Effects of La2O3 on the Mixed Higher Alcohols Synthesis from Syngas

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Effects of La2O3 on the Mixed Higher Alcohols Synthesis from Syngas over Co Catalysts: A Combined Theoretical and Experimental Study Vanessa M. Lebarbier,† Donghai Mei,† Do Heui Kim,† Amity Andersen,† Jonathan L. Male,† Johnathan E. Holladay,† Roger Rousseau,*,† and Yong Wang*,†,‡ † ‡

Pacific Northwest National Laboratory, Richland, Washington 99352, United States Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99614, United States ABSTRACT: The promoting role of lanthanum oxide (La2O3) in the catalytic synthesis of mixed higher (C2
C6) alcohols from syngas on Co-based catalysts was investigated using a combination of in situ and ex situ X-ray diffraction (XRD), photoelectron spectroscopy (XPS), catalyst reactivity performance studies, and ab initio molecular dynamics (AIMD) simulations. XRD measurements show that doping with La (0.5 wt %) onto activated carbon (AC) supported Co catalyst enhances the Co2C phase formation, whereas the Co2C phase formation is largely suppressed on alumina supported Co catalyst. A strong correlation of the selectivity toward alcohols with the ratio of Co2C/Co particles present in the catalysts was observed. AIMD simulations suggest that on AC supports La exists as an oxide phase in the form of small clusters in the vicinity of Co particles. It was found that Co2C formation is energetically favorable, especially for smaller Co particles because of the formation of surface carbide. Theoretical mechanistic studies indicate that oxygenated hydrocarbons can be formed on these catalysts by multiple routes involving the formation of CHxO and CHxCO species at the interface between the La2O3 phase and Co/Co2C. A detailed comparison with previous findings in the literature as well as discussion of the implications of these results upon the improvement of the selectivity of these catalysts toward higher alcohols is presented.

1. INTRODUCTION With the depletion of petroleum reserves and increasing concern about global climate change, catalytic conversion of coal and biomass-derived syngas into mixed alcohols may play an important role in the production of environmentally benign renewable energy sources.1,2 Instead of producing transportation fuels via the Fischer
Tropsch (FT) synthesis, syngas could be used to produce more valuable mixed higher alcohols such as ethanol, propanol, or butanol as the hydrogen energy carrier and the feed sources for fuel cell applications or as blend stocks in gasoline with similar or beneficial octane rating.3,4 Although extensive efforts had been carried out in the past decades, the low yield and the poor selectivity toward oxygenated hydrocarbons, especially the higher alcohols, remain the major technical barriers.2 For commercialization of this process, significant improvement of the catalytic performance for alcohol synthesis catalysts is critical. Among the various known catalysts, the modified FT catalysts have been explored for direct conversion of syngas to mixed higher alcohols.5
8 By adding a suitable metal (Ir, Re, Pt, Cu) and alkali (Li, K, Cs) promoters to SiO2 or Al2O3-supported FT catalysts such as Co, Fe, or Ni, the yield and the selectivity of mixed higher alcohols are pronouncedly improved.1,2,9
12 However, methane is the dominant product of the hydrocarbon products with selectivity as high as 50%, although most of the produced mixed alcohols are the straight chain primary alcohols.2 r 2011 American Chemical Society

Matsuzaki et al. have studied the effects of various transition metals on the activity and selectivity of C2+ oxygenates over Co/ SiO2 catalysts.9 It was claimed that the highly dispersed metallic Co0 particles are the main active sites for the C2+ oxygenate formation and that the transition-metal promoter facilitates the reduction of the cobalt oxide (Co2+) to the metallic Co0 by hydrogen spillover. However, this conclusion is in disagreement with the conclusion of ref 11, which suggests that partially reduced Co sites are responsible for C2+ oxygenate formation. Takeuchi et al. investigated the promoting effects of alkaline metal oxides on the selectivity of C2+ oxygenates over Co/SiO2 catalysts6,11 and found that the selectivity of C2+ oxygenates increased to ∼60% for the alkaline metal-oxide-promoted Co/SiO2 catalysts, compared with only 20% for the unpromoted Co/SiO2 catalysts. Furthermore, these authors attributed the promoting effects of alkaline earth metal oxides, the ability to control the Co oxidation state and stabilize oxygenated intermediates.11 The promoting effect of La2O3 on the Co-based catalysts had been previously reported.12
14 Barrault et al. found that La2O3 dramatically enhanced the specific activity over carbon-supported Co catalysts. The increased activity was attributed to the formation of new active sites by the addition of La2O3.13 This is inconsistent Received: April 29, 2011 Revised: July 15, 2011 Published: August 16, 2011 17440

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The Journal of Physical Chemistry C with the experiments showing little structural or activity changes observed upon La doping Co/Al2O3 catalysts,15 which raises questions about the role of the support and preparation technique in allowing the formation of possible new active sites. Recently, Jiao et al. reported that the mixed linear C1
C18 alcohols can be produced using activated carbon (AC) supported Co catalysts.5,16 In their study, the authors have shown that the addition of 0.5% La to the Co/AC catalyst increases the CO conversion from 13.5 to 21.4% and enhances the alcohol selectivity from 22.2 to 38.9%.5 These authors claim that the interaction between Co and La2O3 increases the Co dispersion and decreases the reducibility of the Co/AC catalysts and that the addition of La2O3 promotes the formation of a Co2C phase, which is responsible for the improved alcohol selectivity.16 However, this assumption was challenged by recent density functional theory (DFT) calculations, which indicate that the formation of Co2C phase will increase methane formation and may lead to deactivation of the Co catalysts.17 The reaction mechanism for CO hydrogenation to mixed higher alcohols (ethanol) over the Co-based catalysts is assumed to be similar to that on Rh-based catalysts,1 which involves CO dissociation, hydrogenation of C1 species into CHx, CO insertion into adsorbed CHx to form acyl species (CHxCO), and their subsequent hydrogenation. The acetate formed by acyl oxidation was suggested as the key reaction intermediate in the ethanol formation from CO hydrogenation.18 Another reaction route for CO hydrogenation to ethanol was based on the insertion of CH2 into adsorbed CH2
O species forming CH2
CH2
O intermediates, followed by further hydrogenation into ethanol.19 Choi and Liu investigated the ethanol synthesis mechanism for CO hydrogenation on Rh(111) using DFT calculations20 and proposed that CO hydrogenation to formyl (HCO) instead of CO dissociation is the rate-limiting step. These authors propose that a surface-bound formyl species is hydrogenated to methoxy (CH3O), which could then dissociate into CH3 + O, and the key reaction step is CO insertion into CH3 forming acetyl (CH3CO).20 Mei et al. recently studied the ethanol synthesis from syngas on the Rh/Mn/SiO2 catalysts using first-principlesbased kinetic Monte Carlo simulations.21 It was proposed that selectivity toward C2+ oxygenates depends on the relative rates of CO + CHx (x = 0∼3) coupling reactions and CHx hydrogenation, although hundreds of elementary reaction steps were considered in the reaction network. Cheng et al., based on detailed DFT mechanistic studies of oxygenated hydrocarbons on Co surfaces, have suggested that CO coupling to hydrocarbon chains may be responsible for the formation of long chain alcohols.22 Herein a combined theoretical and experimental study of the mixed higher alcohols synthesis from syngas using La2O3 promoted Co catalysts is presented. The major objective of this work is to understand the roles of the La2O3 promoter and various support materials (Al2O3 vs AC) in the formation of mixed higher alcohols using modified Co FT catalysts. To attain this goal, tightly coupled studies employing catalyst synthesis, in situ and ex situ characterization, reactivity measurements and theoretical modeling were performed. The technical details for all experimental and theoretical studies are reported in Section 2 and the results are presented and discussed in Section 3. Specifically, both experimental and theoretical studies are presented in order to elucidate the structures of the catalysts under operating conditions and how the structural change affect catalytic performance (selectivity, activity).

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2. MATERIALS AND METHODS 2.1. Catalyst Synthesis, Characterization, and Reactivity Studies. 2.1.1. Catalysts Preparation. The catalyst denoted

15Co1Zr0.5La/AC was synthesized by impregnating the support, an AC made from coconut shell (G202 M from PICA), with an aqueous solution containing cobalt nitrate, lanthanum nitrate, and zirconium nitrate, as described in ref 16. The Co, Zr, and La loadings are, respectively, 15, 1, and 0.5 wt %. After impregnation, the catalyst was dried at 80 °C for 8 h in static air and then calcined at 350 °C for 3 h in Argon flow (ramp 5 °C/min). To determine the role of the support, we synthesized a 15Co1Zr0.5La/Al2O3 catalyst in a similar way using an Al2O3 support from Engelhard (Al-3945e1120). The samples denoted 15Co/AC and 15Co0.5La/AC were synthesized by the same method as the 15Co1Zr0.5La/AC catalyst using an aqueous solution of cobalt nitrate and La nitrate, respectively. Note that Zr is added to the catalysts to inhibit deactivation but does not influence the catalytic activity or selectivity toward alcohol. (See Section 3.2.1). Furthermore, the nomenclature “spent”, used below, refers to a catalyst that has been pretreated, followed by exposure to the reaction conditions, where pretreatment is performed either in H2 at 430 °C for 12 h or first in H2 at 430 °C for 12 h then under CO at 225 °C for 24 h. 2.1.2. In Situ X-ray Diffraction. The in situ X-ray diffraction (XRD) experiments were carried out at beamline X7B of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The detailed setup of these experiments has been described elsewhere.23 A small amount of sample was placed in a glass tube and heated from 27 to 430 °C for 4 h while continuously flowing a 5% H2 in He gas mixture at atmospheric pressure (i.e., 1 MPa). After 6 h at 430 °C in H2/He flow, gas was switched to 5% CO/He at 225 °C. XRD patterns were collected in situ every 4 min during that period. 2.1.3. Ex Situ X-ray Diffraction. X-ray powder diffraction patterns were recorded using a Philips X’pert MPD (model PW3040/00) diffractometer with copper anode (KR1 = 0.15405 nm) and a scanning rate of 0.016° per second for 2θ = 20
70°. The diffraction patterns were analyzed using Jade (Materials Data, Livermore, CA) and the powder diffraction file database (International Center for Diffraction Data, Newtown Square, PA). 2.1.4. In Situ X-ray Photoelectron Spectroscopy. In situ X-ray photoelectron spectroscopy (XPS) measurements were carried out in a catalytic side chamber, designed in house, and attached to a Physical Electronics Quantum 2000 scanning ESCA microprobe. Sample powders were thermally treated in the side chamber and transferred in vacuo to an analyzer for measuring photoelectron energy spectra. The treatment condition is the same as in situ XRD experiment, which applies H2 to the samples at 430 °C for 6 h, followed by CO at 225 °C for 12 h. This system uses a focused monochromatic Al Ka X-ray (1486.7 eV) source and a spherical section analyzer. The instrument has a 16element multichannel detector. The X-ray beam used was a 100 W, 100 mm diameter beam, which was rastered over a 1.3 mm by 0.2 mm rectangle on the sample. The X-ray beam is incident normal to the sample, and the photoelectron detector was at 45° off normal. Wide scan data were collected using a pass energy of 117.4 eV. The binding energy (BE) scale was calibrated using the Cu2p3/2 feature at 932.62 ( 0.05 eV and Au 4f at 83.96 ( 0.05 eV for known standards. The sample experienced variable degrees of charging. Low-energy electrons and low-energy 17441

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The Journal of Physical Chemistry C Ar+ ions were used to minimize this charging. The BE scale was referenced to the carbon from the support (C 1s = 284.5 eV). 2.1.5. Reactivity. The catalysts were tested for the synthesis of higher alcohol from syngas using a microchannel reactor (stainless steel 316) with the channel dimensions of 6.5 cm  1.06 cm  0.12 cm. The microchannel reactor was configured for high-pressure down-flow mode. The schematic diagram of the reactor system and microchannel reactor assembly was similar to those previously described.24 Experiments were conducted under the same conditions as those reported in ref 16, which are 225 °C, GHSV = 500 h
1, and P = 3 MPa. The catalyst (566 mg) was reduced at 430 °C for 12 h in H2 flow prior to the test. After reduction, the temperature was decreased to 225 °C, and the premixed syngas was fed into the reactor. The feed composition was CO/H2/N2 33.5:62.3:4.2 (vol %). The presence of N2 served as the internal standard for conversion and selectivity purposes. The gas products were separated using MS-5A, PPQ, and OV1 columns and analyzed online by means of a Quad Micro GC equipped with a TCD. The liquid products were collected in a cold trap at 0 °C. The organic phase was analyzed using an HP 5890 GC equipped with an FID detector and a DB5MS column. The aqueous phase was analyzed using an HPLC GC equipped with a water 2414 refractive index detector and a Bio-Rad Aminex HPX-87H ion exclusion column (300 mm 7.8 mm). 2.2. Computational Details. DFT calculations were carried out, employing a gradient-corrected functional for exchange and correlation,25 as implemented in the CP2K package.26
28 Core electrons are modeled as norm-conserving pseudopotentials with wave functions expanded in a double-zeta valence-polarized Gaussian basis set and a plane-wave auxiliary basis with a 300 Ry energy cutoff. Calculations of all reaction barriers were performed using the climbing image nudged-elastic-band (CINEB) method employing 8
12 system replicas.29,30 All simulations were performed on ∼1 nm diameter Co60La2O4 and Co60C30La2O4 clusters. (See Figure 8 and Section 3.1.2 for a discussion of La oxide stoichiometry). The role of ZrO2 is not explored in the current cluster models because reactivity studies (see Section 3.2.1) show that it has little influence on the activity/selectivity of the catalyst. All cluster structures used for reaction calculations were obtained by an AIMD-based simulated annealing procedure, wherein the cluster is first equilibrated at T = 600
1000 K for a duration of 2 to 3 ps, followed by cooling to T = 0 K over the duration of a 1 to 2 ps trajectory. Low atomic masses of 2
4 atomic mass units are used in this procedure to accelerate the AIMD effectively, allowing for an increase in the amount of phase space explored during the course of this relatively short trajectory. All cluster studies are
simulation cell, which leaves >5 Å
performed in a cubic 20 Å between a cluster and its periodic image. The fully optimized clusters are typically well-faceted and provide well-defined sites that result in reaction energies comparable to bulk surfaces. (See Section 3.2.2 for detailed comparison with literatures.) AIMD simulations of the cluster on γ-Al2O3 and AC are performed on a periodic surface slab model. For the former, a fully hydroxylated (3  2) γ-Al2O3(100) surface slab of four Al2O3 layers that was employed as taken from previous work was used.31,32 The hydroxylated surface was chosen because of the fact that at the 225 °C of the reactivity studies γ-Al2O3(100) remains hydroxylated, as are other prominent surface facets such as (110).31,32 Thus, the hydroxylated surface would serve as a better representation of γ-Al2O3 (and other phases of alumina)

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Figure 1. In situ XRD patterns for the 15Co0.5La/AC catalysts recorded during the increase in the temperature from 27 to 430 °C in H2 flow at atmospheric pressure.

under the reported reaction conditions. For AC support, a bilayer graphene model surface with orthorhombic super cell dimen
was used.31,32 The planar graphene layer is sions of 19.6  15.0 Å expected to be significantly more inert than actual AC supports, which contain edges, defects, and organic functional groups. Therefore the reactivity reported in Section 3.1.2 is expected to
vacuum layer be more pronounced on real AC supports. A 20 Å between slabs is employed in all simulations. Calculations are started from partially optimized geometries (max force component of 0.01 a.u) consisting of the cluster chemisorbed on top of the surface. All simulations are run in the NVT ensemble for 20 ps of equilibrated trajectory after an initial 4
8 ps of equilibration time.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Catalysts. To understand how the modified FT catalyst performs the synthesis of higher alcohols, we first characterized the catalysts using in situ XRD and XPS as well as ex situ XRD. These results are compared with a detailed thermodynamics analysis of the catalysts based on theoretical DFT calculations. 3.1.1. In Situ X-ray Diffraction and Photoelectron Spectroscopy. Figure 1 shows the XRD patterns recorded when increasing the temperature from 27 to 430 °C in H2 flow for the 15Co0.5La/AC catalyst. The XRD pattern acquired at 131 °C contains only peaks due to a Co3O4 phase (2θ = 6.3, 7.5, 9.0, 11.8, 12.8°). Further increase in temperature to 210 °C gives rise to the gradual increase in the peaks characteristic of CoO (2θ = 7.4, 8.5, 12.2°) in addition to the disappearance of the peaks due to Co3O4. At 233 °C, peaks characteristic of a Co° cubic phase (2θ = 8.9) arise, whereas the intensity of the peaks due to CoO decreases. The phase transformation from CoO to Co° occurs continuously up to 430 °C, where peaks characteristic of CoO phase disappear entirely. Because the in situ XRD experiment simulates the pretreatment procedure, it provides useful information about how the Co phase changes from Co3O4 to Co°. It can be summarized that only the Co° phase exists after pretreatment at 430 °C in H2 flow. After pretreatment, the temperature was cooled to 225 °C, and the 15Co0.5La/AC sample was exposed to 5% CO. XRD patterns obtained as a function of temperature under these 17442

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Figure 2. In situ XRD patterns (a) 15Co0.5La/AC catalyst recorded during the treatment in CO at 225 °C (atmospheric pressure) and (b) XRD patterns recorded after treatment in CO flow at 225 °C for 8 h (atmospheric pressure) for the 15Co0.5LA/AC and the 15Co/AC catalysts.

conditions are shown in Figure 2a. The cubic metallic Co phase is the initial phase after pretreatment with H2 at 430 °C. When CO is exposed to the sample at 225 °C, there is a significant change in the Co phase from Co° cubic to Co2C phase (2θ = 7.5, 8.6, 9.2, 11.3°). The transformation occurs slowly for a duration of up to 460 min. Hence, treatment in CO flow after the initial reduction of the catalyst favors the formation of Co2C. For the 15Co/AC catalyst, the same experiment was conducted. The XRD pattern recorded after pretreatment in H2 and CO treatment for 8 h is presented Figure 2b. One can see that for the XRD pattern of the 15Co/AC sample, the peaks characteristic of Co2C are hardly detectable compared with the XRD pattern of the 15Co0.5La/ AC catalyst. These results suggest that under the elevated temperature and pressure at which the catalyst operates (i.e., under P = 3 MPa, T = 225 °C), formation of Co2C phase could be facilitated by the presence of La. XPS technique gives some information concerning the surface composition of the catalysts, as opposed to XRD, which is a bulk sensitive technique. Thus, as a complement to these XRD studies, in situ XPS was carried out over the Co/AC and Co
La0.5/AC samples. The samples were treated with H2 at 430 °C for 6 h, followed by the treatment with CO at 225 °C for 12 h at atmospheric pressure. Figure 3 shows the C 1s region of Co/AC and Co-0.5La/AC samples after CO treatment. The main peak at 284.5 eV originates from the carbon support. Only the spectrum recorded for the 15Co0.5La/AC sample contains the peak at 283.2 eV, which is assigned to carbidic carbon. In situ XPS results correspond well to in situ XRD, confirming that La promotes the formation of Co2C not only in the bulk but also on the surface. 3.1.2. Ex Situ X-ray Diffraction. The in situ measurements have suggested that treatment in CO before reaction (i.e., after reduction in H2) could favor the formation of Co2C during reaction. To assess this hypothesis, we conducted a comparison of XRD patterns for the spent 15Co1Zr0.5La/AC exposed to two sets of pretreatment conditions: (1) after reduction in H2 and (2) after reduction in H2 and treatment in CO at 225 °C for 24 h. The XRD patterns are presented (Figure 4a). For both XRD patterns, peaks characteristic of Co° (2θ = 44.27°) and Co2C (2θ = 41.25, 42.7, and 45.7°) are detected. The intensity of the peaks characteristic of Co2C, relative to the peak at 44.2°, which is attributed to Co°, is higher for the XRD pattern for the sample exposed to CO before reaction. This indicates that the Co2C/Co

Figure 3. In situ C 1s XPS spectra recorded after treatment in CO flow at 225 °C for 12 h (atmospheric pressure) for the 15Co0.5LA/AC and the 15Co/AC catalysts.

ratio (i.e., number of Co2C particles/number of Co° particles) is increased for the catalyst that was treated in CO before reaction. Hence, these results are in agreement with the in situ XRD measurements and show that treatment of the catalyst under CO before exposure to the reaction conditions helps the formation of Co2C. The bandwidth of the peaks characteristic of Co2C and Co° is similar for both XRD patterns, signifying that the Co2C and Co° particles are the same size. Consequently, the increase in the Co2C/Co° ratio for the sample treated in CO is indicative of an increase in the number of Co2C particles and a decrease in the number of Co° particles. The in situ XRD measurements have indicated that the formation of Co2C by treatment in CO of Co° is easier for the 15Co/AC catalyst promoted with La. It suggests that under reaction conditions, formation of Co2C could be more facile for the 15Co0.5La/AC catalyst than for the 15Co/AC catalyst. The ex situ XRD patterns recorded for the spent 15Co/AC and 15Co0.5La/AC catalysts are presented in Figure 4b. For both samples, the XRD patterns show peaks characteristic of Co° and Co2C. For the 15Co/AC sample, the presence of Co2C is more evident than from the in situ XRD patterns (Figure 2), which may be due to the fact that the in situ XRD patterns were recorded at atmospheric pressure where the carburization of Co° 17443

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Figure 4. Ex situ XRD patterns: (A) Recorded for the spent 15Co1Zr0.5La/AC catalyst that was (a) reduced in H2 before reaction and (b) reduced in H2 followed by treatment in CO at 225 °C for 24 h before reaction. (B) XRD patterns recorded for the spent 15Co0.5La/AC and 15Co/AC catalysts.

Table 1. Reaction Energies for Reduction of La by H2 as Described in Reaction R1 x

ΔECo/ΔECo2C (kcal/mol)

ΔGCo/ΔGCo2C (kcal/mol)a

0


53/
72


25/
44

2


99/
61


71/
33

4

19/35

47/63

a

Free energy is estimated from the reaction energy corrected for the thermal translation and rotational entropy/energy of the gas-phase species H2 and H2O at T = 500 °C

Figure 5. Ex situ XRD patterns recorded for the spent 15Co1Zr0.5La/ AC and 15Co1Zr0.5La/Al catalysts.

is more difficult. It can be seen from Figure 4b that the intensity of the peaks characteristic of Co2C, relative to the peak at 44.2° attributed to Co°, is higher for the 15Co0.5La/AC catalyst than for the 15Co/AC catalyst, corroborating the results from in situ analysis that the Co2C/Co° ratio is higher for the sample with additional La promoter. To study the influence that the nature of the support has on the catalyst structure (i.e., nature of the cobalt species Co° and/ or Co2C), the ex situ XRD patterns for the spent 15Co1Zr0.5La/ AC and the 15Co0.5La/Al catalysts have been examined. The XRD patterns (Figure 5), indicate that both catalysts present peaks characteristics of Co°, but peaks due to Co2C are observed only for the 15Co1Zr0.5La/AC catalyst, indicating that only the AC support favors the carburization of the Co° particles. These results are in agreement with the literature, showing that the nature of the support, Al2O3, TiO2, or SiO2, strongly affects the formation of Co2C.33 3.1.3. Theoretical Thermodynamics Analysis. Experimental characterization has identified several key structural features of

these catalysts: (i) Co exists as both Co metal and Co2C phase with the amount of the latter strongly dependent on the presence of La. (ii) Exposure to CO in the pretreatment stage also facilitates the formation of Co2C. (iii) Oxide supports such as Al2O3 suppress Co2C formation, whereas AC supports promote it. Here these observations are analyzed in terms of energetics obtained from DFT simulations to determine their atomic level origin. First, the chemical environment of the La species is considered by examining its structure and oxidation state under the experimental conditions, where the presence of excess H2 may lead to partial reduction of La. This is achieved by examining the energy of the chemical reaction R1 La2 Ox þ 2H2 OðgÞ f La2 Oxþ2 þ 2H2 ðgÞ

ðR1Þ

La atoms are supported on either a Co60 or a Co60C30 cluster. Reaction energies and free energies at T = 500 °C are reported in Table 1. Not surprisingly, La2Ox is extremely resistant to reduction for the clusters (x < 4), owing to the very large exothermicity of reaction R1, which results in a strong preference for the stabilization of an oxide phase even at T = 500 °C. From our reactivity data, it was found that the most stable stoichiometry/ structure places two La together in a La2O4 oxide patch. Each La is bound to three oxygen atoms, two of which bridge La atoms with a relatively long (>2.0 Å
) Co
O distance and a third that bridges a La and a Co. Simple electron counting rules thus suggest that there is a formal oxidation state of III on each La. Because of the small amount of La atoms present, a true bulk-like 17444

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Figure 6. Structure of model catalysts obtained from AIMD simulated annealing: (a) Co60La2O4 and (b) Co60C30La2O4. Atoms are represented as: Co (dark blue), La (light blue), O (red), and C (gray).

La2O3 phase is not expected. Two Co atoms at boundary between the oxide and the cluster are also partially oxidized (formally Co+), owing to the existence of O atoms that bridge a Co and a La atom (Figure 6). The La2O4 oxide “patch” is found to be dynamically stable up to very high temperatures (T > 600 °C). AIMD simulations of the cluster in the gas phase at temperatures exceeding 300 °C show no breakup of the patch on a 10 ps time scale, attesting to the high thermal stability Overall, in accord with our experimental findings La on both Co and Co2C is expected to be fully oxidized5,16 and exist as oxide patches. The role of the support on the structure of the Co/La2O4 particles is investigated by AIMD simulations at 300 °C on representative alumina and carbon substrates. Specifically, for alumina, a fully hydroxylated γ-Al2O3, and for AC, a graphene bilayer, were employed. (See Section 2.2 for discussion.) The initial and final structures resulting from these studies are illustrated in Figure 7. For each support, two scenarios are considered where the oxide patch is oriented directly toward or oriented away from the surface, denoted (t) and (a), respectively. To discriminate the energetics of the two simulations, we employed the expectation value of the Kohn
Sham energy, , because it represents both the electronic and vibrational energies at finite temperature; note that this quantity does not include an entropic term and is thus not a free energy. On γ-Al2O3 was found to be ∼60 kcal/mol lower in energy for the case where the La2O4 patch was oriented toward (t) the alumina surface relative to when it is oriented away (a) from it. Selected radial distribution functions (RDFs), which give a volume-weighted probability distribution of contact distances between pairs of atoms, and their integrals are given in Figure 8 to illustrate the resulting structure changes. In both simulations, the La oxide strongly interacts with the surface hydroxyl groups, as evidenced by an increase in La
O coordination number from 3 in the unsupported cluster to ∼4 on the surface. Only a few of the Co atoms form a Co
O bond and become oxidized by the alumina surface. In general, the structure of the Co cluster remains largely that of a chemisorbed Co60 cluster. (See Figure 7.) The La oxide patch retains its structural stability during the MD run, but there is a pronounced separation of this patch away from the Co, as evidenced by an increasing Co
La distance of 2.8 Å
for the unsupported cluster to 3.3 (a) and 3.4 Å
(t) and a concurrent drop in the La
Co coordination number from 7 in the unsupported cluster to 5 (t). This suggests the onset of phase segregation between Co and the oxide patch, with the latter preferring to be attached to the alumina surface. This is in contrast with the observed behavior of the Co60La2O4 cluster on graphene. The trajectory (t) shows no cluster surface interaction, as the Co60La2O4 cluster desorbs from the

Figure 7. Initial and final structures from T = 300 °C AIMD simulation of Co60La2O4 on (a) γ-Al2O3 with La2O4 patch oriented toward (t) the surface (b) γ-Al2O3 with La2O4 patch oriented away (a) from the surface. (c) Graphene with La2O4 patch oriented away (a) from the surface.

surface entirely and is 130 kcal/mol higher than for the situation where Co directly interacts with the graphene (a). In
Co
C this latter simulation, eight Co atoms form multiple 2.0 Å bonds with the support; see RDF in Figure 9. Although the La2O4 patch exhibits large amplitude structural fluctuations greater than those seen in the unsupported cluster, it retains its integrity and does not dissociate on the cluster surface, nor does it interact with the graphene support. These AIMD simulations underscore an important point. On AC, the La/oxide patch remains stable, but the Co cluster becomes carbonized, whereas on oxide supports the La oxide cluster appears to be passivated by surface hydroxyls (possibly even phase-segregated from the cluster), and Co remains metallic. This is in accord with our observations based on XRD, which show an increase in carbide formation on AC supports. The question of how La, in the form of an oxide patch, may influence the formation of Co2C phase is now addressed. To do this, we consider the energy of two chemical reactions carbide formation from bulk C: 2Co þ xCðgrapheneÞ f Co2 Cx and carbide formation from CO reduction: 2Co þ xCOðgÞ þ xH2 ðgÞ f Co2 Cx þ xH2 O

ðR2Þ

ðR3Þ

In reaction R2, the energetic preference for carbide formation from solid C such as the support material is evaluated, whereas reaction R3 addresses the possible role of CO in carbide 17445

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Figure 8. Selected radial distribution function for γ-Al2O3/Co60La2O4 at T = 300 °C: (a) La
O and (b) La
Co. Number next to each peak is its integral.

Figure 9. Selected radial distribution function for graphene/Co60La2O4 at T = 300 °C. (a) La
La and (c) Co
C.

Figure 10. Reaction energies for R2 (a) and R3 (b) as a function of carbon concentration x. The dashed line represents the calculated value based on a bulk Co solid.

formation in the catalyst pretreatment. (See Figure 10a,b, respectively.) For both reactions, a bulk solid Co thermodynamic reference is compared with energetics obtained from clusters so that one can infer the role of the high surface area of the nanoparticle upon phase stability. In addition, the cluster with/ without a lanthanum oxide (La2O3) component is examined to determine the extent to which La can directly influence the phase stability. For reaction R2, it is found that the bulk Co reaction is only mildly exothermic, with an ΔE of
1 kcal/mol, in good agreement with the
0.5 kcal/mol estimated from tabulated thermodynamic data.34 Using the same methodology for a Co60 and Co60La2O4 cluster, it is found that reaction R2 shows an enhanced exothermicity such that for x = 1 (i.e., stoichiometric

Co2C) the reaction energy is
8 kcal/mol. The Co60C30x clusters obtained from AIMD-simulated annealing trajectories show a pronounced propensity toward forming surface carbides. For example, for the Co60C10 species (x = 0.33), two initial structures were employed: one where the 10 carbon atoms were placed inside the cluster and a second where they were dispersed upon the surface. After a short (2p) AIMD trajectory at 700 °C, followed by quenching to absolute zero, both initial configurations resulted in clusters exclusively bearing surface carbide, which were isoenergetic to within 2 kcal/mol. Hence, the enhanced thermodynamic stability of the carbide phase in the clusters can be attributed to the presence of surface carbide. The energy profiles for reaction R2 are essentially identical for the cluster with and without the oxide patch. This indicated that 17446

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Table 2. CO Conversion, Selectivities, and Alcohol Products Distribution for Each Catalysta selectivity (%) catalyst 15Co1Zr0.5La/AC

CO conversion (%) b

alcohol products distribution (%)

CO2

hydrocarbons

alcohols

C1

C2
C5

C6+ 35.3

55

3.7

75.6

21.7

13.6

51.1

15Co1Zr0.5La/ACc

21.5

6.4

71.1

22.5

27.9

72.1

0

15Co0.5La/AC

58

2.0

77.6

20.4

11.2

57.1

31.7

15Co/AC

29.2d

4.4

82.4

13.2

15Co1Zr0.5La/Al

12.5d

12.3

83.4

4.5

22.6

77.4

0

22

76.5

1.6

a T = 225°C, P = 3 MPa, and GHSV = 500 h
1. b Reactivity data collected after reduction in H2. c Reactivity data collected after reduction in H2 and CO treatment for 24 h. d Represents the CO conversion to CO2, hydrocarbons from C1 to C16 and alcohols. Some waxes were produced, and the total CO conversion (including the waxes) is equal to 74 and 84%, respectively, for 15Co/AC and 15Co1Zr0.5La/Al catalysts

carbide formation is indeed favored in systems consisting of small Co particles but that the oxide patch has little direct chemical influence on the energetics of this reaction. For all Co80C30x clusters, the La2O4 patch retained its structural integrity during all simulated annealing trajectories, indicating that it is also stable as a patch on Co2C particles. This observation supports the conclusion16 that La oxide most likely enhances Co2C formation. A decrease in Co particle size leads to a larger surface area and an enhanced thermodynamic stability for carbide formation. For reaction R3, tabulated thermodynamic data indicate that this reaction for bulk Co leading to Co2C should be exothermic by
32 kcal/mol, whereas the current approach gives a value of
55 kcal/mol. This reaction energy indicates an even stronger thermodynamic driving force for carbide formation as a result of CO reduction than from elemental carbon and that Co2C formation is enhanced for the smaller cluster relative to bulk such that at x = 1 ΔE =
61 kcal/mol. However, the BE of CO on Co is estimated to be 38 kcal/mol, which implies that at T = 225 °C there may be significant kinetic competition from CO desorption. The above thermodynamic analysis is in substantial agreement with the experimental observations. Taken together, they suggest that (i) La is found to show a thermodynamic preference to exist in an oxide phase with oxidation state 3+, with the extra finding that these oxides form a stable patch on the surface of Co and Co2C nanoparticles. (ii) There is a net thermodynamic driving force for the formation of Co2C phase, which is enhanced by the formation of surface carbide on nanoparticles. Co2C from CO reduction at pretreatment is also favorable thermodynamically. (iii) Support effects are critical. Oxide supports, such as alumina, have a strong affinity for the La2O3 phase, resulting in its segregation from Co. Given the conclusion above about the La2O3 influencing the particle size, this may lead to a suppression of carbide phase on oxide surfaces.16 AC promotes carbide phases and does not disrupt the integrity of the La2O3 patch. 3.2. Reactivity Studies. Now that a correlation between the catalyst structure and the influence of thermodynamic variables and support effects has been established it is logical to examine the correlation between structure and catalytic activity and selectivity toward higher alcohols. 3.2.1. Experimental Reactivity Studies. Table 2 presents the CO conversion and product selectivity for each catalyst of the present study. Note that no significant loss of activity was

observed for a period of 72 h on stream, except for the 15Co0.5La/AC catalyst with a loss of 24% of conversion. For the 15Co1Zr0.5La/AC catalyst, Table 2 shows the reactivity data collected (1) after reduction in H2 (15Co1Zr0.5La/ACb in Table 2) and (2) after reduction in H2, followed by a treatment in CO (15Co1Zr0.5La/ACc in Table 2). The additional treatment in CO leads to a decrease in the CO conversion from 55 to 21.5%. The XRD measurements have shown that the number of Co° particles is lower when the sample is treated in CO before reaction. The decrease in the CO conversion is likely due to a decrease in the number of Co° particles. Hence, these results suggest, in agreement with the literature,35,36 that the CO dissociation is easier on Co0 than on Co2C. From Table 2, it can be seen that production of long-chain alcohols (i.e., C6+ alcohols) is inhibited when the sample is treated in CO before reaction. This behavior could be due to a lower number of Co0 particles (i.e., higher number of Co2C particles) and suggests that Co0 is the active species for the C chain growth. Note that a slight increase in the alcohol selectivity is observed for the sample treated in CO and presenting the highest Co2C/Co° ratio. Comparing the catalysts 15Co0.5La/AC and 15Co1Zr0.5La/ AC (without pretreatment with CO), it is noticeable that the CO conversion and alcohol selectivity are very similar. This suggests that Zr has little (direct or indirect) influence on the catalyst stability, and preliminary data (not shown) suggest that it may serve a role in preventing catalyst deactivation. Both catalysts exhibit similar Co2C/Co ratios, which suggests that Co2C is the structural component that determines the catalytic performance and that Zr does not influence carbide formation to the extent of La. The comparison between the reactivity data obtained for the 15Co0.5La/AC catalyst and the 15Co/AC catalyst shows that the CO conversion is higher for the unpromoted sample (84 vs 58%). This is likely due to a higher number of Co° particles, as suggested by the XRD patterns, for the 15Co/AC; the Co° particles being active for the CO dissociation. The alcohol selectivity is higher for the 15Co0.5La/AC sample than for the 15Co/AC sample (20.4 vs 13.2%). Because of the presence of waxes for the 15Co/AC sample, the alcohol yield is notably lower for the unpromoted sample and equal to 3.7 versus 11.8% for the La-promoted sample. The XRD measurements have shown that the Co2C/Co° ratio is higher for the sample promoted with La. Hence, these results also suggest that the Co2C species could, at least in part, be responsible for the alcohol formation. To examine the influence of the support, we compared the reactivity for the 15Co1Zr0.5La/AC and the 15Co1Zr0.5La/Al. The CO conversion is higher for the 15Co1Zr0.5La/Al sample 17447

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Table 3. Selected Binding Energies, Eb, of Surface Species Relevant to Higher Alcohol Synthesis on Co60La2O4 and Co60C30La2O4 Particlesa Eb

Co60La2O4

Co60C30La2O4

site

COb

Coc

38

27

La

0c

0c

Coc

6


8

La O


12
17


21
7

Ce

Coc

77

116

f

Coc

95


4

La


10


3

Coc

151

140

Lah

93

39

O

109/146i

107/125i

Co La

98 75

69 54

O

90k/94l

62k/64l 33

Hd

O

CHg

Figure 11. Evolution of the alcohol selectivity with the increase of the Co2C/Co° ratio determined from the intensity of the XRD peaks characteristic of Co2C at 42.7° and Co at 44.27°.

presenting the highest number of Co° particles, in agreement with the discussion above on the effectiveness of the Co° for the CO dissociation. The alcohol selectivity is five times lower for the sample supported on the Al2O3 support (4.5%) than for the sample supported on the AC support (21.7%). Because a significant amount of waxes were produced with the 15Co1Zr0.5La/Al catalyst, the alcohol yield is extremely low and equal to 0.57%, whereas it is equal to 10.82% for the 15Co1Zr0.5La/AC catalyst. Consequently, the alcohol productivity is negligible for the 15Co1Zr0.5La/Al catalyst, and it appears active for the FT reaction rather than for the higher alcohol synthesis. This is not expected to be due to alumina acting as an alcohol dehydration catalyst because this reaction is expected to occur at much higher temperatures and gas pressures than employed in the current study. Because the XRD patterns have pointed out the absence of the peaks characteristic of Co2C for the 15Co1Zr0.5La/Al, these results suggest again that the Co2C particles are active for the alcohol production. To assess the relationship between the amount of Co2C and alcohol production, we have plotted the evolution of the alcohol selectivity with the increase in the Co2C/Co° ratio in Figure 11. There is an increase in the alcohol selectivity with the increase in the Co2C/Co° ratio, which suggests that the Co2C particles are the active species for alcohol formation. For FT synthesis over supported cobalt catalysts, only a few studies have reported the formation of alcohol in the presence of Co2C,33,37 and it is likely that the formation of alcohol is a minor product and hardly detectable because the catalysts do not present a sufficient number of Co2C particles. It is also likely that in the present study the Co2C/Co ratio (as determined by XRD) is high because of the AC support facilitating the Co2C formation. For FT synthesis over supported cobalt catalysts, it has been reported that Co2C leads to a deactivation of the catalyst.33,38 The results presented in this study illustrate the inability of Co2C for dissociating CO. Hence, the decrease in the CO conversion reported in the literature, for the FT synthesis over Co catalysts, may be due to an increase in the number of Co2C particles, which are inactive for CO dissociation, combined with an associated decrease in the number of Co° particles, which are active for CO dissociation.

Eb

species

CH2j

CH3m

c

c

Co

38

La

41

16

O

41

13

a

Negative values of Eb indicate species unstable with respect to desorption into the gas phase. b Neither C bound or O bound CO bind to La. c High coordinated Co site on hexagonally closed packed surface facet. d Relative to 1/2H2(g). e Relative to C(g). f Relative to 1/2 triplet 021 (g). g Relative to CH(g). h Bridging La and O. i Inserts into La
O bond/inserts into Co
O bond. j Relative to triplet CH2(g). k CH2O, C bound to La. l CH2O C bound to Co. m Relative to CH3(g).

3.2.2. Theoretical Mechanistic Studies. To elucidate the role of La2O3 in the selectively toward higher alcohols, we have conducted DFT studies of chemical reactivity on Co and Co2C particles. The mechanism of syngas conversion to hydrocarbons and short chain alcohols on both metal and carbide phase of Co has been examined theoretically in refs 17 and 22. Therefore, the focus of the current section will be to highlight observations that are specific to mixed La/Co catalysts. The relative BEs, Eb, of several key species at various reaction sites on the Co and Co2C particles are compared; see Table 3. On Co, CO has an Eb =38 kcal/mol, but H (relative to 1/2H2(g)) is much less weakly bound Eb = 6 kcal/mol. Both reactants are bound significantly weaker on the Co2+ sites of Co2C by ∼10 kcal/mol. Furthermore, √values of the reaction energy, ΔECO, and its kinetic barrier, ΔECO , are considered for the dissociation of a chemisorbed CO CO f C þ O

ðR4Þ

On a closest packed faceted surface of the Co cluster, CO dissociates into C and O, both on three-fold hollow sites in close proximity. Reaction R4 is endothermic; ΔECO = 9 kcal/mol, with √ a large energy barrier, and ΔECO = 50 kcal/mol, consistent with previously reported DFT barrier on flat Co(0001) surface.22 As a result, CO dissociation is often found to be kinetically favored only at defect sites such as step edges.39 In contrast, on a similarly √ faceted surface of Co2C, ΔECO = 21 kcal/mol and ΔECO = 56 kcal/mol, indicating that the CO activation is thermodynamically more difficult than on Co. These observations are in accord with the studies of ref 17 as well as the assertion that Co is the active 17448

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phase for CO dissociation based on the reactivity data presented in Table 2. In general, CHx species (x = 1
3) are bound to Co more strongly than Co2C (see Table 3). On both Co and Co2C particles, the process of going from elemental C + H to CH is calculated to be energetically downhill by 120 and 84 kcal/mol, respectively: note that this energy is with respect to surfaceadsorbed H and C. Thus, CH is expected to form readily given the strong thermodynamic driving force. The reaction energy profile for hydrogenation of CH to CH4 on both Co and Co2C is shown in Figure 12. On Co, hydrogenation to methane is a fairly low energy process. The maximum energy barrier of 26 kcal/mol occurs for converting CH to CH3 via the trapped intermediate CH2. On Co2C, this process is also facile, with maximum energy barriers of 22 and 19 kcal/mol for hydrogenation of CH to CH2 and CH3 to CH4, respectively. The relative energies of the intermediates and activation barriers reported here differ from those reported by Cheng et al.,17 obtained on stepped Co/ Cc2C(0001) surfaces, however, the conclusion that methane activation occurs on both materials with a global barrier of approximately 20
30 kcal/mol is the same. For the current discussion, this activation energy is critical in that it implies that any process, which is kinetically competitive with methanation, must have similar or lesser activation barriers. There is a distinct difference between the metallic and carbide phases for the C
C coupling reactions. 2CHx f C2 H2x

favorable (i.e., exothermic) but more so on Co. This observation is in accord with the results of ref 17 as well as the current reactivity studies that suggest that hydrocarbon chain growth should be less active on the Co2C phase. Energy barriers for this process are estimated to be about 32 and 28 kcal/mol for x = 1,2 on Co, indicating that it is kinetically competitive with methanation. The presence of CHx species at the periphery of the oxide patch where these species can insert into a Co
O bond is found to be compatible in energy with CHx directly on either Co or Co2C to within ∼5 kcal/mol, that is, thermally accessible at the 250 °C at which the catalytic reactions are performed (see Table 3). Hydrocarbon chain growth at the periphery on the oxide patch is energetically accessible, as typified by the energy to take a CH2 group from a Co substrate to the periphery of the oxide patch to form an OCH2 group, and subsequently couples to a second CH2 group residing on Co. This reaction scheme has a net exothermic energy of
8 and
29 kcal/mol on the Co and Co2C clusters, respectively, and energy barriers of ca. 25
29 kcal/mol. These barriers are similar in magnitude to those for methanation, and thus this process is expected to be kinetically competitive. These observations are in accord with the mechanistic route proposed by Wang et al.19 involving ethylene oxide intermediates. Another possible route for the formation of oxygenated hydrocarbons is to form a C
C bond via the coupling of CO and CHx, followed by subsequent hydrogenation steps.16 Therefore, it is necessary to consider the energy of the C
C coupling reaction

ðR5Þ

The calculated reaction energy for R5 is found to be
58 and
15 kcal/mol for x = 1,2 on the Co cluster. On Co2C, however, these same reaction energies are
2 and
5 kcal/mol, respectively, implying that on both phases this process is energetically

CHx þ CO f CHx CO

ðR6Þ √

The reaction energy ΔECHxCO and activation energy ΔE CHxCO of reactionR6 on Co and Co2C clusters are reported in Table 4 with selected reaction paths illustrated in Figure 13. Reaction R6 is endothermic by >20 kcal/mol with a relatively high barrier for x = 1
3 on Co. As such, this reaction is not expected to be probable, in agreement with the low alcohol selectivity for FT synthesis on Co. Formation of CHCO species at the periphery of the oxide patch, where CH is bound to a partially oxidized Co center, is mildly endothermic (ΔECHCO = 7 kcal/mol), with an activation energy of 19 kcal/mol. This reaction can be followed by a facile hydrogenations leading to alcohols with kinetics barriers compatible with methantion. For x = 2,3, the reaction barriers are on the order of 35
40 kcal/mol, which would render these species kinetically inaccessible. At the interface between the oxide patch and Co2C, ΔECHxCO is exothermic for x = 1,2,3, with barriers ranging from 10 to 20 kcal/mol; see Table 4.

Figure 12. Calculated energy landscape for hydrogenation of CH to CH4 on Co (black) and Co2C (red) clusters. Calculations performed on Co site on hexagonally closed-packed surface facet.

Table 4. Energies in kilocalories per mole of (R6) on Co and Co2C Clustersa reaction

reaction sites

reaction sites ΔECHxCO

CH + CO T CHCO

CH3 + CO T CH3CO a

ΔE

√

ΔE

CHx CO

√

CHx CO

ΔE

CHx CO

32

65

Co2C


14

26

7

19

Co2C/La2O4 interface


8

10

Co

20

61

Co2C


3

11

Co/La2O4 interface


8

29

Co2C/La2O4 interface


7

18

Co Co/La2O4 interface

69 14

137 34

Co2C Co2C/La2O4 interface

22
14

25 20

Co Co/La2O4 interface

CH2 + CO T CH2CO

√

See also Figure 13. 17449

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One can now speculate on what these results suggest would be a likely strategy for improving upon the performance of these catalysts. Clearly, increasing the Co2C/Co ratio in the catalyst samples appears to be the most likely avenue to enhance selectivity toward alcohols. This may be achievable by decreasing the Co particle size, which the current results suggest will enhance the thermodynamic driving force toward carbide formation. Alternately, one may try to synthesize particles, which will maximize the amount of interfacial boundary between La2O3 and Co/Co2C to exploit some of the low-energy reaction channels suggested by the current theoretical analysis.

’ AUTHOR INFORMATION Figure 13. Calculated energy landscape for CH + CO f CHCO. (a) Co at interface with oxide patch. (b) Co2C at interface with oxide patch. (c) On Co2C. Atom coloring scheme same as Figure 7.

These observations suggest that chemistry at the boundary between Co and La oxide may partially be responsible for the increased presence of oxygenated hydrocarbons in the ensemble of reaction products. What is also striking is that reaction R6 is energetically favorable solely on Co2C also with relatively low barriers ΔECHxCO = 11
26 kcal/mol, which implies that this route for forming oxygenated hydrocarbons is also plausible directly on Co2C. In summary, in accord with the current experimental reactivity studies, theoretical mechanistic studies have found that: (i) CO dissociation is favored on CO as opposed to Co2C in agreement with the pronounced reduction of Co conversion for catalysts with the highest Co2C/Co ratio. (ii) Hydrocarbon chain growth can occur on both Co and Co2C, but this process on the former is expected to be more energetically favorable in accord with the observation that longer chain alcohols and hydrocarbons are less prevalent as products on samples with less-metallic Co. (iii) Growth of oxygenated hydrocarbon chains can occur on Co2C as a result of C
C coupling reactions between CO and CHx species, which correlated with the observed selectivity reported in Section 3.2.1. (iv) La2O3 may also play a role in the formation of oxygenates. Stable CHx species can exist at the boundary between Co or Co2C and the oxide phase. These species can undergo kinetically facile C
C coupling reactions with either CO or CHx species, either of which could lead to oxygenated hydrocarbons.

4. CONCLUSIONS This combination of in situ and ex situ catalysis characterization, reactivity studies, and theoretical calculations presents a detailed picture of the role of La2O3 doping on Co FT catalysts. La2O3 promotes carbide formation. As observed experimentally, carbide formation is directly correlated with selectivity to higher alcohols in that the samples with the highest Co2C/Co ratios have the highest observed selectivity toward alcohols. There is theoretical evidence that La2O3 may also be influencing the chemistry of the FT process by creating low-energy reaction channels for C
C coupling at the boundary between oxide and Co substrate. Support effects are critical in that AC supports promote carbide formation, whereas oxides supports, such as alumina, inhibit it.

Corresponding Author

*(R.R.) Tel: (509) 372-6092. Fax: (509) 375-2644. E-mail: [email protected]. (Y.W.) Tel: (509) 371-6273. Fax: (509) 375-2644. E-mail: [email protected].

’ ACKNOWLEDGMENT We thank Sun Yangzhou (CNOOC) for valuable discussions. This work was supported by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Biomass program. Sample characterization and calculations were supported in part by a user proposal from the W. R. Wiley Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory (PNNL). The TR-XRD experiment was carried out at beamline X7B of the NSLS at Brookhaven National Laboratory. PNNL is operated for the U.S. DOE by Battelle Memorial Institute under Contract No. DE-AC05-76RLO 1830. Computational resources were provided by the Molecular Science Computing Facility (EMSL) and the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory. ’ REFERENCES (1) Spivey, J. J.; Egbebi, A. Chem. Soc. Rev. 2007, 36, 1514. (2) Subramani, V.; Gangwal, S. K. Energy Fuels 2008, 22, 814. (3) Antolini, E. J. Power Sources 2007, 170, 1. (4) Xu, C. W.; Wang, H.; Shen, P. K.; Jiang, S. P. Adv. Mater. 2007, 19, 4256. (5) Jiao, G. P.; Ding, Y. J.; Zhu, H. J.; Li, X. M.; Li, J. W.; Lin, R. H.; Dong, W. D.; Gong, L. F.; Pei, Y. P.; Lu, Y. Appl. Catal., A 2009, 364, 137. (6) Takeuchi, K.; Hanaoka, T.; Matsuzaki, T.; Sugi, Y.; Reinikainen, M. Appl. Catal. 1991, 73, 281. (7) Takeuchi, K.; Matsuzaki, T.; Arakawa, H.; Sugi, Y. Appl. Catal. 1985, 18, 325. (8) Takeuchi, K.; Matsuzaki, T.; Hanaoka, T. A.; Arakawa, H.; Sugi, Y.; Wei, K. J. Mol. Catal. 1989, 55, 361. (9) Matsuzaki, T.; Takeuchi, K.; Hanaoka, T.; Arawaka, H.; Sugi, Y. Appl. Catal., A 1993, 105, 159. (10) Sugi, Y.; Matsuzaki, T.; Arakawa, H.; Takeuchi, K.; Hanaoka, T. Stud. Surf. Sci. Catal. 1993, 77, 381. (11) Takeuchi, K.; Matsuzaki, T.; Arakawa, H.; Hanaoka, T.; Sugi, Y. Appl. Catal. 1989, 48, 149. (12) Volkova, G. G.; Yurieva, T. M.; Plyasova, L. M.; Naumova, M. I.; Zaikovskii, V. I. J. Mol. Catal. A: Chem. 2000, 158, 389. (13) Barrault, J.; Guilleminot, A.; Achard, J. C.; Paulboncour, V.; Percheronguegan, A. Appl. Catal. 1986, 21, 307. (14) Cai, Z.; Li, J. L.; Liew, K.; Hu, J. C. J. Mol. Catal. A: Chem. 2010, 330, 10. 17450

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