Platinum-alumina Catalysts Research Articles

Bridging the Gap between Theory and Experiments - Nano-structural Changes in Supported Catalysts under Operating Conditions

Computational approaches have been limited to examining catalytic processes using models that have been greatly simplified in comparison to real catalysts. Experimental studies, especially on emission treatment catalysts, have primarily focused on fully formulated systems. Thus, there remains a knowledge gap between theory and experiments. We combine the power of theory and experiment for atomistic design of catalytically active sites that can translate the fundamental insights gained directly to a catalyst system suitable for technical deployment. In this article, we describe our results on a model platinum-alumina catalyst that is a common constituent of emission treatment catalysts such as three-way, NO/dx trap, oxidation, and HC-SCR catalysts. We present theoretical and experimental studies of the oxidation and reactivity of Pt catalyst clusters towards O, CO, and NO/dx. Our theoretical studies indicate that the reaction energetics are strongly dependent on the size of the clusters as well as the extent of oxidation of the clusters, and the energetics of CO and NO oxidation may be more favorable on the oxidized clusters than metallic clusters because of the weakened adsorption of O, CO and NO. Experimentally, we have observed that the aberration-corrected HA-ADF STEM images of Pt/gg-alumina support show that there are singlemore » atoms, 2-3 atom clusters, and several 10-20 atom clusters of Pt. We also found that the Pt particles size has an impact on CO oxidation initiation and completion temperatures. Substrate effects were studied for equivalent Pt particle size distributions on both gu-alumina and gg-alumina supports. Particle size effects were investigated on Pt/gg-alumina catalysts with Pt particle size distribution centered at 1 nm and 12 nm, respectively. We will describe our results on substrate and Pt particle size effects. In addition, we will also present our study of nano-structural changes in model catalysts on exposure to various reaction conditions.« less

Steam reforming of biodiesel by-product to make renewable hydrogen

The aim of this paper was to investigate the viability of steam reforming the combined glycerol and water by-product streams of a biodiesel plant. A platinum alumina catalyst was used to optimise the operating conditions for glycerol steam reforming and mass spectroscopy was chosen to measure reformer gas yield. The problem is that glycerol steam reforming is relatively untested even with pure glycerol and the by-product quality may be too poor. The strategy was therefore to optimise the process using pure glycerol and compare the performance with by-product glycerol. To test catalyst degradation caused by carbon deposition, a Solid Oxide fuel cell (SOFC) was used as a separate reformer and electrical performance was measured to indicate carbon deposition. This is the first time a SOFC has been run on glycerol. The results showed that thermodynamic theory can be used to predict reformer performance. At high temperatures high gas yield can be reached (almost 100%) and selectivities of 70% (dry basis) obtained. The optimum conditions for glycerol reforming were 860 °C temperature (maximum tested), 0.12 mols/min glycerol flow per kg of catalyst and 2.5 steam/carbon ratio. Reforming catalysts lasted for several days of continuous operation with minimal degradation, 0.4% of feed deposited. By-product glycerol performed slightly worse with a lower yield and more carbon deposition, 2% of feed. The results show that glycerol steam reforming is a viable alternative use for glycerol and potentially a better option than purification.

Heterogeneous Asymmetric Reactions. 29. Enantioselective Hydrogenation of Ethyl Benzoylformate over Dihydrocinchonidine-Modified Platinum–Alumina Catalyst in Acetic Acid

The enantioselective hydrogenation of ethyl benzoylformate to (R)-ethyl mandelate over dihydrocinchonidine (DHCD)-modified Pt/Al2O3 catalyst in acetic acid was studied as a function of modifier concentration, hydrogen pressure and reaction temperature. The maximum enantioselectivity obtained under optimized conditions (DHCD concentration 1 mmol dm-3, 25 bar H2, 0 °C, AcOH/toluene 1:1) was 98% ee. The difference between the rates of racemic and enantioselective hydrogenation was less significant than in the case of ethyl pyruvate. This indicates that the high reaction rate in the enantioselective heterogeneous hydrogenation of α-ketoesters is not a necessary condition of the chiral induction.

Structure of a Platinum–Alumina Catalyst Prepared from the Carbonyl Cluster H2[Pt3(CO)6

The state of a platinum carbonyl cluster in an initial aqueous acetone solution and its transformations on the surface of aluminum oxide in the course of catalyst preparation were studied by EXAFS spectroscopy. It was found that water enters the polynuclear framework of the dissolved cluster (the Pt–O distance is 2.55 A, where O is the oxygen atom of water). Structural changes in the supported cluster in the course of catalyst preparation exhibited a strong interaction of platinum with alumina (the Pt–O distance is 1.92–1.95 A), beginning at the step of H2[Pt3(CO)6]5 adsorption. This interaction was retained upon the subsequent high-temperature treatments of the catalyst. The structures of samples prepared from platinum carbonyl and chloroplatinic acid were significantly different. In the former case, a surface prototype was formed from the initial cluster; in the latter case, the sample consisted of platinum metal clusters of a considerable size.

Kinetic models of catalyst deactivation in paraffin dehydrogenation

The results of studying the deactivation of both unpromoted platinum–alumina catalysts and those promoted with K, Li, In, Sn, and W in the dehydrogenation of lower and higher paraffins are discussed. The main reason for catalyst deactivation is found to be coke formation. The rate laws of coke formation and paraffin dehydrogenation in the non-steady-state regime of the reaction are derived. The catalyst sulfuring is found to enhance its stability. The effects of oxygen and water impurities on the reactions and coke formation are studied.

N 2O formation during organic nitro compounds’ hydrolysis on platinum based catalysts

Hydrolysis reaction products of 1- and 2-nitropropane and 2-methyl-2-nitropropane were compared on a platinum–alumina catalyst, in the conditions of NO lean reduction by propene (gas phase, low concentrations and in the presence of water and oxygen). N 2O formed during the reaction decreased in the order 1-nitropropane > 2-nitropropane ⪢ 2-methyl-2-nitropropane, i.e. following the degree of substitution of the carbon atom in the α position of the nitro group. The Nef (19) reaction mechanism is proposed to explain this observation.

Hydrogenation of aromatics on modified platinum–alumina catalysts

Using a pulsed-micro catalytic reactor benzene and toluene were hydrogenated to cyclohexane and methylcyclohexane, respectively, in a temperature range 50–250°C. Modified catalysts containing 0.35% Pt–Al 2O 3 were investigated. Modification was carried out via (a) introducing a second metal (Ir, Rh, Re and U) and (b) fluorination and chlorination with different halogen contents of 1,3 and 6 wt%. The study revealed that (1) all catalysts show good catalytic activities in the temperature range 125–150°C, except Rh catalyst which is very active even at room temperature, (2) introducing either Ir or Re enhances Pt activity, while U inhibits this activity, (3) halogenation promotes the catalyst activity and (4) alkyl-substitution enhanced aromatic ring hydrogenation.

Influence of water and hydrocarbon processed in feedstream on the three-way behaviour of platinum-alumina catalysts

The influence of water and the nature of the hydrocarbon -methane, ethylene, propene and 1-butene- processed in the feedstream on the three-way behaviour of a ceria-modified alumina supported platinum catalyst is investigated. Experiments were carried out with static feedstreams with air-to-fuel ratios between 14.13 and 15.17 as well as with oxidizing-reducing cycled feedstreams simulating the real automobile exhaust mixture. Conclusions are drawn on the importance of water-gas shift and steam reforming reactions and their relationship with inhibition self-poisoning effects of carbon monoxide and/or hydrocarbons. Comparison of data of the prepared catalyst and those obtained at the same operational conditions with two commercial catalysts, with only platinum/alumina and platinum-rhodium/ceria-alumina respectively, allows confirmation of the effects of ceria and establishes the role of rhodium when added to the catalyst.

Study of Pt/alumina catalysts preparation

Platinum-alumina catalysts were prepared at 20°C by using a method in which hexachloroplatinic anions from solution were adsorbed on θ-alumina in the pH range 1–10 and with contact times between 1 and 24 h. An adsorption maximum was observed at pH around 3–4. From measurements of electrophoretic mobility, the isoelectric point of θ-alumina was determined to be about 8.5. Comparison with literature data indicated that γ- and θ-alumina exhibit similar adsorption properties and that the amount of Pt deposited is controlled by pH and contact time for both systems. The adsorption appears to proceed initially as a physisorption process with relatively strong specific interaction or chemisorption taking over at longer contact times. Pt θ -alumina samples with 0.3 to 2.2 wt% Pt were prepared. Wet calcination at 570°C led to a decrease in the Cl content of the samples, while the Pt content was unaffected.

Catalytic Oxidation of Dichloromethane, Chloroform, and Their Binary Mixtures over a Platinum Alumina Catalyst

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTCatalytic Oxidation of Dichloromethane, Chloroform, and Their Binary Mixtures over a Platinum Alumina CatalystDouglas M. Papenmeier and Joseph A. RossinCite this: Ind. Eng. Chem. Res. 1994, 33, 12, 3094–3103Publication Date (Print):December 1, 1994Publication History Published online1 May 2002Published inissue 1 December 1994https://doi.org/10.1021/ie00036a026RIGHTS & PERMISSIONSArticle Views512Altmetric-Citations38LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

Modification of platinum-alumina catalysts. Effect of the addition of lithium to platinum in the dehydrogenation of cyclohexane

The modification of platinum-alumina catalysts by 0.1 wt% lithium does not affect to a large extent the dehydrogenating properties of platinum. However, when lithium is added to platinum through impregnation by nitrate salt in a higher content, residual nitrate species can remain on the catalyst surface, even after calcination at 773 K. These nitrate species can be coreduced with the platinum precursor and be associated to a significant sintering of the platinum particles.

129Xe nuclear magnetic resonance studies of supported platinum-alumina catalysts

129Xe nuclear magnetic resonance spectra of xenon adsorbed on supported platinum-alumina catalysts have been examined. The state of metallic platinum particles (Pto) was shown to be different for Pt/Al2O3 and Pt/Al2O3(Cl) samples. Some assumptions about the mutual arrangement of supported metallic particles have been made.

Catalytic synthesis of C-alkylimidazoles over platinum-alumina catalysts

The synthesis of C-alkylimidazoles from 1,2-diamines and carboxylic acids over bifunctional platinum—alumina catalysts has been studied. It has been shown that this method is effective for the synthesis of 2-alkyl and 2,4-dialkylimidazoles including imidazoles with long-chain alkyls. The effect of the reaction temperature, space velocity of the flow of the raw materials, and dilution by hydrogen on the yield of product has been examined for the example of the synthesis of 2-methylimidazole from ethylenediamine and acetic acid, and the stability of the catalyst in continuous reaction cycles with intermediate oxidative regeneration has been studied. The composition of the accompanying products has been established and a mechanism proposed for their formation.

Reforming of pyrolysis gasoline over platinum-alumina catalysts containing MFI type zeolites

Reforming of pyrolysis gasoline, a highly olefinic feed, has been carried out over Pt-alumina catalysts containing small amounts of MFI type zeolites. The incorporation of these zeolites helps in lowering the coke deposition on the catalysts, thereby increasing their life. Further, the products are virtually free of aliphatics. Due to the large concentration of aromatics in the feed, significant aromatic gain is obtained only at low pressures of operation.

Catalyst characterization by a probe reaction. Cyclopropane hydrogenolysis and benzene hydrogenation on platinum-alumina catalysts

Ethylene hydrogenation by pulse is used as a probe reaction for the study of cyclopropane hydrogenolysis and benzene hydrogenation using platinum on alumina catalyst. It is shown that cyclopropane hydrogenolysis is a non-demanding reaction, because all the irreversibly adsorbed hydrogen atoms which are active for ethylene hydrogenation are also active for cyclopropane hydrogenolysis. The number of surface hydrogen atoms exceeds that of total platinum atoms in the catalyst by a factor of about 2.5; benzene, on the contrary, at room temperature, reacts with only a small fraction of them. A pulse technique is used to study the mechanism of benzene hydrogenation. The results suggest the following mechanistic features: (a) Benzene hydrogenation is a demanding reaction. (b) The rate-determining step of the reaction is the formation of a σ-complex between the benzene ring and a metal atom in the presence of irreversibly adsorbed hydrogen. (c) The formation of a π-complex cannot be an initial condition for the benzene hydrogenation. (d) Addition of hydrogen to the σ-complex is fast and requires hydrogen in the gas phase.

Infrared and adsorption study of strong metal-support interaction in diluted platinum-alumina catalysts

Diffuse reflectance IR spectroscopy along with H2 and CO adsorption measurements were applied to the study of 0.3% Pt/-γ-Al2O3 catalyst precalcined in an air flow at 1023 K and then reduced at 773 and 973 K in hydrogen. Reduction of the catalyst at 973 K results in formation of supported metallic Pt particles in SMSI (strong metal-support interaction) state, as evidenced by the sharp decrease of Pt ability to chemisorb H, at room temperature as well as the significant lowering of thermal stability of linear Pt-CO complexes. Spectroscopically the effect of SMSI is manifested by the slight increase of the singleton frequency of linearly adsorbed CO from 2050 to 2060 cm−1 together with the decrease of the dipole-dipole shift from 40 to 15 cm−1 and by a large decrease of absorption band extinction.

Deuteron NMR study of the conversion of vinylidene to ethylidyne: evidence for support effects in platinum-alumina catalysts

The authors have used deuterium NMR to study the conversion of vinylidene (CCH{sub 2}), the stable room-temperature species formed from acetylene adsorbed on Pt, plus hydrogen to ethylidyne (CCH{sub 3}). These experiments were performed using small Pt metal clusters supported on {eta}-alumina typical of industrial catalysts. For temperatures above 300 K, exchange between hydrogen atoms in the product ethylidyne surface species was observed. The authors also show that adsorption of C{sub 2}H{sub 2} on the Al{sub 2}O{sub 3} support makes a negligible contribution to the {sup 13}C signal, confirming earlier work by Wang, contrary to results quoted by Ellis et al. for magic angle spinning studies.

Dehydrogenation of piperazine into pyrazine on alumina-platinum catalysts

Dehydrogenation of piperazine on alumina-platinum catalysts at 360–400°C was investigated. It was found that the alumina-platinum catalyst has low selectivity in the formation of pyrazine (yield ≤43%), and dehydrogenation is accompanied by decomposition and coke formation and products of decomposition in the formation of alkylpyrazines. The alumina-platinum catalyst in In2O3 and Re2O7 additives permits obtaining pyrazine with a yield of up to 80%. Dehydrogenation of piperazine is accompanied by reactions of decomposition, dehydroisomerization, and alkylation to a small degree, resulting in the formation of pyrrole, methylimidazole, and alkylpyrazines.

Dehydrogenation of isobutane in the presence of a Zr 2Fe hydrogen acceptor

Earlier it was shown that the isobutene yield during dehydrogenation of isobutane on Pt/AI203 at temperatures ranging from 300 to 400C can be increased by shifting the thermodynamic equilibrium of the reaction through the removal of the hydrogen formed by hydride-forming metals Ti and Zr [1] and intermetallic compounds ZrNi and ZrCo [2]. However, the isobutene yields obtained in these conditions did not exceed 10-11 mol.% and were dictated by the kinetic and thermodynamic limitations of the adsorption capacity of the metals and intermetallic compounds tested, and also by the reduction in the stability of the aluminaplatinum catalyst when hydrogen is removed from the reaction flow. In the present study, an oxide chromium catalyst was used as the isobutane dehydrogenation catalyst. This is less sensitive to the presence of hydrogen in the reaction mixture than platinum catalytic systems. IntermetaUic compound Zr2Fe, which is capable of absorbing hydrogen at temperatures above 400C [3], was tested as the hydrogen acceptor.

Mechanism of dehydrocyclization of n-alkanes over platinum—alumina Catalysts

In order to examine whether the metal function or the acid function is “criticalrd for the reforming reaction, the dehydrocyclization of n-hexane, n-heptane and n-octane was carried out over acidic and non acidic platinum—alumina catalysts. It is concluded that the metal function is the “critical” parameter with the C6 and C7 alkanes whereas both the metal and acid could be important with n-octane.