The promoting effect of La, Mg, Co and Zn on the activity and stability of Ni/SiO2 catalyst for CO2 reforming of methane

CO₂ reforming of methane into synthesis gas over Ni/SiO₂ catalysts promoted by La, Mg, Co and Zn was investigated. The catalysts were prepared by impregnation method and characterized by XRD, TPR, SEM and TG-DTA techniques. Ni-La/SiO₂ catalyst was found to exhibit high activity and excellent stability with the addition of suitable amount of La promoter, which increased the dispersion of NiO and the interaction between NiO and SiO₂. Two different types of carbon species, namely, easily oxidized carbonaceous species and inert carbon, were observed on the surface of the used catalysts. The inert carbon deposited on Ni-Mg/SiO₂ catalyst may be the main reason for its deactivation, while the principal reason for the deactivation of Ni-Co/SiO₂ catalyst might be the sintering of metallic Ni. The addition of La and Mg decreased the contribution of reverse water-gas shift reaction, leading to higher H₂ yield.     

Combined catalytic partial oxidation and CO2 reforming of methane over ZrO2-modified Ni/SiO2 catalysts using fluidized-bed reactor

Nickel on zirconium-modified silica was prepared and tested as a catalyst for reforming methane with CO₂ and O₂ in a fluidized-bed reactor. A conversion of CH₄ near thermodynamic equilibrium and low H₂/CO ratio (1<H₂/CO<2) were obtained without catalyst deactivation during 10 h, in a most energy efficient and safe manner. A weight loading of 5 wt% zirconium was found to be the optimum. The catalysts were characterized using X-ray diffraction (XRD), H₂-temperature reaction (H₂-TPR), CO₂-temperature desorption (CO₂-TPD) and transmission election microscope (TEM) techniques. Ni sintering was a major reason for the deactivation of pure Ni/SiO₂ catalysts, while Ni dispersed highly on a zirconium-promoted Ni/SiO₂ catalyst. The different kinds of surface Ni species formed on ZrO₂-promoted catalysts might be responsible for its high activity and good resistance to Ni sintering.     

Evaluation of Ni/Y2O3/Al2O3 catalysts for hydrogen production by autothermal reforming of methane

Ni/xY₂O₃–Al₂O₃ (x = 5, 10, 15, 20 wt%) catalysts were prepared by sequential impregnation synthesis. The catalytic performance for the autothermal reforming of methane was evaluated and compared with Ni/γ-Al₂O₃ catalyst. The physicochemical properties of catalysts were characterized by X-ray diffraction (XRD), Transmission electron microscope (TEM), X-Ray Photoelectron Spectrometer (XPS), Thermo Gravimetric Analyzer (TGA) and H₂-temperature programmed reduction techniques (TPR). The decrease of nickel particle size and the change of reducibility were found with Y modification. The CH₄ conversion increased with elevating levels of Y₂O₃ from 5% to 10%, then decreased with Y content from 10% to 20%. Ni/xY₂O₃–Al₂O₃ catalysts maintained high activity after 24 h on stream, while Ni/Al₂O₃ had a significant deactivation. The characterization of spent catalysts indicated that the addition of Y retarded Ni sintering and decreased the amount of coke.     

Syngas production from CH4 dry reforming over Co–Ni/Al2O3 catalyst: Coupled reaction-deactivation kinetic analysis and the effect of O2 co-feeding on H2:CO ratio

The dry and oxidative dry reforming of CH₄ over alumina-supported Co–Ni catalysts were investigated over 72-h longevity experiments. The deactivation behaviour at low CO₂:CH₄ ratio (≤2) suggests that carbon deposition proceeds via a rapid dehydropolymerisation step resulting in the blockage of active sites and loss in CO₂ consumption. In particular, at high temperatures of 923 K and 973 K, a ‘breakthrough’ point was observed in which deactivation that was previously slow suddenly accelerated, indicating rapid polymerisation of deposited carbon. Only with feed CO₂:CH₄ > 2 or with O₂ co-feeding was coke-induced deactivation eliminated. In particular, O₂ co-feeding gave improved carbon removal, product H₂:CO ratios more suitable for downstream GTL processing and stable catalytic performance. Conversion-time data were adequately fitted to the generalised Levenspiel reaction-deactivation model. Activation energy estimate (66–129 kJ mol⁻¹) was dependent on the CO₂:CH₄ ratio but representative of other hydrocarbon reforming reactions on Ni-based catalysts.     

Stability improvements of Ni/α-Al2O3 catalysts to obtain hydrogen from methane reforming

Ni catalysts supported on commercial α-Al₂O₃ modified by addition of CeO₂ and/or ZrO₂ were prepared in the present work. Since the principal objective was to evaluate the behavior of these systems and the support effect on the stability, methane reforming reactions were studied with steam, carbon dioxide, partial oxidation and mixed reforming. Results show that catalysts supported on Ce–Zr–α-Al₂O₃ composites present better reforming activity and stability noticeably higher than in the case of the reference support. With respect to composites, the presence of mixed oxides of Ce_xZr_1−xO₂ type facilitates the formation of active phases with higher interaction. This fact reduces the deactivation by sintering conferring to the system a higher contribution of adsorbed oxygen species, favoring the deposited carbon elimination. These improvements resulted in being dependent on the Ce:Zr ratio of the composite, thus obtaining more stable catalysts for Ce:Zr = 4:1 ratios.     

Steam reforming of methanol over Cu/ZnO/ZrO2/Al2O3 catalyst

Steam reforming of methanol was investigated over Cu–ZnO–ZrO₂–Al₂O₃ catalysts at 473 and 573 K. The Cu:Zn:(Al + Zr) molar ratio was 3:3:4; however, the Zr:Al molar ratio was varied and the catalysts were pretreated at different calcination and reduction temperatures. The synthesized catalysts were characterized by N₂ physisorption, temperature-programmed reduction with H₂ (H₂-TPR), X-ray diffraction, oxidized surface TPR, and infrared spectroscopy after carbon monoxide chemisorption. The crystalline size of Cu decreased on increasing the calcination temperatures from 573 to 623 K and increased on increasing the reduction temperatures from 523 to 573 K. Among the tested catalysts, the Cu–ZnO–ZrO₂ catalyst exhibited the highest and lowest hydrogen-formation rates at 473 and 573 K, respectively. After the reaction at 573 K, all the tested catalysts exhibited an increase in the Cu crystalline size, causing the catalyst deactivation. Among the tested catalysts, the Cu–ZnO–ZrO₂–Al₂O₃ catalyst, where the Cu:Zn:Al:Zr molar ratio was 3:3:2:2, showed the highest and most stable catalytic activity at 573 K. Cu dispersion and catalyst composition affected the catalytic performance for steam reforming of methanol.     

Cu/ZnO/Al2O3 water–gas shift catalysts for practical fuel cell applications: the performance in shut-down/start-up operation

The Cu/ZnO/Al₂O₃ catalysts were prepared by the coprecipitation method, and were evaluated in the water–gas shift (WGS) reaction. The effects of the calcination temperature on the BET surface area and crystallite size were characterized. In WGS reaction, the Cu/ZnO/Al₂O₃ catalysts suffered from continuous deactivation in shut-down/start-up operation – the daily requirement for mobile and residential fuel cell systems. Among them, the Cu/ZnO/Al₂O₃ catalyst prepared at the calcination temperature of 450 °C showed the best activity and stability, with the decrement of the CO conversion of only 12.8% after three shut-down/start-up cycles. Deactivation of the Cu/ZnO/Al₂O₃ catalysts is attributed to the blocking or deterioration of the active sites by Zn₆Al₂(OH)₁₆CO₃·4H₂O resulting from the degeneration of the oxides under cyclic operations. Removal of the hydroxycarbonate species by calcination in air followed by re-reduction could restore the steady-state WGS activity; however, the regenerated catalyst underwent much severe deactivation in subsequent shut-down/start-up operation.     

Hydrogen production by steam reforming of liquefied natural gas (LNG) over mesoporous Ni-Al2O3 aerogel catalyst prepared by a single-step epoxide-driven sol-gel method

A mesoporous Ni-Al₂O₃ aerogel catalyst was prepared by a single-step epoxide-driven sol-gel method and a subsequent supercritical CO₂ drying method (NA-ES catalyst). For comparison, a mesoporous Ni-Al₂O₃ aerogel catalyst was also prepared by a single-step alkoxide-based sol-gel method and a subsequent supercritical CO₂ drying method (NA-AS catalyst). Differences in physicochemical properties and catalytic activities of mesoporous Ni-Al₂O₃ aerogel catalysts in the steam reforming of liquefied natural gas (LNG) were investigated. Textural properties of Ni-Al₂O₃ aerogel catalysts were affected by the preparation method. Nickel species were highly dispersed in alumina through the formation of nickel aluminate phase in both NA-ES and NA-AS catalysts. However, chemical states of Al atoms in both catalysts were quite different. In addition, nickel species in the NA-ES catalyst exhibited high reducibility and high dispersion compared to those in the NA-AS catalyst. In the steam reforming of LNG, NA-ES catalyst exhibited a better catalytic performance than NA-AS catalyst in terms of LNG conversion and hydrogen yield, although both catalysts showed a stable catalytic performance during the reaction without deactivation behavior. Furthermore, NA-ES catalyst with small average nickel diameter suppressed water-gas shift reaction. Reducibility and dispersion of nickel species served as important factors determining the catalytic performance of the catalysts.     

Effect and behavior of cerium oxide in Ni/γ-Al2O3 catalysts on autothermal reforming of methane: CeAlO3 formation and its role on activity

Nickel supported γ-alumina (Ni/γ-Al₂O₃) catalysts are well-known to be highly active on the autothermal reforming of methane, but to be unstable due to coke deposition. Cerium oxide (CeO₂) is one of promising promoter to overcome the fast deactivation of nickel-based catalysts by coke formation. Herein, catalytic behavior of CeO₂ over Ni/γ-Al₂O₃ catalysts on the autothermal reforming of methane was investigated. The catalytic activity was maintained for 100 h with H₂/CO molar ratio of 1.9. The formation of CeAlO₃ is observed at the reduction and reaction conditions. In this work, it was found that the formation of CeAlO₃ promoted the catalytic oxidation toward CO₂ and prevented the formation of α-Al₂O₃ and nickel-aluminate, resulting in stable activity for autothermal reforming of methane.     

Preparation of highly stable bimetallic Ni–Cu catalyst for simultaneous production of hydrogen and fish-bone carbon nanofibers: Optimization,effect of catalyst preparation methods and deactivation

This paper presents the preparation of highly stable nano-porous Ni–Cu catalysts for simultaneous production of CO_x–free hydrogen and carbon nano-fibers. The main features of this work focuses on the optimization, methods of catalyst preparation and application of an experimental model for deactivation. The fresh catalysts and the deposited carbon were characterized by SEM, TEM, XRD and Raman spectroscopy. Whatever to be the preparation methods, performance tests showed that the presence of Cu as promoter in Ni–Cu–MgO catalysts, enhanced the catalytic activity, substantially at higher temperatures with the best result obtained for Ni–Cu–MgO catalyst prepared by one step sol- gel method, reaching a hydrogen concentration of 70 vol% (160.51 mol H₂/mol Ni-1 h) and a smaller value of I_D/I_G (less imperfection) for produced carbon nano-fibers at 670 °C. Detailed rate-based model for deactivation of catalyst was found to be dependent on the time, reaction temperature and partial pressure of methane and indicated that the reaction of deactivation could be modeled by a simple hyperbolic model.     

Effect of Pr addition on the properties of Ni/Al2O3 catalysts with an application in the autothermal reforming of methane

Ni/xPr-Al₂O₃ (x = 5, 10, 15, 20 wt%) catalysts with an application in autothermal reforming of methane were prepared by sequential impregnation synthesis; its catalytic performance was evaluated and compared with that of Ni/γ-Al₂O₃ catalyst; the physicochemical properties of the catalysts were characterized by X-ray diffraction (XRD), Transmission electron microscope (TEM), X-Ray Photoelectron Spectrometer (XPS), Thermo Gravimetric Analyzer (TGA) and H₂-temperature programmed reduction techniques (TPR). The results showed that Pr addition promoted the reduction of nickel particle size on the surface. TPR experiments suggested a heterogeneous distribution of nickel oxide particles over xPr-Al₂O₃ supports and the promotion of NiO reduction by Pr modification. The CH₄ conversion increased with elevating levels of Pr addition from 5% to 10%, then decreased with Pr content from 10% to 20%. For the stability catalytic tests, Ni/xPr-Al₂O₃ catalysts maintained the high activity after 48 h while Ni/Al₂O₃ had a significant deactivation.     

Effect of support composition and metal loading on Au catalyst activity in steam reforming of methanol

Gold (Au) supported on CeO₂–Fe₂O₃ catalysts prepared by the deposition-coprecipitation technique were investigated for steam reforming of methanol (SRM). The 3 wt% Au/CeO₂–Fe₂O₃ sample calcined at 400 °C achieved 100% methanol conversion and 74% hydrogen yield due to a strong Ce–Fe interaction in the active solid solution phase, Ce_xFe_1−xO₂. The sintering of Au particles was observed when the highest metal content of 5 wt% was registered, which worsened the SRM activity. According to the TPR and TPO analysis, it was found that the transformation of the α-Fe₂O₃ structure in the mixed oxides and the coke deposition were the main factors for the rapid deactivation of the catalyst.     

Catalytic activity and characterizations of Ni/K2TixOy–Al2O3 catalyst for steam methane reforming

Nickel catalysts supported on the K₂Ti_xO_y–Al₂O₃ were prepared by the wet impregnation method for steam methane reforming to produce hydrogen. X-ray diffraction, N₂ physisorption, scanning electron microscopy with energy dispersive spectroscopy, the H₂ temperature-programed reduction technique, and X-ray photoelectron spectroscopy were employed for the characterization of catalyst samples. The results revealed that the performance of the Ni/K₂Ti_xO_y–Al₂O₃ catalysts was comparable to that of commercial FCR-4 for steam methane reforming under the mild condition. In particular, a catalytic stability test at 800 °C and in the reactant flow with the steam-to-carbon (S/C) feed ratio of 1.0 indicated that the Ni/K₂Ti_xO_y–Al₂O₃ catalysts were more active, thermally stable and resistant to deactivation than the non-promoted Ni/Al₂O₃. It is considered that the appropriate interaction strength between nickel and the modified support and proper K₂Ti_xO_y phases with a surface monolayer coverage achieved at ca. 15 wt.% loading in the support play important roles in promoting the steam methane reforming activity as well as suppressing the sintering of the catalyst.     

On the activity and stability of Sr3NiPtO6 and Sr3CuPtO6 as electrocatalysts for the oxygen reduction reaction in a polymer electrolyte fuel cell

Sr₃NiPtO₆ and Sr₃CuPtO₆ were evaluated as low-platinum alternative oxygen reduction catalysts in a solid polymer electrolyte fuel cell at 80 °C. The oxides were synthesised using a new method based on an organometallic precursor route. The electrochemical evaluation showed similar oxygen reduction performance for Sr₃NiPtO₆ and Sr₃CuPtO₆, with a slightly higher activity for Sr₃NiPtO₆. In comparison with the oxides, the oxygen reduction activity for a commercial Pt/C catalyst was approximately 10 times higher. XRD analysis of the used electrodes revealed that the oxides were not stable in the PEMFC environment, and converted into platinum during operation. Elemental analysis of the used electrodes also showed a difference in platinum formation, where the platinum content on the surface of the electrode facing the gas diffusion layer was several times higher for Sr₃NiPtO₆ than Sr₃CuPtO₆. This indicates that the Sr₃NiPtO₆ electrode may be more susceptible to platinum migration.     

CeO2–ZrO2-promoted CuO/ZnO catalyst for methanol steam reforming

The Cu-based catalysts with different supports (CeO₂, ZrO₂ and CeO₂–ZrO₂) for methanol steam reforming (MSR) were prepared by a co-precipitation procedure, and the effect of different supports was investigated. The catalysts were characterized by means of N₂ adsorption–desorption, X-ray diffraction, temperature-programmed reduction, oxygen storage capacity and N₂O titration. The results showed that the Cu dispersion, reducibility of catalysts and oxygen storage capacity evidently influenced the catalytic activity and CO selectivity. The introduction of ZrO₂ into the catalyst improved the Cu dispersion and catalyst reducibility, while the addition of CeO₂ mainly increased oxygen storage capacity. It was noticed that the CeO₂–ZrO₂-containing catalyst showed the best performance with lower CO concentration, which was due to the high Cu dispersion and well oxygen storage capacity. Further investigation illuminated that the formation of CO on CuO/ZnO/CeO₂–ZrO₂ catalyst mainly due to the reverse water gas shift. In addition, the CuO/ZnO/CeO₂–ZrO₂ catalyst also had excellent reforming performance with no deactivation during 360 h run time and was used successfully in a mini reformer. The maximum hydrogen production rate in the mini reformer reached to 162.8 dm³/h, which can produce 160–270 W electric energy power by different kinds of fuel cells.     

Steam reforming of iso-octane toward hydrogen production over mono- and bi-metallic Cu–Co/CeO2 catalysts: Structure-activity correlations

Τhe feasibility of tailoring the iso-octane steam reforming activity of Cu/CeO₂ catalysts through the use of Co as a second active metal (Cu_20−xCo_x, where x = 0, 5, 10, 15, 20 wt%), is investigated. Characterization studies, involving N₂ adsorption–desorption at −196 °C (BET), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and Temperature Programmed Reduction (H₂-TPR), were carried out to reveal the impact of the morphological, structural and surface properties of the catalysts on the reforming performance. The results showed that reforming activity was monotonically increased upon increasing cobalt loading. The Co/CeO₂ catalyst demonstrated the optimum performance with a H₂ yield of 70–80% in the 600–800 °C temperature interval. The Co/CeO₂ catalyst exhibited also excellent stability at temperatures above 700 °C, while Cu-based catalysts rapidly deactivated in long term stability tests. A close correlation between surface/redox properties and steam reforming efficiency was established. The lower reducibility of Co/CeO₂ catalysts, associated with the formation of Co³⁺ species, in Co₃O₄-like phase, can be accounted for the enhanced carbon tolerance of Co-based catalysts. Furthermore, the high concentration of surface oxygen species on Co/CeO₂ catalysts can be considered for their enhanced performance. On the other hand, the Cu-induced easier reducibility of bimetallic catalysts, in conjunction with carbon deposition and active phase sintering can be accounted for their inferior steam reforming performance. Irreversible changes in the redox properties of Cu-based catalysts, taking place under reaction conditions, could be resulted to ceria deactivation thus hindering the redox process to keep on.     

Hydrogen production from ethanol steam reforming over Ir/CeO2 catalysts: Enhanced stability by PrOx promotion

We studied ethanol steam reforming over Ir/Ce_0.9Pr_0.1O₂ and Ir/CeO₂ catalysts comparatively with respect to activity and stability. We found that PrO_x-doping have significantly promoted the oxygen storage capacity and thermal stability of the catalysts by incorporation into the ceria lattice. Ethanol was readily converted to hydrogen, methane and carbon oxides at 773 K over the Ir/Ce_0.9Pr_0.1O₂ catalyst, and this is 100 K lower than that found for the Ir/CeO₂ catalyst. Moreover, the PrO_x-doped catalyst was stable toward ethanol steam reforming at 923 K for 300 h without an apparent variation in ethanol conversion and product distribution. However, the severe aggregation of ceria particles and heavy coke deposition were observed on the Ir/CeO₂ catalyst, resulting in remarkable deactivation under the same reaction conditions.     

Comparative studies on steam gasification of ash-free coals and their original raw coals

Catalytic gasification of raw coals at mild condition is not realized yet mainly due to deactivation of catalysts via their irreversible interaction with mineral matters in coal. As a means to achieve repeated use of catalysts, four different ash-free coals (AFCs) containing less than 0.2 wt% ash are produced in this work. Steam gasification of ash-free coals (AFCs) and their parent raw coals of various ranks ranging from lignite (Eco) to coking coal (Posco) is performed in a fixed bed reactor at 700–900 °C. Regardless of the rank of the parent raw coals, all the AFCs behave like a highly carbonized coal such that their gasification rate are similarly slow and they exhibit relatively low H₂/CO ratio. The steam gasification and associated CO to CO₂ conversion of the AFCs are, however, significantly enhanced by K₂CO₃, resulting in the higher H₂/CO and CO₂/CO molar ratio.     

Studies on B sites in Fe-doped LaNiO3 perovskite for SCR of NOx with H2

A series of LaNi_1−xFe_xO₃ (x = 0.0, 0.2, 0.4, 0.7, and 1.0) perovskites were synthesized and characterized by X-ray diffraction (XRD), N₂ physisorption, scanning electron microscopy (SEM), H₂-temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The perovskites were investigated for selective catalytic reduction of NO_x by hydrogen (H₂-SCR). It is shown that Fe addition into LaNiO₃ leads to a promoted efficiency of NO_x removal, as well as a high stability of perovskite structure. Moreover, easy reduction of Ni³⁺ to Ni²⁺ with the aid of appropriate Fe component mainly accounts for the enhanced activity. Meanwhile, deactivation of the sulfated catalysts is due to that sulfates mainly deposit on active Ni component while doping of Fe can protect Ni to some extent at the expense of partial sulfation.     

Carbon nanotube formation during propane decomposition on boron-modified Co/Al2O3 catalysts: A kinetic study

In recent years, catalytic decomposition of light hydrocarbons has been explored as an interesting alternative to produce CO_x-free hydrogen along with valuable carbonaceous nanomaterials. In this contribution, we report on the effect of B on the catalytic performance of Co/Al₂O₃ catalysts during propane decomposition. Unpromoted and B-promoted (0.5–5 wt.% B) Co catalysts were prepared by incipient wetness impregnation and thoroughly characterized using XRD, XPS, TPR, H₂ chemisorption, CO-FTIR, Raman, TPO and TEM. The kinetic data were analysed using a phenomenological kinetic model. The kinetic results indicate the existence of an optimal amount of boron (ca. 2%) that maximizes both activity and stability of the catalyst. Based on the analysis of the kinetic parameters, it can be concluded that the presence of boron slows down the carburization and carbon diffusion steps, decreasing the amount of CNTs formed. However, the addition of Boron simultaneously decreases the formation of encapsulating coke, which is the cause of the catalyst deactivation.