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Unexpected C-C Bond Cleavage Mechanism in Ethylene Combustion at Low Temperature: Origin and Implications Hai-Feng Wang, Dong Wang, Xiaohui Liu, Yanglong Guo, Guanzhong Lu, and Peijun Hu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00764 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 9, 2016
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Hai-Feng Wang,† Dong Wang,† Xiaohui Liu,† Yang-Long Guo,† Guan-Zhong Lu,*,† P. Hu*,†,‡ †
Key Laboratory for Advanced Materials, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai, 200237, China ‡ School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast, BT9 5AG, UK ABSTRACT: To achieve low-temperature C=C bond activation has been of interest in heterogeneous catalysis, and to understand the subject, i.e. establishing the mechanism and identifying the origin, is desirable. Herein, taking the CH2CH2 combustion on spinel Co3O4(110) as an example, we report a systematic investigation on the C-C bond breaking processes using first principles calculations. An unexpected pathway for C-C cracking, named as the valency-saturation-driven mechanism, is determined, and the high activity of Co3O4 in catalysing CH2CH2 combustion at low temperature is rationalized. More importantly, some basic C-C bond activation rules on metal oxides with isolated single-atom sites, which differ from the traditional metal catalysis with multi-atom active sites, are revealed. The understandings derived from this work may underpin the structure-activity relationship in oxide catalysis.
KEYWORDS: C-C bond activation, DFT, valency-saturation-driven mechanism, heterogeneous catalysis, Co3O4
To activate the C-C bond is one of the most fundamental processes in chemistry owing to its wide applications.1-6 In particular, searching for highly active catalysts for low-temperature hydrocarbon oxidation, which involves the essential C-C bond cleavage step, is of great importance in heterogeneous catalysis. However, efficient catalysts towards the total oxidation of hydrocarbon containing two carbons or above at low temperatures has rarely been reported. Interestingly, Hao and co-workers reported that mesoporous Co3O4(110) can exhibit considerable activity toward ethylene combustion at 0 ℃.1 Despite the known excellent activity for CO oxidation,7-9 it is very surprising that Co3O4 can crack the strong C=C bond and complete the CH2CH2 combustion at such a low temperature. Currently, the mechanism of C-C bond cleavage on metal oxides is still missing and the knowledge of metal oxide catalysis is very limited. Specifically, the general structure-activity relations of metal oxides in promoting C-C bond cleavage remains highly desired. Herein, we report a systematic theoretical investigation on the complex C-C bond breaking processes of CH2CH2 on Co3O4(110), aiming to reveal the activity origin of Co3O4(110) for the C-C bond breakage at low temperatures as well as the general C-C bond activation rules on metal oxides. Regarding the C-C bond cleavage, the dissociation of hydrocarbons on metal surfaces is usually accomplished by dehydrogenation to form unsaturated C2 species.2,3,10-13 For example, on Pt(111) it was found that the C-C bond cleavages of ethane and ethylene were achieved with a
barrier of 0.9 eV after dehydrogenation to form CHC species, while on Pt(211) the lowest barrier of C-C cleavage is 1.10 eV via CH3CH2 species.10 Because of the high barriers, both reactions are not expected to occur at low temperatures. In ethanol electro-oxidation, the C-C bond breakage was proposed to proceed through CH2CO → CH2+CO or CHCO → CH+CO on Pt(111) and Pt(211), or via the oxametallacyclic species CH2CH2OH with a barrier of 1.29 eV on ternary Rh/Pt/SnO2(110).12,13 However, on the transition-metal oxides, the insight into how the C=C bond is cleaved is rarely reported. In this work, we carried out extensive first principles calculations to investigate ethylene oxidation on Co3O4(110). It was found that, to the best of our knowledge, all the mechanisms reported in the literature cannot be used to explain the high activity of reaction on the surface at low temperature. Instead, an unexpected valency-saturation-driven mechanism, was identified. More importantly, a general insight into the CC cleavage mechanism on metal oxides and metals was quantitatively discussed in terms of catalyst structure and bonding properties, which could provide a fundamental understanding of metal oxides for the chemistry of C-C activation.
All the spin-polarized calculations have been performed with Perdew-Wang 91 functional14 using the VASP code.15,16 The project-augmented wave (PAW) method was used to represent the core-valence electron interaction. The valence electronic states were expanded in plane wave basis sets with energy cutoff at 450 eV. The
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Co3O4(110) surfaces were modeled as a p(11/2) periodic slab with nine atomic layers and the vacuum between slabs is ~15 Å . The bottom two layers are fixed, and all other atoms are fully relaxed. For these surface slabs, a 231 k-point mesh and a Gaussian smearing of 0.05 eV was used. For the bulk structure, 444 k-point mesh was used and the lattice constant of spinel Co3O4 is optimized to be 8.08 Å , agreeing well with the experimental result 8.05 Å .17 It is worth noting that, as the antiferromagnet, the stable spin configuration of Co3O4 suggested by Selloni et al.18 for the interior layers of the model slab was used. The transition states were searched using a constrained optimization scheme,19-21 and were verified when (i) all forces on atoms vanish; and (ii) the total energy is a maximum along the reaction coordination but a minimum with respect to the rest of the degrees of freedom. The force threshold for the optimization and transition state search was 0.05 eV/ Å . Our previous works9 confirm that the above settings can give reasonable results. Specifically, the PBE+U approach (U=2.0 eV for Co(3d) orbital) with the on-site Coulomb correction included was tested, which indicates that DFT can give the same trend as DFT+U in describing the CO oxidation on Co3O4(110)-B surface. Here, taking some key elementary reactions in the C-C bond cracking of CH2CH2 as example, we tested and compared the corresponding barriers given by DFTPW91 and PBE+ U (U=2.0 eV), as shown in Table S1. It shows that +U correction does not give different reaction picture as the common DFT, despite small differences in the quantitative value of the barriers. The details on the computational settings and model construction of Pt(111) are reported in the supporting information.
To model Co3O4(110), the most stable termination configuration was investigated with isolated fourcoordinated Co4c3+ in the octahedral sites and two- and three-coordinated lattice O exposed (denoted as O2c and O3c, respectively), which constitute the main active sites (see Figure 1).9 As the first step, the adsorption of CH2CH2 on Co3O4(110) was calculated. It is found that CH2CH2 prefers to adsorb at the Co3+ site in a π-adsorption configuration, and the chemisorption energy was as high as 1.18 eV (0.61 eV in terms of adsorption free energy considering the temperature effect), which is comparable with that (0.90 eV) on the metal Co(0001) surface at the C2H4 coverage of 1/4 ML.22 Then possible pathways for the direct C-C bond cleavage of C2H4 were considered, as shown in Scheme 1a. Firstly, the direct decomposition of C2H4 into two *CH2 species at the Co3+ site in a germinal adsorption configuration was found to be significantly endothermic by 3.53 eV and thus can be ruled out at low temperature. Alternatively, we examined the possibility of C-C bond cleavage through *CHCH3 configuration from inner-molecule 1,2-H shift of C2H4. It was found that the 1,2-H shift reaction is endothermic by 1.61 eV, and the decomposition of *CHCH3 into *CH and *CH3 requires an energy of 2.08 eV. Secondly, the C2H4 might approach the lattice O to form four-member ring species (denoted as OME), which is an
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Figure 1. (a) Scheme of ethylene combustion on Co3O4(110); (b) the surface structure (top view) of Co3O4(110), showing four-coordinated Co4c3+, the nearest and next-nearest neighboured two-coordinated O (NNO2c and NNN-O2c) and the three-coordinated O3c. Blue, green, red and pink balls indicate the surface and subsurface Co, surface and subsurface O, respectively. (c) the C2H4 adsorption structure and possible activation pathways. important immediate in ethylene partial oxidation by Ag catalyst,23 and decomposes with the help of lattice O. However, the formation of OME needs overcoming a high barrier of 1.10 eV and furthermore, the decomposition of OME into *CH2 and CH2O* (see Scheme 1a) is strongly endothermic (ΔE=2.78 eV). In addition, OME transformation into CH3CHO that may help C-C dissociation was also tested with a barrier above 2 eV, indicating that the OME involved approach to break the C-C bond is not easy at 0 ℃. From the above discussions, it is clear that the direct conversion of C2H4 into C1 species is difficult on Co3O4(110) at experimental temperature (0 ℃); to break the C-C bond, the activation of C-H bond appears to be a prerequisite. Thus, we investigated how the first C-H bond breaks on Co3O4(110), as illustrated in Scheme 1b, and three possible pathways were taken into account: (i) C2H4 dehydrogenation at Co3+; (ii) C2H4 dehydrogenation assisted by the nearest-neighbor lattice O2c (NN-O2c) or O2 species to form CHCH2 adsorbed at Co3+ and lattice OH or OOH; and (iii) C2H4 dehydrogenation with the help of the next nearest-neighbor lattice O2c (NNN-O2c). For pathway i, the reaction enthalpy is 1.86 eV, and thus it can be expected to be difficult with a high barrier (Ea >1.86 eV). C2H4 dehydrogenation with NN-O2c requires a barrier of 1.80 eV and the adsorbed lattice O2 species shows no evident enhancement in dissociating C-H bond (Ea =1.78 eV). In comparison, the NNN-O2c can attract H with a lower barrier of 1.53 eV, despite such a barrier is still too high for the reaction to occur at 0 ℃. Interestingly, when the Co3+ directly linking with the NNN-O2c was covered by C2H4, the barrier would be largely reduced to 0.62 eV. Considering the high coverage of C2H4 on Co3O4(110) due to its strong adsorption, such a dissociation mode for the first C-H bond of C2H4 would be feasible. The barrier reduction can be rationalized by the bond competing effect:
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as the Co3+ bonding with NNN-O2c adsorb strongly a C2H4 molecule, the bond strength of Co3+-O2c(NNN) would be considerably weakened, and thus the NNN-O2c would have stronger ability to accept H and promote C2H4 dehydrogenation.
Scheme 1. (a) Possible routes for the direct C-C bond cleavage of C2H4 without pre-activation of C-H bond. (b) Dehydrogenation mechanism of C2H4 into CH2CH assisted by Co3+, NN-O2c or NNN-O2c at low and high coverage of C2H4. (c) Indirect dissociation pathways for C-C cleavage via kinds of unsaturated CHxCHy species starting from CH2CH. Starting with CH2CH species from CH2CH2 dehydrogenation, the possible C-C bond cleavage reactions following the typical mechanisms on metal surfaces were systemically investigated, which corresponds to CHxCHy → CHx + CHy (0≤ x ≤2, 0≤ y < 2; see Scheme 1c). It shows that all these reactions are strongly endothermic on Co3O4(110). For example, the dissociation of CHCH2 into *CH and *CH2 is endothermic as high as 3.81 eV, while the dissociation pathway of CHC → C+CH favored on metal surfaces is endothermic by 4.61 eV. Therefore, these typical C-C bond cleavage pathways can be ruled out. Alternatively, CH2CH could dehydrogenate into CHCH with a reasonable barrier of 0.65 eV. However, CHCH still cannot break its C-C bond with a reaction energy of 5.67 eV. But interestingly, CH2CH can react with lattice O to form CH2CHO with a lower barrier; for the two-coordinated O2c, the barrier is 0.49 eV, and for the three-coordinated O3c, the barrier is as low as 0.24 eV. Therefore, CH2CH preferably couples with O3c rather than further dehydrogenation, which accords with the fact that Co3O4(110) exhibits good redox properties for CO oxidation.7-9 The higher reactivity of O3c versus O2c towards CH2CH is analogous with CO oxidation on Co3O4(110), and the origin can be traced to the larger bond competing effect in the transition state of O2c case.9 Regarding the formed CH2CHO, it can hardly decompose into CH2 and CHO at Co3+ site owing to the high reaction energy (0.92 eV). Instead, it could go on dehydrogenation following (i) CH2CHO +O2c → CHCHO +
O2cH or (ii) CH2CHO +O2c → CH2CO + O2cH. The transition states of these two reactions were located (see Figure S2), which give the barriers of 0.99 eV and 0.68 eV, respectively. Furthermore, the formation of CH2CO through path ii is exothermic by 0.34 eV and thus exhibits higher stability relative to CHCHO whose formation is endothermic (see Figure 2). Therefore, CH2CHO prefers to dehydrogenate acyl-H rather than α-H in CH2 group. In addition, CH2CO and CHCHO could further dehydrogenate to form CHCO species through breaking the α-H and acyl-H, respectively. The former has a lower barrier (0.52 eV) compared to the latter (0.73 eV). Overall, it can be seen from the energy profiles in Figure 2 that the conversion of CH2CHO to CHCO is favored to follow the acetyl pathway: CH2CHO → CH2CO → CHCO.
Scheme 2. Proposed oxidation-cracking mechanism for CH2CH conversion into CO2. The green arrows indicate the most favored reaction routes, and red and black ones are the less favored routes. Considering that CHCO still can hardly crack its C-C bond and is difficult to dehydrogenate owing to the high barrier over 0.80 eV, we estimated the possibility of CHCO oxidation. It is worth noting that, with the formation of CHCO, the oxygen vacancy at the O3c site will be formed and has to be refilled prior to further oxidation. We find that O2 can efficiently adsorb in the O vacancy with a side-on configuration, giving an adsorption energy of 2.64 eV. Significantly, CHCO can easily react with the adsorbed O2 to form CHOCO with a low barrier (0.30 eV, see Figure S2 for the structures of the transition state and CHOCO). Moreover, once formed, CHOCO can readily break its C-C bond into CHO* and CO* at the Co4c3+ site, requiring a barrier as low as 0.14 eV. Alternatively, it can also dehydrogenate into COCO with a barrier of 0.45 eV, which then decomposes spontaneously into 2CO*. Regarding the conversion of CHO and CO species, our previous work has demonstrated that CO can easily react with the lattice O to release CO2.9 For CHO, it can dehydrogenate at Co3+ site to form CO, or react with the lattice O to form formate (HCOO) which then dissociates to yield CO2. The calculations indicate that the dehydrogenation into CO is an easy and more favored process, corresponding to a low barrier (0.13 eV) in comparison to that of CHO reacting with O3c (0.50 eV). Overall, the formed C1 species, either CO or CHO, can readily be converted into CO2. Based on the above discussions, we propose the following overall reaction pathway of ethylene total oxidation on Co3O4(110) (Scheme 2). Firstly, the adsorbed ethylene dehydrogenates to form CH2CH assisted by the next-nearest
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neighbored O2c. Secondly, CH2CH reacts preferentially with the lattice O3c to produce CH2CHO, which then dehydrogenates twice to yield CHCO. Thirdly, CHCO is further oxidized to form CHOCO, which can easily break C-C bond to produce C1 species (CO and CHO). Finally, the C1 species are oxidized to CO2. One can see that the formation of oxygen-containing group resulting from oxidation reaction, instead of dehydrogenation into highly unsaturated C2 species, is crucially required to accomplish the C-C bond cleavage on Co3O4 at low temperature.
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Table 1. Decomposed data calculated for various dissociation reactions on Co3O4(110) surface. The unit is eV. Note: see the main text for the definitions of Ebond, Eads and Eco-ads. ΔH
Eads
Ebond
Eco-ads
CHxCHy → CHx + CHy CH3C→C+CH3
1.73
3.56
4.25
6.08
CH2C→C+CH2
2.79
1.74
7.72
6.66
CHC→C+CH
4.61
3.25
8.53
7.17
CH3CH→CH+CH3
2.08
2.71
4.70
5.33
CH2CH→CH+CH2
3.81
1.97
7.84
6.00
CHCH→CH+CH
5.67
1.12
10.87
6.32
CH2CH2→CH2+CH2
3.54
1.18
7.97
5.61
CCH2→C+CH2
2.79
1.74
7.72
6.66
CHxCHO → CHx + CHO CH3CHO→CH3+CHO
0.21
0.01
3.94
3.73
CH2CHO→CH2+CHO
1.60
2.40
4.12
4.92
CHCHO→CH+CHO
2.00
3.41
4.27
5.69
CCHO→C+CHO
0.38
2.09
4.50
6.21
CH3CO→CH3+CO
-0.09
1.92
1.13
3.15
CH2CO→CH2+CO
0.90
0.78
4.15
4.02
CHCO→CH+CO
2.90
2.14
4.31
4.31
COCHO→CO+CHO
-0.90
CHxCO → CHx + CO
Figure 2. Energy profiles of the conversion from CH2CH to CHCO following two different oxidationdehydrogenation pathways, as well as the conversion of CHCO into the CO (CHO) through oxidation-cracking mechanism. Inserts are the optimized structures of the key intermediate species such as CH2CHO, CH2CO, CHCHO, CHCO, COCHO and the decomposed products CO/CHO. Having obtained the above results, we are in the position to shed light on the principles for determining how the C-C bond breaks for species containing hydrocarbons or oxygenates. As shown in Table 1, we considered four kinds of reactions and analyzed their reaction enthalpies on Co3O4(110): (i) CHxCHy → CHx + CHy; (ii) CHxCHO → CHx + CHO; (iii) CHxCO → CHx + CO; and (iv) COCHO → CO + CHO. We can see that all the reactions which generate C1 fragments with the valence of C atom being lower than 3 are strongly endothermic. Approximately, the higher the unsaturation degree of C atom is, the more endothermic the reaction is. Quantitatively, the reaction energy depends on the C-C bond energy (Ebond) of C2 species and its adsorption energy (Eads) on Co3+, as well as the co-adsorption energy of C1 species after dissociation (Ecoads), i.e. ΔE = Ebond + Eads – Eco-ads. Generally, the larger bond order of C-C bond in C2 species corresponds to a stronger bond energy of C-C bond, despite of the larger adsorption energy of C2 species on the four-coordinated Co4c3+; meanwhile, the generated highly unsaturated C1 species are hard to be stabilized on the isolated Co4c3+ via forming a double or triple bond. In contrast, only the species CH3CO and COCHO are able to break their C-C bond exothermically, in which the produced C1 species CH3, CO or CHO can readily adsorb at Co4c3+ site to reach the valency saturation of C atom by forming Co-C single bond.24
COCHO → CO + CHO 1.55
0.83
3.28
On transition metal surfaces such as Pt(111), CHxCHy can dissociate into C1 species through the step CHC→ CH+C, which needs a moderate barrier (0.90 eV).10 By contrast, CHxCHy (0≤x, y≤2) cannot dissociate at the Co4c3+ site on Co3O4(110), which is very endothermic by at least 1 eV. What is the origin of the difference between the Co3O4 and Pt(111) surfaces to dissociate C2 hydrocarbons? For CHxCHy→CHx+CHy, the reaction energy depends largely on the adsorption energies of CHx and CHy; the stronger the surface binds with CHx and CHy, the more easily the reaction possibly occurs. To further understand the results, we investigated the adsorption energies of CHx (x=1~3), CHO and CO species on Co3O4(110) and Pt(111) (see Figure 3). We can see that the adsorption energies of C, CH and CH2 on Co3O4(110) are largely reduced in comparison with Pt(111); particularly for C and CH, their adsorption are weakened by ~2.5 eV. However, the adsorption energies of CH3, CHO and CO change by 0.38 eV at most. This result can be understood in terms of structural effect. On metal surfaces, C and CH prefers to occupy the hollow site and CH2 prefers to adsorb at the bridge site to achieve saturated configurations. On the other hand, for CH3, CHO and CO, as long as they adsorb on the top site, the C atom can be stabilized. By contrast, on Co3O4(110), the Co4c3+ cations were separated far away from each other and thus can be considered as being isolated, which can only consequently form a single-bond adsorption configuration. Therefore, C, CH and CH2 adsorbed at the isolated Co3+ site are expected to be unstable. The cleavage of C-C bond of C2 species at the single-
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atom metal or cation site at low temperature should be accomplished when the C atom in the dissociated C1 species is stabilized at a tetravalency, forming a closed outermost valence shell (named as the valency-saturation driven rule), which could be consistent with the homogenous catalysis under mild conditions. Following these rules, possible C2 species which can crack at low temperature are CH3CHO, CH3CO, COCHO and COCO.
species oxidation, and finally enhance the lowtemperature activity. This understanding may be used to explain why the nano-gold catalyst with high oxidative capacity was found to enhance the overall activity of CH2CH2 combustion on Co3O4 at low temperature.1
In summary, we reported a systematic investigation on the C-C bond cracking processes of CH2CH2 on Co3O4(110) using of DFT calculations. Different from the usual C-C activation processes on metal surfaces, an unexpected valence-driving mechanism (CHxCHy → → COCHO → CO+CHO), was identified. Moreover, a general insight into C-C cleavage on metal oxides and metals was quantitatively discussed in terms of catalyst structure and bond saturation of C at the atomic level. The results may provide a fundamental understanding of metal oxide catalysis, and the uncovered mechanism may assist the further catalyst design for C-C activation and oxidation.
Supporting Information. DFT calculation details, structures of key intermediates. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 3. Comparison of adsorption energies of various C1 species at the single-atom Co3+ on Co3O4(110) and multiatom Pt(111). The proposed oxidation-cracking mechanism may be of significance for designing the low-temperature catalysts for hydrocarbons (CnHm, n≥2) combustion. Considering that on many metal oxides the cations are isolated, if one wants to crack C-C bond at low temperature, C atom should be oxidized to be in the form of CHO or CO. This requirement demands actually that the oxides should possess excellent oxidative capacity to provide lattice O or easily activate the external O2 species, in addition to the strong binding ability of metal cations. Similar suggestions may also apply for the transition metal catalysts. For example, on Pt(111), the C-C bond cleavage of ethylene was accomplished via CCH dissociation, which corresponds to a barrier of 0.9 eV and thus limits the lowtemperature activity. If one is to decrease the dissociation barrier of C-CH bond, the earlier transition metals in the periodic table or Pt particles with smaller sizes, which possess larger adsorption ability and thus stronger bondcracking ability,25-28 are required, following the general BEP relation. However, the subsequent oxidation (C/CH +O → CO/CHO→CO2) would be hindered, owing to the strong binding ability on early metals.25,26,28 In fact, Pt is known the best metal catalyst for CO oxidation at medium-high temperature and at low temperature its catalytic activity is limited, resulting from the high barrier of the surface coupling reaction between CO and O.25,27 With the new C-C bond cleavage mechanism found in this work, we propose that decreasing the adsorption of catalysts, i.e. increasing the oxidation ability, may efficiently accomplish the C-C bond cracking and the subsequent C1
*
[email protected];
[email protected] This work was supported by 973 Program (2013CB933201), NSFC (21421004, 21303052, 21333003), Shanghai Rising-Star Program (14QA1401100) and Chen-Guang project (13CG24), Fundamental Research Funds for the Central Universities. HFW also thanks the Special Program for Applied Research on Super Computation of the NSFC-Guandong Joint Fund (the second phase) for computing time.
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