Palladium-catalysed vapour phase aerobic acetoxylation of toluene to benzyl acetate

Supported Pd–Sb acetoxylation catalysts with different Pd and Sb amounts as well as varying supports were prepared by impregnation technique. The contents of Pd and Sb are varied over a wide range, for instance Pd is varied from 0.5 to 20 wt% by keeping Sb content at 8 wt%. In a similar way, Sb content is varied from 4 to 20 wt% by keeping Pd loading constant at 10 wt%. Four different supports such as TiO₂, γ-Al₂O₃, SiO₂ and ZrO₂ are applied at constant Pd (10 wt%) and Sb (8 wt%) contents. Catalytic performance of these solids is evaluated for the gas phase acetoxylation of toluene to benzyl acetate (BA) at T = 210 °C and p = 2 bar. XPS revealed a considerable loss of both Pd and Sb in the near-surface region in the used catalysts. TEM showed that Pd particles exhibit spherical morphology and their size increased dramatically in the spent catalysts compared to their corresponding fresh ones. Monometallic catalysts showed very poor acetoxylation performance but high total oxidation, which results in an increase of the yield of CO_x up to ca. 50%. However, combination of both Pd and Sb was found to suppress total oxidation and thereby enhance the acetoxylation performance with high BA selectivity of ≥85%. Catalytic activity was observed to increase continuously with increase in Pd loading. The catalyst with the highest Pd loading (20 wt% Pd) displayed the best performance (toluene conversion = > 90%, BA yield = > 75%). The activity is however decreasing with time-on-stream due to coke deposits. Nevertheless, the deactivated samples can be regenerated in air to restore their maximum activity. Nature of support, content of co-components showed strong influence on the catalytic performance.     

Sulphide-induced deactivation of Pd/Al2O3 as hydrodechlorination catalyst and its oxidative regeneration with permanganate

Pd-based catalysts have become important in environmental catalysis for their ability to hydrodechlorinate a wide range of chlorinated organic contaminants in water under ambient conditions. The success of their application in the remediation practice, e.g. for groundwater treatment, is often hindered by the sensitivity of Pd to poisoning by sulphur compounds. In this study, the stability and sulphide-induced deactivation behaviour of a highly active Pd/Al₂O₃ catalyst was investigated. The specific activity of Pd for the hydrodechlorination of chlorobenzene corresponds to rate coefficients up to k_Pd = 350 L g⁻¹ min⁻¹. The totally deactivated catalyst, resultant of sulphide poisoning, was regenerated with potassium permanganate. The pH value, as a key parameter which may influence the degree of deactivation as well as the efficiency of catalyst regeneration, was evaluated. Results show that in clean water the Pd/Al₂O₃ catalyst showed no inherent deactivation regardless of the ageing time and the pH value of the catalyst suspension. The degree of catalyst poisoning effected by 1.8–5.4 μM sulphide, corresponding to molar ratios of S:Pd_surface = 1.5–8.5, was observed to be higher under neutral and alkaline than under acidic conditions. The exposure of the catalyst to higher sulphide concentration of 14.2 μM resulted in complete catalyst deactivation regardless of the pH conditions. However, the efficacy of permanganate as oxidative regenerant for the fouled catalyst showed strong pH-dependence. A regeneration time of 10–30 min at low pH was sufficient to recover completely the high catalytic activity of Pd/Al₂O₃ for the hydrodechlorination reaction.     

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.     

Performance of Pd catalysts supported on nanocrystalline α-Al2O3 and Ni-modified α-Al2O3 in selective hydrogenation of acetylene

Nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ have been prepared by sol–gel and solvothermal methods and employed as supports for Pd catalysts. Regardless of the preparation method used, NiAl₂O₄ spinel was formed on the Ni-modified α-Al₂O₃ after calcination at 1150 °C. However, an addition of NiO peaks was also observed by X-ray diffraction for the solvothermal-made Ni-modified α-Al₂O₃ powder. Catalytic performances of the Pd catalysts supported on these nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ in selective hydrogenation of acetylene were found to be superior to those of the commercial α-Al₂O₃ supported one. Ethylene selectivities were improved in the order: Pd/Ni-modified α-Al₂O₃–sol–gel > Pd/Ni-modified α-Al₂O₃-solvothermal ≈ Pd/α-Al₂O₃–sol–gel > Pd/α-Al₂O₃-solvothermal  Pd/α-Al₂O₃-commerical. As revealed by NH₃ temperature program desorption studies, incorporation of Ni atoms in α-Al₂O₃ resulted in a significant decrease of acid sites on the alumina supports. Moreover, XPS revealed a shift of Pd 3d binding energy for Pd catalyst supported on Ni-modified α-Al₂O₃–sol–gel where only NiAl₂O₄ was formed, suggesting that the electronic properties of Pd may be modified.     

Conversion of biomass to fuel: Transesterification of vegetable oil to biodiesel using KF loaded nano-γ-Al2O3 as catalyst

KF-impregnated nanoparticles of γ-Al₂O₃ were calcinated and used as heterogeneous catalysts for the transesterification of vegetable oil with methanol for the synthesis of biodiesel (fatty acid methyl esters, FAME). The ratio of KF to nano-γ-Al₂O₃, calcination temperature, molar ratio of methanol/oil, transesterification reaction temperature and time, and the concentration of the catalyst were used as the parameters of the study. A methyl ester yield of 97.7 ± 2.14% was obtained under the catalyst preparation and transesterification conditions of KF loading of 15 wt%, calcination temperature of 773 K, 8 h of reaction time at 338 K, and using 3 wt% catalysts and molar ratio of methanol/oil of 15:1. This relatively high conversion of vegetable oil to biodiesel is considered to be associated with the achieved relatively high basicity of the catalyst surface (1.68 mmol/g) and the high surface to volume ratio of the nanoparticles of γ-Al₂O₃.     

Carbon nanotube-supported Pd–ZnO catalyst for hydrogenation of CO2 to methanol

A type of Pd–ZnO catalysts supported on multi-walled carbon nanotubes (MWCNTs) were developed, with excellent performance for CO₂ hydrogenation to methanol. Under reaction conditions of 3.0 MPa and 523 K, the observed turnover-frequency of CO₂ hydrogenation reached 1.15 × 10⁻² s⁻¹ over the 16%Pd_0.1Zn₁/CNTs(h-type). This value was 1.17 and 1.18 times that (0.98 × 10⁻² and 0.97 × 10⁻² s⁻¹) of the 35%Pd_0.1Zn₁/AC and 20%Pd_0.1Zn₁/γ-Al₂O₃ catalysts with the respective optimal Pd_0.1Zn₁-loading. Using the MWCNTs in place of AC or γ-Al₂O₃ as the catalyst support displayed little change in the apparent activation energy for the CO₂ hydrogenation, but led to an increase of surface concentration of the Pd⁰-species in the form of PdZn alloys, a kind of catalytically active Pd⁰-species closely associated with the methanol generation. On the other hand, the MWCNT-supported Pd–ZnO catalyst could reversibly adsorb a greater amount of hydrogen at temperatures ranging from room temperature to 623 K. This unique feature would help to generate a micro-environment with higher concentration of active H-adspecies at the surface of the functioning catalyst, thus increasing the rate of surface hydrogenation reactions. In comparison with the “Parallel-type (p-type)” MWCNTs, the “Herringbone-type (h-type)” MWCNTs possess more active surface (with more dangling bonds), and thus, higher capacity for adsorbing H₂, which make their promoting action more remarkable.     

Single step direct coating of 3-way catalysts on cordierite monolith by solution combustion method: High catalytic activity of Ce 0.98Pd0.02O2-δ

A new process of coating of active exhaust catalyst over γ-Al₂O₃ coated cordierite honeycomb is reported here. The process consists of (a) growing γ-Al₂O₃ on cordierite by solution combustion of Al(NO₃)3 and oxylyldihydrazide (ODH) at 600 °C and active catalyst phase Ce_0.98Pd_0.02O_2-δ on γ-Al₂O₃ coated cordierite again by combustion of ceric ammonium nitrate and ODH with 1.2 × 10⁻³ M PdCl₂ solution at 500 °C. Weight of active catalyst can be varied from 0.02 to 2 wt% which is sufficient but can be loaded even up to 12 wt% by repeating dip dry combustion. Adhesion of catalyst to cordierite surface is via oxide growth which is very strong. About 100% conversion of CO is achieved below 80 °C at a space velocity of 880 h⁻¹. At much higher space velocity of 21,000 h⁻¹, 100% conversion is obtained below 245 °C. Activation energy for CO oxidation is 8.4 kcal/mol. At a space velocity of 880 h⁻¹ 100% NO conversion is attained below 185 °C and 100% conversion of ‘HC’ C₂H₂ below 220 °C. At same space velocity 3-way catalytic performance over Ce_0.98Pd_0.02O_2-δ coated monolith shows 100% conversion of all the pollutants below 220 °C with 15% of excess oxygen. In this method, handling of nano material – powder is avoided.     

Carbon incorporation and deactivation of MgO(0 0 1) supported Pd nanoparticles during CO oxidation

We have investigated the structure and composition of the model catalyst system Pd/MgO(0 0 1) during oxidation, CO reduction and CO oxidation at near atmospheric pressures by a combination of in situ X-ray diffraction and ex situ transmission electron microscopy and spectroscopy techniques. From the in situ X-ray experiments, we find: (a) the Pd nanoparticles with 9 nm in diameter transform into epitaxial PdO above 10⁻¹ mbar O₂ pressure at 570 K, (b) the oxidation process can be reverted by CO exposure, recovering Pd nanoparticles in their initial orientation, and (c) during CO oxidation in a mixture of 50 mbar O₂ and 50 mbar CO a new phase is evolving with lattice constant close to the MgO substrate value, which we assign to expanded Pd nanoparticles forming upon carbon incorporation. Ex situ transmission electron microscopy and different spectroscopy techniques uncover the CO₂ induced growth of a disordered overlayer containing C, Mg and O, which forms during CO oxidation and leads to an overgrowth of Pd nanoparticles thereby deactivating the catalyst.     

Low-temperature catalytic combustion of methane over Pd/CeO2 prepared by deposition–precipitation method

Ceria supported 2 wt% Pd catalysts for low-temperature methane combustion were prepared by the impregnation (IM) and deposition–precipitation (DP) methods, which are denoted as Pd–IM and Pd–DP, respectively. DP was found to be an available method for achieving high activity and stability of the Pd/CeO₂ catalyst. The temperatures for methane ignition (T_10%) and total conversion (T_100%) over Pd–DP are 224 and 300 °C at GHSV of 50,000 h⁻¹, which are 83 and 110 °C lower than the corresponding temperatures of Pd/Al₂O₃. X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) analyses show that palladium species in Pd–DP is highly dispersed, positively charged and difficultly reduced. Raman spectra disclosed that the largest concentration of defects and/or oxygen vacancies was formed in Pd–DP catalyst. A kind of cationic PdO^δ+ sites with higher binding energies than PdO are in close vicinity to the oxygen vacancies in the CeO₂ support and might act as the active centers for methane oxidation. Furthermore, the deactivation and steam aging tests for Pd–DP showed that the performance of this type of palladium was very stable and could be repeatedly recovered after several long time aging tests.     

Characterization of Pd impregnated on metal/silica-pillared H-keyaites (M-SPK,M = Ti,Zr) catalysts for partial oxidation of methane to hydrogen

Pd(5) impregnated metal/silica-pillared H-keyaites (M-SPK, M = Ti, Zr) catalysts were prepared for the partial oxidation of methane (POM) to hydrogen. The catalysts were characterized by BET, TEM, SAXS and XPS. In addition, the catalytic yield of the POM to hydrogen over Pd(5) impregnated on M-SPK and Pd(5)/Al₂O₃, commercial catalyst were investigated in a fixed bed flow reactor under (Ed atmosphere. BET-specific surface areas, average pore sizes and nitrogen adsorption/desorption isotherms were 284.3–396.2 m²/g, 3.3–3.8 nm and type B on type IV isotherms for Pd(5)/M-SPK(M = Ti, Zr), and 90.5 m²/g, 8.3 nm and type E on type IV isotherm for Pd(5)/Al₂O₃, respectively. TEM images of SPK and Pd(5)/SPK showed the formation of mesoporous layer compounds, as well as the homogenous dispersion of Pd particles on the surface. SAXS peaks at 0.13 Å for fresh Pd(5)/SPK were maintained without being broken, even after about 53 h in stream at 973 K. XPS showed the existence of two oxidation states for Pd (Pd⁰ and Pd²⁺) on the surface of the catalyst, depending on the carrier, whereas the presence of Ti and Zr in SPK induced a change in the oxidation state (O²⁻, O⁻) of the catalyst. The yield values of the POM to hydrogen over Pd(5)/M-SPK(M = Ti, Zr) were 64.9% and 55.8%, respectively, at 973K, CH₄/O₂ = 2, GHSV = 8.4 × 10⁴ ml/g_cat h, and these values were kept constant even after 70 h in stream. These results confirm that Ti and Zr in SPK frame induced oxidation states of Pd, and that the yield of Pd(5)/M-SPK positively regulates the POM to hydrogen.     

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.     

Selective oxidation of propane and ethane on diluted Mo–V–Nb–Te mixed-oxide catalysts

Te-free and Te-containing Mo–V–Nb mixed oxide catalysts were diluted with several metal oxides (SiO₂, γ-Al₂O₃, α-Al₂O₃, Nb₂O₅, or ZrO₂), characterized, and tested in the oxidation of ethane and propane. Bulk and diluted Mo–V–Nb–Te catalysts exhibited high selectivity to ethylene (up to 96%) at ethane conversions <10%, whereas the corresponding Te-free catalysts exhibited lower selectivity to ethylene. The selectivity to ethylene decreased with the ethane conversion, with this effect depending strongly on the diluter and the catalyst composition. For propane oxidation, the presence of diluter exerted a negative effect on catalytic performance (decreasing the formation of acrylic acid), and α-Al₂O₃ can be considered only a relatively efficient diluter. The higher or lower interaction between diluter and active-phase precursors, promoting or hindering an unfavorable formation of the active and selective crystalline phase [i.e., Te₂M₂₀O₅₇ (M = Mo, V, and Nb)], determines the catalytic performance of these materials.     

ESR study of alumina-supported palladium catalyst

The ESR spectra of differently pretreated 0.97 wt% Pd/Al₂O₃ catalyst showed very broad signal atg 2.10 assigned to Pd⁺ ions. The intensity of this signal is stronger after pretreatments at higher temperatures (500–600 °C). This result appears to support our earlier idea (ref. [2]) as to an important role of electron-deficient palladium as an active centre in catalyzing the reaction of neopentane hydrogenolysis.     

Development of Monolithic Catalysts with Low Noble Metal Content for Diesel Vehicle Emission Control

Monolith washcoated catalysts with potential for diesel emission control have been developed. Two types of catalysts have been prepared for further study: (1) MnO _x supported on granulated -Al₂O₃, (2) MnO _x supported on cordierite monolith washcoated with -Al₂O₃. Both catalysts have been calcined at 500 and 900 °C and subsequently modified by doping with 0.1–1.0 wt% of Pt or Pd. The influence of the concentration of both manganese oxide (0–10 wt%) and noble metals Pt and Pd in the range 0–1.0 wt% on the catalytic activity in methane oxidation has been studied. Comparison of the catalytic activity of MnO _x /Al₂O₃ and MnO _x + Pt(Pd)/Al₂O₃ with that of a standard 1 wt%Pt/Al₂O₃ catalyst shows the existence of a synergetic effect. This effect is more pronounced for the samples calcined at 900 °C. The developed monolithic catalysts MnO _x + Pt(Pd)/Al₂O₃ demonstrate higher activity and thermal stability (up to 900 °C) compared to the commercial monolithic catalyst (TWC's).     

Novel BN supported bi-metal catalyst for oxydehydrogenation of propane

A novel boron nitride (BN) supported Pt-Sn catalyst was used for the oxydehydrogenation of propane. BN is a graphite-like inert support which provides negligible interaction with metals. The Pt-Sn/BN catalysts were prepared by co-incipient wetness impregnation with various Sn loadings. A commercial support γ-Al₂O₃ was chosen to compare with BN. PtSn alloys were formed due to the partially reduced Sn in Pt-Sn/BN catalyst in H₂ at 400 °C. Furthermore, the crystalline phases of PtSn and SnPt₃ alloys were also observed from the XRD patterns of Pt-Sn/BN catalysts. However, PtSn alloys were not detected in Pt-Sn/γ-Al₂O₃ by XRD. The Sn addition clearly improved the activity and propylene selectivity of Pt-Sn/BN at 600 °C. The more the Sn loading, the higher the selectivity and yield of propylene were. A maximum yield of propylene (38.3%) was achieved on Pt-Sn (0.75 wt%)/BN catalyst at the start of reaction. The catalysts, Pt-Sn/γ-Al₂O₃, deactivated more rapidly than Pt-Sn/BN. The activity and selectivity enhancement are attributed to the formation of PtSn and/or SnPt₃ alloy particles on the BN support. Compared with the hydrophilic γ-Al₂O₃, the hydrophobic BN surface can expel H₂O during the oxidation of hydrogen resulting in the activity increase.     

Preparation and characteristics of pretreated Pt/alumina catalysts for the preferential oxidation of carbon monoxide

The polymer electrolyte membrane fuel cell (PEMFC) needs purified hydrogen fuel from hydrocarbon reforming and water-gas shift (WGS) reaction. Concentration of CO should be 10 ppm level to avoid poisoning of the platinum anode electrode. For this, preferential oxidation of carbon monoxide (PROX) reaction is essential. In this study, a novel pretreatment technique was applied to a conventional Pt/γ-Al₂O₃ catalyst. Oxygen-treated, water-treated, and conventional Pt/γ-Al₂O₃ catalyst were prepared and their performances in the PROX reaction were investigated in a simulated hydrogen-rich reaction conditions. Our results showed that catalytic activity of the oxygen-treated 5% Pt/γ-Al₂O₃ catalyst for the CO conversion increased dramatically especially at the low temperature below 100 °C. The enhancement is attributed to the formation of well-dispersed small Pt particles.     

Ethylene epoxidation in low-temperature AC corona discharge over Ag catalyst: Effect of promoter

In this work, the epoxidation of ethylene using a low-temperature corona discharge system was investigated with various reported catalytically active catalysts: Ag/α-Al₂O₃, Cs–Ag/α-Al₂O₃, Cu–Ag/α-Al₂O₃, and Au–Ag/α-Al₂O₃. It was experimentally found that the investigated catalysts could improve the ethylene conversion and the ethylene oxide (EO) yield and selectivity for the corona discharge system, particularly 1 wt.% Cs–12.5 wt.% Ag/α-Al₂O₃ and 0.2 wt.% Au–12.5 wt.% Ag/α-Al₂O₃. The power consumption per EO molecule produced in the corona discharge system, combined with the superior bimetallic catalysts, was much lower than that of the sole corona discharge system and that of the corona discharge system combined with the monometallic Ag catalyst.     

Highly selective and stable bimetallic catalysts supported on different materials for n-butane dehydrogenation

A study about the performance of Pt(0.3 wt%)Sn(0.3 wt%) catalysts supported on different materials in n-butane dehydrogenation is reported in this paper. The materials used as supports were γ-Al₂O₃, ZnAl₂O₄ spinel, MgAl₂O₄ spinel and spheres of α-Al₂O₃ with a washcoating of γ-Al₂O₃. The syntheses of both spinels leaded to very pure ZnAl₂O₄ and MgAl₂O₄ supports. The material prepared by washcoating showed the presence of an uniform and homogeneous layer of γ-Al₂O₃ (with a thickness between 12 and 18 μm) deposited on the spheres of α-Al₂O₃.The best behavior in activity, selectivity and stability through five severe cycles was achieved by bimetallic PtSn catalysts supported on MgAl₂O₄ spinel and on the material prepared by washcoating. The very good performance of these catalysts through the different cycles of reaction-regeneration can be due to the presence of metallic phases which preserve the strong intermetallic interaction along the different treatments, thus avoiding segregation processes.     

Deactivation and coke formation on palladium and platinum catalysts in vegetable oil hydrogenation

Deactivation of palladium and platinum catalysts due to coke formation was studied during hydrogenation of methyl esters of sunflower oil. The supported metal catalysts were prepared by impregnating γ-alumina with either palladium or platinum salts, and by impregnating α-alumina with palladium salt. The catalysts were reused for several batch experiments. The Pd/γ-Al₂O₃ catalyst lost more than 50% of its initial activity after four batch experiments, while the other catalysts did not deactivate. Samples of used catalysts were cleaned from remaining oil by repeated extractions with methanol, and the amount of coke formed on the catalysts was studied by temperature-programmed oxidation. The deactivation of the catalyst is a function of both the metal and the support. The amount of coke increased on the Pd/γ-Al₂O₃ catalyst with repeated use, but the amount of coke remained approximately constant for the Pt/γ-Al₂O₃ catalyst. Virtually no coke was detected on the Pd/α-Al₂O₃ catalyst. The formation of coke on Pd/α-Al₂O₃ may be slower than on the Pd/γ-Al₂O₃ owing to the carrier’s smaller surface area and less acidic character. The absence of deactivation for the Pt/γ-Al₂O₃ catalyst may be explained by slower formation of coke precursors on platinum compared to palladium.     

Hydrogenation of methyl propionate over Ru–Pt/AlOOH catalyst: Effect of surface hydroxyl groups on support and solvent

A ruthenium–platinum bimetallic catalyst supported on boehmite was prepared by co-impregnation and hydrothermal reduction and characterized by XRD, TEM and TG–DTG. Reduction time of the catalyst affected the conversion of γ-Al₂O₃ to boehmite and the specific surface area of the catalyst, and consequently influenced the catalytic performance of the catalyst. Under the same conditions, the Ru–Pt/AlOOH catalyst showed much higher activity and selectivity than the Ru–Pt/γ-Al₂O₃ in aqueous hydrogenation of methyl propionate. The selectivity to 1-propanol of 97.8% could be obtained at methyl propionate conversion of 89.1% over Ru–Pt/AlOOH at 453 K under 5 MPa of H₂ for 6 h. It is postulated that the high performance of this novel catalyst is related to the cooperation of the hydroxyl groups of support surface and water solvent.