J. Phys. Chem. C 2010, 114, 21493–21503
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Density Functional Study of Ethanol Decomposition on Rh(111) Ming Li,† Wenyue Guo,*,† Ruibin Jiang,† Lianming Zhao,† Xiaoqing Lu,§ Houyu Zhu,† Dianling Fu,† and Honghong Shan*,‡ College of Physics Science and Technology, China UniVersity of Petroleum Dongying, Shandong 257061, People’s Republic of China, State Key Laboratory for HeaVy Oil Processing, China UniVersity of Petroleum Dongying, Shandong 257061, People’s Republic of China, and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, People’s Republic of China ReceiVed: July 22, 2010; ReVised Manuscript ReceiVed: October 3, 2010
Ethanol decomposition on Rh(111) is systematically investigated using periodic density functional theory (DFT) calculations. The various adsorption modes of the intermediates involved are located. It is determined that ethanol adsorbs weakly on the Rh(111) surface. CH3CH2O, CH, and H prefer 3-fold sites with adsorption energies of 49.9, 146.6, and 64.3 kcal/mol, respectively. CO binds stably at the top site with a binding energy of 42.5 kcal/mol. CH2CH2O (3-fold-η1(Cβ)-η1(O)) and CHCO (3-fold-η2(Cβ)-η1(CR)) are inclined to adsorb on the surface to make the C and O atoms saturated. For the other intermediates, adsorption configurations are bridge-η1(Cβ)-η1(O) for CH2CHO, 3-fold-η1(Cβ)-η1(CR)-η1(O) for CH2CO, and 3-fold-η2(Cβ)-η1(O) for CHCHO. For intermediates going along the decomposition pathway, energy barriers for the Cβ-H and C-C bond scission are gradually decreased; however, for the CR-H or C-O bond cleavage, the energy barrier decreases first and then rises, presenting a V-shaped curve. The most favorable decomposition route for ethanol on Rh(111) is CH3CH2OH f CH3CH2O f CH2CH2O f CH2CHO f CH2CO f CHCO f CH + CO f C + CO, in which the dehydrogenation of CH3CH2O is the rate-determining step. 1. Introduction The dwindling supplies of fossil fuels, along with environmental concerns, have stimulated intensive research and development in the area of hydrogen energy.1 Ethanol has become as an attractive liquid hydrogen source, because it is not only nontoxic and producible from renewable sources, but it also has storage and handling safety.2 Using ethanol for economical hydrogen production, however, requires the development of catalytic reformers with high stability, efficiency, and selectivity for ethanol steam reforming (ESR),3,4 which has been one of the major challenges in catalysis. Recently, Rh-based catalysts promoted by certain promoters and/or oxides have been found to efficiently dissociate the C-C bond of ethanol and display unique efficiency and selectivity in catalyzing ESR.3 In general, ESR involves ethanol decomposition.4 The study of ethanol decomposition can provide some insight into the reforming process5 and, therefore, has been extensively performed experimentally.6-9 On Rh(100), ethanol was found to adsorb molecularly at 130 K and the oxametallacycle (CH2CH2O) intermediate was determined to be formed via decomposition by high-resolution electron energy-loss spectroscopy (HREELS).6 On Rh(111), temperature-programmed desorption (TPD) and laser-induced thermal desorption (LITD) spectra showed that the desorption products are exclusively carbon monoxide and hydrogen;7,8 HREELS data explicitly excluded the formation of acetaldehyde, and the oxametallacycle * Authors to whom correspondence should be addressed. Tel.: 86-546839-6634. Fax: 86-546-839-7511. E-mail:
[email protected] (W.G.),
[email protected] (H.S.). † College of Physics Science and Technology, China University of Petroleum. § Department of Physics and Materials Science, City University of Hong Kong. ‡ State Key Laboratory for Heavy Oil Processing, China University of Petroleum.
intermediate was again proposed.7 High-resolution core level spectroscopy (HRCLS) study indicated the enhancement effect of the steps on ethanol dehydrogenation.9 Mavrikakis et al. reviewed the decomposition of ethanol on Group VIII and IB metals and proposed a possible decomposition pathway on Rh(111), i.e.,10
CH3CH2OH f CH3CH2O f CH2CH2O f CH2 + CO f CO + C Despite these experimental efforts, it remains puzzling why ethanol prefers to dehydrogenate to CH2CH2O instead of CH3CHO on Rh surfaces and how CH2CH2O further transforms to the final products. From the theoretical point of view, ethanol adsorption and decomposition on the Rh-based surfaces have also been studied.11-14 The adsorption and formation of hydrogen bond of ethanol on Rh(111) were studied by density functional theory (DFT).11 Using the UBI-QEP calculation, Vesselli et al. proposed that ethanol dissociates to formaldehyde via direct C-C bond cleavage on Rh(111).12 Wang et al. elucidated the ethanol decomposition on some selected metals by both the DFT calculations and electronic structure analyses.13 By combining Brønsted-Evans-Polyani (BEP) correlations with the scaling relations, Ferrin et al. generated a potential energy surface (PES) for the C-C and C-O bond-breaking in CHxCHyOHz (x ) 0-3, y ) 0-2, z ) 0-1) on several transition metals.14 Obviously, to date, there is no agreement regarding where and how ethanol dissociates to C1 species on Rh surfaces. Also, the generally accepted atomic-level descriptions of the decomposition process, as well as systematic characterization of the relevant intermediates, are still not achieved. To fully resolve the puzzles, a complete kinetic description of ethanol decomposition on Rh surfaces is desired. In this work,
10.1021/jp106856n 2010 American Chemical Society Published on Web 11/10/2010
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J. Phys. Chem. C, Vol. 114, No. 49, 2010
we study explicitly the most possible reaction pathways of ethanol decomposition on the Rh(111) surface within the selfconsistent DFT framework. Our main purpose is to map out the decomposition network including the structures and energetics of the relevant intermediates as well as the decomposition PES. This helps to shed light on the complicated nature of the surface-mediated ethanol decomposition, as well as provide important clues for gaining insight into the ESR process. 2. Computational Details Calculations were performed in the frame of DFT with program package DMol3 in Materials Studio of Accelrys, Inc.,15-17 using the exchange-correlation functional of the GGAPW91 approximation.18,19 To take the relativity effect into account, the density functional semicore pseudo-potential (DSPP)20 method was employed for the Rh atoms, whereas the C, H, and O atoms were treated with an all-electron basis set. The valence electron functions were expanded into a set of numerical atomic orbital by a double-numerical basis with polarization functions (DNP). A Fermi smearing of 0.005 Hartree and a real-space cutoff of 4.5 Å were used to improve the computational performance. Test calculations for the adsorption of ketenyl and carbon monoxide, which showed larger realspace cutoff (5.5 Å) and lower Fermi smearing (0.0005 Hartree), do not lead to remarkable energy changes (32.9 kcal/mol and
