Mechanistic Study of Pt-Catalyzed Electrooxidation of HCOOH in Acid

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Mechanistic Study of Pt-Catalyzed Electrooxidation of HCOOH in Acid Medium: Kinetic Considerations on Effect of Solvation Lihui Ou, Junxiang Chen, Yuandao Chen, and Junling Jin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09386 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 12, 2018

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The Journal of Physical Chemistry

Mechanistic Study of Pt-Catalyzed Electrooxidation of HCOOH in Acid Medium: Kinetic Considerations on Effect of Solvation

Lihui Ou1, Junxiang Chen2*, Yuandao Chen1, Junling Jin1

1College

of Chemistry and Materials Engineering, Hunan Province Cooperative Innovation Center for the

Construction & Development of Dongting Lake Ecologic Economic Zone, Hunan University of Arts and Science, Changde 415000, China. 2CAS

Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of

Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, China.

Abstract: The HCOOH oxidation on Pt(111) under gas-phase and the present simulated electrochemical environment have been studied systematically by combining density functional theory (DFT) with mean-field kinetic analysis. Our present results indicate that HCOOH oxidation in acid aqueous-phase conditions is substantially different from that in gas-phase. Using the present models, the detailed oxidation pathways are evaluated on Pt(111) under the effect of solvation. The present calculations show that HCOOH oxidation mainly proceeds via direct oxidation involving COOH intermediate, and HCOO is only a spectator in gas-phase, whereas it mainly occurs via direct pathway including HCOO intermediate at the present simulated acid electrochemical interfaces and CO is a poisoning species that can block surface active sites, which can qualitatively and rationally explain some conflicting experimental and theoretical findings at present. Furthermore, the origin of solvation effect on HCOOH oxidation mechanisms is revealed. Our present simulated electrochemical interfaces may be able to partially represent the real electrocatalytic systems for HCOOH oxidation in acid solution. The models we used and the corresponding analysis may be able to be used to simulate other electrocatalytic reactions.



To whom correspondence should be addressed. Lihui Ou: E-mail: [email protected]. Phone: (86-736)7186115.

Junxiang Chen: E-mail: [email protected].

ACS Paragon Plus Environment

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1. INTRODUCTION The electrocatalytic oxidation of formic acid (HCOOH) over the past decades has attracted great interest due to the possibility of developing direct formic acid fuel cells (DFACs) with high power densities, nontoxic nature of HCOOH and fast oxidation kinetics, which could replace conventional combustion engines that used in vehicles and be also power sources for portable electronic devices.1-8 Pt is known as most excellent electrocatalyst for HCOOH oxidation since Pt surface can give lower overpotential as compared with other metals. Thus, Pt is extensively used as the anode electrocatalysts in DFACs.9-14 At present, it is generally accepted that HCOOH is electrochemically oxidized to CO2 on Pt electrode through dual-pathway mechanisms composing of indirect and direct pathways,15-20 in which the indirect pathway involves the dehydration of HCOOH into CO, which then oxidized to CO2, whereas the direct pathway proceeds through serial dehydrogenation of HCOOH into CO2. However, the detailed mechanisms on HCOOH oxidation on Pt at the atomic level have not yet been fully understood. Debates still exist on some unsolved key problems, such as which is main pathway, what intermediates are involved in direct pathway, and how CO intermediate is formed, etc. Numerous experimental investigations for HCOOH electrooxidation have presented different views on which pathway may be optimal and what may be relevant intermediates. The previous in situ infrared reflection-absorption spectroscopy (IRAS) clearly showed that the adsorbed CO formed through HCOOH dehydration on Pt, which is a poisonous species for HCOOH oxidation, confirming the existence of indirect pathway.24, 25 Identity of reaction intermediates in direct pathway is particularly controversial. COOH species have long been assumed to be the intermediates,15-17, 26 for example, in situ IR spectroscopy studies from Sun et al. speculated that COOH is a possible reaction intermediate on Pt electrode in acid solution.15 By using cyclic voltammograms and surface-enhanced infrared absorption spectroscopy (SEIRAS) in an attenuated total reflection (ATR) mode in acid medium, Osawa et al. observed adsorbed HCOO species as the active intermediate in direct pathway on Pt, which is then oxidized into CO2.10, 11, 27-29 Moreover, they further confirmed that HCOOH is oxidized to CO2 through HCOO intermediate by isotope-labeled formic acids (H12COOH and


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H13COOH),30 in which HCOO oxidation into CO2 is considered to be the rate-determining step. The consistent conclusions are also obtained in acid solution on the Pt electrode by the cyclic voltammograms studies from Herrero et al., in which HCOO species are involved in oxidation mechanisms and plays a key role and indirect pathway through CO intermediate is negligible.31-34 However, the study from Behm et al. and Chen et al. showed that HCOO species are not reaction intermediates for the direct pathway on the Pt electrode, but probably site-blocking spectator species during HCOOH oxidation.35-37 Chen et al. proposed that the weakly adsorbed HCOOH precursor is directly oxidized into CO2 by direct HCOOH oxidation mechanism through the intermediates of neither HCOO nor CO on Pt, showing that neither the indirect pathway by CO intermediate nor the direct pathway by HCOO intermediate is dominant for HCOOH oxidation, instead the adsorbed COOH intermediate is probably formed by C-H bond activation as the rate-determining step for the direct pathway based on the electrochemical data and kinetic model.9, 38 Thus, although the previous experimental studies paid the extensive and prolonged attention to HCOOH oxidation pathways on Pt, the mechanistic details have remained controversial. To clarify the above conflicting experimental studies, numerous theoretical simulations based on first-principles were performed on HCOOH oxidation mechanisms. Due to the complexity of interaction between HCOOH and H2O molecules and the difficulty to simulate solid/liquid interfaces theoretically, unfortunately, there are few first-principles studies reported on HCOOH oxidation mechanisms in Pt/H2O interface. How to model the H2O/metal interface is also an important challenge at present in the modeling of electrocatalytic reaction. Even so, some theoretical calculations under aqueous-phase environment were still used to study HCOOH oxidation on Pt(111). For example, based on the density functional theory (DFT) calculations, the results obtained by Neurock et al.39 showed that the pathway through HCOO intermediate has a large activation energy (ca. 1.1 eV) and COOH is the most probably reaction intermediate during HCOOH oxidation on Pt(111) in aqueous-phase, which further transform into CO2 requires low activation energy, thus suggesting that the direct pathway through COOH intermediate is active, supporting the abovementioned experimental studies by Chen et al.38 By using continuum solvation model, Liu et al. also studied HCOOH oxidation at Pt/H2O interface,40 showing that HCOO species can promote the adsorption of HCOOH in a CH-down configuration, in which the presence of pre-adsorbed HCOO species could be as a catalyst to facilitate



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the weakly adsorbed HCOOH moving into the proper configuration for HCOOH direct oxidation. Notably, the both theoretical studies from Neurock et al. and Liu et al. suggested HCOO is only an active precursor due to the high stability of the adsorbed HCOO species and the difficulties for further oxidizing into CO2. More recently, Jacob et al. studied HCOOH oxidation mechanisms on Pt(111) under electrochemical environment and compared with oxidation reactions in gas-phase using DFT calculations,41 showing that HCOOH oxidation occurs through a single mechanism on Pt(111) in gas-phase, where only HCOO is considered as the intermediate, whereas HCOOH electrocatalytic oxidation proceeds through a dual-pathway mechanism involving HCOO and COOH intermediates on Pt(111), in which the barriers of these dual-pathway are almost identical. Furthermore, the similar results were also obtained on idealized terraced surfaces, suggesting that the hydrogen-bonding character of H2O that including in aqueous-phase can stabilize or destabilize charge transfer processes, thus strongly influencing both thermodynamics and kinetics for specific reaction steps. The most recent DFT study from Herrero et al. showed that HCOOH oxidation mainly proceeds through HCOO intermediate by including implicit and explicit solvation effects, in which only an explicit H2O molecule is considered for convenience.34 As implied by these theoretical investigations, still no consistent HCOOH oxidation mechanisms are obtained on Pt electrode theoretically, and the detailed mechanistic information has not yet been fully understood. Moreover, the earliest theoretical studies do not focused on acid ice-like H2O environment on Pt(111), which may be more able to represent real electrocatalytic systems and reflect the hydrogen-bonding interaction between H2O molecules and intermediates. Therefore, it is of importance to study HCOOH oxidation on Pt(111) on atomic level under aqueous-phase environment, in order to rationally design the sustainable and cost-effective Pt-based alloy electrocatalysts. In the modeling of electrochemical interface, solvent effect is essential, while plays an important role in electrocatalytic reaction. However, the role of solvent effect in HCOOH oxidation has not yet been systematically carried out. To elucidate the above conflicting mechanism between experiments and theoretical calculations and study the influences of solvation on the oxidation pathways, HCOOH oxidation mechanisms were investigated by using DFT under electrochemical environment on Pt(111) in present paper. We used bilayer H2O structure chosen to fill up the vacuum region to model the aqueous-phase environment, in which the H2O molecules were initially oriented in a hexagonal bilayer with the molecular dipole plane nearly parallel to


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the (111) surface and subsequently optimized. Considering HCOOH oxidation usually performs in acid medium, hydrated proton was simulated in Pt/H2O interface, which is also used to adjust the electrode potentials. A number of unique mechanistic information that included different intermediates was obtained. By comparing with pathways that occurred in gas-phase, our present studies can clarify the important role of explicit H2O molecules on oxidation reactions and further elucidate this complicated electrocatalytic oxidation mechanisms.

2. MODELS AND COMPUTATIONAL DETAILS All calculations were implemented on the basis of the periodic DFT slab approach by using the generalized gradient approximation with the Perdew–Burke–Ernzerhof exchange correlation functional.42 The nuclei and core electrons were described by ultrasoft pseudopotentials.43 A plane-wave basis set was used to solve self-consistently the Kohn-Sam equations. The plane-wave basis was set finite by using a kinetic energy cutoff of 30 Ry and a charge-density cutoff of 300 Ry. The smearing technique of Methfessel–Paxton was used to treat the Fermi surface with a parameter of 0.02 Ry.44 Calculations have been carried out by using the PWSCF codes that included in the Quantum ESPRESSO distribution,45 while XCRYSDEN graphical package has been used to produce the figures of the chemical structures.46-48 Pt(111) crystal plane is generally chosen as the representative surface for both experimental and theoretical studies since it is the most stable among various exposed basal planes of nanoparticles. Given the complexity of real HCOOH electrocatalytic systems, HCOOH oxidation reactions in acid aqueous-phase have been studied to probe the mechanism of low-temperature HCOOH fuel cells, in which 12 explicit H2O molecules with bilayer structure chosen to fill up the vacuum region were used to model the aqueous-phase environment. In fact, in the case of Pt(111), the formation of an ordered H2O bilayer structure in a hexagonal arrangement with 2/3 monolayer saturation coverage with respect to the surface normal had been demonstrated by LEED, TDS, SEM, XAS and XPS along with DFT calculations in previous experimental and theoretical studies.49-51 Our present solvation model is on the basis of the previous experimental and theoretical studies on structure and orientation of H2O on Pt(111). However, many different H2O structures may also exist, which all are approximate in energy.52 Since all energies of interest in this study are energy differences, which are not sensitive to the accurate model of H2O as long as the same model is consistently used and a reasonable model in a local minimum



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structure is choose when calculating the energy differences. The calculating systems were modeled including specific intermediates to study the adsorption of reactant HCOOH and their following reactions. Considering the coverage is 2/3 of H2O monolayer (ML), a (3x3) slab with nine metal atoms per layer was created. Prior to calculations, the thickness of Pt(111) slab was tested. The total energies of three-, four-, five-, and six-layer Pt(111) slab were calculated, and the total energy differences were obtained between four- and three-layer slab, five- and four-layer slab, six- and five-layer slab, in which the total energy differences between five- and four-layer slab, six- and five-layer slab are basically consistent. In principle, the as many as possible slab layers should be used to model the Pt(111) surface to achieve exact calculations in super-cell systems. However, the increase of the slab layers will add computational cost and decrease the computational efficiency. Thus, Pt(111) surface was modeled with theoretical equilibrium lattice constant of 3.99 Ǻ by using four metal layers.51 Using (33) uniformly shifted k-meshes for (33) slabs, Brillouin-zone integrations were performed with the special-point technique, which was tested to converge to a subset of the relative energies reported herein. Vacuum layers 16Ǻ in thickness were added above the top layer of slab, which is sufficiently large to ensure that the interactions between repeated slabs in a direct normal to the surface are negligible. The Pt atoms in the bottom two layers are fixed at the theoretical bulk positions, whereas the top two layers are allowed to relax and all the other structural parameters have been optimized to minimize the total energy of the system. Structural optimization was performed until the Cartesian force components acting on each atom were brought below 10-3 Ry/Bohr and the total energy was converged to within 10-5 Ry. The saddle points and minimum energy paths (MEPs) were located by using the climbing image nudged elastic band (CI-NEB) method.53, 54 The quasi-Newton method was employed to optimize the transition state images from the NEB calculations. Optimizing the transition state images minimizes the forces in finding the saddle point. The activation and reaction energies were calculated with applied zero point energy (ZPE) corrections. Density functional perturbation theory within the linear response was used to study the vibrational properties to consider the ZPE contributions to the activation and reaction energies.55 The ZPEs were calculated using the PHONONS code that included in the Quantum ESPRESSO distribution.45 The calculation procedure can be describes briefly as follows. The dynamical matrices of the initial state, transition state and final state are first obtained for each oxidation pathway, which are then Fourier-transformed to real space with the constructed


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force-constant matrices to obtain phonon density of states. Finally, ZPEs for various states can be evaluated by phonon density of states. Additionally, the entropies were also partially considered, in which the entropies for gaseous molecules can be obtained by the literature of Nørskov et al.,56, 57 whereas the entropies of the adsorbed surface species are ignored since the their contributions to activation and reaction energies are little and almost negligible, as shown in previous theoretical study.56

3. RESULTS To distinguish HCOOH oxidation mechanisms under gas- and acid aqueous-phase environment and determine the effect of solvation on reaction pathways, the various possible pathways based on these both environments are discussed on Pt(111). The origins of solvation effect on oxidation mechanisms are also systematically illustrated. 3.1 HCOOH Oxidation in Gas-phase 3.1.1 The Most Stable Adsorption Configurations of Adsorbed Species The possible intermediates involved in HCOOH oxidation pathways on Pt(111) are firstly investigated. The most stable adsorption configurations and adsorption energies of five possible species (HCOOH, COOH, HCOO, CO and CO2) in HCOOH oxidation are shown in Figure 1. The adsorption energies are calculated in gas-phase according to Eads = Eadsorbate/Pt(111) – EPt(111) – Eadsorbate, in which Eadsorbate/Pt(111) is the total energy of adsorbed adsorbates on Pt(111), EPt(111) is the energy of clean Pt(111) surface, and Eadsorbate is the energy of adsorbates. A more negative Eads value indicates a stronger adsorption.

Eads = -0.13 eV (a)

Eads = -2.34 eV

Eads = -2.38 eV

Eads = -1.89 eV

Eads = -0.07 eV

(b)

(c)

(d)

(e)

Figure 1. The optimized adsorption configurations and adsorption energies (Eads, eV) of the possible species involved in HCOOH oxidation on Pt(111): (a) HCOOH, (b) COOH, (c) HCOO, (d) CO, (e) CO2. Gray, red, brown, and pink



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spheres represent Platinum, Oxygen, Carbon, and Hydrogen atom, respectively.

On Pt(111), HCOOH prefers to physically adsorb in an upright fashion with CH group pointing down toward the surface and OH group pointing up, as shown in Figure 1 (a). The calculated adsorption energy is -0.13 eV. Our present HCOOH adsorption configuration is in well agreement with previous studies from Liu et al.,40 in which HCOOH prefers to adsorb on Pt(111) with CH-down configuration. COOH prefers to adsorb at a top site of Pt through C atom with OH group pointing up, as shown in Figure 1 (b), and the adsorption energy is -2.34 eV. HCOO adsorbs in a bidentate configuration through two O atoms binding at two neighboring Pt atoms with an adsorption energy of -2.38 eV, see Figure 1(c). As far as we know, a poor job of treating CO adsorption on Pt(111) is done through DFT calculations. For example, experimental results indicate that the preferred adsorption sites of CO are bridge and top sites whereas DFT calculations showed that fcc hollow sites are more favorable.58, 59 Our present calculation also indicated that CO prefers to adsorb at the fcc hollow site through C atom on Pt(111), as shown in Figure 1(d). Nonetheless, it is interesting to note that the calculated adsorption energy of -1.89 eV in our present study for CO on Pt(111) is in well agreement with a value of -1.75 eV that determined in experiments. CO2 is nearly parallel to the Pt(111) surface in a linear configuration and weakly physisorbs on the surface and the adsorption energy is as small as -0.07 eV. These most stable adsorption configurations are used to perform subsequent minimum energy pathway (MEP) analysis in gas-phase. 3.1.2 The MEP of HCOOH Oxidation For the first oxidation of HCOOH, three possibilities are considered on Pt(111): formations of HCOO, COOH and CO intermediates through O-H, C-H bond scission and dehydration, respectively. Based on the MEP analysis (See Figure S1), the activation energy for HCOO formation is calculated as 0.20 eV with the reaction energy of -0.41 eV. The C-H bond activation to form COOH is also exothermic by -0.53 eV, and has an activation energy of 0.19 eV. However, HCOOH dehydration into CO has a considerably higher activation energy of 1.27 eV although this process is calculated to be stronger exothermic by -0.97 eV on Pt(111) in gas-phase. Thus, our present studies show that HCOO and COOH intermediates may be all able to be easily formed on Pt(111) during HCOOH oxidation since the formations of these both intermediates is exothermic and have extremely low activation energies, which can be easily overcome by thermo-activated process and may be parallel pathways on Pt(111), implying that indirect pathway through CO intermediate may be impossible to


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occur under gas-phase environment. Accordingly, the final oxidation production CO2 may be able to be formed through HCOO and COOH intermediates dehydrogenation. As shown in Figure S2, the O-H bond activation step of COOH intermediate to form CO2 is exothermic by -0.21 eV with an activation energy of 0.42 eV, the C-H bond activation from HCOO intermediate is also exothermic by -0.43 eV but has a significantly higher activation energy of 1.04 eV. Although CO2 formation through HCOO is more exothermic than that through COOH oxidation, the activation energy for COOH oxidation into CO2 is significantly lower than that for HCOO oxidation process. Simultaneously, we noted that CO intermediate may be also able to be formed through COOH dehydroxylation with a relatively lower activation energy of only 0.34 eV, which further oxidation into CO2 is exothermic by -0.43 eV and has a low activation energy of 0.66 eV. Thus, our present studies show that the final production CO2 may be able to be formed by COOH dehydrogenation and intermediate CO that formed through COOH dehydroxylation further oxidation on Pt(111) in gas-phase, whereas HCOO is only a spectator. Based on the adsorption configuration of HCOO, we can speculate that the rigid molecular configuration with the C-H bond oriented along the surface normal make it very difficult to activate the C-H bond.39 Thus, intermediate COOH further oxidation may be more favorable on Pt(111) than that of HCOO. Overall, the reaction pathways with the lowest activation energies are the most critical in determining HCOOH oxidation mechanism on Pt(111). Based on the above MEP analysis, two optimized energy pathways are obtained in gas-phase, as shown in Figure 2. One is COOH pathway, in which COOH is yielded through HCOOH dehydrogenation, and then further oxidized into CO2 through dehydrogenation, namely, HCOOH* → (COOH + H)*, COOH* → CO2 + H*, the rate determining step is COOH* → CO2 + H* with an activation energy of 0.42 eV; another is defined as CO pathway, in which intermediate CO is formed by COOH dehydroxylation, and finally leading to CO2 formation, namely, HCOOH* → (COOH + H)*, COOH* → (CO + OH)* → CO2 + H*, the rate determining step is (CO + OH)* → CO2 + H* with an activation energy of 0.66 eV. Our present results in gas-phase confirmed the abovementioned experimental study from Chen et al., namely, HCOOH initial oxidation may proceed through C-H bond activation to form COOH intermediate on Pt electrodes.38



0.8

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COOH* CO2

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Figure 2. The overall energy diagram for (a) COOH and HCOO pathways; (b) CO pathway on Pt(111) in gas-phase.

3.2 HCOOH Electrocatalytic Oxidation Mechanisms 3.2.1 The Modeling of Electrochemical Interfaces In contrast to the gas/solid interface, the theoretical investigation of atomic level HCOOH electrocatalytic oxidation mechanisms in electrocatalysis occurring in aqueous/solid interface is quite challenging since electrode potential and aqueous-phase environment must be considered, which are the critical factors influencing electrocatalytic reactions. For modeling HCOOH electro-oxidation reactions, acid medium is simulated by introducing H atoms into H2O bilayer structure which can separate into protons and electrons. Furthermore, the surface charge and the electrode potential can be varied through the change of the concentration of protons.60-62 The electrode potential (U) can be approximately obtained by referring the work function (Φ) of the Pt/H2O system to the experimental work function of standard hydrogen electrode (SHE) based on the equation, U =Φ/e – 4.44, in which work function (Φ) is calculated according to the equation, Φ = VVacuum – EFermi. Only an H atom is introduced into H2O bilayer for the sake of simplicity in the present paper, the calculated work function is 4.87 eV and the corresponding electrode potential is 0.43 V (vs. SHE), which is converted to the RHE scale at pH = 3.75 (pKa value of HCOOH and the oxidation performance is maximal at a pH close to the pKa)28 with URHE = USHE + 0.059 pH in order to compare with experimental results, ca. 0.65 V (vs. RHE), just being in range of the low experimental potentials of HCOOH electro-oxidation into CO2. Using the model with explicit H2O bilayer


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structure and electrical field effects from hydrated protons have been reported to play an important role in oxygen reduction, hydrogen evolution and CO2 reduction reactions.60-62 3.2.2 The Most Stable Adsorption Configurations of Reactive Species in Aqueous-phase Under acid aqueous-phase environment, the most stable adsorption configurations and adsorption energies of five possible species (HCOOH, COOH, HCOO, CO and CO2) during HCOOH oxidation are shown in Figure 3. Considered that the relaxation of H2O molecules in acid aqueous-phase contributes a significant portion to the energy variation in the geometric optimization of system, the total energy of Pt/aqueous-phase system was used as a reference to calculate the value of adsorption energy. Therefore, we define the adsorption energy Eads for HCOOH and intermediates on Pt(111) under acid aqueous-phase environment as Eads = Etotal – EPt(111)/aqueous-phase – Eadsorbate, in which Etotal is the total energy of adsorbed reactive species on Pt(111) in acid aqueous-phase, and EPt(111)/aqueous-phase is the total energy of Pt/aqueous-phase system.

Eads = -0.42 eV (a)

Eads = -2.06 eV (b)

Eads = -2.58 eV

Eads = -1.84 eV

(c)

(d)

Eads = +0.12 eV (e)

Figure 3. The Optimized adsorption configurations and Adsorption Energies (Eads, eV) of the possible species involved in HCOOH oxidation on Pt(111) under acid aqueous-phase environment: (a) HCOOH, (b) COOH, (c) HCOO, (d) CO, (e) CO2.

HCOOH still prefers to adsorb on the Pt(111) surface in a CH-down configuration with an adsorption energy of -0.42 eV under acid aqueous-phase environment, as shown in Figure 3(a), which is well consistent with that observed in gas-phase. However, the adsorption is significant stronger than in gas-phase due to solvation effect. As observed in gas-phase, the adsorption of COOH at a top site of Pt through C atom with OH group pointing up is still the most stable configuration under the present simulated electrochemical interfaces, as shown in Figure 3(b), and the corresponding adsorption energy is -2.06 eV, which is slightly lower than that in



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gas-phase. Similarly, HCOO still adsorbs in a bidentate configuration through two O atoms binding at two neighboring Pt atoms in acid aqueous-phase with an adsorption energy of -2.58 eV, see Figure 3(c). In acid aqueous-phase, our present calculations also indicated that CO prefers to adsorb at the fcc hollow site through C atom on Pt(111) with the calculated adsorption energy of -1.84 eV, as shown in Figure 3(d), the value of adsorption energy is nearly equal with that in gas-phase. Under acid aqueous-phase environment, however, the geometry configuration of CO2 is significantly changed with a slightly positive adsorption energy of +0.12 eV, which is no longer a linear configuration due to influence of solvation, as shown in Figure 3(e), implying the chemisorbed CO2 is unstable and easy to desorb. 3.2.2 The MEP of HCOOH Oxidation in Acid Aqueous-phase During HCOOH electro-oxidation in acid aqueous-phase, H atoms removed from HCOOH remain at the electrochemical interface, either as a hydrated proton (Heterolytic pathway) or as an explicitly adsorbed H species on Pt electrode (Homolytic pathway). To model and maintain nearly constant concentration of proton and corresponding electrode potential, homolytic pathways for HCOOH electro-oxidation are considered in our present study, namely, Langmuir-Hinshelwood HCOOH oxidation mechanisms. Furthermore, our previous study on CH3OH decomposition in aqueous-phase showed that the homolytic and heterolytic pathways may be able to take place simultaneously on Pt electrode.63 Starting from HCOOH with the CH-down configuration on Pt(111), the MEP analysis is carried out for three possibilities (See Figure S3), as illustrated in gas-phase. The activation energy for dehydration of HCOOH produces CO is ca. 0.25 eV, and the reaction is highly exothermic by -1.78 eV. The C-H and O-H bonds activation processes in HCOOH that leading to the formation of adsorbed COOH and HCOO intermediates are relatively lower exothermic by -0.37 and -0.01 eV, respectively. The corresponding activation energies of respective process are 0.65 and 0.58 eV. In comparison, the activation energies for dehydrogenation are nearly equal, but larger than that of dehydration process. Thus, we conclude that indirect pathway that through CO intermediate is able to be more easily to take place on Pt(111) in acid solution. However, we also note that the activation energies for the formation of COOH and HCOO intermediates are very low, only ca. 0.60 eV, which can be easily overcome at room temperature, indicating that the direct pathway through these both intermediates is also possible to occur. Therefore, the indirect and direct pathways may be able to occur simultaneously on Pt(111) at the present simulated electrochemical interfaces.


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For the final oxidation production CO2 formation on Pt(111) in acid aqueous-phase, starting from COOH and HCOO intermediates, the O-H and C-H bond activation steps into CO2 are exothermic by -1.10 and -1.46 eV with activation energies of 0.05 and 0.42 eV, respectively, in which the C-H bond activation from HCOO has a slightly higher activation energy, but can be overcome easily via a thermo-active process, as shown in Figure S4(a). Simultaneously, we noted that CO formation through COOH dehydroxylation requires a relatively higher activation energy of 0.45 eV than HCOOH dehydration, showing that CO is mainly produced by HCOOH initial oxidation at the present simulated electrochemical interfaces (See Figure S4b). Subsequently, CO is further oxidized into CO2 with a low activation energy of 0.59 eV. Thus, our present studies showed that the final production CO2 can be formed by COOH, HCOO dehydrogenation and intermediate CO that formed through HCOOH dehydration further oxidation on Pt(111) based on the MEP analysis in acid aqueous-phase, namely, direct and indirect pathways may be able to take place simultaneously due to the low activation energies of various reaction steps. 0.8

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0.0

Energy/eV

Energy/eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.4 -0.8 TS2

-1.2 -1.6

-2.0

CO2

Indirect Pathway CO*

-2.0

0

2

4

6

8

10

12

14

16

0

Reaction Coordiate (a)

2

4

6

8

10

12

14

16

Reaction Coordiate (b)

Figure 4. The overall energy diagram for (a) Direct pathway through COOH and HCOO intermediates; (b) Indirect pathway on Pt(111) in acid aqueous-phase.

Based on the above MEP analysis, HCOOH oxidation mechanisms are obtained on Pt(111) at the present applied electrode potential, as shown in Figure 4. One is direct pathways including COOH and HCOO intermediates, in which COOH and HCOO is produced through HCOOH dehydrogenation, and then further oxidized into CO2 through dehydrogenation, namely, HCOOH* → (COOH + H)*, COOH* → CO2 + H* and



Page 14 of 33

HCOOH* → (HCOO + H)*, HCOO* → CO2 + H*, the rate determining step are HCOOH dehydrogenation with activation energies of ca. 0.60 eV; another is indirect pathway including CO intermediate, in which CO is formed by HCOOH dehydroxylation, and leading to CO2 formation, namely, HCOOH* → CO* + H2O, (CO + OH)* → CO2 + H*, the rate determining step is CO further oxidation into CO2 with an activation energy of 0.59 eV. Our present MEP results under acid aqueous-phase environment confirmed previous experimental and theoretical studies on COOH and HCOO species as intermediates and poison effect of CO intermediate on pure Pt electrode.9-27, 30, 35, 39-41 However, our present studies in acid solution showed that CO is mainly formed by HCOOH dehydration on Pt(111), rather than dehydroxylation of COOH as observed in gas-phase.

4. DISCUSSION 4.1 Analysis for the Optimal HCOOH Oxidation Pathways with Mean-Field Kinetics Given the present studies on Pt(111) in gas-phase, we found that HCOOH oxidation mechanism should be considered as two different pathways through the above MEP analysis, each starting from adsorbed HCOOH with CH-down configuration. The direct pathway possibly proceeds by a strongly adsorbed COOH intermediate formed through initial C-H bond activation step of HCOOH, in which HCOO intermediate is only a spectator due to significantly higher activation energy for its further oxidation into CO2. The CO pathway operates by CO intermediate, which is from COOH dehydroxylation, rather than a generally previous accepting product of HCOOH dehydration, thus named as CO pathway in our present work. Based on the overall energy diagram in gas-phase (See Figure 3), we observed that the activation energies in overall optimal pathways for HCOOH oxidation into CO2 are only ca. 0.20 and 0.25 eV for both different pathways on Pt(111), respectively, seeming to suggest that these two pathways may both be able to take place simultaneously and easily in gas-phase due to extremely low activation energies. In acid electrochemical interfaces modeled by solvent effect and electrode potential, the preferred adsorption configuration of HCOOH is almost not changed. The direct pathways can proceed via COOH and HCOO intermediates, in which HCOO is no longer a spectator due to very low activation energy induced by H2O hydrogen bond character for HCOO further oxidation into CO2 in acid medium compared with that in gas-phase. The indirect pathways occur through CO intermediate formed by HCOOH dehydration, in which the


Page 15 of 33

source of CO is inconsistent with that in gas-phase. On the basis of overall energy diagram (See Figure 4), the activation energies for the optimal HCOOH oxidation mechanism through COOH and HCOO intermediates are ca. 0.65 and 0.60 eV, respectively, whereas which is higher than that of indirect pathways through CO intermediate, ca. 0.25 eV, seeming to indicate that at the present simulated electrode potential indirect pathways are more facile to occur on Pt(111) based on the above MEP analysis. Given that the activation energies are also not too high, only ca. 0.60 eV, which are also easy to be overcome by thermo-activated process, thus the dual-pathway mechanisms of HCOOH oxidation appear to be present on Pt(111) under acid aqueous-phase environment. 0.8

0.8

TS46

1. HCOOH+*

TS23

0.0

2. HCOOH*

TS35

TS36

4. HCOO*

-0.4

3. COOH*

1: *+HCOOH 2: HCOOH* 3: COOH* 4: HCOO* 5: CO* 6:*+CO2

-0.8 -1.2 -1.6

TS23 1. HCOOH+*

0.4

TS56

TS24

5. CO* 6. *+CO2

Energy/eV

0.4

Energy/eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TS36

TS24 TS25

0.0

2. HCOOH*

3. HCOO*

-0.4

TS46

4. COOH*

-0.8

1: *+HCOOH 2: HCOOH* 3: COOH* 4: HCOO* 5: CO* 6:*+CO2

-1.2 -1.6

TS56 6. *+CO2

5. CO*

-2.0

-2.0 -2

0

2

4

6

8

10

-2

0

Reaction Coordiate

2

4

6

8

10

Reaction Coordinate

(a)

(b)

Figure 5. The calculated reaction network during HCOOH oxidation in (a) gas-phase and (b) acid aqueous-phase.

To obtain more accurate HCOOH oxidation pathways in gas- and acid aqueous-phase, mean-field kinetic analysis are proposed. To do such kinetic analysis, we should firstly put some simplifications on the reaction model. For all the associated reactions, the key is the transform among intermediates, so all of them can be simplified to the form i→j. For example, the reaction HCOOH* → CO* + H2O can be simplified to HCOOH*→CO*, and dehydrogenation reaction like HCOOH* → (HCOO + H)* (actually representing the reaction HCOOH* → HCOO* + H+ + e-) can be simplified to HCOOH* → HCOO*. Since the omitted parts are always H2O or H+, which are all considered to have the concentration of 1 mol/L, in writing the kinetic expression through mass action law, their concentrations can all be omitted. The forward and backward reaction kinetic constants Kij and Kji are calculated by the following definition:

Kij   k BT / h  exp   TSij  I i  / RT 


(1)


Page 16 of 33

where the TSij and Ii represent the energies of the transition state and reactant in the reaction i→j, respectively. As an exception, for adsorption reaction HCOOH→HCOOH*, the reaction is assumed to be fast so that we use the average energy of HCOOH and HCOOH* as the transition state energy. Then, according to the above calculated results, the whole reaction mechanisms are summarized in Figure 5, which is actually a HCOOH oxidation catalytic network. The reaction rate constant, which is calculated by Equation (1), is relative with reaction rate. In general, the reaction pathway is possibly more favorable with larger values of rate constant. The calculated values of Kij are summarized in Table S1. The associated values of rate constant for all pathways are plotted in Figure 6. The results show that the oxidations of HCOOH mainly proceed through direct oxidation including COOH intermediate since COOH formation has larger rate constant. Simultaneously, the notably slower rate constant is also observed for HCOO further oxidation into CO2 although HCOO formation has only slightly lower rate constant than COOH formation. Thus, the kinetic analysis suggests that the direct pathway through COOH intermediate is more favorable on Pt(111), whereas the direct pathway through HCOO intermediate only contributes little to HCOOH oxidation in gas-phase. At the present simulated acid electrochemical interfaces, although the indirect pathway through HCOOH dehydration into CO intermediate is more facile to occur due to higher rate constant than the direct pathways, CO further oxidation has significantly lower rate constant than COOH and HCOO further oxidation into CO2, as shown in Figure 6. Thereinto, HCOO formation and further oxidation has the moderate and largest rate constant, respectively; suggesting that the direct pathway involved HCOO intermediate may be major on the Pt electrode in acid solution.

(a)


Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) Figure 6. The schematic HCOOH oxidation pathway and the calculated rate constants: (a) in gas-phase; (b) in acid aqueous-phase.

Based on the kinetic mass action law, the corresponding coverage of adsorbed species can be solved under the steady state in order to determine which species may block the surface active sites and further confirm the above kinetic analysis, in which the coverage of all intermediates would not change with time. On this condition, the associated coverage can be calculated through the method proposed by King and Altman,64 which introduced a matrix to represent the reaction kinetics. The corresponding matrixes in gas- and acid aqueous-phase are written by equations (2) and (3), respectively.

 K 21 - K36 - K 46 - K56   1   0   K12 +K 65 +K 63 +K 64      - K12 K 21  K 23  K 24  K32  K 42 0  2   0     K 63  K53   3    0  - K 23 K32  K35  K36 0       K 64 - K 24 0 K 46 +K 42 0  4   0    1 1 1 1 1   5  1  

 K 21  K36  K 46  K56   1   0   K12 +K 65 +K 63 +K 64       K32  K 42  K52    2   0  - K12 K 21  K 23  K 24  K 25    K 63  K 23 K32  K36 0 0   3    0        K 64  K 24 0 K 46 +K 42 0  4   0    1 1 1 1 1   5  1  

(2)

(3)

where the number 1, 2, 3, 4, 5 and 6 indicate the *+HCOOH, HCOOH*, COOH*, HCOO*, CO* and *+CO2 (See Figure 5), respectively, and θ1 to θ5 mean the coverage of the blank sites, HCOOH*, COOH*, HCOO*, and CO*, respectively. The matrixes can be solved by Cramer’s rule and the corresponding values of coverage in gas- and acid aqueous-phase are obtained in equations (4) and (5), respectively, from which we understand that during HCOOH oxidation in gas-phase, the Pt surface is dominated by HCOO* intermediate due to notably higher coverage value than others. Combining with rate constant for HCOO further oxidation, it can be



Page 18 of 33

speculated that HCOO intermediate may be able to block surface active site and the direct oxidation of HCOOH through COOH is more favorable pathway, further confirming that HCOO is only a spectator, in which the present defined CO pathway is also possible to occur due to the relatively lower rate constant of CO formation by COOH and lower CO coverage. In fact, the previous studies from Chen et al. also showed that bridge bonded HCOO is a site-blocking spectator by experimental measurement and mathematical modeling and HCOOH is directed oxidized to CO2 through an adsorbed COOH intermediate.36-38 The strongly adsorbed bidentate HCOO on the Pt electrode may lead to its accumulation on the surface and very slow oxidation rate, as observed the most negative value of adsorption energy (-2.38 eV) in Figure 1(c), which is also confirmed in previous experimental study from John et al.65 However, in acid electrochemical interfaces, we observe that the surface is covered by monolayer adsorbed CO intermediate, which further explain why the CO formation and further oxidation have the largest and smallest rate constant, respectively. Thus, it can be concluded that CO mainly acts as a poisoning species during HCOOH oxidation at the Pt electrode in acid solution. Our present results at the simulated electrochemical interfaces are consistent with the previous experimental studies from Chen et al.,9, 66 in which the CO formation is fast, but CO oxidation rate is negligible at the present applied electrode potential, thereby leading to the observed notable CO accumulation with time up to saturation and concluding that the indirect pathway through HCOOH dehydration into CO and its further oxidation into CO2 only represents a minor pathway and contributes less than 0.01% to the total HCOOH oxidation rate. Simultaneously, we also note that HCOO intermediate has the lowest coverage among various adsorbed species at the present simulated electrode potential, implying HCOO may be able to be fast oxidized into CO2, which can be confirmed by the above calculated larger rate constant, as shown in Figure 6. The results are also well agreeable with the recent experimental studies from Osawa et al.28, 29 Their results showed that the observed HCOO concentration is very small, but a considerable current is given in acidic solution by HCOO oxidation, thus concluding that HCOOH oxidation through HCOO intermediate is the major pathway, which is responsible for the majority of the HCOOH oxidation current.


Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10   8.7  10   1   *   7     1.31  10    2   HCOOH*   4      3   COOH*  9.51  10       1     9.99  10  4   HCOO*        6  5   (CO+OH)*   9.21  10 

14   7.47  10   1   *   4.19  107        2   HCOOH*   11    3    COOH*   3.92  10       14     3.74  10  4   HCOO*          5   CO*   1.00 

(4)

(5)

Based on the above MEP and mean-field kinetic analysis, we have already shown that the solvent effect substantially influences HCOOH oxidation pathways and corresponding kinetics. In gas-phase, the Pt surface become covered with HCOO intermediate since HCOO formation has significantly lower rate constant than its further oxidation, namely, it forms faster than it can be removed. Therefore, HCOO species predominantly is a spectator and the direct oxidation through COOH intermediate is more favorable. However, HCOO further oxidation has the largest rate constant at the present simulated electrochemical interfaces, showing the direct pathway via HCOO intermediate is more facile to occur. Simultaneously, in acid aqueous-phase, the accumulation of adsorbed CO on Pt(111) is well established at the present applied electrode potential, which is able to confirm that adsorbed CO produced by HCOOH dehydration is a poisoning species identified from previous IRAS experimental studies,24, 26, 67 and explain why the measured HCOOH oxidation currents are still low even if at low overpotentials on pure Pt electrodes. Our present conclusions in acid medium are inconsistent with the previous theoretical studies at Pt/H2O interfaces from Neurock et al. and Liu et al.,39, 40 in which they concluded that COOH is the intermediate of direct oxidation and HCOO is a spectator on the Pt surface. The discrepancy may be that the acid medium and kinetics analysis are not considered in the previous theoretical studies from Neurock et al. and Liu et al. However, the present study is partially consistent with the most recent DFT studies from Jacob et al. and Herrero et al.,34,

41

in which HCOO is identified as key intermediate for

HCOOH oxidation. Our present conclusions are also partially consistent with recent experimental studies in acid solution based on the above analysis. The present simulated electrochemical interfaces may be able to partially represent the real electrocatalytic systems for HCOOH oxidation in acid solution at the applied electrode potential of 0.65 V (vs. RHE), corresponds to a low potential. In fact, the CO oxidation rate increases with potential due to enhanced OH formation at higher potentials. For example, the studies from Chen and Behm et al. showed that the CO oxidation rate is negligible at low electrode potentials,9 which can result in the CO accumulation, whereas the rates for CO further oxidation and HCOOH dehydration are of comparable magnitude



Page 20 of 33

at higher potentials. Thus, we can speculate that the indirect pathway through HCOOH dehydration into CO and its further oxidation into CO2 may be able to occur at higher electrode potentials, which will be considered in our future work. Additionally, HCOOH oxidation mechanisms will be also performed in the presence of specific adsorption of anion in the future, which is more representative for the modeling of real electrochemical interfaces.

(a)

(b)

Figure 7. The difference electron density maps for HCOOH (a) in gas-phase and (b) in acid aqueous-phase.

4.2 The Origin of Solvation Effect on Oxidation Mechanisms 4.2.1 The Geometry Parameters Analysis of HCOOH Molecule As discussed above MEP and kinetics, the solvent effect play an extremely critical role in determining HCOOH electro-oxidation mechanisms, particularly initial oxidation step of HCOOH on Pt(111). In gas-phase, HCOOH oxidation prefers to proceed through initial C-H bond activation steps into COOH intermediate, whereas O-H bond cleavage into HCOO occurs at acid solution. To ascertain the origin of difference of oxidation pathways between gas- and acid aqueous-phase environment, the geometry configurations of HCOOH on Pt(111) are firstly analyzed. As shown in Table S2, compared with isolated HCOOH molecule, the geometry parameters are almost not changed when HCOOH adsorbed on Pt(111) in gas-phase, indicating physisorbed HCOOH, which is able to be confirmed by extremely low adsorption energy of -0.13 eV. Under acid aqueous-phase environment, the geometry parameters of HCOOH take place significant change due to solvation effect, stronger adsorbed HCOOH with an adsorption energy of -0.42 eV on Pt(111) can confirm this phenomenon. Notably, the bond length of C-H1 in gas-phase is slightly stretched, while it is shortened in acid


Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


aqueous-phase in contrast with that of isolated HCOOH, which make the scission of C-H1 bond into COOH intermediate more easily occur in gas-phase; the bond length of O2-H2 is only slightly stretched in gas-phase, while it is considerably stretched in acid solution; the bond angle of C-O2-H2 is significantly increased that can make O2-H2 bond more easily cleavage into HCOO intermediate in acid solution, all showing that the interactions of HCOOH with H2O molecules can play a crucial role in HCOOH oxidation due to effect of hydrogen bond on geometry configuration of HCOOH, thus leading to different oxidation pathways and further confirming above MEP and kinetic analysis between gas-phase and acid medium. 4.2.2 The Electronic Structure Analysis The different HCOOH oxidation pathways in gas- and acid aqueous-phase are also indicated by the difference in electron densities given in Figure 7. In gas-phase, there is a significant electron density increase between HCOOH and Pt(111) surface, whereas such an electron accumulation is not observed in acid aqueous-phase, instead, the significant electron interactions are seen between HCOOH and H2O molecules under acid aqueous-phase environment. These different electronic properties clearly indicate that the interactions between HCOOH and Pt(111) in gas- and acid aqueous-phase can lead to different oxidation mechanisms. Thereinto, the significant electron accumulation between H1 and Pt(111) in gas-phase make the scission of C-H1 bond easier, whereas the cleavage of C-H1 bond may be more difficult to occur due to no significant electron interactions between H1 and Pt(111) in acid aqueous-phase, which are able to be confirmed by the above analysis on geometry parameters, namely, the bond length of C-H1 is stretched in gas-phase and the corresponding value becomes shorter compared with isolated HCOOH molecule in acid aqueous-phase. Simultaneously, the significant electron interactions between O2-H2 bond and H2O molecules make the bond angle of C-O2-H2 significantly increase. Thus, the existence of hydrogen bonds and interactions of HCOOH with H2O molecules under acid solution may result in more easily cleavage of O2-H2 bond. Our present conclusions of electron density difference further ascertain the above MEP and kinetic analysis. As demonstrated above, the HCOOH adsorption behavior and oxidation mechanisms may be able to be determined by electronic interactions among the Pt(111) surface, HCOOH and H2O molecule. Thus, the quantative analysis of electronic structures facilitate the further understanding the origin of H2O solvation effect on Pt(111) surface toward HCOOH oxidation. Based on the projected electron densities of states, the Löwdin



Page 22 of 33

charge (the number of valence electron) of HCOOH on Pt(111) can be obtained from Löwdin population analysis in gas- and aqueous-phase. Table 1 gives the electron gains (Δq) for C, O and H atoms in HCOOH molecule, respectively, Table 2 gives the average electron gains (Δq) for surface Pt atoms in HCOOH adsorbed Pt(111) in gas- and acid aqueous-phase, respectively, which were obtained by subtracting the Löwdin charge of the corresponding component of clean Pt(111) surface and isolated HCOOH molecule from that in the optimized structure. In the mean time, the electron gains (Δq) of H2O cluster at the electrochemical interfaces are also given in Table 2. A gain of electron by the component will be implied by a positive value of Δq. The analysis indicates that C, O2 and H1 atoms gain electrons, whereas O1 and H2 atoms lose electrons for HCOOH adsorbed on Pt(111) in gas-phase. Simultaneously, we also observed that surface Pt atoms gain electrons in gas-phase. The present results confirm the above difference electron density analysis on electron accumulation between H1 and Pt(111) surface in gas-phase since H1 and surface Pt atoms all gain electrons. Simultaneously, the electronic repulsive interactions between C and H1 make C-H1 bond cleavage more easily occur, and the electronic attractive interactions between O2 and H2 make O2-H2 bond cleavage more difficultly occur in gas-phase. However, we noticed that the nature of electronic interactions in acid solution were practically not the same as those observed in gas-phase. The total net electrons of O2 and H2 atoms are positive since p orbital can gain more electrons although s orbital loses electrons, and the strongest repulsive interactions between O2 and H2 than others make O2-H2 bond cleavage the easiest occur in acid solution. Furthermore, it was noticed that surface Pt atoms and H2O cluster under acid aqueous-phase environment all lose electrons, which should be transferred to HCOOH molecule. The corresponding results can be confirmed by the more total net electrons of HCOOH molecule in acid aqueous-phase, as shown in Tables 1 and 2. In the mean time, the present results are also able to explain why no electron accumulation between H1 and Pt(111) in acid medium. Based on Löwdin population analysis, we can conclude that the significant different electron interactions in gas- and acid aqueous-phase may lead to the difference of HCOOH oxidation pathways. Table 1 The electron gains (Δq) of total, s and p orbitals of C, O and H atoms in HCOOH under gas- and aqueous-phase environment. Difference of Electron/Atom (Δq) Total


s

p

Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C

+0.0019

+0.0010

+0.0010

O1

-0.0107

-0.0001

-0.0107

O2

+0.0005

+0.0002

+0.0002

H1

+0.0164

+0.0164

+0.0000

H2

-0.0009

-0.0009

-0.0000

Total

+0.0072

+0.0166

-0.0095

C

-0.0009

+0.0075

-0.0084

O1

+0.0701

-0.0056

+0.0756

O2

+0.0084

-0.0145

+0.0228

H1

-0.0034

-0.0034

-0.0000

H2

+0.0304

+0.0304

+0.0000

Total

+0.1046

+0.0144

+0.0900

Gas-phase

Aqueous-phase

Table 2 The average electron gains (Δq) of total, s and d orbitals of surface Pt atoms in HCOOH adsorbed Pt(111) under gas- and aqueous-phase environment; the electron gains (Δq) of total, s and p orbitals of H2O cluster in HCOOH adsorbed Pt(111) under aqueous-phase environment. Average Difference of Electron/Atom (Δq)

Gas-phase

Total

s

p

d

Pt

+0.0045

+0.0008

/

+0.0036

Pt

-0.0044

-0.0025

/

-0.0019

H2O Cluster

-0.0665

-0.0431

-0.0233

/

Aqueous-phase

Simultaneously, the present calculated lower work function of Pt(111) surface at electrochemical interfaces than that in gas-phase (4.87 eV vs. 6.01 eV) also explains why the Pt(111) surface under acid aqueous-phase environment tends to lose electrons, leading to an enhanced back-donation of electrons from surface Pt atoms to the HCOOH molecule compared to that in gas-phase. Thus, the solvation effect plays a crucial role in determining HCOOH oxidation mechanisms, which can change the oxidation pathways, as demonstrated in above MEP and kinetic analysis.



3.5

0.6 0.5

Clean Pt(111) in gas-phase HCOOH-adsorbed Pt(111)

3.0

s orbital of surface Pt atom

2.5

PDOS

PDOS

0.4 0.3 0.2

0.0 -10

2.0 Clean Pt(111) in gas-phase HCOOH-adsorbed Pt(111)

1.5 1.0

0.1

0.5 -8

-6

-4

-2

0

2

4

6

d orbital of surface Pt atom

0.0 -10

8

-8

-6

-4

-2

0

2

4

6

8

E-Ef/eV

E-Ef/eV (a)

(b)

Figure 8. Partial density of states of surface Pt atom on clean and HCOOH-adsorbed Pt(111) in gas-phase: (a) s orbital of surface Pt atom; (b) d orbital of surface Pt atom. 3.5

0.6 Clean Pt(111) in aqueous-phase HCOOH-adsorbed Pt(111)

0.5

3.0

s orbital of surface Pt atom

2.5

PDOS

0.4

PDOS

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

0.3 0.2

Clean Pt(111) in aqueous-phase HCOOH-adsorbed Pt(111)

1.5 1.0

0.1 0.0 -10

2.0

0.5 -8

-6

-4

-2

0

2

4

6

8

E-Ef/eV

0.0 -10

d orbital of surface Pt atom

-8

(a)

-6

-4

-2

0

E-Ef/eV

2

4

6

8

(b)

Figure 9. Partial density of states of surface Pt atom on clean and HCOOH-adsorbed Pt(111) in acid aqueous-phase: (a) s orbital of surface Pt atom; (b) d orbital of surface Pt atom.

In order to further determine the origin of the solvation effect and understand the electron transfer between the HCOOH and the Pt(111) surface, we performed partial density of states (PDOS) analysis of clean and HCOOH-adsorbed Pt(111) surface in the present study since it can describe the number of electrons per interval of energy at each energy level that are available to be occupied. A high PDOS at a specific energy level means that there are many electrons available for occupation. As shown in Figure 8, s and d states of surface Pt atom in HCOOH-adsorbed Pt(111) in gas-phase are almost identical with those in clean Pt(111), implying that only


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slightly electron transfer occurs between HCOOH and Pt(111) surface, which can be confirmed by physisorbed HCOOH on Pt(111) with extremely low adsorption energy of -0.13 eV. However, under acid aqueous-phase environment, s and d states of surface Pt atom in HCOOH-adsorbed Pt(111) below Fermi energy level have lower PDOS compared with clean Pt(111) surface (See Figure 9), meaning that the electrons are transferred into HCOOH, which are well agreeable with the above corresponding Löwdin population analysis. The electron transfer into HCOOH is also able to be confirmed by the move into lower energy level of PDOS of O atoms in HCOOH compared with isolated HCOOH molecule, as shown in Figures S5 and S6. Thus, the observed lower PDOS of the s and d states of surface Pt atom and lower energy level of PDOS of O atoms leads to stronger adsorption of HCOOH on Pt(111) in acid aqueous-phase.

5. CONCLUSIONS The Pt-catalyzed oxidation mechanisms of HCOOH have been systematically studied in gas-phase and the present simulated electrochemical interfaces, involving the influences from aqueous-phase environment, which plays a crucial role in determining electrocatalytic reaction mechanisms. By combining the MEP and mean-field kinetic analysis, we can conclude that HCOOH oxidation mainly proceeds by direct oxidation via COOH intermediate, whereas HCOO intermediate is only a spectator in gas-phase. In contrast, HCOOH oxidation mainly occurs via direct pathway including HCOO intermediate at the present simulated acid electrochemical interfaces. Our present studies show that CO can be formed and accumulated in acid electrochemical interfaces, thus which can lead to blocking and poisoning of surface active sites, and explaining why the experimentally measured HCOOH oxidation currents are still low even if at low overpotentials on pure Pt electrodes. The origin of solvation effect on HCOOH oxidation mechanisms is illustrated systematically. The significant different geometry parameters of HCOOH molecule and electron interactions between gas- and acid aqueous-phase may lead to the difference of HCOOH oxidation mechanisms. The present calculated lower work function of Pt(111) surface in acid aqueous-phase can explain why the Pt(111) surface tends to lose electrons, leading to an enhanced back-donation of electrons from surface Pt atoms to HCOOH. Thus, the relevance of the role played by the solvation effect is well established, and completed Pt-catalyzed HCOOH oxidation mechanisms are revealed in the present study. Our present simulated electrochemical interfaces may be able to partially represent



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the real electrocatalytic systems for HCOOH oxidation in acid solution at the present electrode potential. In fact, the indirect pathway through HCOOH dehydration into CO and its further oxidation into CO2 may be able to occur due to OH formation at higher electrode potentials, which will be considered in our future work. Additionally, HCOOH oxidation mechanisms will be also performed in the presence of specific adsorption of anion in future, which is more representative for the modeling of real electrochemical interfaces.



ASSOCIATED CONTENT

The MEP analysis for HCOOH first oxidation to form CO, HCOO and COOH intermediates on Pt(111) in gas- and acid aqueous-phase; the MEP analysis for COOH and HCOO further oxidation to form CO2 production, COOH further oxidation into CO intermediate, and CO further oxidation into CO2 on Pt(111) in gas- and acid aqueous-phase; the calculated values of kinetic rate constant Kij in gas- and acid aqueous-phase; the geometry parameters of HCOOH molecule on Pt(111) in gas- and acid aqueous-phase; the Partial density of states of O atoms in HCOOH molecule in acid aqueous-phase are summarized in Supporting Information. This material is available free of charge on the ACS Publications website at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.



ACKNOWLEDGEMENTS

This work was supported by the Hunan Provincial Natural Science Foundation of China [grant number 2018JJ2273]; the Outstanding Youth Foundation of the Education Department of Hunan Province [grant number 16B178]; and the National Natural Science Foundation of China [grant number 21303048].



REFERENCES


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TOC Graphic

0.8

0.8

TS TS

HCOOH+*

0.0

0.0

TS

HCOOH*

HCOO*

-0.4

COOH*

-0.8

TS HCOOH+*

0.4

TS

CO*

*+CO2

-1.2

Energy/eV

0.4

Energy/eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TS

HCOOH*

HCOO*

-0.4 COOH*

-0.8 -1.2

*+CO2

-1.6

-1.6

CO*

-2.0

-2.0 -2

0

2

4

6

8

10

-2

0

Reaction Coordiate


2

4

6

8

Reaction Coordinate

10

Mechanistic Study of Pt-Catalyzed Electrooxidation of HCOOH in Acid

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