以类水滑石为前体的Ni催化剂及其催化甲烷水蒸气重整制氢:催化活性和动力学(英文)

Ni/Mg–Al catalysts derived from hydrotalcite-type precursors were prepared by a co-precipitation technique and applied to steam reforming of methane. By comparison with Ni/γ-Al2O3 and Ni/α-Al2O3 catalysts prepared by in-cipient wetness impregnation, the Ni/Mg–Al catalyst presented much higher activity as a result of higher specific surface area and better Ni dispersion. The Ni/Mg–Al catalyst with a Ni/Mg/Al molar ratio of 0.5:2.5:1 exhibited the highest activity for steam methane reforming and was selected for kinetic investigation. With external and inter-nal diffusion limitations eliminated, kinetic experiments were carried out at atmospheric pressure and over a temperature range of 823–973 K. The results demonstrated that the overal conversion of CH4 and the conversion of CH4 to CO2 were strongly influenced by reaction temperature, residence time of reactants as wel as molar ratio of steam to methane. A classical Langmuir–Hinshelwood kinetic model proposed by Xu and Froment (1989) fitted the experimental data with excellent agreement. The estimated adsorption parameters were consistent thermodynamical y.     

Ni catalysts with Mo promoter for methane steam reforming

NiO/Al₂O₃ catalyst precursors were prepared by simultaneous precipitation, in a Ni:Al molar ratio of 3:1, promoted with Mo oxide (0.05, 0.5, 1.0 and 2.0 wt%). The solids were characterized by adsorption of N₂, XRD, TPR, Raman spectroscopy and XPS, then activated by H₂ reduction and tested for the catalytic activity in methane steam reforming.The characterization results showed the presence of NiO and Ni₂AlO₄ in the bulk and Ni₂AlO₄ and/or Ni₂O₃ and at the surface of the samples.In the catalytic tests, high stability was observed with a reaction feed of 4:1 steam/methane. However, at a steam/methane ratio of 2:1, only the catalyst with 0.05% Mo remained stable throughout the 500 min of the test.The addition of Mo to Ni catalysts may have a synergistic effect, probably as a result of electron transfer from the molybdenum to the nickel, increasing the electron density of the catalytic site and hence the catalytic activity.     

Steam reforming of ethanol over Pt–Ni Catalysts

A parametric study was conducted over Pt–Ni/δ-Al₂O₃ to explore the effect of Pt and Ni contents on the ethanol steam reforming characteristics of the bimetallic catalyst. Experiments with catalysts having 0.2–0.3 wt%Pt and 10–15 wt%Ni contents indicated that the best ethanol steam reforming performance is achieved over 0.3 wt%Pt–15 wt%Ni/δ-Al₂O₃. Kinetics of ethanol steam reforming was studied over this catalyst in the 673–823 K interval using differential and integral methods of data analysis. A power-function rate expression was obtained with reaction orders of 1.01 and −0.09 in ethanol and steam, respectively, and the apparent activation energy of ethanol steam reforming over 0.3 wt%Pt–15 wt%Ni/δ-Al₂O₃ was calculated as 59.3 ± 2.3 kJ mol⁻¹.     

Steam reforming of ethanol over Ni/support catalysts for generation of hydrogen for fuel cell applications

The paper reports experimental results concerning the influence of the support nature (TiO₂, ZnO, Al₂O₃ and Al₂O₃–Fe₂O₃) of nickel catalysts on their activity, selectivity and coking phenomenon in the steam reforming of ethanol in the range of 570–870 K. The chemical transformations of ethanol occurring on the catalyst support make its chemical nature an important factor affecting the productivity and selectivity of the process. It was found that the most suitable supports in nickel catalysts designed for hydrogen generation in the steam reforming of ethanol are ZnO and TiO₂. Taking into consideration both the efficiency of hydrogen generation and the intensity of carbon deposition, the optimum temperature of the process of the steam reforming of ethanol is below 750 K. An improvement in the selectivity of hydrogen generation and diminishing of the formation of undesirable products may be obtained by promoting nickel catalysts with sodium.     

Effect of silicates on the structure of Ni-containing catalysts obtained from hydrotalcite-type precursors

New nickel hydrotalcite-like compounds with silicates as interlayer anions used as catalyst precursors in the catalytic partial oxidation of methane were prepared by the coprecipitation method. The properties of these materials were compared with those of compounds obtained from carbonate-containing materials. The precursors and calcined samples were characterized by powder X-ray diffraction, FT-IR and Vis/UV/NIR spectroscopies, thermal analyses (DTA and TG), temperature programmed reduction (TPR) and N₂ adsorption/desorption at −196 °C. The results show that the incorporation of silicates in the lamellar compounds modifies the structural and textural properties of the precursors. After calcination, silicates – which are non-volatile anions – contribute to the final structure of the catalysts, which form a new forsterite-like phase, increasing their specific surface area but not altering the reducibility of the nickel species.     

Steam reforming of methane under water deficient conditions over gadolinium-doped ceria

The catalytic properties of gadolinium-doped ceria (CGO) in methane steam reforming were studied. Catalytic tests were carried out between 750 and 900 °C, for H₂O/CH₄ ratios varying between 0.1 and 1, pretreated in H₂O/N₂, N₂ and H₂/N₂. Above 800 °C, slight deactivation with time on stream was observed except for the H₂-pretreated sample. Surface area measurements, O₂ adsorption at room temperature and O₂-temperature programmed oxidation experiments were performed after catalytic testing. Changes in both surface area and redox properties of CGO were observed and related to catalytic deactivation. Hydrogen is thought to play a key role in catalytic activity and deactivation process.     

Cobalt catalysts derived from hydrotalcite-type precursors applied to steam reforming of ethanol

Catalysts derived from Co/Mg/Al hydrotalcite-type precursors modified with La and Ce were characterized by XANES and tested in ethanol steam reforming. The reaction data showed that, with a molar ratio of water:ethanol = 3:1 in the feed, addition of Ce and La favored acetaldehyde production. Increasing the water content (water:ethanol = 5:1) decreased the acetaldehyde formation by favoring the adsorption of water molecules on these samples, enhancing the acetaldehyde conversion.     

High combustion activity of methane induced by reforming gas over Ni/Al2O3 catalysts

During the reactions related to oxidative steam reforming and combustion of methane over -alumina-supported Ni catalysts, the temperature profiles of the catalyst bed were studied using an infrared (IR) thermograph. IR thermographical images revealed an interesting result: that the temperature at the catalyst bed inlet is much higher under CH₄/H₂O/O₂/Ar = 20/10/20/50 than under CH₄/H₂O/O₂/Ar = 10/0/20/70; the former temperature is comparable to that over noble metal catalysts such as Pt and Pd. Based on the temperature-programmed reduction and oxidation measurements over fresh and used catalysts, the metallic Ni is recognized at the catalyst bed inlet under CH₄/H₂O/O₂/Ar = 20/10/20/50, although it is mainly oxidized to NiAl₂O₄ under CH₄/H₂O/O₂/Ar = 10/0/20/70. This result indicates that the addition of reforming gas (CH₄/H₂O = 10/10) to the combustion gas (CH₄/O₂ = 10/20) can stabilize Ni species in the metallic state even under the presence of oxygen in the gas phase. This would account for its extremely high combustion activity.     

Catalytic performance and QXAFS analysis of Ni catalysts modified with Pd for oxidative steam reforming of methane

Pd–Ni bimetallic catalysts prepared by co-impregnation and sequential impregnation methods were compared in the catalytic performance in oxidative steam reforming of methane. The sequential impregnation was more effective to the suppression of hot spot formation. According to the structural analysis by in situ quick-scanning X-ray absorption fine structure (QXAFS) during the temperature programmed reduction, the sequential impregnation method gave the bimetallic particles with higher Pd surface composition because of the low possibility of the Pd–Ni bond formation. Higher surface composition of Pd with higher reducibility than Ni is connected to the enhancement of the catalyst reducibility and the suppression of the hot spot formation.     

Novel circulating fast fluidized-bed membrane reformer for efficient production of hydrogen from steam reforming of methane

The coupling of steam reforming and oxidative reforming of methane for the efficient production of hydrogen is investigated over Ni/Al₂O₃ catalyst in a novel circulating fast fluidized-bed membrane reformer (CFFBMR) using a rigorous mathematical model. The removal of product hydrogen using palladium hydrogen membranes “breaks” the thermodynamic equilibrium barrier exists among the reversible reactions. Oxygen can be introduced into the adiabatic CFFBMR for oxidative reforming by direct oxygen (or air) feed and through dense perovskite oxygen membranes. The simulations show that high productivity of hydrogen can be obtained in the CFFBMR. The combination of these two different processes does not only enhance the hydrogen productivity but also save the energy due to the exothermicity of the oxidative reforming. Based on the preliminary investigations, four parameters (number of hydrogen membranes, number of oxygen membranes, direct oxygen feed rate and steam-to-carbon feed ratio) are carefully chosen as main variables for the process optimization. The optimized result shows that the hydrogen productivity (moles of hydrogen produced per hour per m³ of reactor) in the novel CFFBMR is about 8.2 times higher than that in typical industrial fixed-bed steam reformers.     

High stability of Ce-promoted Ni/Mg-Al catalysts derived from hydrotalcites in dry reforming of methane

Ce-promoted Ni/Mg-Al catalysts were synthesized by means of a methodology that involves the doping of Ni-Mg-Al mixed oxides derived from hydrotalcites with [Ce(EDTA)]⁻ and subsequent thermal decomposition. The effect of the nominal load of Ce in the catalytic performance of the materials was studied. The solids were characterized by means of XRD, BET area, TPR-H₂, TPD-CO₂, chemical analysis by ICPs, TGA, SEM and TEM and were evaluated in CO₂ reforming of methane at 700 °C. The results indicate the partial reconstruction of the periclase phase during the doping with [Ce(EDTA)]⁻ and the formation of a mixture of crystalline periclase and fluorite phases after the calcination. Catalysts with particle sizes of Ni⁰ between 5 and 9 nm were obtained. Ce presents a promote effect in the degree of reduction of Ni and the amount and strength of the basic sites. It was evident a beneficial effect of cerium in the catalytic activity and selectivity of the doped materials. The increase of the nominal Ce load between 1 and 10% causes no considerable effect in the catalytic activity and selectivity or in the size of crystallite in these materials but in the inhibition of the coke formation. The catalysts show excellent catalytic performance under drastic conditions of reaction and long operation times. The Ce-doped Ni/Mg-Al catalyst is stable up to 100 h of reaction using a feed mixture of CH₄/CO₂/He 10/10/80 at 24 L g⁻¹ h⁻¹, up to 20 h of reaction using CO₂/CH₄ 20/20 at 48 L g⁻¹ h⁻¹ and up to 15 h of reaction using CO₂/CH₄ 40/40 at 96 L g⁻¹ h⁻¹. The filamentous coke formation is demonstrated on the surface of the catalyst when gas of dilution in the reactants is not used. The developed method of synthesis becomes an interesting methodology for obtaining catalysts for CO₂ reforming of methane.     

Catalytic performance and characterization of Pt-Ni bimetallic catalysts for oxidative steam reforming of methane

Effects of methane oxygen mixture addition to steam reforming of methane and subsequent removal of the methane oxygen mixture from the oxidative steam reforming of methane on catalytic performance were investigated using monometallic Ni and Pt catalysts and two Pt-Ni bimetallic catalysts. Hysteresis with respect to the addition and removal of the methane oxygen mixture was observed clearly on a Pt-Ni bimetallic catalyst prepared by co-impregnation method and the Ni catalyst. In contrast, no hysteresis was observed for a Pt-Ni catalyst that was prepared by sequential impregnation method. Combined with characterization results obtained using EXAFS analysis and FTIR of CO adsorption, Pt-Ni catalyst was prepared by sequential impregnation is formed Pt-Ni alloy particles, where Pt atoms are segregated on the surface, enhances the reducibility of Ni drastically and this is related to the behavior without hysteresis.     

Hydrogen Production from Catalytic Steam Reforming of Bio-Oils: A Critical Review

In the future, hydrogen will be an important energy carrier and industrial raw material. Catalytic steam reforming of bio-oils is a promising and economically viable technology for hydrogen production. However, during the reforming process, the catalysts are rapidly deactivated due to coke formation and sintering. Thus, maintaining the activity and stability of catalysts is the key issue in this process. Optimized operation conditions could extend the catalyst lifetime by affecting the coke morphology or promoting coke gasification. This article summarizes the recent developments in the field of catalytic steam reforming of bio-oils, focusing on the operation conditions, the properties of the catalysts, and the effects of the catalyst supports. The expected insights into the catalytic steam reforming of bio-oils will provide further guidance for hydrogen production from bio-oils.     

用于燃料电池的甲醇水蒸气重整反应制氢的研究

研究了甲醇水蒸气重整制氢反应过程中各种因素对Cu/ZnO/Al2O3催化剂的活性和选择性的影响.结果表明:Cu/Zn比为2.0的催化剂在250℃反应时,催化剂效果较好,最合适反应条件是:压力0.1 MPa,温度250℃,n(H2O)∶n(CH3OH)=1.0～1.2,液体流速0.1 mL/min.在Cu/ZnO/Al2O3催化剂上,甲醇水蒸气重整、甲醇分解和水气转换反应随反应条件的不同而发生相互抑制或促进作用.     

The effect of CeO2 structure on the activity of supported Pd catalysts used for methane steam reforming

Palladium (Pd) supported on CeO₂-promoted γ-Al₂O₃ with various CeO₂ (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO₂ structure and dispersion on the γ-Al₂O₃ support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO₂ structure. Pd is present on the oxidized CeO₂-promoted catalysts as Pd⁰, Pd⁺ and Pd²⁺, at ratios strongly dependent on CeO₂ structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO₂ and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO₂. FTIR spectra of adsorbed CO revealed that after pre-treatment under H₂ or in the presence of amorphous CeO₂, partial encapsulation of Pd particles occurs. CeO₂ structure influences the CH₄ steam reforming reaction rates. Crystalline CeO₂ and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO₂ as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al₂O₃ or on amorphous (reduced) CeO₂/Al₂O₃ catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO₂/γ-Al₂O₃ than on Pd/γ-Al₂O₃, which is attributed to a catalytic synergism between Pd and CeO₂. The low rates on the reduced Pd/CeO₂/Al₂O₃ catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration.     

Biogas reforming over La-NiMgAl catalysts derived from hydrotalcite-like structure: Influence of calcination temperature

Hydrotalcite-like compound with general formula [M(II)_1 − xM(III)_x(OH)₂]^x+(Aⁿ⁻_x/n) ^· mH₂O, where Aⁿ⁻ is the compensation anion, has been used as precursor of active catalysts for biogas reforming. This precursor was calcined at six different temperatures between 250 and 750 °C and the resulting catalysts were tested in order to evaluate the influence of the calcination temperature on the catalytic activity and stability. XRD characterization showed that from 250 °C the hydrotalcite structure is no longer detected, leading to Mg(Ni,Al)O solid solutions, where no peaks related to lanthanum appear. An increase on the calcination temperature increased the grain size and cell parameter value. 50 h-catalytic tests were carried out at 700 °C, CH₄:CO₂ molar ratio of 1:1 and a mass/feed alimentation ratio (W/F) of 0.4 mg min cm^− 3. Used catalysts were characterized by temperature programmed oxidation (TPO), scanning electron microscopy (SEM) and Raman spectroscopy in order to obtain information about coke deposition. Catalytic tests highlighted the great influence of calcination temperature over catalytic activity and stability, having found that, as a general trend, calcination temperatures below 750 °C decrease both the stability and catalytic activity, with the exception of the catalyst calcined at 550 °C, where a higher activity was achieved but with a comparatively low stability.     

Steam reforming of n-hexadecane over noble metal-modified Ni-based catalysts

Steam reforming of n-hexadecane, a main constituent of diesel, over noble metal-modified Ni-based hydrotalcite catalyst was carried out in a temperature range of 700–950 °C, at an atmospheric pressure with space velocity of 10,000–100,000 h⁻¹ and feed molar ratio of H₂O/C = 3.0. The catalysts were prepared by a co-precipitation and dipping methods. The noble metal-modified Ni-based hydrotalcite catalyst displayed higher resistance for the sintering of active metal than the Ni-based hydrotalcite catalyst prepared by the conventional method. It was found that the Rh-modified Ni-based catalysts showed high resistance to the formation of carbon compared to Ni-based catalysts. The results suggest that Rh-modified Ni-based catalyst can be applied for the steam reforming (SR) reaction of diesel.     

Characterization and activity in dry reforming of methane on NiMg/Al and Ni/MgO catalysts

Dry reforming of methane has been investigated on two series of catalysts either prepared by co-precipitation: n(Ni_xMg_y)/Al, Ni_xMg_y and Ni_xAl_y or prepared by impregnation: Ni/MgO (mol% Ni = 5, 10). The catalysts, calcined at 600–900 °C, were characterized by different techniques: BET, H₂-TPR, TPO, XRD, IR, and TEM-EDX analysis. The surface BET (30–182 m² g⁻¹) decreased with increasing the temperature of calcination, after reduction and in the presence of Mg element. The XRD analysis showed, for n(Ni_xMg_y)/Al catalysts, the presence of NiAl₂O₄ and NiO–MgO solid solutions. The catalyst reducibility decreased with increasing the temperature of pretreatment. The n(Ni_xMg_y)/Al catalysts were active for dry reforming of methane with a good resistance to coke formation. The bimetallic catalyst Ni_0.05Mg_0.95 (calcined at 750 °C and tested at 800 °C) presents a poor activity. In contrast, the 5% Ni/MgO catalyst, having the same composition but prepared by impregnation, presents a high activity for the same calcination and reaction conditions. For all the catalysts the activity decreased with increasing the temperature of calcination and a previous H₂-reduction of the catalyst improves the performances. The TPO profiles and TEM-EDX analysis showed mainly four types of coke: CH_x species, surface carbon, nickel carbide and carbon nanotubes.     

Pure hydrogen production by methane steam reforming with hydrogen-permeable membrane reactor

Low temperature steam reforming of methane mainly to hydrogen and carbon dioxide (CH₄ + 2H₂O → 4H₂ + CO₂) has been performed at 773 and 823 K over a commercial nickel catalyst in an equilibrium-shift reactor with an 11-μm thick palladium membrane (Mem-L) on a stainless steel porous metal filter. The methane conversion with the reactor is significantly higher than its equilibrium value without membrane due to the equilibrium-shift combined with separation of pure hydrogen through the membrane. The methane conversion in a reactor with an 8-μm membrane (Mem-H) is similar to that with Mem-L, although the hydrogen permeance through Mem-H is almost double of that through Mem-L. The amount of hydrogen separated in the reaction with Mem-H is significantly large, showing that the hydrogen separation overwhelms the hydrogen production because of the insufficient catalytic activity.     

Catalytic activity of atomized Ni3Al powder for hydrogen generation by methane steam reforming

The catalytic activity of Ni₃Al for methane steam reforming was investigated for the first time using its atomized powder. It was found that the activity was significantly enhanced by the combined pretreatment of acid and alkali leaching, while it was quite low for the as-received powder. The high activity was attributed to the formation of fine Ni particles on the porous surface of the powder.