Steam Reforming of Ethylene Glycol over MgAl2O4 Supported Rh, Ni

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Steam Reforming of Ethylene Glycol over MgAl2O4 Supported Rh, Ni, and Co Catalysts Donghai Mei, Vanessa Lebarbier Dagle, Xing Rong, Dr. Karl Albrecht, and Robert Alexander Dagle ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01666 • Publication Date (Web): 03 Dec 2015 Downloaded from http://pubs.acs.org on December 11, 2015

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Steam Reforming of Ethylene Glycol over MgAl2O4 Supported Rh, Ni, and Co Catalysts Donghai Mei1,*, Vanessa Lebarbier Dagle2, Rong Xing2, Karl O. Albrecht2, and Robert A. Dagle2,* 1

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Fundamental and Computational Sciences Directorate, Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352, USA

Energy and Environmental Directorate, Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352, USA

Abstract Steam reforming of ethylene glycol (EG) over MgAl2O4 supported metal (15 wt.% Ni, 5 wt.% Rh, and 15 wt.% Co) catalysts were investigated using combined experimental and theoretical methods. Compared to highly active Rh and Ni catalysts with 100% conversion, the steam reforming activity of EG over the Co catalyst is comparatively lower with only 42% conversion under the same reaction conditions (500˚C, 1 atm, 119,000 h-1, S/C=3.3 mol). However, CH4 selectivity over the Co catalyst is remarkably lower. For example, by varying the gas hour space velocity (GHSV) such that complete conversion is achieved for all the catalysts, CH4 selectivity for the Co catalyst is only 8%, which is much lower than the equilibrium CH4 selectivity of ~ 24% obtained for both the Rh and Ni catalysts. Further studies show that varying H2O concentration over the Co catalyst has a negligible effect on activity, thus indicating zeroorder dependence on H2O. These experimental results suggest that the supported Co catalyst is a promising EG steam reforming catalyst for high hydrogen production. To gain mechanistic insight for rationalizing the lower CH4 selectivity observed for the Co catalyst, the initial decomposition reaction steps of ethylene glycol via C-O, O-H, C-H, and C-C bond scissions on the Rh(111), Ni(111) and Co(0001) surfaces were investigated using density functional theory (DFT) calculations. Despite the fact that the bond scission sequence in the EG decomposition on the three metal surfaces varies, which leads to different reaction intermediates, the lower CH4 selectivity over the Co catalyst, as compared to the Rh and Ni catalysts, is primarily due to the

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higher barrier for CH4 formation. The higher S/C ratio enhances the Co catalyst stability, which can be elucidated by the facile water dissociation and an alternative reaction path to remove the CH species as a coking precursor via the HCOH formation. Keywords: ethylene glycol; steam reforming; density functional theory; cobalt, rhodium, nickel *Corresponding authors: D. Mei, [email protected]; R. Dagle, [email protected]


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1. Introduction Hydrogen production via the catalytic steam reforming of biomass-derived oxygenates has been recognized as one economically feasible and environmentally benign way for the efficient utilization of renewable energy resources.1-3 The biomass-derived oxygenates obtained from fast pyrolysis or liquefaction is generally a mixture of oxygenated compounds including carboxylic acids, aldehydes, ketones, alcohols, and phenols.4 As the simplest polyol model compound, ethylene glycol (EG) can be produced by catalytic hydrogenation of cellulose or cellulosic oxygenates.5-7 Aqueous-phase and steam reforming of EG using supported metal catalysts have been extensively studied.5, 8-20 Davda et al. investigated the activity and selectivity of aqueous-phase reforming of EG over a series of silica supported transition metal catalysts.9 They found that the reforming activity decreases in the order of Pt~Ni > Ru > Rh~Pd > Ir. The H2 selectivity is low over the supported Rh, Ru and Ni catalysts at the expense of high alkane selectivity, while the H2 selectivity over Pt and Pd catalysts are relatively higher than the other four metal catalysts. Pt and Pd catalysts have been suggested as promising EG steam reforming catalysts for high H2 selectivity.9 Li et al. studied the effect of support on EG steam reforming activity over Ni-based catalysts.5 They found higher Ni dispersion enhances the activity by facilitating C-C bond breaking. Karim et al. found that the higher Co0 surface fraction is responsible for the low CH4 selectivity in ethanol steam reforming using MgO supported Co catalysts.21 The steam-to-carbon ratio (S/C) also plays an important role in the EG steam reforming reaction. It has been found that conversion increased from 46.3% to 96.7% with increasing stoichiometric S/C ratio from 1 to 9 at 400ºC over Ni/Al2O3 catalyst.20 H2 selectivity increased from 55.6% to 89.6% at the expense of CH4 formation.


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It has been generally postulated that the steam reforming of EG proceeds with EG complete decomposition into CO and H2 via CHx (x=1-3) fragments (Eq. 1), followed by watergas shift (WGS) reaction leading to the formation of CO2 and H2 (Eq. 2). Accompanying with this major reaction route, several side reactions such as methanation (Eq. 3), hydrogenolysis (Eq. 4), as well as dehydrogenation could also occur, leading to the formation of alkanes and oxygenated hydrocarbons (Eq. 5).6 For the purpose of hydrogen production, the desired catalysts are those that could efficiently cleave the C-C, C-H and O-H bonds of EG leading to the formation of more atomic H and CO or CO2 while keep the C-O and C-C bonds of EG intact. In order to enhance H2 selectivity, the hydrogenation of CHx to CH4 and C2H6-xO2-y oxygenated hydrocarbons (x=1-5; y=0-1) should be limited. HOCH 2CH 2OH ↔ 2CO + 3H 2

(1)

H 2O + CO ↔ CO2 + H 2

(2)

CO + 3H 2 ↔ CH 4 + H 2O

(3)

HOCH 2CH 2OH + H 2 ↔ C2 H 5OH + H 2O

(4)

HOCH 2CH 2 OH ↔ C 2 H 6 − x O 2 − y + x / 2 ⋅ H 2

(5)

Density functional theory (DFT) calculations and microkinetic modeling have been carried out to gain fundamental insights into the catalytic reactivity on the understanding of plausible reaction mechanisms for EG steam reforming on well-defined model metal catalysts.1112, 22-23

Due to the complexity of EG decomposition reaction network and a few hundred of

possible reaction intermediates, the detailed analysis of all elementary steps in the decomposition


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reaction network of EG using DFT calculations is impractical. Instead, a simplified kinetic model with limited reaction intermediates involved in the EG decomposition was proposed.11, 22 DFT results combined with semi-empirical Brønsted-Evans-Polanyi relationships and linear scaling relationships, Salciccioli et al. found that the initial decomposition of EG proceeds first via O-H bond cleavage, followed by C-H bond and the second O-H bond scission on Pt while both O-H bonds will initially be broken on PtNi surfaces.12 Very recently, Christiansen and Vlachos found that EG steam reforming kinetics on Pt is nearly the same as EG decomposition kinetics. The added steam (water) only contributed in the following WGS reaction.8 In the present work, EG steam reforming was investigated over MgAl2O4 supported Rh, Ni and Co catalysts at 500ºC. MgAl2O4 was chosen as the support for its better resistance to carbon formation in steam reforming reactions as compared to Al2O3.24-26 We found the CH4 selectivity on the Co catalyst is much lower than its equilibrium value obtained on the Rh and Ni catalysts. Hence, the MgAl2O4-supported Co catalyst had a much higher H2 yield, making it a promising catalyst for EG steam reforming for hydrogen production. To understand the origin of the low CH4 formation on the Co catalyst, the EG decomposition, methanation, and WGS reaction over Rh(111), Ni(111) and Co(0001) surfaces were calculated using DFT calculations. Since previous theoretical and experimental studies suggested that dihydroxyethylene (HOCHCHOH) is the most likely reaction intermediate in the initial decomposition of EG on metal catalyst surfaces,5, 8-9, 11-12, 22 EG decomposition pathways were explored by calculating the initial decomposition of HOCHCHOH via all possible C-C, C-O, C-H, O-H bond scissions on the Rh(111), Ni(111) and Co(0001) surfaces.


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2. Experimental Procedures and Computational Details 2.1 Catalyst preparation Spinel-supported Rh, Ni and Co catalysts were prepared by incipient wetness impregnation of MgAl2O4 (Sasol Puralox 30/140) with solution of Rh nitrate (10 wt% Rh in nitric acid), Ni nitrate hexahydrate, Co nitrate hexahydrate dissolved in deionized water. The target metal loading was 5 wt% for Rh and 15 wt% for Ni and Co catalysts. After impregnation, the catalysts were dried at 120ºC for 8 hours and calcined under static air at 500ºC for 4 hours. 2.2 Catalyst characterizations Nitrogen adsorption was measured at 77 K with an automatic adsorptiometer (Micromeritics ASAP 2000). The samples were pretreated at 110°C for 12 hour under vacuum. The surface areas were determined from adsorption values for five relative pressures (P/P0) ranging from 0.05 to 0.2 using the BET method. Scanning transmission electron microscopy (STEM) measurements were conducted with a FEI Titan 80-300 operated at 300 kV. All images were digitally recorded using a charge-coupled device (CCD) camera and were analyzed using Gatan Digital Micrograph. In general, the STEM sample preparation involved mounting of powder samples on copper grids covered with lacey carbon support films and loaded them into the STEM chamber. STEM images were collected from at least 5 different locations on the grid. The amount of solid carbon deposited on the spent catalysts was measured by a Shimadzu Total Carbon Analyzer (TOC-5000A with a SSM-5000A Solid Sample Module). X-ray powder diffraction spectra were recorded using a Philips X’pert MPD (Model PW3040/00) diffractometer with copper anode (Kα1 = 0.15405 nm) and a scanning rate of 0.002° per second between 2θ = 10°- 70°. The diffraction patterns were analyzed using Jade 5 (Materials Data Inc.,


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Livermore, CA) and the Powder Diffraction File database (International Center for Diffraction Data, Newtown Square, PA). TPR experiments were performed on an automated catalyst characterization unit (Micromeritics Autochem 2910) equipped with a TCD detector. The catalyst (0.1 g) was loaded in a U-type quartz tube. Then, a 5% H2/Ar mixture was flown through the sample starting from 20 °C and heating up to 800 °C with a ramp of 10 °C/ min. 2.3 Activity measurements Catalytic activity tests were conducted in an 8 mm inner diameter Inconel fixed-bed reactor. For each test 0.01-0.10 g of catalyst (60-100 mesh) was diluted with α-Al2O3 (60-100 mesh) for a 10:1 dilution and loaded between two layers of quartz wool inside the reactor. A Ktype thermocouple was placed in the middle of the reactor bed for measurement of the catalyst bed temperature. The catalyst was reduced at 850°C for 8 hours using a 10% H2/N2 gas mixture prior to the test. To compare the activity of the different catalysts under EG steam reforming condition a pre-mixture of EG and H2O was introduced into the system using a HCLP pump and microchannel vaporizer. N2 carrier gas was fed into the system using a Brooks mass flow controller. The gas products were analyzed on-line by means of an Agilent Micro GC which separated the gases using MS-5A and PPU columns and quantified the gases with a TCD detector. EG conversion is defined by moles of converted EG moles/EG moles of fed. The C1 product (CO, CO2, CH4) selectivity is defined on the carbon basis as follows: (C moles of product)i/(moles of EG fed×2). The carbon balance, calculated as carbon in products/carbon in fed in the reactor, was at least 90% for all experiments.


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2.4 Computational details All calculations were performed using spin-polarized DFT within the generalized gradient approximation (GGA) as implemented in the Vienna ab initio simulation package (VASP).27-29 The core and valence electrons were represented by the projector augmented wave (PAW) method30-31 with a kinetic cutoff energy of 400 eV. The exchange correlation functional was described by the Perdew−Burke−Ernzerhof (PBE) functional.32 Our XRD patterns show that (111) plane is one of the major planes on the Ni and Co catalysts. For the supported Rh catalyst, the metal particle size is too small to detect any plane information. Actually, due to the smallest surface energies, the (111) plane (facet) is generally the most exposed and stable surface plane on the transition metals such as Ni, Co and Rh, which are identified by both experimentally and theoretically.33-35 As such, we chose Rh(111), Ni(111) and Co(0001) planes as the catalyst surfaces for ethylene glycol steam reforming in this work. The ground-state atomic geometries of clean and the adsorbed M(111) (M= Rh, Ni, Co(0001)) systems were obtained by minimizing the forces on each atom to below 0.03 eV/Å. A periodic (2×2) supercell M(111) surface slab with four atomic layers was used in this work. During the geometric optimizations and the transition state searching processes, the adsorbate(s) and the metal atoms in the top two surface layers were allowed to relax while the metal atoms in the bottom two surface layers of the surface slab were fixed. A 15 Å vacuum layer was inserted between the M(111) surface slab in the z direction to avoid interaction between images. To ensure the accuracy of calculations, the effects of slab thickness (up to six atomic layers) and different Monkhorst-Pack (MP) mesh sampling were tested. A (3×3×1) MP sampling schedule was found to be accurate to reach total energy convergence of C-C. The CH bond cleavage of HOCHCHOH leading to the HOCCHOH is most likely to be broken at first with the lowest barriers of 36 and 41 kJ/mol on Co(0001) and Ni(111). The C-O and O-H bond scissions are competitive while the C-C bond of HOCHCHOH intermediate is the most difficult to break on Co(0001) and Ni(111) surfaces with the highest barriers of 104 and 103 kJ/mol. On


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Rh(111), the bond-breaking sequence follows a different order. The C-C bond is most likely the easiest bond to be cleaved, followed by the O-H and C-H bonds. The C-O of HOCHCHOH, however, becomes the last bond to be broken. Therefore, the formation and decomposition of the CHyO (y=1,2) species will be the dominant route in the EG decomposition on the Rh catalyst. While on the Ni and Co catalysts, more complex reaction paths associated with the decomposition of C2HyOz (y=1,3; z=1,2) into HCOH, HCO, CHx, H, OH and CO species are expected. This is consistent with our experimental observation that a small amount of C2H4 was identified on the Co catalyst. 3.3 Comparison of methanation on supported Rh, Ni and Co catalysts Experimental results indicate that the major products from the EG steam reforming over MgAl2O4 supported Rh, Ni and Co catalysts are H2, CH4, CO, and CO2 (Figure 4). Hence, although the bond scission sequences of EG may be different on the three metal surfaces as suggested by DFT, the majority of the adsorbed EG does undergo eventual complete decomposition To understand the lower CH4 selectivity on the Co catalysts as compared to the Rh or Ni catalysts, the dissociation reaction intermediates of HCOH, HCO and COH on Rh(111), Ni(111), and Co(0001) surfaces were studied using DFT calculations. The calculated activation barriers and reaction energies is summarized in Table 3. Rh, Ni and Co all exhibit similar energy barriers for the possible dissociation steps of HCOH via C-H, O-H and C-O bond scissions, leading to the formation of COH, HCO and CH+OH. The calculated activation barriers for the C-H bond scission of HCOH are much lower than the barriers for the other two bond-breaking possibilities, indicating the COH formation is the dominant from HCOH dissociation on three metal surfaces. The formed COH species then


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further dissociates into CO+H via the O-H bond-breaking. The C-O bond-breaking of COH leading to C+OH is unlikely due to the extremely high barriers of 232, 207, and 196 kJ/mol on Rh(111), Ni(111), and Co(0001) surfaces. For HCO species, our DFT results listed in Table 3 show that HCO will dissociate into CO+H via the C-H bond-breaking. The calculated activation barriers for HCO→CO+H are 17, 30 and 66 kJ/mol on Rh(111), Ni(111) and Co(0001), respectively. The C-O bond-breaking of HCO leading to CH+O is inhibited due to the high activation barriers (119 kJ/mol on Rh(111), 121 kJ/mol on Ni(111) and 90 kJ/mol on Co(0001)). Therefore, we conclude that HCOH, HCO and COH species derived from the EG decomposition will produce CO and hydrogen on all three metal catalysts in the dominant way. On the other hand, HCOH could dissociate into CH+OH. As shown in Table 3, the activation barriers for this step are 64 and 73 kJ/mol on Rh(111) and Co(0001), respectively, which is lower than the barrier of 92 kJ/mol on Ni(111). This suggests that the CH formation from HCOH dissociation is slightly easier on Rh(111) and Co(0001) than Ni(111). However, our calculations show that HCOH dissociation into CH+OH is slightly endothermic (+5 kJ/mol) on Co(0001) while it is exothermic (−26 kJ/mol) on the Rh(111) and Ni(111) surfaces. Therefore, the formation of CH species is reversible on Co(0001). In contrast to the Rh and Ni catalysts, this suggests less CH formation on the Co catalyst. Once the CH is formed on three metal surfaces, consecutive hydrogenation steps of CH leading to CH4 formation prevail while the CH dissociates into C and H is difficult. The calculated methanation energy profiles on three metal surfaces are shown in Figure 13. Clearly, the CH species is the lowest energy position in the methanation energy landscapes for all three metal catalysts, suggesting the CH species is the most abundant species on the catalyst surfaces involving in the methanation reaction.37-39


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Our experimental results show the CH4 selectivity on the supported Co catalyst is much lower than the Rh and Co catalysts. In principle, whatever the hydrogenation and dehydrogenation rates of CHx in the methanation on three metal surfaces are, the CH4 selectivity in the EG steam reforming process is determined by the CH3 hydrogenation, which is the final step leading to the CH4 formation.40-42 Table 4 lists our calculation results of methanation on three metal surfaces. The activation barrier of the CH3 hydrogenation on Rh(111) is 65 kJ/mol, which is lower than the barriers of 86 and 105 kJ/mol on Ni(111) and Co(0001) surfaces. This indicates the methanation reactivity order is Rh(111) > Ni(111) > Co(0001). Hence, the lowest methane selectivity would be expected for the Co catalyst based on the DFT calculated barrier of the last CH3 + H barrier, which is consistent with experimental results. The low CH4 selectivity over supported Co catalysts has been also found in the steam reforming of other oxygenates such as ethanol21 and acetone.43 3.4 Water activation and its role in catalyst stability The high yield of hydrogen observed with the Co catalyst is at the expense of the CH4 selectivity. On one hand, the dehydrogenation of CHx (x=2,3) instead of hydrogenation toward CH4 formation is desired (low CH4 selectivity); on the other hand, the dehydrogenation of CHx to CH or C is a possible mechanism for coking of the catalyst surface. Hence, the rapid removal of CH and C species by reacting with hydroxyl or atomic oxygen from H2O dissociation is very important for the stability of steam reforming catalysts. This, in turn, requires steam reforming catalysts with facile water dissociation. Catalyst deactivation due to the sintering and coking is one of most common issues for steam reforming reactions of hydrocarbons and oxygenates using supported metal catalysts.44-46


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Our experimental results indicated that increasing the S/C ratio enhanced the catalyst stability by decreasing the rate of deactivation for the MgAl2O4-supported Co catalyst in the EG steam reforming. This suggests that the higher water concentration is beneficial due to the decreased rate of coke deposits. To understand the steam (water) effect on the observed Co catalyst stability, water adsorption and dissociation on the Rh(111), Ni(111) and Co(0001) surfaces were calculated using DFT. Consistent with previous DFT calculations,47-50 water adsorption strength on the three metal surfaces is in the range of −30~−40 kJ/mol. The calculated activation barrier and reaction energy for water dissociation on Co(0001) is 54 and −36 kJ/mol. Compared to the water dissociation on Rh(111) and Ni(111) surfaces shown in Table 5, water dissociation into hydroxyl and hydrogen atom on the Co(0001) surface is both thermodynamically and kinetically more feasible on the Co catalyst. The hydroxyl produced by water dissociation could react with the most abundant CH species forming HCOH. Interestingly, we also note that the HCOH formation via OH+CH on the Co(0001) surface (Table 3) is slightly exothermic (−5 kJ/mol) while this step is endothermic (+26 kJ/mol) on both Rh(111) and Ni(111) surfaces. The calculated activation barrier of 68 kJ/mol for the HCOH formation on Co(0001) is much lower than the corresponding barriers of 118 kJ/mol on Ni(111) and 90 kJ/mol on Rh(111). Compared with the CH hydrogenation barrier (CH+H→CH2) of 64 kJ/mol, it is clear that the CH hydrogenation and the HCOH formation are competitive on the Co(0001) surface. On the other hand, our calculation results suggest that CO hydrogenation is kinetically more feasible than the HCOH formation on the Rh(111) and Ni(111) surface. As a result, unlike the Rh and Ni catalysts, the facile HCOH formation provides a new channel to consume the most abundant surface CH species.51 The formed HCOH then dissociates into COH with a very low barrier (10 kJ/mol). As shown in Table 3, the calculated activation barrier for COH dissociation into CO+H


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via the O-H bond scission is 92 kJ/mol, which is much lower than the barrier of 196 kJ/mol for COH dissociation into C+OH via the C-O bond scission. In this way, the CH species is converted into CO and H via the HCOH instead of being hydrogenated into methane or dehydrogenated to the C deposit. This indicates that the hydroxyl and atomic oxygen from facile water activation on the Co catalyst surface enhances the catalyst stability by removing the possible coking precursor of CH species, which is consistent with our experiments that the catalyst deactivation was not observed for the supported Co catalyst at higher S/C ratio condition. Finally, the relatively facile water activation on the Co catalyst may also slightly contribute to the low CH4 selectivity. Contrasting to the Rh and Ni catalysts, the abundant CH species on the Co catalyst can be converted into CO via hydroxylation channel (HCOH) in a competitive way. As the result, The CH species coverage will be lower on the Co catalyst than the Rh and Ni catalysts. This indirectly decreases the hydrogenation reaction rates leading to the CH4 formation. Since the activation barriers for both CH and CH2 hydrogenation steps are low (64 and 61 kJ/mol), it is assumed that this competitive hydroxylation path plays a minor role in the low CH4 selectivity. This is confirmed by our experimental observation that the effect of the water concentration on the CH4 selectivity is barely noticeable although it was found that the CH4 selectivity is only 3.5% at the S/C ratio of 6 while the CH4 selectivity is about 8% at the S/C ratio of 3.3 with different EG conversions. 4. Conclusions The steam reforming of ethylene glycol over MgAl2O4 supported Rh, Ni and Co catalysts were studied using a combined experimental and theoretical method. Compared to the Rh and Ni


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catalysts, CH4 selectivity using the Co catalyst is remarkably lower although the steam reforming activity is also lower. At complete conversion, the CH4 selectivity of EG steam reforming on the Co catalyst is 8%, which is much lower than the equilibrium CH4 selectivity of 23.8% obtained on the Rh and Ni catalysts. DFT calculations were carried out to study the initial decomposition reaction steps of HOCHCHOH via C-O, O-H, C-H, and C-C bond scissions on the Rh(111), Ni(111) and Co(0001) surfaces. Unlike the Rh catalyst where the C-C bond-breaking is the first step of HOCHCHOH, the bond scission sequence for HOCHCHOH on both the Ni and Co catalysts are in the same order: C-H > C-O ≈ O-H > C-C. The bond scission sequence variations may lead to various dissociated oxygenated fragments, such as HCOH, COH and HCO. The further dissociation of these three oxygen containing C1 fragments along with the methanation and water activation steps was also explored. These DFT results suggest that the lower CH4 selectivity over the Co catalyst versus the Rh and Ni catalysts is primarily due to the higher barrier for CH3 hydrogenation step to form methane. The relatively facile water activation on the Co catalyst enhances the catalyst stability at higher S/C ratio condition by provide an alternative reaction path for the most abundant CH species via the CH hydroxylation forming HCOH intermediate instead of CH hydrogenation. Supporting Information The TEM images (Figure S1) and particle size distributions (Figure S2) for MgAl2O4 spinel supported Ni, Rh, and Co catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.


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Acknowledgements This work was financially supported by the United States Department of Energy (DOE)’s Bioenergy Technologies Office (BETO) and performed at the Pacific Northwest National Laboratory (PNNL). PNNL is a multi-program national laboratory operated for DOE by Battelle Memorial Institute. Computing time and advanced catalyst characterization use was granted by a user proposal at the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL). EMSL is a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at PNNL. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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(13) Shabaker, J. W.; Davda, R. R.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. J. Catal. 2003, 215, 344-352. (14) Shabaker, J. W.; Huber, G. W.; Davda, R. R.; Cortright, R. D.; Dumesic, J. A. Catal. Lett. 2003, 88, 1-8. (15) Skoplyak, O.; Barteau, M. A.; Chen, J. G. Surf. Sci. 2008, 602, 3578-3587. (16) Skoplyak, O.; Barteau, M. A.; Chen, J. G. G. J. Phys. Chem. B 2006, 110, 1686-1694. (17) Tupy, S. A.; Chen, J. G. G.; Vlachos, D. G. Top. Catal. 2013, 56, 1644-1650. (18) Tupy, S. A.; Karim, A. M.; Bagia, C.; Deng, W. H.; Huang, Y. L.; Vlachos, D. G.; Chen, J. G. G. ACS Catal. 2012, 2, 2290-2296. (19) Zhang, C.; Zhang, P.; Li, S.; Wu, G.; Ma, X.; Gong, J. Phys. Chem. Chem. Phys. 2012, 14, 3295-3298. (20) Hu, X.; Lu, G. Appl. Catal. B Environ. 2009, 88, 376-385. (21) Karim, A. M.; Su, Y.; Engelhard, M. H.; King, D. L.; Wang, Y. ACS Catal. 2011, 1, 279-286. (22) Kandoi, S.; Greeley, J.; Simonetti, D.; Shabaker, J.; Dumesic, J. A.; Mavrikakis, M. J. Phys. Chem. C 2011, 115, 961-971. (23) Yu, W.; Porosoff, M. D.; Chen, J. G. Chem. Rev. 2012, 112, 5780-5817. (24) Basagiannis, A. C.; Verykios, X. E. Appl. Catal. B Environ. 2008, 82, 77-88. (25) Mei, D.; Glezakou, V.-A.; Lebarbier, V.; Kovarik, L.; Wan, H.; Albrecht, K. O.; Gerber, M.; Rousseau, R.; Dagle, R. A. J. Catal. 2014, 316, 11-23. (26) Mei, D. H.; Lebarbier, V. M.; Rousseau, R.; Glezakou, V. A.; Albrecht, K. O.; Kovarik, L.; Flake, M.; Dagle, R. A. ACS Catal. 2013, 3, 1133-1143. (27) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15-50. (28) Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54, 11169-11186. (29) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 14251-14269. (30) Blochl, P. E. Phys. Rev. B 1994, 50, 17953-17979. (31) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758-1775. (32) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671-6687. (33) Hammer, B.; Norskov, J. K., Theoretical surface science and catalysis - Calculations and concepts. In Adv. Catal., Gates, B. C.; Knozinger, H., Eds. 2000; Vol. 45, pp 71-129. (34) Norskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Nature Chem 2009, 1, 37-46. (35) Besenbacher, F.; Chorkendorff, I.; Clausen, B. S.; Hammer, B.; Molenbroek, A. M.; Nørskov, J. K.; Stensgaard, I. Science 1998, 279, 1913-1915. (36) Rong, X.; Dagle, V. L.; Flake, M.; Kovarik, L.; Albrecht, K. O.; Deshmane, C.; Dagle, R. A. Catal. Today 2015, submitted. (37) van Santen, R. A.; Ghouri, M. M.; Shetty, S.; Hensen, E. M. H. Catal. Sci. Technol. 2011, 1, 891-911. (38) van Santen, R. A.; Markvoort, A. J.; Filot, I. A. W.; Ghouri, M. M.; Hensen, E. J. M. Phys. Chem. Chem. Phys. 2013, 15, 17038-17063. (39) van Santen, R. A.; Neurock, M.; Shetty, S. G. Chem. Rev. 2010, 110, 2005-2048. (40) Cheng, J.; Gong, X.-Q.; Hu, P.; Lok, C. M.; Ellis, P.; French, S. J. Catal. 2008, 254, 285-295. (41) Gong, X. Q.; Raval, R.; Hu, P. Surf. Sci. 2004, 562, 247-256.


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(42) Gong, X. Q.; Raval, R.; Hu, P. J. Chem. Phys. 2005, 122. (43) Sun, J. M.; Mei, D. H.; Karim, A. M.; Datye, A. K.; Wang, Y. ChemCatChem 2013, 5, 1299-1303. (44) Sehested, J. Catal. Today 2006, 111, 103-110. (45) Trimm, D. L. Catal. Today 1997, 37, 233-238. (46) Trimm, D. L. Catal. Today 1999, 49, 3-10. (47) Pozzo, M.; Carlini, G.; Rosei, R.; Alfè, D. J. Chem. Phys. 2007, 126, 164706. (48) Phatak, A. A.; Delgass, W. N.; Ribeiro, F. H.; Schneider, W. F. J. Phys. Chem. C 2009, 113, 7269-7276. (49) Fajín, J. L. C.; D. S. Cordeiro, M. N.; Gomes, J. R. B. J. Phys. Chem. A 2014, 118, 5832-5840. (50) Luo, W.; Asthagiri, A. J. Phys. Chem. C 2014, 118, 15274-15285. (51) Qi, Y.; Yang, J.; Duan, X.; Zhu, Y.-A.; Chen, D.; Holmen, A. Catal. Sci. Technol. 2014, 4, 3534-3543.


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ACS Catalysis

Table Captions Table 1 Carbon deposition and the particles sizes for the 5%Rh/MgAl2O4, 15%Ni/MgAl2O4 and 15%Co/MgAl2O4 after the stability measurement. Table 2 Initial decomposition steps of HOCHCHOH intermediate on Rh(111), Ni(111) and Co(0001) surfaces. The calculated activation barriers (Ea) and reaction energies (∆H) are in kJ/mol. Table 3 Dissociation of HCOH, HCO, and COH on Rh(111), Ni(111) and Co(0001) surfaces. The calculated activation barriers (Ea) and reaction energies (∆H) are in kJ/mol. Table 4 Methanation on Rh(111), Ni(111) and Co(0001) surfaces. The calculated activation barriers (Ea) and reaction energies (∆H) are in kJ/mol. Table 5 Water dissociation on Rh(111), Ni(111) and Co(0001) surfaces. The calculated activation barriers (Ea) and reaction energies (∆H) are in kJ/mol.

Figure Captions Figure 1 Temperature programmed Reduction (TPR) profiles for 15% Co/MgAl2O4, 15% and 5% Rh/MgAl2O4.

Ni/MgAl2O4

Figure 2 XRD patterns for the for15% Co/MgAl2O4 and 15% Ni/MgAl2O4 after reduction treatment Figure 3 Evolution of the ethylene glycol conversion as a function of the amount of carbon fed (moles of C in ethylene glycol) per gram of catalyst for 5%Rh/MgAl2O4, 15%Ni/MgAl2O4 and 15%Co/MgAl2O4. T= 500ºC, S/C=3.3, P= 1 atmosphere, Ethylene glycol/H2O/N2: 10/67/23 (molar). GHSV varied between 119,000 and 952,000 h-1 to compare catalyst stability at similar starting conversions. Figure 4 Comparison of the catalytic performance of 5%Rh/MgAl2O4, 15%Ni/MgAl2O4 and 15%Co/MgAl2O4 for ethylene glycol steam reforming at 500ºC, S/C=3.3, P= 1 atmosphere, GHSV = 119,000h-1, Ethylene glycol/H2O/N2 =10/67/23 (molar). Figure 5 Comparison of the CH4 selectivity (red) at 100% conversion (blue) for 5%Rh/MgAl2O4, 15%Ni/MgAl2O4 and 15%Co/MgAl2O4 for ethylene glycol steam reforming at 500ºC, S/C=3.3, P= 1 atmosphere, Ethylene glycol/H2O/N2=10/67/23 (molar). The dotted line corresponds to the CH4 selectivity at equilibrium. GHSV was varied to compare the catalysts at the same conversion (i.e.100% conversion). Figure 6 Effect of the GHSV on the EG conversion and CH4 selectivity for 15%Co/MgAl2O4 at 500ºC, S/C=3.3 P= 1 atmosphere, Ethylene glycol/H2O/N2 =10/67/23 (molar). Figure 7 Effect of the H2O concentration on the EG conversion and CH4 selectivity for 15%Co/MgAl2O4 at 500ºC, P= 1 atmosphere, GHSV = 119,000h-1, Ethylene glycol/H2O/N2 : 10/67/23 (molar). Figure 8 Evolution of the ethylene glycol conversion as a function of the amount of carbon fed (moles of C in ethylene glycol) per gram of catalyst for 15%Co/MgAl2O4. S/C =6, T= 500ºC, P= 1 atmosphere, Ethylene glycol/H2O/N2: 10/67/23 (molar). GHSV = 152,000 h-1. Figure 9 Schematic representations of HOCHCHOH intermediate on Rh(111), Ni(111) and Co(0001) surfaces. The top and side views are shown for each structure.


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Page 24 of 36

Figure 10 Initial decomposition of HOCHCHOH intermediate on the Rh(111) surface. IS, TS and FS are initial state, transition state, and final state, respectively. Figure 11 Initial decomposition of HOCHCHOH intermediate on the Ni(111) surface. Figure 12 Initial decomposition of HOCHCHOH intermediate on the Co(0001) surface. Figure 13 Potential energy surface diagram of methanation on Rh(111), Ni(111) and Co(0001) surfaces. The listed numbers (kJ/mol) are the forward/reverse activation barriers for each elementary step.


24

Page 25 of 36

Intensity (a.u.)

15%Co/MgAl2O4

5%Rh/MgAl2O4

15%Ni/MgAl2O4

0

100

200

300

400

500

600

Temperature ( C)

Figure 1 Temperature programmed Reduction (TPR) profiles for 15% Co/MgAl2O4, 15% and 5% Rh/MgAl2O4.

Ni/MgAl2O4

MgAl2O4

Coº Coº

(Intensity a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

reduced 15% Co/MgAl2O4

Niº Niº

reduced 15%Ni/MgAl2O4

10

20

30

40

50

60

70

80

2 theta ( Degrees)

Figure 2 XRD patterns for the for15% Co/MgAl2O4 and 15% Ni/MgAl2O4 after reduction treatment.


25

ACS Catalysis

EG conversion (%)

100

80 15% Ni/MgAl2O4 ( ) 60 15%Co/MgAl2O4 ( ) 40

20 5%Rh/MgAl2O4 ( ) 0 0

10

20

30

40

50

60

Mol C fed/ gram catalyst Figure 3 Evolution of the ethylene glycol conversion as a function of the amount of carbon fed (moles of C in ethylene glycol) per gram of catalyst for 5%Rh/MgAl2O4, 15%Ni/MgAl2O4 and 15%Co/MgAl2O4. T= 500ºC, S/C=3.3, P= 1 atmosphere, Ethylene glycol/H2O/N2: 10/67/23 (molar). GHSV varied between 119,000 and 952,000 h-1 to compare catalyst stability at similar starting conversions.

100

EG Conversion/ Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 36

conversion CO CO2 CH4 C2H4

80

60

40

20

0 5%Rh/MgAl2O4

15%Ni/MgAl2O4

15%Co/MgAl2O4

Figure 4 Comparison of the catalytic performance of 5%Rh/MgAl2O4, 15%Ni/MgAl2O4 and 15%Co/MgAl2O4 for ethylene glycol steam reforming at 500ºC, S/C=3.3, P= 1 atmosphere, GHSV = 119,000h-1, Ethylene glycol/H2O/N2=10/67/23 (molar).


26

Page 27 of 36

EG conversion/ CH4 Selectivity (%)

100

80

60

40

20

0 5%Rh/MgAl2O4

15%Ni/MgAl2O4

15%Co/MgAl2O4

Figure 5 Comparison of the CH4 selectivity (red) at 100% conversion (blue) for 5%Rh/MgAl2O4, 15%Ni/MgAl2O4 and 15%Co/MgAl2O4 for ethylene glycol steam reforming at 500ºC, S/C=3.3, P= 1 atmosphere, Ethylene glycol/H2O/N2=10/67/23 (molar). The dotted line corresponds to the CH4 selectivity at equilibrium. GHSV was varied to compare the catalysts at the same conversion (i.e.100% conversion).

100

EG conversion / CH4 selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

80

60

Conversion

40

CH4 equilibrium at complete conversion

20 CH4 0 0

20,000

60,000

100,000

140,000

GHSV (h-1) Figure 6 Effect of the GHSV on the EG conversion and CH4 selectivity for 15%Co/MgAl2O4 at 500ºC, S/C=3.3 P= 1 atmosphere, Ethylene glycol/H2O/N2=10/67/23 (molar).


27

ACS Catalysis

EG Conversion/ CH4 selectivity (%)

100

80

60 conversion 40

20 CH4 0 60

65

70

75

80

85

90

H2O concentration (molar %) Figure 7 Effect of the H2O concentration on the EG conversion and CH4 selectivity for 15%Co/MgAl2O4 at 500ºC, P= 1 atmosphere, GHSV = 119,000h-1, Ethylene glycol/H2O/N2 = 10/67/23 (molar).

100

EG Conversion/ CH4 selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

80

conversion

60

40

20 CH4 0 0

5

10

15

20

25

30

35

40

45

Moles of carbon fed /gram catalyst Figure 8 Evolution of the ethylene glycol conversion as a function of the amount of carbon fed (moles of C in ethylene glycol) per gram of catalyst for 15%Co/MgAl2O4. S/C =6, T= 500ºC, P= 1 atmosphere, Ethylene glycol/H2O/N2=10/67/23 (molar). GHSV = 152,000 h-1.


28

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ACS Catalysis

Rh(111)

Ead = −98 kJ/mol

Ni(111)

Ead = −174 kJ/mol

Co(0001)

Ead = −41 kJ/mol

Figure 9 Schematic representations of HOCHCHOH intermediate on Rh(111), Ni(111) and Co(0001) surfaces. The top and side views are shown for each structure.


29

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

IS

d(C-C)=1.42 Å

Page 30 of 36

TS

d(C-C)=1.90 Å

FS

d(C-C)=4.05 Å

(a) C-C bond scission: HOCHCHOH ↔ HOCH + CHOH

d(C-O)=1.45 Å

d(C-O)=2.31 Å

d(C-O)=3.38 Å

(b) C-O bond scission: HOCHCHOH ↔ HOCHCH + OH

d(C-H)=1.09 Å

d(C-H)=1.56 Å

d(C-H)=2.93 Å

(c) C-H bond scission: HOCHCHOH ↔ HOCHCOH + H

d(O-H)=1.00 Å

d(O-H)=1.37 Å

d(O-H)=3.26 Å

(d) O-H bond scission: HOCHCHOH ↔ HOCHCHO + H

Figure 10 Initial decomposition of HOCHCHOH intermediate on the Rh(111) surface. IS, TS and FS are initial state, transition state, and final state, respectively.


30

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ACS Catalysis

IS

d(C-C)=1.41 Å

TS

d(C-C)=1.96 Å

FS

d(C-C)=2.76 Å


d(C-O)=1.40 Å

d(C-O)=1.86 Å

d(C-O)=3.93 Å


d(C-H)=1.12 Å

d(C-H)=1.62 Å

d(C-H)=2.54 Å


d(O-H)=1.00 Å

d(O-H)=1.40 Å

d(O-H)=2,72 Å


Figure 11 Initial decomposition of HOCHCHOH intermediate on the Ni(111) surface.


31

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

IS

d(C-C)=1.44 Å

Page 32 of 36

TS

d(C-C)=1.97 Å

FS

d(C-C)=2.70 Å


d(C-O)=1.42 Å

d(C-O)=1.88 Å

d(C-O)=3.04 Å


d(C-H)=1.13 Å

d(C-H)=1.62 Å

d(C-H)=2.73 Å


d(O-H)=1.00 Å

d(O-H)=1.37 Å

d(O-H)=3.26 Å


Figure 12 Initial decomposition of HOCHCHOH intermediate on the Co(0001) surface.


32

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ACS Catalysis

Figure 13 Potential energy surface diagram of methanation on Rh(111), Ni(111) and Co(0001) surfaces. The listed numbers (kJ/mol) are the forward/reverse activation barriers for each elementary step.


33

ACS Catalysis

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Page 34 of 36

Table 1 Carbon deposition and the particles sizes for the 5%Rh/MgAl2O4, 15%Ni/MgAl2O4 and 15%Co/MgAl2O4 after the stability measurement. Catalyst

Carbon deposition (coke)

Particle size (nm) by TEM*

per carbon fed

reduced

spent

15% Ni/MgAl2O4

1.83×10-4

8.3

11.7

15% Co/MgAl2O4

6.15×10-5

9

9

5% Rh/MgAl2O4

3.51×10-5

2

2

*TEM images and particle size distribution can be found in the SI, Figure S1-S2.

Table 2 Initial decomposition steps of HOCHCHOH intermediate on Rh(111), Ni(111) and Co(0001) surfaces. The calculated activation barriers (Ea) and reaction energies (∆H) are in kJ/mol. Rh(111)

Ni(111)

Co(0001)

Ea

∆H

Ea

∆H

Ea

∆H

C −C HOCHCHOH ← → HOCH + CHOH

70

−25

103

+53

104

+54

C −H HOCHCHOH ← → HOCCHOH + H

109

−3

41

−7

36

−4

O−H HOCHCHOH ← → HOCHCHO + H

85

+31

89

+16

87

+18

C −O HOCHCHOH ← → HOCHCH + OH

134

+9

83

−36

78

−82

C −O +O − H HOCHCHOH ←  → OCHCH + OH 2

96

−94

85

−89

90

−37

Reactions


34

Page 35 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Table 3 Dissociation of HCOH, HCO, and COH on Rh(111), Ni(111) and Co(0001) surfaces. The calculated activation barriers (Ea) and reaction energies (∆H) are in kJ/mol. Rh(111)

Ni(111)

Co(0001)

Reactions

Ea

∆H

Ea

∆H

Ea

∆H

C− H HCOH ← → H + COH

7

−67

28

−112

10

−72

O−H HCOH ← → HCO + H

78

−47

67

−47

81

−37

C−O HCOH ← → CH + OH

64

−26

92

−26

73

+5

C− H HCO ← → H + CO

17

−113

30

−115

66

−56

C−O HCO ← → CH + O

119

−25

121

−20

90

−58

C−O COH ← → C + OH

232

+63

207

+58

196

−22

O−H COH ← → CO + H

93

−68

54

−91

92

−84

Table 4 Methanation on Rh(111), Ni(111) and Co(0001) surfaces. The calculated activation barriers (Ea) and reaction energies (∆H) are in kJ/mol. Rh(111)

Ni(111)

Co(0001)

Reactions

Ea

∆H

Ea

∆H

Ea

∆H

C+H C + 4 H ← → CH + 3H

68

−36

86

−53

82

−49

C+H CH + 3H ← → CH 2 + 2 H

66

+63

71

+43

64

+37

C+H CH 2 + 2 H ← → CH 3 + H

73

+28

71

+3

61

−15

C+H CH 3 + H ← → CH 4 ( g )

65

+7

86

−41

105

−2

Table 5 Water dissociation on Rh(111), Ni(111) and Co(0001) surfaces. The calculated activation barriers (Ea) and reaction energies (∆H) are in kJ/mol. Rh(111)

Ni(111)

Co(0001)

Ea

∆H

Ea

∆H

Ea

∆H

O−H H 2O ← → OH + H

97

+16

86

−11

54

−36

O−H OH ← → O + H

78

−8

105

+3

95

−6

Reactions


35

ACS Catalysis

TOC

100

EG conversion/ CH4 Selectivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 36

80

60

40

20

0 5%Rh/MgAl2O4

15%Ni/MgAl2O4

15%Co/MgAl2O4

Low CH4 selectivity was found for steam reforming of ethylene glycol over spinel supported Co catalyst.


36

Steam Reforming of Ethylene Glycol over MgAl2O4 Supported Rh, Ni

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