J.W. Shabaker

Aqueous-phase reforming of methanol and ethylene glycol over alumina-supported platinum catalysts

The rates of aqueous-phase reforming of methanol and ethylene glycol to form H2 and CO2 were measured under kinetically controlled reaction conditions at temperatures of 483 and 498 K over alumina-supported platinum catalysts. Results show that the rates of formation of H2 from aqueous solutions of methanol (from 1 to 10 wt%) are similar to the rates of conversion of ethylene glycol, suggesting that CC bond cleavage is not rate limiting for ethylene glycol reforming. Aqueous-phase reforming of both oxygenated hydrocarbons over Pt/Al2O3 leads to nearly 100% selectivity for the formation of H2 (compared to the formation of alkanes), suggesting that methanation or Fischer–Tropsch reactions involving CO/CO2 and H2 do not appear to be important over platinum-based catalysts under the conditions of the present study. The rate of production of hydrogen is higher order in methanol (0.8) compared to ethylene glycol (0.3–0.5), and the reaction is weakly inhibited by hydrogen (−0.5 order) for both feedstocks. The rates of aqueous-phase reforming of methanol and ethylene glycol show apparent activation barriers of 140 and 100 kJ/mol, respectively, from 483 K and 22.4 bar total pressure to 498 K and 29.3 bar total pressure. Low levels of CO (<300 ppm) are detected in the gaseous effluents from aqueous-phase reforming of methanol and ethylene glycol over alumina-supported Pt catalysts, suggesting that water–gas shift processes are operative under the aqueous-phase reforming conditions of this study. The observed reaction kinetics for ethylene glycol of this study can be explained by a reaction scheme involving quasi-equilibrated adsorption of ethylene glycol, water, H2, and CO2, combined with irreversible steps involving dehydrogenation of adsorbed ethylene glycol to form adsorbed C2O2 species, cleavage of the CC bond to form adsorbed CO species, further dehydrogenation leading to adsorbed CO∗, and removal of adsorbed CO∗ by water-gas shift. Aqueous-phase reforming of methanol may take place by a similar reaction scheme, without the step involving cleavage of the CC bond. The nearly first-order reaction kinetics with respect to methanol can be explained by weaker adsorption of methanol compared to molecular adsorption of ethylene glycol.

Aqueous-Phase Reforming of Ethylene Glycol Over Supported Platinum Catalysts

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 TiO2, Al2O3, carbon, SiO2, SiO2-Al2O3, ZrO2, CeO2, and ZnO. High activity for the production of H2 by aqueous-phase reforming was observed over Pt-black and over Pt supported on TiO2, carbon, and Al2O3 (i.e., turnover frequencies near 8-15 min-1 at 498 K); moderate catalytic activity for the production of hydrogen is demonstrated by Pt supported on SiO2-Al2O3 and ZrO2 (turnover frequencies near 5 min-1); and lower catalytic activity is exhibited by Pt supported on CeO2, ZnO, and SiO2 (H2 turnover frequencies lower than about 2 min-1). Pt supported on Al2O3, and to a lesser extent ZrO2, exhibits high selectivity for production of H2 and CO2 from aqueous-phase reforming of ethylene glycol. In contrast, Pt supported on carbon, TiO2, SiO2-Al2O3 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-1 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.

11 DFT and experimental studies of C-C and C-O bond cleavage in ethanol and ethylene glycol on Pt catalysts

Abstract Density functional theory (DFT) studies of ethanol decomposition were conducted on Pt(111) slabs to investigate the structure and energetics of dehydrogenated ethanol species and transition states for cleavage of C-C and C-O bonds. The ketenyl (CHCO) species has the lowest energy transition state for C-C bond scission, and the energy required to form this species (plus adsorbed H-atoms) from gaseous ethanol is 4 kJ/mol. The lowest energy transition state for C-O bond scission is the 1-hydroxyethylidene (CH 3 COH) species, and the energy required to form this species from gaseous ethanol is 42 kJ/mol. In contrast, the lowest energy transition state for C-C bond serssion in ethane is the adsorbed ethylidene (CH 3 CH) species, and the energy required to form this species from gaseous ethane is 125 kJ/mol. Thus cleavage of the C-C bond in adsorbed species derived from ethanol on Pt(111) should be faster than cleavage of the C-O bond, and cleavage of the C-C bond in ethanol should also be faster than cleavage of the C-C bond in ethane over Pt(111). Reaction kinetics studies were conducted for aqueous-phase reforming of ethylene glycol over Pt/SiO 2 at temperatures of 483 and 498 K and at a total pressure of 22 bar. This catalyst exhibited high activity and selectivity for production of H 2 , with low rates of alkane production. It appears that catalysts based on Pt may be promising materials for the selective production of hydrogen by aqueous phase reforming of oxygenated hydrocarbons, such as ethylene glycol.