Tailoring performance of Co–Pt/MgO–Al2O3 bimetallic aerogel catalyst for methane oxidative carbon dioxide reforming: Effect of Pt/Co ratio

Co–Pt/MgO–Al₂O₃ bimetallic aerogel catalysts were synthesized via a sol-gel combined with supercritical drying method. The catalysts were characterized by XRD, BET, HRTEM, STEM-HAADF, XPS, H₂-TPR, H₂-TPD, TG/DSC, FESEM and their catalytic performances in CH₄ oxidative CO₂ reforming were evaluated. The H₂ spillover effect between Pt and Co enhanced the reducibility of the catalyst, while the strong metal-support interaction (SMSI) effect in the bimetallic aerogel catalysts confined the agglomeration of metal particles. Pt/Co ratio played a key role on the existence of surface metal species, leading to different catalytic performances. The optimal Pt/Co ratio was Pt/Co = 0.02 w/w, on which a 50% higher activity in terms of CH₄ conversion than monometallic Co or Pt aerogel catalysts was obtained. Whereas the impregnated catalyst with an identical composition showed a much lower activity. The Co–Pt aerogel catalysts also showed high resistance to inactive carbon formation. The oxidation temperature of the carbon species deposited on the spent Co–Pt aerogel catalyst was only 275 °C and no filamentous or graphitic carbon was identified, disclosing that the formation of inactive carbon was inhibited due to the synergy between Co and Pt and the SMSI effect.     

High-performance Ni–SiO2 for pressurized carbon dioxide reforming of methane

The SiO₂ and Ni–SiO₂ were synthesized via the complex-decomposition method by using different organic acids as the complexing agent and fuel. The Ni-supported SiO₂ from different sources was prepared by the incipient impregnation method. The Ni–SiO₂ and Ni/SiO₂ were comparatively evaluated for carbon dioxide reforming of methane (CDR) under severe conditions of CH₄/CO₂ = 1.0, T = 750 °C, GHSV = 53200 mL g⁻¹ h⁻¹, and P = 0.1–1.0 MPa. The materials were fully characterized by XRD, XPS, TEM, TG-DSC, H₂-TPR, and N₂ adsorption-desorption at −196 °C. It was found that the complexing agent and preparation method of the catalyst significantly affected its surface area, the size and dispersion of Ni, the reduction behavior, and the coking and sintering properties, which determine the activity and stability of the catalyst for CDR. As a result, a highly active and stable Ni–SiO₂ for pressurized CDR was obtained by optimizing the complexing agent.     

Eclectic trimetallic Ni–Co–Ru catalyst for the dry reforming of methane

Ru-promoted Ni–Co catalysts supported on MgO–Al₂O₃ are tested for dry reforming activity in a fixed-bed reactor at atmospheric pressure. The effect of temperature and contact time is investigated, and the catalysts maintain high stability for up to 47 h on stream. The support surface area significantly affects dispersion, and whisker carbon is observed in spent samples with pore sizes higher than 15 nm. H₂-TPR studies reveal the presence of spinels that are not completely reducible, yet the catalysts give remarkable activity. Four synthesis methods are tested for the support, with the sol-gel method in neutral conditions providing the best performance, with a small compromise on activity but a significant improvement in catalyst stability and no whisker carbon formation. Further work will optimize the Ni/Co ratio in the active phase to decrease the carbon buildup on the catalysts.     

Ni–Fe/Mg(Al)O alloy catalyst for carbon dioxide reforming of methane: Influence of reduction temperature and Ni–Fe alloying on coking

Various Ni–Fe/Mg(Al)O alloy catalysts were obtained by calcination of Ni–Fe–Mg–Al hydrotalcite-like compounds, followed by reduction at different temperatures (973–1173 K). The characterizations of XRD and STEM-EDX suggest that the resulting Ni–Fe alloy particles are composition-uniform and size-controllable. The alloy composition is little affected by the reduction temperature, whereas the particle size (5.8–8.2 nm) increases with the increase of reduction temperature. This property is ascribed to the homogeneous distribution of nickel and iron species during the catalyst preparation. All of the Ni–Fe/Mg(Al)O alloy catalysts show relatively high and stable activity for CH₄–CO₂ reforming during 25 h of investigation at 773–1073 K. Particularly, the 973 K-reduced catalyst exhibits higher coke-resistance due to its smaller particle size. E_a-CH₄ and CH₄-TPSR measurements indicate that Ni–Fe alloying inhibits CH₄ dissociation. It is considered that during DRM CH₄ is dissociated at the Ni sites and CO₂ may be activated at the metal-support interface as well as the Fe sites. Ni–Fe alloying may inhibit CH₄ dissociation and/or promote CO₂ activation, thus contributing to the suppression of coke deposition.     

Hydrogen production from carbon dioxide reforming of methane over highly active and stable MgO promoted Co–Ni/γ-Al2O3 catalyst

CoNi/Al₂O₃ and MgCoNi/Al₂O₃ catalysts are investigated for hydrogen production from CO₂ reforming of CH₄ reaction at the gas hourly space velocity of 40,000 mL g⁻¹ h⁻¹. The MgO promoted CoNi/Al₂O₃ catalyst shows much higher conversions (97% for CO₂ and 95% for CH₄ at 850 °C) than the CoNi/Al₂O₃ catalyst. In addition, the stability is maintained for 200 h in CO₂ reforming of CH₄. The outstanding catalytic activity and stability of the MgO promoted CoNi/Al₂O₃ catalyst is mainly due to the basic nature of MgO, an intimate interaction between Ni and the support, and rapid decomposition/dissociation of CH₄ and CO₂, resulting in preventing coke formation in CO₂ reforming of CH₄.     

Co–Ni alloy supported on CeO2 as a bimetallic catalyst for dry reforming of methane

Dry reforming of methane (DRM) is an effective route to convert two major greenhouse gas (CH₄ and CO₂) to syngas (H₂ and CO). Herein, in this work, monometallic Ni/CeO₂ and a series of bimetallic Co–Ni/CeO₂ catalysts with Co/Ni ratios between 0 and 1.0 have been tested for DRM process at 600–850 °C, atmospheric pressure and a CH₄/CO₂ ratio of 1. The catalysts were characterized by X-ray diffraction, hydrogen-temperature programmed reduction, CO₂-Temperature programmed desorption, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The catalyst with a Co/Ni ratio of 0.8 (labeled as 0.8 Co–Ni/CeO₂) exhibited the highest catalytic activity (CH₄ and CO₂ initial conversion for 80% and 85% at 800 °C, respectively) and the highest stability (less carbon deposition after 600min). This improved activity can be attributed to the Co–Ni alloy, which formed after reduction. Its weak chemisorption with hydrogen results in inhibition of reverse water gas shift reaction. In addition, Co-promoted the adsorption of surface oxygen enhances carbon removal, making it more stable.     

Kinetics of methane decomposition to COx-free hydrogen and carbon nanofiber over Ni–Cu/MgO catalyst

Kinetic modeling of methane decomposition to CO_x-free hydrogen and carbon nanofiber has been carried out in the temperature range 550–650 °C over Ni–Cu/MgO catalyst from CH₄–H₂ mixtures at atmospheric pressure. Assuming the different mechanisms of the reaction, several kinetic models were derived based on Langmuir–Hinshelwood type. The optimum value of kinetic parameters has been obtained by Genetic Algorithm and statistical analysis has been used for the model discrimination. The suggested kinetic model relates to the mechanism when the dissociative adsorption of methane molecule is the rate-determining stage and the estimated activation energy is 50.4 kJ/mol in agreement with the literature. The catalyst deactivation was found to be dependent on the time, reaction temperature, and partial pressures of methane and hydrogen. Inspection of the behavior of the catalyst activity in relation to time, led to a model of second order for catalyst deactivation.     

Co–Ni/WC-AC catalysts for dry reforming of methane: The role of Ni species

To improve the DRM reaction performance of the catalysts, a series of Co–Ni/WC-AC catalysts are prepared by impregnation using WC-AC as the support. The structural features of the fresh and spent catalysts are characterized by BET, XRD, H₂-TPR, XPS and TG. The results show that the introduction of Ni in the 20Co/WC-AC catalyst promotes the conversion of W species to WC. Further, WC enhances the interaction between the active metal and the support. Thus, the activity and sintering resistance of Co–Ni/WC-AC catalysts are improved. It is also found that the introduction of different ratios of Ni has a significant effect on the chemical environment (oxygen environment) on the catalyst surface.10Co–10Ni/WC-AC catalysts showed high surface O_α and O_β contents of 26% and 53%, respectively. The catalyst shows excellent catalytic performance. The conversion of CH₄ and CO₂ is stable at about 84% and 85% at 800 °C.     

High performance of Mg–La mixed oxides supported Ni catalysts for dry reforming of methane: The effect of crystal structure

A series of mixed Mg–La oxide supports with various Mg²⁺/La³⁺ mole ratios were prepared via co-precipitation of Mg and La nitrates, and then impregnated to form 5 wt.% Ni catalysts. The as-prepared catalysts were evaluated in DRM reaction for 200 h and characterized by means of in situ DRIFTS, XRD, TEM, CO₂-TPD, XPS, and TGA. It was found that the interaction of suitable amount of MgO with La₂O₃ stabilized cubic La₂O₃ species in catalysts, which has high basicity to adsorb CO₂ forming monoclinic La₂O₂CO₃ (Ia) species in DRM reaction. The introduction of MgO also created surface oxygen ions (i.e. O^–). Both monoclinic La₂O₂CO₃ (Ia) and surface oxygen species are able to oxidize and remove deposited carbon, keeping the Ni catalyst at high activity and stability. Low Mg²⁺/La³⁺ ratios generated hexagonal La₂O₃ and La₂O₂CO₃ (II) in DRM reaction. The hexagonal La₂O₂CO₃ (II) did not play significant role in carbon removal so that the catalysts deactivated fast.     

Paper-structured catalyst containing CeO2–Ni flowers for dry reforming of methane

In this paper, ceria flowers containing nano-sized catalyst Ni particles were prepared on alumina-silica fiber network for production of hydrogen from biogas. The CeO₂ flowers were prepared using hydrothermal method and then NiO was loaded on the CeO₂ flowers by impregnation method. The Paper-structured catalysts (PSCs) were prepared from alumina-silica fibers and the CeO₂–NiO flowers using conventional paper making method. The loaded NiO particles were uniformly dispersed on the CeO₂ flowers with the use of polyvinylpyrrolidone as a dispersion enhancer, which was observed by FE-SEM and EDX analysis. The NiO particles were then reduced into Ni by using H₂. The PSCs containing CeO₂–Ni flowers with various Ni contents (2.1, 3.4, and 4.6%) were used for dry reforming of CH₄. It was found that 3.4% amount of Ni on the PSC was suitable for reforming reaction, and the higher amount of Ni (4.6%) did not increase the CH₄ conversion. The PSC with the CeO₂ flowers had porous structure and large surface area leading to the better dispersion of the Ni particles with smaller size. This helped increase in catalytic performance, prevention of agglomerated particle catalysts at high temperature and coke forming after a long time operation. The CH₄ conversion of the PSCs containing CeO₂–NiO flowers in the dry reforming of CH₄ was much higher (nearly 90%) with a smaller Ni content in comparison with the PSC without the CeO₂ flowers (with higher Ni content of 8.6%). Moreover, the PSCs with the flowers exhibited an excellent catalytic stability with the degradation of CH₄ conversion of only 3.1% after 50 h of reforming. In addition, the high oxygen storage capacity and oxygen mobility of CeO₂ resulted in a partial removal of coke forming on the catalyst particles during reforming. This indicated that the catalytic activity of the Ni particles dispersed on the CeO₂ flowers for dry reforming of CH₄ was superior to that of various Ni-based catalyst systems which had much higher Ni contents. Therefore, it is possible to use the PSCs containing CeO₂–Ni flowers to generate hydrogen for use as fuels from dry reforming of CH₄.     

The effect of the absence of Ni,Co, and Ni–Co catalyst pretreatment on catalytic activity for hydrogen production via oxidative steam reforming of ethanol

This article presents a study of the catalytic performance of Ni, Co, and Ni–Co–Mg–Al mixed oxides obtained from hydrotalcite precursors for the oxidative steam reforming of ethanol (OSRE) when no pretreatment (pre-reduction) is accomplished. Two catalysts (a Ni-based monometallic and an equimolar Ni–Co-based catalyst) achieve in situ reduction over shorter time periods compared with the other bimetallic catalysts and also, exhibit the best catalytic activity. On the contrary, the monometallic Co catalyst did not exhibit good catalytic performance, likely because of the existence of resistant spinel phases to soft reduction processes and/or to the re-oxidation of Co. The equimolar presence of Ni and Co generates a synergistic effect evidenced by the increase in the reducibility, basicity, and mobility of electrophilic oxygen species of the solid. The results yield important information for better understanding the catalytic system under study.     

The effect of NH3·H2O addition in Ni/SBA-15 catalyst preparation on its performance for carbon dioxide reforming of methane to produce H2

Three kinds of Ni catalysts loaded on SBA-15 were prepared by impregnation method, adding NH₃·H₂O or C₂H₂O₄ in the impregnant, and applied to CO₂ reforming (DRM) of methane to generate hydrogen at 750 °C. The results showed that the catalyst prepared with NH₃·H₂O addition exhibited high activity and stability. TEM, H₂-TPR, XPS, XRD, and TG-MS characterization indicated that with addition of NH₃·H₂O (0.1 mol L^?1) in the preparation process, small nickel oxide particles, with average size of about 7 nm, were obtained on the catalyst, and the strong association between support and Ni species made them uniformly dispersed, enabling the capacity to resist carbon deposition and metal sintering, which contributed to the excellent activity, stability and selectivity for the DRM to H₂ production.     

The effect of different atmosphere treatments on the performance of Ni/Nb–Al2O3 catalysts for methane steam reforming

Methane steam reforming is currently the most widely used hydrogen production reaction in industry today. Ni/Nb–Al₂O₃ catalysts were prepared by treatment under H₂, N_2, and air atmosphere prior to reduction and applied for methane steam reforming reaction at low temperature (400–600 °C). The hydrogen-treated catalysts increased catalytic activity, with 55.74% methane conversion at S/C = 2, GSVH of 14400 mL g^?1 h^?1 and 550 °C. The H₂ atmosphere treatment enhanced the Ni–Nb interaction and the formation of stable, tiny, homogeneous Ni particles (6 nm), contributing to good activity and stability. In contrast, the catalysts treated with nitrogen and air showed weaker interactions between Ni and Nb species, whereas the added Nb covered the active sites, which caused the decrease in activity. Meanwhile, carbon accumulation was also observed. This work is informative for preserving small nano-sized nickel particles to enhance catalytic performance.     

Nickel–silica core@shell catalyst for methane reforming

Steam reforming of methane from biogas at a small scale could potentially provide a source of hydrogen for applications such as electricity generation via fuel cells. The efficiency of the reforming process is dependent upon an effective catalyst and thus this work aimed to produce a highly active catalyst for methane reforming which is resistant to deactivation. A nickel–silica core@shell catalyst was synthesized by a deposition–precipitation method. The catalyst was characterized by XRD, SEM, TPR and infrared spectroscopy techniques. TEM analyses of sections of the catalyst embedded in resin confirmed that the catalyst had a core@shell structure. Both forms of nickel phyllosilicate 1:1 and 2:1 were identified in the catalyst structure. The performance of the catalyst in methane steam reforming was investigated. The catalyst showed relatively high methane conversion 85% at 750 °C.     

Highly coke resistant Ni–Co/KCC-1 catalysts for dry reforming of methane

Nanofibrous KCC-1 supported Ni–Co bimetallic catalysts were investigated for dry reforming of methane for syngas generation. Monometallic catalysts such as Ni/KCC-1 and Co/KCC-1, and a series of bimetallic Ni–Co/KCC-1 catalysts were prepared by impregnation and co-impregnation method, respectively. All the catalysts were characterized by XRD, FT-IR, HR-SEM, FE-SEM, XPS, FT-Raman, BET, UV–Visible DRS and AAS techniques. Monometallic nickel supported catalyst contains NiO as an active phase, whereas bimetallic nickel catalysts contain Ni₂O₃, and NiCo₂O₄ on the surface. In the case of cobalt loaded catalysts, spinel Co₃O₄ is the dominant active species, apart from NiCo₂O₄. The addition of cobalt in Ni/KCC-1 has a pronounced effect on the crystallite size, surface area and active species. The hydrogen pretreatment of the catalyst produces bimetallic Ni–Co alloy on the surface. The catalytic activities of the bimetallic catalysts towards dry reforming of methane are better than monometallic catalysts. Mesoporous silica-based KCC-1 offers easy accessibility to the entire surface moieties due to its fibrous nature and the presence of channels, instead of pores. The 2.5%Ni-7.5%Co/KCC-1 showed the maximum CH₄ and CO₂ conversion along with a remarkably low H₂/CO ratio. The life-time test confirms the high thermal stability of the catalysts at 700 °C for 8 h, with less deactivation due to coke formation. The spent catalysts were characterized by XRD, TGA, FT-Raman, and FE-SEM to understand the structural and chemical changes during the reaction. The insignificant D band and G band of graphitic carbon in FT-Raman spectra for the highly active 2.5%Ni-7.5%Co/KCC-1 and 5%Ni–5%Co/KCC-1 catalysts along with TGA results containing 12% weight loss confirms the minimum coke deposition, formation of amorphous carbon and highest coke resistance. The fibrous support restricts the sintering and aggregation of nickel particles as well the deposition of coke. The addition of amphoteric cobalt increases the activity and stability of the catalysts. Ni–Co/KCC-1 with high coke resistance seems to be a promising catalyst for dry reforming of methane.     

Coke-resistance over Rh–Ni bimetallic catalyst for low temperature dry reforming of methane

Methane and carbon dioxide can be converted into syngas using the prospective dry reforming of methane technology. Carbon deposition is a major cause of catalyst deactivation in this reaction, especially at low temperature. The superior stability of bimetallic catalysts has made their development more and more appealing. Herein, a series of bimetallic RhNi supported on MgAl₂O₄ catalysts were synthesized and used for low temperature biogas dry reforming. The results demonstrate that the bimetallic RhNi catalyst can convert CH₄ and CO₂ by up to 43% and 52% over at low reaction temperature (600 °C). Moreover, the reaction rate of CH₄ and CO₂ of RhNi–MgAl₂O₄ remains stable during the 20 h long time stability test, most importantly, there was no obviously carbon deposition observed over the spent catalyst. The enhanced coking resistance should be attributed to the addition of a little amount of noble metal Rh can efficiently suppress dissociation of CH_X1 species into carbon, and the high surface areas of MgAl₂O₄ support can also promote the adsorption and activation of carbon dioxide to generate more O1 species. Balancing the rate of methane dissociation and carbon dioxide activation to inhibit the development of carbon deposition is a good strategy, which provides a guidance for design other high performance dry reforming of methane catalysts.     

Effect of ceria content in Ni–Ce–Al catalyst on catalytic performance and carbon/coke formation in dry reforming of CH4

In this study, to enhance the catalytic activity and minimize the carbon/coke formation, cerium incorporated alumina-supported new Ni–Ce–Al catalysts were investigated in dry reforming of biogas. Ni–Ce–Al catalysts were synthesized by the modified one-pot sol-gel method in an inert environment. To determine the effect of the Ce/Al ratio on physicochemical properties of catalysts and their activity, Ni (5 wt %) catalysts with different Ce/Al ratios (0/1, 1/2, 1/1, 2/1 b y wt.) were tested in a fixed-bed flow reactor using an equimolar ratio of CH₄/CO₂/Ar gas mixture. To explain the correlation between catalytic activity and catalyst properties, characterization studies were carried out by using N₂ adsorption-desorption isotherms, XPS, XRD, SEM-EDS, TEM, ICP-OES, DRIFT, O₂-TPO and TGA methods before and after activity tests. XRD analysis showed that both CeO₂ and γ-Al₂O₃ crystalline phases with metallic Ni having different peak intensities were separately formed in all the catalysts. Among the catalysts of different Ce/Al ratios, the Ni–1Ce–1Al catalyst containing equal amounts of Al and Ce has the smallest CeO₂ crystallite size. This catalyst also has the highest pore diameter and volume. XPS analysis showed that Ce incorporation into the Ni-based Al₂O₃ catalyst decreased the NiAl₂O₄ formation. DRIFT analysis indicated that the addition of ceria into the support material decreased the Lewis acidity of alumina. During 25-h long-term activity test, the conversions of CH₄ and CO₂ obtained with the catalyst Ni–1Ce–1Al (Ce/Al ratio is 1/1) were approximately the same and averaged 76% and 85%, respectively. This behavior of the catalyst indicates its high stability. TGA, XRD and SEM analyzes performed with the used Ni–1Ce–1Al and Ni–Al catalysts show very little carbon formation in the catalyst containing equal weights of Ce and Al compared to the catalyst without Ce. This result shows that the addition of cerium to the catalyst structure prevents carbon deposition due to its special oxygen mobility. This study showed that the crystallite size of CeO₂ in the catalyst structure strongly influences preventing carbon accumulation in the dry reforming reaction.     

Natural gas reforming of carbon dioxide for syngas over Ni–Ce–Al catalysts

A series of Ni–Ce–Al composite oxides with various Ni molar contents were synthesized via the refluxed co-precipitation method and used for natural gas reforming of CO₂ (NGRC) for syngas production. The effect of Ni molar content, reaction temperature, feed gas ratio and gas hourly space velocity (GHSV) on the Ni–Ce–Al catalytic performance was investigated. The Ni₁₀CeAl catalyst was selected to undergo 30 h stability test and the conversion of CH₄ and CO₂ decreased by 2.8% and 2.6%, respectively. The characterization of the reduced and used Ni₁₀CeAl catalyst was performed using BET, H₂-TPR, in-situ XRD, TEM, and TGA-DTG techniques. The in-situ XRD results revealed that Ce₂O₃, CeO₂ and CeAlO₃ coexisted in the Ni₁₀CeAl catalyst after reduction at 850 °C for 2 h. The results of the TEM analysis revealed that the Ni particle size increased after the NGRC reaction, which mainly caused the catalyst deactivation.     

Combined steam and CO2 reforming of methane over Co–Ce/AC-N catalyst: Effect of preparation methods on catalyst activity and stability

An appropriate preparation method is the essential to improve the catalyst performance. In this study, Co–Ce/AC-N catalysts were fabricated on N-doped activated carbon supports by impregnation, sol-gel, precipitation and mix methods, respectively. It was used to catalyze the combined steam and dry reforming of methane (CSDRM). The effects of different preparation methods on the catalyst performance were investigated by means of N₂ adsorption-desorption, XRD, H₂-TPR, TEM, CO₂-TPD, FTIR and XPS. Compare with the catalysts prepared by other methods, the catalyst prepared by impregnation exhibits a large surface area, high active metal dispersion, and strong metal-support interaction. Meanwhile, it also has strong basic sites and abundant oxygen vacancies. These greatly improve the activity and stability of the catalyst. The conversions of CH₄ and CO₂ at 650 °C were achieved 71.6% and 64.4%, and H₂/CO was retained at 1.5.     

Mixed reforming of methane over Ni–K/CeO2–Al2O3: Study of catalyst performance and reaction kinetics

Syngas can be effectively produced by mixed reforming of methane (MRM). In this work, the performance of Ni–K/CeO₂–Al₂O₃ catalyst in this process was investigated in a fixed-bed reactor in the 923–1073 K range. Both potassium and ceria are renowned for improving the performance of Ni catalyst in the reforming process. The influence of reaction conditions (viz. temperature, space time, feed composition and time-on-stream) on the conversion of two reactants CH₄ and CO₂, yield of the products H₂ and CO and the H₂/CO ratio in syngas were studied. At T = 1073 K and W/Q₀ = 0.17 g-h/L (here, W and Q₀ denote catalyst mass and volumetric flow rate of feed), conversions of CH₄ and CO₂ were 91.2 and 80.1%. When S/C ratio (or steam-to-carbon ratio) in feed increased from 0.2 to 0.5 mol/mol, H₂/CO ratio at T = 1073 K changed from 1.32 to 2.14 mol/mol. The catalyst performed stably for 50 h of time-on-stream. Reaction kinetics was studied between 973 and 1073 K and power law kinetic model was suggested. The apparent activation energy values for consumption of CH₄ and CO₂ were found to be 33.3 and 45.5 kJ/mol, respectively. This work is expected to aid catalyst development and reactor design for the MRM process.