Confirmation of sulfur tolerance of bimetallic Pd–Pt supported on highly acidic USY zeolite by EXAFS

The sulfur tolerance (i.e., degree of sulfidation) of Pd and Pt in sulfided bimetallic Pd–Pt catalysts (Pd : Pt mole ratio of 4 : 1) supported on USY (ultrastable Y) zeolites (SiO₂/Al₂O₃ = 10.7, 48, and 310) was investigated using an extended X‐ray absorption fine structure (EXAFS) method. The sulfidation of the catalysts was done in a 1000 ppm H₂S–2% H₂/N₂ stream at 573 K for 0.5 h. In the Fourier transforms of Pd K‐edge and Pt L_III‐edge EXAFS spectra, both of the peaks due to metallic Pd and to metallic Pt for the Pd–Pt/USY (SiO₂/Al₂O₃ = 10.7) catalyst remained most after sulfidation. Further, the results of the Fourier transforms confirmed that the sulfur tolerance of both Pd and Pt decreased with increasing SiO₂/Al₂O₃ ratio, suggesting that Pd and Pt become sulfur‐tolerant when Pd–Pt bimetallic particles are supported on highly acidic USY zeolite. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of calcination temperature on the hydrolysis of carbonyl sulfide over hydrotalcite-derived Zn–Ni–Al catalyst

A novel catalyst for low temperature hydrolysis of carbonyl sulfide (COS) was prepared by thermal decomposition of Zn–Ni–Al hydrotalcite-like compounds (HTLCs). As the key factors of catalyst activity, effects of calcination temperature have been studied. The samples were carefully characterized by XRD, FTIR, SEM, CO₂-TPD and N₂ adsorption/desorption. Results showed that HTLCs calcined at 350 °C exhibited excellent activity due to the production of more M–O pairs which are active sites of the hydrolysis of COS. However, calcination at 500 °C led to the destruction of pore structure and reduction of active sites, ultimately led to a lower COS conversion.     

Kinetics of methane combustion over Pt and Pt–Pd catalysts

A kinetic study was performed over thermally aged and steam-aged Pt and Pt–Pd catalysts to investigate the effect of temperature, and methane and water concentrations on the performance of catalysts in the range of interest for environmental applications. It was found that both catalysts permanently lose a large portion of their initial activity as result of exposure to 5 vol.% water in the reactor feed. Empirical power-law and LHHW type of rate equations were proposed for methane combustion over Pt and Pt–Pd catalysts respectively. Optimization was used to determine the parameters of the proposed rate equations using the experimental results. The overall reaction orders of one and zero in methane and water concentration was found for stabilized steam-aged Pt catalyst in the presence and absence of water. The apparent self-inhibition effect caused by methane over Pt–Pd catalyst in the absence of water was associated with the inhibiting effect of water produced during the combustion of methane. A significant reversible inhibition effect was also observed over steam-aged Pt–Pd catalyst when 5 vol.% water vapor was added to the reactor feed stream. A significant reduction in both activity and activation energy was observed above temperatures of approximately 550 °C for steam-aged Pt–Pd catalyst in the presence of water (the activation energy dropped from a value of 72.6 kJ/mol to 35.7 kJ/mol when temperature exceeded 550 °C).     

Kinetic behavior of hydrogenation of aromatics in diesel fuel over silica–alumina-supported bimetallic Pt–Pd catalyst

The kinetics and long-term stability test of the aromatic hydrogenation of diesel fuel were studied on SiO₂–Al₂O₃ supported bimetallic Pt–Pd catalyst. The tests on the influence of operating parameters and kinetics studies were carried out using Pt (0.5 wt.%)–Pd (1.0 wt.%)/SiO₂–Al₂O₃, which provided the highest catalytic activity in a previous paper [Applied Catalysis A: General, 192 (2000) 253] with hydrotreated light cycle oil (LCO)/straight-run light gas oil (SRLGO) feedstocks containing 30 vol.% aromatics/100 wppm sulfur and 34 vol.% aromatics/420 wppm sulfur under numerous conditions. The results on this catalyst obtained at different LHSV showed an excellent fit to first-order kinetics. The apparent activation energy was determined to be 92 kJ/mol. Long-term stability test demonstrated the excellent stability of this catalyst. The products during the long-term stability test are of good quality, with the upgraded color, the increased cetane index, and sufficiently decreased sulfur content.     

Steam reforming of methanol over Pt–Zn alloy catalyst supported on carbon black

Steam reforming of methanol over Zn-promoted Pt catalyst supported on an electrically conductive carbon black has been investigated after H₂ reduction at 873 K. X-ray diffraction measurement showed that Pt–Zn alloy was formed on the carbon black (C). The Zn-promoted Pt/C catalyst showed higher activity and selectivity to CO₂ compared with unpromoted Pt/C catalyst. Methyl formate was formed over the Zn-promoted Pt/C catalyst in decomposition of methanol (without water). This suggests that steam reforming of methanol over the Zn-promoted Pt/C catalyst can proceed via methyl formate, which is different from that of the unpromoted Pt/C catalyst.     

NO presence effects on the reduction of N2O by CO over Al–Pd–Co oxide catalyst

Nitrous oxide (N₂O) is known as one of the greenhouse gases of which the emission levels are to be controlled and also an ozone-depleting material in the stratosphere. For more than a last couple of decades, various catalysts have been investigated for the reduction of N₂O emission. However, most of those catalysts require reaction temperatures as high as 450 °C even with the use of effective reducing agents. Furthermore, NO, which is usually present with N₂O, significantly interferes with the removal efficiency of N₂O. Al–Pd–Co oxide catalyst which is a type of mixed metal oxides (MMO) has shown 100% conversion of N₂O at temperatures as low as 200 °C using CO. This paper examines the effect of NO on the reduction of N₂O with CO over Al–Pd–Co oxide catalyst. The experiments were carried out in the temperature range of 100–500 °C with space velocity of 10,000–50,000 h^?1. Though the efficiency of N₂O reduction is decreased significantly when NO is present, the efficiency increases when sufficient CO is supplied. In this study, the reduction mechanisms of N₂O and NO by CO have been confirmed, and it was shown that MMO catalyst can simultaneously remove N₂O and NO using CO with high efficiency.     

Microstructure of a Pd/ceria–zirconia catalyst after high-temperature aging

Ga/ZSM-5 is an effective catalyst for the conversion of dilute (3%) ethylene-in-methane reactant streams into aromatic hydrocarbons at 500–550°C. A Ga loading as low as 0.5 wt% is sufficient to obtain maximum yields of aromatic products. At 520°C, an ethylene conversion of 93%, with an aromatics selectivity of 81%, was obtained over a 5 wt% Ga/ZSM-5 catalyst. The conversion of ethylene into aromatics over Ga/ZSM-5 catalysts involves a complex sequence of oligomerization, isomerization, cracking, and cyclization reactions that occur on Brønsted acid zeolites in the zeolite. The role of the gallium, which exists as both Ga³⁺ at zeolitic exchange sites and as Ga₂O₃ within the channels and on the external surface of the calcined catalyst, is to promote dehydrogenation of the acid-catalyzed oligomerization and cyclization products.     

Influence of the crystalline structure of ZrO2 on the metallic properties of Pt in Pt/WO3–ZrO2 catalysts

The influence of the crystalline structure of ZrO₂ on the metallic properties of Pt, when supported on WO₃–ZrO₂, was studied. Pt supported on tetragonal zirconia loses its metallic properties while when supported on monoclinic zirconia it presents good metallic activities. WO₂,^2- deposited on amorphous Zr(OH)₄ before calcination generates an active material for n‐butane isomerization. The larger the fraction of the tetragonal phase of zirconia in this material, the higher the isomerization activity and the lower the metallic activity of Pt. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pd oxidation under UHV in a model Pd/ceria–zirconia catalyst

A model planar catalyst was prepared by depositing Pd onto a thick (few m) film of ceria–zirconia in ultrahigh vacuum (UHV), and the oxidation state of Pd and its support was determined by X-ray photoelectron spectroscopy, following thermal treatments in UHV, oxygen, or carbon monoxide. It was found that Pd could be oxidized simply by heating the catalyst in UHV, indicating that transfer of oxygen from the support to the metal is both thermodynamically favorable and facile.     

Pd–Pt bimetallic catalyst supported on SAPO‐5 for catalytic combustion of diluted methane in the presence of water vapor

The examination of the combustion of a trace amount of methane over Pd‐ion‐exchanged silicoaluminophosphate‐5 (Pd‐SAPO‐5) at low temperature in the presence of water vapor reveals that water vapor strongly depresses methane combustion and causes a significant decrease in catalytic activity with time. The newly prepared bimetallic catalyst Pd–Pt‐SAPO‐5, on the other hand, shows higher performance for methane combustion under similar conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Liquid phase catalytic hydrodechlorination of aryl chlorides over Pd–Al-MCM-41 catalyst

Supported Pd catalysts were prepared by direct hydrothermal (DHT) or template-ion exchange (TIE) method on Al-substituted MCM-41 as the support and tested in the hydrodechlorination of aryl chlorides such as 4-chloroanisole in liquid phase, together with impregnated Pd catalysts as references. When Pd was loaded on Al-MCM-41 with the Si/Al ratio of 150 by the TIE method, the catalyst showed the highest activity among the Pd catalysts prepared. PdO originally formed on the catalyst surface was in situ reduced to Pd metal during the reaction as the active form for the dechlorination. The activities of the supported Pd catalysts well correlated with the Pd dispersion on the TIE or impregnated Pd catalysts, whereas the DHT catalysts showed relatively high activities independently on the Pd dispersions. It was confirmed that the dechlorination proceeded by a heterogeneous mechanism catalyzed by Pd metal particles sized less than 10 nm on the surface of the catalyst. Al substitution for Si on MCM-41 was effective for the loading of Pd metal with the high dispersion, none the less Pd was located on the surface of the TIE catalyst particles and no significant effect of mesoporous structure on the reaction was observed. Methanol was the most profitable as the solvent among the various solvents tested. Various types of arylchlorides bearing hydroxy, methoxy, methyl, nitro and phenylcarbonyl group at the p-position were efficiently dechlorinated over Pd–Al-MCM-41 catalyst at 40 °C. Electrophilic attack of arylchloride was proposed as the rate determining step, where ionic mechanism positively worked and electron-releasing substituents increased the catalytic activity.     

New catalytic functions of Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys in the conversions of methanol

Pd and Pt supported on ZnO, Ga₂O₃ and In₂O₃ exhibit high catalytic performance for the steam reforming of methanol, CH₃OH+H₂OCO₂+3HH₂, and the dehydrogenation of methanol to HCOOCH₃, 2CH₃OHHCOOCH₃+2HH₂. Combined results with temperature-programmed reduction (TPR) and XRD method revealed that Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys were produced upon reduction. Over the catalysts having the alloy phase, the reactions proceeded selectively, whereas the catalysts having metallic phase exhibited poor selectivities.     

Influence of CO2 co-feeding on Fischer–Tropsch fuels production over carbon nanofibers supported cobalt catalyst

Nowadays, the syngas which is obtained from the reforming of coal, biomass or natural gas contain significantly amounts of CO₂ that cannot be separated and consequently, it can take part into the Fischer–Tropsch (FTS) catalytic activity. Therefore, the presence of CO₂ in the syngas flow should be taken into account. In the present study, the FTS CO hydrogenation process was compared to that of CO₂ on a carbon nanofibers supported Co catalyst. The influence of CO₂ content in the feed stream (H₂/CO/CO₂ ratio) on the reaction performance in terms of conversion and selectivity to the different products was described. Both the support and the prepared catalyst were characterized by nitrogen adsorption–desorption, temperature-programmed reduction (TPR) and X-ray diffraction (XRD). Results showed that CO hydrogenation was controlled by a Fischer–Tropsch regime, whereas CO₂ hydrogenation was controlled by a methanation process. When feed was composed of CO and CO₂ mixtures, the catalytic activity decreased with respect to that obtained with a CO₂-free feed stream. Moreover, the presence of CO₂ in feed stream favored the formation of lighter hydrocarbons and could block the production of further CO₂ via Water-Gas-Shift (WGS) reaction.     

Dehydrogenation of methylcyclohexane over Pt supported on Mg–Al mixed oxides catalyst: The effect of promoter Ir

To enhance the hydrogen release during hydrogen storage, several Pt–Ir supported on Mg–Al mixed oxide catalysts were prepared and then applied into the dehydrogenation of methylcyclohexane (MCH) in this study. The effects of iridium content, reduction temperature on the activity and stability of the catalysts were studied in detail. In the presence of Ir, metal particle size was decreased and electron transfer between Ir and Pt was observed. High reduction temperature increased the metallic Ir content but enlarged the particle size of active sites. During the dehydrogenation reaction on Pt–Ir bimetallic catalyst, MCH was efficiently converted into toluene and PtIr-5/Mg–Al-275 exhibited the highest activity. After prolonging the residence time and raising the reaction temperature to 350 °C, the conversion and hydrogen evolution rate were increased to 99.9% and 578.7 mmol·(g Pt)⁻¹·min⁻¹, respectively. Moreover, no carbon deposition was observed in the spent catalyst, presenting a high anti-coking ability and good potential for industrial application.     

The effect of oxygen on the aromatization of methane over the Mo/HZSM‐5 catalyst

The aromatization of methane over a Mo/HZSM‐5 catalyst was carried out in the presence of oxygen. It is shown that the addition of a small amount of oxygen is beneficial to improve the durability of the catalyst. UV‐Raman spectra disclose that the carbonaceous deposits formed on the HZSM‐5 are mainly polyolefinic and aromatic, while that on the Mo/HZSM‐5 is mainly polyaromatic. The small amount of O₂ added may partly remove the coke deposits on the active sites and keep the catalyst as MoO_xC_y/HZSM‐5, thus resulting in an improvement of the catalytic performance of the Mo/HZSM‐5 catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrogen–deuterium isotope effects in the reactions of chlorobenzene and benzene on a Pt/γ-Al2O3 catalyst

The kinetic isotope effect for combustion of a C₆H₅Cl/C₆D₅Cl mixture on Pt/-Al₂O₃ was found to be close to unity between 520 and 580 K. However, in the presence of an excess of heptane, an isotope effect of 1.5 was found between 460 and 490 K. For the combustion of a C₆H₆/C₆D₆ mixture the k_H/k_D value was around 2 between 404 and 439 K. The results show that in the combustion of chlorobenzene per se, C–H bond activation is not a rate-determining step. On Pt sites, C–Cl bond scission probably occurs already at low temperatures. The chlorine and the phenyl group cannot easily react further. Chlorine on the surface is active in chlorination, which is shown by the formation of C₆D₅Cl in an experiment with C₆H₅Cl and C₆D₆. Only at a certain temperature is the chlorine removed, partly as polychlorinated benzenes. The removal of chlorine from the catalyst allows oxygen to take part in the reaction, which determines the rate of the combustion of chlorobenzene. When heptane is present, Cl is removed from the surface and C–H bond scission can become rate determining, as is also the case in the combustion of C₆H₆/C₆D₆. Upon (partial) combustion of C₆H₅Cl/C₆D₅Cl and C₆H₆/C₆D₆ mixtures on a Pt/-Al₂O₃ catalyst, hydrogen–deuterium exchange occurs on the -Al₂O₃ support.     

Influence of Indium Content on the Properties of Pt–Re/Al2O3 Naphtha Reforming Catalysts

The influence of indium on the properties of Pt–Re/Al₂O₃ catalysts used in naphtha reforming is studied. The addition of indium to the Pt–Re/Al₂O₃ catalyst produces a big decrease of acidity. It also produces an inhibition of the metal function, i.e., dehydrogenation and hydrogenolysis activity. The reaction of n-C₅ isomerization shows that indium addition decreases the total activity of the Pt–Re catalyst but increases the selectivity to the i-C₅ isomers. The selectivity to low cost light gases (C₁–C₃) is particularly decreased. The reaction of n-C₇ reforming showed that addition of indium increases the stability of the catalyst and the selectivity to aromatics, and decreases the production of light gases.     

Influence of Cesium in Pt/NaCsβ on the Physico-Chemical and Catalytic Properties of the Pt Clusters in the Aromatization ofn-Hexane

Various Pt/NaCsβ catalysts with decreasing Na/Cs ratios and a Pt/KL sample taken as a reference catalyst for aromatization ofn-hexane, have been prepared by exchange of NaCsβ and KL zeolites in a Pt tetraammine solution then calcination and reduction. The increase of the Cs content in the NaCsβ zeolitic support results in a decrease of the cyclohexane adsorption and Pt exchange capacities. The Pt/Csβ and Pt/KL samples show similar behaviors which strongly differ from those of the Na-containing Pt/NaCsβ samples in terms of (i) reducibility of the Pt ions after calcination, (ii) shape of the FTIR spectra of CO adsorbed on the Pt clusters after reduction, and (iii) catalytic activity of these clusters in the conversion ofn-hexane. In particular, the selectivity to aromatization is much higher on the Pt/Csβ and Pt/KL catalysts than on the Na-containing Pt/NaCsβ ones. The reasons for the specific behaviors of the Pt/Csβ and Pt/KL samples are discussed.     

Benzylacetate synthesis by oxyacetoxylation of toluene on Pd–Bi binary catalyst

Benzylacetate synthesis from toluene, acetic acid and oxygen on Pd–Bi binary catalyst was studied in the liquid phase. By incorporation of Bi with Pd, both the activity and selectivity were improved. Especially better stability was obtained with the catalyst having Pd/Bi = 3. Deactivation of the catalyst was investigated in detail by XRD, XPS, TEM, elemental analysis, EPMA and so on. Comparing the used catalyst with the fresh one, it was indicated that the main cause of deactivation was the dissolution of Pd into the reaction mixture from the most outer surface of the catalyst. By adopting proper reaction conditions to prevent the Pd dissolution, the catalyst having Pd/Bi = 3 was suggested to be used as an industrial catalyst. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of the support composition on the hydrogenation of acrolein over Ag/SiO2–Al2O3 catalysts

The gas phase hydrogenation of acrolein over supported silver catalysts has been investigated with a focus on the influence of the support acidity. Acidity has been varied by preparing silver catalysts supported on silica/alumina supports with varying SiO₂/Al₂O₃ ratio. After the catalytic experiments the Ag catalysts exhibit similar particle sizes, as revealed with TEM (transmission electron microscopy). The acidity of the samples was estimated using TPD of adsorbed ammonia which gives the total acidity of the samples, furthermore by IR of adsorbed pyridine to identify the Brønsted and Lewis acidity. No Brønsted acidity was found, and the Lewis acidity showed a clear dependence on the support composition. It is shown that a high total acidity and a high amount of strong Lewis acid sites on the catalysts cause a low conversion of acrolein and low selectivity to allyl alcohol. The interaction of silver with the support or effects of the metal–support perimeter are discussed as possible reasons for this behaviour.