Oxidation of glucose in suspensions of moderately hydrophobized palladium catalysts

In absorption with fast chemical reaction, the reaction occurs only in the liquid-side film at the interface whereas the gas concentration in the bulk of the liquid is about zero. The absorption might be more strongly enhanced if the catalyst concentration in the film was locally increased. For the oxidation of glucose to gluconic acid on suspended Pd/Al₂O₃ particles, particle adhesion at the gas–liquid interface was promoted by moderate hydrophobization with trichloromethylsilane (TCMS). The silanization had no significant effect on the catalyst activity studied under kinetic control. In the mass transfer controlled regime, enhancement of the absorption rate by the hydrophobized Pd/Al₂O₃ catalyst particles occurred at very low catalyst loadings. This can be attributed to locally higher catalyst concentration in the liquid film.     

Kinetic Study of Heterogeneously Catalyzed Glucose Oxidation in a Stirred Cell

Liquid phase oxidation of glucose to gluconic acid using Bi promoted Pd catalysts was studied in a stirred cell. Gas‐liquid mass transfer limitations are observed at lower but not at higher rotational speeds. The conversion and the deactivation of the Pd catalyst depend on the O₂ concentration in the liquid phase. With decreasing glucose concentration, the reaction rate decreases leading to higher oxygen concentration in the liquid, which deactivates the catalyst due to over‐oxidation. Severe mass transfer limitations even at low Pd loadings could be attributed to intraparticle or liquid‐to‐solid mass transfer.     

Effect of Palladium on Iron Fischer–Tropsch Synthesis Catalysts

Studies were conducted to investigate the effect of Pd on the Fischer–Tropsch Synthesis (FTS) selectivity, activity and kinetics as well as on the water–gas shift activity of an iron catalyst. Two palladium promoted catalysts (Pd0.002/Fe100 and Pd0.005/Fe100) were prepared from a base Fe100/Si5.1 (atomic ratio) catalyst. Results of FTS over the two palladium promoted catalysts were compared to those obtained from the K/Fe/Si base catalyst and a Cu/K/Fe/Si catalyst. The results indicate that Pd enhanced the FT activity while the selectivity for CO₂ and CH₄ changed little compared to the results for the base catalyst and the Cu promoted catalyst. Palladium promotion had a negative effect on the C₂—C₄ olefin to paraffin ratio. Pd promotion led to a higher WGS rate than the other two catalysts at high syngas conversions. A higher WGS rate compared to the FTS rate was obtained only for the Pd promoted catalysts. The FTS rate constant for the Pd promoted catalyst is higher than the base catalyst but lower than for the Cu promoted catalyst.     

Formation of an intermetallic compound Pd3Te with deactivation of Te/Pd/C catalysts for selective oxidation of sodium lactate to pyruvate in aqueous phase

The addition of a trace amount of Te promoted the activity of Pd/C in the liquid-phase oxidation of lactic acid, but the Te/Pd/C catalyst, for which Pd₃Te crystalline phase extended over the bulk at a higher Te-doping above Te/Pd = 0.3 (atom), was again inactive. A powder XRD evidence for Pd₃Te is given.     

Gas-phase hydrogenation of maleic anhydride to butyric acid over Cu/TiO2/γ-Al2O3 catalyst promoted by Pd

The promoter effect of palladium on the Cu/TiO₂/γ-Al₂O₃ catalyst was investigated for the gas-phase selective hydrogenation of maleic anhydride to butyric acid at atmospheric pressure. The results show that Pd is added rarely into the Cu/TiO₂/γ-Al₂O₃ catalyst for the hydrogenation of maleic anhydride, the higher selectivity to butyric acid can be obtained. In the absence of Pd (or Cu) in the Cu–Pd/TiO₂/γ-Al₂O₃ catalyst, the selectivity to butyric acid (BA) is nearly zero. Using the Cu–Pd/TiO₂/γ-Al₂O₃ (Pd/Cu=3/100 (atom)) catalyst, 56.2% selectivity to BA and 100% conversion of maleic anhydride were obtained at 280 °C.     

Flame-synthesized LaCoO3-supported Pd: 2. Catalytic behavior in the reduction of NO by H2 under lean conditions

A 0.5 wt% Pd/LaCoO₃, prepared by flame-spray pyrolysis (FP), was tested as catalyst for the low-temperature selective reduction of NO by H₂ in the presence of excess O₂. In particular, the effect of the precalcination and prereduction temperature on catalytic activity was compared with that of a similar Pd/LaCoO₃ sample prepared by impregnation with a Pd solution of FP-prepared LaCoO₃. The FP-made catalyst allowed full NO conversion at 150 °C, with 78% selectivity to N₂, thus outperforming the catalytic behavior of the corresponding sample prepared by impregnation. The higher activity of the FP-made catalyst has been attributed to the formation of segregated Co metal particles, not present in the impregnated sample, formed during the precalcination at 800 °C, followed by reduction at 300 °C. Two reaction mechanisms can be deduced from the temperature-programmed experiments. The first of these, occurring at lower temperatures, indicates cooperation between the Pd and Co metal particles, with formation of active nitrates on cobalt, successively reduced by hydrogen spillover from Pd. The second, occurring at higher temperature, allows 50% conversion of NO, with >90% selectivity to N₂, and involves N adatoms formed by dissociative NO adsorption over Pd. Prereduction at 600 °C led to a slight increase in catalytic activity, due to the formation of a PdCo alloy, which is more stable on reoxidization compared with Pd alone. Moreover, the cooperative reaction mechanism seems to be favored by the proximity of Co and Pd in metal particles.     

AFM studies of Pd silica supported thin film catalysts

AFM has been used to study the effects of pretreatment gases on Pd/SiO₂ supported thin film catalysts during 1,3-butadiene hydrogenation. The Pd/SiO₂ catalyst, treated with O₂ followed by H₂ at 450°C, has an initial conversion of 85% and a surface morphology of 60 x 65 nm² Pd grains and only deactivates slightly. After a second treatment, the reactivity was fully recovered and the surface morphology exhibits a redispersion of the Pd grains. The catalyst with a similar initial reactivity and morphology but only treated in H₂, shows a decrease in activity and coalescence of Pd grains after repeated treatment. XPS studies have shown that the O₂ and H₂ treated Pd/SiO₂ catalyst has a lower Pd binding energy than the H₂ treated Pd/SiO₂. The effect of the substrate thickness and composition is also reported.     

Hydrodechlorination of tetrachloroethene over Pd/Al2O3: influence of process conditions on catalyst performance and stability

The influence of process parameters (temperature, pressure, hydrogen flow rate, and nature of solvent) on both activity and stability of a 0.5% Pd on alumina catalyst used for tetrachloroethene (TTCE) hydrodechlorination in an organic matrix was studied. In the range of temperature studied (250–350 °C), higher temperatures lead to higher initial activity but faster deactivation. Increasing hydrogen flow rates, up to 0.8 L/min (STP), produce higher activity and stability of the catalyst, whereas pressure in the range 0.5–2 MPa has no significant effect. In all the cases, both hydrodechlorination and hydrogenation of the double bond take place, yielding ethane as the main product. Concerning to the solvent, there is no difference in the initial catalytic activity for either toluene or n-decane, but n-decane leads to faster catalyst deactivation.The effect of temperature and space time in TTCE conversion at the period of constant catalytic activity can be modelled by a kinetic model assuming first order for TTCE and zero-order for H₂.Finally, the performance of the Pd alumina-supported catalyst is compared with that of a Pd carbon-supported catalyst with the same metal load, used in previous works. Although the carbon-supported catalyst yields higher initial conversion, the alumina-supported catalyst is more resistant to deactivation.     

Liquid‐Phase Catalytic Hydrogenation of p‐Chlorobenzophenone to p‐Chlorobenzhydrol over a 5 % Pd/C Catalyst

The kinetics of the liquid‐phase catalytic hydrogenation of p‐chlorobenzophenone have been investigated over a 5 % Pd/C catalyst. The effects of hydrogen partial pressure (800–2200 kPa), catalyst loading (0.4–1.6 gm dm^–3), p‐chlorobenzophenone concentration (0.37–1.5 mol dm^–3), and temperature (303–313 K) were studied. A stirring speed > 20 rps has no effect on the initial rate of reaction. Effects of various catalysts (Pd/C, Pd/BaSO₄, Pd/CaCO₃, Pt/C, Raney nickel) and solvents (2‐propanol, methanol, dimethylformamide, toluene, xylene, hexane) on the hydrogenation of p‐chlorobenzophenone were also investigated. The reaction was found to be first order with respect to hydrogen partial pressure and catalyst loading, and zero order with respect to p‐chlorobenzophenone concentration. Several Langmuir‐Hinshelwood type models were considered and the experimental data fitted to a model involving reaction between adsorbed p‐chlorobenzophenone and hydrogen in the liquid phase.     

The enhancement effect of MoOx on Pd/C catalyst for the electrooxidation of formic acid

Molybdenum oxide (MoO_x) was added to a Pd/C catalyst using a novel two-step procedure. The enhancement effect of MoO_x on Pd/C catalyst for the electrooxidation of formic acid was verified by electrochemical experiments. Compared to the Pd/C catalyst, the experimental results showed that the addition of MoO_x could significantly enhance the electrocatalytic performances for the electrooxidation of formic acid. Significant improvements in electrocatalytic activity and stability were primarily ascribed to the effect of MoO_x on the Pd catalyst. In addition to the large specific surface area, the hydrogen spillover effect is speculated to have accelerated the electrooxidation rate of formic acid in the direct pathway.     

Toluene destruction over nanometric palladium supported ZSM-5 catalysts: influences of support acidity and operation condition

The effects of catalyst physicochemical property and operation condition on toluene destruction over Pd/Z-x (Z: ZSM-5; x: n _Si/Al) were extensively studied. The support acidity has important impact on active phase dispersion and reaction product desorption. Pd/Z-25 shows the highest catalytic activity with toluene complete conversion at 220 °C, which is about 60 °C lower than that of Pd/Z-300. Both Pd loading and space velocity are key experimental factors determining toluene oxidation activity. The water vapor has a significant negative effect on the oxidation reaction, especially for the catalyst with higher Al content. The activity of the used Pd/Z-25 could not regain its initial level after removal of water vapor due to the formation of coke. Pd⁰ and Pd²⁺ species have a synergetic effect on toluene oxidation, and the catalytic activity is primarily correlated to the support acidity, the active phase dispersion, and the CO₂ desorption capability.     

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.     

Formation and catalytic activity of partially oxidized Pd nanoparticles

Combining multi molecular beam (MB) experiments and in-situ time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we have studied the formation and catalytic activity of Pd oxide species on a well-defined Fe₃O₄ supported Pd model catalyst. It was found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (~10⁻⁶ mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface. Their formation and reduction is fully reversible. As a consequence, the Pd interface oxide layer acts as an oxygen reservoir providing oxygen for catalytic surface reactions. In addition to the Pd interface oxide, the formation of surface oxides was also observed for temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature resulting in partially oxidized Pd particles between 500 and 600 K. It is shown that the catalytic activity of the model catalyst for CO oxidation decreases significantly with increasing surface oxide coverage independent of the composition of the reactants. We address this deactivation of the catalyst to the weak CO adsorption on Pd surface oxides, leading to a very low reaction probability.     

Reactivation of spent Pd/AC catalyst by supercritical CO2 fluid extraction

In this article, we reported a nondestructive and environmentally friendly method for the reactivation of a spent Pd/AC catalyst for the hydrogenation of benzoic acid by using supercritical CO₂ (scCO₂) fluid extraction. The effects of reactivation conditions, such as extraction temperature, pressure, CO₂ flow rate, and time, on the activity of the reactivated Pd/AC catalyst, were presented. The catalyst was characterized by N₂ physisorption, laser particle size analysis, and transmission electron spectroscopy, and the liquid extract was analyzed by GC-MS. It is found that scCO₂ fluid extraction was very efficient in eliminating organic substances blocking the pores of the catalyst, while did not affect noticeably the granule size of the catalyst and the particle size of Pd. The reactivated Pd/AC catalyst regained more than 70% of the activity of the fresh 5.0 wt % Pd/AC catalyst, and has been successfully used in an industrial unit for the hydrogenation of benzoic acid. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Highly Active Carbon‐supported PdSn Catalysts for Formic Acid Electrooxidation

PdSn/C catalysts with different atomic ratios of Pd to Sn were synthesised by a NaBH₄ reduction method. Electrochemical tests show that the alloy catalysts exhibit significantly higher catalytic activity and stability for formic acid electrooxidation (FAEO) than the Pd/C catalyst prepared with the same method. XRD and TEM indicate that a particle‐size effect is not the main cause for the high performance. XPS confirms that Pd is modified by Sn through an electronic effect which can decrease the adsorption strength of poisonous intermediates on Pd and thus promote the FAEO greatly.     

Oxygen-induced Restructuring of a Pd/Fe3O4 Model Catalyst

We have studied the influence of oxygen on the structure and morphology of a Pd/Fe₃O₄ model catalyst using molecular beam (MB) methods, IR reflection absorption spectroscopy (IRAS) and scanning tunneling microcopy (STM). The model catalyst was prepared under ultrahigh vacuum (UHV) conditions by physical vapor deposition (PVD) and growth of Pd nanoparticles on an ordered Fe₃O₄ thin film on Pt(111). It is found that surface oxides are formed on the Pd nanoparticles even under mild oxidation conditions (temperatures of 500 K and effective oxygen partial pressures of around 10⁻⁶ mbar). These surface oxides are initially generated at the Pd/Fe₃O₄ interface and, subsequently, are formed at the Pd/gas interface. The process of formation and reduction of surface and interface oxides on the Pd particles is fully reversible in that all oxides formed can be fully reduced. As a result, the oxide phase acts like a storage medium for oxygen during oxidation reactions, as probed via CO oxidation. The process of surface and interface oxidation is directly connected with the onset of a non-reversible sintering process of the Pd particles. It is suggested that this sintering process occurs via a mobile Pd oxide species, which is stabilized by interaction with the Fe₃O₄ support. The restructuring is monitored via STM and IRAS using CO as a probe molecule. In addition to a decrease in particle density and Pd surface area, a reshaping of the particles occurs, which is characterized by the formation of well-ordered crystallites and with a relatively large fraction of (100) facets. After a few oxidation/reduction cycles at 500 K, the sintering process becomes very slow and the system shows a stable behavior under conditions of CO oxidation.     

Palladium Catalysts for Fatty Acid Deoxygenation: Influence of the Support and Fatty Acid Chain Length on Decarboxylation Kinetics

Supported metal catalysts containing 5?wt% Pd on silica, alumina, and activated carbon were evaluated for liquid-phase deoxygenation of stearic (octadecanoic), lauric (dodecanoic), and capric (decanoic) acids under 5?% H₂ at 300?°C and 15?atm. On-line quadrupole mass spectrometry (QMS) was used to measure CO?+?CO₂ yield, CO₂ selectivity, H₂ consumption, and initial decarboxylation rate. Post-reaction analysis of liquid products by gas chromatography was used to determine n-alkane yields. The Pd/C catalyst was highly active and selective for stearic acid (SA) decarboxylation under these conditions. In contrast, SA deoxygenation over Pd/SiO₂ occurred primarily via decarbonylation and at a much slower rate. Pd/Al₂O₃ exhibited high initial SA decarboxylation activity but deactivated under the test conditions. Similar CO₂ selectivity patterns among the catalysts were observed for deoxygenation of lauric and capric acids; however, the initial decarboxylation rates tended to be lower for these substrates. The influence of alkyl chain length on deoxygenation kinetics was investigated for a homologous series of C₁₀?CC₁₈ fatty acids using the Pd/C catalyst. As fatty acid carbon number decreases, reaction time and H₂ consumption increase, and CO₂ selectivity and initial decarboxylation rate decrease. The increase in initial decarboxylation rates for longer chain fatty acids is attributed to their greater propensity for adsorption on the activated carbon support.     

TAP Investigation of NO Adsorption on Pd/Al2O3: Effect of Thermal Aging

This paper deals with the aging effect on the kinetics of the NO decomposition on a commercial Pd/Al₂O₃ catalyst. Temporal analysis of products experiments are discussed in the light of a selected mechanism involving the recombination of two NO_ads species to form N₂O, which is an intermediate in N₂ formation. Experiments over the fresh catalyst indicate a strong metal/support interface, with a spill-over effect, which is difficult to model. Thermal aging had a detrimental effect over this interface, the kinetic features depending mainly on the metallic Pd sites. The different heats of adsorption and activation energies are proven consistent with other theoretical studies. The mechanism led to a high surface coverage for O ad-atoms.     

Impregnated carbon based catalyst for protection against carbon monoxide gas

Activated carbon based palladium impregnated catalyst (Pd/C) was prepared for the reactive removal of carbon monoxide (CO) gas under ambient air conditions. For this, active carbon of 1250 m²/g surface area was impregnated with palladium salt to get Pd/C catalyst, containing palladium from 4.0 to 8.0% (w/w). Catalytic efficiency of the catalyst against CO gas was determined under dynamic conditions by passing CO–air mixture to the fixed bed of the Pd/C catalyst. Results indicated that Pd/C catalyst was continuously adsorbing and actively removing CO gas during the course of the palladium catalyzed reaction, i.e., CO + 1/2 O₂ → CO₂ and was found capable of providing excellent protection against CO gas. Moisture content (humidity) of inlet CO–air mixture indicated it to be an important factor affecting the CO removal efficiency of the catalyst, as an increase in humidity after the CO breakthrough resulted in to the activation of the catalyst due to the generation of hydroxyl groups and enhanced protection by the regeneration of the catalyst. Study indicated that Pd/C catalyst works as a catalytic converter, i.e., the continuous conversion of CO to CO₂ using atmospheric oxygen and moisture. In order to determine the shelf life, the Pd/C catalyst was also evaluated for its performance after accelerated ageing at 70 °C and 50% relative humidity (RH) for 3.75 and 7.5 months. The catalyst was found to be working efficiently for 3.75 months but after 7.5 months it could not provide 100% protection against CO gas, however, the same catalyst started giving 100% protection after regeneration. Hence, studies indicated the Pd/C catalyst to be a promising catalyst for the reactive removal of CO gas in enclosed spaces/compartments, coal mines, fire accidents and for getting the protection for longer duration under ambient air conditions.     

Pd nanoparticles supported on carbon-modified rutile TiO2 as a highly efficient catalyst for formic acid electrooxidation

In this article, Pd nanoparticles supported on carbon-modified rutile TiO₂ (CMRT) as a highly efficient catalyst for formic acid electrooxidation were investigated. Pd/CMRT catalyst was synthesized by using liquid phase reduction method in which Pd nanoparticles was loaded on the surface of CMRT obtained through a chemical vapor deposition (CVD) process. Pd/CMRT shows three times the catalytic activity of Pd/C, as well as better catalytic stability towards formic acid electrooxidation. The enhanced catalytic property of Pd/CMRT mainly arises from the improved electronic conductivity of carbon-modified rutile TiO₂, the dilated lattice constant of Pd nanoparticles, an increasing of surface steps and kinks in the microstructure of Pd nanoparticles and slightly better tolerance to the adsorption of poisonous intermediates.