Methanol Decomposition to Synthesis Gas Over Supported Pd Catalysts Prepared from Hydrotalcite Precursors

Supported Pd catalysts were prepared by solid-phase crystallization (spc) starting from MgAl hydrotalcite anionic clay minerals as the precursors, and were tested for the methanol decomposition to synthesis gas. The precursors based on [Mg₆Al₂(OH)₁₆CO₃ ^2-] · 4H₂O were prepared by coprecipitation from raw materials containing Pd²⁺, Mg²⁺ and Al³⁺ ions as the components of hydrotalcite. The precursors were thermally decomposed and reduced to afford supported Pd catalysts on MgAl mixed oxide. Pd-supported catalysts as a reference were also prepared by the impregnation (imp) method. The spc-Pd catalysts thus prepared afforded highly dispersed Pd metal particles and showed higher activity as well as lower activation energy than the imp-Pd catalysts. When the precursor was prepared under mild conditions, finer particles of Pd metal were formed over the catalyst, resulting in a high activity. In the case of spc-Pd catalysts, carbon monoxide adsorbed on Pd easily desorbed, compared with imp-Pd catalysts. It is likely that the high activity is due to the highly dispersed and stable Pd metal particles and the easy desorption of carbon monoxide.     

Methanol decomposition to synthesis gas over supported Pd catalysts prepared from synthetic anionic clays

Supported Pd or Rh catalysts were prepared by the solid-phase crystallization method starting from hydrotalcite anionic clay minerals based on [Mg₆Al₂(OH)₁₆CO ₂ ²⁻ ]·4H₂O as the precursors. The precursors were prepared by a coprecipitation method from the raw materials containing Pd²⁺ and various trivalent metal ions which can replace each site of Mg²⁺ and Al³⁺ in the hydrotalcite. Rh³⁺ was also used for preparing the catalyst as comparison. The precursors were then thermally decomposed and reduced to form supported Pd or Rh catalysts and used for the methanol decomposition to synthesis gas. Among the precursors tested, use of Mg–Cr hydrotalcite containing Pd²⁺ resulted in the formation of efficient Pd supported catalysts for the production of synthesis gas by selective decomposition of methanol at low temperature. Although Pd²⁺ cannot well replace the Mg²⁺ site in the hydrotalcite, the Pd supported catalyst (Pd/Mg–Cr) prepared by the solid-phase crystallization method formed highly dispersed Pd metal particles and showed much higher activity than that prepared by the conventional impregnation method. When the precursor was prepared under mild conditions, more fine particles of Pd metal were formed over the catalyst, resulting in high activity. It is likely that the high activity may be due to the highly dispersed and stable Pd metal particles assisted by the role of Cr as the co-catalyst. This revised version was published online in November 2006 with corrections to the Cover Date.     

DRIFT study of CO chemisorption on organometallics-derived Pd/MgO catalysts: the effect of chlorine

Magnesia-supported palladium catalysts were prepared from chemical vapour deposition (CVD) of [Pd(C₃H₅)(C₅H₅)] and incipient wetness impregnation of [Pd(C₃H₅)Cl]₂ and [Pd(acac)₂]. DRIFT spectroscopy of adsorbed CO on prereduced catalysts indicates that the electronic state of metal particles depends on the preparation methodology and markedly on the organometallic precursor. Inn-heptane reforming at 500°C, the highest activity and selectivity were shown by the CVD-based system. Chloride ions deriving from the impregnation solvent exchange with surface hydroxyls. Acidic Mg-Cl sites are thus formed, which induce a beneficial effect on the catalytic properties. The reforming activity collapsed when a chlorine-containing precursor was used, due to a partial coverage of the palladium surface with chemisorbed chlorine atoms.     

Preparation, active phase composition and Pd content of perovskite-type oxides

Perovskite-type oxides, containing Pd, were prepared via a combined sol–gel and combustion synthesis method and were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and BET specific surface area (SSA). Their activity as three-way catalysts for the abatement of carbon monoxide, methane and nitrogen oxides, emitted from stoichiometrically operating natural-gas-fuelled vehicles, was investigated under simulated exhaust conditions. The preparation conditions concerning the complexing agent, the use of additives and the thermal treatment of the precursor solutions, were investigated. The La_0.91Mn_0.85Ce_0.24Pd_0.05O_z and La_1.034Mn_0.966Pd_0.05O_z phases were the most active. Their activation by high-temperature hydrothermal treatment was ascribed to the migration of Pd out of the perovskite lattice and the formation of segregated PdO. The role of Pd was crucial for the catalytic activity of the active phase. A low Pd content favored the dispersion of oxidized Pd^x+, 2 ≤ x ≤ 4, and thus, enhanced catalytic activity. Oxidized Pd^x+ with x > 2 appeared to be less active than Pd²⁺. The catalytic activity of La_1.034Mn_0.966Pd_0.05O_z increased significantly when 8 ppm SO₂ were introduced in the reaction mixture.     

Thermal Stable Pd/Ce0.2Y0.8O2-δ Catalysts for CO and CH4 Oxidation

A series of Pd/Ce_0.2Y_0.8O_2-δ catalysts were prepared and tested for CO and CH₄ oxidation. It is found that the Pd/Ce_0.2Y_0.8O_2-δ catalysts are thermal stable, and the catalytic activity enhances with increasing calcination temperature. The enhancement of the activity is due to the decomposition of surface carbonate species and the increase in average oxidation state of Pd.     

Characteristics and Catalytic Properties of Pd/SiO2 Synthesized by One-step Flame Spray Pyrolysis in Liquid-phase Hydrogenation of 1-Heptyne

In this study, Pd/SiO₂ catalysts with 0.5–10 wt.% Pd loadings were prepared by one-step flame spray pyrolysis (FSP) and characterized by N₂ physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), CO chemisorption, and X-ray photoelectron spectroscopy (XPS). The average cluster/particles size of Pd as revealed by TEM were ca. 0.5–3 nm. The turnover frequencies (TOFs) of the flame-made catalysts decreased from 66.2 to 4.3 per s as Pd loading increased from 0.5 to 10 wt.%, suggesting that the catalytic activity was dependent on Pd particle/cluster size. However, there were no appreciable influences on 1-heptene selectivity. The flame-made Pd/SiO₂ showed better properties than the conventional prepared catalysts. Their advantages are not only the presence of large pores that facilitates diffusion of the reactants and products, but also the high-catalytic activity of as-synthesized catalysts so that further pretreatment is not necessary.     

Influence of Pd Precursor and Method of Preparation on Hydrodechlorination Activity of Alumina Supported Palladium Catalysts

A series of alumina supported Pd catalysts were prepared by the novel deposition-precipitation method adopting the chloride precursor (DP-Cl) of Pd and varying the metal content from 0.25 to 1.0 wt%. The catalytic properties of prepared catalysts were studied by various characterization techniques such as N₂ adsorption, CO chemisorption, TPR, XRD, XPS, and TEM techniques. The activity and stability of the catalysts were evaluated for the gas phase hydrodechlorination (HDC) of chlorobenzene operating at atmospheric pressure. At 1 wt% of Pd the catalyst showed higher chlorobenzene conversion with good stability when tested for a period of 25 h, whereas the other catalysts exhibited a loss in activity with time. In order to elucidate the exceptional activity and stability of this catalyst, a few more catalysts with 1 wt% Pd were prepared by impregnation technique and also using a non-chloride precursor, palladium nitrate. The 1 wt% DP-Cl catalyst again was found to be the best among the others. The activity and stability of the DP-Cl catalyst was also found to be superior to two low-dispersed catalysts, each with 10 wt% Pd, prepared by conventional impregnation method using the chloride and nitrate as the precursors. The characterization results reveal that the high activity and stability of the DP-Cl catalyst is related to the formation of electron deficient Pd species and its stabilization in the octahedral vacancies of alumina.     

Wet oxidation of wastewater containing hydrocarbons by novel supported Pd catalysts

Pd catalysts supported on TiO₂, ZrO₂, ZSM-5, MCM-41 and activated carbon were used in catalytic wet oxidation of hydrocarbons such as phenol, m-cresol and m-xylene. It was found that the Pd/TiO₂ catalyst was highly effective in the wet oxidation of hydrocarbon. The activities of catalysts with various hydrocarbon species, catalyst support, oxidation state of catalyst performed in a 3-phase slurry reactor show that reaction on Pd surface is more favorable than that in aqueous phase and that the active site is oxidized Pd in catalytic wet air oxidation of hydrocarbons. Based on the experimental results, a plausible reaction mechanism of wet oxidation of hydrocarbons catalyzed over Pd/TiO₂ catalyst was proposed. This catalyst is superior to other oxide catalysts because it suppressed the formation of hardly-degradable organic intermediates and polymer.     

Synthesis of Pd/C Catalyst by Modified Polyol Process for Formic Acid Electrooxidation

Pd catalyst supported on Vulcan XC‐72 carbon black was prepared by a modified polyol process. Its performance was compared with that of Pd/C catalyst prepared by impregnation reduction method by using NaBH₄ as a reducing agent for formic acid electrooxidation. Their physical characterisations were tested by means of energy dispersive analysis of X‐ray, X‐ray diffraction and transmission electron micrographs. Their activities were presented by cyclic voltammetry and chronoamperometry. The results show that the particle sizes of Pd/C catalysts prepared by modified polyol process and impregnation reduction method are 3.9 and 7.9 nm, respectively. The size dispersion of the former is narrower and more homogeneous than that of the latter. However, both of Pd/C catalysts display the characteristic diffraction peaks of a Pd face‐centred cubic (f.c.c.) crystal structure. The results of electrochemical measurements present that the Pd/C catalyst prepared by modified polyol process has the higher electrocatalytic activity and stability for formic acid electrooxidation in comparison to the Pd/C one by impregnation reduction method due to the particle size effect, and its peak current density of CV and the current of chronoamperometric curve at 1,000 s reach 33.2 and 11.2 mA cm^–2, respectively.     

Characterization of supported Pd-Pt catalysts by chemical probes

The temperature programmed (palladium) hydride decomposition appeared very effective method for diagnosing the extent of alloying in Pd-Pt/SiO₂ catalysts. Introduction of Pt to Pd catalysts moves the TPHD peak gradually towards lower temperatures and the shift in hydride decomposition peak is accompanied by a decrease in the amount of released hydrogen. The catalysts prepared the direct redox method showed much better alloy homogeneity than the samples prepared by coimpregnation. The results of probing with other techniques, i.e. catalytic probing with hydrodechlorination of CCl₂F₂ and XRD, were in line with TPHD data. The catalytic probing showed the presence of synergistic effect, a clear indication of Pd-Pt mixing. XRD data of post-reaction samples showed carbon incorporation into Pd-rich phase, whereas separate Pt-rich phase did not experience any bulk carbiding.     

Synthesis and physicochemical characterization of nanostructured Pd/ceria‐clinoptilolite catalyst used for p‐xylene abatement from waste gas streams at low temperature

BACKGROUND: The effect of Pd loading, xylene concentration and GHSV on xylene oxidation was tested over Pd/CeO₂(30%)‐clinoptilolite nanocatalysts at low temperatures. The catalysts were prepared by acid treatment of clinoptilolite, followed by the incipient wetness method of synthesized ceria and modified clinoptilolite in PdCl₂ solution. The synthesized nanocatalysts were characterized by XRD, FESEM, EDAX, TEM, BET, FTIR and TG‐DTG analysis. RESULTS: The XRD patterns confirmed the formation of crystalline ceria with an average crystallite size of 11.8 nm. FESEM images showed nanostructures in cavities of natural zeolite, brought about by ceria incorporation and acid activation. TEM analysis showed high dispersion of Pd with a size distribution between 6.6 and 36.7 nm. The quantitative analysis showed that the specific surface area of Pd(1%)/CeO₂(30%)‐clinoptilolite was 77 m² g^?1. The results showed that Pd(1%)/CeO₂(30%)‐clinoptilolite is the most appropriate catalyst, with the conversion more than 90% at 275 °C. CONCLUSIONS: Experimental results established effective performance and durability for the catalysts. As a result, clinoptilolite modification and ceria incorporation significantly altered the samples' morphology at nanoscale, improving the structure of composites and distribution of noble metals. A reaction path was suggested based on the adsorption‐migration of species to reveal the mechanism of p‐xylene oxidation over nanocatalysts. © 2012 Society of Chemical Industry     

Synergistic effect of Pd in methane combustion PdMnOx/Al2O3 catalysts

The catalytic methane combustion was investigated over alumina-supported monometallic and bimetallic palladium and manganese oxide catalysts. The catalytic activity of these systems showed that palladium incorporation on MnO_x/Al₂O₃ catalyst leads to an enhancement in methane combustion. The higher catalytic activity of the PdMn/Al₂O₃ catalysts is related to a greater mobility of lattice oxygen in manganese oxide in the presence of palladium. These bimetallic catalysts also showed a significant improvement in catalysts stability with respect the monometallic ones. Surface analysis of the used catalysts revealed less amount of coke and Mn/Al and Pd/Al atomic ratios almost unchanged, which is indication of absence of active phase sintering.     

Activation of Supported Pd Metal Catalysts for Selective Oxidation of Hydrogen to Hydrogen Peroxide

Catalytic activity of supported Pd metal catalysts (Pd metal deposited on carbon, alumina, gallia, ceria or thoria) showing almost no activity in the liquid-phase direct oxidation of H₂ to H₂O₂ (at 295 K) in acidic medium (0.02 M H₂SO₄) can be increased drastically by oxidizing them using different oxidizing agents, such as perchloric acid, H₂O₂, N₂O and air. In the case of the Pd/carbon (or alumina) catalyst, perchloric acid was found to be the most effective oxidizing agent. The order of the H₂-to-H₂O₂ conversion activity for the perchloric-acid-oxidized Pd/carbon (or alumina) and air-oxidized other metal oxide supported Pd catalysts is as follows: Pd/alumina < Pd/carbon < Pd/CeO₂ < Pd/ThO₂ < Pd/Ga₂O₃. The H₂ oxidation involves lattice oxygen from the oxidized catalysts. The catalyst activation results mostly from the oxidation of Pd metal from the catalyst producing bulk or sub-surface PdO. It also caused a drastic reduction in the H₂O₂ decomposition activity of the catalysts. There exists a close relationship between the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity in the oxidation process and the H₂O₂ decomposition activity of the catalysts; the higher the H₂O₂ decomposition activity, the lower the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity.     

Nanosized Pd/Pt and Pd/Rh Catalysts for Naphthalene Hydrogenation and Hydrogenolysis/Ring-opening

Pd/Rh and Pd/Pt catalysts supported on two different mesoporous materials – a Zr-doped MCM-41-type silica [Si/Zr = 5 w/w (SiZr)] and a commercial silica-alumina [Si/Al = 40:60 w/w (SiAl)] – were prepared by incipient wetness impregnation using nanosized suspensions of alloy particles prepared by polyol-mediated synthesis in diethylene glycol (DEG). The catalytic behaviour of these catalysts was investigated in the hydrogenation and hydrogenolysis/ring-opening of naphthalene at 6.0 MPa, by checking the role of both the main reaction conditions (temperature, contact time and H₂/naphthalene molar ratio) and increasing amounts of dibenzothiophene (DBT). The catalysts supported on SiAl showed higher activity than catalysts supported on SiZr, thus suggesting that activity is favoured by higher acidity of the support and/or higher interaction of the nanosized metal particles with the support. While using the SiZr support, weaker metal-support interactions took place by forming catalysts with bigger metal and/or metal oxide particles. Besides, the catalyst with lowest noble-metal content (0.3 wt.%) (SiAl-0.3Pd/Pt-5) had the greatest acidity and metal surface and, consequently, the highest activity. Furthermore, it exhibited a good thiotolerance in presence of increasing amounts of DBT in the feed, thus maintaining a high catalytic activity in the hydrogenation of naphthalene, although with decreased yield in trans- and cis-decalin (decahydronaphthalene or DeHN) and high-molecular-weight compounds (H.M.W.), with a corresponding increased yield in the partially hydrogenated tetralin (tetrahydronaphthalene or TeHN).     

Characterization study of CeO2 supported Pd catalyst for low-temperature carbon monoxide oxidation

Catalysts consisting of palladium supported on cerium dioxide (Pd/CeO₂) were prepared and used for carbon monoxide oxidation in a stoichiometric mixture of carbon monoxide and oxygen. Pd/CeO₂ exhibits high catalytic activity for the oxidation of CO, showing markedly enhanced catalytic activities due to the combined effect of palladium and cerium dioxide. The Pd/CeO₂ catalyst is superior to Pd/ZrO₂, Pd/Al₂O₃, Pd/TiO₂, Pd/ZSM-5 and Pd/SiO₂ catalysts with regard to the activity under the conditions examined. The catalysts were characterized by means of XRD and TPR. The position of the H₂-TPR peak shifts to lower temperature with increasing Pd loading from 0.25 to 2.0%. CeO₂ inhibits the hydrogen reduction of PdO. CO-TPR measurements have shown the existence of three peaks. The low-temperature peak (α) is due to the Pd hydroxide species. The β peak has been attributed to finely dispersed PdO. The high-temperature peak (γ) has been attributed to crystal phase PdO. Crystal phase PdO is more difficult to reduce by CO than finely dispersed PdO. On the basis of the catalytic activity and CO-TPR results, we conclude α species (Pd hydroxide) mainly contribute to the catalytic activity for low-temperature CO oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

FTIR study of the oxidation reaction of CO with O2 over bimetallic Pd–Rh/SiO2 catalysts in an oxidized state

CO species adsorbed on the surface of oxidized bimetallic Rh–Pd catalysts, prepared by coimpregnation and sequential impregnation methods, were analyzed in situ by IR spectroscopy, during the reaction of CO with O₂ in an oxidizing atmosphere. The results show that the two methods of impregnation lead to the existence of oxidized Rh on the surface of the bimetallic catalyst, however, in the case of the sequential impregnation method, the Pd surface is more reduced than in the case of catalysts prepared by coimpregnation. The simultaneous presence of reduced Pd and oxidized Rh that occurs in the catalysts prepared by sequential impregnation allows the existence of a synergistic effect similar to that proposed in the literature for the Pt–Rh system. The lower degree of oxidation of the Pd in the catalysts prepared by sequential impregnation, is mainly due to the fact that the Pd in these catalysts comes from the organic precursor palladium acetylacetonate, while in the catalysts prepared by coimpregnation, the Pd comes from the precursor PdCl₂. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of Pd/Al2O3 pretreatment on catalytic activity in cyclopentane/deuterium exchange

Big variations in overall activity and product selectivity in the cyclopentane/deuterium exchange reaction were found in effect of various pretreatments of two chlorine‐free Pd/γ‐Al₂O₃ catalysts. The most important changes are observed when severely prereduced (at 600 °C) Pd/Al₂O₃ catalysts have been reoxidised and mildly rereduced: the multiple type of exchange, typical of mildly pretreated Pd catalysts, is replaced by a stepwise mode, and a big increase in catalytic activity occurs. At this state, the Pd/γ‐Al₂O₃ catalysts retain some water (as surface hydroxyls) generated by reoxidation and mild reduction. Deuterium spillover from Pd onto alumina and changes in acidity of alumina are invoked to rationalize the kinetic results. Changes in the state of Pd after various pretreatments, as probed by temperature‐programmed hydride decomposition, can hardly be correlated with changes in the catalytic behaviour in the exchange reaction. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Stability of bimetallic Bi–Pd and Pb–Pd carbon-supported catalysts during their use in glyoxal oxidation

The incorporation of Bi or Pb as promoting elements in Pd-based carbon-supported catalysts drastically increases the catalytic activity in the selective oxidation of glyoxal into glyoxylic acid. Because partial dissolution of the promoter was clearly demonstrated by atomic absorption analysis of the reaction medium, experiments are performed to examine the stability of these catalysts. Dissolution tests in the presence of the individual constituents of the reaction medium (glyoxal, glyoxylate, glycolate, oxalate) were carried out in air or nitrogen to identify the factors responsible for Pb or Bi leaching. Pb- or Bi-promoted Pd/C catalysts were prepared by thermal degradation of acetate-type precursors and characterized by X-ray diffraction and X-ray photoelectron spectroscopy before and after their use in glyoxal oxidation. Promoter leaching increases with the reaction time. Monometallic Bi/C and Pb/C catalysts were found to lose smaller amounts of promoting agent than the bimetallic M–Pd/C (M=Bi, Pb) catalysts. Losses are more pronounced from Pb–Pd/C catalysts than from their Bi-based partners. Both glyoxal and glyoxylate seem to be among the main factors responsible for the promoter losses in relation to their complexing properties.     

The influence of the crystal structure of TiO2 support material on Pd catalysts for formic acid electrooxidation

The influence of the crystal structure of TiO₂ support material on Pd catalyst-mediated formic acid electrooxidation was investigated. Pd/TiO₂ catalysts were synthesized by loading Pd on TiO₂ with different crystal structures obtained by calcinations at different temperatures. Electrochemical tests showed that TiO₂ with the rutile structure improved the catalytic activity of Pd nanoparticles toward formic acid electrooxidation. Physicochemical and electrochemical characterizations revealed that the enhancement of Pd/TiO₂ (rutile) catalytic activity arose from uniform dispersion of Pd nanoparticles, an increase in surface-active sites, and good tolerance to the adsorption of poisonous intermediates (such as CO_ad, COOH_ad and so on).