Syngas production via combined oxy-CO2 reforming of methane over Gd2O3-modified Ni/SiO2 catalysts in a fluidized-bed reactor

In this research, Ni/SiO₂ catalyst was modified with different amount of Gd₂O₃ and characterized with temperature-programmed desorption of CO₂ (CO₂-TPD) and NH₃ (NH₃-TPD), temperature-programmed reduction with H₂ (H₂-TPR) and X-ray diffraction (XRD). It was found that Gd₂O₃-modified Ni/SiO₂ catalysts possessed higher CO₂ adsorption and activation ability due to the formation of surface carbonate species. H₂-TPR and XRD characterizations found that the strong interaction among nickel, Gd₂O₃ and SiO₂ took place, which improved the dispersion of Ni. Gd₂O₃-modified Ni/SiO₂ catalysts exhibited higher activity and stability for the combined oxy-CO₂ reforming of methane in fluidized-bed reactor. The H₂/CO ratio in produced syngas could be controlled via controlling reaction temperature and CO₂/O₂ ratio in feed.     

Enhancement of pyrolysis gasoline hydrogenation over Pd-promoted Ni/SiO2-Al2O3 catalysts

Pyrolysis gasoline upgrading by hydrogenation and ring opening was investigated over highly loaded Ni catalysts supported on amorphous silica-alumina and incorporating promoters as Pd, seeking a higher aromatic reduction of this feedstock in order to meet stringent fuel regulations. The effect of Ni loading and Pd component on the activity of those systems was evaluated in a fixed bed reactor under the following operating conditions: T = 573 and 673 K, H₂:PyGas molar ratio = 10, P = 5.0 MPa, WHSV = 4 h⁻¹. The catalyst properties, measured by several characterization techniques (ICP-AES, XRD, N₂ adsorption-desorption isotherms, TPR, H₂-TPD, CO chemisorption, XPS, FTIR spectroscopy of adsorbed pyridine and NH₃-TPD), were related to their catalytic activity and selectivity. Interestingly, the increase in Ni loading from 24.4 to 33.2 Ni wt.% has a negative effect on both hydrogenation and ring opening activities, as it causes a drop in the BET surface area and a decrease in metal-support interaction, with a negative bearing on catalyst stability. On the other hand, the addition of Pd has a positive effect for hydrogenation, linked with the higher electronegativity of Pd⁰ species compared to those of Ni⁰, as well as with a greater stability of Pd-promoted catalysts during on-stream conditions. A linear correlation has been found between the total amount of desorbed H₂, as determined from H₂-TPD experiments on freshly reduced catalysts, and the initial turnover frequency.     

Catalytic conversion of methane and CO2 to synthesis gas over a La2O3-modified SiO2 supported Ni catalyst in fluidized-bed reactor

In this contribution, a commercial spherical SiO₂ was modified with different amounts of La₂O₃, and used as the support of Ni catalysts for autothermal reforming of methane in a fluidized-bed reactor. Nitrogen adsorption, XRD and H₂-TPR analysis indicated that La₂O₃-modified SiO₂ had higher surface area, strengthened interaction between Ni and support, and improved dispersion of Ni. CO₂-TPD found that La₂O₃ increased the alkalescence of SiO₂ and improved the activation of CO₂. Coking reaction (via both temperature-programmed surface reaction of CH₄ (CH₄-TPSR) and pulse decomposition of CH₄) disclosed that La₂O₃ reduced the dehydrogenation ability of Ni. CO₂-TPO, O₂-TPO (followed after CH₄-TPSR) confirmed that only part amount of carbon species derived from methane decomposition could be removed by CO₂, and O₂ in feed played a crucial role for the gasification of the inactive surface carbons. Ni/xLa₂O₃-SiO₂ (x = 10, 15, 30) possessed high activity and excellent stability for autothermal reforming of methane in a fluidized-bed reactor.     

The role of carbon surface chemistry in N2O conversion to N2 over Ni catalyst supported on activated carbon

The effect of acidic treatments on N₂O reduction over Ni catalysts supported on activated carbon was systematically studied. The catalysts were characterized by N₂ adsorption, mass titration, temperature-programmed desorption (TPD), and X-ray photoelectron spectrometry (XPS). It is found that surface chemistry plays an important role in N₂O-carbon reaction catalyzed by Ni catalyst. HNO₃ treatment produces more active acidic surface groups such as carboxyl and lactone, resulting in a more uniform catalyst dispersion and higher catalytic activity. However, HCl treatment decreases active acidic groups and increases the inactive groups, playing an opposite role in the catalyst dispersion and catalytic activity. A thorough discussion of the mechanism of the N₂O catalytic reduction is made based upon results from isothermal reactions, temperature-programmed reactions (TPR) and characterization of catalysts. The effect of acidic treatment on pore structure is also discussed.     

The hydrocracking of n-decane over bifunctional Ni-H3PW12O40/SiO2 catalysts

A series of bifunctional Ni-H₃PW₁₂O₄₀/SiO₂ catalysts for the hydrocracking of n-decane were designed and prepared. The evaluation results of the catalysts show that Ni-H₃PW₁₂O₄₀/SiO₂ catalysts possess a high activity for hydrocracking of n-decane and an excellent tolerance to the sulfur and nitrogen compounds in the feedstock. Under the reaction conditions: reaction temperature 300 °C; H₂/n-decane volume ratio of 1500; total pressure of 2 Mpa and the LHSV 2 h⁻¹, the conversion of n-decane over reduced 5%Ni-50%H₃PW₁₂O₄₀/SiO₂ catalysts is as high as 90%, the C₅⁺ selectivity equal to 70%. In order to reveal the structure and nature of the catalysts, a number of characterizations including XRD, Raman, H₂-TPD, NH₃-TPD, XPS and FT-IR of pyridine adsorption were carried out. The characteristic results show that the high activity of the catalysts and high C₅⁺ selectivity can be related to the unique structure of the H₃PW₁₂O₄₀ and its suitable acidity.     

Reforming of methane and coalbed methane over nanocomposite Ni/ZrO2 catalyst

Nanocomposite Ni/ZrO₂-AN catalyst consisting of comparably sized Ni metal and ZrO₂ nanoparticles is studied in comparison with zirconia- and alumina-supported Ni catalysts (Ni/ZrO₂-CP and commercial Ni/Al₂O₃-C) for steam reforming of methane (SRM) and for combined steam and CO₂ reforming of methane (CSCRM). The reactions are performed under atmospheric pressure with stoichiometric amounts of H₂O and CH₄ or (H₂O + CO₂) and CH₄ at 1073 K. Under a wide range of methane space velocity (gas hourly space velocity of methane GHSV_CH4 = 12,000–96,000 ml/(h g_cat.), the nanocomposite Ni/ZrO₂-AN catalyst always shows higher activity and stability for both SRM and CSCRM reactions. The two supported Ni catalysts (Ni/ZrO₂-CP and Ni/Al₂O₃-C) exhibit fairly stable catalysis under low GHSV_CH4 but they are easily deactivated under high GHSV_CH4 and become completely inactive when they are reacted for ca.100 h at GHSV_CH4 = 48,000 ml/(h g_cat.). The CSCRM reaction is carried out with different H₂O/CO₂ ratios in the reaction feed while keeping the molar ratio (H₂O + CO₂)/CH₄ = 1.0, the results prove that the nanocomposite Ni/ZrO₂-AN catalyst can be highly promising in enabling a catalytic technology for the production of syngas with flexible H₂/CO ratios (ca. H₂/CO = 1.0–3.0) to meet the requirements of various downstream chemical syntheses.     

Catalytic reforming of model tar compounds from hot coke oven gas with low steam/carbon ratio over Ni/MgO-Al2O3 catalysts

The catalytic reforming of toluene and naphthalene was performed to investigate the possibility for directly converting tar components from hot coke oven gas (COG) with lower steam/carbon (S/C) molar ratios to light fuel gases. The NiO/MgO-Al₂O₃ catalysts reduced exhibited excellent catalytic activity, stability and sulphur tolerance. The effects of various reaction conditions and S/C ratios on the catalytic performance were investigated in detail. Toluene and naphthalene were completely converted into small gas molecules at 700-800 °C and S/C = 0.28. An appropriate amount of steam benefited the methanation reaction of CO and H₂. The effects of N₂, CH₄ or CO in COG were also discussed. Relative to N₂, CO contributed to the conversion of toluene and the formation of CH₄, but the opposite was true for CH₄. The sulphur tolerance was tested by adding H₂S in the feed gas. The reaction results were explained by a water cycle mechanism.     

Influence of the co-feeding of CO, H2, CO2 or H2O in the partial oxidation of methane over Ni and Rh supported catalysts

The influence of the addition of 5 vol.% of carbon monoxide, hydrogen, carbon dioxide or water to the feed of partial oxidation of methane was investigated over Ni/γ-Al₂O₃ and Rh/γ-Al₂O₃ catalysts. In addition to catalytic tests, thermodynamic calculations were performed to predict the effect of these gas co-feeds. Compared to the thermodynamic trends, differences in the influence of the co-feeding on catalytic performances were observed between both catalysts. Co-feeding of CO, H₂, CO₂ or H₂O can modify the oxidation state and dispersion of the metal component of the catalysts during reaction, and as a consequence, their performances. Changes in catalysts can be due to dynamic processes occurring during reaction. It is suggested to take these processes into account in a more complex kinetic equation for the reactions involved.     

Sequential production of H2 and CO over supported Ni catalysts

Methane decomposition into H₂ and carbon nanofibers at 823 K and subsequent gasification of the carbon nanofibers with CO₂ into CO at 923 K were performed over supported Ni catalysts (Ni/SiO₂, Ni/TiO₂ and Ni/Al₂O₃). Supported Ni catalysts were deactivated for CH₄ decomposition with time on stream due to deposition of a large amount of carbon nanofibers. Subsequent contact of CO₂ with carbon nanofibers on the deactivated catalysts resulted in the formation of CO with a conversion of the carbons higher than 95%. In addition, gasification with CO₂ regenerated the activity of supported Ni catalysts for CH₄ decomposition, indicating that H₂ formation through CH₄ decomposition and CO formation through gasification with CO₂ could be carried out repeatedly. Conversions of carbon nanofibers into CO were kept higher than 95% in the repeated gasification over all the catalysts, while change in the catalytic activity for CH₄ decomposition with the repeated cycles depended on the kind of catalytic supports. Catalytic activity of Ni/SiO₂ for CH₄ decomposition was high at early cycles, however, the activity decreased gradually with the repeated cycles. On the other hand, Ni/TiO₂ and Ni/Al₂O₃ showed high activity for CH₄ decomposition and the activity was kept high during the repeated cycles. These changes of catalytic activities for CH₄ decomposition could be explained by changes in particle sizes of Ni metal, i.e. Ni metal particles in Ni/SiO₂ aggregated into ones larger than 150 nm with the repeated cycles, while the particle sizes of Ni metal in Ni/TiO₂ and Ni/Al₂O₃ remained at an effective range for CH₄ decomposition (60-100 nm).     

Role of CeO2 in Ni/CeO2–Al2O3 catalysts for carbon dioxide reforming of methane

Ni catalysts supported on γ-Al₂O₃, CeO₂ and CeO₂–Al₂O₃ systems were tested for catalytic CO₂ reforming of methane into synthesis gas. Ni/CeO₂–Al₂O₃ catalysts showed much better catalytic performance than either CeO₂- or γ-Al₂O₃-supported Ni catalysts. CeO₂ as a support for Ni catalysts produced a strong metal–support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO₂ had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/γ-Al₂O₃ catalysts for this reaction. A weight loading of 1–5 wt% CeO₂ was found to be the optimum. Ni catalysts with CeO₂ promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO₂-promoted catalysts are attributed to the oxidative properties of CeO₂.     

Combination of CO2 reforming and partial oxidation of methane to produce syngas over Ni/SiO2 prepared with nickel citrate precursor

The reaction of combination of CO₂ reforming and partial oxidation of methane to produce syngas (CRPOM) was tested over Ni/SiO₂ catalysts which were prepared via incipient-wetness impregnation using precursors of nickel citrate and nickel nitrate. The catalysts were characterized by X-ray powder diffraction analysis (XRD) and H₂-temperature-programmed reduction (H₂-TPR) techniques. It was shown that the nickel citrate precursor strengthened interaction between NiO and support to form nickel silicate like species which could be reduced to produce small crystallites of metallic nickel at high temperatures. The Ni/SiO₂ prepared with the nickel citrate precursor exhibited good catalytic performances for its highly dispersed metallic nickel derived from the nickel silicate species.     

Selective hydrogenation of acetylene on Pd/SiO2 catalysts promoted with Ti, Nb and Ce oxides

Transition-metal oxides added to Pd/SiO₂ improve significantly the activity and the ethylene selectivity of the catalyst in acetylene hydrogenation, which is caused by the interaction between the oxides and the Pd surface similar to the case of the oxide-supported catalysts. It has been confirmed through experiments that metal oxides spread on and modify both geometrically and electronically the Pd surface after the catalyst is reduced at 500°C. Such a behavior of metal oxides in the catalyst is correlated well with their promotional effect on the catalyst performance. That is, the oxides on the Pd surface retard the sintering of the dispersed Pd particles, suppress the adsorption of ethylene in the multiply-bound mode, and facilitate the desorption of ethylene produced by acetylene hydrogenation. Among the three metal oxides examined in this study, Ti oxide is found to have the most promotional effect.     

Catalytic and electrocatalytic production of H2 from propane decomposition over Pt and Pd in a proton-conducting membrane-reactor

In the present study, the C₃H₈ decomposition reaction is studied in a double-chamber proton-conducting cell-reactor. The proton conductor was a strontia–ceria–ytterbia perovskite disk of the form: SrCe_0.95Yb_0.05O₃₋. The working electrode-catalyst was either Pt or Pd, in the form of a polycrystalline film. The reaction temperature varied between 923 and 1023 K and the partial pressure of propane in the feed gas was 2 kPa. The hydrogen produced by the reaction was electrochemically transported through the walls of the proton-conducting disk, to the outer reactor-chamber. The two electrodes (Pt and Pd) are compared in terms of their catalytic and electrocatalytic activities.     

Modelling of citral hydrogenation kinetics on an Ni/Al2O3 catalyst

The kinetics of citral hydrogenation in ethanol over an Ni/Al₂O₃ catalyst was studied in a slurry reactor operating at atmospheric pressure and at a temperature range of 60–77°C. Citronellal was the primary reaction product, whereas the amounts of unsaturated alcohols were very minor. Citronellol was the dominating product, generated mainly through the hydrogenation of the carbonyl group of citronellal. Based on the experimental data, a kinetic model was developed for hydrogenation. The model comprises competitive and rapid adsorption steps as well as rate-determining hydrogenation steps. The mass transfer limitation of hydrogen was included in the mathematical model. The kinetic parameters and the mass transfer parameter of hydrogen were estimated from the experimental data. A comparison of the model predictions with the experimental data revealed that the proposed kinetic approach gave a satisfactory reproduction of the data.     

Partial oxidation of methane to syngas over Ni/MgO, Ni/CaO and Ni/CeO2

Partial oxidation of methane to syngas at atmospheric pressure and 750°C was examined over Ni/MgO, Ni/CaO and Ni/CeO₂ catalysts with nickel loading of 13 wt%. All catalysts had similar high conversion of methane and high selectivity to syngas, which nearly approached the values predicted by thermodynamic equilibrium. However, only Ni/MgO showed high resistance to carbon deposition under thermodynamically severe conditions (CH₄/O₂ = 2.5, a higher CH₄ to O₂ ratio than the stoichiometric ratio). Its catalytic activity remained stable during 100 h of reaction, with no detectable carbon deposition. The oxidation of carbon deposited from pure CH₄ decomposition and from pure CO disproportionation was investigated by in situ TPO-MS study which showed that both were effectively inhibited over Ni/MgO. In addition, the catalysts were characterized by TPR, XRD and XPS. It was revealed that the excellent performance of Ni/MgO resulted from the formation of an ideal solid solution between NiO and MgO. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effective kinetics of 3-hydroxypropanal hydrogenation over Ni/SiO2/Al2O3 catalyst

Concentration-time curves at 45-80°C and 2.60-5.15 MPa were measured, in a spinning basket reactor, to model the hydrogenation of 3-hydroxypropanal (HPA) to 1,3-Propanediol (PD) over Ni/SiO₂/Al₂O₃ catalyst pellets. A mathematical model whose parameters are effective diffusion coefficients and intrinsic kinetic parameters is proposed to describe this process and to avoid the dependence of the model parameters on the catalyst particle size. This model fits the experimental data reasonably and allows a reliable scale up of this process in comparison to other empirical models.     

Integrated coal pyrolysis with CO2 reforming of methane over Ni/MgO catalyst for improving tar yield

A new process to integrate coal pyrolysis with CO₂ reforming of methane over Ni/MgO catalyst was put forward for improving tar yield. And several Chinese coals were used to confirm the validity of the process. The experiments were performed in an atmospheric fixed-bed reactor containing upper catalyst layer and lower coal layer to investigate the effect of pyrolysis temperature, coal properties, Ni loading and reduction temperature of Ni/MgO catalysts on tar, water and char yields and CH₄ conversion at fixed conditions of 400 ml/min CH₄ flow rate, 1:1 CH₄/CO₂ ratio, 30 min holding time. The results indicated that higher tar yield can be obtained in the pyrolysis of all four coals investigated when coal pyrolysis was integrated with CO₂ reforming of methane. For PS coal, the tar, water and char yield is 33.5, 25.8 and 69.5 wt.%, respectively and the CH₄ conversion is 16.8%, at the pyrolysis temperature of 750 °C over 10 wt.% Ni/MgO catalyst reduced at 850 °C. The tar yield is 1.6 and 1.8 times as that in coal pyrolysis under H₂ and N₂, respectively.     

Characterizations and activities of the nano-sized Ni/Al2O3 and Ni/La–Al2O3 catalysts for NH3 decomposition

A series of nano-sized Ni/Al₂O₃ and Ni/La–Al₂O₃ catalysts that possess high activities for NH₃ decomposition have been successfully synthesized by a coprecipitation method. The catalytic performance was investigated under the atmospheric conditions and a significant enhancement in the activity after the introduction of La was observed. Aiming to study the influence of La promoter on the physicochemical properties, we characterized the catalysts by N₂ adsorption/desorption, XRD, H₂-TPR, chemisorption and TEM techniques. Physisorption results suggested a high specific surface area and XRD spectra showed that nickel particles are in a highly dispersed state. A combination of XRD, TEM and chemisorption showed that Ni⁰ particles with the average size lower than 5.0 nm are always obtained even though the Ni loading ranged widely from 4 to 63%. Compared with the Ni/Al₂O₃ catalysts, the Ni/La–Al₂O₃ ones with an appropriate amount of promoter enjoy a more open mesoporous structure and higher dispersion of Ni. Reduction kinetic studies of prepared catalysts were investigated by temperature-programmed reduction (TPR) method and the fact that La additive partially destroyed the metastable Ni–Al mixed oxide phase was detailed.     

The kinetics of the catalytic hydrogenolysis of ethane over Ni/SiO2

The rate of hydrogenolysis of ethane over a Ni/SiO₂ catalyst, studied over a large range of pressure and of temperature, is shown to be related to the degree of hydrogen coverage θ_H, by the equation: with K₀ nearly equal to the number of ethane molecules colliding with the Ni surface, E₀ = 14 ? 1 kcal/mole, Y = ?1 ? 2 and X = 15 ? 2. The rate-limiting step is believed to be the irreversible, dissociative adsorption of ethane on an ensemble of at least 12 adjacent Ni atoms, free from adsorbed hydrogen, resulting in the complete cracking of C₂H₆: C₂H₆ + 12Ni → 2

Irreversible adsorption of ethane is assumed to compete with the reversible adsorption of hydrogen. This mechanism, which is compared with those proposed earlier, is in good agreement with data on ethane adsorption studied by magnetic methods, and with a study of ethane hydrogenolysis over NiCu/SiO₂ catalysts.     

Ni/活性炭催化剂上还原途径及其催化性能

采用浸渍法制得Ni/活性炭催化剂,在惰性气氛中将其进行了不同温度的热处理,利用X射线衍射(XRD)及热重分析(TG-DTG)等手段进行了表征,在250℃,1 MPa下考察了不同还原途径对甲醇羰基化合成醋酸的影响。XRD谱图与TG-DTG曲线表明,随着热处理温度的升高,Ni(NO_3)_2于200～300℃分解,NiO则于450℃开始被还原。活性炭促进了NiO还原为金属Ni,而且热处理温度越高还原越充分。催化活性评价结果说明,惰性气氛下利用活性炭促进作用还原后的催化活性不如经还原气氛H_2处理后的高。同时两种还原方法均表明,热处理或还原温度过高,会导致催化活性下降。