NO SCR reduction by hydrogen generated in line on perovskite-type catalysts for automotive diesel exhaust gas treatment

This paper deals with the preparation (by solution combustion synthesis, SCS), characterization (by XRD, BET, FESEM and TPD/R analyses), catalytic activity evaluation (in a temperature-programmed reaction—TPRe apparatus), and the assessment of the reaction mechanism of NO reduction by H₂ in the presence of oxygen on a series of perovskite-type catalysts belonging to the LaFeO₃ family (LaFeO₃, La_0.8Sr_0.2FeO₃, Pd/La_0.8Sr_0.2FeO₃, La_0.8Sr_0.2Fe_0.9Pd_0.1O₃, La_0.7Sr_0.2Ce_0.1FeO₃, Pd/La_0.7Sr_0.2Ce_0.1FeO₃, La_0.7Sr_0.2Ce_0.1Fe_0.9Pd_0.1O₃). The catalysts have been studied in the 25-350 °C temperature range. Significant catalytic activities were measured at 150-250 °C. Among the catalysts screened, La_0.8Sr_0.2Fe_0.9Pd_0.1O₃, showed the best performance. Hence, it was deposited directly over a ceramic honeycomb monolith by in situ SCS and then tested in a lab-scale test rig. A mechanistic analysis is presented concerning the relationship between the observed activity and the reducibility of the B site, determined from TPR experiments, as well as the correlation between the observed oxygen inhibition and the proposed NO_x reduction mechanism.Some final conclusions are drawn on the perspective of the practical application of the investigated after-treatment route for diesel exhaust gases.     

异氟尔酮在蛋壳型Pd/Al2O3催化剂上的选择性加氢

采用等体积浸渍法制备出蛋壳型Pd/Al2O3催化剂,采用连续加氢固定床微反装置研究了异佛尔酮(IP)的选择性加氢反应,考察了载体焙烧温度、蛋壳层厚度、Pd负载量及粒子大小、溶剂等反应条件对Pd/Al2O3催化剂上IP选择性加氢的影响.结果表明,蛋壳型Pd/Al2O3是IP选择性加氢制备3,3,5-三甲基环己酮(TMCH)的优异催化剂,反应条件缓和,IP转化率及TMCH选择性均可达到99.5％以上,具有良好的工业应用前景.     

Separation of hydrogen adsorption and absorption on Pd thin films

Hydrogen sorption at Pd films of 20-80 nm deposited on a polycrystalline gold electrode was studied in sulfuric and perchloric acid. Assuming that the hydrogen adsorption does not vary with the Pd films thickness, hydrogen adsorption/absorption charges in Pd were separated in the two contributions in the hydrogen-poor α-Pd-H phase. The results are compared to those obtained at Pd monolayers on Au(1 1 1). The adsorption on polycrystalline Pd begins at potentials more negative than on 0.8 ML Pd on Au(1 1 1) and is not much affected by the nature of anion (sulfate or perchlorate), contrary to the thin layers on Au(1 1 1). The absorption charge in α-PdH phase in the potential range of 0.08-0.15 V was found to be similar to that at a 25 μm Pd foil in this potential range while at more positive potentials it is larger. In the presence of crystal violet which adsorbs at the electrode surface it was found that some residual H adsorption exists. There is more hydrogen absorbed in Pd in the presence of crystal violet in the hydrogen-poor α phase but in the hydrogen-rich β phase the amount of hydrogen is the same.     

Vapour phase hydrogenation of olefins by formic acid over a Pd/C catalyst

It has been found that ethylene and propylene could be effectively hydrogenated by formic acid vapour over a Pd/carbon catalyst at low temperatures (<440 K). Surface hydrogen formation from formic acid is the rate-determining step for this hydrogenation reaction. Interaction of this hydrogen with the olefins is then fast. The conversion of formic acid in the presence of either of the olefins at any temperature is higher than in their absence. This has been explained by a much lower surface hydrogen concentration in the presence of the olefins. Direct experiments have confirmed that hydrogen inhibits the formic acid decomposition. Water vapour addition has a small positive effect on the decomposition of formic acid as well as on the hydrogenation of the olefins with formic acid. Catalysts consisting of gold supported on carbon or titania are both active in the production of hydrogen from formic acid. However, in contrast to the Pd/C catalyst, neither gives hydrogenation of the olefins with this acid.     

Electrochemical generation of hydrogen peroxide using surface area-enhanced Ti-mesh electrodes

Ti-mesh electrodes coated with Ti were obtained by using an electrophoretic deposition (EPD) method. The Ti coating was porous and showed a good adherence to the Ti-mesh surface, due to sintering of Ti particles during thermal treatment at 900 °C. The Ti-coated mesh electrode has a BET surface area of 3.5 m²/g, about four times larger than that of the bare electrode. The surface area-enhanced Ti-mesh electrode was applied in electrical generation of hydrogen peroxide. It was shown that the rate of hydrogen peroxide generation increased drastically compared to the fresh electrode, since the larger electrode surface area enhanced not only current density, but also the oxygen mass transfer rate.     

Hydrogenation of olefins catalyzed by Pd(II) complexes containing a perfluoroalkylated S,O-chelating ligand in supercritical CO2 and organic solvents

Pd(II) complexes with the tridentate ligands heptadecafluorodecyl-thiophene-2-carboxylate (L₁ = SOSR_f1, R_f1 = (CH₂)₂(CF₂)₇CF₃) and heptadecafluorononil thiophene-2-carboxylate (L₂ = SOOR_f2, R_f2 = (CH₂)(CF₂)₇CF₃) have been synthesized, and their catalytic activities for the homogeneous hydrogenation of olefins have been studied with molecular hydrogen in scCO₂ and organic solvents. Both perfluoroalkylated-heterocycle ligands potentially have three donor atom sets (SOO/SOS). Depending on the soft-hard interaction between palladium and the donor atoms of the ligands, different activities for the Pd(II) complexes were observed. The nature of the solvent had an effect upon the hydrogenation of olefins. The effects of temperature and H₂ pressure on the hydrogenation of styrene were also investigated.     

裂解汽油一段加氢Pd系和新型Ni系催化剂加氢性能对比及影响因素浅析

对裂解汽油一段加氢Pd系和新型Ni系催化剂,通过工业应用情况分析和实验室加氢性能评价,催化剂物性分析表征和热重分析认为:新型Ni系催化剂加氢和抗结炭性能优异,此优良的性能在于新型Ni系催化剂的载体结构和活性相结构特征。改进载体孔结构可借助大孔容焦作用提高Pd系催化剂的抗积炭性能,但不能实现长周期运转。对杂质含量较高的原料,新型Ni系催化剂适应性良好。     

Electrochemical sensors for detection of hydrogen in air: model of the non-Nernstian potentiometric response of platinum gas diffusion electrodes

The potentiometric response of three different platinum gas diffusion electrodes deposited on H₃PO₄ doped polybenzimidazole (PBI) was investigated under humidified atmospheres that contained H₂ or mixtures of H₂ and O₂. Continuum modelling was used to analyse the response. It is shown that the non-Nernstian response under H₂H₂ON₂ mixtures can be explained by a difference of water activity on both sides of the membrane. Under H₂O₂N₂ mixtures, the oxygen mass transport parameters have a strong effect on the electrode sensitivity.     

Hydrogenation of 2-Butyn-1,4-diol in the Presence of Functional Crosslinked Resin Supported Pd Catalyst. The Role of Polymer Properties in Activity/Selectivity Pattern

Functional gel type resins of various crosslinking degrees (3–20%) with C=O and carboxylic groups were used as the supports for Pd catalysts (0.5–2 wt% Pd). The role of polymer properties was studied in the hydrogenation of 2-Butyne-1,4-diol (B3-D) to alkene (B2-D) and alkane (B1-D). Hydrogenation was studied at atmospheric pressure of hydrogen using THF, H₂O and THF + H₂O mixtures as the solvents. Systematic studies were carried out to determine the role of the type of solvent, crosslinking degree of polymer, the content of Pd in catalysts, initial B3-D concentration and the procedure of catalyst reduction in activity/selectivity behaviour of catalysts. Swelling degree of polymer matrix under the catalytic run exhibits crucial role in the activity and selectivity to alkene, B2-D. In the presence of highly expanded catalyst (THF solvent, 3% crosslinking degree, 1 wt% Pd) the alkyne, B3-D, is hydrogenated to alkene, B2-D, with selectivity ca. 85% up to high B3-D conversion (90%). The suppression of alkene to alkane hydrogenation in the stage of B3-D is ascribed to high ability of Pd centres in the Pd/OFP catalysts to strong adsorption of alkyne substrate. It may also be related to steric hindrances of polymer in the vicinity of active Pd centres. At small content of added water (5% by vol.) to THF the catalysts offer very attractive performance in terms of activity and 98% selectivity to alkene. Water facilitates interactions of B3-D with functional groups of polymer that leads to better expansion of polymer matrix and more effective suppression of alkene hydrogenation in the alkyne stage.     

Monitoring hydrogen absorption in Pd electrodes by means of electric and electrochemical signals

A reliable, nondestructive methodology to determine the actual hydrogen content in hydrogen storage electrodes has been developed. This methodology is based on relative electric resistance measurements, backed up by measurements of electrode potential, since both quantities are or are derived from electric signals, easy to handle and use in automatic data acquisition lines. The setup and procedure for relative resistance measurements, both under electrolysis and at open circuit conditions, will be described and discussed in terms of possible error sources and their relevant uncertainties, on the basis of the results obtained for the Pd–H system, chosen as model system. The use of electric and electrochemical signals to determine the current–potential distribution along the electrode as a function of the cell geometry and the working conditions will also be presented.     

Use of Water as a Solvent in Directing Hydrogenation Reactions of Aromatic Acids over Pd/carbon Nanofibre Catalysts

By conducting the catalytic hydrogenation over water miscible Pd/carbon nanofibre catalysts, selective ring hydrogenation of aromatic acids can be performed in aqueous solution without the need to provide protection for the external acid function.     

Hydrogen recovery from cyclohexane as a chemical hydrogen carrier using a palladium membrane reactor

A palladium membrane reactor was applied to recover the hydrogen from cyclohexane as one of the promising chemical hydrogen carriers. The operation conditions of the palladium membrane reactor to obtain a higher hydrogen recovery were predicted by computer simulation. As a result, it was shown that the hydrogen recovery rate becomes higher as the pressure on the hydrogen permeation side is lowered below atmospheric pressure or as the reaction pressure increases. This was confirmed experimentally. As the perm-side pressure was lowered, the conversion as well as the hydrogen recovery rate at 573 K was found to increase. About 80% of the hydrogen contained in cyclohexane, depending on the operation condition was successfully recovered.     

Hydrogen oxidation kinetics on model Pd/C electrodes: Electrochemical impedance spectroscopy and rotating disk electrode study

This work reports on the kinetics of the hydrogen oxidation reaction (HOR) on model Pd nanoparticles supported on a low surface area carbon substrate. Two Pd/C samples, with the average particle size 2.6 and 4.0 nm were used. The structure of the catalysts was characterized with the ex situ (electron microscopy) and in situ (electrochemical) methods. We utilized the electrochemical impedance spectroscopy (EIS) and the rotating disk electrode (RDE) voltammetry to study the kinetics of the HOR on Pd/C. The relevance of these techniques for elucidating the kinetics and the mechanism of the HOR on Pd/C was explored. The experimental results suggest that the catalytic activity of Pd in the HOR is more than 2 orders of magnitude lower than that of Pt, and does not depend on the particle size in the range from 2.6 to 4.0 nm. Computational modeling of the experimental steady-state (RDE) and non-steady-state (EIS) data shows that the reaction kinetics can be adequately described within Heyrovsky-Volmer mechanism, with the rate constants υ_0H = (8.8 ± 1.5) × 10⁻¹⁰ mol cm⁻² s⁻¹ and υ_0V = (1.0 ± 0.3) × 10⁻⁸ mol cm⁻² s⁻¹. The model suggests that underpotentially deposited hydrogen H_UPD is unlikely to be the active intermediate H_ad of the HOR. It is concluded that the surface coverage of H_ad deviates from that of H_UPD with increasing overpotential, and the lateral interactions within H_ad adlayer are weak.     

Direct synthesis of hydrogen peroxide from hydrogen and oxygen over a Pd core-silica shell catalyst

Pd core–silica shell particles (Pd@SiO₂) were prepared by encapsulating Pd colloids with a silica shell through the Stöber method. The palladium core particles were well dispersed (Dispersion = 43%) and had uniform size (4 nm) and shape inside the porous silica shell. Pd@SiO₂ showed good catalytic activity (554 mmol H₂O₂/g Pd·h) for the direct synthesis of H₂O₂, which was better than those of impregnated Pd catalysts (Pd/SiO₂ and Pd/Al₂O₃). It is expected that the stabilization of less coordinated Pd crystals in a highly dispersed state by core-shell formation is effective for the improvement of H₂O₂ production.     

Improved performances of electrodes based on Cu-loaded copper hexacyanoferrate for hydrogen peroxide detection

Copper hexacyanoferrate thin films were potentiodynamically deposited on glassy carbon electrodes and employed for the chronoamperometric detection of hydrogen peroxide at 0.0 V vs. SCE. A new experimental procedure based on the intercalation of copper ions inside the structure of the films led to sensors with improved performances. The modified electrode displayed an increased sensitivity to hydrogen peroxide detection together with a wider linear response range and a lower detection limit. Furthermore, even if the electrode response decreased with use, the initial activity could be restored by performing a new intercalation procedure.     

The study of hydrogen electrosorption in layered nickel foam/palladium/carbon nanofibers composite electrodes

In the present work, the process of hydrogen electrosorption occurring in alkaline KOH solution on the nickel foam/palladium/carbon nanofibers (Ni/Pd/CNF) composite electrodes is examined. The layered Ni/Pd/CNF electrodes were prepared by a two-step method consisting of chemical deposition of a thin layer of palladium on the nickel foam support to form Ni/Pd electrode followed by coating the palladium layer with carbon nanofibers layer by means of the CVD method. The scanning electron microscope was used for studying the morphology of both the palladium and carbon layer. The process of hydrogen sorption/desorption into/from Ni/Pd as well as Ni/Pd/CNF electrode was examined using the cyclic voltammetry method. The amount of hydrogen stored in both types of composite electrodes was shown to increase on lowering the potential of hydrogen sorption. The mechanism of the anodic desorption of hydrogen changes depending on whether or not CNF layer is present on the Pd surface. The anodic peak corresponding to the removal of hydrogen from palladium is lower for Ni/Pd/CNF electrode as compared to that measured for Ni/Pd one due to a partial screening of the Pd surface area by CNF layer. The important feature of Ni/Pd/CNF electrode is anodic peak appearing on voltammetric curves at potential ca. 0.4 V more positive than the peak corresponding to hydrogen desorption from palladium. The obtained results showed that upon storing the hydrogen saturated Ni/Pd/CNF electrode at open circuit potential, diffusion of hydrogen from carbon to palladium phase occurs due to interaction between carbon fibers and Pd sites on the nickel foam support.     

Electrogeneration of hydrogen peroxide in seawater and application to disinfection

The cathodic electrogeneration of hydrogen peroxide in seawater by means of oxygen reduction on a gas diffusion cathode was studied. The effects on the reaction yield of several operative parameters such as cell design, medium composition, anolyte concentration, pH and working potential were investigated. Results indicate that in a two-compartment cell notable concentrations of hydrogen peroxide are obtained with a constant yield in a wide range of charge. Lower catholyte pH values, obtainable by means of the anolyte choice, mitigate the decrease in the efficiency due to cathode fouling. Application of hydrogen peroxide electrogeneration to seawater disinfection was also tested. Comparative tests conducted using both commercial and electrogenerated hydrogen peroxide, either alone or combined with iron in Fenton’s treatment, are also presented.     

Enhanced electrochemical reduction of hydrogen peroxide on silver paste electrodes modified with surfactant and salt

The modification of silver paste electrodes with a combination of dodecylbenzenesulfonic acid and KCl has been shown to lead to significant enhancements of the electrochemical reduction of hydrogen peroxide. The catalytic enhancement was shown to be dependent on the concentration of the surfactant/salt solution, which resulted in increases of some 80-fold in amperometric response to hydrogen peroxide at −0.1 V vs Ag/AgCl, pH 6.8 over unmodified silver paste. Physical analysis showed modifications to both the surface morphology and chemical composition of the silver paste electrode surface. However, BET and electrochemical analysis revealed no significant change in surface area. It is suggested that the enhanced catalysis may result from the formation of stabilised surfactant/salt structures at the metal electrode surface. The electrode was also shown to be suitable for the amperometric detection of hydrogen peroxide with a limit of detection of 1.1 × 10⁻⁶ M (S/N = 3).     

An integrated purification and production of hydrogen with a palladium membrane-catalytic reactor

In this paper, we presented an integrated production and purification process of hydrogen by the use of a defect-free palladium membrane. Hydrogen could be purified from a variety of mixtures providing the purity of 3–7 N depending on the feeding stream. The permeation parameters are accurately predicted by a separation model as established. The membrane is prepared by electroless plating and is stable among 300–400°C. Using an active catalyst, the rate of steam reforming of methanol was found to be significantly faster than that without a membrane module. In the steam reforming of methane, the reaction temperature was lowered to 500°C to achieve a conversion of 45%, which is 15% higher than the thermodynamic equilibrium conversion.     

Two conversion maxima at 373 and 573 K in the reduction of nitrogen monoxide with hydrogen over Pd/TiO2 catalyst

Supporting Pt and Pd catalysts have been examined for the reduction of NO with H₂ in the presence of oxygen and moisture. All catalysts showed a conversion maximum in the NO reduction at around 373 K. An additional conversion maximum was found to appear at around 573 K over several metal oxides supporting Pd, and Pd/TiO₂ gave the highest conversion at around 573 K among the catalysts tested. In the reaction at 373 K, NO might be reduced directly by H₂ both on Pt and Pd catalysts to give N₂ and N₂O. At the conversion maximum of the Pd/TiO₂ catalyst at 575 K, however, in situ generated NO₂ seems to react with H₂.