Palladium-Based Catalysts with Improved Sulphur Tolerance for Diesel-Engine Exhaust Systems

Palladium-based catalysts have been developed for the diesel exhaust system with an emphasis on their sulphur tolerance during the simultaneous oxidation of CO and HC. Promising materials include Au–Pd–Pt, Co–Pd–Pt and Ni–Pd–Pt supported on Al₂O₃ or TiO₂ and Pd–Pt/MoO₃–Al₂O₃ with the Al₂O₃ support modified with MoO₃ monolayer.     

Characterisation and microstructure of Pd and bimetallic Pd–Pt catalysts during methane oxidation

The catalytic oxidation of methane was studied over Pd/Al₂O₃ and Pd–Pt/Al₂O₃. It was found that the activity of Pd/Al₂O₃ gradually decreases with time at temperatures well below that of PdO decomposition. The opposite was observed for Pd–Pt/Al₂O₃, of which the activity decreases slightly with time. Morphological studies of the two catalysts showed major changes during operation. The palladium particles in Pd/Al₂O₃ are initially composed of smaller, randomly oriented crystals of both PdO and Pd. In oxidising atmospheres, the crystals become more oxidised and form larger crystals. The activity increase of Pd–Pt/Al₂O₃ is probably related to more PdO being formed during operation. The particles in Pd–Pt/Al₂O₃ are split into two different domains: one with PdO and the other likely consisting of an alloy between Pd and Pt. The alloy is initially rich in palladium, but the composition changes to a more equalmolar Pd–Pt structure during operation. The ejected Pd is oxidised into PdO, which is more active than its metallic phase. The amount of PdO formed depends on the oxidation time and temperature.     

Lab Scale Fixed-Bed Reactor for Operando X-Ray Absorption Spectroscopy for Structure Activity Studies of Supported Metal Oxide Catalysts

Lab scale fixed-bed reactor is applied for operando transmission X-ray absorption spectroscopy (XAS) for structure–activity studies of supported metal oxide catalysts under real reaction conditions. This setup includes many properties of an optimal fixed-bed reactor for operando transmission XAS studies. For instance, it is usable in a wide range of temperature (up to 1,000 °C), pressure and space velocity. Besides, this operando setup can be used for transmission XAS measurements in a wide edge energy range. The potential of this reactor for operando transmission XAS is demonstrated by, as examples, the three-way catalytic performance of Pd/Al₂O₃/CeZrO₂ and Rh/Al₂O₃.     

Oscillatory CO Oxidation Over Pt/Al2O3 Catalysts Studied by In situ XAS and DRIFTS

Fresh and mildly aged Pt/Al₂O₃ model diesel oxidation catalysts with small and large noble metal particle size have been studied during CO oxidation under lean burn reaction conditions to gain more insight into the structure and oscillatory reaction behaviour. The catalytic performance, CO adsorption characteristics using in situ DRIFTS and oxidation state using in situ XAS were correlated. Stable and pronounced oscillations only occurred over the catalyst with smaller particle sizes. Characteristic for this catalyst are low-coordinated surface Pt sites (more corner and edge atoms) which seem to become oxidized at elevated temperature as evidenced by in situ DRIFTS and in situ XAS. In situ XAS further uncovered that the oxidation of the Pt surface starts from the end of the catalyst bed and the oxidation state oscillates like the catalytic activity.     

Improvement of thermal stability of NO oxidation Pt/Al2O3 catalyst by addition of Pd

The present work has been undertaken to tailor Pt/Al₂O₃ catalysts active for NO oxidation even after severe heat treatments in air. For this purpose, the addition of Pd has been attempted, which is less active for this reaction but can effectively suppress thermal sintering of the active metal Pt. Various Pd-modified Pt/Al₂O₃ catalysts were prepared, subjected to heat treatments in air at 800 and 830 °C, and then applied for NO oxidation at 300 °C. The total NO oxidation activity was shown to be significantly enhanced by the addition of Pd, depending on the amount of Pd added. The Pd-modified catalysts are active even after the severe heat treatment at 830 °C for a long time of 60 h. The optimized Pd-modified Pt/Al₂O₃ catalyst can show a maximum activity limited by chemical equilibrium under the conditions used. The bulk structures of supported noble metal particles were examined by XRD and their surface properties by CO chemisorption and EDX-TEM. From these characterization results as well as the reaction ones, the size of individual metal particles, the chemical composition of their surfaces, and the overall TOF value were determined for discussing possible reasons for the improvement of the thermal stability and the enhanced catalytic activity of Pt/Al₂O₃ catalysts by the Pd addition. The Pd-modified Pt/Al₂O₃ catalysts should be a promising one for NO oxidation of practical interest.     

Arc Plasma Processing of Pt and Pd Catalysts Supported on γ-Al2O3 Powders

Direct deposition of Pt and Pd nanoparticles onto γ-Al₂O₃ powders was studied by using a pulsed arc plasma process under vacuum to use them as an automotive catalyst. As deposited Pt catalyst exhibited a higher metal dispersion and thus a higher catalytic activity for CO oxidation, compared to the conventional Pt/Al₂O₃ prepared by wet impregnation. In contrast, Pd/Al₂O₃ prepared by the arc plasma method was less active because of its metallic state of Pd with a lower dispersion. A weak interaction between precious metals and γ-Al₂O₃ is not enough for thermal stabilization of as deposited nanoparticles during ageing in a stream of 10% H₂O in air at 900 °C.     

EXAFS characterization of bimetallic Pt–Pd/SiO2–Al2O3 catalysts for hydrogenation of aromatics in diesel fuel

Bimetallic Pt–Pd/SiO₂–Al₂O₃ catalysts exhibited much higher activities in aromatic hydrogenation of distillates than monometallic Pt/SiO₂–Al₂O₃ and Pd/SiO₂–Al₂O₃ catalysts. The studies of extended X‐ray absorption fine structure (EXAFS) indicated that there was an interaction between Pt and Pd in the Pt–Pd/ SiO₂–Al₂O₃ catalyst. Furthermore, from the EXAFS, it was assumed that the active metal particle on the Pt–Pd/SiO₂–Al₂O₃ catalysts is composed of the “Pd dispersed on Pt particle” structure. Regarding both the activities of aromatic hydrogenation and the EXAFS results, it was concluded that the Pd species dispersed on Pt particles were responsible for the high activity of the bimetallic Pt–Pd/SiO₂–Al₂O₃ catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H₂ over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H₂ reaction Pt/Al₂O₃>Pd/Al₂O₃>Rh/Al₂O₃ and for the NO+CO reaction Rh/Al₂O₃>Pd/Al₂O₃> Pt/Al₂O₃. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H₂ reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al₂O₃ during the NO+H₂ reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N₂O) irrespective of the nature of the reducing agent (CO or H₂). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N₂ via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H₂+O₂ reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.     

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.     

Complete Oxidation of Methane at Low Temperature over Pt Catalysts Supported on High Surface Area SnO2

The catalytic activity of Pt catalysts supported on high surface area tin(IV) oxide in the complete oxidation of CH₄ traces under lean conditions at low temperature was studied in the absence and in the presence of water (10 vol.%) or H₂S (100 vol.ppm). Their catalytic properties were compared to those of Pd/Al₂O₃ and Pt/Al₂O₃. In the absence of H₂S in the feed, Pt/SnO₂appears as a very promising catalyst for CH₄ oxidation, being even significantly more active under wet conditions than the best reference catalyst, Pd/Al₂O₃. Catalysts steamed-aged at 873 K were also studied in order to simulate long term ageing in real lean-burn NGV exhaust conditions. To this respect, Pt/SnO₂ is slightly less resistant than Pd/Al₂O₃. In the presence of H₂S, Pt/SnO₂catalysts are rapidly and almost completely poisoned, comparably to Pd/Al₂O₃and the catalytic activity is hardly restored upon oxidising treatment below 773 K. A synergetic effect between Pt and specific surface SnO₂sites active in CH₄oxidation is proposed to explain the superior catalytic behaviour of Pt/SnO₂.     

Aqueous-phase reforming of ethylene glycol over supported Pt and Pd bimetallic catalysts

More than 130 Pt and Pd bimetallic catalysts were screened for hydrogen production by aqueous-phase reforming (APR) of ethylene glycol solutions using a high-throughput reactor. Promising catalysts were characterized by CO chemisorption and tested further in a fixed bed reactor. Bimetallic PtNi, PtCo, PtFe and PdFe catalysts were significantly more active per gram of catalyst and had higher turnover frequencies for hydrogen production (TOF_H2) than monometallic Pt and Pd catalysts. The PtNi/Al₂O₃ and PtCo/Al₂O₃ catalysts, with Pt to Co or Ni atomic ratios ranging from 1:1 to 1:9, had TOF_H2 values (based on CO chemisorption uptake) equal to 2.8–5.2 min⁻¹ at 483 K for APR of ethylene glycol solutions, compared to 1.9 min⁻¹ for Pt/Al₂O₃ under similar reaction conditions. A Pt₁Fe₉/Al₂O₃ catalyst showed TOF_H2 values of 0.3–4.3 min⁻¹ at 453–483 K, about three times higher than Pt/Al₂O₃ under identical reaction conditions. A Pd₁Fe₉/Al₂O₃ catalyst had values of TOF_H2 equal to 1.4 and 4.3 min⁻¹ at temperatures of 453 and 483 K, respectively, and these values are 39–46 times higher than Pd/Al₂O₃ at the same reaction conditions. Catalysts consisting of Pd supported on high surface area Fe₂O₃ (Nanocat) showed the highest turnover frequencies for H₂ production among those catalysts tested, with values of TOF_H2 equal to 14.6, 39.1 and 60.1 min⁻¹ at temperatures of 453, 483 and 498 K, respectively. These results suggest that the activity of Pt-based catalysts for APR can be increased by alloying Pt with a metal (Ni or Co) that decreases the strengths with which CO and hydrogen interact with the surface (because these species inhibit the reaction), thereby increasing the fraction of catalytic sites available for reaction with ethylene glycol. The activity of Pd-based catalysts for APR can be increased by adding a water-gas shift promoter (e.g. Fe₂O₃).     

Marked Effect of Mo and Fe Addition Upon Liquid Phase Methanol Reforming with Water Over Al2O3 Supported Pt Catalysts

Successively impregnated Pt–Mo/Al₂O₃ and Pt–Fe/Al₂O₃ catalysts exhibited large enhancement effect in H₂ formation rate of liquid phase methanol reforming. Added Mo oxide forms monolayer on Al₂O₃ and facilitates the higher dispersion of Pt particles. In the case of Fe, formation of some surface bimetallic clusters between Pt and Fe was confirmed by XAS analysis, which causes the enhancement effect of H₂ formation in MeOH–H₂O reaction.     

Particle Size Effect on CH4 Oxidation Over Noble Metals: Comparison of Pt and Pd Catalysts

Intrinsic catalytic activities (TOF values) in CH₄ complete oxidation under lean conditions were estimated as a function of Pt and Pd particle sizes (d_m) for two series of Pt/Al₂O₃ and Pd/Al₂O₃ catalysts. Comparison of TOF ~ f(d_m) relationships revealed significant difference between Pt and Pd catalysts. For Pt catalyst TOF showed tendency to increase by 2–3 times with increasing particle size from 1 to ca 3 nm and remained constant, when Pt particles became larger than 3 nm. On the other hand, for Pd catalyst TOF increased almost linearly when particle size grew from 1 to 20 nm. These different tendencies were attributed to the different mechanisms of CH₄ oxidation over Pt and Pd catalysts: Langmuir–Hinshelwood and Mars-Van Krevelen respectively.     

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.     

CO selective oxidation in hydrogen-rich gas over platinum catalysts

The kinetics of CO oxidation in hydrogen-rich gas on Pt/mordenite (Pt/MD) or Pt/Al₂O₃ were investigated over a wide range of CO (0.4–1.8%) and O₂ concentrations (0.26–1.14%). The integral flow measurements showed that both the catalysts that could remove CO from 1% to ppm-level Pt/MD had a wider operation temperature range than Pt/Al₂O₃, especially towards lower temperatures.     

Selective hydrogenation of (E)-2-hexenal to (E)-2-hexen-1-ol over Co-based bimetallic catalysts

Hydrogenation of (E)-2-hexenal was carried out in a liquid phase using Co-based bimetallic catalysts (M–Co/Al₂O₃, M=Pd, Pt, Ru, Rh, Sn, Fe, or Cu). Pd–Co/Al₂O₃ showed the highest activity among the catalysts tested and catalyzed the hydrogenation of CC bond predominantly to produce hexanal and 1-hexanol. Pt–Co/Al₂O₃ was more active than monometallic Co/Al₂O₃ for the hydrogenation of CO bond. The excellent result, 92% selectivity to (E)-2-hexen-1-ol formation at 90% conversion, was obtained by the hydrogenation over Pt–Co/Al₂O₃ bimetallic catalyst. No improved activities were observed for the other bimetallic catalysts.     

Catalytic Hydroprocessing of p-Cresol: Metal, Solvent and Mass-Transfer Effects

A systematic study of the comparative performances of supported Pt, Pd, Ru and conventional CoMo/Al₂O₃, NiMo/Al₂O₃, NiW/Al₂O₃ catalysts as well as the effects of solvent, H₂ pressure and temperature on the hydroprocessing activity of a representative model bio-oil compound (e.g., p-cresol) is presented. With water as solvent, Pt/C catalyst shows the highest activity and selectivity towards hydrocarbons (toluene and methylcyclohexane), followed by Pt/Al₂O₃, Pd and Ru catalysts. Calculations indicate that the reactions in aqueous phase are hindered by mass-transfer limitations at the investigated conditions. In contrast, with supercritical n-heptane as solvent at identical pressure and temperature, the reactant and H₂ are completely miscible and calculations indicate that mass-transfer limitations are eliminated. All the noble metal catalysts (Pt, Pd and Ru) show nearly total conversion but low selectivity to toluene in supercritical n-heptane. Further, conventional CoMo/Al₂O₃, NiMo/Al₂O₃ and NiW/Al₂O₃ catalysts do not show any hydrodeoxygenation activity in water, but in supercritical n-heptane, CoMo/Al₂O₃ shows the highest activity among the tested conventional catalysts with 97?% selectivity to toluene. Systematic parametric investigations with Pt/C and Pt/Al₂O₃ catalysts indicate that with water as the solvent, the reaction occurs in a liquid phase with low H₂ availability (i.e., low H₂ surface coverage) and toluene formation is favored. In supercritical n-heptane with high H₂ availability (i.e., high H₂ surface coverage), the ring hydrogenation pathway is favored leading to the high selectivity to 4-methylcyclohexanol. In addition to differences in H₂ surface coverage, the starkly different selectivities between the two solvents may also be due to the influence of solvent polarity on p-cresol adsorption characteristics.     

Steady State Isotopic Transient Kinetic Analysis of Flameless Methane Combustion over Pd/Al2O3 and Pt/Al2O3 Catalysts

The SSITKA measurements were performed in the steady state of complete methane oxidation on the Pd/Al₂O₃ and Pt/Al₂O₃ catalysts. It was found that the number of intermediates and their average life-time on the catalyst surface changes with the increase of reaction temperature. On the Pd/Al₂O₃ catalyst there is larger number of active centres than on Pt/Al₂O₃ catalyst which permits the course of methane oxidation at lower temperatures.     

Hydrogenation of the Aromatics and Olefins in FCC Gasoline During Deep Desulphurisation

The applicability of four catalyst with different composition (conventional and new generation CoMo/Al₂O₃, new composition Pt,Pd/USY, Pt/H-Mordenite) catalysts was investigated for selective desulphurization of different sulphur containing FCC gasolines. The new generation CoMo/Al₂O₃ and the new composition Pt,Pd/USY were found to have favourable hydrodesulphurisation activity. The reactions of some C₅-C₆ olefin and aromatic hydrocarbons are discussed under the conditions of deep desulphurisation, highlighting the effects of that on the octane number.     

Chloride‐induced migration of supported platinum and palladium across phase boundaries

Previous work showed that calcination in O₂ of physical mixtures of Fe₂O₃ and supported Pt leads to a strong reduction enhancement of the Fe₂O₃, but that a much smaller effect was observed with supported Pd. The present results show that a strong reduction enhancement could be achieved by pretreating Pt/Al₂O₃ or Pd/Al₂O₃ with NH₄Cl and then decomposing NH₄Cl, before mixing the solid with Fe₂O₃. Such pretreatment with NH₄Cl has no effect on SiO₂ or zeolite‐supported metals, because only Al₂O₃ retains chloride ions at its surface. In the physical mixtures, chlorides migrate from Al₂O₃ to Fe₂O₃ at elevated temperature and form a volatile compound, presumably FeCl₃. Layered‐bed experiments show that this FeCl₃ sublimes, and that its chemical interaction with Pt or Pd on any support results in the formation of mobile Pt– or Pd–chloro complexes that reach Fe₂O₃ particles by surface migration. After exposure to an H₂ flow, the complexes are reduced, and Pt or Pd particles are formed on the Fe₂O₃, enhancing its reduction by H spillover. These metal particles on the Fe₂O₃ have been identified by TEM and X‐ray energy dispersive spectroscopy (EDS). Abundant formation of PdFe alloys upon reduction is verified by TPR/TPD, indicating that almost all Pd has interacted with the volatile Fe chloride. In the absence of a transition metal, chloride ions retard the reduction of Fe₂O₃. This revised version was published online in July 2006 with corrections to the Cover Date.