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.     

High Efficiency of Noble Metal and Metal Oxide Catalyst Systems for the Selective Reduction of NO with Propene in Lean Exhaust Gas

An investigation was conducted of noble metal and metal oxide catalysts deposited on Al₂O₃. The noble metals Pt, Pd, Rh the metal oxides CuO, SnO₂, CoO, Ag₂O, In₂O₃, catalysts were examined. Also investigated were noble metal Pt, Pd, Rh-doped In₂O₃/Al₂O₃ catalysts prepared by single sol–gel method. Both were studied for their capability to reduce NO by propene under lean conditions. In order to improve the catalytic activity and the temperature window, the intermediate addition propene between a Pt/Al₂O₃ oxidation and metal oxide combined catalyst system was also studied. Pt/Al₂O₃ and In₂O₃/Al₂O₃ combined catalyst showed high NO reduction activity in a wider temperature window, and more than 60% NO conversion was observed in the temperature range of 300–550 °C.     

Hydrodeoxygenation of guaiacol on noble metal catalysts

Hydrodeoxygenation (HDO) performed at high temperatures and pressures is one alternative for upgrading of pyrolysis oils from biomass. Studies on zirconia-supported mono- and bimetallic noble metal (Rh, Pd, Pt) catalysts showed these catalysts to be active and selective in the hydrogenation of guaiacol (GUA) at 100 °C and in the HDO of GUA at 300 °C. GUA was used as model compound for wood-based pyrolysis oil. At the temperatures tested, the performance of the noble metal catalysts, especially the Rh-containing catalysts was similar or better than that of the conventional sulfided CoMo/Al₂O₃ catalyst. The carbon deposition on the noble metal catalysts was lower than that on the sulfided CoMo/Al₂O₃ catalyst. The performance of the Rh-containing catalysts in the reactions of GUA at the tested conditions demonstrates their potential in the upgrading of wood-based pyrolysis oils.     

High efficiency of noble metal and metal oxide catalyst systems for the selective reduction of NO with propene in lean exhaust gas

An investigation was conducted of noble metal and metal oxide catalysts deposited on Al₂O₃. The noble metals Pt, Pd, Rh the metal oxides CuO, SnO₂, CoO, Ag₂O, In₂O₃, catalysts were examined. Also investigated were noble metal Pt, Pd, Rh-doped In₂O₃/Al₂O₃ catalysts prepared by single sol–gel method. Both were studied for their capability to reduce NO by propene under lean conditions. In order to improve the catalytic activity and the temperature window, the intermediate addition propene between a Pt/Al₂O₃ oxidation and metal oxide combined catalyst system was also studied. Pt/Al₂O₃ and In₂O₃/Al₂O₃ combined catalyst showed high NO reduction activity in a wider temperature window, and more than 60% NO conversion was observed in the temperature range of 300–550 °C.

     

Nano-gold supported on Fe2O3: A highly active catalyst for low temperature oxidative destruction of methane green house gas from exhaust/waste gases

A number of nano-gold catalysts were prepared by depositing gold on different metal oxides (viz. Fe₂O₃, Al₂O₃, Co₃O₄, MnO₂, CeO₂, MgO, Ga₂O₃ and TiO₂), using the homogeneous deposition precipitation (HDP) technique. The catalysts were evaluated for their performance in the combustion of methane (1 mol% in air) at different temperatures (300–600 °C) for a GHSV of 51,000 h⁻¹. The supported nano-gold catalysts have been characterized for their gold loading (by ICP) and gold particle size (by TEM/HRTEM or XRD peak broadening). Among these nano-gold catalysts, the Au/Fe₂O₃ (Au loading = 6.1% and Au particle size = 8.5 nm) showed excellent performance. For this catalyst, temperature required for half the methane combustion was 387 °C, which is lower than that required for Pd(1%)/Al₂O₃ (400 °C) and Pt(1%)/Al₂O₃ (500 °C) under identical conditions. A detailed investigation on the influence of space velocity (GHSV = 10,000–100,000 cm³ g⁻¹ h⁻¹) at different temperatures (200–600 °C) on the oxidative destruction of methane over the Au/Fe₂O₃ catalyst has also been carried out. The Au/Fe₂O₃ catalyst prepared by the HDP method showed much higher methane combustion activity than that prepared by the conventional deposition precipitation (DP) method. The XPS analysis showed the presence of Au in the different oxidation states (Au⁰, Au¹⁺ and Au³⁺) in the catalyst.     

Co/Al2O3 catalysts promoted with noble metals for production of hydrogen by methane steam reforming

The effect of noble metal addition on the catalytic properties of Co/Al₂O₃ was evaluated for the steam reforming of methane. Co/Al₂O₃ catalysts were prepared with addition of different noble metals (Pt, Pd, Ru and Ir 0.3 wt.%) by a wetness impregnation method and characterized by UV–vis spectroscopy, temperature programmed reduction (TPR) and temperature programmed oxidation (TPO) of the reduced catalysts. The UV–vis spectra of the samples indicate that, most likely, large amounts of the supported cobalt form Co species in which cobalt is in octahedral and tetrahedral symmetries. No peaks assigned to cobalt species from aluminate were found for the promoted and unpromoted cobalt catalysts. TPO analyses showed that the addition of the noble metals on the Co/Al₂O₃ catalyst leads to a more stable metallic state and less susceptible to the deactivation process during the reforming reaction. The Co/Al₂O₃ promoted with Pt showed higher stability and selectivity for H₂production during the methane steam reforming.     

Controlled modification of Pt/Al2O3 for the preferential oxidation of CO in hydrogen: A comparative study of modifying element

The catalytic performance of a series of Pt/Al₂O₃ catalysts, modified with Cr, Mn, Fe, Co, Ni, Cu and Sn, has been tested for the preferential oxidation of CO in hydrogen. The promoters were deposited onto the surface of a 5 wt.% monometallic Pt/Al₂O₃ catalyst using a controlled surface approach, to give a nominal promoter:Pt surface atomic ratio of 1:2 (corresponding to typically 0.15–0.25 wt.% of the promoting metal). The aim of this approach was to selectively create the Pt-promoter oxide interfacial sites considered to be important for the non-competitive dual-site mechanism proposed for such promoted catalysts. In this mechanism the promoting oxide is believed to act as an active oxygen provider, providing oxygen for the oxidation of the CO on the Pt. The catalysts were characterised using TEM, EDX, ICP-AES and CO chemisorption and results suggest that the promoter was successfully deposited on to the Pt surface. Even at the low loadings of promoter used, significant enhancement was observed in the catalytic performance of the PROX reaction in a simulated reformate mixture, for the Fe- and Co-promoted catalysts in particular (and to a lesser extent the Mn, Sn, Cu- and Ni-promoted catalysts), highlighting the successful preparation of the Pt-promoting metal oxide interfacial sites. The Mn-promoted catalyst, however showed no enhancement in the absence of water suggesting that the form of the promoting metal oxide may be particularly important for promotion of Pt for the PROX reaction.     

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.     

Catalytic oxidation for carbon-black simulating diesel particulate matter over promoted Pt/Al2O3 catalysts

Catalytic oxidation activity of carbon-black (CB) simulating the soot of diesel particulate matters to CO₂ over 3Pt/Al₂O₃, 3Pt5Mn/Al₂O₃ and 3Pt/30Ba–Al₂O₃ catalysts is investigated with model gases of diesel emission. In case of the large amount of CB compared to the amount of catalyst (3/1, w/w) in the mixture sample, insufficient oxygen at the point of sudden increase in the amount of CO₂ is leaded to the partial oxidation using the lattice oxygen of the catalyst. And the peaks of CO₂ after the first peak were attributed to the regional combustion of the CB, which was not in contact with catalyst particles. The fresh 3Pt5Mn was estimated to the oxidation states on the catalyst surface by XPS. For used sample at 700 °C, the BEs of Pt 4d5 was revealed to metallic state Pt(0) (314.4 eV) in a predominant levels compared with Pt(II) (317.3 eV). While BEs of Mn 2p were similar to that obtained from the fresh 3Pt5Mn. It is suggested that Pt is in charge of the roles in CB-oxidation, using the lattice oxygen of the catalyst. Two-stage catalytic system with the strategies of promoting the soot oxidation and NO_x reduction, simultaneously, were composed of the CB oxidation catalyst and the diesel oxidation catalyst. The catalytic oxidation of CB was accelerated by activated oxidants and exothermic reaction resulted from the diesel oxidation catalyst, which lies in upstream of two-stage. But the system with the CB oxidation catalyst sited in the upstream showed the initiation of CB oxidation at a lower temperature than the other case. Two-stage catalytic system composed of 3Pt5Mn with CB in the upstream and DOC in the downstream showed high oxidation activity with 95% consumption rate of CB to the total loaded CB in the range of 100–500 °C during the TPR process.     

Cu/Al2O3 catalysts for soot oxidation: Copper loading effect

Cu/Al₂O₃ catalysts with metal loading from 0.64 to 8.8 wt.% have been prepared and characterized by different techniques: N₂ adsorption at −196 °C (BET surface area), ICP (Cu loading), XRD, selective copper surface oxidation with N₂O (Cu dispersion), TPR-H₂ (redox properties), and XPS (copper surface species). The catalytic activity for soot oxidation has been tested both in air and NO_x/O₂. The activity in air depends on the amount of easily-reduced Cu(II) species, which are reduced around 275 °C under TPR-H₂ conditions. The amount of the most active Cu(II) species increases with the copper loading from Cu_1% to Cu_5% and remains almost constant for higher copper loading. In the presence of NO_x, the first step of the mechanism is NO oxidation to NO₂, and the catalytic activity for this reaction depends on the copper loading. For catalysts with copper loading between Cu_1% and Cu_5%, the catalytic activity for soot oxidation in the presence of NO_x depends on NO₂ formation. For catalysts with higher copper loading this trend is not followed because of the low reactivity of model soot at the temperature of maximum NO₂ production. Regardless the copper loading, all the catalysts improve the selectivity towards CO₂ formation as soot oxidation product both under air and NO_x/O₂.     

Electro-reduction of oxygen and electro-oxidation of methanol at Pd monolayer-modified macroporous Pt electrode

With polystyrene latex spheres self-assembled on indium tin oxide-coated glass electrode as templates, highly ordered macroporous Pt was prepared by electrochemical deposition. Then, the macroporous Pt was modified by Pd monolayer involving the galvanic displacement of Cu monolayer formed by under-potential deposition on macroporous Pt. Electrocatalytic properties of the Pd-modified macroporous Pt electrode for oxygen reduction were investigated by cyclic voltammetry and chronoamperometry in O₂-saturated solution containing 0.1 M HClO₄. Methanol electro-oxidation on the Pd-modified macroporous Pt surfaces in 0.5 M H₂SO₄ containing 1 M CH₃OH was studied by cyclic voltammetry. The corresponding results showed that Pd-modified macroporous Pt electrode had negative catalytic activity for methanol oxidation in compared with macroporous Pt. However, Pd-modified macroporous Pt electrode had positive electrocatalytic activity to O₂ reduction.     

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.     

A comparative study of the selective oxidation of NH3 to N2 over gold, silver and copper catalysts and the effect of addition of Li2O and CeOx

This paper describes the selective oxidation of ammonia into nitrogen over copper, silver and gold catalysts between room temperature and 400 °C using different NH₃/O₂ ratios. The effect of addition of CeO_x and Li₂O on the activity and selectivity is also discussed. The results show that copper and silver are very active and selective toward N₂. However the multicomponent catalysts: M/Li₂O/CeO_x/Al₂O₃ (M: Au, Ag, Cu) perform the best. On all three metal containing catalysts the activity and selectivity is influenced by the particle size and the interaction between metal particles and support.     

Modulation Excitation X-Ray Absorption Spectroscopy to Probe Surface Species on Heterogeneous Catalysts

The advantages and open questions of the combination of modulation excitation spectroscopy and phase sensitive detection (PSD) with X-ray absorption spectroscopy (XAS) for the analysis of heterogeneous catalysts at work are reviewed. The characteristic spectral signatures of two different edges (Pd K and Pt L₃) are described in relation to the red-ox chemistry of Pd/Al₂O₃ and Pt/Al₂O₃ with respect to NO reduction by CO and CO oxidation, respectively. Both examples demonstrate that PSD makes XAS sensitive to potentially active species for the catalytic reaction.     

Dependence of Synergetic Effect of Palladium–Manganese-Hexaaluminate Combustion Catalyst on Nature of Palladium Precursor

A synergetic effect in the methane oxidation activity of palladium and manganese hexaaluminate was studied over Pd-modified manganese-hexaaluminate catalysts, prepared by incipient wetness impregnation and calcined at 1,200?°C. The magnitude of the synergetic effect is found to be depends on the palladium precursor: it is higher for palladium nitrate and palladium acetate than for tetrachloropalladic acid. The Pd/MnLaAl₁₁O₁₉ catalysts were characterized by X-ray diffraction, X-ray microanalysis, transmission electron microscope and temperature-programmed reduction with hydrogen. These data were compared with the properties of Pd/Al₂O₃ catalysts. At variation of Pd-precursors, a minor trend to the decrease of the Pd particle size was observed at transition from the ex-chloride Pd/MnLaAl₁₁O₁₉ catalyst with uniform Pd-distribution profile to the ex-nitrate and ex-acetate catalysts with egg-shell Pd-distribution. Slightly smaller size of metal palladium particles in the ex-nitrate and ex-acetate catalysts leads to the formation of larger amount of PdO dispersed on their surface during oxygen-pretreatment in H₂-TPR experiments (Pd/PdO atomic ratio was 1/4) and under methane-oxidation mixture in comparison with ex-chloride catalysts (Pd/PdO?=?4/1). The palladium addition to manganese-hexaaluminate changes strongly its redox properties, as result Mn³⁺ reduction to Mn²⁺ take place about 100?°C below that of pure hexaalunimate. The latter indicate probably on the higher oxygen mobility in Pd-modified manganese-hexaaluminate. A higher PdO/Pd ratio formed in the ex-nitrate and ex-acetate Pd-modified manganese-hexaaluminate catalysts together with the high oxygen mobility provide the synergetic effect in methane oxidation activity at light-off temperature region. The high catalytic activity of manganese-hexaaluminate ensures methane combustion efficiency of the Pd-modified manganese-hexaaluminate catalysts at temperature above 700?°C.     

In situ DRIFTS study of the effect of structure (CeO2–La2O3) and surface (Na) modifiers on the catalytic and surface behaviour of Pt/γ-Al2O3 catalyst under simulated exhaust conditions

The nature and relative populations of adsorbed species formed on the surface of un-promoted and sodium-promoted Pt catalysts supported either on bare Al₂O₃ or CeO₂/La₂O₃-modified Al₂O₃, were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under simulated automobile exhaust conditions (CO + NO + C₃H₆ + O₂) at the stoichiometric point. The DRIFT spectra indicate that interaction of the reaction mixture with the Pt/Al₂O₃ catalyst leads mainly to formation of formates and acetates on the support and carbonyl species on partially positively charged Pt atoms (Pt^δ+). Although enrichment of Al₂O₃ with lanthanide elements (CeO₂ and La₂O₃) does not significantly modify the carboxylate species formed on the support, it causes significant modification of the oxidation state of Pt, as indicated by the appearance of a substantial population of carbonyl species on reduced Pt sites (Pt⁰–CO). This modification of the Pt component is enhanced when Na-promotion is used, leading to formation of carbonyl species only on electron enriched Pt (i.e., fully reduced Pt⁰ sites) and to the formation of NCO on these Pt entities (2180 cm⁻¹). The latter are thought to result from enhanced NO dissociation at Na-modified Pt sites. These results correlate well with observed differences in the catalytic performance of the three different systems.     

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.     

An XPS evidence of the effect of the electronic state of Pd on CH4 oxidation on Pd/γ–Al2O3 catalysts

In this work, we investigated the catalytic activity of 2% Pd/γ–Al₂O₃ and 2% Pd–1% Sn/γ–Al₂O₃ for CH₄ oxidation in lean conditions in the presence and in the absence of SO₂ in the reaction feed. The catalysts were studied by the Pd3d_5/2 electron binding energy values determined by XPS analysis. Sulfates formation and/or tin addition to Pd/Al₂O₃ resulted in an increase of the Pd3d_5/2 electron binding energy. Results showed a direct relation between Pd activity for CH₄ oxidation and the degree of oxidation of Pd species.     

Fe2O3/Al2O3 catalysts for the N2O decomposition in the nitric acid industry

Fe₂O₃ catalysts supported on Al₂O₃ were used to remove nitrous oxide from the nitric acid plant simulated process stream (containing O₂, NO and H₂O). Catalysts were prepared by the coprecipitation method and were characterized for their physico-chemical properties by BET, XRD, AFM and TPR analysis. A strong influence of the post-preparation heating conditions on the structural and catalytic properties of the catalysts has been evidenced. Laboratory tests revealed 95% conversion of N₂O at temperature 750 °C and a slight decrease in activity in the presence of H₂O and NO. The catalysts were inert towards decomposition of NO. The pilot-plant reactor and real plant studies (up to 3300 h time-on-stream) confirmed high activity and very good mechanical stability of the catalysts as well as no decomposition of nitric oxide.     

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).