Characterization and reactivity of MoO3 supported on AlPO4

A series of MoO₃/AlPO₄ catalysts with molybdena content varying from 2 to 16 wt% were prepared and characterized by low temperature oxygen chemisorption (LTOC), ammonia chemisorption, X-ray diffraction (XRD) and electron spin resonance (ESR). Maximum O₂ uptake was observed at 6 wt% MoO₃ loading indicating the completion of monolayer. The ESR results are in conformity with LTOC and XRD data. The activities of the catalysts were tested in methanol partial oxidation and are correlated with their surface characteristics wherever possible.IICT Communication No. 3188.     

Pt-CeO2/C anode catalyst for direct methanol fuel cells

In order to develop a cheaper and durable catalyst for methanol electrooxidation reaction, ceria (CeO₂) as a co-catalytic material with Pt on carbon was investigated with an aim of replacing Ru in PtRu/C which is considered as prominent anode catalyst till date. A series of Pt-CeO₂/C catalysts with various compositions of ceria, viz. 40 wt% Pt-3–12 wt% CeO₂/C and PtRu/C were synthesized by wet impregnation method. Electrocatalytic activities of these catalysts for methanol oxidation were examined by cyclic voltammetry and chronoamperometry techniques and it is found that 40 wt% Pt-9 wt% CeO₂/C catalyst exhibited a better activity and stability than did the unmodified Pt/C catalyst. Hence, we explore the possibility of employing Pt-CeO₂ as an electrocatalyst for methanol oxidation. The physicochemical characterizations of the catalysts were carried out by using Brunauer Emmett Teller (BET) surface area and pore size distribution (PSD) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. A tentative mechanism is proposed for a possible role of ceria as a co-catalyst in Pt/C system for methanol electrooxidation.     

Pt–Pd bimetallic catalysts for methane emissions abatement

Bimetallic Pd–Pt catalysts with constant 2 wt% metal loading and varying Pt/Pd ratios were prepared, characterized and studied in the catalytic combustion of methane at low temperature under lean conditions in view of their use for CH₄ abatement from lean-burn NGV heavy duty vehicles exhausts. The influence of mild steam ageing featuring long-term use of the catalysts was also addressed together with their tolerance to H₂S. Catalysts were characterised by Transmission Electron Microscopy and Temperature Programmed Oxidation experiments. Experimental data agreed to suggest an interaction between Pd and Pt in Pd-rich catalysts, thus explaining their improved catalytic activity, even after mild ageing, compared to reference Pd/Al₂O₃. This interaction has no effect on the sulfur tolerance.     

Reducibility and CO hydrogenation over Pt and Pt-Co bimetallic catalysts encaged in NaY-zeolite

Temperature programmed techniques (TPR, TPD) and X-ray diffraction (XRD) have been used to study ion migration and location as well as reducibility of platinum and cobalt ions encapsulated in Pt/NaY, Co/NaY and Pt-Co/NaY zeolites prepared by ion exchange. The temperature required to reduce Co²⁺ in NaY was significantly lowered by the presence of Pt and dependent upon the relative locations of Pt and Co ions in zeolite cages. The exact location was controlled by the calcination condition and the metal contents. For bimetallic catalyst with low Pt content (0.5 wt% Pt and 0.9 wt% Co), the TPR results indicated that reduction of Co²⁺ ions in the vicinity of Pt shifted toward lower temperature, while that of Co²⁺ staying alone was not affected. With high Pt loading (4.5 wt% Pt, 0.7 and 2.6 wt% Co), however, most of the Co²⁺ ions were reduced by means of Pt at temperature below 723 K after calcination at 573 K. The temperature for Pt reduction in bimetallic catalysts was somewhat higher than Pt/NaY and increased with Co atomic fraction, indicating that mixed oxide, PtCo _x O _y , might be formed during calcination. After reduction in hydrogen at 723 K, highly dispersed metal particles were formed. These fine particles were most probably confined inside zeolite cages as indicated by the absence of XRD peak for all samples after calcination and reduction. Surface composition of the bimetallic particles may be different for catalysts with similar Pt content but different Co loading. Accordingly, H/Pt ratios of 1.0 and 0.72 for catalysts with low and high Co content, respectively, were shown by hydrogen chemisorption. It was further supported by the increase in TPD peak intensity with Co loading in the high temperature range, which was related to the reoxidation of Co in bimetallic particles by surface hydroxyl groups. Preliminary results on CO hydrogenation demonstrated that activity and methanol selectivity were higher on Pt-Co bimetallic catalysts than either over monometallic Pt or Co catalyst, which was consistent with the Pt enhanced Co reduction and formation of Pt-Co bimetallic particles.     

Combined XPS and TPR study of sulfur removal from a Pt/BaO/Al2O3 NO x storage reduction catalyst

Pt/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts (1 wt% Pt, 10 wt%BaO) were sulfated under conditions simulating a real NSR catalyst operation. Comparative TPR and XPS studies of sulfur removal from Pt/Al₂O₃ and Pt/BaO/Al₂O₃ catalysts indicate that the sulfur removal from Al₂O₃ surface precedes reductive decomposition of BaSO₄ (250–400 °C). Barium sulfate decomposition started with further increase in desulfation temperature at the point of surface atomic ratio Ba:S = 1 (~450^o). Simultaneously, an intensive formation of sulfide species on the catalyst surface was observed. Thermodynamic analysis of the desulfation process allows us to hypothesize that barium sulfide formation may hinder sulfur removal under reducing conditions.     

Gold dispersed on TiO2 and SiO2: adsorption properties and catalytic behavior in hydrogenation reactions

Titania-supported Au catalysts were given both low temperature reduction and high temperature reduction at 473 and 773 K, respectively, and their adsorption and catalytic properties were compared to identically pretreated Pt/TiO₂ catalysts and pure TiO₂ samples as well as Au/SiO₂ catalysts. This was done to determine whether a reaction model proposed for methanol synthesis over metals dispersed on Zn, Sr and Th oxides could also explain the high activities observed in hydrogenation reactions over MSI (Metal-Support Interaction) catalysts such as Pt/TiO₂. This model invokes O vacancies on the oxide support surface, formed by electron transfer from the oxide to the metal across Schottky junctions established at the metal-support interface, as the active sites in this reaction. The similar work functions of Pt and Au should establish similar vacancy concentrations, and O₂ chemisorption indicated their presence. However, these Au catalysts were completely inactive for CO and acetone hydrogenation, and ethylene hydrogenation rates were lower on the supported Au catalysts than on the supports alone. Consequently, this model cannot explain the high rate of the two former reactions over TiO₂-supported Pt although it does not contradict models invoking specialinterfacial sites.     

Effect of the support on the mechanism of partial oxidation of methane on platinum catalysts

The partial oxidation of methane was studied on Pt/Al₂O₃, Pt/ZrO₂, Pt/CeO₂ and Pt/Y₂O₃ catalysts. For Pt/Al₂O₃, Pt/ZrO₂ and Pt/CeO₂, temperature programmed surface reaction (TPSR) studies showed partial oxidation of methane comprehends two steps: combustion of methane followed by CO₂, and steam reforming of unreacted methane, while for Pt/Y₂O₃ a direct mechanism was observed. Oxygen Storage Capacity (OSC) evaluated the reducibility and oxygen transfer capacity of the catalysts. Pt/CeO₂ catalyst showed the highest stability on partial oxidation. The results were explained by the higher reducibility and oxygen storage/release capacity which allowed a continuous removal of carbonaceous deposits from the active sites, favoring the stability of the catalyst, For Pt/Al₂O₃ and Pt/ZrO₂ catalysts the increase of carbon deposits around or near the metal particle inhibits the CO₂ dissociation on CO₂ reforming of methane. Pt/Y₂O₃ was active and stable for partial oxidation of methane, and its behavior was explained by a change in the reaction mechanism.     

Effect of the support on the mechanism of partial oxidation of methane on platinum catalysts

The partial oxidation of methane was studio on Pt/Al₂O₃, Pt/ZrO₂, Pt/CeO₂ and Pt/Y₂O₃ catalysts. For Pt/Al₂O₃, Pt/ZrO₂ and Pt/CeO₂, temperature programmed surface reaction (TPSR) studies showed partial oxidation of methane comprehends two steps: combustion of methane followed by CO₂ and steam reforming of unreacted methane, while for Pt/Y₂O₃ a direct mechanism was observed. Oxygen Storage Capacity (OSC) evaluated the reducibility and oxygen transfer capacity of the catalysts. Pt/CeO₂ catalyst showed the highest stability on partial oxidation. The results were explained by the higher reducibility and oxygen storage/release capacity which allowed a continuous removal of carbonaceous deposits from the active sites, favoring the stability of the catalyst. For Pt/Al₂O₃ and Pt/ZrO₂ catalysts the increase of carbon deposits around or near the metal particle inhibits the CO₂ dissociation on CO₂ reforming of methane. Pt/Y₂O₃ was active and stable for partial oxidation of methane and its behaviour was explained by a change in the reaction mechanism.     

Water‐promoted ammonia oxidation by a platinum amine complex in zeolite HZSM‐5 catalyst

In this study it was found that [Pt(NH₃)₄]HZSM‐5 is an active catalyst for the oxidation of ammonia at low temperature that, in contrast with other catalysts, becomes more active in the presence of water. Furthermore, the selectivity to nitrogen was found to increase when water is present. This revised version was published online in July 2006 with corrections to the Cover Date.     

Combustion of Methane at Low Temperature over Pd and Pt Catalysts Supported on Al2O3, SnO2 and Al2O3-Grafted SnO2

Pd and Pt catalysts supported on alumina, tin(IV) oxide and tin(IV) oxide grafted on alumina were prepared, characterised and tested with respect to the low-temperature combustion of methane after reduction in H₂ and ageing under reactants at 600°C. In the case of Pd, the use of SnO₂ or SnO₂-based supports led to catalysts slightly less active than Pd/Al₂O₃. In contrast, SnO₂ was found to strongly promote the oxidation of methane over Pt catalysts with respect to Pt/Al₂O₃, even after ageing under reactants. When Pt was supported on SnO₂ grafted on Al₂O₃, the activity was found at most similar to or, after ageing, lower than Pt/Al₂O₃. This negative effect was discussed, being partly related to the sintering of SnO₂ under reactants observed by FTIR and XRD.     

Relationship between the surface characteristics of Pt catalyst and catalytic performance on the H2 SCR

This research investigated how the physical and chemical properties of Pt/TiO₂-based catalysts with high activity in SCR reaction are affected by the preparation conditions (type of TiO₂, Pt content and calcination temperature) using XRD, BET and TPR analysis. The catalyst preparation conditions that achieve optimum reactivity were identified by examination of how the physical and chemical properties relate to catalytic activity. According to the results, Pt content over 2 wt% causes a phenomenon in which Pt agglomeration increases linearly according to the surface area of the limited support. However, Pt content over 3 wt% showed an increase in reducibility in the low temperature region that is proportional to the absolute amount of Pt has increased. Moreover, although increased calcination temperature did not result in phase transition of the TiO₂ support, it did lead to reduction of the surface area by increasing crystallinity and sintering of Pt.     

Preparation of platinum-doped hollow spheres and their electrocatalytic activity in water electrolysis

Pure TiO₂ hollow spheres were prepared by using poly(styrene-methacrylic acid) latex particles as template; thereafter, titania hollow spheres were coated by platinum with an appropriate amount of choloroplatinic acid solution to obtain Pt/TiO₂ catalysts. The morphology and structure of nonstructural Pt/TiO₂ hollow spheres were characterized by BET, XRD, TGA, SEM and TEM analysis. In the samples, a remarkably uniform layer of Pt consisting of particles from 5 to 70 nm in size was formed over TiO₂ hollow spheres. We found the electrocatalytic nature of the samples by cyclic voltammetric experiment in acidic solution. The anodic peak current density of 20 wt% Pt-loaded TiO₂ hollow particles was observed 2.5 times higher than that of 5 wt% Pt/TiO₂ in the same experimental condition. Also, the anodic current density of 20 wt% Pt/TiO₂ hollow spheres calcined at various temperatures followed the order: 400 °C≈500 °C>600 °C. The electrocatalytic activity of the Pt-loaded TiO₂ hollow spheres depends on the amount of atomic platinum present in the sample; a higher concentration of platinum results in a larger current density value in anodic sweep, resulting in more oxygen production during electrolysis. Pt/TiO₂ hollow sphere catalysts have also shown long term electrocatalytic stability in acidic media.     

Effect of lithium and residual nitrate species on platinum dispersion in Pt/Al2O3 catalysts

The effect of lithium (ex LiNO₃) on the metallic dispersion of 0.8 wt% Pt/Al₂O₃ catalysts, prepared by different impregnation techniques, was investigated by temperature programmed reduction (TPR) and the frontal method of H₂ chemisorption. The temperature at which platinum precursor is reduced at a maximum rate (543 K) was not modified by 0.1 wt% lithium addition, whatever the preparation technique used. The dispersion values of platinum (70–90%), after reduction at 773 K, were slightly dependent on the preparation procedure. After the addition of 0.8 wt% lithium the TPR profile presented two well defind peaks and the dispersion values (20–50%), measured after reduction at 773 K, presented a significant decrease. The results are linked with the presence of residual nitrate ions, that had not been eliminated during calcination at 773 K in air, but had been decomposed under the reducing atmosphere of the TPR experiment.     

Characterization of tungsten oxide supported on TiO2 and activity for acid catalysis

Tungsten oxide-titania catalysts were prepared by drying powdered Ti(OH)4 with ammonium metatungstate aqueous solution, followed by calcining in air at high temperature. Characterization of prepared catalysts was performed by using FTIR, Raman, XPS, XRD, and DSC and by measuring surface area. Upon the addition of tungsten oxide to titania up to 20 wt%, the specific surface area and acidity of catalysts increased in proportion to the tungsten oxide content due to the interaction between tungsten oxide and titania. Since the TiO₂ stabilizes the tungsten oxide species, for the samples equal to or less than 20 wt%, tungsten oxide was well dispersed on the surface of titania, but for the samples containing 25 wt% or above 25 wt%, the triclinic phase of WO₃ was observed at calcination temperature above 400 °C. The catalytic activities for 2-propanol dehydration and cumene dealkylation were correlated with the acidity of catalysts measured by ammonia chemisorption method. This paper was presented at the 8th APCChE (Asia Pacific Confederation of Chemical Engineering) Congress held at Seoul between August 16 and 19, 1999.     

Promoting Effect of Pt Addition to V2O5/ZrO2 Catalyst on Reduction of NO by C3H6

The effect of Pt addition to a V₂O₅/ZrO₂ catalyst on the reduction of NO by C₃H₆ has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Pt loading promoted the catalytic activity remarkably. FTIR spectra of NO adsorbed on the catalysts doped with Pt show the presence of two different types of Pt sites, Pt oxide and Pt cluster, on the surface. The amount of these sites depends on Pt contents and the catalyst state. Pt atoms highly disperse on the surface as Pt oxide at low Pt content, being aggregated into Pt metal clusters by increasing Pt amount or reducing the catalysts. The spectral behavior of V=O bands on the surface also supports the formation of Pt clusters. It is concluded that Pt promotes the NO–C₃H₆ reaction through a reduction–oxidation cycle between its oxide and cluster form.     

Enhanced Sulfur Tolerance of Bimetallic PtPd/Al2O3 Catalysts for Hydrogenation of Tetralin by Addition of Fluorine

A series of mono- and bi-metallic Pt-Pd/Al₂O₃ samples with and without F were studied as aromatic hydrogenation catalysts. The effects of changing the order of impregnation of the Pt precursor and F as well as varying the calcination temperature (300–500 °C) were investigated. Temperature programmed reduction (TPR) results demonstrate the presence of a higher fraction of dispersed metal precursor species left on the surface from the impregnation (PtO _x Cl _y ) on the Pt/Al₂O₃ sample calcined at high temperature. The impregnation of F before the Pt precursor significantly decreases the interaction between the metal and the support. However, this decrease is not observed when F is impregnated after the metal precursor. For the bimetallic Pt-Pd catalysts, the sample prepared adding F before the metal show a higher degree of Pt-Pd interaction than either the parent Pt-Pd/Al₂O₃ catalyst or the one prepared with F added later. TPD of ammonia result show the increase in strong acid sites when F is present. Activity tests for tetralin hydrogenation in the presence of 350 ppm dibenzothiophene indicate a better sulfur tolerance for all F-promoted catalysts, especially Pt-Pd.     

ESR, FT-Raman spectroscopic and ethanol partial oxidation studies on MoO3/SnO2 catalysts made by metal oxide vapor synthesis

A series of SnO₂-supported MoO₃ catalysts were prepared by the metal oxide vapor synthesis (MOVS) technique. ESR studies indicated the presence of highly dispersed Mo⁵⁺ species in both octahedral and tetrahedral coordination environments at all the loadings studied. At the highest MoO₃ loading of 12 wt%, the formation of MoO₃ microcrystallites was indicated from the lower intensity of the ESR signal. Raman studies also showed the presence of well dispersed surface molybdate species up to 4.4 wt% MoO₃ loading, and the peaks corresponding to microcrystallites of molybdena were observed at 12 wt% MoO₃ loading. The ethanol partial oxidation activities of the catalysts increased with increase in MoO₃ loading and the catalyst with 4.4 wt% molybdena content showed the highest activity; all the MOVS catalysts showed 100% selectivity to acetaldehyde at low conversions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective oxidation of H2S to sulfur over vanadia supported on mesoporous zirconium phosphate heterostructure

Vanadium oxide supported on mesoporous zirconium phosphate catalysts has been synthesized, characterized and tested in the selective oxidation of H₂S to sulfur. The nature of the vanadium species depends on the V-loading of catalyst. Catalysts with a V-content lower than 4wt% present both isolated vanadium species and V₂O₅ crystallites. However, V₂O₅ crystallites have been mainly observed in catalysts with higher V-content, although the presence of isolated V-species on the surface of the metal oxide support cannot be completely ruled out. The catalytic behaviour also depends on V-loading of catalysts. Thus, while the catalytic activity of catalysts can be related to the number of V-sites, the catalyst decay is clearly observed in samples with low V-loading. The characterization of catalysts after the catalytic tests indicates the presence of sulfur on the catalyst, which is favoured on catalysts with low V-loading. However, a clear transformation of V₂O₅ to V₄O₉ can be proposed according to XRD and Raman results of used catalysts with high V-loading. The importance of V⁵⁺–O–V⁴⁺ pairs in activity and selectivity is also discussed.     

Synergistic effect of CeO2 modified Pt/C catalysts on the alcohols oxidation

The effect of the addition of CeO₂ to Pt/C catalysts on electrochemical oxidation of alcohols (methanol, ethanol, glycerol, ethylene glycol) was studied in alkaline solution. The ratios of Pt to CeO₂ in the catalysts were optimised to give the better performance. The electrochemical measurements revealed that the addition of CeO₂ into Pt-CeO₂/C catalysts could significantly improve the electrode performance for alcohols oxidation, in terms of the reaction activity and the poisoning resistance, due to the synergistic effect. The electrode with the weight ratio of Pt to CeO₂ equals 1.3:1 with platinum loading of 0.30 mg/cm² showed the highest catalytic activity for oxidation of ethanol, glycerol and ethylene glycol.     

Methanol Synthesis from CO2 Using Skeletal Copper Catalysts Containing Co-precipitated Cr2O3 and ZnO

Skeletal Cu-Cr₂O₃-ZnO catalysts have been prepared by leaching CuAl₂ alloy particles at 273 K using 6.1 M aqueous NaOH solutions containing sodium chromate (Na₂CrO₄) and sodium zincate (Na₂Zn(OH)₄). The presence of sodium chromate and sodium zincate in the caustic solution was found to affect the pore structure and surface areas of the resulting catalysts. Both BET and Cu surface areas were increased by increasing the concentration of Na₂CrO₄ and of Na₂Zn(OH)₄.Increasing the Na₂CrO₄ level from 0 to 0.06 M in a 6.1M NaOH solution containing 0.2M Na₂Zn(OH)₄ caused the content of ZnO in the catalyst to decrease from 8.8 to 3.0 wt% whilst increasing the Cr₂ O₃ content from 0 to 1.7 wt%, indicating that the presence of Na₂CrO₄ in the leach liquor not only resulted in deposition of a Cr compound but also inhibited precipitation of zinc hydroxide onto skeletal Cu catalysts. On the other hand, increasing the concentration of Na₂Zn(OH)₄ from 0 to 0.6 M in a 6.1 M NaOH solution containing 0.008 M Na₂ CrO₄ resulted in increasing the ZnO loading from 0 to 8.9wt% with an almost constant content of Cr₂ O₃ (1.3 ± 0.2%) in the catalysts, revealing that sodium zincate only led to precipitation of zinc hydroxide and did not suppress Cr₂O₃ formation.Hydrogenation of CO₂ was studied using a gas mixture of 24% CO₂ in H₂ at a total pressure of 4MPa, space velocities up to 210000L kg^-1h^-1 and temperatures in the range 493-533K. The catalysts were found to be both highly active and selective for methanol synthesis. This study confirms the role of ZnO in promoting the activity of copper for methanol synthesis from CO₂ and improving the selectivity by inhibiting the reverse water-gas shift reaction. The role of Cr₂O₃ is to improve the structural development of high surface area skeletal copper.