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Titolo:
First-principles analysis of the hydrogenation of carbon monoxide over palladium
Autore:
Neurock, M;
Indirizzi:
Univ Virginia, Dept Chem Engn, Charlottesville, VA 22903 USA Univ Virginia Charlottesville VA USA 22903 Charlottesville, VA 22903 USA
Titolo Testata:
TOPICS IN CATALYSIS
fascicolo: 3-4, volume: 9, anno: 1999,
pagine: 135 - 152
SICI:
1022-5528(1999)9:3-4<135:FAOTHO>2.0.ZU;2-M
Fonte:
ISI
Lingua:
ENG
Soggetto:
FISCHER-TROPSCH SYNTHESIS; DENSITY-FUNCTIONAL THEORY; SUPPORTED COBALT CATALYSTS; SINGLE-CRYSTAL SURFACES; CO HYDROGENATION; METHANOL SYNTHESIS; PD(111) SURFACE; CROTONALDEHYDE HYDROGENATION; RH(111)-(2X2)O SURFACES; SPECTROSCOPIC EVIDENCE;
Keywords:
density functional theory; quantum chemical calculations; hydrogenation; methanol synthesis; CO chemisorption; formaldehyde; methoxide; formyl; CO; Pd(111); reaction paths; activation barriers;
Tipo documento:
Review
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
101
Recensione:
Indirizzi per estratti:
Indirizzo: Neurock, M Univ Virginia, Dept Chem Engn, Charlottesville, VA 22903 USA Univ Virginia Charlottesville VA USA 22903 ville, VA 22903 USA
Citazione:
M. Neurock, "First-principles analysis of the hydrogenation of carbon monoxide over palladium", TOP CATAL, 9(3-4), 1999, pp. 135-152

Abstract

The reaction paths for the hydrogenation of CO to methanol over Pd-x (x = 1-4 and 19) cluster models were examined using first-principle density functional quantum chemical calculations. The predicted adsorption energies forthe most favorable binding modes for CO, H-2, HCO, H3CO, CH3OH, C, O and Hon a Pd-19 model Pd(111) clusters were -147, -62, -340, -51, -195, -33, -610, -349 and -251 kJ/mol, respectively. The most favorable modes for CO, CH3O, H, C and O on Pd(111) were all found to be the 3-fold fcc site. The most favorable modes for the formyl and formaldehyde surface intermediates at low coverage were the 3-fold (eta(2)mu(3)), and the di-sigma sites, respectively. At higher surface coverages, however, the atop eta(1) (C) and the pimodes for the formyl and formaldehyde intermediates were more likely. The computed adsorption energies were subsequently used to compute overall reaction energies for the hydrogenation of CO to methanol. The initial hydrogenation of CO to the eta(1) (C) HCO intermediate was found to be +52 kJ/mol endothermic and has been speculated as a possible rate-limiting step. The remaining surface hydrogenation steps become increasingly more exothermic as more hydrogen was added. The elementary steps of formyl to formaldehyde, formaldehyde to methoxide and methoxide to methanol were computed to be -9, -26 and -33 kJ/mol, respectively. The overall energy for CO dissociation wasfound to be highly unlikely at +260 kJ/mol and a clear indication that methanation and chain growth chemistry is not very likely over Pd. The most favorable reaction coordinate for the hydrogenation of CO to the eta(1) (C) formyl intermediate was that which proceeds over a single Pd site where there is a migratory insertion of the CO into a Pd-H bond. The barrier for thispath was computed to be +78 kJ/mol on the Pd-19 cluster. There was a very weak dependence on cluster size. This is a likely indication that this reaction is structure insensitive. A second path which involved the coupling ofH and CO over a bridge site was found to be +130 kJ/mol which is less likely, but may also occur under different conditions.

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Documento generato il 15/07/20 alle ore 08:46:26