Kinetic study of isotopic exchange reaction between hydrogen and water vapor over a pt/sdbc hydrophobic catalyst sheet

The isotopic exchange reaction between hydrogen and water vapor was studied for the temperature range 313 K to 353 K using 2.2% platinum supported on a hydrophobic styrene-divinylbenzene copolymer (SDBC) sheet. With increasing relative humidity, the exchange rate increased, giving a stepped profile with a knee shape, and the reaction order with respect to hydrogen increased from almost zero to first order. In order to explain this unusual behavior, a simple kinetic model was proposed assuming that the water molecules are adsorbed in a multilayer on the platinum surface, and are subject to the exchange equivalent to finite number of layers. On the other hand, this multilayer of adsorbed water inhibits the transfer of HD to the platinum surface.     

Kinetics of iodine poisoning of hydrophobic Pt/SDBC catalyst for hydrogen isotopic exchange reaction

Changes with time in the isotopic exchange reaction rate between H₂ and HDO(v) over a hydrophobic Pt/SDBC (styrene-divinylbenzene copolymer) catalyst induced by iodine have been studied experimentally. The addition of a trace amount of I₂ to the reactor caused a remarkable decrease in the activity of the catalyst. The measurement results by means of the gas chromatographic pulse technique show that the H₂ adsorption onto the catalyst surface was strongly inhibited by iodine. A kinetic model for the poisoning was developed in terms of the competitive adsorption of I₂ with H₂ onto the catalytic active site. The experimental results were explained reasonably by the proposed kinetic model based on the site balance by assuming that I₂ adsorbs non-dissociatively on the active site. The poisoning could be prevented by the conversion of I₂ to I^? or IO₃^?through redox reactions.     

Diffusional effects in wetproofed catalysts for isotopic exchange between hydrogen gas and water vapour

Deuterium transfer rates between water vapour and hydrogen gas have been obtained for platinized carbon powder (Pt-C) and Teflon wetproofed platinized carbon films. The pore diffusional resistance in the catalyst films was assessed by calculating the effectiveness factor, ?, from a mathematical model for simultaneous diffusion and first order reversible reaction kinetics. Wetproofing the Pt-C catalyst with Teflon increased the intrinsic rate of reaction, but the effectiveness factor decreased with increasing catalyst layer thickness. Pore size and surface area distributions and water vapour adsorption isotherms of the Pt-C/Teflon catalyst films were similar to those of the Pt-C itself. Since Teflon had negligibly small surface area and very large pores compared to the carbon support, wetproofing did not cause significant changes to the physical characteristics of the platinized carbon.     

Continuous hydrogen evolution from cyclohexanes over platinum catalysts supported on activated carbon fibers

The continuous production of hydrogen from cyclohexanes is achieved effectively using Pt/ACF (ACF = activated carbon fiber) catalysts in a fixed-bed flow reactor. The Pt catalysts are more effective than a Pd/ACF catalyst for the reaction. Besides cyclohexane, methylcyclohexane, 1,4-dimethylcyclohexane, and p-menthane can also be employed as hydrogen source in the reaction system.     

Characterization of supported platinum by a probe reaction

A probe reaction is proposed to measure the number of active hydrogen atoms on platinum supported catalysts. Adsorbed hydrogen is titrated by ethylene pulses. The reaction product, ethane, desorbs quantitatively and can be measured directly by gas chromatography. This determination yields the total number of active hydrogen atoms per Pt atom (NAH). The method was tested on a set of Pt/Al₂O₃ catalysts. The results show that NAH can exceed several times the number of metal atoms on the catalyst and increases with the time of catalyst treatment with hydrogen at high temperature. The turnover rate for ethylene hydrogenation is constant when referred to NAH, but not when referred to the percentage of exposed metal atoms, as determined by conventional methods.     

Kinetics of the reaction between carbon dioxide and hydrogen over a ruthenium catalyst in a continuous stirred tank reactor

The catalytic reaction between carbon dioxide and hydrogen over a surface coated ruthenium catalyst was investigated in a stirred tank reactor, under isothermal conditions from 200 to 300°C. The kinetic data obtained were analysed by the Hougen-Watson method. Non-linear least squares analysis was used to discriminate among 28 rate models. The following Hougen-Watson type rate equation was selected.     

The oxidation of ethylene over a supported platinum catalyst

The complete oxidation of ethylene over a 0.3% Pt/Al₂O₃ catalyst, at ethylene concentration of ≤ 1.0 mol/m³ and oxygen concentrations of 0.16 to 8.6 mol/m³, was studied in a recycle reactor at temperatures of 362 to 472 K. The activity of the catalyst was influenced by pretreatment and reaction conditions causing fluctuations in rate measurements. Nevertheless, a simple rate function, r =Ae^?E/RT[O₂]/[C₂H₄], fitted the data as well as more complicated rate functions.     

Rates of isotopic exchange between hydrogen and aminomethane catalyzed by potassium methylamide

Pseudo first order kinetic constants for the exchange of deuterium between hydrogen and aminomethane were determined from measurements of the rates of absorption of hydrogen deuteride into liquid aminomethane flowing over a sphere. Potassium methylamide dissolved in the amine catalyzed the reaction. Kinetic constants range from 130 to 650 sec^?1 at temperatures from —30° to —10°C and catalyst concentrations of 4.95 to 15.8 g K/kg amine.     

Deuterium exchange between hydrogen and water in a trickle bed reactor

The catalyzed exchange of deuterium between hydrogen and liquid water has been studied as the basis for a heavy water production process. Platinum catalyst which had been waterproofed with Teflon was tested in a 0.2 m diameter trickle bed reactor at pressures and temperatures up to 6 MPa and 440 K. Extensive experimental data were used to test a model of the system which was developed from fundamental principles. It was found that mass transfer plays a very important role in the overall exchange and the conventional theory of vapour/liquid mass transfer does not adequately describe the absorption process. Modelling of the data has resulted in the postulation of a second method of mass transfer whereby HDO transfers directly from the catalyst to the bulk liquid phase.     

A comparative study of the water gas shift reaction over platinum catalysts supported on CeO2, TiO2 and Ce-modified TiO2

WGS reaction has been investigated on catalysts based on platinum supported over CeO₂, TiO₂ and Ce-modified TiO₂. XPS and XANES analyses performed on calcined catalysts revealed a close contact between Pt precursors and cerium species on CeO₂ and Ce-modified TiO₂ supports. TPR results corroborate the intimate contact between Pt and cerium entities in the Pt/Ce–TiO₂ catalyst that facilitates the reducibility of the support at low temperatures while the Ce–O–Ti surface interactions established in the Ce-modified TiO₂ support decreases the reduction of TiO₂ at high temperature. The changes in the support reducibility leads to significant differences in the WGS activity of the studied catalysts. Pt supported on Ce-modified TiO₂ support exhibits better activity than those corresponding to individual CeO₂ and TiO₂-supported catalysts. Additionally, the Ce–TiO₂-supported catalyst displays better stability at reaction temperatures higher than 573 K that observed on pure TiO₂-supported counterpart. Activity measurements, when coupled with the physicochemical characterization of catalysts suggest that the modifications in the surface reducibility of the support play an essential role in the enhancement of activity and stability observed when Pt is supported on the Ce-modified TiO₂ substrate.     

Low temperature isotopic exchange of molecular oxygen via the reaction of NO,NH3 and O2 over supported vanadia and molybdena catalysts

This study describes the behaviour of NO, NH₃ and¹⁸O₂ over supported molybdena and vanadia catalysts. The study was carried out using differential scanning calorimetry. The catalysts show different behaviour with respect to the reaction of NO, NH₃ and O₂ and similar behaviour with respect to the formation of water.     

Catalytic hydrogen transfer of ketones over atomic layer deposited highly-dispersed platinum nanoparticles supported on multi-walled carbon nanotubes

Hydrogen transfer of ketones, catalyzed by highly-dispersed platinum nanoparticles supported on multi-walled carbon nanotubes (MWCNTs), was studied. Pt nanoparticles were deposited on gram quantities of non-functionalized MWCNTs by atomic layer deposition (ALD) at 300 °C, using (methylcyclopentadienyl) trimethylplatinum and oxygen as precursors. TEM analysis showed that ~ 1.4 nm Pt nanoparticles were highly dispersed on MWCNTs. The heterogeneous hydrogen-transfer reactions of acetophenone indicated that an acetophenone conversion of 100% and a 1-phenylethanol selectivity of 99.0% could be obtained with a ketone to Pt mass ratio of 24,690 and a ketone to KOH mass ratio of 22 at 150 °C for 5 h. The selectivity of the Pt/MWCNT catalyst was higher than that of the commercial Pt/C catalyst, due to the highly-dispersed, uniform Pt nanoparticles and the unique porous structures of the Pt/MWCNT catalyst. The high stability of the Pt/MWCNT catalyst was demonstrated by reutilization of the catalyst. The high reactivity and selectivity of this catalyst for hydrogen transfer reduction were also demonstrated for other ketones.     

Kinetics of hydrogen evolution reaction on a stainless steel electrode

The kinetic parameters for the hydrogen evolution reaction on a stainless steel substrate have been obtained from a study of the steady-state polarization curves as well as the galvanostatic transients. The high Tafel slope obtained in the steady-state polarization measurements was ascribed to the presence of an oxide film present on the surface of the stainless steel electrode.     

Selective hydroisomerization of n-dodecane over platinum supported on SAPO-11

Effects of loading method and amount of platinum on the hydroisomerization of n-dodecane were investigated. Platinum was supported on SAPO-11 by incipient wetness, ion exchange, or atomic layer deposition. Characterization of catalysts was carried out using N₂ adsorption, XRD, NH-TPD, TPR, and pyridine-IR. The dispersion of the Pt (IE 0.2)/SAPO-11 and the Pt (ALD 0.6)/SAPO-11 are 0.65 and 0.59, respectively, which are much higher than those of the catalysts prepared by incipient wetness method. The Pt(IW)/SAPO-11 catalyst exhibited the best isomerization activity in the hydroisomerization of n-dodecane, which is attributed to the highest reducibility.     

Dissociation of dihydrogen and hydrogen sulfide over a sulfided NiMo-alumina catalyst as evidenced by D2S-H2 isotopic exchange

To investigate the possible role of surface sulfur in the activation of dihydrogen, the reaction of D₂S with H₂ was carried out at 423 K on a presulfided NiMo-Al₂O₃ catalyst. The incorporation of D atoms in H₂ shows that the catalyst is capable of dissociating H₂S and H₂ into species which by recombination make the exchange of hydrogen atoms possible between these molecules. This can be considered as evidence of the involvement of surface sulfur in the activation of dihydrogen.     

The promotion of carbon monoxide oxidation by hydrogen on supported platinum catalyst

This paper reports the results of a study of the combined oxidation of hydrogen and carbon monoxide on a platinum diesel oxidation catalyst. Experimental results for the light-off curves obtained in a monolith supported catalyst are shown for different concentrations. The presence of hydrogen is shown to promote the oxidation of CO, with the largest effect shown for the first small addition of hydrogen, then a progressively decreasing effect as the hydrogen concentration is increased. The enhancement effect can be successfully simulated using a kinetic model in which the activation energy for the desorption of CO is decreased as a function of hydrogen adsorption. Hydrogen is allowed to adsorb on sites only accessible to hydrogen and not CO or oxygen.     

Residual hydrogen in supported platinum catalysts and its influence on their catalytic properties

When Al₂O₃-, SiO₂-, or TiO₂-supported Pt catalysts were reduced above 400°C, there was a sharp decrease in their hydrogenolysis activity for n-pentane at 400°C. The chemisorption of H₂ on the catalyst at 20°C also decreased as if a part of the surface Pt atoms was no longer accessible to chemisorption. A few air-H₂ cycles at 20°C restored part of the hydrogenolysis activity and H₂ chemisorption capacity lost on reduction of the catalyst at higher temperatures. An air oxidation at 500°C followed by reduction at 400°C fully restored the original properties of the catalyst. The uptake of hydrogen by oxidized catalysts above 400°C was more than would correspond to the topmost layer of the Pt surface only. In the case of Pt-TiO₂, a reduction of the carrier was shown to occur at about 500°C by means of temperature-programmed desorption and reduction studies. The attenuation of the catalytic properties of the three types of catalysts studied seems to be caused mainly by a stronger chemisorption of H₂ on Pt since such effects are observed on Pt-b1ack also. Self-inhibition by strongly chemisorbed H₂ seems to be a widespread phenomenon in catalysis.     

Surface studies on catalysts used for deuterium exchange between hydrogen and water vapour

Platinum/charcoal and nickel/chromium oxide catalysts for the isotopic exchange of deuterium between hydrogen and water vapour were prepared under optimum experimental conditions. The catalytic activity determination of both indicated higher activity for Pt/charcoal than for Ni/Cr₂O₃. BET measurements of the specific surface area and pore structure showed higher values for Pt/charcoal than for Ni/chromia. Chemisorption data of both catalysts allowed the determination of the specific metallic surface area, the degree of dispersion and the average particle size of the supported metals. A comparison between the results of the catalytic activity, BET measurements and chemisorption was made and indicated close agreement.