PdO x /Pd at Work in a Model Three-Way Catalyst for Methane Abatement Monitored by Operando XANES

The oxidation state of palladium in a model Pd/ACZ three-way catalyst was monitored by synchronous XANES and mass spectrometry during two consecutive heating (to 850 °C) and cooling (to 100 °C) cycles under stoichiometric conditions simulating exhaust after treatment of a natural gas engine. During heating in the first cycle, PdO reduction occurred around 500 °C and the initial fully oxidized state of Pd was never recovered upon heating and cooling cycles. A mixed Pd²⁺/Pd oxidation state was at work in the second cycle. Hence, the operando XANES study reveals that the PdO _x /Pd pair exists in a working catalyst but is less active than the catalyst in its initial state of fully oxidized palladium. It is also evident from XANES spectra that ceria–zirconia promotes re-oxidation of metallic Pd, thus reasonably sustaining catalytic activity after exposure to high temperatures.     

Pd-Perovskite Catalysts for Methane Emissions Abatement: Study of Pd Substitution Effects

Several perovskite-type oxide catalysts (LaMnO₃, LaMn_0.95Pd_0.05O₃, LaMn_0.9Pd_0.1O₃, LaMn_0.85Pd_0.15O₃, 6wt%Pd-LaMnO₃) were prepared, characterized, and tested as catalysts for methane oxidation. The half conversion temperature of methane over the best catalyst (LaMn_0.85Pd_0.15O₃) selected was 425 °C respect to 485 °C for LaMnO₃. This catalyst and the 6wt%Pd-LaMnO₃ one were then deposited on cordierite monoliths and tested. Half methane conversion (T ₅₀) was achieved at about 300 °C (GHSV = 10000 h^?1) for both catalytic converters. Conversely, the perovskite catalyst substituted with Pd showed a better thermal-proof property than that supporting dispersed Pd.     

Electrooxidation of formic acid on carbon supported PtxPd1−x (x = 0-1) nanocatalysts

In this work, carbon supported Pt_xPd_1−x (x = 0-1) nanocatalysts were investigated for formic acid oxidation. These catalysts were synthesized by a surfactant-stabilized method with 3-(N,N-dimethyldodecylammonio) propanesulfonate (SB12) as the stabilizer. They show better Pt/Pd dispersion and higher catalytic performance than the corresponding commercial catalysts. Furthermore, the electrocatalytic properties of Pt_xPd_1−x/C were found to depend strongly on the Pt/Pd deposition sequence and on the Pt/Pd atomic ratio. At a lower potential, formic acid oxidation current on co-deposited Pt_xPd_1−x/C catalysts increase with increasing Pd surface concentration. Nanoscale Pd/C is a promising formic acid oxidation catalyst candidate for the direct formic acid fuel cell.     

Liquid-phase oxidation of alcohols with oxygen catalysed by modified palladium(II) oxide

Benzyl and trans-cinnamyl alcohols are heterogeneously oxidised to the corresponding aldehydes by O₂ in liquid phase at 100 °C and ambient pressure using hydrous binary Pd^II–M oxides (M=Co^III, Fe^III, Mn^III and Cu^II) as catalysts. Modification of Pd^II oxide with transition metal cations greatly improves the catalytic activity and selectivity to aldehydes, Co^III and Fe^III being the most effective promoters. In benzyl alcohol oxidation in toluene solution, the Pd–Co system gives 85–100% selectivity to aldehydes at 53–95% alcohol conversion in 15–60 min reaction time. The catalyst can be re-used without loss of its activity and selectivity. The presence of a certain amount of water in the catalysts is essential for their performance. From TGA, the composition of the optimal Pd–Co catalyst can be approximated as PdO·(0.13–1.0)CoO(OH)·(2–3)H₂O. The oxidation of alcohols on Pd–M oxide catalysts is accompanied by transfer hydrogenation and decarbonylation side reactions, which is similar to the oxidation on the palladium metal. This indicates that the oxidation of alcohols on Pd–M oxide catalysts occurs via a dehydrogenation mechanism, with hydrogen being present on the catalyst surface.     

LaCo1−x Pd x O3 Perovskite-Type Oxides: Synthesis, Characterization and Simultaneous Removal of NO x and Diesel Soot

A series of naometric perovskite catalysts LaCo_1?x Pd _x O₃ (x = 0, 0.01, 0.03) were prepared via a solution combustion synthesis route using metal nitrates as oxidizers and urea as fuel. It is essential to add a certain amount of ammonia aqueous solution to Pd²⁺ ions solution in the catalyst preparation process. Homogeneous nanoparticles LaCo_1?x Pd _x O₃ catalysts with the sizes in the range of 68–122 nm were obtained and characterized by using of XRD, BET, H₂-TPR, XPS, SEM and TEM. Pd was successfully introduced into the LaCoO₃ perovskite lattices. Further information was obtained by using XPS upon the LaCo_0.97Pd_0.03O₃ (with NH₄OH) sample after H₂-TPR. The results revealed that surface Pd was reduced to the metallic state at the end of the first step in the H₂-TPR experiment, and some surface Co could be reduced to metallic Co simultaneously. The catalytic properties were investigated for simultaneous NO _x -soot removal reaction. The performance of LaCo_1?x Pd _x O₃ catalysts were greatly improved by the partial substitution of Pd. The maximum NO conversion into N₂ and the ignition temperature of soot are 32.8% and 265 °C, respectively.     

Effect of sol–gel conditions on BET surface area,pore volume,mean pore radius,palladium dispersion,palladium particle size,and catalytic CO oxidation activity of Pd/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> cryogels

Porous and homogeneous palladium-alumina cryogels were synthesized from aluminium sec-butoxide (ASB) and palladium nitrate through one-pot sol–gel processing in an aqueous system and subsequent freeze drying. In order to optimize the sol–gel conditions so that higher catalytic oxidation activity could be acquired after high temperatures heating, effects of H₂O/ASB, HNO₃/ASB, ethylenediamine/Pd, and urea/ASB ratios on BET surface area, pore volume, mean pore radius, Pd dispersion, Pd diameter, and catalytic CO oxidation activity of the Pd/Al₂O₃ cryogels were examined. It was revealed that optimized molar ratios were ASB:H₂O:HNO₃:urea?=?1:76:0.26:0.29 and ethylenediamine:Pd?=?3.4:1, at the ratios of which higher Pd dispersion and more superior catalytic activity were obtained. It was suggested that maintenance of as large BET surface area and pore volume as possible even after the heating was important to obtain high Pd dispersion, which consequently brought about superior catalytic activity. It was also suggested that porosity of the cryogel also played an important role in suppressing the sintering of palladium. The palladium-alumina cryogel prepared under the optimized sol–gel conditions was compared with corresponding commercial catalyst in regard to Pd particle size, Pd dispersion, PdO reducibility, catalytic CO oxidation activity. It was shown, after heating the cryogel at 800 °C for 5 h, that finer palladium particles with ca. 3.5 nm diameter were dispersed throughout alumina support with higher dispersion (ca. 32%), which was primarily responsible for higher catalytic oxidation activity on the cryogel.     

Influence of PdO content and pathway of its formation on methane combustion activity

The methane oxidation reaction is known to induce changes in the surface structure and composition of Pd catalysts; making it extremely arduous to relate the methane oxidation activity to specific catalyst properties by conventional methods (continuous flow reactor studies). To circumvent this, methodical pulse reactor studies have been undertaken to obtain correlations between the initial methane combustion activity and the catalyst properties (Pd⁰/PdO content and path of PdO formation). While the initial methane combustion activity (at 160–280 °C) continuously increased with increasing PdO concentration (0–100%) in the catalyst, it continuously decreased with increasing Pd⁰ content (0–100%). Controlled studies were undertaken to obtain catalysts with identical PdO content by two pathways: (i) by controlled partial oxidization of Pd⁰/Al₂O₃ and (ii) by controlled partial reduction of PdO/Al₂O₃. Interestingly, for a given PdO content, the catalysts obtained by partial oxidation of Pd⁰/Al₂O₃ showed a significantly superior performance to the catalyst obtained by partial reduction of PdO/Al₂O₃ for all the temperatures investigated. These studies unambiguously show that along with the relative concentration of PdO, the PdO formation pathway is also critical in deciding the methane combustion activity of the catalyst.     

Novel Pd–Au/TiO2 catalyst for the selective catalytic reduction of NOx by H2

Novel Pd–Au/TiO₂ catalyst exhibited high catalytic activity with a wide temperature window for the selective catalytic reduction of NO_x by H₂ in the presence of oxygen. The synergetic effect between Pd and Au contributes to the formation of Pd⁰ and Pd–Au alloy, thus promoting the NO_x reduction to proceed.     

Effect of CeO2 addition into Pd/Zr–Pr mixed oxide on three-way catalysis and thermal durability

XPS shows that the surface chemical state of Pd in Pd/ZrPrO_x is mainly Pd^4 +, which is likely doped in ZrPrO_x. However, the Pd^4 + is segregated out as PdO when Pd/ZrPrO_x is exposed at 1000 °C in air, resulting in the particle growth of Pd and deterioration. It has been found that the mixing Pd/ZrPrO_x with CeO₂ enhances the catalytic activity and thermal durability. Analysis using XPS and HAADF-STEM shows that the mixing effect of ceria is to stabilize Pd^4 + at the high temperature and ceria also functions as an excellent support for the segregated PdO, which migrates from ZrPrO_x to CeO₂.     

Pd/Ce0.9Cu0.1O1.9-Y2O3 catalysts for catalytic combustion of toluene and ethyl acetate

A dual-functional Pd/CuO-CeO₂-Y₂O₃ (Pd/CCY) using CuO-CeO₂ as precursor was prepared and tested for catalytic combustion of toluene and ethyl acetate. It was found that the catalyst is much higher active and thermally stable for combustion of both toluene and ethyl acetate, compared with the catalyst prepared with the conventional method. Therefore, the formation of the CuO-CeO₂ solid solution before coating is the key to high activity and thermal stability. Moreover, Pd/PdO species are the active phase for oxidation of toluene, while CuO or CuO-CeO₂ is responsible for oxidation of ethyl acetate, and the formation of the Pd probably impels the enhancement of activity for the catalyst calcined at 1000 °C.     

Surface Modification of Pd/α-Si3N4 Catalysts Through the Solvent Used During Synthesis. Implications on the CO Chemisorption Properties and Catalytic Performances

Palladium catalysts supported on α-Si₃N₄ were prepared by impregnation with Pd(II)-acetate dissolved either in toluene or in water. The mean metal particle size of ~0.5 wt% Pd catalysts was similar (~5 nm) and independent of the way of preparation. Nevertheless, the two catalysts present very different chemisorption behaviour chemisorptive and catalytic properties. Fourier transformed infrared (FTIR) spectra of adsorbed CO at different temperatures (ranging from room temperature to 300 °C) show a very different behaviour for both catalysts. While the CO adsorption states on the Pd/α-Si₃N₄ prepared in toluene are very similar to those generally measured for silica and/or alumina supported palladium catalysts, CO chemisorbs less strongly on Pd/α-Si₃N₄ prepared in water and on different adsorption sites. The Pd/α-Si₃N₄ catalyst obtained by aqueous impregnation is much less efficient for the methane total oxidation. It is less active and less stable: it deactivates strongly after 3 h on stream at 650 °C. The two catalysts present about the same activity for the 1,3-butadiene hydrogenation after stabilisation at 20 °C. But, the catalyst prepared in water shows a much better selectivity to butenes. The results are discussed in terms of the possible migration of silicon atoms from the silicon nitride support to the surface of the palladium particles, when the catalyst is prepared in water. This is not the case when prepared in an organic solvent.     

Low Temperature Catalytic Oxidation of Binary Mixture of Toluene and Acetone in the Presence of Ozone

This work reports on gas phase catalytic ozonation of a binary mixture of toluene and acetone and compares it with catalytic ozonation of single component acetone and toluene. Catalytic ozonation was conducted at 25–90 °C on MnO_x/γ-Al₂O₃ catalyst. XANES and EXAFS were used to identify formal oxidation state of Mn and local structure of manganese oxide in the catalyst. Absorption energy of Mn K-edge of the catalyst was determined to be 6553.86 eV indicating that the majority of manganese in the catalyst was in 3+ oxidation state. Catalytic ozonation in the mixture was favourable for removal of toluene, and repressive for removal of acetone. This was due to (a) lower apparent activation energy of catalytic ozonation of toluene (E_{a, Toluene}?=?31 kJ mol^?1?<?E_{a, Acetone}?=?40 kJ mol^?1) that led to higher reactivity of toluene with active oxygen species, and (b) inhibitory effect of accumulated carbonaceous byproducts on the acetone removal. Increase of reaction temperature enhanced conversion of both compounds, decreased the gap between toluene and acetone conversions, and improved CO_x yield. Overall degradation pathway of toluene and acetone in the mixture was determined by identifying the reaction intermediates and carbonaceous deposits on the catalyst. The observed mixture effects helped to understand potentials and limitations of catalytic ozonation in treating mixture of VOCs, which will aid in developing commercial air treatment systems.

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Influence of bismuth on the structure and activity of Pt and Pd nanocatalysts for the direct electrooxidation of NaBH4

In the past few years, borohydrides have gathered a lot of attention as an energy carrier for fuel cell application. Numerous investigations on both hydrogen generation and direct oxidation of NaBH₄ have been published. Nonetheless, in our knowledge, only a few catalysts are capable to completely perform the direct oxidation of NaBH₄ at low potentials without hydrogen evolution.In this work, carbon supported Pd_1−xBi_x/C and Pt_1−xBi_x/C nanocatalysts were synthesized by a “water in oil” microemulsion method. The influence of surface modifications of Pt and Pd by Bi on the electrooxidation of sodium borohydride in alkaline media was evaluated. Physical and electrochemical methods were applied to characterize the structure and surface of the synthesized catalysts.It was verified that bismuth is present at the surface of the bimetallic catalysts and that hydrogen adsorption/desorption reactions are strongly limited on Pt and Pd surfaces with high bismuth coverage. Although the onset potential for NaBH₄ oxidation on Pd_xBi_1−x/C catalysts is ca. 0.2 V higher than that for Pd/C, the presence of bismuth on palladium surface influences the reaction mechanism, limiting hydrogen evolution and oxidation in the case of Pd_0.8Bi_0.2 catalyst. On Pt_0.9Bi_0.1 catalyst the onset potential remains unchanged comparing to Pt/C and negligible hydrogen evolution was observed in the whole potential range where the catalyst is active. The number of exchanged electrons was calculated using the Koutecky-Levich equation and it was found that for Pt_0.9Bi_0.1 catalyst, ca. 8 electrons are exchanged per BH₄⁻ ion at low potentials. The presented results are remarkable evidencing that NaBH₄ can be directly oxidized at low potentials with high energy efficiency.     

Synergy between tungsten and palladium supported on titania for the catalytic total oxidation of propane

Titania-supported palladium catalysts modified by tungsten have been tested for the total oxidation of propane. The addition of tungsten significantly enhanced the catalytic activity. Highly active catalysts were prepared containing a low loading of 0.5 wt.% palladium, and activity increased as the tungsten loading was increased up to 6 wt.%. Catalysts were characterised using a variety of techniques, including powder X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction and aberration-corrected scanning transmission electron microscopy. Highly dispersed palladium nanoparticles were present on the catalyst with and without the addition of WO_x. However, the addition of WO_x slightly increases the average palladium particle size, and there was some evidence for the Pd forming epitaxial islands on the support in the tungsten-doped samples. Surface analysis identified a combination of Pd⁰ and Pd²⁺ on a Pd/TiO₂ catalyst, whereas all of the Pd loading was found in the form of Pd²⁺ with the addition of tungsten into the catalysts. At low tungsten loadings, isolated monotungstate and some polytungstate species were highly dispersed over the titania support. The concentration of polytungstate species increased as the loading was increased, and it was also promoted by the presence of palladium. The coverage of the highly dispersed tungstate species over the titania also increased as the tungsten loading increased. Some tungstate species were also found to be associated with the palladium oxide particles, and there was an enrichment of oxidised tungsten species at the peripheral interface of the palladium oxide nanoparticles and the titania. Sub-ambient temperature–programmed reduction experiments identified an increased concentration of highly reactive species on catalysts with palladium and tungsten present together, and we propose that the new WO_x-decorated interface between PdO_x and TiO₂ particles may be responsible for the enhanced catalytic activity in the co-impregnated catalysts.     

Properties and performance of Pd based catalysts for catalytic oxidation of volatile organic compounds

Catalytic oxidations of volatile organic compounds (VOCs) (benzene, toluene and o-xylene) over 1 wt% Pd/γ-Al₂O₃ catalyst were carried out to assess the properties and performance of the Pd based catalyst. The properties of the prepared catalysts were characterized by the Brunauer Emmett Teller (BET) surface area, H₂ chemisorption, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) analyses. The experimental results revealed a significant increase in VOCs conversion with the lapse of the reaction time at certain reaction temperatures. On the other hand, the hydrogen pretreated 1 wt% Pd/γ-Al₂O₃ catalyst, whose shape of conversion curve is similar to the non pretreated catalyst, led the conversion curves for the total oxidation of VOCs to be shifted to lower temperature. It was also found that such increases in VOCs conversion were highly dependent on the oxidation state of Pd and the growth of Pd particles in the catalyst. In addition, in the case of the catalyst consisting of the same oxidation state (PdO/Pd²⁺ or Pd⁰), the particle sizes possibly play a more important role in the catalytic activity. The activity order of 1 wt% Pd/γ-Al₂O₃ catalyst with respect to the VOC molecule was o-xylene > toluene > benzene.     

Mesoporous CexZr1−xO2 solid solutions supported CuO nanocatalysts for toluene total oxidation

Mesoporous Ce_xZr_1−xO₂ solid solutions were prepared by the surfactant-assisted method and used as support of CuO nanocatalysts for catalytic total oxidation of toluene. The prepared CuO/Ce_xZr_1−xO₂ catalysts have a wormhole-like mesoporous structure with high surface area and uniform pore size distribution, and the CuO nanoparticles were highly dispersed on the surface of Ce_xZr_1−xO₂. The doping of ZrO₂ in CeO₂ promotes the dispersion of active copper species and enhances the reducibility of copper species. The effect of Ce/Zr ratio, calcination temperature and CuO loading amount on the catalytic performance of CuO/Ce_xZr_1−xO₂ was investigated in detail. The 400 °C-calcined 8%CuO/Ce_0.8Zr_0.2O₂ catalyst exhibits the highest activity with the complete toluene conversion temperature of 275 °C at the condition of GHSH = 33,000 h⁻¹ and the toluene concentration of 4400 ppm. The interfacial interaction between CuO and the Ce_xZr_1−xO₂ support, highly dispersed CuO nanoparticles and the nature of the support contribute to the high catalytic activity of mesoporous CuO/Ce_xZr_1−xO₂ nanocatalysts.     

Highly Active La1−x K x CoO3 Perovskite-type Complex Oxide Catalysts for the Simultaneous Removal of Diesel Soot and Nitrogen Oxides Under Loose Contact Conditions

The nanometric La_1?x K _x CoO₃ (x = 0–0.30) perovskite-type oxides were prepared by a citric acid-ligated method. The catalysts were characterized by means of XRD, IR, BET, XPS and SEM. The catalytic activity for the simultaneous removal of soot and nitrogen oxides was evaluated by a technique of the temperature-programmed oxidation reaction. In the LaCoO₃ catalyst, the partial substitution of La³⁺ at A-site by alkali metal K⁺ enhanced the catalytic activity for the oxidation of soot particle and reduction of NO _x . The La_0.70K_0.30CoO₃ oxides are good candidate catalysts for the simultaneous removal of soot particle and NO _x . The combustion temperatures for soot particles over the La_0.70K_0.30CoO₃ catalyst are in the range from 289 to 461 °C, the selectivity of CO₂ is 98.4% and the conversion of NO to N₂ is 34.6% under loose contact conditions. The possible reasons that can lead to the activity enhancement for the K-substitution samples compared to the unsubstituted sample (LaCoO₃) were given. The particle size has a large effect on its catalytic performance for the simultaneous removal of diesel soot and nitrogen oxides.     

Performance Comparison of Pt1–xPdx/Carbon Nanotubes Catalysts in Both Electrodes of Polymer Electrolyte Membrane Fuel Cells

The catalytic activity of Pt_1–xPd_x nanoparticles supported on carbon nanotubes (CNTs) was evaluated for both the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) of polymer electrolyte membrane fuel cells (PEMFCs). Using a colloidal method, Pt_1–xPd_x/CNTs catalysts (x = 0, 0.46, 0.76, and 0.9) were prepared, and their physical and electrochemical characteristics were analyzed using a variety of characterization techniques, including XRD, TEM, energy dispersive spectrometer, cyclic voltammetry, and electrochemical impedance spectroscopy. Both Pt and Pd atoms formed a continuous solid solution and thus were randomly mixed in Pt_1–xPd_x nanoparticles. Due to the high hydrogen absorption of Pd, the use of Pd in the catalyst provided an advantage for HOR but a disadvantage for ORR. The Pt_0.53Pd_0.47/CNTs catalyst in the anode and cathode showed the best cell performance of PEMFCs.     

The active phase of Au–Pd/Al2O3 for CO oxidation

Au–Pd/Al₂O₃ catalyst was prepared by modified impregnation method. It was found that the catalyst calcined in air at 473 K showed higher CO oxidation activity in comparison with the catalysts treated at other temperature. Nitrogen adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure spectroscopy (XANES) techniques were employed to study the relationship between the surface/bulk structures of these catalysts and their catalytic performance. The results indicated the higher activity was attributed to the smaller pore volume and co-existence of PdO and Au⁰ in their surface. The formation of Au_xPd_y alloy was unfavorable for the catalytic reaction.     

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.