TWC Behaviour of Platinum Supported on High and Low Surface Area Cerium/Zirconium Mixed Oxides

The catalytic activity as TWC of two Pt catalysts supported on one high and one low surface area Ce_0.68Zr_0.32O₂ mixed oxide has been investigated. The catalyst behaviour was studied both fresh and after thermal ageing, and correlated to the textural and the Pt dispersion changes. The results show that the catalyst with the highest surface area is not necessarily the catalyst which has the best performances as TWC. The different behaviour of the catalysts has been attributed mainly to differences in the platinum dispersion and/or the reducibility of the samples, both related to the platinum/support interaction.     

The characterization of Pt/Sn catalyst for the electrochemical oxidation of methanol

Pt/Sn electrodeposited catalysts have been prepared, characterized and tested for the electro-oxidation of methanol. Catalyst activities were measured in 3 M H₂SO₄ electrolyte between ambient temperature and 95°C. Enhancement in specific activity by a factor of about 50 was found over electrodeposited platinum black. This behaviour is in contrast to that of alloys of platinum and tin which were found to have very low activities compared with platinum catalysts and to be readily corroded in H₂SO₄ electrolyte.ESCA (electron spectroscopy for chemical analysis) studies and Mössbauer spectroscopy showed the majority of the tin in the deposit to be present in an oxidized form. A small amount (17%) was present as a dilute alloy of tin in platinum. Surface area measurements and X-ray powder diffraction indicated that the increase in activity over platinum black was not attributable to smaller platinum particle size. It seems that the combination of platinum and tin results in a decrease in the poisoning effect by strongly adsorbed organic residues. Whether this arises from the operation of a cyclic Sn(II)/Sn(IV) redox system or from modification of the platinum surface remains unresolved.     

Hydrodechlorination of CCl4 over Pt/γ-Al2O3 prepared from different Pt precursors

The platinum particle size on γ-Al₂O₃ prepared from different platinum precursors such as Pt(NH₃)₂(NO₂)₂, H₂PtCl₆, and K₂PtCl₆, and its effect on hydrodechlorination (HDC) of CCl₄ with the variation of calcination temperatures was investigated. It concomitantly affects the products distribution and catalytic stability in HDC of CCl₄. To verify the effects of platinum particle size with different platinum precursors on the product distribution, the catalysts have been characterized by HRTEM and CO chemisorption, FT-IR, and TP methods (TPR, TPD and TPSR). The catalysts with small platinum particles, which possess low coordination number and electron-deficient character, favor the complete dechlorination of CCl₄ and produce CH₄ more selectively. It could be due to the strong adsorption strength of CCl₄ or the decreased activation energy of surface intermediates on small platinum particles. FT-IR studies reveal that the maximum peak position of linear-bonded CO shifted to the higher frequency with the increase of platinum particle size in all catalysts prepared from three different platinum precursors. While the selectivity to CH₄ increased with the decrease in the platinum particle size, the total amount of carbonaceous species on the platinum particles was enhanced. The larger the platinum particle, the higher the selectivity to CHCl₃ was obtained in all tested catalysts under the non-deactivating condition.     

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.     

Deactivation phenomena during catalytic wet air oxidation (CWAO) of phenol over platinum catalysts supported on ceria and ceria–zirconia mixed oxides

Catalytic wet air oxidation (CWAO) of aqueous solution of phenol was carried out with pure oxygen at 160 °C in a stirred batch reactor on platinum supported oxide catalysts (Pt/CeO₂^c calcined at 650 and 800 °C and Pt/Ce_xZr_1 − xO₂ with x = 0.90, 0.75 and 0.50). The catalysts were characterized before (BET, FT-IR spectroscopy, hydrogen chemisorptions, oxygen storage capacity (OSC)) and after reaction (TPO, elementary analysis, GC–MS and DTA–TGA). The results demonstrate a poisoning of the catalysts during CWAO reaction due to the formation of different forms of carbon deposit on the materials: carbonates and polymeric carbon species. This poisoning phenomenon is limited by the introduction of 50% of zirconium into ceria lattice for the catalysts presenting the lowest surface area. Polymeric deposits play a major role in the catalyst deactivation.     

Silicon nitride supported platinum catalysts for the partial oxidation of methane at high temperatures

The catalytic partial oxidation of methane has been studied over platinum silicon nitride supported catalysts in the temperature range of 900–1100°C at a contact time of 0.35 ms at atmospheric pressures. The feed consisted of a mixture of CH₄/O₂/N₂≈2/1/10. The catalysts were prepared by impregnation of platinum bis-acetyl-acetonate on silicon nitride powder (Si₃N₄). The different catalysts were characterized by chemical analysis, XPS and TEM. Minor particle sintering occurs during reaction. Metal losses were observed at 900°C, for catalysts containing 1.0 and 2.2 wt.% of platinum. A catalyst with a low amount of platinum (0.045 wt.%) appeared to be stable at 900°C, as no platinum loss was observed. The high stability of the 0.045 wt.% Pt/Si₃N₄ catalyst could be attributed to particular interactions between the metal and the support.     

Investigation of sulfur poisoning of CNx oxygen reduction catalysts for PEM fuel cells

The role of the transition metal used during the growth of non-noble metal electrochemical oxygen reduction CN_x catalysts was investigated through sulfur treatment, a well-known poison for transition metal-based catalysts. The intent of sulfur poisoning was to show the existence of an electrocatalytic active site in CN_x that did not depend on iron. The sulfur treatment was shown to be effective on a platinum catalyst, as seen by the decreasing onset potential. The same treatment, however, not only showed no negative effect on the CN_x catalyst, but enhanced its performance, as seen by the increase in the onset potential. This suggests that, if there are iron-based active sites in these catalysts, they are either sulfur tolerant or they do not participate in the electrocatalytic oxygen reduction. The deposition of sulfur onto CN_x catalyst was verified by temperature-programmed oxidation and X-ray photoelectron spectroscopy. Iron K-edge X-ray absorption near edge structural analysis of the CN_x catalyst suggested that the iron phase, which was primarily composed of nanometer-sized metallic particles, was unchanged by sulfur poisoning, suggesting that the residual iron left in these materials is not catalytically accessible.     

Studying the role of the state of platinum in Pt/SO4/ZrO2/Al2O3 catalysts in the isomerization of n-hexane

Samples of SO₄/ZrO₂/Al₂O₃ and Pt/Al₂O₃ Pt/Al₂O₃ catalysts and their physical mixtures are prepared, and the catalytic properties of the samples in n-hexane isomerization are studied. The considerable effect of the state of platinum on the catalytic performance of the samples is revealed. IR spectroscopy (CO_ads), oxygen chemisorption, and oxygen-hydrogen titration show that the reduced catalysts contain ionic forms of platinum capable of adsorbing up to three hydrogen atoms per each surface atom of platinum. By means of H/D isotopic exchange, it is found that specific properties of ionic platinum are apparent in the formation of the hydride form of adsorbed hydrogen. It is speculated that the activity and stability of catalysts based on sulfated zirconia in n-hexane isomerization can be attributed to the involvement of ionic and metallic platinum in the activation of hydrogen. The results can be used to develop effective catalysts for the isomerization of C₅–C₆ gasoline fractions in order to obtain the isomerizate as a high-octane additive for modern gasolines.     

A novel sintering resistant and corrosion resistant Pt4ZrO2/C catalyst for high temperature PEMFCs

A novel Pt₄ZrO₂/C catalyst was prepared and compared with 20 wt.% Pt/C in terms of the sintering resistance and corrosion resistance. To evaluate their sintering resistance and corrosion resistance properties, an accelerated ageing test (AAT) was performed. The catalysts before and after AAT were characterized by cyclic voltammetry (CV), rotating disk electrode (RDE) and X-ray diffraction (XRD). After AAT, the dissolution rate of Pt and Zr in H₃PO₄ media (105 wt.%, 204 °C) was characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The electrochemical area (ECA) changes of thin film electrodes based on Pt₄ZrO₂/C and Pt/C catalysts were also evaluated using continuous CV sweep technique. All the results showed that Pt₄ZrO₂/C has higher sintering resistance and corrosion resistance than Pt/C. ‘Anchor effect’ is proposed to explain the enhanced effect of ZrO₂ in Pt₄ZrO₂/C binary catalyst compared with Pt/C that contain platinum alone.     

Preparation and Characterization of Pt/C and Pt/TiO2 Electrocatalysts by Liquid Phase Photodeposition

The preparation of carbon and titanium dioxide supported Pt catalysts through a photochemical and photocatalytic routes were investigated. The catalysts were prepared by irradiation with UV-light (365 nm) at room temperature using H₂PtCl₆ and C₁₀H₁₄O₄Pt (Pt(acac)₂) as platinum precursors. The kinetic studies revealed that H₂PtCl₆ produced metallic platinum faster than Pt(acac)₂ and also showed that the amount of platinum deposited on TiO₂ was higher than on carbon. The samples were characterized by X-ray diffraction, SEM/EDS and cyclic voltammetry. X-ray diffraction permitted to identify the crystallographic (111) and (200) planes from platinum metal on the catalysts synthesized, the intensity of peaks depends of the amount of platinum deposited. SEM/EDS test confirmed what it was found by the kinetics studies. The electrocatalytic activity was compared with a commercial Pt E-Tek catalyst (10 wt%). The electrochemical results showed that Pt/C-AA catalyst synthesized by liquid phase photo-deposition method has stability in acid media and high distribution of the actives sites on the electrode surfaces. It could be considered as a candidate for electro-catalyst for polymer electrolyte fuel cell. The Pt/TiO₂ catalysts did not present electrochemical activity.     

Catalytic activity and long-term stability of palladium oxide catalysts for natural gas combustion: Pd supported on LaMnO3-ZrO2

The catalytic activity and long-term stability of 2% Pd/LaMnO₃-ZrO₂ catalysts for natural gas combustion were deeply investigated. The catalyst, prepared via solution combustion synthesis, was completely characterized (XRD, BET, FESEM/EDS, TPC/TPD/TPR and FT-IR analysis) in the fresh status, and in the aged one, after prolonged treatment under hydro-thermal ageing and S-compounds poisoning (up to 3 weeks of hydro-thermal treatment at 800 °C under a flow of domestic boiler exhaust gases typical composition of 9% CO₂, 18% H₂O, 2% O₂ in N₂, including 200 ppmv of SO₂). An increased catalytic activity towards NG combustion with ageing was detected: the T₅₀, in fact, got lowered from 570 (fresh sample) to 465 °C (after 3 weeks ageing). Highly dispersed Pd centers were predominant on fresh catalyst. Upon ageing, oxygen covered Pd metal particles formed, at the expense of dispersed cationic and zerovalent Pd atoms. The increase in the catalytic activity was associated to the phase modification occurring in the bulk support, where Mn oxides, active towards CH₄ combustion, segregated. Moreover, bands due to sulfate species were detected in aged samples: IR analysis showed that Pd atoms did not interact significantly with these species. The bands of sulfate species decreased in intensity after 3 weeks ageing, likely mostly due to sintering of the catalyst, with the corresponding decrease in the surface area.     

The effect of ageing time on co-precipitated Cu/ZnO/ZrO2 catalysts used in methanol synthesis from CO2 and H2

The effect of suspension ageing time during the catalyst precipitation process on the performance of co-precipitated Cu/ZnO/ZrO₂ catalysts in methanol synthesis from CO₂ and H₂ has been studied. The ageing time influenced greatly the physical and chemical characteristics of the catalysts as well as their activity in the methanol synthesis. Prolonged ageing was advantageous, mainly due to both lower sodium contents and enhanced crystallinity of the catalysts.     

Calorimetric study of the reversibility of CO pollutant adsorption on high loaded Pt/carbon catalysts used in PEM fuel cells

A major obstacle to the broader use of fuel cells is the poisoning of supported Pt catalysts by the CO present in virtually all feeds. In this paper, the microcalorimetry technique was employed to study and compare the CO adsorption properties of different commercial carbon-supported platinum catalysts with high Pt loading, aimed to be used in proton exchange membrane fuel cells (PEMFCs) applications. Combined with other techniques of characterization, such as BET, XRD, TPD-MS and TPR, adsorption microcalorimetry has permitted a better understanding of the studied systems. The pore architecture of Pt/C catalysts was found to influence the kinetics of heat release during CO adsorption. The accessibility of CO molecules to the adsorption sites increased with the mesoporosity of the catalyst. The degree of catalyst poisoning by CO upon successive air/H₂/CO cycles varied between 2 and 30% for the different studied samples. These results confirm that the surface chemistry of the catalyst, and in particular the Pt deposition method, affects the surface site energy distribution and consequently the adsorptive properties towards H₂ and CO. It was found that both H₂ and CO are chemisorbed on the investigated samples. Pt/C powders exhibit higher differential heats of carbon monoxide adsorption in comparison with hydrogen adsorption. A reaction between pre-adsorbed H₂ and CO from the gas phase takes place on Pt/C catalysts as a result of competitive adsorption.     

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.     

Ageing mechanisms on PdOx-based catalysts for natural gas combustion in premixed burners

The ageing effect induced by S-compounds over 2%Pd/CeO₂·2ZrO₂, 2%Pd/LaMnO₃·2ZrO₂ and 2%Pd/BaCeO₃·2ZrO₂ catalysts for CH₄ combustion was investigated; S-compounds are in fact added as odorants in the natural gas network for safety purposes. Pd-based catalysts were prepared by solution combustion synthesis (SCS), starting from metal nitrates/glycine mixtures. Basic characterization (XRD, BET, FESEM analysis), FT–IR studies and catalytic activity tests were performed on powders and after accelerated ageing carried out up to 2 weeks (hydro-thermal treatment at 900 °C under a flow rate with typical domestic boiler exhaust gas composition, 9% CO₂, 18% H₂O, 2% O₂ in N₂, containing also 200 ppmv of SO₂ to emphasize any poisoning effect). Over fresh catalysts, IR analysis of CO adsorption evidenced the formation of highly dispersed Pd metal clusters and Pd ions. With ageing, 2%Pd/CeO₂·2ZrO₂ increased its CH₄ combustion half-conversion temperature (T₅₀, regarded as an index of catalytic activity) from 382 °C—recorded for fresh sample—to 421 °C, attained with the same sample aged two weeks. An unexpected improvement was found instead in the overall performance of 2%Pd/LaMnO₃·2ZrO₂ and 2%Pd/BaCeO₃·2ZrO₂: the T₅₀ in fact lowered from 570 to 450 °C for the first one, and from 512 to 443 °C for the second one, after two weeks ageing. S-hydro-thermal treatment provoked bulk and surface sulfates formation on all aged samples, with a concentration increasing with the exposure time. Prevailing ageing mechanisms seemed to be Pd metallic clusters coalescence, detected over the Ce–Zr system, and surface-bulk sulfates formation, the latter destroying the initial crystallographic structure. In 2%Pd/LaMnO₃·ZrO₂ and 2%Pd/BaCeO₃·ZrO₂ powders the amount of the perovskite phase strongly decreased during ageing, in favor of the formation of bulk sulfate and of oxides.     

A Comparative Study of Organic Cobalt Complex Catalysts for Oxygen Reduction in Polymer Electrolyte Fuel Cells

Reduction of platinum catalysts loading is a central issue in polymer electrolyte fuel cells. As alternatives for platinum, some organic metal chelate compounds are tested as cathode catalysts, such as cobalt aza-complexes or cobalt complexes possessing aminophenyl moieties featured as Co-N₄ or Co-N₂O₂ chelate structures. The way of immobilization of catalysts on the graphite surface influences their stability as well as the performance of oxygen reduction. Heat-treated catalysts supported on graphite at 600°C show much better oxygen reduction abilities than as-received metal complexes. The original chemical structure of metal complexes affects crucially the catalytic ability, though initial structures of molecules are no more intact after the heat treatment. The catalytic activity of these complexes may originate from the central chelate unit CoN₄ on the carbon substrate, and this unit is assumed to constitute the basic coordination site for an oxygen molecule. Electropolymerized catalysts impart a high level of oxygen reduction ability, probably due to the improved molecular orientation for oxygen coordination and formation of good chelate sites on the graphite surface.     

Pt-Al2O3 Cryogel with High Thermal Stability for Catalytic Combustion

The cryogel catalyst of platinum on alumina was prepared from aluminum sec-butoxide and H₂PtCl₆ through the sol-gel technique and subsequent freeze drying. The cryogel catalyst showed higher thermal stability of platinum than the corresponding xerogel or impregnation catalysts, which was ascribed to the more intimately developed platinum-alumina interaction accompanied by the encapsulation of the metal into the alumina cryogel. It was also shown that platinum accessibility was higher on the cryogel than on the xerogel despite the higher thermal stability of the metal on the formed than on the latter. For the VOC combustion, the cryogel exhibited higher activity than the xerogel and impregnation catalysts. Also for the methane combustion the cryogel showed higher activity, although it showed lower activity than the impregnation catalysts above 600 °C. By the addition of ceria as an additive to the cryogel catalyst, the CH₄ combustion activity was improved especially in the temperature region above 600 °C.     

Thermal ageing phenomena and strategies towards reactivation of NO x - storage catalysts

The thermal ageing and reactivation of Ba/CeO₂ and Ba/Al₂O₃ based NO _x -storage/ reduction (NSR) catalysts was studied on model catalysts and catalyst systems at the engine. The mixed oxides BaAl₂O₄ and BaCeO₃, which lower the storage activity, are formed during ageing above 850 °C and 900 °C, respectively. Interestingly, the decomposition of BaCeO₃ in an atmosphere containing H₂O/NO₂ leads again to NO _x -storage active species, as evidenced by comparison of fresh, aged and reactivated Pt-Ba/CeO₂ based model catalysts. This can be technically exploited, particularly for the Ba/CeO₂ catalysts, as reactivation studies on thermally aged Ba/CeO₂ and Ba/Al₂O₃ based NSR catalysts on an engine bench showed. An on-board reactivation procedure is presented, that improved the performance of a thermally aged catalyst significantly.     

Deactivation phenomena of supported platinum catalysts during the hydrogenation of cyclopropane

Cyclopropane hydrogenation reaction was performed in a differential reactor operating at atmospheric pressure, 50‡C and H₂/C₃H₆ ratio of 9, and the deactivation of the catalysts during the reaction was investigated by observing the effects of the supports and the size of platinum crystallites, Owinglo the accumulation of the carbonaceous materials the platinum catalysts dispersed on the acidic supports such as γ-Al₂O₃, SiO₂-Al₂O₃ and Y-zeolite deactivated significantly, while no detectable deactivation was observed for the catalysts on the non-acidic supports of SiO₂ and active carbon. In addition smaller platinum particles on acidic supports were morevulnerable to the catalyst deactivation.     

Hydrogenation of soybean oil over various platinum catalysts: Effects of support materials on trans fatty acid levels

The effects of various support materials on the catalytic performance of supported platinum catalysts for the hydrogenation of soybean oil were examined. There was a linear relationship between the catalytic activity and the platinum dispersion of the platinum catalysts. Among the examined catalysts, Pt/BaSO₄ was effective for the reduction of both trans fatty acid (TFA) and additional saturated fatty acid (ASFA) levels in partially hydrogenated oils (iodine value (IV) = 70). In addition, the relationship between the TFA levels and the electronegativity of the metal ion in the support material was a volcano function.