We present a microkinetic model as well as experimental data for the low-temperature water gas shift (WGS) reaction catalyzed by Pt at temperatures from 523 to 573 K and for various gas compositions at a pressure of 1 atm. Thermodynamic and kinetic parameters for the model are derived from periodic, self-consistent density functional theory (DFT-GGA) calculations on Pt(111). The destabilizing effect of high CO surface coverage on the binding energies of surface species is quantified through DFT calculations and accounted for in the microkinetic model. Deviations of specific fitted model parameters from DFT calculated parameters on Pt(111) point to the possible role of steps/defects in this reaction. Our model predicts reaction rates and reaction orders in good agreement with our experiments. The calculated and experimental apparent activation energies are 67.8 kJ/mol and 71.4 kJ/mol, respectively. The model shows that the most significant reaction channel proceeds via a carboxyl (COOH) intermediate. Formate (HCOO), which has been experimentally observed and thought to be the key WGS intermediate in the literature, is shown to act only as a spectator species.

Aqueous-phase reforming of 10 wt% ethylene glycol solutions was studied at temperatures of 483 and 498 K over Pt-black and Pt supported on TiO 2 , Al 2 O 3 , carbon, SiO 2 , SiO 2-Al 2 O 3 , ZrO 2 , CeO 2 , and ZnO. High activity for the production of H 2 by aqueous-phase reforming was observed over Pt-black and over Pt supported on TiO 2 , carbon, and Al 2 O 3 (i.e., turnover frequencies near 8-15 min ÿ1 at 498 K); moderate catalytic activity for the production of hydrogen is demonstrated by Pt supported on SiO 2-Al 2 O 3 and ZrO 2 (turnover frequencies near 5 min ÿ1); and lower catalytic activity is exhibited by Pt supported on CeO 2 , ZnO, and SiO 2 (H 2 turnover frequencies lower than about 2 min ÿ1). Pt supported on Al 2 O 3 , and to a lesser extent ZrO 2 , exhibits high selectivity for production of H 2 and CO 2 from aqueous-phase reforming of ethylene glycol. In contrast, Pt supported on carbon, TiO 2 , SiO 2-Al 2 O 3 and Pt-black produce measurable amounts of gaseous alkanes and liquid-phase compounds that would lead to alkanes at higher conversions (e.g., ethanol, acetic acid, acetaldehyde). The total rate of formation of these byproducts is about 1-3 min ÿ1 at 498 K. An important bifunctional route for the formation of liquid-phase alkane-precursor compounds over less selective catalysts involves dehydration reactions on the catalyst support (or in the aqueous reforming solution) followed by hydrogenation reactions on Pt.

A microkinetic model for methanol decomposition on platinum is presented. The model incorporates competitive decomposition pathways, beginning with both O-H and C-H bond scission in methanol, and uses results from density functional theory (DFT) calculations [Greeley and Mavrikakis, J. Am. Chem. Soc. 124 (2002) 7193, Greeley and Mavrikakis, J. Am. Chem. Soc. 126 (2004) 3910].Results from reaction kinetics experiments show that the rate of H 2 production increases with increasing temperature and methanol concentration in the feed and is only nominally affected by the presence of CO or H 2 with methanol. The model, based on the values of binding energies, pre-exponential factors and activation energy barriers derived from first principles calculations, accurately predicts experimental reaction rates and orders. The model also gives insight into the most favorable reaction pathway, the rate-limiting step, the apparent activation energy, coverages, and the effects of pressure. It is found that the pathway beginning with the C-H bond scission (CH 3 OH fi H 2 COH fi HCOH fi CO) is dominant compared with the path beginning with O-H bond scission. The cleavage of the first C-H bond in methanol is the rate-controlling step. The surface is highly poisoned by CO, whereas COH appears to be a spectator species.

Chloromethylated poly(styrene) and chloropropylated silica gel have been reacted with α,ω-diamines to give diamine modified catalyst supports which have then been covalently attached to iron() tetrakis(pentafluorophenyl)porphyrin (FeTF 5 PP). In this way the diamino groups provide a simple linker/spacer unit between the support and the catalyst. The diamino-silica has been further modified using trimethylsilyl chloride and by acetylation or acid washing. The polarities of the modified inorganic and organic supports have been measured using Reichardt's dye. Spectroscopic studies reveal that with all the supported catalysts, except that on acid washed diamino-silica where the amines are protonated, the diamino groups reduce the iron() porphyrin to iron(). The supported iron porphyrins have been used to catalyse the oxidation of ethylbenzene by dioxygen. These reactions give the same three products, 1-phenylethyl hydroperoxide, 1-phenylethanol and acetophenone, as the analogous homogeneous oxidation using FeTF 5 PP, suggesting that they proceed by the same mechanism, however, in general they are slower. The overall product yields are limited by the stability/activity of the iron porphyrin and these in turn are very dependent on the length of the linker, the catalyst loading and the microenvironment provided by the support. The role of the diamino groups in the oxidations is discussed.

Production of H 2 for fuel cells is usually accomplished by a multistep process, which starts with catalytic reforming of hydrocarbons [1] or oxygenated hydrocarbons over metal catalysts [2,3] to produce a mixture of H 2 , CO, and CO 2 . These reformate gases are subsequently treated by several steps, such as water-gas shift (WGS) (CO + H 2 O!CO 2 + H 2 ) [4,5] and preferential oxidation of CO in the H 2 -rich gas stream (PROX; CO + 1/2 O 2 !CO 2 ), [6,7] for applications involving proton-exchange membrane (PEM) fuel cells, this oxidation of CO is necessary owing to the strong poisoning effects of CO on Pt-based anodes.[8] While these methods for removing CO from H 2 gas stream are well established, they suffer from several limitations. For example, the WGS reaction is slow at the low temperatures (e.g., 500 K) required to achieve favorable thermodynamics for this reaction, [9] and PROX requires the injection of O 2 (or air) into the H 2 gas stream and consumes a fraction of the H 2 as well. Moreover, these processes for CO conversion often involve the use of platinum or platinum-alloy catalysts, which are expensive and compete with fuel-cell electrodes for the limited supply of this precious metal, compared with the abundant holdings of gold in the world.[10]Recently, we reported the discovery of a room-temperature process for production of electrical energy using a fuel cell containing a carbon anode and operating with an aqueous solution of a polyoxometalate (POM) compound that had been reduced with pure CO over gold-nanotube catalysts. [11] This process for the oxidation and utilization of CO involves the reaction of CO and liquid water with a reducible POM, such as [H 3 PMo 12 O 40 ], which serves as an oxidizing agent for reaction of CO with liquid water and as an energy-storage agent for electrons; the process takes advantage of the high catalytic activities of gold for CO oxidation, [12][13][14] especially in the presence of liquid water, as shown experimentally using gold-nanotube membrane catalysts [15] and as predicted by density functional theory calculations. [16,17] A representative

The oxidation of ethylbenzene with dioxygen catalysed by iron() porphyrins in a solvent free system has been studied over the temperature range 30-110 ЊC. The time dependence of the formation of the three main products, 1-phenylethanol, acetophenone and 1-phenylethyl hydroperoxide, and the fate of the iron porphyrin are interpreted in terms of a free radical autoxidation mechanism. The yields of the oxidation products are determined by the rate of reaction and by the lifetime of the catalyst. Catalyst degradation is shown to involve reaction of the porphyrin ligand with 1-phenylethoxyl and 1-phenylethylperoxyl radicals. The disadvantages of increased induction periods and longer reaction times of the oxidations observed at lower reaction temperatures are counter balanced by increased catalyst turnovers. Less extensive studies on the oxidations of toluene, cumene, (2-methylpropyl)benzene and tert-butylbenzene support the overall mechanism proposed for ethylbenzene.A comparative study using the catalysts iron() 2, 3,7,8,12,13,17,18-octachloro-5,10,15,20-tetrakis-(2,6-dichlorophenyl)porphyrin and iron() tetrakis(pentafluorophenyl)porphyrin and five of its derivatives reveals that halogenation of the β-pyrrole positions markedly increases the activity of the catalysts but not the stability of the porphyrin towards degradation. The highest yields were obtained with the µ-oxodimer of iron() tetrakis(pentafluorophenyl)porphyrin and iron() tetrakis(4-dimethylamino-2,3,5,6-tetrafluorophenyl)porphyrin.

Production of H 2 for fuel cells is usually accomplished by a multistep process, which starts with catalytic reforming of hydrocarbons [1] or oxygenated hydrocarbons over metal catalysts [2,3] to produce a mixture of H 2 , CO, and CO 2 . These reformate gases are subsequently treated by several steps, such as water-gas shift (WGS) (CO + H 2 O!CO 2 + H 2 ) [4,5] and preferential oxidation of CO in the H 2 -rich gas stream (PROX; CO + 1/2 O 2 !CO 2 ), [6,7] for applications involving proton-exchange membrane (PEM) fuel cells, this oxidation of CO is necessary owing to the strong poisoning effects of CO on Pt-based anodes.[8] While these methods for removing CO from H 2 gas stream are well established, they suffer from several limitations. For example, the WGS reaction is slow at the low temperatures (e.g., 500 K) required to achieve favorable thermodynamics for this reaction, [9] and PROX requires the injection of O 2 (or air) into the H 2 gas stream and consumes a fraction of the H 2 as well. Moreover, these processes for CO conversion often involve the use of platinum or platinum-alloy catalysts, which are expensive and compete with fuel-cell electrodes for the limited supply of this precious metal, compared with the abundant holdings of gold in the world.[10]Recently, we reported the discovery of a room-temperature process for production of electrical energy using a fuel cell containing a carbon anode and operating with an aqueous solution of a polyoxometalate (POM) compound that had been reduced with pure CO over gold-nanotube catalysts. [11] This process for the oxidation and utilization of CO involves the reaction of CO and liquid water with a reducible POM, such as [H 3 PMo 12 O 40 ], which serves as an oxidizing agent for reaction of CO with liquid water and as an energy-storage agent for electrons; the process takes advantage of the high catalytic activities of gold for CO oxidation, [12][13][14] especially in the presence of liquid water, as shown experimentally using gold-nanotube membrane catalysts [15] and as predicted by density functional theory calculations. [16,17] A representative

No abstract