Surface modification of solution combustion synthesized Ni/Al2O3 catalyst for aqueous-phase reforming of ethanol

The effect of surface modification of an alumina powder supported nano-scale nickel catalyst used in aqueous-phase reforming of ethanol has been explored in this paper. The Al₂O₃ powder was prepared by a solution combustion synthesis (SCS) route and the surface of the powder was modified by a non-thermal RF plasma treatment using nitrogen gas. Catalysts were coated by an impregnation method. The performances of the unmodified and modified Ni/Al₂O₃ catalysts have been compared from a catalytic activity, selectivity, and microstructural point of view. The catalytic activity results showed that while nature, relative ratio and selectivity of the products both in gas and liquid effluents did not change, catalytic activity (in terms of EtOH conversion and H₂ yield per g) of the sample increased after plasma modification. Microstructural (XRD, surface area) analysis showed that phase content and surface area of unmodified and modified catalysts are similar, while TEM and H₂-chemisorption showed higher metal surface area, higher metal dispersion and lower active metal particle size for the modified sample compared to the unmodified sample. The temperature programmed reduction (TPR) analysis demonstrated stronger support-metal interaction and smaller NiO particles for the modified catalyst at lower heat treatment temperature. The temperature programmed desorption (TPD) of ammonia analysis showed stronger acidity for the modified support, which can explain better dispersion of the metal particles on the modified catalyst compared to the unmodified sample.     

Effect of preparation method on the performance of the Ni/Al2O3 catalysts for aqueous-phase reforming of ethanol: Part II-characterization

In the first part of this paper [1], we have discussed the effect of preparation method on the performance of Ni/Al₂O₃ catalysts for aqueous-phase reforming of ethanol (EtOH). One catalyst was synthesized using a sol–gel method (SG). The other was synthesized by adding nickel nitrate to a solution combustion synthesized alumina support (SCS). Based on the product distribution, we proposed the reaction pathway as a mixture of dehydrogenation of EtOH to acetaldehyde followed by C–C bond breaking to produce CO and CH₄ and oxidation of acetaldehyde to acetic acid followed by decarbonylation to CO₂ and CH₄. CH₄ (C₂H₆ and C₃H₈ also) can form via Fischer–Tropsch reactions of CO/CO₂ with H₂. The CH₄ (C₂H₆ and C₃H₈) reacts to form hydrogen and carbon monoxide through steam reforming, while CO converts to CO₂ mostly through the water-gas shift reaction (WGSR). The SG catalysts showed poorer WGSR activity than the SCS catalysts. The difference of the metal particle size distribution governed by preparation method appeared to be the key factor of controlling catalytic efficiency, but some contradictory results could not be explained.     

Reforming of bioethanol over Ni/Al2O3 and Ni/CeZrO2/Al2O3 catalysts in supercritical water for hydrogen production

Bioethanol was reformed in supercritical water (SCW) at 500 °C and 25 MPa on Ni/Al₂O₃ and Ni/CeZrO₂/Al₂O₃ catalysts to produce high-pressure hydrogen. The results were compared with non-catalytic reactions. Under supercritical water and in a non-catalytic environment, ethanol was reformed to H₂, CO₂ and CH₄ with small amounts of CO and C2 gas and liquid products. The presence of either Ni/Al₂O₃ or Ni/CeZrO₂/Al₂O₃ promoted reactions of ethanol reforming, dehydrogenation and decomposition. Acetaldehyde produced from the decomposition of ethanol was completely decomposed into CH₄ and CO, which underwent a further water-gas shift reaction in SCW. This led to great increases in ethanol conversion and H₂ yield on the catalysts of more than 3-4 times than that of the non-catalytic condition. For the catalytic operation, adding small amounts of oxygen at oxygen to ethanol molar ratio of 0.06 into the feed improved ethanol conversion, at the expense of some H₂ oxidized to water, resulting in a slightly lower H₂ yield. The ceria-zirconia promoted catalyst was more active than the unpromoted catalyst. On the promoted catalyst, complete ethanol conversion was achieved and no coke formation was found. The ceria-zirconia promoter has important roles in improving the decomposition of acetaldehyde, the enhancement of the water-gas shift as well as the methanation reactions to give an extremely low CO yield and a tremendously high H₂/CO ratio. The SCW environment for ethanol reforming caused the transformation of gamma-alumina towards the corundum phase of the alumina support in the Ni/Al₂O₃ catalyst, but this transformation was slowed down by the presence of the ceria-zirconia promoter.     

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.     

Hydrogen production by steam reforming of model bio-oil using structured Ni/Al2O3 catalysts

This study focuses on hydrogen production from the steam reforming of model bio-oil over Ni/Al₂O₃ catalysts prepared in two different geometries (monolith and pellet) using the dip-coating and wet impregnation methods and characterized using Powder X-Ray diffraction, Temperature Programmed Reduction, Scanning Electron Microscopy (SEM) and BET Surface area analysis. The effects of the catalyst geometry and reforming temperatures were studied by carrying out experiments at the optimal conditions of T = (823, 923, 1023) K and S/C ratio = 13 determined from the thermodynamic analysis of the process prior to the experiments using the process simulator PRO-II. The experimental results showed high steady state H₂ yield corresponding to 2.58 and 1.73 mol (out of 5.13 mol) using monolithic and the pelletized catalysts respectively. The product distribution achieved with the monolithic catalyst was closer to the thermodynamic results suggesting a higher selectivity to hydrogen production.     

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.     

Non-thermal plasma assisted synthesis and physicochemical characterizations of Co and Cu doped Ni/Al2O3 nanocatalysts used for dry reforming of methane

Ni/Al₂O₃ nanocatalysts doped with Co and Cu were prepared by co-impregnation and modified by non-thermal plasma. The nanocatalysts were characterized by XRD, FESEM, TEM, EDX dot-mapping, BET, FTIR, TGA-DTG, and XPS analysis. According to XRD and XPS results, good interaction between active phase and support can be observed in both Ni–Co/Al₂O₃ and Ni–Cu/Al₂O₃ nanocatalysts. A uniform morphology, high surface area, and well dispersed particles of active sites in Ni–Co/Al₂O₃ nanocatalyst were observed that shows the effect of cobalt in controlling Ni ensemble size. In contrast Ni–Cu/Al₂O₃ nanocatalyst had no homogenous dispersion of active phase due to sintering of copper particles. The activity measurements illustrated better Ni–Co/Al₂O₃ nanocatalyst activity in comparison to Ni/Al₂O₃ and Ni–Cu/Al₂O₃ in terms of CH₄ and CO₂ conversion. H₂ and CO yield were higher for Ni–Co/Al₂O₃ and higher H₂/Co ratio was obtained as well. Whereas Ni/Al₂O₃ and Ni–Co/Al₂O₃ did not experience deactivation, Ni–Cu/Al₂O₃ suffered from activity loss by ca. 22% and 16% for CH₄ and CO₂ conversion, respectively. Sintering most likely happened in Ni–Cu/Al₂O₃ nanocatalyst due to high temperature of calcination while cobalt by controlling the size of Ni particles, alternated the size of active sites to a size range in which carbon formation was suppressed. Ni/Al ratio from XPS analysis which signifies Ni dispersion on alumina support was 5.15, 9.16, and 6.35 for Ni/Al₂O₃, Ni–Co/Al₂O₃, and Ni–Cu/Al₂O₃ nanocatalysts respectively. The highest ratio of Ni/Al was for Ni–Co/Al₂O₃ nanocatalyst that shows the best coverage of support by Ni active phase in this nanocatalyst.     

Catalytic properties of Ag promoted ZnO/Al2O3 catalysts for hydrogen production by steam reforming of ethanol

Ag promoted ZnO/Al₂O₃ catalysts were prepared by using the incipient wetness impregnation method. The catalytic properties of steam reforming reaction for hydrogen production on the prepared catalysts were evaluated with H₂O:C₂H₅OH molar ratios of 3:1 at 450 °C and atmospheric pressure. Ag promoted ZnO/Al₂O₃ catalysts show higher SRE catalytic activity than ZnO/Al₂O₃ catalysts. H₂ and CH₃CHO are the major products on Ag promoted catalysts, and C₂H₄ is also produced probably due to acid sites on Al₂O₃. SRE mechanism on Ag promoted ZnO/Al₂O₃ catalysts, which contains C-C scission, is different from that on ZnO/Al₂O₃ catalysts. A method based on thermogravimetry (TG), differential scanning calorimetry (DSC) and mass spectrometry (MS) was used to analysis the coking behavior on catalyst surface. The surfaces of Ag promoted ZnO/Al₂O₃ catalysts show two different types of coking, and suffer higher coke deposition during the steam reforming reaction.     

Methanol steam reforming in an Al2O3 supported thin Pd-layer membrane reactor over Cu/ZnO/Al2O3 catalyst

In this experimental study, a membrane reactor housing a composite membrane constituted by a thin Pd-layer supported onto Al₂O₃ is utilized to perform methanol steam reforming reaction to produce high-grade hydrogen for PEM fuel cell applications. The influence of various parameters such as temperature, from 280 to 330 °C, and pressure, from 1.5 to 2.5 bar, is analyzed. A commercial Cu/Zn-based catalyst is packed in the annulus of the membrane reactor and the experimental tests are performed at space velocity equal to 18,500 h⁻¹ and H₂O:CH₃OH feed molar ratio equal to 2.5:1. Results in terms of methanol conversion, hydrogen recovery, hydrogen yield and products selectivities are given. As a best result of this work, 85% of methanol conversion and a highly pure hydrogen stream permeated through the membrane with a CO content lower than 10 ppm were reached at 330 °C and 2.5 bar. Furthermore, a comparison between the experimental results obtained in this work and literature data is proposed and discussed.     

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.     

Effect of Al2O3 content on the adsorptive properties of Cu/ZnO/Al2O3 for removal of odorant sulfur compounds

Cu/ZnO/Al₂O₃ adsorbents for removal of odorant sulfur compounds were prepared with various Al/Cu molar ratios by co-precipitation method. The sulfur removing ability as a function of Al/Cu molar ratio of the adsorbents for t-butyl mercaptan (TBM), tetrahydro thiophene (THT), dimethyl disulfide (DMS) and H₂S were investigated at 250 °C and 6000 h⁻¹ space velocity. Based on the results of adsorption capacity and characterization by various techniques, the optimum Al/Cu ratio for maximum sulfur removal capacity is found to be at Al/Cu molar ratio of 0.15 which possesses the well-dispersed Cu species with high reducibility. The adsorption capacity is highest for H₂S followed by TBM, DMS and THT. The main role of Al₂O₃ component is to provide the dispersion of CuO species homogeneously with small particle formation and high reducibility.     

Selective CO oxidation on Cu promoted Pt/Al2O3 and Pt/Nb2O5 catalysts

Pt–Cu catalysts supported on Al₂O₃ and Nb₂O₅ were studied for use in selective CO oxidation. The addition of copper enhanced the activity and selectivity of Pt–Cu/Nb₂O₅ at lower temperatures when compared to Pt/Nb₂O₅. On the other hand, copper addition was not beneficial in the case of Al₂O₃ supported catalysts.     

Preparation of nickel catalysts supported on CaO.2Al2O3 for methane reforming with carbon dioxide

Nanocrystalline calcium aluminate (CaO.2Al₂O₃) was prepared by a simple co-precipitation method using Poly (ethylene glycol)-block-poly(propylene glycol)-block poly(ethylene glycol) (PEG-PPG-PEG, MW:5800) as surfactant and employed as catalyst support for nickel catalysts in methane reforming with carbon dioxide. The prepared samples were characterized by X-ray diffraction (XRD), N₂ adsorption (BET), Temperature programmed reduction and oxidation (TPR-TPO) and Scanning electron microscopy (SEM) techniques. The results showed that the prepared support has a high potential as support for nickel catalysts in methane reforming with carbon dioxide. The results showed high catalytic activity and stability for the prepared catalysts. Among the prepared catalysts 15% Ni/CaO.2Al₂O₃ was the most active catalyst and showed the highest affinity for carbon formation. In addition, 7% Ni/CaO.2Al₂O₃ possessed high catalytic stability during 50 h time on stream. The TPO analysis revealed that increasing in nickel content increased the amount of deposited carbon over the spent catalysts. SEM results detected only whisker type of carbon for all spent catalysts.     

Nickel catalysts supported on ZrO2, Y2O3-stabilized ZrO2 and CaO-stabilized ZrO2 for the steam reforming of ethanol: Effect of the support and nickel load

Catalysts with various nickel loads were prepared on supports of ZrO₂, ZrO₂–Y₂O₃ and ZrO₂–CaO, characterized by XRD and TPR and tested for activity in ethanol steam reforming. XRD of the supports identified the monoclinic crystalline phase in the ZrO₂ and cubic phases in the ZrO₂–Y₂O₃ and ZrO₂–CaO supports. In the catalysts, the nickel impregnated on the supports was identified as the NiO phase. In the TPR analysis, peaks were observed showing the NiO phase having different interactions with the supports. In the catalytic tests, practically all the catalysts achieved 100% ethanol conversion, H₂ yield was near 70% and the gaseous concentrations of the other co-products varied in accordance with the equilibrium among them, affected principally by the supports. It was observed that when the ZrO₂ was modified with Y₂O₃ and CaO, there were big changes in the CO and CO₂ concentrations, which were attributed to the rise in the number of oxygen vacancies, permitting high-oxygen mobility and affecting the gaseous equilibrium. The liquid products analysis showed a low selectivity to liquid co-products during the reforming reactions.     

Effects of modifying Ni/Al2O3 catalyst with cobalt on the reforming of CH4 with CO2 and cracking of CH4 reactions

Alumina supported nickel (Ni/Al₂O₃), nickel–cobalt (Ni–Co/Al₂O₃) and cobalt (Co/Al₂O₃) catalysts containing 15% metal were synthesized, characterized and tested for the reforming of CH₄ with CO₂ and CH₄ cracking reactions. In the Ni–Co/Al₂O₃ catalysts Ni–Co alloys were detected and the surface metal sites decreased with decrease in Ni:Co ratio. Turnover frequencies of CH₄ were determined for both reactions. The initial turnover frequencies of reforming (TOF_DRM) for Ni–Co/Al₂O₃ were greater than that for Ni/Al₂O₃, which suggested a higher activity of alloy sites. The initial turnover frequencies for cracking (TOF_CRK) did not follow this trend. The highest average TOF_DRM, H₂:CO ratio and TOF_CRK were observed for a catalyst containing a Ni:Co ratio of 3:1. This catalyst also had the maximum carbon deposited during reforming and produced the maximum reactive carbon during cracking. It appeared that carbon was an intermediate product of reforming and the best catalyst was able to most effectively crack CH₄ and oxidize carbon to CO by CO₂.     

Fabrications and evaluations of hydrogen permeation on Al2O3/CeO2/graphene (ACG) composites membrane by Hot Press Sintering (HPS)

Ceramic membrane has high permeation rate of hydrogen and chemical stability. Al₂O₃ indicates stable at high temperature and a relatively large surface area. In addition, Al₂O₃ of porous is used as hydrogen separation membranes support, because of the high hydrogen permeability based on Knudsen diffusion mechanism.     

Influence of surfactants on Fe2O3 nanostructure photoanode

Fe₂O₃ nanostructures photoanodes were prepared via sol–gel spin-coating method onto fluorine-doped tin oxide glass substrates using six different surfactants: polyethylene glycol (PEG-300), Triton X-100, pluronic F127, cetyltrimethylammonium bromide (CTAB), octadecyltrimethylammonium bromide (OTAB) and tetradecyltrimethylammonium bromide (TTAB). The resulting films have thickness from 520 ± 10 to 980 ± 10 nm after calcinations at 450 °C in the air. A comparative study of photocatalytic activity of thin films was performed. The photo-generated samples were determined by measuring the currents and voltages under illumination of UV–vis light. The highest photocurrent density of 1.77 mA/cm² at 1 V/SCE, under illumination intensity of 100 mW cm⁻² from a solar simulator with a global AM 1.5 filter, were produced by TTAB treated sample. The optical properties, morphology, surface roughness and structure of the films were also characterized by UV–visible spectroscopy, SEM, AFM and XRD. The results are consistent with photocatalytic performance: TTAB treated sample has the highest grain size and optical absorption. The improved performance of this sample can be attributed to the crystallinity process of TTAB, which leads to the larger grain size and highest photocatalytic activity. The study demonstrates that photoelectrochemical performance of metal oxide can be improved by simply changing surfactant. The results highlighted the superior performance of cationic surfactants over non-ionic surfactants in preparing Fe₂O₃ photoanodes by sol–gel method. Moreover, the study showed that decreasing hydrocarbon tail of cationic surfactants can increase the crystallite size and improve photocatalytic activity.     

Optical properties of sol–gel deposited Al2O3 films

Highly transparent, uniform and corrosion resistant Al₂O₃ films were prepared on stainless-steel and quartz substrates by the sol–gel process from stable coating solutions using aluminum-sec-butoxide, Al(OBu^s)₃ as precursor, acetylacetone, AcAcH as chelating agent and nitric acid, HNO₃, as catalyzer. Films up to 1000 nm thick were prepared by multiple spin coating deposition, and were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), optical spectroscopy and micro Vickers hardness test. XRD of the film heat treated at 400°C showed that they had an amorphous structure. XPS confirmed that they were stoichiometric Al₂O₃. The refractive index (n) and extinction coefficient (k) were found to be n=1.56±0.01 and k=0.003±0.0002 at 600 nm, respectively. The surface microhardness and corrosion resistance investigations showed that Al₂O₃ films improved the surface properties of stainless-steel substrates.     

Reaction pathway for ethanol steam reforming on a Ni/SiO2 catalyst including coke formation

The effect operating conditions (temperature, space time, steam/ethanol molar ratio, ethanol partial pressure and time on stream) have on the activity and stability of a Ni/SiO₂ catalyst for H₂ production by ethanol steam reforming has been studied in a fluidized bed reactor. This catalyst allows obtaining total conversion above 500 °C, with a steam/ethanol molar ratio of 6 and a space time of 0.138 g_catalysth/g_ethanol. Catalyst deactivation in the 300–500 °C range is due to coke deposition, whose nature (determined by TPH and TPO analysis) mainly depends on reaction temperature. The coke deposited at 300 °C is amorphous and blocks metallic sites, whereas at higher temperatures the coke is mainly filamentous and, although its content increases as reaction temperature is raised to 500 °C, it has a low effect on catalyst deactivation because it does not block metal sites. Above 600 °C the decrease in coke content due to gasification is noticeable, although at this temperature an incipient Ni sintering is observed, which is significant at 700 °C.     

Nickel on a macro-mesoporous Al2O3@ZrO2 core/shell nanocomposite as a novel catalyst for CO methanation

A novel nickel catalyst supported on Al₂O₃@ZrO₂ core/shell nanocomposites was prepared by the impregnation method. The core/shell nanocomposites were synthesized by depositing zirconium species on boehmite nanofibres. This contribution aims to study the effects of the pore structure of supports and the zirconia dispersed on the surface of the alumina nanofibres on the CO methanation. The catalysts and supports were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), H₂ temperature-programmed reduction (H₂-TPR), nitrogen adsorption–desorption, and thermogravimetry and differential thermal analysis (TG-DTA). The catalytic performance of the catalysts for CO methanation was investigated at a temperature range from 300 °C to 500 °C. The results of the characterization indicate that the metastable tetragonal zirconia could be stably and evenly dispersed on the surface of alumina nanofibres. The interlaced nanorods of the Al₂O₃@ZrO₂ core/shell nanocomposites resulted in a macropore structure and the spaces between the zirconia nanoparticles dispersed on the alumina nanofibres formed most of the mesopores. Zirconia on the surface of the support promoted the dispersion and influenced the reduction states of the nickel species on the support, so it prevented the nickel species from sintering as well as from forming a spinel phase with alumina at high temperatures, and thus reduced the carbon deposition during the reaction. With the increase of the zirconia content in the catalyst, the catalytic performance for the CO methanation was enhanced. The Ni/Al₂O₃@ZrO₂-15 exhibited the highest CO conversion and methane selectivity at 400 °C, but they decreased dramatically above or below 400 °C due to the temperature sensitivity of the catalyst. Ni/Al₂O₃@ZrO₂-30 exhibited a high and constant rate of methane formation between 350 °C and 450 °C. The excellent catalytic performance of this catalyst is attributed to its reasonable pore structure and good dispersion of zirconia on the support. This catalyst has great potential to be further studied for the future industrial use.