Hydrogen production from catalytic steam reforming of bio-oil aqueous fraction over Ni/CeO2–ZrO2 catalysts

A series of composite catalysts Ni/CeO₂–ZrO₂ were prepared via impregnation method with Ni as the active metal. A laboratory-scale fixed-bed reactor was employed to investigate the catalyst performance during hydrogen production by steam reforming bio-oil aqueous fraction. Effects of water-to-bio-oil ratio (W/B), reaction temperature, and the loaded weight of Ni and Ce on the hydrogen production performance of Ni/CeO₂–ZrO₂ catalysts were examined. The obtained results were compared with commercial nickel-based catalysts (Z417). The best performance of Ni/CeO₂–ZrO₂ catalyst was observed when the Ni and Ce loaded weight were 12% and 7.5% respectively. At W/B = 4.9, T = 800 °C, H₂ yield reaches the highest of 69.7% and H₂ content of 61.8% were obtained. Under the same condition, H₂ yield and H₂ content were higher than commercial nickel-based catalysts (Z417).     

Oxidative steam reforming of ethanol over Ni/Al2O3 catalysts promoted by CeO2, ZrO2 and CeO2–ZrO2

Oxidative steam reforming of ethanol at low oxygen to ethanol ratios was investigated over nickel catalysts on Al₂O₃ supports that were either unpromoted or promoted with CeO₂, ZrO₂ and CeO₂–ZrO₂. The promoted catalysts showed greater activity and a higher hydrogen yield than the unpromoted catalyst. The characterization of the Ni-based catalysts promoted with CeO₂ and/or ZrO₂ showed that the variations induced in the Al₂O₃ by the addition of CeO₂ and/or ZrO₂ alter the catalyst's properties by enhancing Ni dispersion and reducing Ni particle size. The promoters, especially CeO₂–ZrO₂, improved catalytic activity by increasing the H₂ yield and the CO₂/CO and the H₂/CO values while decreasing coke formation. This results from the addition of ZrO₂ into CeO₂. This promoter highlights the advantages of oxygen storage capacity and of mobile oxygen vacancies that increase the number of surface oxygen species. The addition of oxygen facilitates the reaction by regenerating the surface oxygenation of the promoters and by oxidizing surface carbon species and carbon-containing products.     

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.     

Chemical-looping steam methane reforming over macroporous CeO2–ZrO2 solid solution: Effect of calcination temperature

Chemical-looping steam methane reforming (CL-SMR) is a novel process for the co-production of pure hydrogen and syngas without purification processes. A series of CeO₂–ZrO₂ mixed oxides were prepared by colloidal crystal templating method with calcination temperature increasing from 450 to 850 °C. The structural characteristic and reducibility of CeO₂–ZrO₂ oxygen carriers were investigated by SEM, XRD and TPR techniques and correlated to their reactivity for CL-SMR. The CeO₂–ZrO₂ mixed oxides calcined at low temperatures (e.g., 450 °C) exhibit a better uniform and three-dimensionally ordered macroporous structure, which enhance the mobility of oxygen species, improving the reducibility of CeO₂–ZrO₂ oxygen carriers. The ordered macroporous structure can lead to a high reactivity for CL-SMR, especially for the hydrogen production in water splitting reaction. It was found that the Ce–Zr-450 sample showed the best performance for H₂ production. After ten redox CL-SMR cycles at 800 °C, the Ce–Zr-450 sample still maintained relatively high hydrogen yield and the three-dimensionally ordered macroporous structure remained in good condition, indicating high reactivity and structural stability.     

Reaction characteristics of two-step methane reforming over a Cu-ferrite/Ce–ZrO2 medium

Methane was reformed over a Cu-ferrite/ZrO₂ medium in a two-step process, consisting of a syn-gas production step and a water-splitting step. In the syn-gas production step, increase in Cu content in the Cu_xFe_3−xO₄/ZrO₂ medium suppressed carbon deposition and enhanced the reaction rate for stoichiometries of x ≤ 0.7. In the water-splitting step, the addition of Cu promoted the gasification of the deposited carbon. Furthermore, the addition of Ce as a binder in the Cu_0.7Fe_2.3O₄/Ce–ZrO₂ medium also improved reactivity in the syn-gas production step and yielded the highest reactivity when the molar ratio of Ce/Zr was 3/1. As a result of the co-addition of Cu and Ce, the Cu_0.7Fe_2.3O₄/Ce–ZrO₂ medium showed high durability, with a constant evolution of the synthesis gas and hydrogen in ten repeated cycles. It is thus expected that the Cu-ferrite/Ce–ZrO₂ medium is favourable for two-step methane reforming.     

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.     

Production of hydrogen via steam reforming of biofuels on Ni/CeO2–Al2O3 catalysts promoted by noble metals

The catalytic activity of Ni/CeO₂–Al₂O₃ catalysts modified with noble metals (Pt, Ir, Pd and Ru) was investigated for the steam reform of ethanol and glycerol. The catalysts were characterized by the following techniques: Energy-dispersive X-ray, BET, X-ray diffraction, temperature-programmed reduction, UV–vis diffuse reflectance spectroscopy and X-ray absorption near edge structure (XANES). The results showed that the formation of inactive nickel aluminate was prevented by the presence of CeO₂ dispersed on alumina. The promoting effect of noble metals included a decrease in the reduction temperatures of NiO species interacting with the support, due to the hydrogen spillover effect. It was seen that the addition of noble metal stabilized the Ni sites in the reduced state along the reforming reaction, increasing the ethanol and glycerol conversions and decreasing the coke formation. The higher catalytic performance for the ethanol steam reforming at 600 °C and glycerol steam reforming was obtained for the NiPd and NiPt catalysts, respectively, which presented an effluent gaseous mixture with the highest H₂ yield with reasonably low amounts of CO.     

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.     

Carbon deposition on Ni/ZrO2–CeO2 catalyst during steam reforming of acetic acid

Steam reforming of acetic acid, one model compounds of bio-oil, was studied on the Ni/ZrO₂–CeO₂ catalysts which were prepared by the impregnation method. The results showed that high acetic acid conversion and hydrogen yield were obtained in the temperature range of 650–750 °C when H₂O/HAC ratio was 3. Nevertheless, the catalyst deactivation was caused by carbon deposition eventually with time-on-stream. In order to discuss the behavior of the carbon deposition on the Ni/ZrO₂–CeO₂ catalyst during steam reforming of bio-oil, the structure and morphology of carbon deposition were investigated by BET, XRD, TG/DTA, TPR, SEM and EDX techniques. All the experimental results showed acetone and CO were the important carbon precursors of acetic acid reforming and the graphitic-like carbon was the main type of carbon deposition on the surface of the deactivated 12%Ni/CeO₂–ZrO₂ catalyst.     

Effect of CaO–ZrO2 addition to Ni supported on γ-Al2O3 by sequential impregnation in steam methane reforming

Ni catalysts supported on (CaO–ZrO₂)-modified γ-Al₂O₃ were prepared by sequential impregnation. The effects of varied CaO to ZrO₂ mole ratios at 0, 0.20, 0.35, 0.45, and 0.55 on the activity and stability of the modified Ni catalysts were studied. As a result of using CaO–ZrO₂ as a promoter, each catalyst contained CaO–ZrO₂ at only 5%. γ-Al₂O₃ used as support was modified by CaO–ZrO₂ before the deposition of nickel oxide. The addition of CaO–ZrO₂ at an optimum ratio was expected to improve the stability of Ni catalysts due to the decrease of carbon formation resulting from carbon gasification. All the fresh catalysts were characterized by ICP, XRD, BET surface area, TGA in H₂, and TPR before catalytic testing in steam methane reforming at 600 °C. The spent catalysts were examined by TEM and TGA to observe the catalysts deactivation. The identification of CaO–ZrO₂ phases indicated that CaO and ZrO₂ reacted with each other to be monoclinic solid solution ZrO₂, CaZr₄O₉, CaZrO₃, and CaO corresponding to the phase diagram of CaO–ZrO₂. The existence of CaZrO₃ for 0.55 mol ratio of CaO/ZrO₂ enhanced activity in steam methane reforming because oxygen vacancies in CaZrO₃ greatly preferred the water adsorption creating the favorable conditions for carbon gasification and, then, water gas shift. The prominence and continued existence of these two reactions on the Ni catalysts leads to the particular increase of H₂ yield. Moreover, the increasing amount of CaZrO₃ in the Ni catalysts significantly improved carbon gasification. However, the Ni catalysts with CaZrO₃ showed whisker carbon after catalytic testing; this carbon specie has not been tolerated in steam methane reforming. Therefore, these results significantly differed from the hypothesis.     

Syngas production by CO2 reforming of coke oven gas over Ni/La2O3–ZrO2 catalysts

Syngas production by CO₂ reforming of coke oven gas (COG) was studied in a fixed-bed reactor over Ni/La₂O₃–ZrO₂ catalysts. The catalysts were prepared by sol–gel technique and tested by XRF, BET, XRD, H₂-TPR, TEM and TG–DSC. The influence of nickel loadings and calcination temperature of the catalysts on reforming reaction was measured. The characterization results revealed that all of the catalysts present excellent resistance to coking. The catalyst with appropriate nickel content and calcination temperature has better dispersion of active metal and higher conversion. It is found that the Ni/La₂O₃–ZrO₂ catalyst with 10 wt% nickel loading provides the best catalytic activity with the conversions of CH₄ and CO₂ both more than 95% at 800 °C under the atmospheric pressure. The Ni/La₂O₃–ZrO₂ catalysts show excellent catalytic performance and anti-carbon property, which will be of great prospects for catalytic CO₂ reforming of COG in the future.     

Ni–SiO2 and Ni–Fe–SiO2 catalysts for methane decomposition to prepare hydrogen and carbon filaments

Active and stable Ni–Fe–SiO₂ catalysts prepared by sol–gel method were employed for direct decomposition of undiluted methane to produce hydrogen and carbon filaments at 823 K and 923 K. The results indicated that the lifetime of Ni–Fe–SiO₂ catalysts was much longer than Ni–SiO₂ catalyst at a higher reaction temperature such as 923 K, however, a reverse trend was shown when methane decomposition took place at a lower reaction temperature such as 823 K. XRD studies suggested that iron atoms had entered into the Ni lattice and Ni–Fe alloy was formed in Ni–Fe–SiO₂ catalysts. The structure of the carbon filaments generated over Ni–SiO₂ and Ni–Fe–SiO₂ was quite different. TEM studies showed that “multi-walled” carbon filaments were formed over 75%Ni–25%SiO₂ catalyst, while “bamboo-shaped” carbon filaments generated over 35%Ni–40%Fe–25%SiO₂ catalysts at 923 K. Raman spectra of the generated carbons demonstrated that the graphitic order of the “multi-walled” carbon filaments was lower than that of the “bamboo-shaped” carbon filaments.     

Synthesis gas production from CO2 reforming of methane over Ni–Ce/SiO2 catalyst: The effect of calcination ambience

Ni–Ce/SiO₂ catalysts were prepared by calcination under Ar, CO₂, O₂ and H₂ ambience, and applied in CO₂ reforming of methane for synthesis gas production. BET, XRD, XPS, TPR, SEM, TEM and TPH techniques were employed to characterize the fresh and used catalysts. Highly dispersed nickel oxides bearing stronger interaction with SiO₂ prevented the metal sintering. The formation of reactive carbon species on Ni–Ce/SiO₂ catalyst calcined under Ar ambience effectively promoted the carbon elimination and kept the catalyst more stable. Nevertheless, the oxygen storage capacity of CeO₂ might partly lose on Ni–Ce/SiO₂ calcined under H₂ ambience. As a result, the inhibition of carbon elimination and the deposition of inert carbon were responsible for its partial deactivation.     

Hydrogen production by steam reforming of ethanol over P123-assisted mesoporous Ni–Al2O3–ZrO2 xerogel catalysts

A series of mesoporous Ni–Al₂O₃–ZrO₂ xerogel (denoted as X-NAZ) catalysts were prepared by a P123-assisted epoxide-driven sol–gel method under different P123 concentration (X, mM), and they were applied to the hydrogen production by steam reforming of ethanol. The effect of P123 concentration on the physicochemical properties and catalytic activities of X-NAZ catalysts was investigated. All the catalysts retained a mesoporous structure. Pore volume of the catalysts increased with increasing P123 concentration. Ni surface area and ethanol adsorption capacity of X-NAZ catalysts exhibited volcano-shaped trends with respect to P123 concentration. The trend of hydrogen yield was well matched with the trend of Ni surface area and ethanol adsorption capacity. Thus, Ni surface area and ethanol adsorption capacity of the catalysts served as important factors determining the catalytic performance. Among the catalysts tested, 12-NAZ catalyst with the highest Ni surface area and the largest ethanol adsorption capacity showed the best catalytic performance in the steam reforming of ethanol. In conclusion, an optimal P123 concentration was required for maximum production of hydrogen in the steam reforming of ethanol over X-NAZ catalysts.     

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.     

Iron incorporated Ni–ZrO2 catalysts for electric power generation from methane

On the purpose to perform as functional layer of SOFCs operating on methane fuel, NiFe–ZrO₂ alloy catalysts have been synthesized and investigated for methane partial oxidation reactions. Ni₄Fe₁–ZrO₂ shows catalytic activity comparable to that of Ni–ZrO₂ and superior to other Fe-containing catalysts. In addition, O₂-TPO analysis indicates iron is also prone to coke formation; as a result, most of NiFe–ZrO₂ catalysts do not show improved coking resistance than Ni–ZrO₂. Anyway, Ni₄Fe₁–ZrO₂ (Ni:Fe = 4:1 by weight) prepared by glycine-nitrate process shows somewhat less carbon deposition than the others. However, Raman spectroscopy demonstrates that the addition of Fe does reduce the graphitization degree of the deposited carbon, suggesting the easier elimination of carbon once it is deposited over the catalyst. Ni₄Fe₁–ZrO₂ has an excellent long-term stability for partial oxidation of methane reaction at 850 °C. A solid oxide fuel cell with conventional nickel cermet anode and Ni₄Fe₁–ZrO₂ functional layer is operated on CH₄–O₂ gas mixture to yield a peak power density of 1038 mW cm⁻² at 850 °C, which is comparable to that of hydrogen fuel. In summary, the Ni₄Fe₁–ZrO₂ catalyst is potential catalyst as functional layer for solid-oxide fuel cells operating on methane fuel.     

Hydrogen production by steam reforming of ethanol over mesoporous Ni–Al2O3–ZrO2 xerogel catalysts: Effect of nickel content

Hydrogen production by steam reforming of ethanol over mesoporous Ni–Al₂O₃–ZrO₂ xerogel catalysts (denoted as XNiAZ) with different nickel content (X, wt%) was studied. A single-step epoxide-driven sol–gel method was employed for the preparation of the catalysts. The effect of nickel content of XNiAZ catalysts on their physicochemical properties and catalytic activities was investigated. All the XNiAZ catalysts exhibited a well-developed mesoporous structure and they dominantly showed an amorphous NiO–Al₂O₃–ZrO₂ composite phase, leading to high dispersion of NiO. Nickel surface area and reducibility of XNiAZ catalysts showed volcano-shaped trends with respect to nickel content. Nickel surface area of XNiAZ catalysts played a key role in determining the catalytic performance in the steam reforming of ethanol; an optimal nickel content was required for maximum production of hydrogen. Among the catalysts tested, 15NiAZ catalyst with the highest nickel surface area exhibited the best catalytic performance in the steam reforming of ethanol. In addition, 15NiAZ catalyst showed high and stable hydrogen yields under different total feed rate, demonstrating its potential applicability in large-scale hydrogen production.     

Rh promoted and ZrO2/Al2O3 supported Ni/Co based catalysts: High activity for CO2 reforming,steam–CO2 reforming and oxy–CO2 reforming of CH4

Ni (2.5 wt%) and Co (2.5 wt%) supported over ZrO₂/Al₂O₃ were prepared by following a hydrolytic co-precipitation method. The synthesized catalysts were further promoted by Rh incorporation (0.01–1.00 wt%) and tested for their catalytic performance for dry CO₂ reforming, combined steam–CO₂ reforming and oxy–CO₂ reforming of methane for production of syngas. The catalysts were characterized by using N₂ physical adsorption, XRD, H₂–TPR, SEM, CO₂–TPD, NH₃–TPD, TEM and TGA. The results revealed that ZrO₂ phase was in crystalline form in the catalysts along with amorphous Al oxides. Ni and Co were confirmed to be in their respective spinel phases that were reducible to metallic form at 800 °C under H₂. Ni and Co were well dispersed with their nano-crystalline nature. The catalyst with 0.2% loading of Rh showed superior performance in the studied reactions for reforming of methane. This catalyst also showed good coke resistance ability for dry CO₂ reforming reaction with 3.8 wt% of carbon formation during the reaction as compared to 11.6 wt% carbon formation over the catalyst without Rh. The catalyst performance was stable throughout the reaction time for CH₄ conversions, irrespective of carbon formation with slight decline (～1%) in CO₂ conversion. For dry CO₂ reforming reaction, this catalyst showed good conversion for both CH₄ and CO₂ (67.6% and 71.8% respectively) with a H₂/CO ratio of 0.84, while for the Oxy-CO₂ reforming reaction, the activity was superior with CH₄ and CO₂ conversions (73.7% and 83.8% respectively) and H₂/CO ratio of 1.05.     

Low-temperature partial oxidation of methane over Pd–Ni bimetallic catalysts supported on CeO2

Monometallic Pd and Ni and bimetallic Pd–Ni catalysts supported on CeO₂ are prepared via mechanochemical and conventional incipient wetness impregnation methods and tested for the production of syngas by the partial oxidation of methane. Compared with monometallic Ni/CeO₂ and Pd/CeO₂, bimetallic Pd–Ni/CeO₂ catalysts show considerable higher methane conversion and syngas yield. Additionally, the bimetallic catalysts prepared by ball milling produce syngas at lower temperature. Different preparation parameters, such as metal loading, Pd/Ni ratio, milling energy, milling time and order of incorporation of the metals are examined. The best performance is obtained with a bimetallic catalyst prepared at 50 Hz for 20 min with only 0.12 wt% Pd and 1.38 wt% Ni. Stability tests demonstrate superior stability for bimetallic Pd–Ni/CeO₂ catalysts prepared by a mechanochemical approach. From the characterization results, this is explained in terms of an impressive dispersion of metal species with a strong interaction with the surface of CeO₂.