Exhaust-gas reforming using precious metal catalysts

Rh-only and Rh bimetallic catalysts have been screened for exhaust-gas reforming, under conditions that mimic the output of an autoignition gasoline engine. Propane has been used as a model fuel, with simulated exhaust-gas providing the co-reactants (O₂ and H₂O) needed to generate hydrogen. Based on oxygen-conversion as a measure of light-off, Pt–Rh on ceria–zirconia shows the highest activity. In the presence of SO₂, adsorbed sulphur species do not inhibit the oxidation reactions that induce light-off, but suppress the major pathway to hydrogen (steam reforming). By excluding platinum and using silica-enriched alumina as the underlying support, light-off is delayed, but the steam reforming reaction becomes much more insensitive to the presence of sulphur. The Pt–Rh catalyst is most suited to exhaust-gas reforming systems in which the engine runs on a sulphur-free fuel, whereas the Rh-only catalyst is the better choice when the fuel is conventional gasoline.     

平板状阳极氧化铝催化剂在煤油水蒸汽重整中的催化性能

在草酸溶液中对一种铝基板(Al/Fe-Ni-Cr alloy/Al)采用阳极氧化技术制备了氧化铝多孔膜,考察了扩孔处理、水热处理和焙烧处理对氧化铝膜比表面积的影响,并用FE-SEM、BET等研究了氧化铝膜的微观形态及结构。采用这种材料作为载体通过浸渍法制备了负载钌质量分数1.0%的钌基催化剂Ru/Al,并在固定床反应器上研究Ru/Al催化剂在煤油水蒸汽重整中的催化性能。结果表明,在氧化铝多孔膜载体制备过程中,扩孔处理能有效扩大膜孔,水热处理能让多孔膜形成更为复杂的多孔结构,并极大地提高多孔膜的比表面积。在空气中750℃焙烧50 h后,多孔膜的比表面积仍能维持在原来的60%。催化剂性能测试结果表明,与对比剂Ru A相比,在活性实验中虽然两者活性相似,但在加速劣化实验中,采用阳极氧化技术制备的催化剂Ru/Al比Ru A更稳定。工业应用中对这种载体仍需要改良以减弱烧结程度,因为在水蒸汽气氛于750℃焙烧50 h后,载体的比表面积只有原来的38%。     

Surface modification of Ni catalysts with trace Pt for oxidative steam reforming of methane

Hysteresis of catalytic performance with respect to temperature increasing and decreasing in oxidative steam reforming of methane (CH₄/H₂O/O₂/Ar = 40/30/20/10) over the monometallic Ni catalysts disappeared by the modification with Pt, and the additive effect of Pt by the sequential impregnation method (Pt/Ni) was much more significant than that by the co-impregnation method (Pt + Ni) in terms of catalytic performance and catalyst bed temperature profile. Characterization results by means of TEM, TPR, EXAFS, and FTIR suggest that the Pt atoms on the Pt/Ni catalysts were located more preferably on the surface to form a PtNi alloy than those on the Pt + Ni catalysts. The modification of Ni with Pt suppressed the oxidation of Ni species near the bed inlet in the oxidative steam reforming of methane at 1123 K, although the species on the monometallic Ni catalysts were oxidized under similar conditions. This can be due to the decreased oxidation rate of the species and the increased reduction rate caused by the surface modification of Ni with Pt. Consequently, the PtNi species can be maintained in the metallic state near the bed inlet, and the species can be the active site for the reforming reaction as well as the combustion reaction, which this leads to a lower bed temperature and smaller temperature gradient than those seen for the monometallic Ni catalysts.     

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.     

Trace Ru-doped anodic alumina-supported Ni catalysts for steam reforming of kerosene: Activity performance and electrical-heating possibility

A plate-type Ru-doped anodic alumina-supported Ni catalyst was synthesized to investigate its activity and electrical-heating possibility in steam reforming of kerosene (SRK). The catalyst showed a favorable SRK durability, which was associated with a synergistic effect between Ru and Ni, referring to the formation of Ru–Ni alloy. Under an electrical-heating pattern, the SRK system could reach stability within ca. 15 min, and no deactivation was observed during the 10 h test. Therefore, this plate-type catalyst is very promising, because it is a multipurpose reforming catalyst for SRK as well as SRM and has a swift startup.     

Combined reforming of methane over supported Ni catalysts

The hydrogen exchange for propane-d ₈ adsorbed on zeolite Zn/H-MFI has been studied by¹H MAS NMR spectroscopy in situ within the temperature range of 420–490 K. Kinetic measurements of the H/D exchange between the acidic hydroxyl groups of the zeolite and the adsorbed deuterated propane molecules show that only methyl groups of the alkane are involved in the exchange. Two mechanisms are proposed to rationalize the regioselectivity of the exchange: (i) propane dehydrogenation on Zn-sites followed by protonation of propene by acidic OH groups in accordance to the Markovnikov’s rule and abstraction of deuteride ion from another propane molecule; (ii) the reversible heterolytic dissociative adsorption of propane to form Zn-propyl species and acidic OH groups.     

Activity of different zeolite-supported Ni catalysts for methane reforming with carbon dioxide

The catalytic performance of Ni based on various types of zeolites (zeolite A, zeolite X, zeolite Y, and ZSM-5) prepared by incipient wetness impregnation has been investigated for the catalytic carbon dioxide reforming of methane into synthesis gas at 700 °C, at atmospheric pressure, and at a CH₄/CO₂ ratio of 1. It was found that Ni/zeolite Y showed better catalytic performance than the other types of studied zeolites. In addition, the stability of the Ni/zeolite Y was greatly superior to that of the other catalysts. A weight of Ni loading at 7 wt.% showed the best catalytic activity on each zeolite support; however, the 7% Ni catalysts produced a higher amount of coke than that of two other Ni loadings, 3 and 5%.     

Steam reforming of methane over Ni catalysts prepared from hydrotalcite-type precursors:Catalytic activity and reaction kinetics

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 cataly...     

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.     

Effects of metal support interaction on dry reforming of methane over Ni/Ce-Al2O3 catalysts

Dry (CO₂) reforming of methane is conducted over two newly synthesized Ni20/Ce-γAl₂O₃ and Ni20/Ce-meso-Al₂O₃ catalysts. The x-ray diffraction (XRD) patterns indicated that Ni20/Ce-meso-Al₂O₃ exhibits a better dispersion of nickel, while Ni20/Ce-γAl₂O₃ has larger amounts of nickel crystallites. The temperature programmed desorption (TPD) kinetics analysis indicated that Ni20/Ce-meso-Al₂O₃ had a lesser metal-support interaction than the Ni20/Ce-γAl₂O₃. The thermal gravimetric analysis (TGA) indicated that the incorporation of ceria into the Al₂O₃ matrix helps to stabilize Ni20/Ce-meso-Al₂O₃ during dry reforming of methane. The temperature programmed reduction (TPR) indicated that the synthesized catalysts were sufficiently reducible below 750 °C. A fixed bed reactor evaluation (at 750 °C) showed that both catalysts can facilitate methane reforming to syngas with minimal coking throughout the 30 hours time-on-stream (TOS). However, Ni20/Ce-meso-Al₂O₃ is more promising in terms of prolonged stability for dry reforming applications. Moreover, the syngas yield for Ni20/Ce-γAl₂O₃ is close to equilibrium prediction during the first 1 hour of reaction time.     

Carbon dioxide reforming of methane to synthesis gas over supported Ni catalysts

Carbon dioxide reforming of methane to synthesis gas has been investigated over supported Ni catalysts in the temperature range of 500–850°C. Addition of CaO (10mol%) promoter to the Ni/γ-Al₂O₃ resulted in an increase of reaction rate and an improvement of catalyst stability, which may be related to enhanced reducibility of the promoted catalyst. The kinetic studies show that the overall reaction can be described by a Langmuir-Hinshelwood mechanistic scheme, assuming that methane dissociation is the rate determining step. In addition to adsorbed CO and formate species, three types of carbonaceous species, C, C_β and C_γ, were found to exist on the Ni catalyst. While the active C, species is suggested to be responsible for CO formation, the less active C_β and C_γ species are attributed to causing catalyst deactivation.     

Development of ultra-stable Ni catalysts for CO2 reforming of methane

Carbon deposition behavior in CO₂ reforming of methane, methane decomposition, and CO disproportionation on nickel-magnesia solid solution was investigated by means of thermogravimetric analysis and temperature programmed reaction of deposited carbon with carbon dioxide. It was found that rapid oxidation of CH_x on Ni surface by oxygen species from CO₂ through dissociation at metal-support interface is a key step for the inhibition of carbon formation.     

A novel monolith catalyst of plate-type anodic alumina for the hydrolysis of dimethyl ether

A novel monolith catalyst of plate-type anodic alumina was applied in the dimethyl ether (DME) hydrolysis reaction system. The reactivity of the anodic alumina with hydration treatments in DME hydrolysis reaction was investigated. The preferred hydration-treated temperature was found to be 80 °C and the anodic Al₂O₃/Al monolith exhibited higher activity than the commercial Al₂O₃ in DME hydrolysis reaction. Meanwhile, the anodic Al₂O₃/Al monolith was proven to have higher MeOH effluent mole percentage with less unfavorable side reactions than the ZSM-5 catalyst. The anodic γ-Al₂O₃/Al monolith had just 0.85% coking while the ZSM-5 catalyst had 8.81% after 100 h of continuous experiments.     

A structured catalyst: Noble metal supported on a plate-type zirconia substrate prepared by anodic oxidation for steam reforming of hydrocarbon

A structured catalyst: noble metal supported on a plate-type zirconia substrate was prepared by subjecting a zirconium plate to a process consisting of anodic oxidation in an oxalic acid bath and calcination in the air, followed by rhodium or ruthenium component deposition by the dipping treatment. The catalytic performances of the prepared catalysts were evaluated for steam reforming of n-butane and propane. The substrate surface was significantly corroded by the anodic oxidation and calcination, and a rugged zirconia layer about 100 μm thick was formed. The crystalline state of zirconia was mainly monoclinic and tetragonal. In steam reforming of n-butane, the structured ruthenium catalyst had some activity, while the activity of the rhodium catalyst exceeded that of the commercial catalyst. For the rhodium catalyst, its reforming activity was improved by changing the temperature of dipping bath and the number of dips for adjustment of the rhodium deposition state. The rhodium catalyst prepared by dipping twice at a bath temperature of 25 °C has the largest metal surface area and a higher metal dispersion, which were thought to be the causes for the high performance. In steam reforming of propane, the rhodium catalyst showed a significantly higher activity than the commercial catalyst. The rhodium catalyst was less prone to deterioration of activity due to n-butane and propane reforming.     

SHS-produced Ni-Co-Al-Mg-O catalysts for dry reforming of methane

Oxide catalysts Ni-Co-Al-Mg-O, Ni-Al-Mg-O, Co-Al-O, and Co-Mg-O have been prepared by combustion synthesis and characterized by XRD, Fourier transform IR spectroscopy (FTIR), SEM/EDS, TEM, and nitrogen porosimetry. Their catalytic activity in the process of dry (carbon dioxide) reforming of methane was studied by gas chromatography at different temperatures. Feed gas contained equal amounts of methane, carbon dioxide, and nitrogen and the effect of catalyst composition and temperature on the catalytic activity, selectivity, and product yields were recorded.     

Effect of boron on the stability of Ni catalysts during steam methane reforming

Ni catalysts promoted with 0.5 and 1.0 wt% boron were synthesized, characterized and tested during steam methane reforming, to evaluate the effect of boron on the deactivation behavior. Boron adsorbs on the γ-Al₂O₃ support and on the Ni particles and 1.0 wt% boron is found to enhance the stability without compromising the activity. Catalytic studies at 800 °C, 1 atm, a stoichiometric methane to steam ratio, and space velocities of 330,000 cm³/(h g_cat) show that promotion with 1.0 wt% boron reduces the rate of deactivation by a factor of 3 and increases the initial methane conversion from 56% for the unpromoted catalyst to 61%. Temperature-programmed oxidation (TPO) and scanning electron microscopy (SEM) studies confirm the formation of carbonaceous deposits and illustrate that 1.0 wt% boron reduces the amount of deposited carbon by 80%.     

Investigations on precious metal/alumina interactions on three-way automotive emission control catalysts,BMA catalysts and model samples

Combined XPS and SIMS measurements, partly supplemented by XRD investigations, were utilized to detect precious metal/support interactions on catalysts containing various precious metals. The Klemm-Bronger reaction between Pt and α-Al₂O₃ on methane/ammonia catalysts was used as a standard reference. Evidence was found for different stages of Pt/Al interactions and changing electronic properties of Pt on the surfaces of these Pt/α-Al₂O₃ catalysts by means of XPS and SIMS, and in the bulk material by XRD. The SIMS investigations on Pt and Rh on γ-Al₂O₃ showed that, even at temperatures around 500 °C, Rh/Al interactions can appear to a small extent in the topmost atomic layers, whereas the Pt/γ-Al₂O₃ specimens did not show any measurable effects. After on-road operation, enhanced SIMS cluster-ion signals as well as anomalous XPS signal contributions were measured on the surfaces of used three-way automotive emission control catalysts. These signals did not appear for the fresh catalysts. By analogy with the related results reported on the methane/ammonia catalysts, the ion signals were used as qualitative surface probes indicating the presence of precious metal/support interactions, especially between Rh and Al.     

Ni based on dual-support Mg-Al mixed oxides and SBA-15 catalysts for dry reforming of methane

A series of new dual support Ni catalysts consisting of Mg-Al mixed oxides and SBA-15 was prepared. The performance of the catalysts in the dry reforming of methane to syngas was investigated at 800 °C, atmospheric pressure and gas hourly space velocity of 12000 mL/g_cat · h. The ordered hexagonal mesoporous structure of SBA-15 coated with Mg-Al mixed oxides was found to be maintained. One of the composite catalysts exhibited excellent anti-deposition of carbon and high stability for 500 h on stream. It is concluded that strong metal-support interaction promoted catalyst performance by inhibiting carbon deposition and metal sintering.     

Catalytic test of supported Ni catalysts with core/shell structure for dry reforming of methane

Supported nickel catalysts with core/shell structures of Ni/Al₂O₃ and Ni/MgO-Al₂O₃ were synthesized under multi-bubble sonoluminescence (MBSL) conditions and tested for dry reforming of methane (DRM) to produce hydrogen and carbon monoxide. A supported Ni catalyst made of 10% Ni loading on Al₂O₃ and MgO-Al₂O₃, which performed best in the steam reforming of methane (97% methane conversion at 750 °C) and in the partial oxidation of methane (96% methane conversion at 800 °C), showed also good performance in DRM and excellent thermal stability for the first 150 h. The supported Ni catalysts Ni/Al₂O₃ and Ni/MgO-Al₂O₃ yielded methane conversions of 92% and 92.5%, respectively and CO₂ conversions of 95.0% and 91.8%, respectively, at a reaction temperature of 800 °C with a molar ratio of CH₄/CO₂ = 1. Those were near thermodynamic equilibrium values.