Adsorption and Dissociation of CO on Body-Centered Cubic Transition Metals and Alloys: Effect of Coverage and Scaling Relations

Abstract

The adsorption and dissociation of CO have been calculated on the (100) surfaces of the body-centered cubic transition metals Fe, Mo, Cr, and W and the alloys Fe3Mo and Fe3Cr using density functional theory for two CO coverages, 0.25 and 0.5 ML. A complete analysis of the vibrational frequencies was performed to check whether the calculated structures are stable geometries or transition-state structures. For coverages up to 0.25 ML, carbon monoxide adsorbs molecularly onto all four metals at fourfold hollow sites with tilting angles with respect to the surface normal of 47°, 57°, 57°, and 58° and adsorption energies of −1.53, −2.64, −3.03, and −3.01 eV for Fe, Mo, Cr, and W, respectively. The calculated CO stretching frequencies at this coverage are 1211, 1062, 1037, and 926 cm−1. At higher coverages, CO adsorption does not exhibit significant changes in both adsorption energy and tilting angle on all four metals but leads to blue shifts of the CO frequency for Fe and Cr and red shifts for Mo and W. Furthermore, scaling relations apply to a bent CO species at a surface coverage of 0.25 ML of CO on all four transition metals as well as the metal alloys Fe3Mo and Fe3Cr, in the sense that the heat of adsorption of CO and the activation energy of CO dissociation scale linearly with the heat of adsorption of the carbon as well as both dissociation products.

X-ray emission spectroscopy and density functional study of CO/Fe(100)

“…11 Chemisorption and dissociation of CO on Fe(100) has been studied extensively using density functional theory (DFT) which has confirmed tilted CO at the four-fold hollow site as the most strongly bound and has provided reliable estimates of barriers to diffusion and dissociation. [13][14][15][16][17][18][19][20][21][22] In the present work we extend the analysis by combining experimental measurement of the atom-projected 2p density of states using x-ray emission spectroscopy (XES) with DFT calculations of spectra and bonding. XES is a corelevel spectroscopy measuring the photon emitted when a valence electron decays to fill a previously created core-hole in a non-Auger process.…”

Methane, formaldehyde and methanol formation pathways from carbon monoxide and hydrogen on the (0 0 1) surface of the iron carbide χ-Fe5C2

“…The latter converts the catalyst into iron carbides. Among the mixture of carbide and oxide phases [5][6][7][8] present under process conditions, the Hägg carbide (v-Fe 5 C 2 ) is generally regarded as the active phase for FTS [2,5,6,[9][10][11][12][13][14][15][16][17][18], although other phases such as e-Fe 2.2 C or h-Fe 3 C may exhibit activity as well [9]. Mechanistic studies on CO hydrogenation and FTS on metallic iron surfaces are available in the literature [4,14,[19][20][21][22][23][24][25][26][27][28][29], but studies on the catalytic properties of iron carbides in FTS are scarce, we refer to de Smit and Weckhuysen [13] for a recent review and to other literature [3,5,[8][9][10][11]13,15,24,[30][31][32][33][34][35]…”

Direct versus Hydrogen‐Assisted CO Dissociation on the Fe (100) Surface: a DFT Study

“…The overall energy balance is À0.93 eV in agreement with previous work. [7,9] The activation energy for the direct process is 1.09 eV, which is smaller than the CO dissociation barrier when no hydrogen is present (1.2 eV), showing that the H-atom affects the TS more than the IS. On the other hand, Scheijen et al [7] have found an increase of 0.05 eV in the barrier for CO dissociation when its coverage increased from 0.25 to 0.5 on Fe (100).…”

“…[1,2] Many recent computational studies have addressed the mechanism for this reaction. [3][4][5][6][7] Dissociation of the CO bond is a key step in the initiation of the F-T process. In principle it may occur through one of three pathways: [8] a) CO ads + * !C ads + O ads b) CO ads + H ads $HCO ads + * !HC ads + O ads c) CO ads + Hads$COH ads + * !C ads + OH ads where * stands for the active site.…”

“…In principle it may occur through one of three pathways: [8] a) CO ads + * !C ads + O ads b) CO ads + H ads $HCO ads + * !HC ads + O ads c) CO ads + Hads$COH ads + * !C ads + OH ads where * stands for the active site. The direct reaction (a) has been shown to be energetically feasible on the more reactive surfaces of iron [7,9,10] and on stepped surfaces of cobalt or ruthenium, [8,11] with activation energies in the range of 0.5-1.2 eV. However, on close-packed surfaces, such as Fe (110), Co (0001), and Ru (0001), the activation energies are substantially higher.…”