Stability of Ni and Rh–Ni catalysts derived from hydrotalcite-like precursors for the partial oxidation of methane

In this work, NiMgAl and RhNiMgAl catalysts prepared from HTLCs precursors were investigated for the Partial Oxidation of Methane (POM) at 550 and 750 °C. Samples have been characterized by XRD, TPR, H₂ chemisorption, TPSR analyses, XPS, field emission scanning electron microscopy and Raman spectroscopy. NiMgAl catalysts with high Ni content (40 and 16 wt%) showed high stability and high methane conversion for POM. On the other hand those with lower Ni content (NiHT15 and NiHT25, with 6 and 4 wt%) exhibited low catalytic activity with low H₂/CO ratio (<2) and fast deactivation. In RhNiHT25 (0.6 wt. % Rh), the Ni reducibility was improved, increasing the methane conversion and hydrogen selectivity. In addition, the noticeable increase in stability was related to the absence of carbon deposition after 30 h on stream at 550 °C. These results show that RhNiHT25 is promising for application in membrane reactors to produce high purity hydrogen.     

Partial oxidation of methane on co-precipitated Ni–Mg/Al catalysts modified with copper or iron

Methane transformation to hydrogen and synthesis gas (CO + H₂) by heterogenous catalysts can play an important role to secure the supply of energy, chemicals and fuels in the future. Methane is the main constituent of natural gas and biogas and it is also found in crystalline hydrates at the continental slopes of many oceans. In view of this vast reserves and resources, the use of methane as chemical feedstock has to be intensified. In this present work, (NiMg)Al catalysts doped with Fe or Cu, prepared by co-precipitation method and characterized by different techniques, were studied in the partial oxidation of methane (T_reaction = 750 °C, CH₄/O₂ ratio = 2). The effect of catalyst composition and pre-treatment conditions of these catalysts were investigated. Also, these catalysts show a very high activity and selectivity in the partial oxidation reaction, which depends on the conditions of catalysts preparation. The obtained results indicated increasing of activity and selectivity with decreasing calcination temperature and increasing nickel and aluminium contents in the catalysts composition. The solid doped with iron constituted the best catalyst for the total oxidation of methane and for the water–gas shift reaction. On the other hand, the addition of copper was remarkably improved the catalytic performances of the (NiMg)Al solid. So, the presence of this element supported the partial oxidation of methane with production of syngas (CO + H₂). With the addition of iron or copper for the catalyst composition, we were observed (in our previous work) the possibility of formation of NiM (M = Fe or Cu) alloy which increased nickel particles dispersion. In the case of copper, the reducibility of NiO was also assisted (TPR results) which increased catalytic activity in partial oxidation of methane.     

Metallic Ni monolith–Ni/MgAl2O4 dual bed catalysts for the autothermal partial oxidation of methane to synthesis gas

A dual bed catalyst system consisting of a metallic Ni monolith catalyst in the front followed by a supported nickel catalyst Ni/MgAl₂O₄ has been studied for the autothermal partial oxidation of methane to synthesis gas. The effects of bed configuration, reforming bed length, feed temperature and gas hourly space velocity on the reaction as well as the stability are investigated. The results show that the metallic Ni monolith in the front functions as the oxidation catalyst, which prevents the exposure of the reforming catalyst in the back to the very high temperature, while the supported Ni/MgAl₂O₄ in the back functions as the reforming catalyst which further increases the methane conversion by 5%. A typical 5 mmNi monolith–5mmNi/MgAl₂O₄ dual bed catalyst exhibits methane conversion and hydrogen and carbon monoxide selectivities of 85.3%, 91.5% and 93.0%, respectively, under autothermal conditions at a methane to oxygen molar ratio of 2.0 and gas hourly space velocity of 1.0 × 10⁵ h⁻¹. The dual bed catalyst system is also very stable.     

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.     

Combination of CO2 reforming and partial oxidation of methane to produce syngas over Ni/SiO2 and Ni–Al2O3/SiO2 catalysts with different precursors

Ni/SiO₂ and Ni–Al₂O₃/SiO₂ catalysts were prepared by incipient wetness impregnation using citrate and nitrate precursors and tested with a reaction of combination of CO₂ reforming and partial oxidation of methane to produce syngas (H₂/CO). The catalytic activity of Ni/SiO₂ and Ni–Al₂O₃/SiO₂ greatly depended on interaction between NiO and support. NiO strongly interacted with support formed small nickel particles (about 4 nm for NiSC which is abbreviation of Ni/SiO₂ prepared with Nickel citrate precursor) after reduction. The small nickel particles over NiSC catalysts exhibited a good catalytic performance.     

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.     

Catalytic partial oxidation of CH4–H2 mixtures over Ni foams modified with Rh and Pt

The catalytic partial oxidation (CPO) of methane–hydrogen mixtures in air, intended for the first stage of hybrid radiant catalytic burners, was investigated under self-sustained short contact time conditions on commercial Ni foam catalysts eventually modified with Rh and Pt. The modified catalysts were prepared by a simple novel method based on the spontaneous deposition of noble metals via metal exchange reactions onto those Ni foam substrates. SEM-EDS, electrochemical methods and H₂-TPR analysis were integrated to characterize morphology, surface area of metal deposits and reducibility of foam catalysts before and after exposure to severe conditions in the CPO reactor. In particular Rh forms finely dispersed deposits that retain their high specific surface area at temperatures up ca. 1100 °C. Modification with noble metals enhances stability and reducibility of the Ni foam whereas the overall CPO performance is not significantly improved. Safe operation of the CPO reactor with up to 70% vol. H₂ in the fuel mixture has been achieved by properly increasing the feed equivalence ratio to avoid catalyst overheating, while guaranteeing high methane conversions and a persistent net hydrogen production.     

Effects of Y2O3-modification to Ni/γ-Al2O3 catalysts on autothermal reforming of methane with CO2 to syngas

The effects of Y₂O₃-modification to Ni/γ-Al₂O₃ catalysts on autothermal reforming of methane to syngas were investigated. It was found that the introduction of Y₂O₃ (5%, 8%, 10%) lead to significant improvement in catalytic activity and stability, and the H₂/CO ratio could be adjusted via controlling the O₂/CO₂ ratio of the feed gas. According to the characterization results of catalysts before and after reaction, it was found that the Y₂O₃·γ-Al₂O₃ supported Ni catalysts had higher NiO reducibility, smaller Ni particle size, higher Ni dispersion and stronger basicity than those of the Ni/γ-Al₂O₃ catalysts. The analysis of catalysts after reaction showed that the addition of Y₂O₃ inhibited the Ni sintering, changed the type of coke and decreased the amount of coke on the catalysts. All the experimental results indicated that the introduction of Y₂O₃ to Ni/γ-Al₂O₃ resulted in excellent catalytic performances in autothermal reforming of methane, and Y₂O₃ played important roles in preventing metal sintering and coke deposition via controlling NiO reducibility, Ni particle size and dispersion, and basicity of catalysts.     

CO2 reforming of methane over Ni/SBA-15 prepared with β-cyclodextrin – Role of β-cyclodextrin in Ni dispersion and performance

Ni/SBA-15-CD(1/X) catalysts were prepared by the impregnation of a certain amount of Ni(NO₃)₂ and various contents of β-cyclodextrin (CD), in which 1/X indicates the molar ratio of CD to Ni. The physicochemical properties of the catalysts were characterized by BET, XRD, TEM, TPR and TGA, and their catalytic performance in the CO₂ reforming of methane to syngas was evaluated using a fixed-bed quartz reactor. The characterization results revealed that Ni/SBA-15-CD(1/X) prepared with n(CD)/n(Ni) ratios in the range of 1/66–1/33 possessed smaller NiO particles and exhibited stronger interactions between NiO and SBA-15, whereas NiO particles were not well-dispersed on Ni/SBA-15-CD(1/X) catalysts prepared with further CD addition (1/X = 1/8 and 1/1). The reaction results indicated that the better-dispersed Ni/SBA-15-CD(1/X) catalysts, such as Ni/SBA-15-CD(1/66), Ni/SBA-15-CD(1/50) and Ni/SBA-15-CD(1/33), exhibited higher conversions and stronger abilities to resist carbon deposition. Regarding the role of CD in dispersing Ni particles, it could be speculated that complexes were formed between CD and Ni²⁺, as well as NO₃⁻, which would change the state of Ni species during the impregnation and heat treatment processes.     

Enhanced catalytic performance of Au/CuO–ZnO catalysts containing low CuO content for preferential oxidation of carbon monoxide in hydrogen-rich streams for PEMFC

In this study, effects of Au and CuO loadings in Au/CuO–ZnO nanocatalysts for preferential oxidation of carbon monoxide in H₂-rich streams (PROX) are investigated. CuO–ZnO supports were synthesized by a co-precipitation method. Au was also incorporated into the catalysts by a deposition-precipitation procedure. The catalysts were characterized by XRD, BET surface area, FESEM, HRTEM, H₂-TPR, FTIR, and CO-TPD. 2–10 nm Au nanoparticles are dispersed on CuO–ZnO support and significantly enhance the reducibility of CuO. The Au/CuO–ZnO catalysts containing low amount of CuO were found to be more active for PROX compared to the Au/ZnO catalyst. Moreover, as more CuO is added to Au/ZnO, the CO₂ selectivity increases in the whole PROX temperature range. The catalyst containing 2 wt% Au and 1 wt% CuO on ZnO exhibited the highest activity and selectivity in the operating temperature range of PEM fuel cells. The activity of this catalyst also remained almost intact during 900 min of PROX time on stream at 80 °C.     

Performance and stability of La0.5Sr0.5CoO3−δ perovskite as catalyst precursor for syngas production by partial oxidation of methane

The aim of this work was to investigate the performance and stability of the perovskite La_0.5Sr_0.5CoO_3−δ, as a potential catalyst precursor, for the synthesis gas production by partial oxidation of methane. For this purpose, the catalytic activity of La_0.5Sr_0.5CoO_3−δ was studied as a function of the temperature, flow rate and feed composition. In addition, its stability with the time-on-stream and redox cycles was also explored. Before and after testing, the catalyst precursor was characterized by X-ray diffraction, SEM-EDX and specific surface area (BET). The results evidenced a remarkable catalytic activity due to the stability of the cobalt, which is in a highly disperse state, in its reduced state. The CH₄ conversion and the CO and H₂ selectivities were enhanced with the increase of redox cycles. Finally, the precursor was totally regenerated to the initial perovskite structure under a specific thermal treatment.     

Reactivity and stability of Zr–doped CeO2 for solar thermochemical H2O splitting in combination with partial oxidation of methane via isothermal cycles

Ceria-based oxides have attracted a lot of attention as an attractive redox material because of their large capacity for storing and releasing oxygen in the solar-driven thermochemical water splitting (STWS) process. Nevertheless, the extremely high temperatures and low oxygen partial pressure required to achieve deep degrees of reduction and large temperature swing in a redox cycle introduce challenges in the practical implementation. These above challenges can be addressed in a unique way by integrating partial oxidation of methane into the reduction step. The STWS can therefore operate isothermally at significantly lower temperatures. In this work, the CeO₂-ZrO₂ solid solutions (Ce_1-xZr_xO₂) are synthesized, characterized, and assessed for thermochemical water splitting in combination with partial oxidation of methane. Up to 160 consecutive redox cycles are also conducted in a bench-scale fixed bed. At an operating temperature of 900 °C, methane successfully promotes the reduction of Ce_1-xZr_xO₂ to produce the synthesis gas with a 2:1H₂/CO ratio and 87.86% selectivity. When compared to CeO₂, the thermodynamic fuel generation capability of CeO₂ with Zr⁴⁺ doping is three times greater in the partial oxidation of methane step and water splitting step. Ce_0.8Zr_0.2O₂ (C8Z2) demonstrates the best redox activity in terms of CO and H₂ production in a redox cycle among the various Zr⁴⁺ doping levels. After 160 consecutive redox cycles, C8Z2 is also very robust, maintaining its redox activity. The C8Z2 composite redox solid solution thus exhibits excellent redox activity and long-term redox stability, potentially making it appropriate for STWS in combination with partial oxidation of methane.     

Partial oxidation of methane into syngas (H2 + CO) over effective high-dispersed Ni/SiO2 catalysts synthesized by a sol–gel method

A Ni catalyst supported on mono dispersed silica spheres, Ni/SiO₂-Sph (SG), has been successfully synthesized by a sol–gel method. By comparing it with other Ni catalysts (supported on commercial silica and silica spheres) prepared by an impregnation method, we find that the size of Ni particles and their dispersion are closely related to performances of the catalysts in partial oxidation of methane (POM) into synthesis gas (CO + H₂). Several means such as H₂-TPR, TEM, and XRD are employed to characterize these catalysts. Although the catalyst Ni/SiO₂-Sph (SG) in specific surface area is not large, the Ni particles are the smallest in size (3–5 nm) among the three catalysts, and are uniformly distributed, high dispersed over the silica surfaces, being not much changed as Ni loading. It is notable that the smaller size of the NiO particles is corresponding to the stronger NiO–SiO₂ interactions. The catalyst Ni/SiO₂-Sph (SG) shows the best catalytic performances and the longest lifetime among the three catalysts at the POM conditions.     

A comparative study on the performance of mesoporous SBA-15 supported Pd–Zn catalysts in partial oxidation and steam reforming of methanol for hydrogen production

Using mesoporous SBA-15 (Santa Barbara Amorphous No. 15, a mesoporous material) as support, Pd–Zn nanocatalysts with varying Pd and Zn content were tested for hydrogen production from methanol by partial oxidation and steam reforming reactions. The physico-chemical characteristics of the synthesized SBA-15 support were confirmed by XRD, N₂ adsorption, SEM and TEM analyses. The PdZn alloy formation during the reduction of Pd–Zn/SBA-15 was revealed by XRD and DRIFT study of adsorbed CO. Also, the correlation between Pd and Zn loadings and PdZn alloy formation was studied by XRD and TPR analyses. The metallic Pd surface area and total uptakes of CO and H₂ were measured by chemisorption at 35 °C. The metallic Pd surface area values are in linear proportion with the Pd loading. The formation of PdZn alloy during high temperature reduction was confirmed by a shift in absorption frequency of CO on Pd sites to lower frequency due to higher electron density at metal particles resulted from back-donation. The reduced Pd–Zn/SBA-15 catalysts were tested for partial oxidation of methanol at different temperatures and found that catalyst with 4.5 wt% Pd and 6.75 wt% Zn on SBA-15 showed better H₂ selectivity with suppressed CO formation due to the enhanced Pd dispersion as well as larger Pd metallic surface area. The O₂/CH₃OH ratio is found to play a significant role in CH₃OH conversion and H₂ selectivity. The performance of 4.5 wt% Pd–6.75 wt% Zn/SBA-15 catalyst in steam reforming of methanol was also tested. Comparatively, the H₂ selectivity is significantly higher than that in partial oxidation, even though the CH₃OH conversion is less. Finally, the long term stability of the catalyst was tested and the nature of PdZn alloy after the reactions was found to be stable as revealed from the XRD pattern of the spent catalysts.     

A series of Ni–Al2O3 catalysts derived from layered double hydroxides for vapor phase catalytic exchange between water and hydrogen

Fabricating supported metal catalysts from layered double hydroxides (LDHs) is a promising strategy to develop high-efficient and low-cost materials for water detritiation. In this work, a series of NiAl-LDHs with different Ni/Al ratios were prepared and reduced to obtain Ni-based catalysts with Al₂O₃ support (Ni_x-Al₂O₃) and further tested in vapor phase catalytic exchange (VPCE) process. Results revealed that the activity of catalysts varies with Ni/Al molar ratios and is also affected by textural properties. Remarkably, the Ni₂–Al₂O₃ with the Ni/Al molar ratio of 2 exhibited excellent activity, due to the balanced factors of Ni content, specific surface area, Ni particle size and Al³⁺ configuration proportion. This study sheds some light on reaction mechanism of VPCE process and may be applicable for the rational design of highly efficient catalysts.     

Production of syngas via autothermal reforming of methane in a fluidized-bed reactor over the combined CeO2–ZrO2/SiO2 supported Ni catalysts

Production of syngas via autothermal reforming of methane (MATR) in a fluidized bed reactor was investigated over a series of combined CeO₂–ZrO₂/SiO₂ supported Ni catalysts. These combined CeO₂–ZrO₂/SiO₂ supports and supported Ni catalysts were characterized by nitrogen adsorption, XRD, NH₃-TPD, CO₂-TPD and H₂-TPR. It was found that the combined supports integrated the advantages of SiO₂ and CeO₂, ZrO₂. That is, they have bigger surface area (about 300 m²/g) than pure CeO₂ and ZrO₂, stronger acidity and alkalescence than that of pure SiO₂, and enhanced the mobility of H adatoms. Ni species dispersed highly on these combined CeO₂–ZrO₂/SiO₂ supports, and became more reducible. Ni catalysts on the combined supports possess higher CO₂ adsorption ability, higher methane activation ability and exhibited higher activity for MATR. H₂/CO ratio in product gas could be controlled successfully in the range of 0.99–2.21 by manipulating the relative concentrations of CO₂ and O₂ in feed.     

Preparation and characterization of 5%Ni/γ-Al2o3 catalysts by complexation with NH3 derivatives active in methane steam reforming

5 wt%NiO/γ-Al₂O₃ supported nickel catalysts were prepared by incipient wetness impregnation (IWI) method. Ammonia (NH₃) derivatives aliphatic amines based on: ethylamine (EA), diethylamine (DEA) and triethylamine (TEA) were used as ligands to complex Ni(NO₃)_2,6H₂O nickel nitrates salts. Various techniques including: Thermogravimetric (TGA)- Differential thermal analysis DTA, XRF, SEM, XRD, RTP and BET were used to characterize the physic-chemical properties of the mentioned catalysts. The catalytic performances were evaluated in steam methane reforming reaction at different temperatures ranging from 500 to 800 °C. According to the results, Ni²⁺ ions form strongly different complexes with NH₃, EA, DEA and TEA amines. Based on DRX and TPR measurements, the relative stronger encumbrance due to the bigger aliphatic amines volume of the corresponding intermediate complexes avoids the growing of NiAl₂O₄ spinel phase and leads to a sensitive decrease of the average NiO crystallites size from 15 to 9 nm appearing in high dispersion state and strongly interacting with the support with Strong Metal Support Interaction (SMSI). High catalytic performances are achieved with Ni-Diethylamine and Ni-Triethylamine catalysts at 700 °C in steam reforming reaction (CH₄ conversion = 99%, H₂ yield = 82%) under GHSV 24 × 10³ mL g_cat⁻¹.h⁻¹and H₂O/CH₄ = 3.     

Cermets Ni/(Ce0.9Ln0.1O1.95) (Ln = Gd,La, Nd and Sm) prepared by solution combustion method as catalysts for hydrogen production by partial oxidation of methane

Catalysts based on Ni/(Ce_0.9Ln_0.1O_1.95) (Ln = Gd, La, Nd and Sm) have been developed and tested for hydrogen production by partial oxidation of methane. The synthesis method (SCS, solution combustion synthesis) produces macroporous composite materials composed of ceramic (cer, Ce_0.9Ln_0.1O_1.95) and metallic (met, Ni) phases, without the need of an activation stage prior to the catalytic reaction. The catalysts have been characterized by different techniques: X-ray diffraction, N₂ adsorption-desorption, Hg porosimetry, Scanning Electron Microscopy, Temperature Programmed Reduction, H₂ and O₂ pulse chemisorption, X-ray photoelectron spectroscopy and Raman spectroscopy. With the exception of the lanthanum-loaded catalyst, the catalysts are highly active, selective and stable; being the one doped with gadolinium the most efficient. Correlations structure-activity point out that the excellent catalytic performance is related to the high catalytic surface area per unit mass of catalyst and to an appropriate balance of nickel dispersion to oxygen vacancies of the support.     

Preparation and evaluation of Ni/γ-Al2O3 catalysts promoted by alkaline earth metals in glycerol reforming with carbon dioxide

Glycerol is the main by-product in the biodiesel process and can be considered as a promising and renewable source for hydrogen generation through the reforming process. In this work, catalysts with 15 wt% Ni supported on 3 wt% M ? Al₂O₃ (M = MgO, CaO, SrO, and BaO) were prepared and employed in the glycerol dry reforming (GDR) reaction to produce hydrogen and carbon monoxide. The textural characteristics of the fresh and spent catalysts were determined using the ICP, BET, TPR, TPO, and SEM analyses. Based on the obtained results, the catalyst promoted by SrO had the highest catalytic activity. The results indicated that adding various alkaline-earth oxides into the catalyst support decreased the Ni crystalline size from 17.2 nm to 7.4–10.9 nm. Moreover, all promoted catalysts showed better catalytic performance and the promoted sample with 3 wt% SrO possessed higher stability than unpromoted catalyst during 20 h on stream.