Victor
Fung
a,
Franklin (Feng)
Tao
b and
De-en
Jiang
*a
aDepartment of Chemistry, University of California, Riverside, CA 92521, USA. E-mail: de-en.jiang@ucr.edu; Tel: +1 951 827 4430
bDepartment of Chemical and Petroleum Engineering and Department of Chemistry, University of Kansas, Lawrence, KS 66045, USA
First published on 20th May 2016
Co3O4 is a metal oxide catalyst with weak, tunable M–O bonds promising for catalysis. Here, density functional theory (DFT) is used to study the oxidative dehydrogenation (ODH) of ethane on Co3O4 nanorods based on the preferred surface orientation (111) from the experimental electron-microscopy image. The pathway and energetics of the full catalytic cycle including the first and second C–H bond cleavages, hydroxyl clustering, water formation, and oxygen-site regeneration are determined. We find that both lattice O and Co may participate as active sites in the dehydrogenation, with the lattice-O pathway being favored. We identify the best ethane ODH pathway based on the overall energy profiles of several routes. We identify that water formation from the lattice oxygen has the highest energy barrier and is likely a rate-determining step. This work of the complete catalytic cycle of ethane ODH will allow further study into tuning the surface chemistry of Co3O4 nanorods for high selectivity of alkane ODH reactions.
A wide variety of catalytic materials have been studied for the ODH reaction: earth-abundant metal oxides3–18 appear as cost-effective and more viable alternatives to noble metal catalysts. In particular, spinel cobalt oxide (Co3O4) is a particularly promising candidate which has been found to be very active in many oxidation reactions including those of CO,19 CH4,20 and other hydrocarbons.21 A major factor in the catalytic activity of Co3O4 is the very weak M–O bond among other transition metal oxides22 and the presence of readily generated oxygen vacancies at the surface.20,23 Experimental and theoretical studies have identified a number of stable surfaces on the Co3O4 nanoparticles and nanorods, including the (100), (110) and (111) surfaces.19,21,24–32
Of these surfaces, the high activity of the (110) facet for reactions such as CO oxidation has been widely reported; however, the (111) surface has been found to be readily formed26,29 and more thermodynamically stable28 than the (110) surface. Therefore, the morphology of Co3O4 nanocatalysts may greatly impact their catalytic activity due to the difference in preferred surface orientations and it would be very interesting to understand the change in the reaction mechanism on those different surfaces. DFT has been employed in many previous studies on the surface properties and catalytic activity of Co3O4 that is thought to proceed via the Mars–van Krevelen mechanism involving the lattice oxygen on the surface.33–37 These active oxygen species react and desorb in products to form vacancies which must then be regenerated from gaseous molecular oxygen in order to maintain the catalytic activity of the surface.
Several recent works have shown that modifying the pure Co3O4 surface can improve its performance for alkane activation. These methods include doping transition metals such as Ni to facilitate the oxidation of alkanes20 and depositing nonmetallic elements such as Si and Ge onto the surface of Co3O4 nanorods to tune the selectivity of the catalyst towards the ethane ODH product.38 However, the underlying mechanism of how the modification of the Co3O4 surface by other elements changes the activity and selectivity of the nanocatalyst for ethane ODH remains unclear.
As a first step toward understanding the roles of morphology control and surface modification in Co3O4 nanocatalysts for ethane ODH, herein we present the detailed analysis of the full catalytic cycle from first principles based on the experimental determination of the preferred surface orientation of Co3O4 nanorods. We elucidate the nature of the two C–H activations to produce ethylene, the formation of water, and the regeneration of the oxygen sites to complete the catalytic cycle. These insights will provide a better understanding of the role of the different active surface atoms on a specific surface orientation in determining the selectivity and yield of the ethane ODH pathway that may allow one to design better Co3O4 nanocatalysts.
All DFT calculations were performed using the Vienna ab initio simulation package (VASP).40,41 The on-site Coulomb interaction was included using the DFT+U method by Dudarev et al.42 with a Hubbard parameter of U = 2 eV for Co, which yielded band gaps in good agreement with the experimental results and was used to good effect in previous DFT studies of Co3O4 surface chemistry and catalysis.19,20 The Perdew–Burke–Ernzerhof (PBE)43 form of the generalized-gradient approximation (GGA) was chosen for electron exchange and correlation. The electron–core interaction was described using the projector-augmented wave method (PAW).44,45 A kinetic energy cutoff of 450 eV was used for the planewaves, and the Brillouin zone was sampled using a 3 × 3 × 1 Monkhorst–Pack scheme.46 All calculations in this work were performed with spin polarization. The slab was created containing 11 layers with the bottom 9 layers frozen in their bulk positions. The optimized lattice constant was found to be 8.14 Å, in very good agreement with the experimental value of 8.09 Å.47
The adsorption energies were defined by Eads = Esurface+adsorbate − (Esurface + Eadsorbate), where the energy of the adsorbate Eadsorbate was computed by placing an adsorbate molecule in a cubic cell of 10 Å in side length (in other words, the molecule and its nearest periodic images are separated by about 10 Å vacuum). Transition states were found via a two-step approach, using both the nudged elastic band method (NEB) and the dimer method48 implemented in the VASP-VTST package by Henkelman et al.49 The initial and final states of the reactions were first identified and optimized, and were used to generate 8–10 equally spaced images between them through linear interpolation. These images were then minimized under the NEB constraints to forces of ∼0.4–0.2 eV Å−1 in order to generate an approximate minimum reaction energy path. The highest two energy images along the path were then used to generate a starting geometry for the dimer method, until reaching an energy convergence of 0.05 eV Å−1. Selected transition states were confirmed by vibrational frequency analysis to verify the effectiveness of this method and the convergence criterion.
The Co3O4(111) surface can be terminated into six inequivalent layers; we chose the one with the lowest surface energy based on previous DFT calculations.28 This surface model terminates to yield tetrahedral Co2+ atoms and triply coordinated oxygen atoms on the surface (Fig. 1C). The surface lattice oxygens can be categorized into two types: OA, which are triply coordinated to the subsurface cobalt (Co3+); OB, which are doubly coordinated to the subsurface Co but also singly coordinated to the surface Co (Fig. 1C). After relaxation of this Co3O4(111) surface termination, the surface Co–OB bonds shorten from 1.952 to 1.788 Å. The OA–Co bonds shorten from 1.932 to 1.885 Å. In predicting the reactivity of the two inequivalent lattice O atoms, we find that OA has a lower vacancy formation energy of 2.488 eV and a higher average Bader charge of −0.767 |e|. Meanwhile, OB has a vacancy formation energy of 2.946 eV and an average Bader charge of −0.860 |e|. The average Bader charge of the surface Co was found to be 1.298 |e|. The lower OA vacancy formation energy suggests that the OA oxygen is more active and more likely to react to result in a vacancy.
Both OA and OB vacancies lead to minimal surface rearrangement, with the OA vacancy leading to the least change in surface bond lengths and the OB vacancy leading to a shortening of the remaining two Co–OB bonds from 1.788 to 1.769 Å. These results show that the Co3O4(111) surface can readily generate and refill oxygen vacancies without loss of surface structure, suggesting robustness and stability consistent with experimental and theoretical findings,26,28,50 and making it a good candidate as the model for the catalytic surface.
The less negative Bader charge of OA than that of OB indicates the increased ability of the OA oxygen to further gain electrons, so OA is more electrophilic than OB and thus is the more favored active site for the homolytic fission of the C–H bond in the ethane ODH reaction. The more negative Bader charge of OB suggests that it is more nucleophilic in character and more suitable for the heterolytic fission of the C–H bond as part of the metal–oxygen redox pair, where the metal cation Co2+ participates as the electrophile. Both the homolytic and heterolytic C–H cleavages will be examined next.
We examined both the heterolytic and homolytic pathways on the Co3O4(111) surface. Based on the configuration of the surface cobalt and oxygen, there should be five possible final states. Starting with weakly physisorbed C2H6 on the surface, we found the minimum energy paths and transition states leading to the five final states. These transition and final states are illustrated in Fig. 2, while the energy profiles are shown in Fig. 3. For the heterolytic activation (reactions 1A and B), the transition states were found to form a Co–C2H5–H–O complex. The barrier for the pathway via OB is 1.02 eV, lower than that via OA (1.11 eV), likely due to the fact that OA is slightly farther from Co than OB, which results in a less stable transition state complex. For the homolytic activation (reactions 1C–E), the transition states show the formation of a CH3CH2–H–O complex where the C–H–O angle is almost 180 degrees, while in the final states, the ethyl group attaches to another nearby lattice O. Reaction 1C is activated by the OA oxygen with a barrier of 0.62 eV, whereas reactions 1D and 1E are both activated by the OB oxygen with a similar barrier of 0.98 to 1.03 eV. The stronger oxidizing power of OA than that of OB, as indicated by the charge analysis, corroborates the lower activation energy of the first C–H activation on OA.
Fig. 2 Transition and final states for the first C–H activation on Co3O4(111). Reactions 1A and 1B are Co2+-activated heterolytic reactions. Reactions 1C–E are O-activated homolytic reactions. |
Fig. 3 Energy profiles of the first C–H activation. Reactions 1A and 1B are Co-activated, whereas reactions 1C–1E are O-activated. |
The energy barriers (Ea) and reaction energies (ΔE) of both the heterolytic and homolytic pathways of 1A–E are compared in Table 1, together with those of propane from another study.21 We find that our calculated barriers are in very good agreement with the propane barriers for the same surface. It is apparent that the homolytic pathway is both the thermodynamically and kinetically favored reaction path due to the overall lower barriers as well as the more negative reaction energies. The OA-catalyzed pathway has the lowest barrier by far with a value of 0.62 eV. These results are also in good agreement with the propane DFT results and consistent with the higher bond dissociation energy of the ethane C–H bond.
No. | Reaction pathway | Ethane Ea (eV) | Ethane ΔE (eV) | Propane Ea (eV) (ref. 21) | Propane ΔE (eV) (ref. 21) |
---|---|---|---|---|---|
1A | C2H6(g) → C2H5–Co2+ + OB–H | 1.02 | −0.47 | 0.95 | −0.71 |
1B | C2H6(g) → C2H5–Co2+ + OA–H | 1.11 | −0.33 | N/A | N/A |
1C | C2H6(g) → C2H5–OB + OA–H | 0.62 | −1.07 | 0.41 | −1.41 |
1D | C2H6(g) → C2H5–OB + OB–H | 1.03 | −0.75 | 0.60 | −0.80 |
1E | C2H6(g) → C2H5–OA + OB–H | 0.98 | −1.27 | N/A | N/A |
Fig. 5 Energy profiles of the second C–H activation. Reactions 2A–2C follow the 1A path. Reactions 2D–2F follow the 1C path. |
The barriers and reaction energies of the second C–H activation are compared in Table 2. Most notably, the second C–H activation following the heterolytic pathway (1A then 2C) appears to have lower barriers than that following the homolytic pathway (1C then 2D or 2E). This result is also consistent with the DFT study of propane on the Co3O4(110) surface.21 Reactions 2A, 2C, and 2D via β-hydrogen elimination all lead to ethylene formation. In the case of 2A and 2C, we found that desorption of C2H4 is uphill by about 1.0 eV, while in the case of 2D, C2H4 was found to spontaneously go to the gas phase. Reactions 2B, 2E, and 2F via α-hydrogen elimination generate CH3CH which were found to bind very strongly to the surface Co or O with a strength greater than 2 eV. We expect that the CH3CH intermediate is unlikely to result in ethylene production and more likely to be part of the further oxidation pathways.
No. | Reaction pathway | E a (eV) | ΔE (eV) |
---|---|---|---|
2A | C2H5–Co2+ + OB–H → CH2CH2–Co2+ + 2OB–H | 0.87 | −1.08 |
2B | C2H5–Co2+ + OB–H → CH3CH–Co2+ + 2OB–H | 1.76 | 0.50 |
2C | C2H5–Co2+ + OB–H → CH2CH2–Co2+ + OA–H + OB–H | 0.39 | −1.02 |
2D | C2H5–OB + OA–H → CH2CH2–OB + 2OA–H | 1.26 | 0.07 |
2E | C2H5–OB + OA–H → CH3CH–OB + OA–H + OB–H | 1.26 | 0.33 |
2F | C2H5–OB + OA–H → CH3CH–OB + H–OA–H | 2.47 | 2.34 |
Fig. 6 Initial, transition and final states for hydrogen diffusion across the Co3O4(111) surface: A, diffusion of hydrogen from OA to OB; B, diffusion of hydrogen from OB to an OA–H hydroxyl group. |
After the H–O–H species is formed on the surface (Fig. 6B), it can then desorb to the gas phase. The desorption energy was found to be 0.581 eV, much lower than the lattice-oxygen vacancy formation energy (2.488 eV for the OA atom). Indeed, the addition of two hydrogen atoms on the same lattice oxygen destabilizes the system and lengthens the Co–O bonds. Oxygen vacancies are then formed following the desorption of water.
Fig. 8 Lower panel: The overall energy profile of ODH of ethane to ethylene on Co3O4(111); upper panel: structures (with key distances) corresponding to the transition states a–f in the energy profile (Co, blue; O, red; C, gray; H, white). In the energy profile, the C–H cleavages follow the 1C–2D homolytic path (red) or the 1A–2C heterolytic path (black) with energetics from Fig. 2–5, while the hydrogen diffusion and water formation energies are from Fig. 6. |
A schematic drawing of the complete catalytic cycle for the homolytic path is shown in Fig. 9, composed of the first and second C–H activations, hydrogen diffusion, water formation, and lattice-oxygen regeneration. It starts with C2H6 adsorption, followed by the first C–H cleavage on lattice OA. Then, the second C–H cleavage takes place on another lattice OA, leading to the formation and desorption of C2H4. Next, hydroxyl clusters via hydrogen diffusion and water forms from nearby hydroxyl groups. Water desorbs and leaves lattice-oxygen vacancies, which are refilled by molecular oxygen. The cycle repeats.
Next, we have found the vacancy formation energy and Bader charge of the lattice oxygen to be good descriptors of the catalytic activity of the lattice oxygen. We were able to predict the OA oxygen to have lower barriers for C–H activation than the OB oxygen. We would expect this trend to hold for lattice oxygen atoms of different Co3O4 surfaces, such as (110), (100), and (112). These surfaces could be important for Co3O4 nanocatalysts with different morphologies.
We find that the water formation step of the catalytic cycle on the pure (111) surface appears to be the RDS, while experiments show that changing the alkane will change the overall barriers,54 thus suggesting that the C–H activation barrier is significant to the overall rates. Based on preliminary DFT results, we believe that the hydroxylation of the surface will affect the energy profile by increasing the barriers of C–H activation and reducing the barriers of H2O formation. In this model, we can then expect that the actual experimental energy profile will have the C–H activation and water formation barriers approach in energy at a steady-state hydrogen coverage of the surface. Thus, varying C–H bond strength would in fact affect the rates of the overall ODH reaction.
Experimental findings have indeed shown that surface hydroxylation occurs on the surfaces of metal oxides as the dominant species during and after a reaction.55,56 In addition, water formation and consequently water concentration were found to affect the rates of the ODH/oxidation reactions on many studied metal oxides.55,57–59 This phenomenon occurs for CO and CH4 oxidation on Co3O4 as well.56,60,61 These experimental findings support our conclusion that water formation is an important step of ethane ODH on Co3O4(111). This phenomenon is notably absent in CO oxidation, which is consistent with experimental observations of CO oxidations having much lower barriers and reaction temperatures than alkane oxidation. The coverage effect of surface hydroxyl groups on the catalytic pathways thus warrants further studies.
The experimental work on ethane ODH on Co3O4 nanorods38 has focused on ethylene/CO2 selectivity and ethylene yield as a function of surface modification by Si and Ge oxides. Detailed kinetic data such as reaction rates and apparent activation energy on the unmodified Co3O4 nanorods are still not available for a direct comparison. To compare with the experimental ethylene/CO2 selectivity, the complete pathway of ethane combustion has to be mapped out, which requires substantial and deeper investigation and is beyond the scope of the present work. A joint experimental and computational study is expected in the near future to directly compare the computational results and the experimental data.
The present DFT study focuses on the (111) surface based on the STEM images of our Co3O4 nanorod sample (Fig. 1), but other facets may also exist such as the (100) and (110) surfaces depending upon the preparation methods and calcination temperature. The different facets can affect the energetics of the elementary steps because the oxygen reactivity can be different. For example, the (110)-B surface of Co3O4 has a twofold-coordinate oxygen site, in contrast with the threefold-coordinate oxygen sites on the (111) surface. How this twofold-coordinate oxygen site on the (110)-B surface would change the energetics of ethane ODH steps is a question that we intend to find out in a future study.
This journal is © The Royal Society of Chemistry 2016 |