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The mechanism of methanol dehydrogenation on the PdAu(1 0 0) surface: A DFT study
Applied Surface Science 510 (2020) 145434
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The mechanism of methanol dehydrogenation on the PdAu(1 0 0) surface: A DFT study
T
⁎
Minhua Zhang, Yufei Wu, Yingzhe Yu
Key Laboratory for Green Chemical Technology of Ministry of Education, R&D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, PR China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: PdAu(1 0 0) Methanol Dehydrogenation Mechanism Density functional theory
Density functional theory (DFT) was employed to investigate the mechanism of CH3OH dehydrogenation on the PdAu(1 0 0) surface. The adsorption of intermediates involved in the methanol dehydrogenation process was identified and all the elementary reactions were calculated. The route 3 “CH3OH → CH2OH → CHOH → CHO → CO” is determined as the most preferable pathway, of which the rate-limiting step is the dehydrogenation of CH3OH to CH2OH. PdAu has an enhanced catalytic performance towards MOR comparing to the monometallic Pd and Au systems. The strengthened adsorption of CH3OH and the decreased energy barrier of the rate-limiting step account for the enhanced activity, while the weaker CO adsorption contributes to the better anti-poisoning ability. On the PdAu(1 0 0) surface, Pd atoms play as the active sites while Au strengthens the activity and poisoning tolerance of the neighboring Pd. The synergistic effect of PdAu is explained by the d-band theory. The upshifted d-band center suggests that the Pd atoms on the PdAu(1 0 0) surface are more active than those on the Pd(1 0 0) surface. Our theoretical study reveals the mechanism of CH3OH dehydrogenation on the PdAu(1 0 0) surface, which is in favor of the further optimization of the promising non-platinum PdAu catalysts for DMFCs.
1. Introduction With the increasing demand for energy in the world, it is vital to develop innovative energy technologies for the sustainable progress of the green economy. Direct methanol fuel cells (DMFCs), converting the chemical energy of methanol to the electric energy, are thought to be one of the most prospective power conversion technologies because of their high energy conversion efficiency, high power density and the low emission of pollutants [1–3]. For DMFCs, it is critical to improve the efficiency of the methanol oxidation reaction (MOR), which has been studied on a series of transition metal surfaces [4]. Metal Pt is commonly used as the electrocatalyst for MOR due to its excellent activity [5]. But these Pt catalysts are easy to be deactivated by the CO-like species [6–9]. CO strongly adsorbed on the catalysts can hinder the reactants shifting to the active sites and decrease the reaction rates [10]. Nowadays more and more studies have concentrated on the development of non-Pt catalysts for the electrode reactions in fuel cells [11]. Razmi et al. [12] investigated the electrooxidation of glycerol on Au, Pd and Pt nanoparticles and found that all of them exhibited exceptional activities, which provided us a good guidance to develop more effective and stable electrocatalysts. In recent years, Au has
gradually aroused people’s interests for its application to the fuel cells [13]. Comparing to the generally employed platinum, gold catalysts would not sustain the deactivation by CO during the catalyzed process [14,15]. In addition, Pd is considered as an efficient catalyst for MOR owing to its higher electrocatalytic activity, better anti-poisoning ability and relative lower cost [16]. Aiming to further enhance the catalytic activity, numerous efforts have been devoted to synthesis of novel bimetallic catalysts [17–26], among which PdAu catalysts exhibit good performance. The alloying of Pd and Au improves the capacity of electrocatalyzing over the monometallic catalysts because of the ligand effects and ensemble effects [27,28]. Lots of researchers have carried out experiments to study the electrocatalytic performance of the PdAu catalysts. Wang and coworkers [20] synthesized the nanoporous PdAu catalysts by the method of chemical dealloying ternary AlPdAu, and demonstrated a greatly improved catalytic activity and durability towards DMFCs after the electrochemical measurements, which was significantly better than the NPeAu, NPePd or other well-known Pdbased nanoparticle electrocatalysts. Luo et al. [23] prepared the PdAu alloy nanocatalysts with the different ratios of Au and Pd atoms. They further characterized the structure and morphology of the PdAu
⁎ Corresponding author at: Key Laboratory for Green Chemical Technology of Ministry of Education, R&D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, PR China. E-mail address: [email protected] (Y. Yu).
https://doi.org/10.1016/j.apsusc.2020.145434 Received 13 August 2019; Received in revised form 14 January 2020; Accepted 15 January 2020 Available online 20 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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catalysts and treated them as promising non-Pt anode catalysts for the reason of the greatly enhance activity. PdAu bimetallic catalysts with the octapodal shape, which showed excellent electrocatalytic properties, were fabricated by Hong et al. [29] through a one-pot aqueous synthesis method. The ultrasonic synthesis method was employed by Bunazawa and coworkes [25] towards the PdAu catalysts, the solvent effect was also examined and the ethylene glycol was selected as the optimum one. Hu et al. [30] fabricated the well-crystallized PdAu nanoparticles via a solution plasma technique, which exhibited a improved electrochemical stability comparing to the Pd and Au nanoparticles mixture. In brief, the experiments demonstrated that the PdAu catalysts, with the enhanced activity and stability, were attractive as the non-platinum electrocatalysts for direct methanol fuel cells (DMFCs). However, the mechanism of methanol dehydrogenation on the PdAu is still unclear, which limits its further optimization. Experiments alone cannot elucidate the dehydrogenation process completely, but theoretical study has acted as a crucial role for catalysts optimization by the means of calculation [31]. Zhao et al. examined the mechanism of CH3OH dehydrogenation using DFT on the Pd catalyst [32], which was also calculated on different Au facets [33,34]. However, there is still no comprehensive elucidation of the methanol dehydrogenation mechanism on the PdAu alloy catalyst. Herein, we conduct this DFT study to reveal the mechanism of methanol dehydrogenation on the PdAu (1 0 0) facet which is the most active for the reactions [35–37]. Four routes are proposed as illustrated in Fig. 1, and the most preferable one is determined. This theoretical calculation will be beneficial to the further modifying and application of the promising PdAu catalysts.
Fig. 2. . Top view (upper) and side view (lower) of the optimized PdAu(1 0 0) surface.
et al. [47] suggested that Pd atoms preferred to form the second neighbor ensembles on the PdAu(1 0 0) surface. Thus we substituted two Au atoms with two Pd atoms to form the second neighboring Pd ensemble as shown in Fig. 2. In all calculations, the upper half of layers together with the adsorbates were relaxed while the lower half were fixed. A 15 Å vacuum was used to avoid interactions between the slab and its periodic images. The adsorption energy of intermediates, Eads, was calculated according to
2. Computational details All the DFT calculations were conducted on the principle of spin unrestricted with the Dmol3 program package in Materials Studio of Accelrys Inc [38,39]. To explain the exchange and correlation effects, the generalized gradient approximation (GGA) with the RPBE functional was utilized [40]. The double numerical plus polarization (DNP) was employed and the density functional semicore pseudopotential (DSPP) was used considering the relativity effect. A generalized Gaussian smearing technique [41] of 0.005 Ha was employed to accelerate convergence. A 2 × 2 × 1 grid was adopted to generate k-points according to the Monkhorst-Pack method [42]. The convergence criteria for the energy calculation and geometry optimization were 0.004 Ha/Å, 0.005 Å for maximum force, maximum displacement and 1.0 × 10−6 Ha, 2.0 × 10−5 Ha for the tolerance of self-consistent field (SCF), energy. The PdAu(1 0 0) surface was modeled with a 4 × 4 unit cell consisting of four layers of atoms. The lattice parameter for bulk is 4.18 Å, in good agreement with the experimental value 4.08 Å [43]. Pd atoms were placed on the topmost layer because a phenomenon of surface segregation of Pd in the PdAu bimetallic systems was evidenced [44]. Besides, it was found that the ensembles of isolated Pd atoms were more stable than continuous configurations [45,46]. Furthermore, Gotsis
Eads = EM / S − EM − ES
(1)
where EM/S, EM and ES represented the energies of the surface with intermediates, the intermediates and the PdAu(1 0 0) surface slab, respectively. The transition states were located using the complete linear synchronous transit/quadratic synchronous transit (LST/QST) method [48]. And the frequencies of them are examined to confirm the validity. The reaction energy Er was defined as
Er = EFS − EIS
(2)
The activation barrier Ea was calculated according to
Ea = ETS − EIS
(3)
where EIS, EFS and EIS represented the energies of the initial, final and transition states, respectively. 3. Results and discussions In this section, we clarify the results beginning with the adsorption configurations of the intermediates involved in methanol
Fig. 1. . Reaction network of methanol dehydrogenation on the PdAu(1 0 0) surface. 2
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Table 1 . Adsorption energies, Eads (eV), and structure parameters of the intermediates involved in methanol dehydrogenation on the PdAu(1 0 0) surface. Species
Eads (eV)
Bond length (Å)
CH3OH*
−0.55
CH3O* CH2O* CHO* CH2OH*
−1.13 −0.14 −1.72 −1.42
CHOH*
−2.34
COH*
−2.62
CO* H*
−1.05 −2.45
d(CeO) = 1.45, d(OePd) = 2.57, d(CeH) = 1.10, d(OeH) = 0.97 d(CeO) = 1.41, d(OePd) = 2.04, d(CeH) = 1.11 d(CeO) = 1.22, d(OePd) = 2.68, d(CeH) = 1.11 d(CeO) = 1.20, d(CePd) = 1.98, d(CeH) = 1.12 d(CeO) = 1.39, d(CePd) = 2.09, d(CeH) = 1.10, d(OeH) = 0.98 d(CeO) = 1.32, d(CePd) = 1.95, d(CeH) = 1.10, d(OeH) = 0.99 d(CeO) = 1.32, d(CePd) = 1.94, d(CeAu) = 2.06, d(OeH) = 0.98 d(CeO) = 1.16, d(CePd) = 1.93 d(HeAu) = 1.76, d(HePd) = 1.71
tally well with this point that methanol prefers to be adsorbed on the top site of Pd atom with the formation of OePd bond. The values of OePd, OeC and OeH bonds are 2.57 Å, 1.45 Å and 0.97 Å respectively with a smaller CeO-surface angle of 35° compared with the Pd, Au and other catalysts [34,50], which indicates that the methylidyne H is closer to the surface and easier to be activated on the PdAu(1 0 0) surface. The adsorption energy is calculated to be −0.55 eV comparing to that on the Pd(1 0 0) being −0.42 eV. 3.1.2. For the CHxO(x = 1–3) species The CHxO species are formed through the initial cleavage of OeH bond followed by the subsequent CeH abstractions. Similar to methanol, methoxy(CH3O) is also preferably adsorbed on the top of Pd atom through the OePd bond. The OePd bond is shortened from 2.57 Å to 2.04 Å and the adsorption energy increases to −1.13 eV. The OePd axis is more sloping compared with that of methanol. As for formaldehyde(CH2O), it is generally accepted that CH2O prefers to be adsorbed through a η1-C-η1-O adsorption mode [37]. We placed it at different sites on the PdAu(1 0 0) surface including the top, bridge and hollow sites, and two structures were gained. One is in accordance with the η1-C-η1-O adsorption structure where CH2O binds to the surface through the O and C atom simultaneously. But according to our calculated results, this structure is not stable on the PdAu(1 0 0) surface with a tiny Eads of −0.06 eV. The other configuration is CH2O adsorbed via the long pair electrons of the oxygen atom. Though it is more stable compared with the first, it still desorbs easily due to the smallest adsorption energy of −0.14 eV. This phenomenon is also investigated upon other catalysts, which strongly limits the catalytic performance. However on the PdAu(1 0 0) surface, though CH2O interacts weakly with the surface, there is another “CHOH” route available for methanol dehydrogenation which will be depicted below. A change occurs when CHO is adsorbed. It is CePd bond rather than OePd bond that ties the intermediate with the surface. The adsorption energy is substantially strengthened to −1.72 eV, which is beneficial for the subsequent dehydrogenation reaction. The distances of CeO and CePd bond are 1.20 Å and 1.98 Å, respectively.
Fig. 3. . Most stable configurations of the intermediates involved in methanol dehydrogenation on the PdAu(1 0 0) surface. Yellow, Au atoms; Cyan, Pd atoms; Grey, C atoms; Red, O atoms; White, H atoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
decomposition. Then all the potential elementary reactions in the process are examined. Finally, after determining the mechanism of methanol dehydrogenation on the PdAu(1 0 0) surface, we launch a discussion on the difference between the bimetallic PdAu system and the monometallic Pd and Au systems. 3.1. Adsorption configurations and energies The intermediates adsorbed on the PdAu(1 0 0) surface are depicted in this subsection. There are total nine species generated in the CH3OH dissociation process. They were both placed at different sites on the PdAu(1 0 0) surface and then were calculated to obtain the adsorption energies. The most stable configurations of them are presented in Fig. 3, along with the detailed structure parameters and adsorption energies tabulated in Table 1.
3.1.3. For the CHxOH(x = 0–2) species The successive scissions of CeH bonds result in the generation of the CHxOH species. They are both bound to the surface via the C atom, which are stronger than those via the O atom. Hydroxymethyl(CH2OH) is firstly formed ater CH3OH breaking the CeH bond. CH2OH prefers to locate atop the Pd atom with a strengthened adsorption energy equivalent to −1.42 eV. The methylene and hydroxyl H atoms are both closer to the PdAu(1 0 0) surface, which is facile for their activation. CHOH will be formed if the methylene H is removed. This removal of H contributes to a stronger binding energy of −2.34 eV. The distance of the CePd decreases to 1.95 Å compared with that in CH2OH. And the
3.1.1. For CH3OH It is widely agreed that methanol is adsorbed through the oxygen long pair electrons on the metal surfaces [49]. Our calculated results 3
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CeO axis becomes more parallel with the PdeAu bond seen from the top view, which is conducive to CHOH dehydrogenation to CHO. Apart from CHO, the other product from the dehydrogenation of CHOH is COH. In its most stable structure, COH is found to prefer the bridge site on the PdAu(1 0 0) surface. The lengths of CePd and CeAu bonds are 1.94 Å and 2.06 Å respectively. The adsorption energy is calculated to be −2.62 eV since the C atom has no H neighbors.
Table 2 . Reaction energy, Er (eV), activation barrier, Ea (eV), of each elementary step and frequencies of the transition states involved in the route 1.
3.1.4. For CO and H A strong adsorption of CO can lead to the poisoning of catalysts especially those including metal Pt. While on the PdAu(1 0 0) surface, CO has a moderate adsorption, of which the Eads is calculated to be −1.05 eV. To obtain the most optimized structure and the corresponding adsorption energy, we have calculated different configurations placed at the top, bridge and hollow sites respectively. The results reveal that CO, which binds vertically to the metal surface via the CePd bond, owns the biggest binding energy. We have also examined the H adsorption at different sites. The bridge site is confirmed the most preferable location for H atom with the Eads of −2.45 eV. The HeAu bond and the HePd bond are equivalent to 1.76 Å and 1.71 Å, respectively.
Elementary reaction step
Er (eV)
Ea (eV)
Freq (cm−1)
CH3OH* → CH3O*+H* CH3O* → CH2O*+H* CH2O* → CHO*+H* CHO* → CO*+H*
1.19 −0.18 −0.03 −0.58
1.67 0.52 0.50 0.36
−458.62 −474.72 −888.20 −754.27
route 1 are shown in Fig. 4. Table 2 lists the reaction energies, activation barriers of the reactions and the frequencies of the transition states. CH3OH* → CH3O* + H* The first step of the route 1 is the dehydrogenation reaction of CH3OH to form CH3O and H atom. In the initial state, CH3OH is adsorbed at its most stable site. The hydroxyl H is approaching to the AuePd bridge site. In the transition state, the OeH bond elongates from 0.97 Å to 1.62 Å. The H atom locates at the bridge site. In the final state, CH3O and H atom are coadsorbed on the PdAu(1 0 0) surface at the most stable configuration. The H atom locates at the neighboring bridge site, and CH3O is adsorbed through a 2.05 Å CePd bond. This step is unfavorable in both thermodynamics and kinetics with the reaction energy of 1.19 eV and the barrier energy of 1.67 eV, respectively. CH3O* → CH2O* + H* Then CH3O is dehydrogenated to CH2O. In this process, one H atom is dissociated from the C atom in CH3O, gradually approaching to the top of Au atom. The distance between the leaving H atom and C atom extends from 1.11 Å to 1.68 Å in the transition state. In the final state the CeH bond breaks and the H atom shifts to the bridge site. This step is energetically facile due to its exothermic energy equivalent to −0.18 eV. The reaction barrier is calculated to be 0.52 eV, which suggests that it is also kinetically feasible. CH2O* → CHO* + H* Subsequent reaction is CH2O dehydrogenation to CHO, which is also the third step in the route 2. In the initial state, CH2O is adsorbed through its oxygen long pair electrons. The distance between the C and the H departing from CH2O is 1.11 Å. In the transition state, the H atom
3.2. Reaction pathways In this subsection we will elucidate all the elementary reactions included in methanol dehydrogenation on the PdAu(1 0 0) surface. The most stable structures play as the initial states. And the most stable coadsorption configurations were selected as the final states. All the transition states have been calculated to confirm that each of them has only one imaginary frequency. There are 4 routes for methanol dehydrogenation into CO and H ultimately on the PdAu(1 0 0) surface, which can be divided into the initial OeH bond activation (route 1) and the initial CeH bond activation (route 2, 3, 4) as Fig. 1 shows. 3.2.1. Initial OeH bond activation The route 1 undergoes the cleavage of OeH bond of methanol primarily with the removal of methylidyne H following. The initial, transition and final states of each elementary reaction involved in the
Fig. 4. . Initial, transition and final states of each elementary step involved in the route 1. 4
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links with the C atom, Pd atom and Au atom, of which the bond lengths are 1.52 Å, 2.11 Å and 2.01 Å respectively. The C atom binds to the Pd atom at a distance of 2.11 Å. In the final state, the H atom moves to the neighboring bridge site indicating the abstraction of CeH bond. CHO is adsorbed on the top of Pd atom through the CePd bond, which shortens the distance between CHO and the surface. This step is a little exothermic with the −0.03 eV reaction energy. An activation barrier of 0.50 eV should be overcome in this step, which indicates that this step is kinetically favored. CHO* → CO* + H* This is the last step for the route 1 but also for the route 2 and 3. In the initial state, the axis of CePd is nearly vertical to the surface. Then the H atom is activated and tends to transfer towards the PdeAu bridge site. In the final state, the scission of CeH bond indicates the production of the final products. CO is adsorbed through a CePd bond with the H coadsorbed at the neighboring bridge site. The reaction energy of this step is −0.58 eV and the activation barrier is 0.36 eV, revealing that the dehydrogenation of CHO is not only energetically but also kinetically promoted.
Table 3 . Reaction energy, Er (eV), activation barrier, Ea (eV), of each elementary step and frequencies of the transition states involved in the route 2–4. Elementary reaction step
Er (eV)
Ea (eV)
Freq (cm−1)
CH3OH* → CH2OH*+H* CH2OH* → CH2O*+H* CH2OH* → CHOH*+H* CHOH* → CHO*+H* CHOH* → COH*+H* COH* → CO*+H*
0.86 0.15 0.75 −0.63 1.07 −0.99
1.41 1.11 1.30 0.45 1.10 1.76
−813.41 −251.83 −43.45 −742.64 −816.98 −1041.21
involved in the route 2 to 4 are shown in Fig. 5, with the Er, Ea and frequencies of the transition states listed in Table.3. CH3OH* → CH2OH* + H* Instead of CH3O, CH2OH is produced firstly from methanol for the route 2–4. The initial state of this reaction is the same with that in the CH3OH* → CH3O* + H* step. The H that will be removed in this step is activated by the surface, moving towards the bridge site of the Pd and Au. In the transition state, the distance of the CeH is stretched from 1.10 Å to 1.68 Å. The location of H atom is similar to that in the final state but closer to the Pd atom. And the C atom is binding to the Pd atom. In the final state, CH3O and H are coadsorbed on the PdAu(1 0 0) surface. The CeH bond is further elongated to 2.52 Å indicating the breakage of it and the formation of CH2OH. CH2OH binds to the surface with the 2.13 Å CePd bond rather than the OePd bond. Compared with the reaction of CH3O forming, it is more possible for methanol
3.2.2. Initial CeH bond activation The route 2 to 4 are both initiated from the abstraction of CeH bond in methanol but differ in the intermediate from which the hydroxyl H dissociates. In the route 2, the Hydroxyl H tends to dissociate from CH2OH. While it is inclined to occur from CHOH in the route 3. However in the route 4, the hydroxyl H will not dissociate until the last COH dehydrogenation step. Optimized structures for IS, TS and FS
Fig. 5. . Initial, transition and final states of each elementary step involved in the route 2–4. 5
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dehydrogenated due to their comparable barriers. After the reaction of CH3OH breaking the CeH bond, CH2OH chooses to be dehydrogenated to CH2O in the route 2, while in the route 3 CHOH is formed. But no matter CH2O or CHOH, they are all converted into CHO, which suggests that the route 2 and the route 3 have the same last step. Though the activation barrier of the CHOH forming from CH2OH is a little higher than that of the CH2O forming, the subsequent step of the route 3 is easier compared with the corresponding step of the route 2. From this point of view, the route 2 and the route 3 are both potential pathways for methanol to be dehydrogenated on the PdAu(1 0 0) surface. However, the “CHOH” route has its advantage over the “CH2O” route. The weak adsorption of CH2O makes it a factor that limits the catalytic performance, which is verified on other catalysts [34], since the reaction CH2OH dehydrogenation to CHOH is prohibited on them. While on the PdAu(1 0 0) surface, although the easy desorption of CH2O is not modified, the “CHOH” channel is available for methanol dehydrogenation to avoid this problem. Therefore the route 3 is more effective than the route 2. Moreover, the route 4 is prohibited because the formation of COH has a quite high energy barrier. Above all, methanol prefers to be dehydrogenated on the PdAu(1 0 0) surface through the route 3 “CH3OH → CH2OH → CHOH → CHO → CO”, of which CH3OH dehydrogenation to CH2OH plays as the rate-limiting step with the energy barrier of 1.41 eV.
dehydrogenation into CH2OH on the PdAu(1 0 0) surface due to the decreased activation barrier of 1.41 eV, and the reaction energy of 0.86 eV. These results evidence that the initial CeH cleavage is more preferable on the PdAu(1 0 0) surface, which is consistent with that on Pd but opposite to Au catalysts [34,50]. CH2OH* → CH2O* + H* Two potential dehydrogenation reactions could occur on CH2OH: the scission of OeH to form CH2O (route 2) and the fracture of CeH to form CHOH. The initial state of the step generating CH2O is CH2OH adsorbed atop the Pd atom with the OeH bond of 0.98 Å. In the transition state, the H moves to the hollow site with the lengthened 1.70 Å distance of the OeH bond. The CH2O is adsorbed as the η1-C-η1O configuration in the final state. Lengths of OePd and CePd bonds are 2.42 Å and 2.47 Å, respectively. The reaction energy of this step is 0.15 eV. And the reaction barrier is 1.11 eV. CH2OH* → CHOH* + H* In the course of CHOH formation, the CeH bond breaks instead of the OeH bond. The reaction starts from the most stable configuration of CH2OH. During the reaction, the H atom bound to the C atom moves towards the surface. Finally, CHOH forms and is coadsorbed on the PdAu(1 0 0) surface together with the H atom. This step has a comparable reaction barrier equivalent to 1.30 eV with the other CH2OH dehydrogenation reaction to CH2O. In addition, due to the weak adsorption of CH2O and the strong adsorption of CHOH, CH2OH breaking the CeH bond to produce CHOH is the more effective route on the PdAu (1 0 0) surface. CHOH* → CHO* + H* Similar to CH2OH, two possible reactions could occur on CHOH to generate CHO (route 3) and COH (route 4), respectively. In the initial state, the CeO axis of CHOH becomes more parallel with the PdeAu bond, which is conducive to the removal of the hydroxyl H. During the process of CHO forming, the hydroxyl H shifts to the Au atom with the distance of OeH elongated from 0.99 Å to 1.45 Å. The final state indicates the formation of the CHO species. This step is supported energetically with the exothermic energy of −0.63 eV. The 0.45 eV barrier also makes this step feasible kinetically. CHOH* → COH* + H* In this process, the separated H is in inclined to approach the hollow site with the CeH bond stretched to 1.68 Å. In the final state, COH is adsorbed on the top site of Pd atom, and moves to the bridge site for stable adsorption afterward. It’s quite difficult for CHOH dehydrogenation into COH because of its activation barrier of 1.10 eV compared with the other CHOH dehydrogenation step. The 1.07 eV reaction energy also indicates that this step is prohibited thermodynamically. COH* → CO* + H* COH is dehydrogenated to CO, which is unique to the route 4. In the initial state, COH is adsorbed through the CePd and CeAu bonds at the bridge site. Then the H approaches to the Au atom in the transition state and finally locates atop the Pd atom. Though this step is exothermic, it is unlikely to occur for this step due to the quite high activation barrier of 1.76 eV.
3.4. Comparison with the monometallic systems Experimental researches [20,23,51] demonstrated that the PdAu exhibited not only the greatly enhanced catalytic activity but also the long-term stability toward methanol electrooxidation, which was significantly better than the monometallic Pd and Au. Our calculated results are accordance with the experimental conclusions and can explain the advantages of PdAu from the mechanism aspect. Our calculated results suggest that the methanol dehydrogenation mechanism on the PdAu(1 0 0) surface is similar to that on the Pd(1 0 0) surface but different from the Au(1 0 0) surface. It is consistent with the experiment results carried out by Wang et al. [20], indicating that Pd should act as primary active sites catalyzing the dehydrogenation of methanol while Au enhances the activity and poisoning tolerance of the neighboring Pd. The mechanism of methanol dehydrogenation on the Au(1 0 0) surface was investigated by Hussain and coworkers [34]. Though the pathway was determined as “CH3OH → CH3O → CH2O → CHO → CO”, they revealed that methanol dehydrogenation on the pure Au(1 0 0) is difficult due to the weak adsorption of intermediates and the high reaction barriers. This verifies the point that Pd atoms play as the active sites on the PdAu(1 0 0) surface. Zhao and coworkers [32] examined CH3OH decomposition on the Pd(1 0 0) surface. They found that methanol preferred to dehydrogenate via the route “CH3OH → CH2OH → CHOH → CHO → CO”, of which the rate-limiting step was the CeH bond scission of CH3OH with the barrier of 1.79 eV. Such a route is also preferred on the PdAu(1 0 0) surface based on our computational results, while some improvements resulting from the alloying Au contributes to the greatly enhanced performance of the PdAu catalysts in experiments. The weak adsorption of CH3OH is a critical problem limiting the activity of catalysts for DMFCs [51]. However on the PdAu(1 0 0) surface, the adsorption of CH3OH is strengthened with the binding energy of −0.55 eV comparing with that on Pd(1 0 0) of −0.42 eV [32]. Moreover, the decreased rate-limiting step barrier is also a cause leading to the enhancement of activity. The rate-limiting reaction on the PdAu(1 0 0) surface is the same with that on the Pd (1 0 0), but the value of energy barrier reduces to 1.41 eV from 1.79 eV on Pd(1 0 0). Besides, the weaker CO adsorption energy (PdAu(1 0 0) of −1.05 eV versus Pd(1 0 0) of −2.09 eV) accounts for the long-term stability property. The CO would not cover the catalyst but desorb or react with the OH– to be removed since a strong adsorption could prevent the removal reactions. It’s typical for Au to avoid the deactivation by CO during the reaction process [14,52]. The alloying Au is
3.3. Reaction mechanism on the PdAu(1 0 0) surface In present work, we have calculated the intermediates adsorption and the elementary reactions involved in methanol dehydrogenation on the PdAu(1 0 0) surface as discussed above. Fig. 6 shows the potential energy surfaces of methanol dehydrogenation on the PdAu(1 0 0) surface. The initial OeH bond cleavage or CeH bond cleavage distinguishes the route 1 or route 2 to 4. After the reactant CH3OH adsorbed on the surface, it’s more difficult for it to break the OeH bond primarily compared with the CeH bond. The energy barrier of 1.67 eV versus 1.41 eV indicates that CH3OH on the PdAu(1 0 0) surface tends to be dehydrogenated to CH2OH firstly. Thus the route 1 is forbidden, but the route 2 and 3 are both feasible pathways for methanol to be 6
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Fig. 6. . Potential energy surfaces of methanol dehydrogenation on the PdAu(1 0 0) surface.
4. Conclusion
responsible for the enhancement of anti-poisoning property of the PdAu alloy system, which is also studied by other researchers [53]. To examine the synergistic effect between Pd and Au in the PdAu system, we calculated the d-band centers of the Pd atoms on PdAu (1 0 0) surface and Pd(1 0 0) surface respectively, according to
εd =
∫ ρEdE ∫ ρdE
In this article, the mechanism of methanol dehydrogenation on the PdAu(1 0 0) surface was studied using DFT calculations. The adsorption of intermediates involved in the methanol dehydrogenation process was confirmed. All the elementary reactions have been calculated and there are total 4 routes for methanol to be dehydrogenated on the PdAu (1 0 0) surface. CH3OH is firstly dehydrogenated to CH2OH via the initial CeH bond cleavage, which is more favorable than the OeH bond breakage producing CH3O. The route 2 “CH3OH → CH2OH → CH2O → CHO → CO” and the route 3 “CH3OH → CH2OH → CHOH → CHO → CO” have comparable barriers. However the most preferable path for CH3OH dehydrogenation on the PdAu(1 0 0) surface is determined as the route 3 due to the easy desorption of CH2O. CH3OH dehydrogenation to CH2OH acts as the rate-limiting step with the energy barrier of 1.41 eV. The methanol dehydrogenation mechanism on the PdAu(1 0 0) surface is similar to that on the Pd(1 0 0) surface but different from the Au(1 0 0) surface, indicating that Pd atoms play as the active sites while Au strengthens the activity and durability of the PdAu system. The enhanced adsorption of CH3OH (−0.55 eV on the PdAu(1 0 0) versus −0.42 eV on the Pd(1 0 0)) and the decreased energy barrier of the rate-limiting step (1.41 eV on the PdAu(1 0 0) versus 1.79 eV on the Pd(1 0 0)) account for the strengthened activity of
(4)
where E is the energy with respect to the Fermi level and ρ is the electronic density of states. Nørskov et al. [54] came up with the theory which demonstrated that the d-band centers of transition metals could represent their activities. The DOS and d-PDOS plots of the Pd atoms on PdAu(1 0 0) and Pd(1 0 0) surfaces are shown in Fig. 7. It is obvious that the Pd atoms on PdAu(1 0 0) surface are different in the electronic properties from those on Pd(1 0 0) surface. Then the d-band centers of Pd atoms were calculated to be −1.39 eV on the PdAu(1 0 0) and −1.69 eV on the Pd(1 0 0). The positively shifting by 0.3 eV indicates a higher activity of the Pd atoms on PdAu(1 0 0) surface comparing with those on Pd(1 0 0) surface, which leads to the advantages of the PdAu system.
Fig. 7. . (A) Density of states of the Pd atoms on PdAu(1 0 0) surface and Pd(1 0 0) surface and (B) partial density of states of the d band (dePDOS) and deband centers of the Pd atoms on PdAu(1 0 0) surface and Pd(1 0 0) surface. 7
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the PdAu system. The CO has a weaker adsorption on the PdAu(1 0 0) with the binding energy of −1.05 eV comparing to that on the Pd (1 0 0) of −2.09 eV, which contributes to a better anti-poisoning ability of the PdAu over the Pd. The synergistic effect of the PdAu is explained by the d-band center theory. The upshifted d-band center suggests that the Pd atoms on the PdAu(1 0 0) surface are more active compared with those on the Pd(1 0 0) surface. Our results reveal the mechanism of methanol dehydrogenation on the PdAu(1 0 0) surface and explain the enhanced catalytic performance from the theory aspect, which verifies that PdAu is a promising non-platinum catalyst for DMFCs. This theoretical study will be beneficial to the further modification and application of the PdAu catalysts.
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CRediT authorship contribution statement Minhua Zhang: Conceptualization, Methodology, Resources. Yufei Wu: Conceptualization, Formal analysis, Writing - original draft. Yingzhe Yu: Conceptualization, Validation, Writing - review & editing. References [1] G. Xu, R. Si, J. Liu, L. Zhang, X. Gong, R. Gao, B. Liu, J. Zhang, Directed selfassembly pathways of three-dimensional Pt/Pd nanocrystal superlattice electrocatalysts for enhanced methanol oxidation reaction, J. Mater. Chem. A 6 (2018) 12759–12767. [2] S.S. Munjewar, S.B. Thombre, Effect of current collector roughness on performance of passive direct methanol fuel cell, Renew. Energy 138 (2019) 272–283. [3] H. Li, Q. Fu, L. Xu, S. Ma, Y. Zheng, X. Liu, S. Yu, Highly crystalline PtCu nanotubes with three dimensional molecular accessible and restructured surface for efficient catalysis, Energy Environ. Sci. 10 (2017) 1751–1756. [4] S.S. Munjewar, S.B. Thombre, R.K. Mallick, A comprehensive review on recent material development of passive direct methanol fuel cell, Ionics 23 (2016) 1–18. [5] J.C. Park, C.H. Choi, Graphene-derived Fe/Co-N-C catalyst in direct methanol fuel cells: effects of the methanol concentration and ionomer content on cell performance, J. Power Sources 358 (2017) 76–84. [6] J. Xie, Q. Zhang, L. Gu, S. Xu, P. Wang, J. Liu, Y. Ding, Y. Yao, C. Nan, M. Zhao, Y. You, Z. Zou, Ruthenium-platinum core-shell nanocatalysts with substantially enhanced activity and durability towards methanol oxidation, Nano Energy 21 (2016) 247–257. [7] F. Saleem, B. Ni, Y. Yong, L. Gu, X. Wang, Ultra-small tetrametallic Pt-Pd-Rh-Ag nanoframes with tunable behavior for direct formic acid/methanol oxidation, Small 12 (2016) 5261–5268. [8] Y. Lou, C. Li, X. Gao, T. Bai, C. Chen, H. Huang, C. Liang, Z. Shi, S. Feng, Porous Pt nanotubes with high methanol oxidation electrocatalytic activity based on original bamboo-shaped Te nanotubes, ACS Appl. Mater. Inter. 8 (2016) 16147–16153. [9] X. Peng, D. Chen, X. Yang, D. Wang, M. Li, C.C. Tseng, R. Panneerselvam, X. Wang, W. Hu, J. Tian, Y. Zhao, Microwave-assisted synthesis of highly dispersed PtCu nanoparticles on three-dimensional nitrogen-doped graphene networks with remarkably enhanced methanol electrooxidation, ACS Appl. Mater. Inter. 8 (2016) 33673–33680. [10] X. Wang, L. Chen, B. Li, A density functional theory study of methanol dehydrogenation on the PtPd3(111) surface, Int. J. Hydrogen Energy 40 (2015) 9656–9669. [11] D.S. Su, G. Sun, Nonprecious-metal catalysts for low-cost fuel cells, Angew. Chem. Int. Ed. Engl. 50 (2011) 11570–11572. [12] E. Habibi, H. Razmi, Glycerol electrooxidation on Pd, Pt and Au nanoparticles supported on carbon ceramic electrode in alkaline media, Int. J. Hydrogen Energy 37 (2012) 16800–16809. [13] S. Liu, P. Jin, D. Zhang, C. Hao, X. Yang, Reaction mechanism for methanol oxidation on Au(111): a density functional theory study, Appl. Surf. Sci. 265 (2013) 443–451. [14] C. Yu, F. Jia, Z. Ai, L. Zhang, Direct oxidation of methanol on self-supported nanoporous gold film electrodes with high catalytic activity and stability, Chem. Mater. 19 (2007) 6065–6067. [15] M. Wang, Y. He, R. Li, Z. Ma, Z. Zhang, X. Wang, Electrochemical activated PtAuCu alloy nanoparticle catalysts for formic acid, methanol and ethanol electro-oxidation, Electrochim. Acta 178 (2015) 259–269. [16] L. Chen, L. Lu, H. Zhu, Y. Chen, Y. Huang, Y. Li, L. Wang, Improved ethanol electrooxidation performance by shortening Pd-Ni active site distance in Pd-Ni-P nanocatalysts, Nat. Commun 8 (2017) 14136. [17] Y. Wang, Z.M. Sheng, H. Yang, S.P. Jiang, C.M. Li, Electrocatalysis of carbon blackor activated carbon nanotubes-supported Pd-Ag towards methanol oxidation in alkaline media, Int. J. Hydrogen Energy 35 (2010) 10087–10093. [18] R.M. Modibedi, T. Masombuka, M.K. Mathe, Carbon supported Pd-Sn and Pd-Ru-Sn nanocatalysts for ethanol electro-oxidation in alkaline medium, Int. J. Hydrogen Energy 36 (2011) 4664–4672. [19] Z. Liu, X. Zhang, L. Hong, Physical and electrochemical characterizations of nanostructured Pd/C and PdNi/C catalysts for methanol oxidation, Electrochem. Commun. 11 (2009) 925–928. [20] X. Wang, B. Tang, X. Huang, Y. Ma, Z. Zhang, High activity of novel nanoporous PdAu catalyst for methanol electro-oxidation in alkaline media, J. Alloys Compd. 565 (2013) 120–126.
8
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Full Length Article
The mechanism of methanol dehydrogenation on the PdAu(1 0 0) surface: A DFT study
T
⁎
Minhua Zhang, Yufei Wu, Yingzhe Yu
Key Laboratory for Green Chemical Technology of Ministry of Education, R&D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, PR China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: PdAu(1 0 0) Methanol Dehydrogenation Mechanism Density functional theory
Density functional theory (DFT) was employed to investigate the mechanism of CH3OH dehydrogenation on the PdAu(1 0 0) surface. The adsorption of intermediates involved in the methanol dehydrogenation process was identified and all the elementary reactions were calculated. The route 3 “CH3OH → CH2OH → CHOH → CHO → CO” is determined as the most preferable pathway, of which the rate-limiting step is the dehydrogenation of CH3OH to CH2OH. PdAu has an enhanced catalytic performance towards MOR comparing to the monometallic Pd and Au systems. The strengthened adsorption of CH3OH and the decreased energy barrier of the rate-limiting step account for the enhanced activity, while the weaker CO adsorption contributes to the better anti-poisoning ability. On the PdAu(1 0 0) surface, Pd atoms play as the active sites while Au strengthens the activity and poisoning tolerance of the neighboring Pd. The synergistic effect of PdAu is explained by the d-band theory. The upshifted d-band center suggests that the Pd atoms on the PdAu(1 0 0) surface are more active than those on the Pd(1 0 0) surface. Our theoretical study reveals the mechanism of CH3OH dehydrogenation on the PdAu(1 0 0) surface, which is in favor of the further optimization of the promising non-platinum PdAu catalysts for DMFCs.
1. Introduction With the increasing demand for energy in the world, it is vital to develop innovative energy technologies for the sustainable progress of the green economy. Direct methanol fuel cells (DMFCs), converting the chemical energy of methanol to the electric energy, are thought to be one of the most prospective power conversion technologies because of their high energy conversion efficiency, high power density and the low emission of pollutants [1–3]. For DMFCs, it is critical to improve the efficiency of the methanol oxidation reaction (MOR), which has been studied on a series of transition metal surfaces [4]. Metal Pt is commonly used as the electrocatalyst for MOR due to its excellent activity [5]. But these Pt catalysts are easy to be deactivated by the CO-like species [6–9]. CO strongly adsorbed on the catalysts can hinder the reactants shifting to the active sites and decrease the reaction rates [10]. Nowadays more and more studies have concentrated on the development of non-Pt catalysts for the electrode reactions in fuel cells [11]. Razmi et al. [12] investigated the electrooxidation of glycerol on Au, Pd and Pt nanoparticles and found that all of them exhibited exceptional activities, which provided us a good guidance to develop more effective and stable electrocatalysts. In recent years, Au has
gradually aroused people’s interests for its application to the fuel cells [13]. Comparing to the generally employed platinum, gold catalysts would not sustain the deactivation by CO during the catalyzed process [14,15]. In addition, Pd is considered as an efficient catalyst for MOR owing to its higher electrocatalytic activity, better anti-poisoning ability and relative lower cost [16]. Aiming to further enhance the catalytic activity, numerous efforts have been devoted to synthesis of novel bimetallic catalysts [17–26], among which PdAu catalysts exhibit good performance. The alloying of Pd and Au improves the capacity of electrocatalyzing over the monometallic catalysts because of the ligand effects and ensemble effects [27,28]. Lots of researchers have carried out experiments to study the electrocatalytic performance of the PdAu catalysts. Wang and coworkers [20] synthesized the nanoporous PdAu catalysts by the method of chemical dealloying ternary AlPdAu, and demonstrated a greatly improved catalytic activity and durability towards DMFCs after the electrochemical measurements, which was significantly better than the NPeAu, NPePd or other well-known Pdbased nanoparticle electrocatalysts. Luo et al. [23] prepared the PdAu alloy nanocatalysts with the different ratios of Au and Pd atoms. They further characterized the structure and morphology of the PdAu
⁎ Corresponding author at: Key Laboratory for Green Chemical Technology of Ministry of Education, R&D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, PR China. E-mail address: [email protected] (Y. Yu).
https://doi.org/10.1016/j.apsusc.2020.145434 Received 13 August 2019; Received in revised form 14 January 2020; Accepted 15 January 2020 Available online 20 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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catalysts and treated them as promising non-Pt anode catalysts for the reason of the greatly enhance activity. PdAu bimetallic catalysts with the octapodal shape, which showed excellent electrocatalytic properties, were fabricated by Hong et al. [29] through a one-pot aqueous synthesis method. The ultrasonic synthesis method was employed by Bunazawa and coworkes [25] towards the PdAu catalysts, the solvent effect was also examined and the ethylene glycol was selected as the optimum one. Hu et al. [30] fabricated the well-crystallized PdAu nanoparticles via a solution plasma technique, which exhibited a improved electrochemical stability comparing to the Pd and Au nanoparticles mixture. In brief, the experiments demonstrated that the PdAu catalysts, with the enhanced activity and stability, were attractive as the non-platinum electrocatalysts for direct methanol fuel cells (DMFCs). However, the mechanism of methanol dehydrogenation on the PdAu is still unclear, which limits its further optimization. Experiments alone cannot elucidate the dehydrogenation process completely, but theoretical study has acted as a crucial role for catalysts optimization by the means of calculation [31]. Zhao et al. examined the mechanism of CH3OH dehydrogenation using DFT on the Pd catalyst [32], which was also calculated on different Au facets [33,34]. However, there is still no comprehensive elucidation of the methanol dehydrogenation mechanism on the PdAu alloy catalyst. Herein, we conduct this DFT study to reveal the mechanism of methanol dehydrogenation on the PdAu (1 0 0) facet which is the most active for the reactions [35–37]. Four routes are proposed as illustrated in Fig. 1, and the most preferable one is determined. This theoretical calculation will be beneficial to the further modifying and application of the promising PdAu catalysts.
Fig. 2. . Top view (upper) and side view (lower) of the optimized PdAu(1 0 0) surface.
et al. [47] suggested that Pd atoms preferred to form the second neighbor ensembles on the PdAu(1 0 0) surface. Thus we substituted two Au atoms with two Pd atoms to form the second neighboring Pd ensemble as shown in Fig. 2. In all calculations, the upper half of layers together with the adsorbates were relaxed while the lower half were fixed. A 15 Å vacuum was used to avoid interactions between the slab and its periodic images. The adsorption energy of intermediates, Eads, was calculated according to
2. Computational details All the DFT calculations were conducted on the principle of spin unrestricted with the Dmol3 program package in Materials Studio of Accelrys Inc [38,39]. To explain the exchange and correlation effects, the generalized gradient approximation (GGA) with the RPBE functional was utilized [40]. The double numerical plus polarization (DNP) was employed and the density functional semicore pseudopotential (DSPP) was used considering the relativity effect. A generalized Gaussian smearing technique [41] of 0.005 Ha was employed to accelerate convergence. A 2 × 2 × 1 grid was adopted to generate k-points according to the Monkhorst-Pack method [42]. The convergence criteria for the energy calculation and geometry optimization were 0.004 Ha/Å, 0.005 Å for maximum force, maximum displacement and 1.0 × 10−6 Ha, 2.0 × 10−5 Ha for the tolerance of self-consistent field (SCF), energy. The PdAu(1 0 0) surface was modeled with a 4 × 4 unit cell consisting of four layers of atoms. The lattice parameter for bulk is 4.18 Å, in good agreement with the experimental value 4.08 Å [43]. Pd atoms were placed on the topmost layer because a phenomenon of surface segregation of Pd in the PdAu bimetallic systems was evidenced [44]. Besides, it was found that the ensembles of isolated Pd atoms were more stable than continuous configurations [45,46]. Furthermore, Gotsis
Eads = EM / S − EM − ES
(1)
where EM/S, EM and ES represented the energies of the surface with intermediates, the intermediates and the PdAu(1 0 0) surface slab, respectively. The transition states were located using the complete linear synchronous transit/quadratic synchronous transit (LST/QST) method [48]. And the frequencies of them are examined to confirm the validity. The reaction energy Er was defined as
Er = EFS − EIS
(2)
The activation barrier Ea was calculated according to
Ea = ETS − EIS
(3)
where EIS, EFS and EIS represented the energies of the initial, final and transition states, respectively. 3. Results and discussions In this section, we clarify the results beginning with the adsorption configurations of the intermediates involved in methanol
Fig. 1. . Reaction network of methanol dehydrogenation on the PdAu(1 0 0) surface. 2
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Table 1 . Adsorption energies, Eads (eV), and structure parameters of the intermediates involved in methanol dehydrogenation on the PdAu(1 0 0) surface. Species
Eads (eV)
Bond length (Å)
CH3OH*
−0.55
CH3O* CH2O* CHO* CH2OH*
−1.13 −0.14 −1.72 −1.42
CHOH*
−2.34
COH*
−2.62
CO* H*
−1.05 −2.45
d(CeO) = 1.45, d(OePd) = 2.57, d(CeH) = 1.10, d(OeH) = 0.97 d(CeO) = 1.41, d(OePd) = 2.04, d(CeH) = 1.11 d(CeO) = 1.22, d(OePd) = 2.68, d(CeH) = 1.11 d(CeO) = 1.20, d(CePd) = 1.98, d(CeH) = 1.12 d(CeO) = 1.39, d(CePd) = 2.09, d(CeH) = 1.10, d(OeH) = 0.98 d(CeO) = 1.32, d(CePd) = 1.95, d(CeH) = 1.10, d(OeH) = 0.99 d(CeO) = 1.32, d(CePd) = 1.94, d(CeAu) = 2.06, d(OeH) = 0.98 d(CeO) = 1.16, d(CePd) = 1.93 d(HeAu) = 1.76, d(HePd) = 1.71
tally well with this point that methanol prefers to be adsorbed on the top site of Pd atom with the formation of OePd bond. The values of OePd, OeC and OeH bonds are 2.57 Å, 1.45 Å and 0.97 Å respectively with a smaller CeO-surface angle of 35° compared with the Pd, Au and other catalysts [34,50], which indicates that the methylidyne H is closer to the surface and easier to be activated on the PdAu(1 0 0) surface. The adsorption energy is calculated to be −0.55 eV comparing to that on the Pd(1 0 0) being −0.42 eV. 3.1.2. For the CHxO(x = 1–3) species The CHxO species are formed through the initial cleavage of OeH bond followed by the subsequent CeH abstractions. Similar to methanol, methoxy(CH3O) is also preferably adsorbed on the top of Pd atom through the OePd bond. The OePd bond is shortened from 2.57 Å to 2.04 Å and the adsorption energy increases to −1.13 eV. The OePd axis is more sloping compared with that of methanol. As for formaldehyde(CH2O), it is generally accepted that CH2O prefers to be adsorbed through a η1-C-η1-O adsorption mode [37]. We placed it at different sites on the PdAu(1 0 0) surface including the top, bridge and hollow sites, and two structures were gained. One is in accordance with the η1-C-η1-O adsorption structure where CH2O binds to the surface through the O and C atom simultaneously. But according to our calculated results, this structure is not stable on the PdAu(1 0 0) surface with a tiny Eads of −0.06 eV. The other configuration is CH2O adsorbed via the long pair electrons of the oxygen atom. Though it is more stable compared with the first, it still desorbs easily due to the smallest adsorption energy of −0.14 eV. This phenomenon is also investigated upon other catalysts, which strongly limits the catalytic performance. However on the PdAu(1 0 0) surface, though CH2O interacts weakly with the surface, there is another “CHOH” route available for methanol dehydrogenation which will be depicted below. A change occurs when CHO is adsorbed. It is CePd bond rather than OePd bond that ties the intermediate with the surface. The adsorption energy is substantially strengthened to −1.72 eV, which is beneficial for the subsequent dehydrogenation reaction. The distances of CeO and CePd bond are 1.20 Å and 1.98 Å, respectively.
Fig. 3. . Most stable configurations of the intermediates involved in methanol dehydrogenation on the PdAu(1 0 0) surface. Yellow, Au atoms; Cyan, Pd atoms; Grey, C atoms; Red, O atoms; White, H atoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
decomposition. Then all the potential elementary reactions in the process are examined. Finally, after determining the mechanism of methanol dehydrogenation on the PdAu(1 0 0) surface, we launch a discussion on the difference between the bimetallic PdAu system and the monometallic Pd and Au systems. 3.1. Adsorption configurations and energies The intermediates adsorbed on the PdAu(1 0 0) surface are depicted in this subsection. There are total nine species generated in the CH3OH dissociation process. They were both placed at different sites on the PdAu(1 0 0) surface and then were calculated to obtain the adsorption energies. The most stable configurations of them are presented in Fig. 3, along with the detailed structure parameters and adsorption energies tabulated in Table 1.
3.1.3. For the CHxOH(x = 0–2) species The successive scissions of CeH bonds result in the generation of the CHxOH species. They are both bound to the surface via the C atom, which are stronger than those via the O atom. Hydroxymethyl(CH2OH) is firstly formed ater CH3OH breaking the CeH bond. CH2OH prefers to locate atop the Pd atom with a strengthened adsorption energy equivalent to −1.42 eV. The methylene and hydroxyl H atoms are both closer to the PdAu(1 0 0) surface, which is facile for their activation. CHOH will be formed if the methylene H is removed. This removal of H contributes to a stronger binding energy of −2.34 eV. The distance of the CePd decreases to 1.95 Å compared with that in CH2OH. And the
3.1.1. For CH3OH It is widely agreed that methanol is adsorbed through the oxygen long pair electrons on the metal surfaces [49]. Our calculated results 3
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CeO axis becomes more parallel with the PdeAu bond seen from the top view, which is conducive to CHOH dehydrogenation to CHO. Apart from CHO, the other product from the dehydrogenation of CHOH is COH. In its most stable structure, COH is found to prefer the bridge site on the PdAu(1 0 0) surface. The lengths of CePd and CeAu bonds are 1.94 Å and 2.06 Å respectively. The adsorption energy is calculated to be −2.62 eV since the C atom has no H neighbors.
Table 2 . Reaction energy, Er (eV), activation barrier, Ea (eV), of each elementary step and frequencies of the transition states involved in the route 1.
3.1.4. For CO and H A strong adsorption of CO can lead to the poisoning of catalysts especially those including metal Pt. While on the PdAu(1 0 0) surface, CO has a moderate adsorption, of which the Eads is calculated to be −1.05 eV. To obtain the most optimized structure and the corresponding adsorption energy, we have calculated different configurations placed at the top, bridge and hollow sites respectively. The results reveal that CO, which binds vertically to the metal surface via the CePd bond, owns the biggest binding energy. We have also examined the H adsorption at different sites. The bridge site is confirmed the most preferable location for H atom with the Eads of −2.45 eV. The HeAu bond and the HePd bond are equivalent to 1.76 Å and 1.71 Å, respectively.
Elementary reaction step
Er (eV)
Ea (eV)
Freq (cm−1)
CH3OH* → CH3O*+H* CH3O* → CH2O*+H* CH2O* → CHO*+H* CHO* → CO*+H*
1.19 −0.18 −0.03 −0.58
1.67 0.52 0.50 0.36
−458.62 −474.72 −888.20 −754.27
route 1 are shown in Fig. 4. Table 2 lists the reaction energies, activation barriers of the reactions and the frequencies of the transition states. CH3OH* → CH3O* + H* The first step of the route 1 is the dehydrogenation reaction of CH3OH to form CH3O and H atom. In the initial state, CH3OH is adsorbed at its most stable site. The hydroxyl H is approaching to the AuePd bridge site. In the transition state, the OeH bond elongates from 0.97 Å to 1.62 Å. The H atom locates at the bridge site. In the final state, CH3O and H atom are coadsorbed on the PdAu(1 0 0) surface at the most stable configuration. The H atom locates at the neighboring bridge site, and CH3O is adsorbed through a 2.05 Å CePd bond. This step is unfavorable in both thermodynamics and kinetics with the reaction energy of 1.19 eV and the barrier energy of 1.67 eV, respectively. CH3O* → CH2O* + H* Then CH3O is dehydrogenated to CH2O. In this process, one H atom is dissociated from the C atom in CH3O, gradually approaching to the top of Au atom. The distance between the leaving H atom and C atom extends from 1.11 Å to 1.68 Å in the transition state. In the final state the CeH bond breaks and the H atom shifts to the bridge site. This step is energetically facile due to its exothermic energy equivalent to −0.18 eV. The reaction barrier is calculated to be 0.52 eV, which suggests that it is also kinetically feasible. CH2O* → CHO* + H* Subsequent reaction is CH2O dehydrogenation to CHO, which is also the third step in the route 2. In the initial state, CH2O is adsorbed through its oxygen long pair electrons. The distance between the C and the H departing from CH2O is 1.11 Å. In the transition state, the H atom
3.2. Reaction pathways In this subsection we will elucidate all the elementary reactions included in methanol dehydrogenation on the PdAu(1 0 0) surface. The most stable structures play as the initial states. And the most stable coadsorption configurations were selected as the final states. All the transition states have been calculated to confirm that each of them has only one imaginary frequency. There are 4 routes for methanol dehydrogenation into CO and H ultimately on the PdAu(1 0 0) surface, which can be divided into the initial OeH bond activation (route 1) and the initial CeH bond activation (route 2, 3, 4) as Fig. 1 shows. 3.2.1. Initial OeH bond activation The route 1 undergoes the cleavage of OeH bond of methanol primarily with the removal of methylidyne H following. The initial, transition and final states of each elementary reaction involved in the
Fig. 4. . Initial, transition and final states of each elementary step involved in the route 1. 4
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links with the C atom, Pd atom and Au atom, of which the bond lengths are 1.52 Å, 2.11 Å and 2.01 Å respectively. The C atom binds to the Pd atom at a distance of 2.11 Å. In the final state, the H atom moves to the neighboring bridge site indicating the abstraction of CeH bond. CHO is adsorbed on the top of Pd atom through the CePd bond, which shortens the distance between CHO and the surface. This step is a little exothermic with the −0.03 eV reaction energy. An activation barrier of 0.50 eV should be overcome in this step, which indicates that this step is kinetically favored. CHO* → CO* + H* This is the last step for the route 1 but also for the route 2 and 3. In the initial state, the axis of CePd is nearly vertical to the surface. Then the H atom is activated and tends to transfer towards the PdeAu bridge site. In the final state, the scission of CeH bond indicates the production of the final products. CO is adsorbed through a CePd bond with the H coadsorbed at the neighboring bridge site. The reaction energy of this step is −0.58 eV and the activation barrier is 0.36 eV, revealing that the dehydrogenation of CHO is not only energetically but also kinetically promoted.
Table 3 . Reaction energy, Er (eV), activation barrier, Ea (eV), of each elementary step and frequencies of the transition states involved in the route 2–4. Elementary reaction step
Er (eV)
Ea (eV)
Freq (cm−1)
CH3OH* → CH2OH*+H* CH2OH* → CH2O*+H* CH2OH* → CHOH*+H* CHOH* → CHO*+H* CHOH* → COH*+H* COH* → CO*+H*
0.86 0.15 0.75 −0.63 1.07 −0.99
1.41 1.11 1.30 0.45 1.10 1.76
−813.41 −251.83 −43.45 −742.64 −816.98 −1041.21
involved in the route 2 to 4 are shown in Fig. 5, with the Er, Ea and frequencies of the transition states listed in Table.3. CH3OH* → CH2OH* + H* Instead of CH3O, CH2OH is produced firstly from methanol for the route 2–4. The initial state of this reaction is the same with that in the CH3OH* → CH3O* + H* step. The H that will be removed in this step is activated by the surface, moving towards the bridge site of the Pd and Au. In the transition state, the distance of the CeH is stretched from 1.10 Å to 1.68 Å. The location of H atom is similar to that in the final state but closer to the Pd atom. And the C atom is binding to the Pd atom. In the final state, CH3O and H are coadsorbed on the PdAu(1 0 0) surface. The CeH bond is further elongated to 2.52 Å indicating the breakage of it and the formation of CH2OH. CH2OH binds to the surface with the 2.13 Å CePd bond rather than the OePd bond. Compared with the reaction of CH3O forming, it is more possible for methanol
3.2.2. Initial CeH bond activation The route 2 to 4 are both initiated from the abstraction of CeH bond in methanol but differ in the intermediate from which the hydroxyl H dissociates. In the route 2, the Hydroxyl H tends to dissociate from CH2OH. While it is inclined to occur from CHOH in the route 3. However in the route 4, the hydroxyl H will not dissociate until the last COH dehydrogenation step. Optimized structures for IS, TS and FS
Fig. 5. . Initial, transition and final states of each elementary step involved in the route 2–4. 5
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dehydrogenated due to their comparable barriers. After the reaction of CH3OH breaking the CeH bond, CH2OH chooses to be dehydrogenated to CH2O in the route 2, while in the route 3 CHOH is formed. But no matter CH2O or CHOH, they are all converted into CHO, which suggests that the route 2 and the route 3 have the same last step. Though the activation barrier of the CHOH forming from CH2OH is a little higher than that of the CH2O forming, the subsequent step of the route 3 is easier compared with the corresponding step of the route 2. From this point of view, the route 2 and the route 3 are both potential pathways for methanol to be dehydrogenated on the PdAu(1 0 0) surface. However, the “CHOH” route has its advantage over the “CH2O” route. The weak adsorption of CH2O makes it a factor that limits the catalytic performance, which is verified on other catalysts [34], since the reaction CH2OH dehydrogenation to CHOH is prohibited on them. While on the PdAu(1 0 0) surface, although the easy desorption of CH2O is not modified, the “CHOH” channel is available for methanol dehydrogenation to avoid this problem. Therefore the route 3 is more effective than the route 2. Moreover, the route 4 is prohibited because the formation of COH has a quite high energy barrier. Above all, methanol prefers to be dehydrogenated on the PdAu(1 0 0) surface through the route 3 “CH3OH → CH2OH → CHOH → CHO → CO”, of which CH3OH dehydrogenation to CH2OH plays as the rate-limiting step with the energy barrier of 1.41 eV.
dehydrogenation into CH2OH on the PdAu(1 0 0) surface due to the decreased activation barrier of 1.41 eV, and the reaction energy of 0.86 eV. These results evidence that the initial CeH cleavage is more preferable on the PdAu(1 0 0) surface, which is consistent with that on Pd but opposite to Au catalysts [34,50]. CH2OH* → CH2O* + H* Two potential dehydrogenation reactions could occur on CH2OH: the scission of OeH to form CH2O (route 2) and the fracture of CeH to form CHOH. The initial state of the step generating CH2O is CH2OH adsorbed atop the Pd atom with the OeH bond of 0.98 Å. In the transition state, the H moves to the hollow site with the lengthened 1.70 Å distance of the OeH bond. The CH2O is adsorbed as the η1-C-η1O configuration in the final state. Lengths of OePd and CePd bonds are 2.42 Å and 2.47 Å, respectively. The reaction energy of this step is 0.15 eV. And the reaction barrier is 1.11 eV. CH2OH* → CHOH* + H* In the course of CHOH formation, the CeH bond breaks instead of the OeH bond. The reaction starts from the most stable configuration of CH2OH. During the reaction, the H atom bound to the C atom moves towards the surface. Finally, CHOH forms and is coadsorbed on the PdAu(1 0 0) surface together with the H atom. This step has a comparable reaction barrier equivalent to 1.30 eV with the other CH2OH dehydrogenation reaction to CH2O. In addition, due to the weak adsorption of CH2O and the strong adsorption of CHOH, CH2OH breaking the CeH bond to produce CHOH is the more effective route on the PdAu (1 0 0) surface. CHOH* → CHO* + H* Similar to CH2OH, two possible reactions could occur on CHOH to generate CHO (route 3) and COH (route 4), respectively. In the initial state, the CeO axis of CHOH becomes more parallel with the PdeAu bond, which is conducive to the removal of the hydroxyl H. During the process of CHO forming, the hydroxyl H shifts to the Au atom with the distance of OeH elongated from 0.99 Å to 1.45 Å. The final state indicates the formation of the CHO species. This step is supported energetically with the exothermic energy of −0.63 eV. The 0.45 eV barrier also makes this step feasible kinetically. CHOH* → COH* + H* In this process, the separated H is in inclined to approach the hollow site with the CeH bond stretched to 1.68 Å. In the final state, COH is adsorbed on the top site of Pd atom, and moves to the bridge site for stable adsorption afterward. It’s quite difficult for CHOH dehydrogenation into COH because of its activation barrier of 1.10 eV compared with the other CHOH dehydrogenation step. The 1.07 eV reaction energy also indicates that this step is prohibited thermodynamically. COH* → CO* + H* COH is dehydrogenated to CO, which is unique to the route 4. In the initial state, COH is adsorbed through the CePd and CeAu bonds at the bridge site. Then the H approaches to the Au atom in the transition state and finally locates atop the Pd atom. Though this step is exothermic, it is unlikely to occur for this step due to the quite high activation barrier of 1.76 eV.
3.4. Comparison with the monometallic systems Experimental researches [20,23,51] demonstrated that the PdAu exhibited not only the greatly enhanced catalytic activity but also the long-term stability toward methanol electrooxidation, which was significantly better than the monometallic Pd and Au. Our calculated results are accordance with the experimental conclusions and can explain the advantages of PdAu from the mechanism aspect. Our calculated results suggest that the methanol dehydrogenation mechanism on the PdAu(1 0 0) surface is similar to that on the Pd(1 0 0) surface but different from the Au(1 0 0) surface. It is consistent with the experiment results carried out by Wang et al. [20], indicating that Pd should act as primary active sites catalyzing the dehydrogenation of methanol while Au enhances the activity and poisoning tolerance of the neighboring Pd. The mechanism of methanol dehydrogenation on the Au(1 0 0) surface was investigated by Hussain and coworkers [34]. Though the pathway was determined as “CH3OH → CH3O → CH2O → CHO → CO”, they revealed that methanol dehydrogenation on the pure Au(1 0 0) is difficult due to the weak adsorption of intermediates and the high reaction barriers. This verifies the point that Pd atoms play as the active sites on the PdAu(1 0 0) surface. Zhao and coworkers [32] examined CH3OH decomposition on the Pd(1 0 0) surface. They found that methanol preferred to dehydrogenate via the route “CH3OH → CH2OH → CHOH → CHO → CO”, of which the rate-limiting step was the CeH bond scission of CH3OH with the barrier of 1.79 eV. Such a route is also preferred on the PdAu(1 0 0) surface based on our computational results, while some improvements resulting from the alloying Au contributes to the greatly enhanced performance of the PdAu catalysts in experiments. The weak adsorption of CH3OH is a critical problem limiting the activity of catalysts for DMFCs [51]. However on the PdAu(1 0 0) surface, the adsorption of CH3OH is strengthened with the binding energy of −0.55 eV comparing with that on Pd(1 0 0) of −0.42 eV [32]. Moreover, the decreased rate-limiting step barrier is also a cause leading to the enhancement of activity. The rate-limiting reaction on the PdAu(1 0 0) surface is the same with that on the Pd (1 0 0), but the value of energy barrier reduces to 1.41 eV from 1.79 eV on Pd(1 0 0). Besides, the weaker CO adsorption energy (PdAu(1 0 0) of −1.05 eV versus Pd(1 0 0) of −2.09 eV) accounts for the long-term stability property. The CO would not cover the catalyst but desorb or react with the OH– to be removed since a strong adsorption could prevent the removal reactions. It’s typical for Au to avoid the deactivation by CO during the reaction process [14,52]. The alloying Au is
3.3. Reaction mechanism on the PdAu(1 0 0) surface In present work, we have calculated the intermediates adsorption and the elementary reactions involved in methanol dehydrogenation on the PdAu(1 0 0) surface as discussed above. Fig. 6 shows the potential energy surfaces of methanol dehydrogenation on the PdAu(1 0 0) surface. The initial OeH bond cleavage or CeH bond cleavage distinguishes the route 1 or route 2 to 4. After the reactant CH3OH adsorbed on the surface, it’s more difficult for it to break the OeH bond primarily compared with the CeH bond. The energy barrier of 1.67 eV versus 1.41 eV indicates that CH3OH on the PdAu(1 0 0) surface tends to be dehydrogenated to CH2OH firstly. Thus the route 1 is forbidden, but the route 2 and 3 are both feasible pathways for methanol to be 6
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Fig. 6. . Potential energy surfaces of methanol dehydrogenation on the PdAu(1 0 0) surface.
4. Conclusion
responsible for the enhancement of anti-poisoning property of the PdAu alloy system, which is also studied by other researchers [53]. To examine the synergistic effect between Pd and Au in the PdAu system, we calculated the d-band centers of the Pd atoms on PdAu (1 0 0) surface and Pd(1 0 0) surface respectively, according to
εd =
∫ ρEdE ∫ ρdE
In this article, the mechanism of methanol dehydrogenation on the PdAu(1 0 0) surface was studied using DFT calculations. The adsorption of intermediates involved in the methanol dehydrogenation process was confirmed. All the elementary reactions have been calculated and there are total 4 routes for methanol to be dehydrogenated on the PdAu (1 0 0) surface. CH3OH is firstly dehydrogenated to CH2OH via the initial CeH bond cleavage, which is more favorable than the OeH bond breakage producing CH3O. The route 2 “CH3OH → CH2OH → CH2O → CHO → CO” and the route 3 “CH3OH → CH2OH → CHOH → CHO → CO” have comparable barriers. However the most preferable path for CH3OH dehydrogenation on the PdAu(1 0 0) surface is determined as the route 3 due to the easy desorption of CH2O. CH3OH dehydrogenation to CH2OH acts as the rate-limiting step with the energy barrier of 1.41 eV. The methanol dehydrogenation mechanism on the PdAu(1 0 0) surface is similar to that on the Pd(1 0 0) surface but different from the Au(1 0 0) surface, indicating that Pd atoms play as the active sites while Au strengthens the activity and durability of the PdAu system. The enhanced adsorption of CH3OH (−0.55 eV on the PdAu(1 0 0) versus −0.42 eV on the Pd(1 0 0)) and the decreased energy barrier of the rate-limiting step (1.41 eV on the PdAu(1 0 0) versus 1.79 eV on the Pd(1 0 0)) account for the strengthened activity of
(4)
where E is the energy with respect to the Fermi level and ρ is the electronic density of states. Nørskov et al. [54] came up with the theory which demonstrated that the d-band centers of transition metals could represent their activities. The DOS and d-PDOS plots of the Pd atoms on PdAu(1 0 0) and Pd(1 0 0) surfaces are shown in Fig. 7. It is obvious that the Pd atoms on PdAu(1 0 0) surface are different in the electronic properties from those on Pd(1 0 0) surface. Then the d-band centers of Pd atoms were calculated to be −1.39 eV on the PdAu(1 0 0) and −1.69 eV on the Pd(1 0 0). The positively shifting by 0.3 eV indicates a higher activity of the Pd atoms on PdAu(1 0 0) surface comparing with those on Pd(1 0 0) surface, which leads to the advantages of the PdAu system.
Fig. 7. . (A) Density of states of the Pd atoms on PdAu(1 0 0) surface and Pd(1 0 0) surface and (B) partial density of states of the d band (dePDOS) and deband centers of the Pd atoms on PdAu(1 0 0) surface and Pd(1 0 0) surface. 7
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the PdAu system. The CO has a weaker adsorption on the PdAu(1 0 0) with the binding energy of −1.05 eV comparing to that on the Pd (1 0 0) of −2.09 eV, which contributes to a better anti-poisoning ability of the PdAu over the Pd. The synergistic effect of the PdAu is explained by the d-band center theory. The upshifted d-band center suggests that the Pd atoms on the PdAu(1 0 0) surface are more active compared with those on the Pd(1 0 0) surface. Our results reveal the mechanism of methanol dehydrogenation on the PdAu(1 0 0) surface and explain the enhanced catalytic performance from the theory aspect, which verifies that PdAu is a promising non-platinum catalyst for DMFCs. This theoretical study will be beneficial to the further modification and application of the PdAu catalysts.
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