading Ni/SiO₂ catalysts show high selectivity and moderate activity for RWGS (reverse Water Gas Shift) reaction, likely mainly due to nickel species dispersed in silica exchange sites, as evidenced by visible spectroscopy. High loading Ni/SiO₂ catalysts show both methanation and RWGS but evident short-term deactivation for methanation, attributed to large, segregated Ni metal particles, covered by a carbon veil. Ni on alumina -rich carriers, where nickel disperses forming a surface spinel phase, show high activity and selectivity for methanation, and short-term catalyst stability as well. This activity is attributed to small nickel clusters or metal particles interacting with alumina, formed upon reaction. The addition of SiO₂ in Al₂O₃ support decreases the activity of Ni catalysts in CO₂ methanation, because it reduces the ability of the support to disperse nickel in form of the surface spinel phase, thus reducing the amount of Ni clusters in the reduced catalysts.     

Enhancement of the catalytic performance and coke resistance of alcohol-promoted Ni/MCM-41 catalysts for CH4/CO2 reforming

A series of Ni-based catalysts were prepared by the alcohol-promoted impregnation for CO₂ reforming of methane. In order to illuminate the effects of carbon chain numbers and hydroxyl group numbers on the catalytic performance and coke resistance of Ni-based catalysts, the samples were characterized by XRD, SEM, BET, H₂-TPR, FT-IR, XPS, TG, and TEM. The results show that the introduction of alcohol during impregnation promotes Ni²⁺ species into the channels of MCM-41, thereby strengthening the meatal-support interaction. Besides, the presence of alcohol decreases the particle size of Nickel and increases the surface adsorbed oxygen species over the surface of the support, thus promoting the coke resistance of the catalysts. As a consequence, NM-EG shows the highest catalytic performance, the highest stability, and the best coke resistance in all of the catalysts. This indicates that the main factor influencing the catalytic performance and coke resistance of the catalysts is the number of hydroxyl groups rather than the chain length of the introduced alcohol in the alcohol-promoted impregnation.     

Experimental and model analyses of laminar combustion characteristics of variable composition CO/H2/CH4 mixtures at high N2 and CO2 concentrations

The effect of variable composition CO/H₂/CH₄ mixtures (15%-20% CO, 5%-20% H₂, 0%-15% CH₄) at high diluent ratios (15% CO₂ and 50% N₂) on laminar combustion characteristics has been studied by experiment and numerical simulation. The laminar burning velocities (LBVs) of seven biomass-derived gases in an equivalent ratio of 0.6 to 1.4 have been experimentally measured by the spherical expansion flame method under ambient conditions. The experimental results obtained based on the linear and nonlinear extrapolation methods were compared with the data in the literature and the predictions of four detailed chemical kinetic models (FFCM-1, GRI 3.0, USC II, San Diego 2016). The results show that an increase in the equivalence ratio or a decrease in the H₂ fraction in the mixture is beneficial to the reduction of the LBV difference obtained by the linear and nonlinear extrapolation methods. With the increase of H₂ fraction in the mixture, the highly thermally diffusive fuel significantly enhanced the LBV and the maximum LBV leaned toward the fuel-rich side. For mixtures with a higher CH₄ fraction than H₂, it has the lowest LBV but has the higher adiabatic temperature and heat release. The predictions of the four models show that for all different composition mixtures, San Diego 2016 has over-predicted on the lean side. The FFCM-1 and GRI 3.0 matched better with the experimentally measured LBV of the H₂-rich mixture. With the increase of CH₄ fraction relative to H₂, the prediction of USC II is slightly reduced on the rich side, and all the predictions under stoichiometric conditions are overpredicted compared to the experimental data. Sensitivity analyses are performed on flames of the mixture with different compositions at Φ = 0.8 and 1.2, it is found that with the addition of CH₄ fraction to the mixture, R1 gradually became the most dominating reaction, which has a stronger effect on LBV. Furthermore, the reaction paths and heat release of different composition mixtures under stoichiometric conditions are analyzed. The Markstein lengths of variable composition mixtures at different equivalence ratios are studied. It suggested that the Markstein length gradually decreases with the increase of CH₄ in the fuel, thus the stretched flame speed is more susceptible to flame stretch rate and the flame stability decreases.     

Calculation of CO2, CH4 and H2S solubilities in aqueous electrolyte solution at high pressure and high temperature

This paper reports an investigation into the characterisation of liquid-vapor electrolyte solutions at high pressure and high temperature,A procedure to enable calculations of methane,carbon dioxide and hydrogen sulphide solubilities in brines(0-6m.) for temperature from 25 to 350℃ and for pressures from 1 to 1800 bar is presented.The model is based on Helgeson,Kirkham and Flowers modified equations of state(HKF)and on the semi-empirical interaction model introduced by Pitzer,HKF modified equations of state are used to calculate the reference fugacity of gas species,and the Pitzer ionic interaction model is used to calculate the activity coefficient of dissolved species(i.e.ionic or neutral).The efficiency of the combination of the two models is confirmed by several comparisons with data in the literature.     

Thermal desorption processes of H2 and CH4 from Li2ZrO3 and Li4SiO4 materials absorbed H2O and CO2 in air at room temperature

The thermal desorption processes of hydrogen (H₂) and methane (CH₄) from lithium-based materials, Li₂ZrO₃ and Li₄SiO₄, exposed to air at room temperature of 293 K with a relative humidity of 80%, were investigated using gas chromatography (GC). The GC analysis revealed that the absolute values of the released H₂ and CH₄ gases at 523 K were approximately 7.42 × 10^?6 and 1.54 × 10^?6 ml/g for Li₂ZrO₃, and 3.24 × 10^?6 and 0 ml/g for Li₄SiO₄. The amounts of H₂ and CH₄ released increased with increase in annealing temperatures and considerably depended on absorption properties of water (H₂O) and carbon dioxide (CO₂) present in air at room temperature. The production of CH₄ at low temperature is due to the intermediate species including CH_x precursors produced by the reaction between H split from H₂O and Li₂CO₃ resulting in the CO₂ absorption of Li₂ZrO₃ and Li₄SiO₄ materials.     

Numerical study on effect of CO2 addition in flame structure and NOx formation of CH4‐air counterflow diffusion flames

Numerical study with detailed chemistry has been conducted to investigate the effect of CO₂ addition on flame structure and NO_x formation in CH₄–air counterflow diffusion flame. Radiation effect is found to be dominant especially at low strain rates. The addition of CO₂ makes radiation effect more remarkable even at high‐strain rates. It is, as a result, seen that flame structure is determined by the competition between the radiation and strain rate effects. The important role of CO₂ addition is addressed to thermal and chemical reaction effects, which can be precisely specified through the introduction of an imaginary species. Thermal effect contributes to the changes in flame structure and NO formation mainly, but the effect of chemical reaction cannot be neglected. It is noted that flame structure is changed considerably due to the addition of CO₂, in such a manner, that the path of methane oxidation prefers to take CH₄→CH₃→C₂H₆→C₂H₅ instead of CH₄→CH₃→CH₂→CH. Copyright © 2001 John Wiley & Sons, Ltd.     

Steam reforming of ethanol at moderate temperature: Multifactorial design analysis of Ni/La2O3-Al2O3, and Fe- and Mn-promoted Co/ZnO catalysts

Novel Co (10%) catalysts supported on ZnO and promoted with Fe and Mn (1%) were synthesized and characterized by high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS). Their catalytic activity for steam reforming of ethanol was compared with that of Ni catalysts supported on La₂O₃-Al₂O₃. Experiments at 400 and 500 °C, steam to carbon ratios of 2 and 4, and a wide interval of contact time were analyzed following a multifactorial experimental design. At 500 °C and a steam to carbon molar ratio of 4, complete conversion of ethanol was achieved above a contact time of 200 g min mol⁻¹ for all catalysts. The ratio of selectivity between hydrogen and methane was around 23 mol_H2/mol_CH4 in the Co catalysts, while it approached the thermodynamic equilibrium (5.7 mol_H2/mol_CH4) in the Ni catalysts. The Co catalysts do not promote methane-forming reactions like ethanol cracking and acetaldehyde decarbonilation, nor do they facilitate the reverse methane steam reforming reaction. The catalytic behavior of cobalt is enhanced by promotion with iron or manganese through the formation of bimetallic particles, which facilitates cobalt reducibility. This suggests that Co-Mn/ZnO and Co-Fe/ZnO catalysts have a good potential for their use for ethanol reforming at moderate temperature.     

Theoretical study of brine secondary imbibition in sandstone reservoirs: Implications for H2, CH4, and CO2 geo-storage

In gas geo-storage operations, the injected ex-situ gas will displace the in-situ formation brine and partially occupy the porous space of the target rock. In case of water-wet rock, the displaced formation brine re-imbibes into the in-situ porous space so that the system reaches thermodynamic equilibrium. This process, referred to as ‘secondary imbibition (SI)’, has important influences on the final gas geo-storage performance, as it determines gas loss (e.g., due to capillary forces, “residual trapping”) and injection/withdrawal efficiency. Herein, a fundamental analysis of this SI process in a single capillary tube was performed.Thus, the modified Lucas-Washburn equation was applied to a theoretical analysis, and the effects of gas type, formation depth, organic acid concentration, carbon number, and silica nanofluid on the SI dynamics were assessed. It was found that the SI rate depended on gas type following the order H₂, CH₄, CO₂, and that the SI rate increased with formation depth for H₂ and CH₄, while it decreased for CO₂. Further, the adsorbed organic matter reduced the SI rate, while the silica nanofluid aging accelerated the SI rate.These insights will promote fundamental understanding of gas geo-storage processes. This work thus will provide useful guidance on gas storage capacity optimization and containment security evaluation.     

H2 production from CH4 decomposition: Regeneration capability and performance of nickel and rhodium oxide catalysts

Nickel–lanthanum (LaNiO₃) and nickel–rhodium–lanthanum (LaNi_0.95Rh_0.05O₃) perovskite-type oxide precursors were synthesized by different methodologies (co-precipitation, sol–gel and impregnation). They were reduced in an H₂ atmosphere to produce nickel and rhodium nanoparticles on the La₂O₃ substrate. All samples were tested in the catalytic decomposition of CH₄. Methane decomposed into carbon and H₂ at reaction temperatures as low as 450 °C—no other reaction products were observed. Conversions were in the range of 14–28%, and LaNi_0.95Rh_0.05O₃ synthesized by co-precipitation was the most active catalyst. All catalysts maintained sustained activity even after massive carbon deposition indicating that these deposits are of the nanotube-type, as confirmed by transmission electron microscopy (TEM). The reaction seems to occur in a way that a nickel or rhodium crystal face is always clean enough to expose sufficient active sites to make the catalytic process continue. The samples were subjected to a reduction–oxidation–reduction cycle and in situ analyses confirmed the stability of the perovskite structure. All diffraction patterns showed a phase change around 400 °C, due to reduction of LaNiO₃ to an intermediate La₂Ni₂O₅ structure. When the reduction temperatures reach 600 °C, this structure collapses through the formation of Ni⁰ crystallites deposited on the La₂O₃. Under oxidative conditions, the perovskite system is recomposed with nickel re-entering the LaNiO₃ framework structure accounting for the regenerative capability of these solids.     

CO2 reforming of methane over Ni/ZnAl2O4 catalysts: Influence of Ce addition on activity and stability

The catalytic performance of Ni supported on Ce-promoted ZnAl₂O₄ was evaluated in methane dry reforming. The effect of different nominal loadings of cerium (3, 5 and 7 wt%) in the activity, product yield and stability was studied. Ce presented a promote effect in catalytic activity, product yield and especially in stability. However the catalytic performance was considerably influenced by the amount of cerium. SEM images presented smaller particles and TPR profiles revealed stronger active phase/support interaction by Ce addition which led to increasing methane conversion and decreasing coke deposition. Although high amount of Ce was not in favor of its promoting effect due to aggregation of CeO₂ on the catalyst surface. Among the catalysts investigated, the optimal catalytic activity and stability was achieved over the sample with 5 wt% of cerium.     

Dry reforming of CO2CH4 assisted by high-frequency AC gliding arc discharge: Electrical characteristics and the effects of different parameters

In this work, a knife-shaped gliding arc discharge (GAD) reactor driven by high frequency AC (HFAC) power was employed to convert CO₂ and CH₄. The HFAC GAD exhibited good performance in dry reformation of CO₂CH₄. The development process with the HFAC GAD at a low gas flow rate was recorded and it is discussed with U-I waveforms and discharge images. The effects of input voltage, total gas flow rate, minimum electrode gap distance, and electrode thickness were investigated in terms of the CO₂CH₄ conversion rate, selectivity of CO, H₂, and C₂ hydrocarbons, specific energy density, and energy efficiency. The energy efficiency ranged from 1.58 to 2.21 mmol/kJ under changing operating conditions. The best conversions of CO₂ and CH₄ were achieved as 52.32% and 58.85%, respectively, with an energy efficiency of 1.65 mmol/kJ.     

Pd/NH2-KIE-6 catalysts with exceptional catalytic activity for additive-free formic acid dehydrogenation at room temperature: Controlling Pd nanoparticle size by stirring time and types of Pd precursors

Pd nanoparticle size is one of important factors to determine the catalytic activity of formic acid dehydrogenation catalysts. Thus various approaches to minimization of Pd nanoparticles have been attempted. In this study, we first tried to decrease Pd nanoparticles size and increase Pd dispersion of Pd/NH₂-mesoporous silica (Pd/NH₂-KIE-6) catalysts by controlling only stirring time and types of Pd precursors. It was demonstrated that the stirring time and types of Pd precursors significantly affect the performance of the catalysts. As a result, the Pd/NH₂-KIE-6 exhibited the highest catalytic activity (TOF: 8185 mol H₂ mol catalyst^?1 h^?1) ever reported for additive-free formic acid dehydrogenation at room temperature. In addition, the Pd/NH₂-KIE-6 provided higher TOF even than the case with additives such as sodium formate. Considering that the catalytic activity of Pd-based catalysts for formic acid dehydrogenation was previously controlled by promoter, support type and surface chemistry of supports, controlling the stirring time and types of Pd precursors is novel and very intriguing solutions to go beyond the current kinetic limitation for formic acid dehydrogenation.