Alumina-supported Noble Metal Catalysts Research Articles

Hydrodeoxygenation of acetone–furfural condensation adducts over alumina-supported noble metal catalysts

The hydrodeoxygenation (HDO) of aldol condensation adducts is of key interest in the preparation of biofuels from biomass by catalytic routes. In this work, furfuryldeneacetoneis taken as model of biomass-derived condensation adduct with furanic rings, unsaturations and carbonyl functionalities. Four noble metal catalysts (alumina-supported Ru, Rh, Pd and Pt) were tested. Obtained results show that Rh and Ru catalysts are only active for the hydrogenation of aliphatic double bonds; whereas Pd, and specially Pt catalysts, are active for the total HDO of the adduct, yielding n-octane, with selectivities higher than 30% at 493K after 24h on stream (total conversion and carbon balance closure higher than 90%). Based on these results, a mechanism (considering serial, parallel and equilibrium steps) and the corresponding kinetic model have been proposed for explaining the different selectivity trends observed for these metals.

In situ FTIR study on the formation and adsorption of CO on alumina-supported noble metal catalysts from H2 and CO2 in the presence of water vapor at high pressures

The formation and adsorption of CO from CO(2) and H(2) at high pressures were studied over alumina-supported noble metal catalysts (Pt, Pd, Rh, Ru) by in situ FTIR measurements. To examine the effects of surface structure of supported metal particles and water vapor on the CO adsorption, FTIR spectra were collected at 323 K with untreated and heat (673 K) treated catalysts in the absence and presence of water (H(2)O, D(2)O). It was observed that the adsorption of CO occurred on all the metal catalysts at high pressures, some CO species still remained adsorbed under ambient conditions after the high pressure FTIR measurements, and the frequencies of the adsorbed CO species were lower either for the heat treated samples or in the presence of water vapor. It is assumed that the CO absorption bands on atomically smoother surfaces appear at lower frequencies and that water molecules are adsorbed more preferentially on atomically rough surfaces rather than CO species.

ChemInform Abstract: Methane Oxidation Over Alumina-Supported Noble Metal Catalysts with and without Cerium Additives.

Abstract Laboratory reactor experiments have been conducted to evaluate alumina-supported noble metal catalysts, both in the presence and absence of cerium additives, for their effectiveness in the catalytic oxidation of methane under conditions likely to be encountered in natural-gas vehicle exhaust. Under oxidizing conditions, all of the catalysts promoted the complete oxidation of methane to CO2 and H2O, with the methane oxidation activity ranking given by Pd > Rh > Pt in the absence of Ce and by Rh > Pd ≈ Pt in the presence of Ce. Under reducing conditions, methane oxidation produced substantial amounts of CO and H2 as the principal partial oxidation products. In the absence of Ce, the methane oxidation activity decreases in the order Pd > Rh > Pt, with the tendency to form CO decreasing in the order Rh > Pd > Pt. The activity ranking for methane conversion in reducing feedstreams was not affected by the presence of Ce; however, the addition of Ce to the Pt/Al2O3 and Pd/Al2O3 catalysts almost completely suppressed the formation of the partial oxidation product CO. At a fixed temperature of ≈550°C, the methane conversion over each of the noble metal catalysts goes through a maximum as the feedstream concentration of O2 is varied. The data suggest that O2 inhibits the CH4 oxidation under oxidizing conditions by excluding the more weakly adsorbed species, CH4, from the active sites. Also, methane oxidation experiments in the presence of CO in the feed showed that the methane conversion characteristics of the noble metal catalysts are little affected by the CO.

Catalytic Pyrolysis of Low-Density Polyethylene over Alumina-Supported Noble Metal Catalysts

The effects of Pt/Al2O3 and Rh/Al2O3 catalysts on the pyrolysis of low-density polyethylene (LDPE) at a temperature of 425 °C and residence time of 1 h was investigated in a batch stainless steel a...

Plasma catalytic oxidation of methane on alumina-supported noble metal catalysts

The methane oxidation reaction was studied as a function of temperature for a gas mixture representative of a combined heat power (CHP) within a dielectric barrier discharge (DBD) reactor. The effect of energy deposition was studied, with energy deposition ranging from 36 to 58JL−1, to maintain a low NOx formation at high temperature. In the current study, we also investigated the combined process of a dielectric barrier discharge and γ-alumina noble metal catalysts such as platinum and palladium. In order to separate the catalytic effects from the plasma activation, the work included both purely catalytic and plasma activation processes using a complete CHP mixture with 4% of water in the feed. It was found that the presence of water leads to increase the methane conversion in purely plasma activation processes. Moreover, a higher efficiency for methane oxidation in CHP conditions was observed with the plasma–Pd(X)/Al2O3 systems.

Mechanistic aspects of the water–gas shift reaction on alumina-supported noble metal catalysts: In situ DRIFTS and SSITKA-mass spectrometry studies

Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with mass spectrometry were performed for the first time to study essential mechanistic aspects of the water–gas shift (WGS) reaction over alumina-supported Pt, Pd, and Rh catalysts. In particular, the concentrations (μmol g −1) of active intermediate species found in the carbon-path from CO to the CO 2 product gas (use of 13CO), and in the hydrogen-path from H 2O to the H 2 product gas (use of D 2O) of the reaction mechanism were determined. It was found that by increasing the reaction temperature from 350 to 500 °C the concentration of active species in both the carbon-path and hydrogen-path increased significantly. Based on the large concentration of active species present in the hydrogen-path (OH/H located on the alumina support), the latter being larger than six equivalent monolayers based on the exposed noble metal surface area ( θ > 6.0), the small concentration of OH groups along the periphery of metal-support interface, and the significantly smaller concentration (μmol g −1) of active species present in the carbon-path (adsorbed CO on the noble metal and COOH species on the alumina support and/or the metal-support interface), it might be suggested that diffusion of OH/H species on the alumina support towards catalytic sites present in the hydrogen-path of reaction mechanism might be considered as a slow reaction step. The formation of labile OH/H species is the result of dissociative chemisorption of water on the alumina support, where the role of noble metal is to activate the CO chemisorption and likely to promote formate decomposition into CO 2 and H 2 products. It was found that there is a good correlation between the surface concentration and binding energy of CO on the noble metal (Pt, Pd or Rh) with the activity of alumina-supported noble metal towards the WGS reaction.

Performance of alumina-supported noble metal catalysts for the combustion of trichloroethene at dry and wet conditions

The performance of four different alumina-supported noble metal catalysts (0.5% of Pd, Pt, Rh and Ru, respectively) for the deep oxidation of trichloroethene (1000–2500ppmV, WHSV=55h−1) in air was studied in this work. Experiments were carried out at both dry and wet (20,000ppm of H2O) conditions. Catalysts were compared in terms of activity, selectivity for the different reaction products (CO2, HCl, Cl2, C2Cl4, CCl4 and CHCl3), and stability at reaction conditions.As general trend, the activity of the catalysts decreases in the order Ru≫Pd>Rh>Pt. Concerning to the effect of the water addition, no important effect on the catalyst activity was observed, except in the case of Pt, for which an increase of the catalytic activity was observed. Reaction mechanism (and hence product distribution) is very similar for Rh, Pd and Pt, being in these cases C2Cl4 the only organochlorinated by-product detected. In the case of Ru, the reaction mechanism seems to be quite different, CCl4 and CHCl3 being the main organic by-products.Simple power-law kinetic expressions (first order on trichloroethene concentration for Pd, Rh and Ru, and zeroth order for Pt) provide fairly good fits for catalytic performance of the studied catalysts.Finally, deactivation studies show that both formation of active metal chlorides (especially in the case of Rh) and fouling (especially for Pd and Pt) are the main deactivation causes.

Adsorption and surface reactions of acetaldehyde on alumina-supported noble metal catalysts

Adsorption and further surface transformations of acetaldehyde on Al2O3-supported noble metal catalysts were studied by FTIR spectroscopy. Two forms (H-bridge bonded and bonded to Lewis sites) of molecularly adsorbed acetaldehyde were detected. β-aldolization of acetaldehyde into crotonaldehyde was observed already at 300 K on all surfaces investigated. The formation of adsorbed crotonaldehyde depended on the temperature and on the nature of the metal. The gas phase products (crotonaldehyde, benzene, hydrogen, ethane, ethylene and acetylene) were analyzed by mass spectrometer. Their appearance in the gas phase and their amounts depended on the nature of the metal and on the temperature.

Temperature profiles of alumina-supported noble metal catalysts in autothermal reforming of methane

Temperature profiles of alumina supported Rh, Pt and Pd catalysts were investigated during the reactions related to autothermal reforming of methane by means of IR thermography. From the results of the mixture of reforming gas (CH 4:H 2O = 1:1) with combustion gas (CH 4:O 2 = 1:2), we evaluated the overlap between the reforming and the combustion reaction zones. The most significant overlap was observed over Rh catalyst. In addition, the effect of W/ F and reaction temperature on the temperature profiles and catalytic performance in the autothermal reforming reaction were systematically examined. The overlap between these reaction zones led to a higher methane conversion even at lower W/ F during autothermal reforming of methane; such an overlap was considered to contribute to the inhibition of the hot spot formation. This phenomenon demonstrated an efficient heat transfer from exothermic combustion to endothermic reforming.

2-23.アルミナ担持貴金属触媒によるDMEスチームリフォーミング((7)転換利用III,Session 2 天然ガス・メタンハイドレート等)

2-23.アルミナ担持貴金属触媒によるDMEスチームリフォーミング((7)転換利用III,Session 2 天然ガス・メタンハイドレート等)

Thermogravimetric determination of coke deposits on alumina-supported noble metal catalysts used as hydrodechlorination catalysts

The deactivation of 0.5 wt.% Pd on alumina, 0.5 wt.% Rh on alumina and 0.5 wt.% Pt on alumina catalysts for the hydrodechlorination of tetrachloroethylene (TTCE) in organic matrix has been studied. Experiments were carried out in a continuous fixed-bed reactor at 250°C, 5 bar, 1.3 g catalyst min/mmol TTCE and 0.8 nl/min of hydrogen. The Pd catalyst is the most stable at the studied conditions, followed by Rh and Pt. In order to study the causes of deactivation, fresh and used catalysts were characterised by nitrogen porosimetry, X-ray diffraction (XRD), thermogravimetry (TG), X-ray photoelectron spectroscopy (XPS), temperature-programmed oxidation (TPO) and pulse hydrogen chemisorption. Characterisation results indicate that the deactivation is caused mainly by the formation of carbonaceous deposits, being negligible the effects of poisoning by HCl. It was observed that in addition to the amount of coke, its combustion temperature is an important parameter, showing a higher capacity to deactivate the catalysts the cokes with lower combustion temperatures (obtained from differential thermogravimetry (DTG) profiles).

Enhancement of the catalytic oxidation of hydrogen-lean chlorinated VOCs in the presence of hydrogen-supplying compounds

The complete catalytic oxidation of trichloroethylene (TCE) over alumina-supported noble metal catalysts (Pt and Pd) and in the presence of hydrogen-rich compounds, i.e. water, hexane and toluene was evaluated. Experiments were performed at conditions of lean TCE concentration (around 1000ppm) in air, between 250 and 550°C in a conventional fixed-bed reactor. Hexane and toluene were added to the feedstream in a concentration of around 1000ppm and water concentration varied from 1000 to 15000ppm. TCE oxidation occurred faster in the presence of hexane and toluene over both catalysts. Over palladium catalysts, water did not alter catalytic activity, whereas over platinum catalysts water enhanced TCE oxidation at low temperatures (<400°C) but inhibited it at higher temperatures (>400°C). Selectivity to HCl was much improved by feeding water as a hydrogen-supplying reactant; 7500ppm of water enhanced HCl outputs from 39.4 to 78.0% with Pd, and from 37.5 to 58.9% with Pt. Selectivities to C2Cl4, formed by chlorination of the feed, and Cl2 were greatly reduced. On the other hand water promoted complete oxidation of TCE to CO2, and thus reduced selectivity to CO. In the presence of hexane and toluene, formation of HCl was also enhanced. Hexane showed higher inhibition ability than toluene over both catalysts for the C2Cl4 and Cl2 formation. Unlike in the presence of water, selectivity to CO increased, as a consequence of partial oxidation of both hydrocarbons.

Hydrodesulfurization of dibenzothiophene and hydrogenation of phenanthrene on alumina-supported Pt and Pd catalysts

Hydrodesulfurization (HDS) reactions of dibenzothiophene (DBT) and hydrogenation (HYD) of phenanthrene (PHE) were carried out over several alumina-supported noble metal catalysts at temperatures between 200°C and 400°C and under 50 kg/cm 2 pressure. A dual noble metal catalyst, Pt(2)–Pd(10), showed similar HDS results and better HYD promotion compared with a conventional Co–Mo catalyst. Further, 35 S radioisotope tracer method showed that almost no labile sulfur was accumulated on Pt/Al 2O 3 catalyst in the HDS of DBT or HYD of PHE, although this catalyst showed an HDS activity comparable to that of the Co–Mo catalyst. This indicated that the mechanism of DBT HDS on noble catalysts was different from that on the conventional Mo-based catalyst.

Alumina-Supported Noble Metal Catalysts for Destructive Oxidation of Organic Pollutants in Effluent from a Softwood Kraft Pulp Mill

The effectiveness of alumina-supported noble metal catalysts for the destructive oxidation of organic pollutants in effluent from a softwood kraft pulp mill was evaluated in a slurry reactor at 463 K and an oxygen pressure of 1.5 MPa. The effects of catalyst preparation procedures, such as metal loading, calcination, or reduction treatment on the catalytic activities, were also tested. Alumina-supported palladium catalysts were found to be more effective than supported manganese, iron, or platinum catalysts. The rate of oxidation over Pd/alumina catalyst was significantly higher than that of the uncatalyzed reaction. Adding Ce on the alumina support was found to promote the activity of alumina-supported Pt catalyst but inhibit the activity of alumina-supported Pd catalyst. The reaction mechanisms for the catalytic wet oxidation process and the roles of Ce on catalytic activity for destructive oxidation of organic pollutants in wastewater are discussed.

Effects of rhodium addition on methane oxidation behavior of alumina-supported noble metal catalysts

Laboratory methane oxidation experiments were conducted using a series of catalyst samples prepared by impregnating Rh-free noble metal catalysts (i.e., Pt/Pd, Pt or Pd) with various amounts of rhodium (0.0035, 0.005, and 0.014 wt.-% Rh). The addition of these small amounts of rhodium did not significantly change the temperature required for the onset of the methane oxidation. However, variable-composition experiments conducted at a temperature characteristic of warmed-up catalytic converters (ca. 550°C) reveal that the rhodium addition tends to shift the optimum feedstream stoichiometry (at which a maximum methane conversion occurs) toward more reducing conditions. These Rh-induced effects on methane conversion behavior appear to be independent of how the rhodium is incorporated in the catalyst.

Methane oxidation over alumina-supported noble metal catalysts with and without cerium additives

Laboratory reactor experiments have been conducted to evaluate alumina-supported noble metal catalysts, both in the presence and absence of cerium additives, for their effectiveness in the catalytic oxidation of methane under conditions likely to be encountered in natural-gas vehicle exhaust. Under oxidizing conditions, all of the catalysts promoted the complete oxidation of methane to CO 2 and H 2O, with the methane oxidation activity ranking given by Pd > Rh > Pt in the absence of Ce and by Rh > Pd ≈ Pt in the presence of Ce. Under reducing conditions, methane oxidation produced substantial amounts of CO and H 2 as the principal partial oxidation products. In the absence of Ce, the methane oxidation activity decreases in the order Pd > Rh > Pt, with the tendency to form CO decreasing in the order Rh > Pd > Pt. The activity ranking for methane conversion in reducing feedstreams was not affected by the presence of Ce; however, the addition of Ce to the Pt/Al 2O 3 and Pd/Al 2O 3 catalysts almost completely suppressed the formation of the partial oxidation product CO. At a fixed temperature of ≈550°C, the methane conversion over each of the noble metal catalysts goes through a maximum as the feedstream concentration of O 2 is varied. The data suggest that O 2 inhibits the CH 4 oxidation under oxidizing conditions by excluding the more weakly adsorbed species, CH 4, from the active sites. Also, methane oxidation experiments in the presence of CO in the feed showed that the methane conversion characteristics of the noble metal catalysts are little affected by the CO.

Activity enhancement for the hydrogenation of CO adsorbed on Pt/Al2O3 and Pd/Al2O3 induced by high-temperature reduction

On the basis of the rate constant per active site determined by pulse surface reaction rate analysis (PSRA), the effect of the reduction temperature on the hydrogenation of adsorbed CO was studied for alumina-supported noble metal catalysts. It was found that the reduction temperature influenced the hydrogenation of adsorbed CO in a different way from one noble metal to another. The rate constant on Pt prepared from H2PtCl6 was more than five times larger on reduction at 773 K than at 523 K. Heating the catalyst, exhibiting the enhanced activity, in an O2 atmosphere at 723 K followed by reduction at 523 K resulted in a decrease in the rate constant to near its original value. The rate constant on Pd also varied with heat treatments in a way similar to that for Pt, although the activity enhancement was less pronounced. In contrast, no activity enhancement was observed on Rh and Ru. It was also found that the activity enhancement strongly depended on the metal precursor or preparation conditions : the Pt catalyst prepared from [Pt(NH3)4]Cl2 did not exhibit enhanced activity even after reduction at 773 K. Compared with the effect on the hydrogenation of adsorbed CO, the reduction temperature hardly influenced CO chemisorption for all noble metals examined. On the basis of the results obtained, a possible cause is discussed for the enhanced activity on Pt and Pd and for the absence of enhancement on Rh and Ru.

Improving the poison resistance of supported catalysts

This paper discusses the poison resistance of supported catalysts, in which the poison reacts both with the active component and the catalyst support. For such catalysts, the net rate of the poisoning reaction can be selectively manipulated with respect to the net rate of the main reaction in such a manner that a catalyst with improved poison resistance is obtained. The above statements are illustrated by diffusion-limited lead and phosphorus poisoning experiments using alumina-supported noble metal catalysts. By modifying the pore structure, support surface area, and noble metal impregnation depth of these catalysts, improvements in activity and poison resistance have been attained for automobile exhaust emission-control applications.