The quantum chemical basis of the Fischer-Tropsch reaction

Abstract

Fischer-Tropsch synthesis, transition metal surfaces, CO and CH~ reactivity, electronic features in F-T synthesis, bond strength, activation energy, chemisorption, CO dissociation, extended Hiickel method, ASED method, quantum chemistry.The electronic features determining the reactivity of CO and CH x on transition metal surfaces are reviewed. Focus is on the relevant features that control the Fischer-Tropsch synthesis. The CO dissociation reaction path is controlled by the interaction with the CO bond strength weakening 2~r* orbitals. CH 3 fragment adsorption is controlled by o type molecule fragment orbitals. This directs the CH 3 fragment to the atop adsorption site on those late transition metals that have strongly interacting d-valence electrons. Adsorbed C and O have a stronger bond strength than CH 3 because they have also unoccupied atomic p orbitals available to bonding. Because the bond strength of adsorbed C and O increases more rapidly with depletion of d-valence electron occupation than that of CO, the activation energy for CO dissociation decreases for the corresponding transition metals towards the left of the periodic system. The rate of methanation versus chain growth is controlled by the strength of the M-CH 3 bond versus the activation energy of carbon-carbon bond formation. The first appears to be more sensitive to variations in metal carbon bond strength than the latter.

Structure and reactions of carbon and hydrogen on Ru(0001): A scanning tunneling microscopy study

“…HREELS [11,12] The authors observed CH formation after exposing the surface to H and heating to 370 K. CH 2 , CH 3 or CH 4 were not observed under UHV conditions. Using theoretical calculations Ciobica et al [5,6] found that CH is the most stable species among all CH x (x=0-4).…”

“…It is believed that a particular form of adsorbed carbon is methanated through a series of hydrogenation reactions of intermediate CH x fragments [2][3][4]. These species adsorbed on various metal surfaces have been studied using, for example, high resolution electron energy loss spectroscopy (HREELS) [7][8][9][11][12][13][14][15], reflection adsorption infrared spectroscopy (RAIRS) [16] and temperature programmed desorption (TPD) [7][8][9]13].…”

“…From the slope of the plot, the rate was found to be proportional to I 2.2±0. 4 . This indicates that two electrons are necessary to excite two quanta of C-H stretch oscillation, which result in the rupture of the bond.…”

Aromatization of Methane over Supported and Unsupported Mo-Based Catalysts

“…In the case of supported Pt metals the dehydrogenation of methane occurs readily, yielding a small amount of ethane in addition to the main products, H 2 , and surface carbon (1)(2)(3)(4)(5)(6)(7)(8). The subsequent hydrogenation of the most reactive carbon gives several higher hydrocarbons and even benzene (1)(2)(3)(4)(5)(6)(7)(8)(9). The formation of the latter compounds has been enhanced by adding Cu metal to Rh/SiO 2 catalyst which was attributed to the migration of CH x species from the Rh onto the Cu, and to the ease of the coupling of CH x fragments on copper surface compared to Rh (9).…”

Termikus reakciók vizsgálata arany nanorészecskéket tartalmazó katalizátorokon

“…A hordozós Pt fémek hatékonyan katalizálják a metán bontását, de a szén lerakódás a katalizátorokon korai dezaktiválódáshoz vezet [39][40][41][42][43][44][45] …”