Transient modeling of combined catalytic combustion/CH4 steam reforming

This paper presents an investigation into the complex interactions between catalytic combustion and CH₄ steam reforming in a co-flow heat exchanger where the surface combustion drives the endothermic steam reforming on opposite sides of separating plates in alternating channel flows. To this end, a simplified transient model was established to assess the stability of a system combining H₂ or CH₄ combustion over a supported Pd catalyst and CH₄ steam reforming over a supported Rh catalyst. The model uses previously reported detailed surface chemistry mechanisms, and results compared favorably with experiments using a flat-plate reactor with simultaneous H₂ combustion over a γ-Al₂O₃-supported Pd catalyst and CH₄ steam reforming over a γ-Al₂O₃-supported Rh catalyst. Results indicate that stable reactor operation is achievable at relatively low inlet temperatures (400 °C) with H₂ combustion. Model results for a reactor with CH₄ combustion indicated that stable reactor operation with reforming fuel conversion to H₂ requires higher inlet temperatures. The results indicate that slow transient decay of conversion, on the order of minutes, can arise due to loss of combustion activity from high-temperature reduction of the Pd catalyst near the reactor entrance. However, model results also show that under preferred conditions, the endothermic reforming can be sustained with adequate conversion to maintain combustion catalyst temperatures within the range where activity is high. A parametric study of combustion inlet stoichiometry, temperature, and velocity reveals that higher combustion fuel/air ratios are preferred with lower inlet temperatures (≤500 °C) while lower fuel/air ratios are necessary at higher inlet temperatures (600 °C).     

Structured reactors for kinetic measurements under severe conditions in catalytic combustion over palladium supported systems

The collection of chemical kinetics data in catalytic combustion over very active palladium catalysts under conditions relevant to practical applications (e.g. gas turbine combustors) is extremely difficult, mainly due to strong exothermicity and very fast rate of combustion reactions. Within this purpose in this paper two types of laboratory structured reactors, which closely resemble industrial monolith catalysts, are investigated: (a) the annular reactor, consisting of a catalyst coated ceramic tube, co-axially placed in a quartz tube; (b) the metallic plate-type reactor, consisting of an assembled packet of metallic slabs coated with a ceramic catalytic layer.

The design of the annular reactor configurations for kinetic investigations is first addressed by mathematical modeling. The resulting advantages, including: (i) negligible pressure drops; (ii) minimal impact of diffusional limitations in high temperature–high GHSV experiments; (iii) effective dissipation of reaction heat are then experimentally demonstrated for the case of CH₄ combustion over a PdO/γ-Al₂O₃ catalyst with high noble metal loading (10% (w/w) of Pd).

The feasibility of a near-isothermal operation with the metallic plate-type reactor by an extremely effective dissipation of reaction heat through proper selection of highly conductive support material and of the geometry of the metallic slabs is finally discussed and experimentally demonstrated for the case of combustion of CO at high concentrations over a PdO/γ-Al₂O₃ (3% (w/w) of Pd) catalyst.     

Pt/Pt-Ce/γ-Al2O3催化氧化甲苯研究

通过等体积浸渍法制备单贵金属Pt/γ-Al₂O₃和双金属Pt-Ce/γ-Al₂O₃催化剂,考察Ce对催化剂活性的影响,确定催化剂最优配比。结果表明,当Pt的负载量为质量分数0.5%时,Pt/γ-Al₂O₃催化活性最高;当Pt的负载量为质量分数0.2%,Ce的负载量为质量分数1.0%时,Pt-Ce/γ-Al₂O₃催化剂的催化活性最高。Pt-Ce/γ-Al₂O₃催化剂的甲苯转化率高于Pt/γ-Al₂O₃催化剂。随着Pt负载量增大,催化剂孔容、孔径减小。粉体式催化剂性能优于整体式催化剂,但差别不大;Ce的添加有助于催化剂活性的提升。     

Cu/γ-Al2O3 catalyst for the combustion of methane in a fluidized bed reactor

The applicability of a catalyst based on copper dispersed on γ-Al₂O₃ spheres (1 mm diameter) for fluidized bed catalytic combustion of methane has been assessed. Catalyst properties have been determined by physico-chemical characterization techniques and fixed bed activity tests revealing the presence of a surface CuAl₂O₄ spinel phase, still active and stable in methane combustion after repeated thermal ageing treatments at 800 °C. Methane catalytic combustion experiments have been performed in a 100 mm premixed fluidized bed reactor under lean conditions (0.15–3% inlet methane concentration), showing that complete CH₄ conversion can be attained below 700 °C in a fluidized bed of 1 mm solids with a gas superficial velocity about twice the incipient fluidization velocity.     

A structured test reactor for the evaporation of methanol on the basis of a catalytic combustion

The kinetics of the catalytic deep oxidation of methanol was studied in stacked segments of a turbulence insert (SSTIs) coated with Pt/γ-Al₂O₃ and Pt/zeolite catalysts. These geometries, which are termed a structured test reactor in the heading, are intended to replace plane expanded metal discs (PEMDs) used in the so-called catalytic burner at Research Centre Jülich, up to the present. The catalytic burner performs the low-pollution combustion inter alia of the anode exhaust gases of the fuel cell. SSTIs coated with Pt/γ-Al₂O₃ showed different kinetics for differential conversions as a function of temperature. At low reaction temperatures (100–130°C) ignition kinetics with a very high activation energy of 60.3 kJ/mol was found. In the further course of the reaction, the kinetically controlled reaction with an activation energy of 48.5 kJ/mol was observed. In the range of non-differential conversions, the activation energy decreased significantly, which suggests a limitation of the reaction process by mass transport phenomena. Compared to the PEMDs, clearly higher reaction rates were observed, which enable a reduction of the catalyst mass. This as well as the spatial structure of the SSTIs make it possible to achieve a reduction in volume and weight and thus improved dynamics in constructing a novel catalytic burner. The SSTIs coated with Pt/zeolite showed perceptibly lower reaction rates than the Pt/γ-Al₂O₃ substrates and could thus not compete with the PEMDs.     

Complete benzene oxidation over Pt-Pd bimetal catalyst supported on γ-alumina: influence of Pt-Pd ratio on the catalytic activity

Pt-Pd bimetal catalysts were prepared in order to develop and investigate catalysts with excellent activity and stability for benzene destruction. In the reaction results, the addition of Pt to Pd/γ-Al₂O₃ catalyst brought about the increase of catalytic activity. Moreover, it was effective in preventing the deactivation of the catalysts in benzene combustion. The addition of some amount of Pt made Pd particles available for better benzene combustion. On the contrary, the addition of Pt beyond a certain amount decreases activity because of the Pd active sites overlapped with the Pt active sites. The activity of the catalysts is related to oxidation state of metal, Pd/Al ratio and particle size on γ-Al₂O₃. These effects of Pt addition to Pd catalysts were studied by XPS, XRD, and TEM analyses.     

Catalytic combustion of methane in non-permselective membrane reactors with separate reactant feeds

Catalytic combustion of methane over Pd and Pt/SiO₂/-Al₂O₃ membranes was studied in the temperature range 300–650 °C. Fuel and oxygen were fed at opposite membrane sides. In order to improve reactor controllability the -Al₂O₃ membranes were impregnated with SiO₂ sol resulting to smaller pore size. Methane conversions up to 100% for the palladium membrane and up to 42% for the platinum membrane were achieved. The results indicated a transition from kinetic to mass transfer control within the temperature range investigated. This was accompanied by reduction of methane slip from tube to shell side with increasing temperature. CO and H₂ were detected in the product gases of the palladium membrane. Their concentration could be reduced by applying a trans-membrane pressure difference. Low concentrations of CO were observed for the Pt/SiO₂/-Al₂O₃ membrane, while no CO or H₂ were detected for a Pd/-Al₂O₃ membrane operating in dead-end configuration.     

Catalytic combustion concept for gas turbines

Catalytic combustion for gas turbines was investigated, based on a partial catalytic combustion section followed by a homogeneous combustion zone. A pressurized test rig (<25 bar) was built to test the influence of various parameters on this concept using Pd and Pt catalysts.

The pressure influence on the apparent catalytic reaction rate was of the order 0.4, assuming that the reaction kinetics could be described by a power rate function which was of first order with respect to methane. Pd catalysts showed a pressure-dependent temperature for the transition of the active PdO to the much less active Pd. Combining Pd and Pt within one catalyst resulted in a considerably lower transition temperature.

Homogeneous combustion reactions set on from 650°C, depending on the methane concentration, pressure and flow. With inlet temperatures above 800°C the homogeneous combustion always started. At outlet temperatures below 1050°C high CO concentrations could be measured. At higher temperatures the CO, CH₄ and NO_x concentrations were lower than 5 ppm. During several experiments total conversion of CH₄ and CO was observed.     

The oxidizing role of CO2 at mild temperature on ceria-based catalysts

For thermodynamic reasons, CO₂ has always been considered as inert at mild reaction temperatures (300 °C). In this study, we show that CO₂ may be used as a valuable compound for the catalytic combustion of methane (CCM), if ceria-based materials are used as support for the palladium active phase. Adding CO₂ in the feed significantly improves performances of ceria-zirconia supported catalysts. On the contrary, catalytic performances are inhibited on Pd/γ-Al₂O₃. Inhibition can be avoided by mixing the Pd/γ-Al₂O₃ catalyst with some CeO₂ evidencing cooperation phenomena between both catalysts. In situ DRIFTS experiments show that the inhibition of the alumina-supported catalyst is not due to formation of carbonates species. After an in situ reducing pre-treatment, pure CO₂ is able to rapidly oxidize reduced Pd/Ce_0.21Zr_0.79O₂ catalyst at 300 °C. Dissociation of CO₂ on Ce_0.21Zr_0.79O₂ would be responsible for the oxidation process. Thus, CO₂ helps in replenishing the O reservoir (OSC) of the Ce-Zr-O support which is normally consumed by reductants such as CH₄, H₂ or other HC's. XPS experiments show enrichment in oxygen species bound to Ce (Low BE O1s) on the surface of ceria-zirconia when working in the presence of CO₂. Implications of these results on the behavior of ceria-containing catalysts can be important for practical applications, e.g., in automotive exhaust catalysis.     

Ce改性Pd基整体式催化剂的结构特征及其甲烷催化燃烧性能

分别以拟薄水铝石和添加Ce的拟薄水铝石制备铝溶胶,经过堇青石（Cord）表面涂覆和Pd溶液浸渍,得到浸渍法和溶胶法Ce改性的Pd/γ-Al₂O₃/Cord整体式催化剂。采用XRD、SEM和XPS等对催化剂进行表征,评价其甲烷催化燃烧反应性能,并考察Ce的不同添加方式对催化剂结构和反应性能的影响。结果表明,适量Ce的添加可提高Pd基整体式催化剂的甲烷催化燃烧性能,溶胶法优于浸渍法。随着Ce添加量的增加,浸渍法改性的Pd基催化剂催化性能有所降低,溶胶法则呈现先升高后降低的趋势。溶胶法中Ce的添加物与γ-Al₂O₃涂层充分融合,提高了涂层的热稳定性和活性组分的分散度,0.5Pd/γ-Al₂O₃(3.0Ce)/Cord催化剂催化性能最优。     

贵金属催化剂用于低浓度甲烷的去除

制备不同负载量的Pd基和Pt基催化剂,建立催化剂活性评价装置,考察贵金属Pd和Pt负载量对甲烷转化率的影响,结果表明,甲烷转化率最高的Pd和Pt负载质量分数分别为1.25%和2%,相同负载质量分数0.1%时,Pd基催化剂的甲烷转化率优于Pt基催化剂。催化剂的BET比表面积大小不能反映催化剂的催化活性,二者之间无线性关系。     

Effect of La on the thermal stability of Pd/γ-Al2O3 catalytic membranes

Lanthanum-doped Pd/γ-Al₂O₃ and Pd/γ-Al₂O₃ membranes were prepared by sol-gel methods. The thermal stability of the unsupported Pd/γ-Al₂O₃ and La/Pd/γ-Al₂O₃ membranes was investigated with BET (including average pore size, pore volume and BET surface area), XRD, and DTA techniques. The average pore size of the Pd/γ-Al₂O₃ membranes increased sharply after sintering at temperatures higher than 1000°C. Addition of 3 mol% lanthanum can raise the temperature of the γ-Al₂O₃ to-Al₂O₃ phase transformation significantly. This improves the thermal stability of the Pd/γ-Al₂O₃ catalytic membranes.     

助剂Ba对Pd/Al_2O_3和Pt/Al_2O_3催化剂的C1-C3烷烃催化燃烧性能的影响

通过浸渍法制备了Al_2O_3负载的Pd和Pt催化剂,考察催化剂的甲烷、乙烷和丙烷催化燃烧活性,以及助剂Ba对催化性能的影响。对于Pd/Al_2O_3催化剂,加入Ba使活性物种PdO颗粒变大和还原温度升高,形成更稳定的PdO活性物种,是Pd-Ba/Al_2O_3催化剂活性提升的主要原因。对于Pt/Al_2O_3催化剂,加入Ba助剂使活性物种Pt0含量降低,PtO_x与Al_2O_3载体相互作用增强,使PtO_x物种更难被还原为Pt~0,导致Pt-Ba/Al_2O_3催化剂活性降低。Pd和Pt催化剂催化烷烃氧化反应活性规律一致:丙烷乙烷甲烷。Pd/Al_2O_3催化剂有利于C—H键活化,Pt/Al_2O_3催化剂有利于C—C键活化。Pt/Al_2O_3催化剂对C1-C3烷烃氧化活性的差别明显大于Pd/Al_2O_3催化剂。Pt/Al_2O_3催化剂对碳比例高的烷烃活性更高。     

Fuel-rich catalytic combustion of methane in zero emissions power generation processes

A novel catalytic combustion concept for zero emissions power generation has been investigated. Catalysts consisting of Rh supported on ZrO₂, Ce-ZrO₂ or -Al₂O₃ were prepared and tested under fuel-rich conditions, i.e. for catalytic partial oxidation (CPO) of methane. The experiments were performed in a subscale gas-turbine reactor operating at 5 bar with exhaust gas-diluted feed mixtures.

The catalyst support material was found to influence the light-off temperature significantly, which increased in the following order Rh/Ce-ZrO₂ < Rh/ZrO₂ < Rh/-Al₂O₃. The Rh loading, however, only had a minor influence. The high activity of Rh/Ce-ZrO₂ is probably related to the high dispersion of Rh on Ce-ZrO₂ and the high oxygen mobility of this support compared to pure ZrO₂. The formation of hydrogen was also found to increase over the catalyst containing ceria in the support material.     

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.     

CNG engines exhaust gas treatment via Pd-Spinel-type-oxide catalysts

Four spinel-type catalysts AB₂O₄ (CoCr₂O₄, MnCr₂O₄, MgFe₂O₄ and CoFe₂O₄) were prepared and characterized by XRD, BET, TEM and FESEM techniques. The activity of these catalysts towards the combustion of methane was evaluated in a temperature-programmed combustion (TPC) apparatus. Spinel-type-oxides containing Cr at the B site were found to provide the best results. The half-conversion temperature of methane over the CoCr₂O₄ catalyst was 376 °C with a W/F = 0.12 g s/cm⁻³. On the basis of temperature-programmed oxygen desorption (TPD) analysis as well as of catalytic combustion runs, the prevalent activity of the CoCr₂O₄ catalyst could be explained by its higher capability to deliver suprafacial, weakly chemisorbed oxygen species. This catalyst, promoted by the presence of 1 wt% of palladium deposited by wet impregnation, was lined on cordierite monoliths and then tested in a lab-scale test rig. The combination of Pd and CoCr₂O₄ catalysts enables half methane conversion at 330 °C (GHSV = 10,000 h⁻¹), a performance similar to that of conventional 4 wt% Pd-γ-Al₂O₃ catalysts but enabled with just a four-fold lower amount of noble metal.     

The role of the oxidic support on the deactivation of Pt catalysts during the CO2 reforming of methane

Pt supported on γ-Al₂O₃, TiO₂ and ZrO₂ are active catalysts for the CO₂ reforming of methane to synthesis gas. The stability of the catalysts increased in the order Pt/γ-A1₂O₃ < Pt/TiO₂ < Pt/ZrO₂. For all catalysts, the decrease in activity with time on stream is caused by carbon formation, which blocks the active metal sites for reaction. With Pt/TiO₂ and Pt/ZrO₂, deactivation started immediately after the start of the reaction, while the Pt/γ-A1₂O₃ catalyst showed an induction period during which carbon was accumulated without affecting the catalytic activity.     

Premixed metal fibre burners based on a Pd catalyst

As an alternative to previously developed catalytic FeCrAlloy fibre mat burners based on perovskite catalysts, new catalytic burners have been developed based on Pd catalyst on lantana-stabilised Al₂O₃ and different fibre structures (NIT100A, NIT100S and NIT200S by ACOTECH NV). All development steps are considered, shifting from catalyst preparation (based on combustion synthesis of γ-Al₂O₃) to the optimisation of lantana and Pd loadings, from the definitions of the best catalyst-deposition conditions (washcoating) to the catalytic burners performances, determined in an ad hoc developed combustion chamber. The results show almost half pollutants emissions and better performance compared to various non-catalytic counterparts, especially as far as CO and NO_x emissions are concerned. Some flame instability problems were though registered, especially for one of the catalytic burner mattresses employed, at low specific power inputs and excesses of air (<375 kW/m² and <12%, respectively). Further, PdO/Pd transition is shown to influence the dynamic behaviour of the catalytic burners.     

Catalytic combustion of methane over supported bimetallic Pd catalysts: Effects of Ru or Rh addition

A series of Pd/γ-Al₂O₃ catalysts with various amounts of Ru or Rh with, and/or without, BaO were prepared by successive incipient wetness impregnation. The catalysts were investigated for the catalytic methane combustion before, and after, H₂S poisoning in an oxygen-rich atmosphere. The addition of ruthenium enhanced the catalytic activity for methane oxidation even after H₂S poisoning while maintaining the initial catalytic activity of the fresh catalyst. These results are explained in terms of dispersion of palladium by ruthenium and poisoning resistance of ruthenium. The addition of rhodium did not improve the overall activity in methane oxidation.     

CO气相偶联合成草酸二甲酯Pd/α-Al2O3催化剂失活与再利用

作为合成气制乙二醇关键步骤之一,CO与亚硝酸甲酯合成草酸二甲酯备受关注。综述了近年来CO气相偶联合成草酸二甲酯Pd/α-Al₂O₃催化剂失活与再利用研究进展,探讨催化剂再利用工艺存在的问题,指出应根据在工业应用中出现的问题对Pd/α-Al₂O₃催化剂进行失活研究,在此基础上开发针对性的再生工艺;钯催化剂回收方面萃取法和吸附法逐渐成为研究重点,高效、低耗、短流程绿色工艺的开发是失活钯催化剂再利用的发展方向。