Alumina-supported Platinum Catalysts Research Articles

The Influence of Reduction Methods and Conditions on the Activity of Alumina-Supported Platinum Catalysts for the Liquid Phase Hydrogenation of Benzaldehyde in Ethanol

The activities of supported metal catalysts depend on various preparation variables, including the method of reduction. A variety of reduction procedures can be applied to the preparation of supported metal catalysts. Previously, the authors used a solid-liquid reduction by sodium tetrahydroborate solution for preparing supported platinum catalysts. In this reduction, platinum precursors adsorbed on supports were brought into contact with the reducing solution. The alumina-supported platinum catalysts prepared in this way were found to display interesting activities in the liquid-phase hydrogenation of {alpha},{beta}-unsaturated aldehydes; they were highly selective to the formation of unsaturated alcohols. The selective hydrogenation of C=O bonds of {alpha},{beta}-unsaturated aldehydes is difficult to achieve with platinum catalysts without using some additives like tin and iron. The maximum temperature that the supported platinum catalysts went through was 110{degrees}C, required for the removal of water. This thermal history is a possible reason for the catalytic activity observed. Following those observations, in the present work, the authors have further examined the influence of reduction procedures by using hydrazine as well as sodium tetrahydroborate and different temperatures common during gas-phase reduction with hydrogen. The catalytic activity has been tested by the liquid-phase hydrogenation of benzaldehyde (BAL) in ethanol under mild conditions.more » 12 refs., 3 figs.« less

Influence of moisture on transient oxidation and reduction of alumina-supported platinum

The transient surface behavior on oxidation and reduction of a partially oxidized alumina-supported platinum catalyst containing residual water have been observed by AC electrical conductance (G) measurements.

Activity for liquid-phase hydrogenation of α,β-unsaturated aldehydes of supported platinum catalysts prepared through low-temperature reduction

Alumina-supported platinum catalysts were prepared through the reduction of adsorbed metal precursors by sodium tetrahydroborate solution at room temperature and by hydrogen at high temperatures. The activity of the catalysts prepared was examined for liquid-phase hydrogenation of crotonaldehyde (CRAL) and 3-methylcrotonaldehyde (MCRAL) under mild conditions. In both cases of CRAL and MCRAL, the selectivity of the low-temperature reduced (LR) catalyst toward unsaturated alcohol (UOL) is higher than that of the high-temperature reduced (HR) catalyst. This is in accordance with the previous result on the hydrogenation of cinnamaldehyde, though the total activities of the LR and HR catalysts are comparable. The surface morphology of platinum particles of the LR catalyst seems to be different from that of the HR catalyst and this may be of significance for the enhanced UOL selectivity of the LR catalyst.

Unprecedented effects of solvent and temperature of preparation on hydrosilylating properties of alumina-supported platinum catalyst

It is shown that the solvent and the temperature used during the coordination step in the preparation of an alumina-supported platinum catalyst have large effects on their observed hydrosilylating properties. These unprecedented observations lead to the preparation of a highly active and reusable catalyst.

Competitive reaction of methylcyclohexane and n-hexane over alumina-supported platinum, iridium and ruthenium catalysts

The conversion of methylcyclohexane (MCH), n-hexane (nH) or a 1/1 mixture of the two hydrocarbons was studied over alumina-supported Pt, Ir and Ru catalysts with metal dispersions varying from 10 to 100%, and prepared from the acetylacetonate precursors. The reaction was carried out between 410 and 650 K. The conversion of nH is strongly inhibited in the presence of MCH, which remains the most abundant reactant adsorbed on the metal particles. Neither the activity, nor the selectivity for MCH conversion, were modified by the presence of nH in the feed. During MCH conversion, the dehydrogenation to toluene is the only reaction observed on Pt catalysts. On Ir and Ru catalysts, ring opening and deep hydrogenolysis to lower alkanes operate, the extent of which depends on the metal and the size of the particles.

Aromatics Reduction over Supported Platinum Catalysts. 1. Effect of Sulfur on the Catalyst Deactivation of Tetralin Hydrogenation

The effects of sulfur on the deactivation of {gamma}-alumina-supported platinum catalysts for aromatics reduction were investigated. The catalytic test reactions were carried out in a continuous fixed-bed reactor at 270 C and 180--480 psig. Both fresh and used catalysts were characterized by elemental analysis, fast Fourier transform infrared (FFT-IR) spectroscopy, temperature-programmed reduction (TPR), and electron probe microanalysis (EPMA). A Langmuir-Hinshelwood reaction model, which is based on a chemisorption scheme with irreversible surface reaction control for tetralin hydrogenation and reversible surface reaction for sulfur poisoning, was proposed to describe the deactivation kinetics. The kinetic results show that the reactivation rate constant decreases with the severity of sulfur poisoning, while the deactivation rate constant is comparatively unrelated. The TPR results are consistent with the catalytic performance test; a severely sulfur-poisoned catalyst needs to be reduced at higher temperature. FFT-IR spectroscopy characterizing CO adsorbed on the fresh and sulfur-poisoned catalysts indicated that the bond strength between CO and platinum was weakened with an increase of sulfur poisoning. EPMA spectroscopy characterizing the metal profiles of the catalysts indicated a metal migration during sulfur poisoning.

Decomposition of the Precursor [Pt(NH3)4](OH)2, Genesis and Structure of the Metal-Support Interface of Alumina Supported Platinum Particles: A Structural Study Using TPR, MS, and XAFS Spectroscopy

During the preparation of alumina supported platinum catalysts, the precursor [Pt(NH3)4](OH)2 decomposes to a neutral Pt(NH3)zO species during the drying process at 120 C. Treatment in flowing hydrogen at 180 C leads to partial reduction of the platinum ammine complex and formation of platinum metal particles. A large increase in metal particle size is observed after a treatment under flowing H2 at 200 C. The final reduction at 350 C causes the total disappearance of the platinum precursor with a further increase in platinum particle size. The direct reduction at 350 C yields the biggest metal particles (35 A) while calcination before reduction produces a much higher dispersion (metal particle diameter 10 A). The beneficial effect of calcination, already observed by many authxs when using [Pt(NH3)4](OH)2 as a precursor for the preparation of highly dispersed WyA1203, can now be explained because this treatment avoids the formation of the mobile neutral Pt(NH3)zO complex. The metal particles produced by treatment in flowing hydrogen at 180 C present a metal-oxygen contribution at 2.7 A formed at the metal-support interface. This long distance is assumed to be caused by the presence of hydrogen in the metal-support interface based upon our results in combination with other TPD and EXAFS studies. A second metal-oxygen contribution with similar coordination number is detected at 3.86 A. This is a consequence of the presence of the first shell metal-oxygen at 2.7 8, and implies a [ 11 11 epitaxy in the metal-support interface.

Investigation of methane oxidation on Pt/Al 2O 3 catalysts under transient reaction conditions

The oxidation of methane on alumina-supported platinum catalysts has been investigated for various gas mixtures as a function of the state of oxidation or reduction of the platinum surface. It has been shown that the activity for methane oxidation goes through a sharp maximum a few seconds after the introduction of the reactants to either a pre-reduced or a pre-oxidised catalyst. The results are inter-preted in terms of a Langmuir-Hinshelwood mechanism in which the optimum rate of reaction corresponds to a platinum surface partially covered by both oxygen- and carbon-containing species. The position of the maximum in activity is very reproducible for pre-reduced catalysts but varies in an imprecise manner for pre-oxidised catalysts. These effects are thought to be related to morphological changes in the reduced and subsequently re-oxidised small platinum particles. It appears that even after re-oxidation at 300°C these platinum particles have a strong ‘memory’ of previous reduction treatments.

A study of platinum-supported catalysts through hyperfine interactions

The effect of indium addition on alumina-supported platinum catalysts was investigated by measurements of hyperfine interactions. Via lime differential perturbed angular correlation spectroscopy (TDPAC) on111Cd, Pt/Al2O3 catalysts were studied in the flow of a heptane/H2 gas stream. The results indicate that some amount of indium sticks to platinum which is then dispersed on the support surface. The amount of In that is free from platinum is mobile under reaction conditions, being capable of diluting platinum particles and draining off coke precursors from the platinum surface.

Selective oxidation of methanol in exhaust gas containing aromatic compounds

The activity of exhaust catalysts of methanol and gasoline mixture fueled automobiles is inhibited by coexistent aromatic compounds contained in the exhaust stream. One approach to avoid this inhibition could be the utilization of a molecular-sieve effect. It seems probable that the migration of aromatic compounds, which have larger molecular sizes than that of methanol, will be blocked by these supports and methanol will selectively reach the active sites of the catalysts. Thus, we examined the simultaneous oxidation of methanol and toluene over some ion-exchanged Pt/zeolite catalysts. Experimental results showed that ion-exchanged Pt/zeolite catalysts are less affected by coexistent toluene and have a higher selectivity for methanol oxidation compared with an alumina-supported platinum catalyst, suggesting that the use of these catalysts is effective for oxidizing methanol in the exhaust stream containing aromatic compounds. Among the catalysts tested, a Pt/H-MOR catalyst was found to be the most effective for the oxidation of methanol and effectively catalyzed the reaction in the presence of toluene even at low temperature.

Effects of alumina preparation solvents on the thermostability and oxidation activity of alumina-supported palladium and platinum catalysts

Various alumina powders were prepared by the complexing agent-assisted sol-gel method and the influence of the complexing agents on the thermostability of alumina-supported palladium and platinum catalysts and on their oxidation of carbon monoxide and methane was investigated. Three alumina-supported palladium catalysts were prepared by an impregnation method. The highest carbon monoxide and methane conversion efficiencies were obtained with a Pd/MPD(2-methylpentane-2,4-diol)-alumina catalyst followed by a Pd/IPA (isopropyl alcohol)-alumina and Pd/commercial alumina catalysts. Two alumina-supported platinum catalysts were prepared, one from a commercial alumina and the other was obtained from aluminium isopropoxide using MPD. The platinum catalyst supported on MPD-alumina showed higher carbon monoxide and methane conversion efficiencies than the platinum catalyst supported on the commercial alumina. It was found that the sol-gel method using MPD as a complexing agent improves the thermostability of alumina-supported palladium and platinum catalysts.

Mechanism of the selective reduction of nitrogen monoxide on platinum-based catalysts in the presence of excess oxygen

A range of alumina-supported platinum catalysts have been prepared and investigated for the selective reduction of nitrogen monoxide in the presence of a large excess of oxygen. Steady-state microreactor experiments have demonstrated that these catalysts are very active and selective for the reduction of nitrogen monoxide by propene at temperatures as low as 200°C. There does not appear to be a simple correlation between the activity for nitrogen monoxide reduction and the platinum surface area. Instead it is found that there is a very good inverse correlation between the maximum nitrogen monoxide reduction activity and the temperature. The most active catalysts for selective nitrogen monoxide reduction are those that generate activity at the lowest temperature. The technique of temporal analysis of products (TAP) has been used to obtain detailed mechanistic data about the selective nitrogen monoxide reduction reaction on an alumina-supported platinum catalyst. Using carbon monoxide, hydrogen or propene as reductant it has been demonstrated that the predominant mechanism for selective nitrogen monoxide reduction involves the decomposition of nitrogen monoxide on reduced platinum metal sites, followed by the regeneration of the active platinum sites by the reductant. In the decomposition step it has been shown that oxygen from nitrogen monoxide is retained on the surface of the platinum and blocks the surface for further adsorption/reaction of nitrogen monoxide; it has been observed that oxidised platinum catalysts are not active for the nitrogen monoxide reduction reaction. Under typical operating conditions, propene is a far more efficient reductant than either carbon monoxide or hydrogen. The greater efficiency of propene as a reductant is explained on the basis of the additional reducing power of the propene molecule, which can react with as many as nine adsorbed oxygen atoms, ensuring that 'patches' of reduced platinum are available for nitrogen monoxide adsorption/reaction. A small additional activity of reduced platinum in the presence of propene, which is not observed when carbon monoxide or hydrogen is used as reductant, has been explained on the basis of a second mechanism involving the carbon-assisted decomposition of nitrogen monoxide at sites on the reduced platinum adjacent to adsorbed carbon-containing moieties, believed to be fragments from adsorbed propene molecules. A model for the selective reduction of nitrogen monoxide on alumina-supported platinum catalysts is presented which is capable of explaining all the results obtained in this work and in the published literature on this subject.

High Surface Area Platinum-Titania Aerogels: Preparation, Structural Properties, and Hydrogenation Activity

High surface area platinum-titania aerogels with marked meso- to macroporosity have been synthesized via the sol-gel-aerogel route. An acid-catalyzed titania gel was prepared from tetrabutoxy-titanium(IV) with methanol as solvent. The platinum precursor solutions added after the redispersion of the titania gel were either PtCl 4, (NH 4) 2PtCl 6 or Pt(acac) 2 dissolved in protic solvents. Platinum metal particles formed upon high-termperature supercritical drying. The platinum-titania aerogels have a BET surface area of 150 to 190 m 2 g −1 after thermal pretreatments up to 673 K and the titania matrix consists of well-developed anatase crystallites of about 8-9 nm mean size. Depending on the platinum precursor used, the volume-weighted-mean particle size, determined by TEM, varies in the range 3.6 to 68 nm, consistent with XRD results for the platinum component. All aerogel samples showed a pronounced stability of both the titania matrix and the platinum particles towards air or hydrogen at temperatures up to 673 K. Thermal analysis, combined with mass spectroscopy, revealed that the untreated catalysts contain a considerable amount of entrapped organic impurities after the high-temperature supercritical drying. For the characterization of the activity and the accessibility of platinum particles the liquid phase hydrogenations of trans-stilbene and benzophenone were used as test reactions. Compared to a commercial alumina-supported platinum catalyst, the untreated 2-5 wt% platinum-titania catalysts derived from (NH 4) 2PtCl 6- and especially PtCl 4- precursor solutions exhibit a markedly higher catalytic activity. In general, air pretreatments at 573 K or above had either no or promoting influence on activity. In contrast, pretreatments in hydrogen produced either no or detrimental activity change.

N2O Reduction by Co over an Alumina-Supported Pt Catalyst: Steady-State Multiplicity

Steady-State multiplicity of the N2O + CO reaction over an alumina supported platinum catalyst has been observed over the temperature range 461-520 K. The boundaries of the steady-state multiplicity region have been mapped on a N2O-CO feed composition diagram, and the effect of temperature on multiplicity was determined. Four kinetic models have been examined, and it was found that only a model based on carbon monoxide self-exclusion from the platinum catalyst was able to describe all of the observed behavior. The CO self-exclusion effect requires that an adsorbed CO molecule exclude other CO molecules from an area equivalent in size to NCO surface Pt atoms, where NCO is slightly greater than unity. The proposed mechanism consists of several elementary steps, namely, the reversible adsorption of CO, the irreversible dissociative adsorption of N2O to form adsorbed atomic oxygen and gaseous N2, and the reaction of adsorbed carbon monoxide and atomic oxygen to form gaseous CO2. Due to the CO self-exclusion effect, N2O can still adsorb on a Pt surface saturated with CO and therefore the model predicts CO conversions as high as the experimental values for high concentrations of CO in the feed, something which is not possible with the other three mechanisms which were examined. For this model, the steady-state multiplicity can be described by an explicit rate function.

Preparation, activity and durability of promoted platinum catalysts for automotive exhaust control

The effects of the addition of calcia, ceria and lanthana to alumina-supported platinum catalysts on the simultaneous control of hydrocarbon, carbon monoxide and nitrogen oxide automobile emissions (three-way catalyst behaviour) were analyzed. The activity of the prepared samples was determined with steady-state, reducing and oxidizing, simulated feedstreams as well as with a cycled oxidizing-reducing feedstream averaged at the stoichiometric conditions which resembled the exhaust air/fuel fluctuations in a closed-loop emission control system. Activity of the catalysts was also analyzed after conducting accelerated thermal and chemical ageing in order to test their durability. Under normal operating conditions of the automobile engine, Pt/Al 2O 3 catalysts promoted by rare-earth oxides are able to achieve high HC, CO and NO conversions. The behaviour of the catalysts in the cold start period was determined by analysis of light-off temperatures and a comparison was made with those corresponding to some commercial samples and others reported in the literature. The catalysts prepared in this work showed lower light-off temperatures than those of commercial and reported Pt/Al 2O 3 catalysts but these temperatures were not so low as with PtRh/Al 2O 3. In all cases, the prepared catalysts resulted in a better resistance to accelerated ageing. Samples with ceria showed the best resistance to accelerated ageing.

Preparation of alumina-supported platinum catalyst at ambient temperature for selective synthesis of cinnamyl alcohol by liquid-phase cinnamaldehyde hydrogenation

An alumina-supported platinum catalyst, indicating high activity to selective synthesis of cinnamyl alcohol in cinnamaldehyde hydrogenation, was prepared at ambient temperature by a novel method with the reduction of metal precursors adsorbed on the support by sodium tetrahydroborate solution.

Liquid-phase oxidation of 1-methoxy-2-propanol with air III: Chemical deactivation and oxygen poisoning of platinum catalysts

The deactivation behaviour of carbon and alumina supported platinum catalysts has been studied during the liquid-phase oxidation of 1-methoxy-2-propanol to methoxy-acetone, with air as oxidant. Two different deactivation procedures could be distinguished: deactivation due to strong adsorption of by-products (chemical deactivation) and deactivation which originated from over-oxidation (oxygen poisoning). Among the various parameters which can influence the deactivation behaviour of platinum catalysts, the influence of the nature of the support and of the base have been considered in detail. Both the basic functional groups of activated carbon and the presence of a strong base resulted in similar chemical deactivation. The continuous formation and decomposition of a polymeric carbonyl by-product compound was followed using Chromatographic and spectroscopic methods. The decomposition and removal of this by-product from the catalyst surface was favoured by the presence of sufficient oxygen and consequently different response behaviour to a change in oxygen supply was observed for oxygen-poisoned and chemically deactivated catalysts. A comparative study of the oxidation of 2-propanol, 2-butanol and 1-methoxy-2-propanol to the corresponding ketones demonstrated the strong influence of the electron-withdrawing a-methoxy group on the deactivation behaviour.

Non-Classfcal Kinetiics in Hydrogen-Deuterium Exchange Reaction on Metal Catalyst

AbstractMonte Carlo simulations of the A2 + B2→ P (with P = AA, AB, and BB) reaction on various monodisperse island systems provided various models for the reaction H2 + D2→ HD on alumina supported platinum catalysts. Anomalous reaction kinetics and reaction orders due to the effects of diffusion, reactant correlation, and ensemble character of the catalyst in low dimensional systems are observed not only at early time but also at steady state. Concentration effects and spillover studies indicate that surface diffusion and hydrogen spillover may be responsible for the anomalous behaviors.

Coated silica as support for platinum catalyst: II. Hydrogen chemisorption and activity for combustion

Samples of 0.5 wt.-% platinum supported on silica, alumina, titania and lanthana, and on coated supports (Al 2O 3/SiO 2, TiO 2/SiO 2 and La 2O 3/SiO 2) with threshold monolayer loadings, were characterized by hydrogen-oxygen titration, temperature-programmed desorption (TPD) of hydrogen and catalytic combustion of methane. All the samples show depressed chemisorption when reduced at 773 K, compared to reduction at 573 K. Surface platinum atom concentrations for Pt/TiO 2 and Pt/La 2O 3 are around one order of magnitude higher than those for Pt/SiO 2 and Pt/Al 2O 3. But when coated on silica, the oxide coatings caused greatly depressed surface platinum atom concentrations even lower than that on pure silica support. With Pt/Al 2O 3/SiO 2 and Pt/La 2O 3/SiO 2, hydrogen desorption profiles can be attributed to hydrogen spilt over onto alumina and lanthana, respectively. But for Pt/TiO 2/SiO 2, the desorbed hydrogen seems to originate from spilt over hydrogen on silica. This may imply full or nearly full coverage of silica by the oxides in the former two cases, and partial coverage in the latter case. Differences in TPD profiles of LTR and HTR samples agree well with chemisorption data. Titania and lanthana supported catalysts have much higher specific activity and TOF than silica and alumina supported platinum catalysts for methane combustion. When coated on silica, TOFs were increased while specific activities were lowered relative to those of pure oxides. Titania and titania coated silica, when used as supports for platinum, yield the most active catalysts for catalytic combustion of methane.

KINETIC AND TRACER STUDIES ON METHANOL CONVERSION OVER SUPPORTED PLATINUM CATALYSTS

Hydrogen-deuterium exchange in methanol over Al2,O3, platinum black and alumina supported platinum catalysts have been investigated by means of mass spectrometry. The hydrogen-deuterium exchange proceeds faster over Pt/Al2O3 than either platinum black or alumina, and the rate (as well as the rate of dimethyl ether formation) increases with platinum dispersion. From the reactions CH3OH + D2, CD3OH + H2 and CD3OH + D2 the different exchange rates of the CH3 and OH groups (R1, and R2, respectively) were determined. For all catalysts studied the exchange in the OH group was two or three orders of magnitude faster than CH3 group. Exchange in the methyl group needs higher activation energy: R1/R2 ratio increases with temperature. Catalyst pretreatment has an important effect: higher activation temperature results in a higher R1/R2 ratio. A methematical method is suggested for the evaluation of exchange rates of a molecule with two different types of hydrogen and a model is given for the interpretation of measured data: methanol is activated mainly on Al2,O3, forming surface methoxide groups on the Lewis acidic sites. This methoxide can react with deuterium atoms having been activated over platinum sites. With increasing dispersion the metal-support interface is enhanced which leads to higher rate of exchange.