Nano-crystalline Ceria Catalysts for the Abatement of Polycyclic Aromatic Hydrocarbons

Nano-crystalline cerium oxide catalysts have been prepared by precipitation and evaluated for the total catalytic oxidation of naphthalene, which is a polycyclic aromatic hydrocarbon (PAH). Ceria synthesised by precipitation with urea was the most active catalyst for oxidation of naphthalene to carbon dioxide. The urea precipitated CeO₂ demonstrated over 90% naphthalene conversion to carbon dioxide at 175°C (100 ppm naphthalene, GHSV=25,000 h⁻¹), whilst ceria precipitated via a carbonate only gave 90% conversion at 275°C. Comparison with known high activity total oxidation catalysts, Mn₂O₃ and 0.5% Pt/γ-Al₂O₃, showed that the urea precipitated CeO₂ was a more effective catalyst for naphthalene total oxidation. At temperatures below those required to achieve catalytic activity the adsorption capacity of urea precipitated ceria for naphthalene was considerably greater than any of the other catalysts examined. The high adsorption capacity of the material provides the advantage that it can be used as a combined catalyst and adsorbent to remove PAHs from waste streams.     

Ceria and Gold/Ceria Catalysts for the Abatement of Polycyclic Aromatic Hydrocarbons: An In Situ DRIFTS Study

Gold catalysts supported on nano-crystalline ceria prepared by deposition precipitation have been characterised and tested for the total oxidation of naphthalene. Two different precipitation methods were used to prepare the nano-crystalline ceria supports and it was observed that although both supports were active materials for naphthalene oxidation, ceria synthesized by homogeneous precipitation with urea was markedly more active than CeO₂ precipitated by carbonate. The addition of gold to both active CeO₂ catalysts resulted in different effects for the total oxidation of naphthalene. Gold addition promotes the naphthalene conversion to CO₂ when ceria is prepared by precipitation with carbonates, whilst the light off temperature is shifted towards higher temperatures when gold is added to ceria synthesized by the urea method. This behaviour has been related to a change in the support characteristics and a removal of the carbonate surface species, when gold is deposited onto the ceria support.     

The role of support on the performance of platinum-based catalysts for the total oxidation of polycyclic aromatic hydrocarbons

A range of Pt supported catalysts have been evaluated for the total oxidation of naphthalene. Catalysts contained 0.5 wt% Pt on a range of supports (γ-Al₂O₃, TiO₂, SiO₂, SnO₂, and CeO₂₎. SiO₂ was the best support, the 0.5%Pt/SiO₂ catalyst showing a conversion to carbon dioxide of over 90% at 200 °C (100 vppm naphthalene, GHSV = 45,000 h⁻¹). The catalyst also showed a considerably higher activity (in the temperature range 100–175 °C) than a CeO₂ catalyst recently reported to be one of the most effective catalysts for the total oxidation of naphthalene. The high activity of the 0.5%Pt/SiO₂ catalyst has been attributed to the relatively low dispersion and relatively large size of Pt particles. Furthermore, due to the acidic and non-reducible nature of the SiO₂, platinum is expected to have a weak interaction with the support. XPS data identified the presence of Pt⁰ on the surface and this contributes to the high activity.     

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-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.     

Kinetic modeling of CO oxidation on Pt/CeO2 in a gradientless reactor

Cerium oxide is a major additive in three-way catalysts used in emission control of automobile exhaust. Pt/CeO₂ was studied in order to better understand the role of ceria in promoting CO oxidation reaction. The kinetics of carbon monoxide oxidation on Pt/cerium oxide catalyst, was studied over the temperature range 100–170°C. Steady state kinetic measurements of CO oxidation were obtained in a computer controlled micro-CSTR reactor. Activation energies were reported to vary between 39·5 and 51·2 kJ mol⁻¹. At low concentrations of either reactant (CO, O₂) and total conversion, the catalyst exhibited multiple steady states, similar to the multiplicity behavior of Pt/Al₂O₃. The total conversion was reached at 120°C. In comparison, the total conversion at low reactant concentrations was reached at a temperature of 148°C for the alumina-supported catalyst. Langmuir–Hinshelwood mechanisms gave a good fit to the data. However, no single rate expression could effectively describe the CO oxidation data over the whole concentration in the product of the CSTR reactor. The facts gathered indicate that oxygen adsorbed on interfacial Pt/Ce sites and ceria lattice oxygen provides oxygen for CO oxidation. Cerium oxide has been found to lower CO oxidation activation energy, enhance reaction activity and tends to suppress the usual CO inhibition effect.     

The Oxidation of Methane at Low Temperatures Over Zirconia-Supported Pd,Ir and Pt Catalysts and Deactivation by Sulfur Poisoning

Abstract  

The oxidation of methane over zirconia-supported Pd, Ir, and Pt catalysts at low temperatures under an oxidizing atmosphere and the effect of SO₂ on this reaction were investigated. An Ir–Pt/ZrO₂ catalyst exhibited high activity and low deactivation by SO₂. Characterization of the catalyst indicated the presence of a highly oxidized Pt species that was stabilized by the addition of Ir.     

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.     

Influence of preparation conditions of nano-crystalline ceria catalysts on the total oxidation of naphthalene, a model polycyclic aromatic hydrocarbon

Nano-crystalline ceria catalysts prepared by homogeneous precipitation with urea were tested for the total oxidation of naphthalene, a model polycyclic aromatic hydrocarbon (PAH). Systematic variation of preparation conditions, including calcination temperature, calcination time and aging time, resulted in differences in surface area, reducibility, morphology and crystallite size of the CeO₂ catalyst and hence differences in catalytic performance. A combination of high surface area, small crystallite size and high oxygen defect concentration was found to favor the efficiency of the ceria catalysts for the total oxidation of naphthalene. Optimum preparation conditions for this study included: aging time of 12 h, calcination temperature of 500 °C and a calcination time of 6 h.     

Thin‐Film Catalytic Coating of a Microreactor for Preferential CO Oxidation over Pt Catalysts

Different Pt‐based catalyst layers have been prepared and tested in a stacked foil microreactor for CO oxidation and preferential oxidation of CO in presence of hydrogen. The reactions were performed on Pt without support by impregnation of a pre‐oxidized microstructured metal plate, Pt/Al₂O₃ and Pt/CeO₂ based on sol methods as well as Pt/nano‐Al₂O₃, a combined method of sol‐gel and nanoparticle slurry coating. The ceria based sol‐gel catalyst was much more active for CO oxidation than alumina based sol‐gel catalysts at low temperature. However, total oxidation was only obtained at higher temperature on the alumina based catalysts. The combined method seems to have advantages in terms of less internal mass transfer limitation when trying to increase the catalyst coating thickness based on sol‐gel approaches due to no reduction of CO selectivity up to 300 °C reaction temperature. Experiments on CO oxidation with the Pt/CeO₂ catalyst have been conducted in an oxygen supply microreactor to evaluate the catalyst performance under sequential oxygen supply to reaction zone (CO excess).     

The Mechanism of N2O Decomposition on Fe-ZSM-5: An Isotope Labeling Study

The role of Fe promoters has been investigated on Pd/ceria, Pt/ceria and Rh/ceria catalysts for the water–gas shift (WGS) reaction in 25 torr of CO and H₂O under differential reaction conditions. While no enhancement was observed with Pt and Rh, the activity of Pd/ceria increased by as much as an order of magnitude upon the addition of an optimal amount of Fe. Similarly, the addition of 1 wt% Pd to an Fe₂O₃ catalyst increased the WGS rate at 453 K by a factor of 10 over that measured on Fe₂O₃ alone, while the addition of Pt or Rh to Fe₂O₃ had no effect on rates. The amount of Fe that was necessary to optimize the rates increased with Pd loading but was independent of the order in which Fe and Pd were added to the ceria. Increased WGS activity was also observed upon the addition of Fe to Pd supported on Ce_0.5Zr_0.5O₂. XRD measurements, performed after running the catalyst under WGS conditions, show the formation of a Fe–Pd alloy, even though similar measurements on an Fe/ceria catalyst showed that Fe₃O₄ was the stable phase for Fe in the absence of Pd. Possible implications of these results on the development of new WGS catalysts are discussed.     

A DFT Study on the Adsorption of Formic Acid and Its Oxidized Intermediates on (100) Facets of Pt,Au, Monolayer and Decorated Pt@Au Surfaces

Abstract  

In this work we have systematically characterized the adsorption of formic acid, its decomposed intermediates and products on the (100) surfaces of Pt, Au, monolayer and decorated Pt@Au surfaces. The calculated thermodynamic results validate our previous experimental results that the decorated Pt@Au surface may facilitate formic acid oxidation compared with the benchmark Pt catalyst; while the monolayer Pt@Au surface is not suitable for formic acid oxidation.     

Selective Oxidation of Glycerol Catalyzed by Rh/Activated Carbon: Importance of Support Surface Chemistry

Abstract  

Noble metal catalysts (Pt, Ir, Pd, Rh, Au) supported on activated carbon were assessed for glycerol oxidation. Rhodium is a highly efficient catalyst when the support has neutral or basic properties. The surface chemistry of activated carbon plays a key role in the performance.     

Deactivation and regeneration of Pt/γ-alumina and Pt/ceria–alumina catalysts for methane combustion in the presence of H2S

Ceria has been widely explored as an additive in alumina-supported precious metal catalysts due to a number of unique properties. The success of ceria and ceria-based materials is mainly attributed to the unique combination of an elevated oxygen transport capacity coupled with the ability to shift easily between reduced and oxidised sates. In this study the influence of CeO₂ addition to a Pt/Al₂O₃ catalyst for low temperature (<540 °C) methane oxidation in an oxidising environment has been investigated. The resistance to H₂S-poisoning and influence on catalyst regeneration by oxidation or reductive treatments has been studied. The addition of CeO₂ to the support creates an increase in the level of activity based primarily on the oxygen storage capacity offered by the cerium oxide, causing an increase in oxygen activation. The ceria–alumina-supported catalyst showed a greater shift to poorer activity upon exposure to H₂S. It appears sulphur compounds react with the oxygen storage component causing a decrease in oxygen transfer, removing any benefit offered by the ceria. However, the level of Pt-agglomeration and support changes were reduced with the incorporation of ceria, emphasising the stabilising effect and promotion of metal particle dispersion associated with ceria. In order to obtain the maximum benefit of ceria addition to the support structure in terms of activity a reductive pretreatment is required. Upon exposure to a reducing atmosphere, it appears a Pt–CeO₂ interaction generates greater levels of activity.     

CGO-based electrochemical catalysts for low temperature combustion of propene

This study has shown that the phenomenon of electrochemical promotion of catalysis or NEMCA effect can be used to activate a metal catalyst for the propene deep oxidation in the presence of water in the feed and in both stoichiometric and lean-burn conditions. The electrochemical catalysts were based on sputtered Pt films interfaced with gadolinium-doped ceria solid electrolyte. This system allows implementing NEMCA effect at quite low temperatures and electrochemical activation of propene deep oxidation was evidenced at temperatures as low as 190 °C. In addition, a new design of electrochemical catalysts was proposed by depositing a interlayer of strontium-doped lanthanum manganite between the CGO dense support and an ultra-thin coating of Pt. This concept allows to drastically decrease the Pt loading since the electronic conductivity is ensured by the perovskite layer.     

Low-Temperature Water–Gas Shift: Doping Ceria Improves Reducibility and Mobility of O-Bound Species and Catalyst Activity

Abstract  

A series of platinum loaded catalysts supported on cation (Me)-doped cerium dioxide (Me = Ba, La, Y, Hf and Zn) was prepared by co-precipitation of the Me-nitrates and impregnation of a Pt precursor. Low temperature water–gas shift activity depends on the nature of dopant employed, varying in the order of Ba > Y > Hf > La > undoped ceria > Zn. TPR-XANES measurements with flowing hydrogen reveal that adding dopants to ceria facilitate ceria reduction and increases the extents of both surface shell and bulk reduction of ceria. Experimental results confirm past theoretical models that dopants enhance both O-mobility and reducibility of ceria. DRIFTS measurements of the transient decomposition of formates in steam suggest that formate half-life follows the trend Zn > undoped ceria > La > Hf > Y > Ba, indicating that the formate decomposition rate is enhanced by the addition of most of the dopants tested. Taken together, the results suggest that dopant addition improves the WGS rate by increasing the O-mobility of O-bound associated intermediates. Therefore, less Pt and Ce, which are expensive, is required to achieve comparable levels of activity.     

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.     

Total oxidation of volatile organic compounds by vanadium promoted palladium-titania catalysts: Comparison of aromatic and polyaromatic compounds

Vanadium oxide, palladium oxide and mixed Pd/V-supported on titania catalysts have been prepared and tested in the total oxidation of volatile organic compounds (VOCs). A comparative study with two different aromatic VOCs (benzene and naphthalene) has been carried out. For benzene, the mixed Pd/V-catalysts presented the highest catalytic activity. However, whilst studies with benzene led to the formation of CO₂ only, the total conversion of naphthalene to CO₂ was not achieved throughout the full temperature range for naphthalene conversion. A naphthalene conversion to CO₂ of 99% was obtained over Pd/TiO₂, V/TiO₂ and Pd/V/TiO₂ catalysts at 275, 325 and 300 °C, respectively. Therefore, the requirements for an effective benzene total oxidation catalyst cannot be readily extrapolated to larger polycyclic aromatic compounds, as in the naphthalene oxidation the most active catalyst from an environmental point of view is Pd supported on TiO₂.     

3D Ridge-Valley Structure of a Pt-Ceria Catalyst: HRTEM and EELS Spectrum Imaging

Nano-ridge and valley structures formed as a result of self-aligned individual ceria nano-crystals, and HRSTEM/EELS revealed the 3-dimensional morphology profiles of the ridge and valley structures of the ceria support. After preparation of a Pt/ceria catalyst, HRSTEM indicated the preferential alignment of Pt nanoparticles inside the nano-valleys of the ceria support. EELS analyses confirmed changes in the oxidation state of ceria in the vicinity of Pt nanoparticles that are present at different depth in the valleys.