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A theoretical study on the role of water and its derivatives in acetic acid steam reforming on Ni(111)
Applied Surface Science 419 (2017) 114–125
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A theoretical study on the role of water and its derivatives in acetic acid steam reforming on Ni(111) Zhen-Yi Du a,b , Yan-Xiong Ran a , Yun-Peng Guo a , Jie Feng a , Wen-Ying Li a,∗ a Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Training Base of State Key Laboratory of Coal Science and Technology Jointly Constructed by Shanxi Province and Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan 030024, PR China b Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 19 March 2017 Accepted 1 May 2017 Available online 2 May 2017 Keywords: Acetic acid Steam reforming DFT calculations Solvent effect Dehydrogenation
a b s t r a c t Catalytic steam reforming of acetic acid can be divided into two steps, i.e. acetic acid decomposition followed by water gas shift. While theoretical studies have been devoted to these two individual reactions, the role of water and its derivatives in the reforming process, especially in CH3 COOH decomposition, remains largely unknown. In this study, a thorough investigation of the effects of the solvent water and its derived O*/OH* species on some key dehydrogenation steps on Ni(111) is carried out using density functional theory. The involved dehydrogenation species include O−H bond scission species H2 O*, CH3 COOH*, trans-COOH* and C−H bond scission species CH3 CO*, CH3 C*, CH2 C*. The results show that the pre-adsorbed O*, OH*, and H2 O* species not only affect the adsorption stability of these species, but also influence their dehydrogenation reactivity. O* and OH* species can both enhance the O−H bond scission, and the promotional effect of O* is superior to OH*. Nevertheless, H-abstraction from C−H bond by O* and OH* are both hindered except for CH3 CO* dehydrogenation in the presence of OH*. Furthermore, the solvent water notably weakens O−H bonds, yet exhibits negligible effect on the C−H bond breakage. Analogously, the solvent effect of CH3 COOH* on O−H bond scission is also investigated. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With the increasing demand for energy and growing awareness of environmental protection, the sustainable development of renewable biomass energy has attracted considerable attention in recent years. Bio-oil produced via biomass pyrolysis is highly oxygenated and requires deep hydrogenation before being used as transportation fuels. Thus large amount of hydrogen is needed and the hydrogen source becomes a research of interest [1,2]. Catalytic steam reforming of bio-oil itself can be a sustainable hydrogen source, because this process does not rely on the traditional fossil fuels as the starting materials [3–5]. Acetic acid is widely used as the model compound for studying bio-oil steam reforming, as it is one of the most abundant compounds and contains various functional groups that can represent the properties of bio-oil [6–9]. Nickel-based catalysts have been widely used in acetic acid steam reforming experimentally for its low cost, high stability and activity, and high hydrogen selectivity [10–13]. High hydrogen selectivity is always the ultimate goal of acetic acid steam reform-
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (W.-Y. Li). http://dx.doi.org/10.1016/j.apsusc.2017.05.003 0169-4332/© 2017 Elsevier B.V. All rights reserved.
ing. In fact, the efficiency of the hydrogen production depends on the activation of C−H and O−H bonds. Generally speaking, acetic acid steam reforming (1) can be basically divided into the following two reactions, i.e. acetic acid decomposition (2) and water gas shift reaction (WGS) (3) [14]: CH3 COOH + 2H2 O → 2CO2 + 4H2 ;Ho298k = 131.50 kJ/mol CH3 COOH → 2CO + CO + H2 O →
2H2 ;Ho298k
CO2 + H2 ;Ho298k
= 213.76 kJ/mol
= −41.13 kJ/mol
(1) (2) (3)
From the total reaction Eq. (1), it is clearly understood that hydrogen originates from acetic acid and water via acetic acid decomposition and WGS, respectively. Meanwhile, two hydrogenproducing routes yield the equivalent amount of objective product H2 , indicating that the two paths are equally important. In Eqs. (2) and (3), water seems to be involved only in the WGS reaction, but unrelated to acetic acid decomposition. In fact, the role of water in the reforming system is of great significance. On the one hand, water is a key reactant, and based on our previous calculations [15], water derived O* and OH* species indeed play a crucial role in carbon elimination and WGS. On the other hand, water itself is fundamentally a solvent, where the solvent effect on the dissociation pathways can not be neglected.
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Regarding the effects of water derived active oxygen-containing species (O* and OH*) on dehydrogenation reactions, O* and OH* species commonly participate in the H-abstraction steps directly, yielding co-adsorbed OH* and H2 O*, respectively. Huang et al. [16] reported that in the presence of O* and OH* species, three dehydrogenation reactions are almost consistently promoted. In methanol steam reforming on Cu catalysts [17,18], the presence of co-adsorbed O* and OH* species not only altered the reaction pathways, but also decreased the activation barriers of some dehydrogenation steps. However, Syu et al. [19] found that surface oxygen species slightly elevated the reaction barriers of ethanol decomposition on Rh(111), either for O−H or C−H bond cleavage. Thus, the influence of water derived O* and OH* species on acetic acid decomposition remains ambiguous and worth studying in depth. To our best knowledge, the role of water and its derivatives in acetic acid decomposition, especially in dehydrogenation steps, has not been systematically studied. Chang et al. [20–22] have done a series of work on the water promotion mechanism in heterogeneous catalysis. In methanol steam reforming on PdZn(111) [21], they pointed out that the solvent effect of water not only improved the adsorption stability of reaction species, but also reduced the activation energies of six dehydrogenation steps involving both O−H and C−H bond scission. Boucher et al. [23] revealed that the addition of hydrogen-bonded water complexes facilitated the O−H bond cleavage on copper catalysts, thus resulting in the conversion of methanol to formaldehyde. Nevertheless, the catalytic role of water for dehydrogenation steps has not reached a consensus. Density functional theory (DFT) calculations showed that with the presence of co-adsorbed water molecule on Rh(111), the O−H bond breakage of alcohols was favored, whereas the C−H bond breakage was inhibited [24]. The work by Zaffran et al. [25] studying polyalcohol dehydrogenation on several transition metal catalysts in the solvent water environment came to a similar conclusion. Furthermore, water just acts as a spectator without being involved in the H-abstraction reactions. The promotional role of water in dehydrogenation pathways is typically ascribed to the hydrogen bonding interaction with the dehydrogenation species. In order to unravel the role of water on acetic acid decomposition on Ni(111), the effects of water derived O* and OH* species and the solvent water molecule on the dehydrogenation reactions are all considered in this work. Our previous study [15] indicated that two competitive decomposition pathways of acetic acid exist on Ni(111), that is, the ketene formation path: CH3 COOH* → CH3 COO* → CH3 CO* → CH2 CO* → CH2 * + CO* → CH*, and CH3 C formation path: CH3 COOH* → CH3 COO* → CH3 CO* → CH3 C* → CH2 C* → CHC*, both followed with WGS: CO* + OH* → cisCOOH* → trans-COOH* → CO2 * + H*. Therefore, we choose six key dehydrogenation routes of H2 O*, CH3 COOH*, trans-COOH* and CH3 CO*, CH3 C*, CH2 C* to represent O−H bond and C−H bond breakage, respectively. To further explore the solvent effect on dehydrogenation reactions, the solvent effect of acetic acid on H-abstraction from O−H bond is also investigated. This work provides theoretical insights in how the solvents, water and acetic acid, affect acetic acid decomposition, and helps to gain an indepth understanding of acetic acid steam reforming on Ni-based catalysts.
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Table 1 Adsorption energies Ead /eV of the dehydrogenation species on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces. Species
H2 O CH3 COOH trans-COOH CH3 CO CH3 C CH2 C
Ead /eV Ni(111)
O/Ni(111)
OH/Ni(111)
H2 O/Ni(111)
−0.40 −0.41 −2.27 −2.10 −5.66 −4.22
−0.58 −0.36 −2.15 −2.07 −5.66 −4.18
– −0.29 −2.44 −2.02 −5.38 −4.01
−0.63 −0.45 −2.46 −2.19 −5.51 −4.23
[28,29]. The localized double numerical basis set combined with polarization functions (DNP) is employed for all the atoms to expand the Kohn-Sham orbitals. DFT semi-core pseudopots (DSPP) [30] is applied to treat the core electrons of metal atoms. An orbital cutoff of 4.5 Å and a thermal smearing width of 0.001 hartree (Ha) are adopted with the total energies extrapolated to 0 K. A Ni(111) surface is modeled by a periodic 4 × 4 supercells, three-layer slab with a vacuum thickness of 20 Å. The top two layers together with the adsorbates are free to relax, and the bottom layer is frozen. A Monkhorst-Pack k-point grid of 3 × 3 × 1 is sampled for the Brillouin zone integration [31]. Considering the magnetic property of metallic Ni and the unsaturated intermediates, spin-polarized calculations are performed. The convergence criterion of total energy, maximum displacement, and maximum force tolerances are set to the values of 2 × 10−5 Ha, 5 × 10−3 Å, and 4 × 10−3 Ha/Å, respectively. The radical species in gas phase are calculated in 20 × 20 × 20 Å3 cubic boxes. The adsorption energy Ead of an adsorbate is calculated as follows: For an isolated adsorbate adsorption, Ead = Eadsorbate/slab − Eslab − Eadsorbate For the co-adsorption of adsorbate and A (O, OH, or H2 O), Ead = E(adsorbate+A)/slab − EA/slab − Eadsorbate where Eadsorbate/slab and E(adsorbate+A/slab) represent the total energies of the adsorbate on clean and O*, OH*, or H2 O* pre-adsorbed Ni(111) surface, respectively; Eslab and EA/slab are the energies of clean Ni(111) surface and O*, OH*, or H2 O* pre-adsorbed Ni(111) surface, respectively; Eadsorbate means the energy of radical species in gas phase. By these definitions, a negative Ead value refers to an exothermic process. More negative Ead values represent stronger adsorption states on the Ni(111) surface. For an elementary reaction, the transition state (TS) is optimized by using the complete linear synchronous transit (LST) and quadratic synchronous transit (QST) approach [32,33]. The activation energy Ea is defined as the energy difference between the TS and the initial state (IS), and the reaction enthalpy change H is the energy difference between the final state (FS) and the IS.
3. Results and discussion 2. Computational methods All periodic DFT calculations are carried out by using Dmol3 code [26,27] as implemented in the Materials Studio program package (version 5.5) from Accelrys. The electron exchange and correlation function are calculated within the generalized gradient approximation (GGA) in the form of Perde-Burke-Ernzerhof (PBE) functional
The most stable adsorption configurations of H2 O, CH3 COOH, CH3 CO, CH3 C, CH2 C, and trans-COOH on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces are investigated. The corresponding adsorption energies are displayed in Table 1. To explore the regularity of dehydrogenation reactions, two dehydrogenation reaction modes are investigated during acetic acid steam reforming, i.e., H-abstraction from O−H bond and C−H bond.
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Fig. 1. H-abstraction from O−H bond of water on the clean Ni(111), O/Ni(111) and H2 O/Ni(111) surfaces.
3.1. Effects of water and its derivatives on the H-abstraction from O−H bond 3.1.1. H-abstraction from the O−H bond of water on Ni(111) Water is the starting reactant in acetic acid steam reforming, and therefore the adsorption and dissociation of water molecule with the presence of O*, OH* species and co-adsorbed water on the Ni(111) surface are investigated (Fig. 1). As shown in Fig. 1a (IS1-1), an isolated water molecule is weakly adsorbed at the top site through O atom on the Ni(111) surface with an adsorption energy of −0.40 eV. For the water decomposition to yield OH* and deep dissociation product O* species, the calculated activation energies are 0.97 eV and 1.19 eV, respectively, and both reactions exhibit the slight exothermic property. The lower activation barrier of OH* formation indicates that OH* species is easier to be formed than O* species on the clean Ni(111) surface (Fig. 1b), which is consistent with the reported results [34]. On the O/Ni(111) surface, hydrogen bond is formed between the co-adsorbed O atom and the H atom of water with an O···H distance of 1.91 Å, which is in the typical range of moderate hydrogen bonds (1.5–2.2 Å) [35], leading to a strong interaction between water and the metallic surface, as O−Ni bond length is shortened from 2.25 Å (IS1-1) to 2.16 Å (IS1-3) in Fig. 1c. Therefore, water adsorbs more stably on the O/Ni(111) surface (−0.58 eV) than the clean Ni(111) surface (−0.40 eV). For water dehydrogenation in the presence of O* species, an adjacent hydrogen of water moves to the co-adsorbed O* atom with two OH* species generated. Compared with single water dehydrogenation, the activation energy of this process dramatically decreases to 0.66 eV, whereas the reaction energy is inversely changed to an endothermic energy of 0.38 eV.
When water adsorbs on the H2 O/Ni(111) surface, as shown in Fig. 1d (IS1-4), an additional water molecule can not be adsorbed. Consequently, a visualized hydrogen bond interaction between two water molecules is constructed on the H2 O/Ni(111) surface. The distance of the O1 −Ni bond and the hydrogen bond O2 . . .H are calculated to be 2.10 Å and 1.72 Å, respectively (IS1-4), both of which are shorter than those on the O/Ni(111) surface. This strong hydrogen bonding interaction results in a higher binding energy of −0.63 eV (in absolute value) and an activation of O−H bond in water, which is elongated from 0.97 Å in the gas phase to 1.01 Å in IS1-4. Therefore, in this case, the activation barrier is lowered by 0.15 eV than the single-water dissociation. The adjacent water just acts as a spectator without being involved in the dehydrogenation reaction, from which we observe that hydrogen bonding interaction indeed plays a catalytic role in O−H bond cleavage of water. In brief, for water decomposition on Ni(111), OH* and O* species are formed, and OH* species takes up the major part, as OH* is more favorable to be produced than O*, particularly in the O- and H2 O-assisted situations. 3.1.2. H-abstraction from the O−H bond of acetic acid on Ni(111) For the decomposition route of acetic acid on Ni(111), our previous work [15] thoroughly investigated five different decomposition pathways, and the results show that the dehydrogenation of hydroxyl is both kinetically and thermodynamically preferred. In this section, H-abstraction from the O−H bond in CH3 COOH with the aid of O*, OH*, and H2 O* are addressed (Fig. 2). As displayed in Fig. 2a, CH3 COOH* sitting at the top site of Ni(111) via carbonyl O atom (IS2-1) decomposes into adsorbed
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Fig. 2. H-abstraction from the O−H bond of acetic acid on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces.
CH3 COO* and H. The calculated activation barrier and reaction energy is 0.45 eV and −0.43 eV, respectively. The breaking O−H bond distance is stretched from 1.01 Å in IS2-1 to 1.75 Å in TS21, and finally the atomic H* is located at the neighboring fcc site. On the O/Ni(111) surface, the hydroxyl group of CH3 COOH* slightly tilts towards the atomic O*, forming a short hydrogen bond of 1.62 Å (IS2-2). The atomic O* can abstract the hydrogen atom from the hydroxyl to yield OH*, along with the co-adsorbed CH3 COO* generated (Fig. 2b). According to the computational results, the activation energy of this reaction (0.28 eV) is reduced by 38%, revealing that O−H bond scission by O* species is kinetically more favorable than its direct decomposition path. As stated above, OH* is the most abundant species in water decomposition on the Ni(111) surface. Thus, the effects of coadsorbed OH* species on the O−H bond breakage of CH3 COOH* is explored as shown in Fig. 2c. The reaction begins with the co-adsorption of CH3 COOH* and OH* (IS2-3), followed with the co-adsorbed OH* species interacting with the hydrogen atom of the hydroxyl in CH3 COOH* to generate adsorbed H2 O*. It is worth
noting that the adsorption (−0.29 eV) is somewhat weaker than an isolated CH3 COOH*. This OH* assisted O−H bond cleavage reaction needs to overcome a 0.37 eV energy barrier, which is lower than that of the direct dehydrogenation, but a little higher than that of O* addition. Therefore, it can be deduced that OH* species can also activate the H-abstraction from the O−H bond. In the presence of co-adsorbed H2 O*, the calculated adsorption energy of CH3 COOH* is found to be −0.63 eV, higher than the case without co-adsorbed H2 O* (in absolute value), indicating that the interaction between CH3 COOH* and the Ni slab is enhanced with the presence of water. This can be ascribed to the hydrogen bonding interaction between the co-adsorbed H2 O* and CH3 COOH*. As shown in IS2-4 of Fig. 2d, the two species are likely to form a CH3 COOH. . .H2 O* complex via the hydrogen bond with the length of 1.66 Å. Theoretically, it is likely for the H atom in water to interact with the hydroxyl oxygen of CH3 COOH*, nevertheless, the optimized configuration proves to be unstable and thus is not investigated, which is in accordance with the case of HCOOH* [21]. For the O−H bond scission of CH3 COOH*, water is not involved
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Fig. 3. H-abstraction from O−H bond of trans-COOH on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces.
and keeps intact as it is in the dehydrogenation step, however, the activation energy is decreased to the same degree as that assisted by the co-adsorbed O*. Furthermore, the presence of water favors the dissociation of O−H bond thermodynamically with the reaction energy of −0.62 eV. In addition, the hydrogen bond exists through the overall dehydrogenation process. As stated above, it is obvious that the presence of water not only stabilizes the adsorption of CH3 COOH*, but also activates the O−H bond of acetic acid due to the hydrogen bonding interaction. 3.1.3. H-abstraction from the O−H bond of trans-COOH on Ni(111) Water gas shift reaction is a crucial stage in acetic acid steam reforming, and determines the reaction rate of the overall steam reforming process. Our reported results [15] have illustrated the mechanism of this process on Ni(111), that is, CO* + OH* → cisCOOH* → trans-COOH* → CO2 * + H*, where the combination of CO* and the enriched OH*species is the rate determining step and the decomposition of trans-COOH* is the key step to generate hydrogen atom and finally obtain the desired hydrogen product. Therefore, trans-COOH* dehydrogenation via the O−H bond cleavage in the presence of O*, OH*, and H2 O* on Ni(111) is carried out as well and the results are shown in Fig. 3. The optimized structure of an isolated trans-COOH* on Ni(111) is displayed in IS3-1. The carbonyl O and C atoms are situated at the top and bridge sites, respectively, and the hydroxyl points towards the catalytic Ni surface. This trans configuration is more energetically feasible for O−H bond scission than the cis configuration, due
to its smaller steric hindrance. The activation energy is 1.06 eV and the reaction is nearly energetically neutral with an enthalpy change of 0.02 eV. Finally, as shown in FS3-1, the produced CO2 * forms a bent structure and H atom binds at the fcc site on the Ni(111) surface. On the O/Ni(111) surface, the adsorption configuration of transCOOH* changes in the presence of co-adsorbed O* species. By a comparison between IS3-1 and IS3-2 in Fig. 3, the carbonyl C atom moves from the bridge site to the top site when an adjacent O* is coadsorbed. This can be attributed to the strong interaction between the two species by forming a hydrogen bond via the hydrogen atom in trans-COOH* and the co-adsorbed O atom. The hydrogen bond length is 1.90 Å, within the scope of typical hydrogen bond length. In addition, the activation barrier is dramatically reduced to 0.33 eV, revealing a significant promoting role of O* species on hydroxyl dehydrogenation. In the TS3-2 structure, the hydrogen atom is shared by the hydroxyl O atom and the co-adsorbed O*, where trans-COOH* serves as a H-donor and atomic O* acts as a Hacceptor. The breaking O−H bond is elongated from 0.99 Å in IS3-2 to 1.20 Å in TS3-2. The hydrogen atom moves to the atomic oxygen and then an OH* is formed with O−H inclined to the oxygen of the co-adsorbed CO2 *. Similar to the O* species, the OH* species is also reactive in abstracting the hydrogen atom from trans-COOH* to produce CO2 * and H2 O*, and the reaction path is shown in Fig. 3c. According to our calculation results, the co-adsorbed OH* species not only affects the adsorption of trans-COOH*, but also plays a significantly role in the dehydrogenation step. On the one hand, trans-COOH* inter-
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Fig. 4. H-abstraction from C−H bond of CH3 CO on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces.
acts stronger with the Ni(111) surface with an adsorption energy of −2.44 eV compared with that of −2.27 eV in the case without co-adsorbed OH*, and the intimate interaction between the two species results in an inclined structure of OH* rather than an upright geometry. On the other hand, the dehydrogenation of trans-COOH* promoted by the co-adsorbed OH* species is rather facile with a low activation energy of 0.49 eV. Meanwhile, the reaction is transformed into a mild exothermic process with the reaction energy of −0.12 eV. For the solvent effect of water on trans-COOH* dehydrogenation, a proposed reaction pathway (transCOOH* + H2 O* → CO2 * + H* + H2 O*) is reported in Fig. 3d. The co-adsorbed water molecule serves as a by-stander on the surface, whereas the enhanced adsorption energy of trans-COOH* with water (−2.46 eV) proves its striking role. As shown in IS3-4, the hydrogen bond formed via the attractive interaction of the hydrogen atom in water and the carbonyl O atom might contribute to the change of adsorption. Despite of this, the presence of solvent water also results in an apparent reduction of O−H scission barrier to 0.83 eV. Though the activation barrier reduction is incomparable with the considerable effects of co-adsorbed O* and OH* species, the water assistance makes the reaction thermodynamically more favorable. It is worth noting that a stronger hydrogen bond (1.77 Å) is established between the water and the produced CO2 *, which leads to the resulting CO2 * adsorbed at a top site instead of a more stabilized bridge site.
3.2. Effects of water and its derivatives on H-abstraction from C−H bond 3.2.1. H-abstraction from the C−H bond of acetyl on Ni(111) Acetyl appears to be an essential species during the decomposition of acetic acid on various catalyst surfaces, such as Pd(111) [36], Co(111) [37], Co stepped [38], and Ni(111) [15] surfaces. Previous theoretical studies have proved that the decomposition of acetyl into important intermediate ketene (CH2 CO) is a crucial dehydrogenation step and has been the rate determining step on Co stepped [38] and Ni(111) [15] surfaces, making acetyl enriched on the catalytic surface. Therefore, analogous to H-abstraction from O−H bond, the effect of O*, OH*, and H2 O* assistance on the C−H bond cleavage of CH3 CO* is explored. The corresponding geometries of ISs, TSs, and FSs, activation energy Ea /eV and reaction enthalpy change H/eV are presented in Fig. 4. An isolated CH3 CO* binds at two neighboring Ni atoms via carbonyl C and O atoms with a strong adsorption energy of −2.10 eV. As the rate determining step during acetic acid decomposition, the activation barrier of this H-abstraction from C−H bond of CH3 CO* to obtain ketene is as high as 1.33 eV, together with a disfavored thermodynamic process of 0.49 eV heat absorption. In the TS4-1 of Fig. 4a, the breaking C−H bond is stretched from 1.10 Å (IS4-1) to 1.41 Å. The resulting CH2 CO* is likely to adsorb at the top and bridge sites via two unsaturated C atoms and the fallen hydrogen atom is situated at the neighboring fcc site. Although the reaction is unfavorable in kinetics and thermodynamics, a large number of detectable ketene in experimental studies [13,39] con-
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firms the essentiality and necessity of this C−H bond scission pathway. As a consequence, co-adsorbed O*, OH*, and H2 O* assisted H-abstraction from the C−H bond of CH3 CO* is synthetically examined as follows. For the O/Ni(111) surface, the adsorption energy of CH3 CO* is −2.07 eV, nearly the same as a single CH3 CO* in the absence of co-adsorbed O* (−2.10 eV), indicating that there is no significant interaction between the two co-adsorbed species and it is impossible to form a hydrogen bond with a long O. . .H distance of 2.92 Å as shown in IS4-2. Based on our calculations, in the presence of co-adsorbed O* species, the H-abstraction from CH3 CO* to yield CH2 CO* and OH* is seriously restricted both in kinetics and thermodynamics with an enormous activation barrier of 4.41 eV and an endothermic reaction energy of 0.96 eV. The considerable elevation of reaction barrier suggests that O* is inactive for H-abstraction from C−H bond scission of acetyl, and the decomposition pathway tends to be favored through direct dehydrogenation without O* involved. The C−H bond is stretched from 1.10 Å in IS4-2 to 2.12 Å in TS4-2. Another active species OH* is considered in this C−H bond dissociation process in Fig. 4c. The adsorption energy of CH3 CO* with the presence of OH* is −2.02 eV, which is slightly lower than an isolated CH3 CO* (in absolute value), indicating a slight repulsive interaction between the co-adsorbed CH3 CO* and OH*. When OH* abstract a hydrogen from CH3 CO* to generate ketene and water, the reaction is easier than the direct dissociation. The activation energy and the reaction enthalpy are calculated to be 1.18 eV and 0.64 eV, respectively. The resulting H2 O* is weakly adsorbed on the top site of Ni(111), and the O−Ni bond length is as long as 2.43 Å. What needs to be emphasized is that the involved OH* species not only lowers the activation barrier of this rate determining step, but also improves the selectivity of ketene in acetic acid steam reforming. Meanwhile, the amount of by-product acetone, which is produced via an indirect formation path (CH3 CO* + CH* → CH3 COCH* → CH3 COCH2 * → CH3 COCH3 *) [15], can be reduced as CH3 CO* can be consumed faster with the assistance of hydroxyl. On the H2 O/Ni(111) surface, H2 O* and CH3 CO* sit at their own stable sites individually, and the higher adsorption energy of −2.19 eV (in absolute value) reveals an attractive interaction between them. A hydrogen bonding with carbonyl O atom is formed with the distance of 1.88 Å, which contributes to the change of adsorption energy. For the solvent effect of water on the reaction, water is still not involved and just acts as a by-stander in the dehydrogenation reaction. Coincidently, the activation energy and the reaction energy maintain the same values in the cases with and without the introduction of water (Fig. 4d). Therefore, contrary to the catalytic role of solvent water on the H-abstraction from O−H bond, there seems to be no promoting effects of water on the C−H bond cleavage of CH3 CO*. 3.2.2. H-abstraction from the C−H bond of CH3 C on Ni(111) As stated in the introduction, CH3 C and CH2 CO formation derived from acetyl are the two competing reaction pathways with the same energetic barrier in kinetics during acetic acid decomposition on the Ni(111) surface. Despite that CH3 C formation is more thermodynamically preferred, the subsequent decomposition of CH2 CO is much more facile than CH3 C. Thus, CH3 C* species is inevitably accumulated as a carbon deposition precursor. Therefore, H-abstraction from C−H bond of CH3 C* with the assistance of pre-adsorbed O*, OH*, and H2 O* are worth studying. To begin with, an upright configuration of CH3 C* is formed on the fcc site through its extremely unsaturated C atom. The binding energy is as high as −5.66 eV. For the C−H bond scission of CH3 C* to produce CH2 C* and atomic H*, as displayed in Fig. 5a, CH3 C* needs to get across a certain steric hindrance to dehydrogenate, that
is, the methyl in vertical structure should bend down to interact with the Ni slab, along with the dissociated H atom located at the neighboring hcp site. This process needs to overcome an energetic barrier of 1.17 eV, and the reaction is endothermic by 0.36 eV. The distance of breaking C−H bond increases from 1.10 Å in IS5-1 to 1.62 Å in TS5-1. When O* is introduced on the Ni(111) surface, the two adsorbed O* and CH3 C* species are independent from each other without any interaction (IS5-2), and thus no hydrogen bond is constructed. This is confirmed by the same adsorption energy with and without O* addition. For the O-assisted C−H bond scission of CH3 C* in Fig. 5b, a larger steric hindrance exists for the reaction to overcome. That is mainly because the formed OH* is likely to binds at a farther fcc site to avoid the repulsive interaction with the CH2 C* product, whereas the dissociated hydrogen atom is favored to locate at an adjacent fcc site in the non-O* involved condition. The activation barrier is calculated to be 3.31 eV, much higher than the dehydrogenation path on the clean Ni(111) surface, and the breaking C−H bond is greatly elongated to 2.44 Å. The situation is particularly similar to that of O* participated in CH3 CO dehydrogenation, indicating that O* abstracting a hydrogen atom from C−H bond seems unlikely to take place. When OH* is involved in C−H bond scission of CH3 C*, the two species exhibit a repulsive interaction (Ead = −5.38 eV). The activation energy and the reaction energy are calculated to be 1.28 eV and 0.38 eV, respectively. The higher activation barrier with OHassisted case than the direct decomposition points out that OH* involved C−H bond breakage of CH3 C* is unfavorable. For the CH3 C* dehydrogenation on the H2 O/Ni(111) surface, as presented in Fig. 5d, the strongly adsorbed CH3 C* and the weakly adsorbed H2 O* are located at their own most stable sites in IS5-4. Apparently, it is impossible to form a hydrogen bond between H2 O* and CH3 C*. On this occasion, the adsorption energy is −5.51 eV, indicating that the bonding strength of CH3 C* with an extra water molecule is slightly weaker than that on the clean Ni(111) surface. Despite of this, water acts as a promoting role of C−H bond scission of CH3 C*, though water does not actually participate in the dehydrogenation reaction. The energetic barrier is reduced from 1.17 eV on the clean Ni(111) surface to 0.83 eV with water existing on the surface, at the same time, the enthalpy change is decreased by 0.13 eV. 3.2.3. H-abstraction from C−H bond of CH2 C on Ni(111) As stated above, the solvent water is active for the H-abstraction from CH3 C*, and thus making CH2 C become the dominating decomposition product. The influence of participated O* and OH* together with non-participated solvent water on H-abstraction from the C−H bond of CH2 C* are thoroughly explored in Fig. 6. On the clean Ni(111) surface, as shown in IS6-1, CH2 C* tends to adsorb with its two unsaturated C atoms. The adsorption is strongly exothermic by −4.22 eV. In the presence of co-adsorbed O* (IS6-2), OH* (IS6-3), and H2 O* (IS6-4) species, the two co-adsorbed adsorbates situate at their own optimized sites individually. The calculated binding energies are −4.18 eV, −4.01 eV, and −4.23 eV, respectively, indicating that the slightly repulsive interactions with the addition of O* and OH* species and no obvious interaction in the presence of water is observed. It is apparent that hydrogen bonding interaction is impossible to form. For the direct dehydrogenation of CH2 C* on the clean Ni(111) surface, the reaction needs to overcome an activation energy of 1.35 eV, along with a moderate heat adsorption of 0.17 eV. The produced CHC* tiles to the surface and the hydrogen atom locates at the adjacent hcp site. When co-adsorbed O* species participates in this H-abstraction reaction (Fig. 6b), the activation barrier is dramatically elevated to an incredible value of 3.33 eV, implying this reaction is impossible to occur. Nevertheless, the high reac-
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Fig. 5. H-abstraction from C−H bond of CH3 C on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces.
tion barrier can be ascribed to the large steric hindrance, in that compared with a hydrogen atom in direct dehydrogenation reaction, the generated OH* species in the case of O* addition has a stronger repulsive interaction with CHC*, resulting in a farther distance between OH* and CHC*. On the OH/Ni(111) surface, OH* species can abstract a hydrogen atom in CH2 C* to form CHC* and H2 O*, and water situates at the top site (Fig. 6c). In this case, the activation energy and reaction energy are calculated to be 1.48 eV and 0.15 eV, respectively. The increased reaction barrier implies that OH-involved C−H bond dissociation of CH2 C* is less favorable than direct dehydrogenation. Furthermore, the solvent water effect on CH2 C* dehydrogenation is also investigated in Fig. 6d. When a neighboring water molecule is involved in this dehydrogenation reaction without hydrogen bonding interaction, the water just stays still and serves as a by-stander in the reaction. The dissociation barrier is 0.19 eV higher than that in the absence of water, and the reaction is nearly thermo-neutral, implying the solvent water has negligible effect on this process.
3.3. Summary of the role of water and its derivatives in acetic acid steam reforming In summary, we have systematically investigated the role of water and its derivatives on acetic acid decomposition on the Ni(111) surface. To be specific, it is the O* and OH* species derived from water decomposition and the solvent effect of water on Habstraction from dehydrogenation species HA, including O−H bond scission of H2 O*, CH3 COOH*, trans-COOH* and C−H bond scission
of CH3 CO*, CH3 C*, CH2 C*. The O-assisted, OH-assisted, and H2 Oassisted dehydrogenation pathways can be summarized as follows: HA ∗ + O∗ → A ∗ + OH∗ HA ∗ + OH∗ → A ∗ + H2 O∗ HA ∗ + H2 O∗ → A ∗ + H ∗ + H2 O∗ where O* and OH* species can abstract a hydrogen atom from HA to form OH* and H2 O*, while co-adsorbed H2 O* species does not participate in the dehydrogenation steps, just stand still to interact with HA via hydrogen bonding interaction except for CH3 C* and CH2 C*. According to our calculation results, co-adsorbed O*, OH*, and H2 O* not only affect the adsorption stability of HA, but also make a difference in H-abstraction reactions. As shown in Fig. 7, hydrogen bonding interaction between water and the oxygen-containing dehydrogenation species (H2 O*, CH3 COOH*, trans-COOH*, CH3 CO*) can notably improve their adsorption stability due to hydrogen bonding interaction except for CH3 C* and CH2 C*. Whereas, the co-adsorbed O* and OH* species affect weakly the adsorption of adsorbates. The activation energies for the dehydrogenation reactions are summarized in Table 2. The involved O* and OH* species can both activate O−H bond, and the promoting role of O* species is stronger than that of OH*. Conversely, for C−H bond scission, O* species is extremely tough to take part in this process due to the large steric hindrance, while the enriched OH* species inhibits the C−H bond cleavage except for CH3 CO* dehydro-
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Fig. 6. H-abstraction from C−H bond of CH2 C on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces. Table 2 Activation energies Ead /eV of the dehydrogenation steps on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces. Surfaces
Ni(111) O/Ni(111) OH/Ni(111) H2 O/Ni(111)
O−H dissociation
C−H dissociation
H2 O
CH3 COOH
trans-COOH
CH3 CO
CH3 C
CH2 C
0.97 – 0.66 0.82
0.45 0.28 0.37 0.28
1.06 0.33 0.49 0.83
1.33 4.41 1.18 1.33
1.17 3.31 1.28 0.83
1.35 3.33 1.48 1.54
genation. Note that H-abstraction from CH3 CO* to form ketene is the rate determining step in acetic acid decomposition on Ni(111), and the presence of OH* alters the rate determining step. Furthermore, the addition of adjacent water molecule can weaken the O−H bond and thus plays a catalytic role in O−H bond scission, which is attributed to the hydrogen bonding interaction. However, the solvent effect of water shows no obvious regularity for C−H bond scission. 3.4. The solvent effect of acetic acid on H-abstraction from O−H bond According to the above discussion and literature results [21,25], the solvent effect of water favors H-abstraction from O−H bond due to the hydrogen bonding interaction between water and the adsorbate, thus it is reasonably extrapolated that the activation energy of O−H bond breakage can be reduced as long as a hydrogen bond is formed. Michel et al. [24] points out that hydrogen bond is formed between the ethanol dimer, and the O−H bond rupture of acceptor
ethanol is facilitated by the hydrogen bond while the C−H bond scission is slightly inhibited. Acetic acid, which is the starting reactant, exists abundantly on the catalyst surface. Acetic acid contains a hydroxyl group and this enables it to form O−H. . .O hydrogen bond with neighboring molecules. Therefore, in the following section, the solvent effect of acetic acid for O−H bond scission of CH3 COOH* and trans-COOH* are explored. 3.4.1. H-abstraction from the O−H bond of acetic acid on Ni(111) As displayed in IS7-2 of Fig. 8, two CH3 COOH* molecules bind individually on the top site of Ni(111) surface via their carbonyl O atoms. One is nearly vertical to the surface as an isolated CH3 COOH* on Ni(111), while the other lies down but preferentially interacts through hydrogen bonding interaction with a O. . .H bond length of 1.94 Å. In this case, the vertical acetic acid molecule acts as the hydrogen bond acceptor with the hydroxyl O atom, and the additional CH3 COOH* is the donor by providing the hydroxyl. The dehydrogenation reaction starts from the acceptor molecule, and the breaking O−H bond is stretched from 1.03 Å in IS7-2 to 1.55 Å
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CH3 COOH* of 2.06 Å compared with that of 1.77 Å for the singleCH3 COOH* adsorption. This strong interaction in the co-adsorbed trans-COOH. . .CH3 COOH* complex originates from the hydrogen bonding interaction, where CH3 COOH* acts as the H-donor, and trans-COOH* is the acceptor. As to the hydrogen bond effect on the H-abstraction from trans-COOH*, the activation barrier of this dehydrogenation reaction is notably decreased from 1.06 eV in the situation without co-adsorbed CH3 COOH* to 0.59 eV, and the energy change is reversed from 0.02 eV to moderate exothermal property of −0.33 eV. Combined with the case of CH3 COOH dehydrogenation, it can be inferred that the solvent CH3 COOH can assist the O−H bond scission via the strong hydrogen bonding interaction.
5. Conclusion Fig. 7. Comparison of Adsorption energies Ead /eV of the dehydrogenation species on the clean Ni(111) and H2 O/Ni(111) surfaces.
in TS7-2. Due to the hydrogen bonding effect, the dehydrogenation reaction is extremely facile with a low activation energy of 0.27 eV and strong heat release of 0.70 eV, revealing a promotional effect of the solvent acetic acid on O−H bond scission. Note that the hydrogen bonding interaction is becoming increasingly stronger from the IS7-2 to FS7-2 based on the reduced H-bond lengths. 3.4.2. H-abstraction from the O−H bond of trans-COOH on Ni(111) The O−H bond scission of trans-COOH with the presence of acetic acid is further examined (Fig. 9). As presented in IS8-2, the trans-COOH* and CH3 COOH* species are situated in their most stable adsorption sites, and the strong intermolecular interaction between the two species makes CH3 COOH* lean towards trans-COOH*, resulting in a longer O−Ni bond of
In this work, the influence of water and water derived O* and OH* species on some key dehydrogenation steps during acetic acid steam reforming on Ni(111) is systematically investigated with density functional theory calculations. Six key dehydrogenation species are selected to perform O−H (H2 O*, CH3 COOH*, trans-COOH*) and C−H (CH3 CO*, CH3 C*, CH2 C*) bond scission. With the assistance of pre-adsorbed O* and OH* species, O−H bond scission becomes much more facile with the activation energy being lowered by 18%–69%, and promotional effect of O* is stronger than that of OH*. However, most C−H bond scission reactions are inhibited with only one exception that the enriched OH* species lowers the reaction barrier of the rate determining step (CH3 CO* → CH2 CO* + H*) in acetic acid decomposition. Water molecules exert a significant solvent effect on the dehydrogenation reactions due to the hydrogen bonds formed with the reactants, resulting in the reduced barrier of O−H bond rupture. However, the solvent effect of water shows no obvious regularity for C−H bond scission. Furthermore, the solvent effect of acetic acid favors the O−H bond scission as well.
Fig. 8. H-abstraction from O−H bond of acetic acid on Ni(111): (a) without the presence of CH3 COOH*, (b) with the presence of CH3 COOH*.
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Fig. 9. H-abstraction from O−H bond of trans-COOH on Ni(111): (a) without the presence of CH3 COOH*, (b) with the presence of CH3 COOH*.
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A theoretical study on the role of water and its derivatives in acetic acid steam reforming on Ni(111) Zhen-Yi Du a,b , Yan-Xiong Ran a , Yun-Peng Guo a , Jie Feng a , Wen-Ying Li a,∗ a Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Training Base of State Key Laboratory of Coal Science and Technology Jointly Constructed by Shanxi Province and Ministry of Science and Technology, Taiyuan University of Technology, Taiyuan 030024, PR China b Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 19 March 2017 Accepted 1 May 2017 Available online 2 May 2017 Keywords: Acetic acid Steam reforming DFT calculations Solvent effect Dehydrogenation
a b s t r a c t Catalytic steam reforming of acetic acid can be divided into two steps, i.e. acetic acid decomposition followed by water gas shift. While theoretical studies have been devoted to these two individual reactions, the role of water and its derivatives in the reforming process, especially in CH3 COOH decomposition, remains largely unknown. In this study, a thorough investigation of the effects of the solvent water and its derived O*/OH* species on some key dehydrogenation steps on Ni(111) is carried out using density functional theory. The involved dehydrogenation species include O−H bond scission species H2 O*, CH3 COOH*, trans-COOH* and C−H bond scission species CH3 CO*, CH3 C*, CH2 C*. The results show that the pre-adsorbed O*, OH*, and H2 O* species not only affect the adsorption stability of these species, but also influence their dehydrogenation reactivity. O* and OH* species can both enhance the O−H bond scission, and the promotional effect of O* is superior to OH*. Nevertheless, H-abstraction from C−H bond by O* and OH* are both hindered except for CH3 CO* dehydrogenation in the presence of OH*. Furthermore, the solvent water notably weakens O−H bonds, yet exhibits negligible effect on the C−H bond breakage. Analogously, the solvent effect of CH3 COOH* on O−H bond scission is also investigated. © 2017 Elsevier B.V. All rights reserved.
1. Introduction With the increasing demand for energy and growing awareness of environmental protection, the sustainable development of renewable biomass energy has attracted considerable attention in recent years. Bio-oil produced via biomass pyrolysis is highly oxygenated and requires deep hydrogenation before being used as transportation fuels. Thus large amount of hydrogen is needed and the hydrogen source becomes a research of interest [1,2]. Catalytic steam reforming of bio-oil itself can be a sustainable hydrogen source, because this process does not rely on the traditional fossil fuels as the starting materials [3–5]. Acetic acid is widely used as the model compound for studying bio-oil steam reforming, as it is one of the most abundant compounds and contains various functional groups that can represent the properties of bio-oil [6–9]. Nickel-based catalysts have been widely used in acetic acid steam reforming experimentally for its low cost, high stability and activity, and high hydrogen selectivity [10–13]. High hydrogen selectivity is always the ultimate goal of acetic acid steam reform-
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (W.-Y. Li). http://dx.doi.org/10.1016/j.apsusc.2017.05.003 0169-4332/© 2017 Elsevier B.V. All rights reserved.
ing. In fact, the efficiency of the hydrogen production depends on the activation of C−H and O−H bonds. Generally speaking, acetic acid steam reforming (1) can be basically divided into the following two reactions, i.e. acetic acid decomposition (2) and water gas shift reaction (WGS) (3) [14]: CH3 COOH + 2H2 O → 2CO2 + 4H2 ;Ho298k = 131.50 kJ/mol CH3 COOH → 2CO + CO + H2 O →
2H2 ;Ho298k
CO2 + H2 ;Ho298k
= 213.76 kJ/mol
= −41.13 kJ/mol
(1) (2) (3)
From the total reaction Eq. (1), it is clearly understood that hydrogen originates from acetic acid and water via acetic acid decomposition and WGS, respectively. Meanwhile, two hydrogenproducing routes yield the equivalent amount of objective product H2 , indicating that the two paths are equally important. In Eqs. (2) and (3), water seems to be involved only in the WGS reaction, but unrelated to acetic acid decomposition. In fact, the role of water in the reforming system is of great significance. On the one hand, water is a key reactant, and based on our previous calculations [15], water derived O* and OH* species indeed play a crucial role in carbon elimination and WGS. On the other hand, water itself is fundamentally a solvent, where the solvent effect on the dissociation pathways can not be neglected.
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Regarding the effects of water derived active oxygen-containing species (O* and OH*) on dehydrogenation reactions, O* and OH* species commonly participate in the H-abstraction steps directly, yielding co-adsorbed OH* and H2 O*, respectively. Huang et al. [16] reported that in the presence of O* and OH* species, three dehydrogenation reactions are almost consistently promoted. In methanol steam reforming on Cu catalysts [17,18], the presence of co-adsorbed O* and OH* species not only altered the reaction pathways, but also decreased the activation barriers of some dehydrogenation steps. However, Syu et al. [19] found that surface oxygen species slightly elevated the reaction barriers of ethanol decomposition on Rh(111), either for O−H or C−H bond cleavage. Thus, the influence of water derived O* and OH* species on acetic acid decomposition remains ambiguous and worth studying in depth. To our best knowledge, the role of water and its derivatives in acetic acid decomposition, especially in dehydrogenation steps, has not been systematically studied. Chang et al. [20–22] have done a series of work on the water promotion mechanism in heterogeneous catalysis. In methanol steam reforming on PdZn(111) [21], they pointed out that the solvent effect of water not only improved the adsorption stability of reaction species, but also reduced the activation energies of six dehydrogenation steps involving both O−H and C−H bond scission. Boucher et al. [23] revealed that the addition of hydrogen-bonded water complexes facilitated the O−H bond cleavage on copper catalysts, thus resulting in the conversion of methanol to formaldehyde. Nevertheless, the catalytic role of water for dehydrogenation steps has not reached a consensus. Density functional theory (DFT) calculations showed that with the presence of co-adsorbed water molecule on Rh(111), the O−H bond breakage of alcohols was favored, whereas the C−H bond breakage was inhibited [24]. The work by Zaffran et al. [25] studying polyalcohol dehydrogenation on several transition metal catalysts in the solvent water environment came to a similar conclusion. Furthermore, water just acts as a spectator without being involved in the H-abstraction reactions. The promotional role of water in dehydrogenation pathways is typically ascribed to the hydrogen bonding interaction with the dehydrogenation species. In order to unravel the role of water on acetic acid decomposition on Ni(111), the effects of water derived O* and OH* species and the solvent water molecule on the dehydrogenation reactions are all considered in this work. Our previous study [15] indicated that two competitive decomposition pathways of acetic acid exist on Ni(111), that is, the ketene formation path: CH3 COOH* → CH3 COO* → CH3 CO* → CH2 CO* → CH2 * + CO* → CH*, and CH3 C formation path: CH3 COOH* → CH3 COO* → CH3 CO* → CH3 C* → CH2 C* → CHC*, both followed with WGS: CO* + OH* → cisCOOH* → trans-COOH* → CO2 * + H*. Therefore, we choose six key dehydrogenation routes of H2 O*, CH3 COOH*, trans-COOH* and CH3 CO*, CH3 C*, CH2 C* to represent O−H bond and C−H bond breakage, respectively. To further explore the solvent effect on dehydrogenation reactions, the solvent effect of acetic acid on H-abstraction from O−H bond is also investigated. This work provides theoretical insights in how the solvents, water and acetic acid, affect acetic acid decomposition, and helps to gain an indepth understanding of acetic acid steam reforming on Ni-based catalysts.
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Table 1 Adsorption energies Ead /eV of the dehydrogenation species on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces. Species
H2 O CH3 COOH trans-COOH CH3 CO CH3 C CH2 C
Ead /eV Ni(111)
O/Ni(111)
OH/Ni(111)
H2 O/Ni(111)
−0.40 −0.41 −2.27 −2.10 −5.66 −4.22
−0.58 −0.36 −2.15 −2.07 −5.66 −4.18
– −0.29 −2.44 −2.02 −5.38 −4.01
−0.63 −0.45 −2.46 −2.19 −5.51 −4.23
[28,29]. The localized double numerical basis set combined with polarization functions (DNP) is employed for all the atoms to expand the Kohn-Sham orbitals. DFT semi-core pseudopots (DSPP) [30] is applied to treat the core electrons of metal atoms. An orbital cutoff of 4.5 Å and a thermal smearing width of 0.001 hartree (Ha) are adopted with the total energies extrapolated to 0 K. A Ni(111) surface is modeled by a periodic 4 × 4 supercells, three-layer slab with a vacuum thickness of 20 Å. The top two layers together with the adsorbates are free to relax, and the bottom layer is frozen. A Monkhorst-Pack k-point grid of 3 × 3 × 1 is sampled for the Brillouin zone integration [31]. Considering the magnetic property of metallic Ni and the unsaturated intermediates, spin-polarized calculations are performed. The convergence criterion of total energy, maximum displacement, and maximum force tolerances are set to the values of 2 × 10−5 Ha, 5 × 10−3 Å, and 4 × 10−3 Ha/Å, respectively. The radical species in gas phase are calculated in 20 × 20 × 20 Å3 cubic boxes. The adsorption energy Ead of an adsorbate is calculated as follows: For an isolated adsorbate adsorption, Ead = Eadsorbate/slab − Eslab − Eadsorbate For the co-adsorption of adsorbate and A (O, OH, or H2 O), Ead = E(adsorbate+A)/slab − EA/slab − Eadsorbate where Eadsorbate/slab and E(adsorbate+A/slab) represent the total energies of the adsorbate on clean and O*, OH*, or H2 O* pre-adsorbed Ni(111) surface, respectively; Eslab and EA/slab are the energies of clean Ni(111) surface and O*, OH*, or H2 O* pre-adsorbed Ni(111) surface, respectively; Eadsorbate means the energy of radical species in gas phase. By these definitions, a negative Ead value refers to an exothermic process. More negative Ead values represent stronger adsorption states on the Ni(111) surface. For an elementary reaction, the transition state (TS) is optimized by using the complete linear synchronous transit (LST) and quadratic synchronous transit (QST) approach [32,33]. The activation energy Ea is defined as the energy difference between the TS and the initial state (IS), and the reaction enthalpy change H is the energy difference between the final state (FS) and the IS.
3. Results and discussion 2. Computational methods All periodic DFT calculations are carried out by using Dmol3 code [26,27] as implemented in the Materials Studio program package (version 5.5) from Accelrys. The electron exchange and correlation function are calculated within the generalized gradient approximation (GGA) in the form of Perde-Burke-Ernzerhof (PBE) functional
The most stable adsorption configurations of H2 O, CH3 COOH, CH3 CO, CH3 C, CH2 C, and trans-COOH on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces are investigated. The corresponding adsorption energies are displayed in Table 1. To explore the regularity of dehydrogenation reactions, two dehydrogenation reaction modes are investigated during acetic acid steam reforming, i.e., H-abstraction from O−H bond and C−H bond.
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Fig. 1. H-abstraction from O−H bond of water on the clean Ni(111), O/Ni(111) and H2 O/Ni(111) surfaces.
3.1. Effects of water and its derivatives on the H-abstraction from O−H bond 3.1.1. H-abstraction from the O−H bond of water on Ni(111) Water is the starting reactant in acetic acid steam reforming, and therefore the adsorption and dissociation of water molecule with the presence of O*, OH* species and co-adsorbed water on the Ni(111) surface are investigated (Fig. 1). As shown in Fig. 1a (IS1-1), an isolated water molecule is weakly adsorbed at the top site through O atom on the Ni(111) surface with an adsorption energy of −0.40 eV. For the water decomposition to yield OH* and deep dissociation product O* species, the calculated activation energies are 0.97 eV and 1.19 eV, respectively, and both reactions exhibit the slight exothermic property. The lower activation barrier of OH* formation indicates that OH* species is easier to be formed than O* species on the clean Ni(111) surface (Fig. 1b), which is consistent with the reported results [34]. On the O/Ni(111) surface, hydrogen bond is formed between the co-adsorbed O atom and the H atom of water with an O···H distance of 1.91 Å, which is in the typical range of moderate hydrogen bonds (1.5–2.2 Å) [35], leading to a strong interaction between water and the metallic surface, as O−Ni bond length is shortened from 2.25 Å (IS1-1) to 2.16 Å (IS1-3) in Fig. 1c. Therefore, water adsorbs more stably on the O/Ni(111) surface (−0.58 eV) than the clean Ni(111) surface (−0.40 eV). For water dehydrogenation in the presence of O* species, an adjacent hydrogen of water moves to the co-adsorbed O* atom with two OH* species generated. Compared with single water dehydrogenation, the activation energy of this process dramatically decreases to 0.66 eV, whereas the reaction energy is inversely changed to an endothermic energy of 0.38 eV.
When water adsorbs on the H2 O/Ni(111) surface, as shown in Fig. 1d (IS1-4), an additional water molecule can not be adsorbed. Consequently, a visualized hydrogen bond interaction between two water molecules is constructed on the H2 O/Ni(111) surface. The distance of the O1 −Ni bond and the hydrogen bond O2 . . .H are calculated to be 2.10 Å and 1.72 Å, respectively (IS1-4), both of which are shorter than those on the O/Ni(111) surface. This strong hydrogen bonding interaction results in a higher binding energy of −0.63 eV (in absolute value) and an activation of O−H bond in water, which is elongated from 0.97 Å in the gas phase to 1.01 Å in IS1-4. Therefore, in this case, the activation barrier is lowered by 0.15 eV than the single-water dissociation. The adjacent water just acts as a spectator without being involved in the dehydrogenation reaction, from which we observe that hydrogen bonding interaction indeed plays a catalytic role in O−H bond cleavage of water. In brief, for water decomposition on Ni(111), OH* and O* species are formed, and OH* species takes up the major part, as OH* is more favorable to be produced than O*, particularly in the O- and H2 O-assisted situations. 3.1.2. H-abstraction from the O−H bond of acetic acid on Ni(111) For the decomposition route of acetic acid on Ni(111), our previous work [15] thoroughly investigated five different decomposition pathways, and the results show that the dehydrogenation of hydroxyl is both kinetically and thermodynamically preferred. In this section, H-abstraction from the O−H bond in CH3 COOH with the aid of O*, OH*, and H2 O* are addressed (Fig. 2). As displayed in Fig. 2a, CH3 COOH* sitting at the top site of Ni(111) via carbonyl O atom (IS2-1) decomposes into adsorbed
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Fig. 2. H-abstraction from the O−H bond of acetic acid on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces.
CH3 COO* and H. The calculated activation barrier and reaction energy is 0.45 eV and −0.43 eV, respectively. The breaking O−H bond distance is stretched from 1.01 Å in IS2-1 to 1.75 Å in TS21, and finally the atomic H* is located at the neighboring fcc site. On the O/Ni(111) surface, the hydroxyl group of CH3 COOH* slightly tilts towards the atomic O*, forming a short hydrogen bond of 1.62 Å (IS2-2). The atomic O* can abstract the hydrogen atom from the hydroxyl to yield OH*, along with the co-adsorbed CH3 COO* generated (Fig. 2b). According to the computational results, the activation energy of this reaction (0.28 eV) is reduced by 38%, revealing that O−H bond scission by O* species is kinetically more favorable than its direct decomposition path. As stated above, OH* is the most abundant species in water decomposition on the Ni(111) surface. Thus, the effects of coadsorbed OH* species on the O−H bond breakage of CH3 COOH* is explored as shown in Fig. 2c. The reaction begins with the co-adsorption of CH3 COOH* and OH* (IS2-3), followed with the co-adsorbed OH* species interacting with the hydrogen atom of the hydroxyl in CH3 COOH* to generate adsorbed H2 O*. It is worth
noting that the adsorption (−0.29 eV) is somewhat weaker than an isolated CH3 COOH*. This OH* assisted O−H bond cleavage reaction needs to overcome a 0.37 eV energy barrier, which is lower than that of the direct dehydrogenation, but a little higher than that of O* addition. Therefore, it can be deduced that OH* species can also activate the H-abstraction from the O−H bond. In the presence of co-adsorbed H2 O*, the calculated adsorption energy of CH3 COOH* is found to be −0.63 eV, higher than the case without co-adsorbed H2 O* (in absolute value), indicating that the interaction between CH3 COOH* and the Ni slab is enhanced with the presence of water. This can be ascribed to the hydrogen bonding interaction between the co-adsorbed H2 O* and CH3 COOH*. As shown in IS2-4 of Fig. 2d, the two species are likely to form a CH3 COOH. . .H2 O* complex via the hydrogen bond with the length of 1.66 Å. Theoretically, it is likely for the H atom in water to interact with the hydroxyl oxygen of CH3 COOH*, nevertheless, the optimized configuration proves to be unstable and thus is not investigated, which is in accordance with the case of HCOOH* [21]. For the O−H bond scission of CH3 COOH*, water is not involved
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Fig. 3. H-abstraction from O−H bond of trans-COOH on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces.
and keeps intact as it is in the dehydrogenation step, however, the activation energy is decreased to the same degree as that assisted by the co-adsorbed O*. Furthermore, the presence of water favors the dissociation of O−H bond thermodynamically with the reaction energy of −0.62 eV. In addition, the hydrogen bond exists through the overall dehydrogenation process. As stated above, it is obvious that the presence of water not only stabilizes the adsorption of CH3 COOH*, but also activates the O−H bond of acetic acid due to the hydrogen bonding interaction. 3.1.3. H-abstraction from the O−H bond of trans-COOH on Ni(111) Water gas shift reaction is a crucial stage in acetic acid steam reforming, and determines the reaction rate of the overall steam reforming process. Our reported results [15] have illustrated the mechanism of this process on Ni(111), that is, CO* + OH* → cisCOOH* → trans-COOH* → CO2 * + H*, where the combination of CO* and the enriched OH*species is the rate determining step and the decomposition of trans-COOH* is the key step to generate hydrogen atom and finally obtain the desired hydrogen product. Therefore, trans-COOH* dehydrogenation via the O−H bond cleavage in the presence of O*, OH*, and H2 O* on Ni(111) is carried out as well and the results are shown in Fig. 3. The optimized structure of an isolated trans-COOH* on Ni(111) is displayed in IS3-1. The carbonyl O and C atoms are situated at the top and bridge sites, respectively, and the hydroxyl points towards the catalytic Ni surface. This trans configuration is more energetically feasible for O−H bond scission than the cis configuration, due
to its smaller steric hindrance. The activation energy is 1.06 eV and the reaction is nearly energetically neutral with an enthalpy change of 0.02 eV. Finally, as shown in FS3-1, the produced CO2 * forms a bent structure and H atom binds at the fcc site on the Ni(111) surface. On the O/Ni(111) surface, the adsorption configuration of transCOOH* changes in the presence of co-adsorbed O* species. By a comparison between IS3-1 and IS3-2 in Fig. 3, the carbonyl C atom moves from the bridge site to the top site when an adjacent O* is coadsorbed. This can be attributed to the strong interaction between the two species by forming a hydrogen bond via the hydrogen atom in trans-COOH* and the co-adsorbed O atom. The hydrogen bond length is 1.90 Å, within the scope of typical hydrogen bond length. In addition, the activation barrier is dramatically reduced to 0.33 eV, revealing a significant promoting role of O* species on hydroxyl dehydrogenation. In the TS3-2 structure, the hydrogen atom is shared by the hydroxyl O atom and the co-adsorbed O*, where trans-COOH* serves as a H-donor and atomic O* acts as a Hacceptor. The breaking O−H bond is elongated from 0.99 Å in IS3-2 to 1.20 Å in TS3-2. The hydrogen atom moves to the atomic oxygen and then an OH* is formed with O−H inclined to the oxygen of the co-adsorbed CO2 *. Similar to the O* species, the OH* species is also reactive in abstracting the hydrogen atom from trans-COOH* to produce CO2 * and H2 O*, and the reaction path is shown in Fig. 3c. According to our calculation results, the co-adsorbed OH* species not only affects the adsorption of trans-COOH*, but also plays a significantly role in the dehydrogenation step. On the one hand, trans-COOH* inter-
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Fig. 4. H-abstraction from C−H bond of CH3 CO on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces.
acts stronger with the Ni(111) surface with an adsorption energy of −2.44 eV compared with that of −2.27 eV in the case without co-adsorbed OH*, and the intimate interaction between the two species results in an inclined structure of OH* rather than an upright geometry. On the other hand, the dehydrogenation of trans-COOH* promoted by the co-adsorbed OH* species is rather facile with a low activation energy of 0.49 eV. Meanwhile, the reaction is transformed into a mild exothermic process with the reaction energy of −0.12 eV. For the solvent effect of water on trans-COOH* dehydrogenation, a proposed reaction pathway (transCOOH* + H2 O* → CO2 * + H* + H2 O*) is reported in Fig. 3d. The co-adsorbed water molecule serves as a by-stander on the surface, whereas the enhanced adsorption energy of trans-COOH* with water (−2.46 eV) proves its striking role. As shown in IS3-4, the hydrogen bond formed via the attractive interaction of the hydrogen atom in water and the carbonyl O atom might contribute to the change of adsorption. Despite of this, the presence of solvent water also results in an apparent reduction of O−H scission barrier to 0.83 eV. Though the activation barrier reduction is incomparable with the considerable effects of co-adsorbed O* and OH* species, the water assistance makes the reaction thermodynamically more favorable. It is worth noting that a stronger hydrogen bond (1.77 Å) is established between the water and the produced CO2 *, which leads to the resulting CO2 * adsorbed at a top site instead of a more stabilized bridge site.
3.2. Effects of water and its derivatives on H-abstraction from C−H bond 3.2.1. H-abstraction from the C−H bond of acetyl on Ni(111) Acetyl appears to be an essential species during the decomposition of acetic acid on various catalyst surfaces, such as Pd(111) [36], Co(111) [37], Co stepped [38], and Ni(111) [15] surfaces. Previous theoretical studies have proved that the decomposition of acetyl into important intermediate ketene (CH2 CO) is a crucial dehydrogenation step and has been the rate determining step on Co stepped [38] and Ni(111) [15] surfaces, making acetyl enriched on the catalytic surface. Therefore, analogous to H-abstraction from O−H bond, the effect of O*, OH*, and H2 O* assistance on the C−H bond cleavage of CH3 CO* is explored. The corresponding geometries of ISs, TSs, and FSs, activation energy Ea /eV and reaction enthalpy change H/eV are presented in Fig. 4. An isolated CH3 CO* binds at two neighboring Ni atoms via carbonyl C and O atoms with a strong adsorption energy of −2.10 eV. As the rate determining step during acetic acid decomposition, the activation barrier of this H-abstraction from C−H bond of CH3 CO* to obtain ketene is as high as 1.33 eV, together with a disfavored thermodynamic process of 0.49 eV heat absorption. In the TS4-1 of Fig. 4a, the breaking C−H bond is stretched from 1.10 Å (IS4-1) to 1.41 Å. The resulting CH2 CO* is likely to adsorb at the top and bridge sites via two unsaturated C atoms and the fallen hydrogen atom is situated at the neighboring fcc site. Although the reaction is unfavorable in kinetics and thermodynamics, a large number of detectable ketene in experimental studies [13,39] con-
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firms the essentiality and necessity of this C−H bond scission pathway. As a consequence, co-adsorbed O*, OH*, and H2 O* assisted H-abstraction from the C−H bond of CH3 CO* is synthetically examined as follows. For the O/Ni(111) surface, the adsorption energy of CH3 CO* is −2.07 eV, nearly the same as a single CH3 CO* in the absence of co-adsorbed O* (−2.10 eV), indicating that there is no significant interaction between the two co-adsorbed species and it is impossible to form a hydrogen bond with a long O. . .H distance of 2.92 Å as shown in IS4-2. Based on our calculations, in the presence of co-adsorbed O* species, the H-abstraction from CH3 CO* to yield CH2 CO* and OH* is seriously restricted both in kinetics and thermodynamics with an enormous activation barrier of 4.41 eV and an endothermic reaction energy of 0.96 eV. The considerable elevation of reaction barrier suggests that O* is inactive for H-abstraction from C−H bond scission of acetyl, and the decomposition pathway tends to be favored through direct dehydrogenation without O* involved. The C−H bond is stretched from 1.10 Å in IS4-2 to 2.12 Å in TS4-2. Another active species OH* is considered in this C−H bond dissociation process in Fig. 4c. The adsorption energy of CH3 CO* with the presence of OH* is −2.02 eV, which is slightly lower than an isolated CH3 CO* (in absolute value), indicating a slight repulsive interaction between the co-adsorbed CH3 CO* and OH*. When OH* abstract a hydrogen from CH3 CO* to generate ketene and water, the reaction is easier than the direct dissociation. The activation energy and the reaction enthalpy are calculated to be 1.18 eV and 0.64 eV, respectively. The resulting H2 O* is weakly adsorbed on the top site of Ni(111), and the O−Ni bond length is as long as 2.43 Å. What needs to be emphasized is that the involved OH* species not only lowers the activation barrier of this rate determining step, but also improves the selectivity of ketene in acetic acid steam reforming. Meanwhile, the amount of by-product acetone, which is produced via an indirect formation path (CH3 CO* + CH* → CH3 COCH* → CH3 COCH2 * → CH3 COCH3 *) [15], can be reduced as CH3 CO* can be consumed faster with the assistance of hydroxyl. On the H2 O/Ni(111) surface, H2 O* and CH3 CO* sit at their own stable sites individually, and the higher adsorption energy of −2.19 eV (in absolute value) reveals an attractive interaction between them. A hydrogen bonding with carbonyl O atom is formed with the distance of 1.88 Å, which contributes to the change of adsorption energy. For the solvent effect of water on the reaction, water is still not involved and just acts as a by-stander in the dehydrogenation reaction. Coincidently, the activation energy and the reaction energy maintain the same values in the cases with and without the introduction of water (Fig. 4d). Therefore, contrary to the catalytic role of solvent water on the H-abstraction from O−H bond, there seems to be no promoting effects of water on the C−H bond cleavage of CH3 CO*. 3.2.2. H-abstraction from the C−H bond of CH3 C on Ni(111) As stated in the introduction, CH3 C and CH2 CO formation derived from acetyl are the two competing reaction pathways with the same energetic barrier in kinetics during acetic acid decomposition on the Ni(111) surface. Despite that CH3 C formation is more thermodynamically preferred, the subsequent decomposition of CH2 CO is much more facile than CH3 C. Thus, CH3 C* species is inevitably accumulated as a carbon deposition precursor. Therefore, H-abstraction from C−H bond of CH3 C* with the assistance of pre-adsorbed O*, OH*, and H2 O* are worth studying. To begin with, an upright configuration of CH3 C* is formed on the fcc site through its extremely unsaturated C atom. The binding energy is as high as −5.66 eV. For the C−H bond scission of CH3 C* to produce CH2 C* and atomic H*, as displayed in Fig. 5a, CH3 C* needs to get across a certain steric hindrance to dehydrogenate, that
is, the methyl in vertical structure should bend down to interact with the Ni slab, along with the dissociated H atom located at the neighboring hcp site. This process needs to overcome an energetic barrier of 1.17 eV, and the reaction is endothermic by 0.36 eV. The distance of breaking C−H bond increases from 1.10 Å in IS5-1 to 1.62 Å in TS5-1. When O* is introduced on the Ni(111) surface, the two adsorbed O* and CH3 C* species are independent from each other without any interaction (IS5-2), and thus no hydrogen bond is constructed. This is confirmed by the same adsorption energy with and without O* addition. For the O-assisted C−H bond scission of CH3 C* in Fig. 5b, a larger steric hindrance exists for the reaction to overcome. That is mainly because the formed OH* is likely to binds at a farther fcc site to avoid the repulsive interaction with the CH2 C* product, whereas the dissociated hydrogen atom is favored to locate at an adjacent fcc site in the non-O* involved condition. The activation barrier is calculated to be 3.31 eV, much higher than the dehydrogenation path on the clean Ni(111) surface, and the breaking C−H bond is greatly elongated to 2.44 Å. The situation is particularly similar to that of O* participated in CH3 CO dehydrogenation, indicating that O* abstracting a hydrogen atom from C−H bond seems unlikely to take place. When OH* is involved in C−H bond scission of CH3 C*, the two species exhibit a repulsive interaction (Ead = −5.38 eV). The activation energy and the reaction energy are calculated to be 1.28 eV and 0.38 eV, respectively. The higher activation barrier with OHassisted case than the direct decomposition points out that OH* involved C−H bond breakage of CH3 C* is unfavorable. For the CH3 C* dehydrogenation on the H2 O/Ni(111) surface, as presented in Fig. 5d, the strongly adsorbed CH3 C* and the weakly adsorbed H2 O* are located at their own most stable sites in IS5-4. Apparently, it is impossible to form a hydrogen bond between H2 O* and CH3 C*. On this occasion, the adsorption energy is −5.51 eV, indicating that the bonding strength of CH3 C* with an extra water molecule is slightly weaker than that on the clean Ni(111) surface. Despite of this, water acts as a promoting role of C−H bond scission of CH3 C*, though water does not actually participate in the dehydrogenation reaction. The energetic barrier is reduced from 1.17 eV on the clean Ni(111) surface to 0.83 eV with water existing on the surface, at the same time, the enthalpy change is decreased by 0.13 eV. 3.2.3. H-abstraction from C−H bond of CH2 C on Ni(111) As stated above, the solvent water is active for the H-abstraction from CH3 C*, and thus making CH2 C become the dominating decomposition product. The influence of participated O* and OH* together with non-participated solvent water on H-abstraction from the C−H bond of CH2 C* are thoroughly explored in Fig. 6. On the clean Ni(111) surface, as shown in IS6-1, CH2 C* tends to adsorb with its two unsaturated C atoms. The adsorption is strongly exothermic by −4.22 eV. In the presence of co-adsorbed O* (IS6-2), OH* (IS6-3), and H2 O* (IS6-4) species, the two co-adsorbed adsorbates situate at their own optimized sites individually. The calculated binding energies are −4.18 eV, −4.01 eV, and −4.23 eV, respectively, indicating that the slightly repulsive interactions with the addition of O* and OH* species and no obvious interaction in the presence of water is observed. It is apparent that hydrogen bonding interaction is impossible to form. For the direct dehydrogenation of CH2 C* on the clean Ni(111) surface, the reaction needs to overcome an activation energy of 1.35 eV, along with a moderate heat adsorption of 0.17 eV. The produced CHC* tiles to the surface and the hydrogen atom locates at the adjacent hcp site. When co-adsorbed O* species participates in this H-abstraction reaction (Fig. 6b), the activation barrier is dramatically elevated to an incredible value of 3.33 eV, implying this reaction is impossible to occur. Nevertheless, the high reac-
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Fig. 5. H-abstraction from C−H bond of CH3 C on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces.
tion barrier can be ascribed to the large steric hindrance, in that compared with a hydrogen atom in direct dehydrogenation reaction, the generated OH* species in the case of O* addition has a stronger repulsive interaction with CHC*, resulting in a farther distance between OH* and CHC*. On the OH/Ni(111) surface, OH* species can abstract a hydrogen atom in CH2 C* to form CHC* and H2 O*, and water situates at the top site (Fig. 6c). In this case, the activation energy and reaction energy are calculated to be 1.48 eV and 0.15 eV, respectively. The increased reaction barrier implies that OH-involved C−H bond dissociation of CH2 C* is less favorable than direct dehydrogenation. Furthermore, the solvent water effect on CH2 C* dehydrogenation is also investigated in Fig. 6d. When a neighboring water molecule is involved in this dehydrogenation reaction without hydrogen bonding interaction, the water just stays still and serves as a by-stander in the reaction. The dissociation barrier is 0.19 eV higher than that in the absence of water, and the reaction is nearly thermo-neutral, implying the solvent water has negligible effect on this process.
3.3. Summary of the role of water and its derivatives in acetic acid steam reforming In summary, we have systematically investigated the role of water and its derivatives on acetic acid decomposition on the Ni(111) surface. To be specific, it is the O* and OH* species derived from water decomposition and the solvent effect of water on Habstraction from dehydrogenation species HA, including O−H bond scission of H2 O*, CH3 COOH*, trans-COOH* and C−H bond scission
of CH3 CO*, CH3 C*, CH2 C*. The O-assisted, OH-assisted, and H2 Oassisted dehydrogenation pathways can be summarized as follows: HA ∗ + O∗ → A ∗ + OH∗ HA ∗ + OH∗ → A ∗ + H2 O∗ HA ∗ + H2 O∗ → A ∗ + H ∗ + H2 O∗ where O* and OH* species can abstract a hydrogen atom from HA to form OH* and H2 O*, while co-adsorbed H2 O* species does not participate in the dehydrogenation steps, just stand still to interact with HA via hydrogen bonding interaction except for CH3 C* and CH2 C*. According to our calculation results, co-adsorbed O*, OH*, and H2 O* not only affect the adsorption stability of HA, but also make a difference in H-abstraction reactions. As shown in Fig. 7, hydrogen bonding interaction between water and the oxygen-containing dehydrogenation species (H2 O*, CH3 COOH*, trans-COOH*, CH3 CO*) can notably improve their adsorption stability due to hydrogen bonding interaction except for CH3 C* and CH2 C*. Whereas, the co-adsorbed O* and OH* species affect weakly the adsorption of adsorbates. The activation energies for the dehydrogenation reactions are summarized in Table 2. The involved O* and OH* species can both activate O−H bond, and the promoting role of O* species is stronger than that of OH*. Conversely, for C−H bond scission, O* species is extremely tough to take part in this process due to the large steric hindrance, while the enriched OH* species inhibits the C−H bond cleavage except for CH3 CO* dehydro-
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Fig. 6. H-abstraction from C−H bond of CH2 C on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces. Table 2 Activation energies Ead /eV of the dehydrogenation steps on the clean Ni(111), O/Ni(111), OH/Ni(111) and H2 O/Ni(111) surfaces. Surfaces
Ni(111) O/Ni(111) OH/Ni(111) H2 O/Ni(111)
O−H dissociation
C−H dissociation
H2 O
CH3 COOH
trans-COOH
CH3 CO
CH3 C
CH2 C
0.97 – 0.66 0.82
0.45 0.28 0.37 0.28
1.06 0.33 0.49 0.83
1.33 4.41 1.18 1.33
1.17 3.31 1.28 0.83
1.35 3.33 1.48 1.54
genation. Note that H-abstraction from CH3 CO* to form ketene is the rate determining step in acetic acid decomposition on Ni(111), and the presence of OH* alters the rate determining step. Furthermore, the addition of adjacent water molecule can weaken the O−H bond and thus plays a catalytic role in O−H bond scission, which is attributed to the hydrogen bonding interaction. However, the solvent effect of water shows no obvious regularity for C−H bond scission. 3.4. The solvent effect of acetic acid on H-abstraction from O−H bond According to the above discussion and literature results [21,25], the solvent effect of water favors H-abstraction from O−H bond due to the hydrogen bonding interaction between water and the adsorbate, thus it is reasonably extrapolated that the activation energy of O−H bond breakage can be reduced as long as a hydrogen bond is formed. Michel et al. [24] points out that hydrogen bond is formed between the ethanol dimer, and the O−H bond rupture of acceptor
ethanol is facilitated by the hydrogen bond while the C−H bond scission is slightly inhibited. Acetic acid, which is the starting reactant, exists abundantly on the catalyst surface. Acetic acid contains a hydroxyl group and this enables it to form O−H. . .O hydrogen bond with neighboring molecules. Therefore, in the following section, the solvent effect of acetic acid for O−H bond scission of CH3 COOH* and trans-COOH* are explored. 3.4.1. H-abstraction from the O−H bond of acetic acid on Ni(111) As displayed in IS7-2 of Fig. 8, two CH3 COOH* molecules bind individually on the top site of Ni(111) surface via their carbonyl O atoms. One is nearly vertical to the surface as an isolated CH3 COOH* on Ni(111), while the other lies down but preferentially interacts through hydrogen bonding interaction with a O. . .H bond length of 1.94 Å. In this case, the vertical acetic acid molecule acts as the hydrogen bond acceptor with the hydroxyl O atom, and the additional CH3 COOH* is the donor by providing the hydroxyl. The dehydrogenation reaction starts from the acceptor molecule, and the breaking O−H bond is stretched from 1.03 Å in IS7-2 to 1.55 Å
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CH3 COOH* of 2.06 Å compared with that of 1.77 Å for the singleCH3 COOH* adsorption. This strong interaction in the co-adsorbed trans-COOH. . .CH3 COOH* complex originates from the hydrogen bonding interaction, where CH3 COOH* acts as the H-donor, and trans-COOH* is the acceptor. As to the hydrogen bond effect on the H-abstraction from trans-COOH*, the activation barrier of this dehydrogenation reaction is notably decreased from 1.06 eV in the situation without co-adsorbed CH3 COOH* to 0.59 eV, and the energy change is reversed from 0.02 eV to moderate exothermal property of −0.33 eV. Combined with the case of CH3 COOH dehydrogenation, it can be inferred that the solvent CH3 COOH can assist the O−H bond scission via the strong hydrogen bonding interaction.
5. Conclusion Fig. 7. Comparison of Adsorption energies Ead /eV of the dehydrogenation species on the clean Ni(111) and H2 O/Ni(111) surfaces.
in TS7-2. Due to the hydrogen bonding effect, the dehydrogenation reaction is extremely facile with a low activation energy of 0.27 eV and strong heat release of 0.70 eV, revealing a promotional effect of the solvent acetic acid on O−H bond scission. Note that the hydrogen bonding interaction is becoming increasingly stronger from the IS7-2 to FS7-2 based on the reduced H-bond lengths. 3.4.2. H-abstraction from the O−H bond of trans-COOH on Ni(111) The O−H bond scission of trans-COOH with the presence of acetic acid is further examined (Fig. 9). As presented in IS8-2, the trans-COOH* and CH3 COOH* species are situated in their most stable adsorption sites, and the strong intermolecular interaction between the two species makes CH3 COOH* lean towards trans-COOH*, resulting in a longer O−Ni bond of
In this work, the influence of water and water derived O* and OH* species on some key dehydrogenation steps during acetic acid steam reforming on Ni(111) is systematically investigated with density functional theory calculations. Six key dehydrogenation species are selected to perform O−H (H2 O*, CH3 COOH*, trans-COOH*) and C−H (CH3 CO*, CH3 C*, CH2 C*) bond scission. With the assistance of pre-adsorbed O* and OH* species, O−H bond scission becomes much more facile with the activation energy being lowered by 18%–69%, and promotional effect of O* is stronger than that of OH*. However, most C−H bond scission reactions are inhibited with only one exception that the enriched OH* species lowers the reaction barrier of the rate determining step (CH3 CO* → CH2 CO* + H*) in acetic acid decomposition. Water molecules exert a significant solvent effect on the dehydrogenation reactions due to the hydrogen bonds formed with the reactants, resulting in the reduced barrier of O−H bond rupture. However, the solvent effect of water shows no obvious regularity for C−H bond scission. Furthermore, the solvent effect of acetic acid favors the O−H bond scission as well.
Fig. 8. H-abstraction from O−H bond of acetic acid on Ni(111): (a) without the presence of CH3 COOH*, (b) with the presence of CH3 COOH*.
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Fig. 9. H-abstraction from O−H bond of trans-COOH on Ni(111): (a) without the presence of CH3 COOH*, (b) with the presence of CH3 COOH*.
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