Active Methanol Oxidation Reaction by enhanced CO Tolerance on Bi

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Active Methanol Oxidation Reaction by enhanced CO Tolerance on Bimetallic Pt/Ir Electrocatalysts using Electronic and Bifunctional Effects Soonchul Kwon, Dong Jin Ham, Taeyoon Kim, Yongju Kwon, Seung Geol Lee, and Min Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09053 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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ACS Applied Materials & Interfaces

Active Methanol Oxidation Reaction by Enhanced CO Tolerance on Bimetallic Pt/Ir Electrocatalysts using Electronic and Bifunctional Effects

Soonchul Kwon,†,† Dong Jin Ham,‡,† Taeyoon Kim,† Yongju Kwon,† Seung Geol Lee, §,* Min Cho ∈,*

†

Department of Civil and Environmental Engineering, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea ‡

Department of Chemical Engineering, Pohang University of Science of Technology, 77 Cheongam-Ro. Nam-Gu. Pohang, Gyeongbuk 37673, Republic of Korea §Department

of Organic Material Science and Engineering, Pusan National University, 2, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea ∈Division

of Biotechnology, Advanced institute of Environment and Bioscience, College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan 54596, Republic of Korea

†These authors contributed equally to this work. *Corresponding authors: [email protected] (S.G. Lee), Phone) +82-51-510-2412, 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

[email protected] (M. Cho), Phone) +82-10-6220-0675

Abstract Platinum-based metal alloys have been generally developed to provide high carbon monoxide resistance in the anodes of direct methanol fuel cell. We report the potential of bimetallic platinum/iridium electrocatalysts to preserve the outstanding carbon monoxide tolerance obtained from both experimental and theoretical inquiries, which represents the enhanced electrochemical performance of methanol oxidation and the in-depth and stepwise investigations for reaction mechanisms, respectively. In this study, the findings highlight the dual-enhancement characteristics of low carbon monoxide adsorption energy (electronic effect) and carbon monoxide oxidative removal (bi-functional effect) compared with various electrocatalysts such as platinum, iridium, and platinum/ruthenium alloy. In addition, the reaction affinity of platinum/iridium alloy for methanol dehydrogenation is also studied in accordance with atomistic properties, such as the adsorption energy and electronic band gap, to understand the electrochemical performance compared to Pt. The results obtained indicate that the platinum/iridium alloy surface played diverse roles in terms of its multi-functional behaviors for carbon monoxide tolerance, including the favorable mechanism of methanol dehydrogenation. It turns out that throughout the theoretical indepth studies, platinum/iridium alloys are promising candidates in terms of the extension for electrocatalytic material designs that differ from Ru in direct methanol fuel cells.

Keywords: Platinum; iridium; methanol oxidation reaction; Carbon monoxide tolerance; bifunctional effect; density functional theory 2 ACS Paragon Plus Environment

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1. Introduction In recent years, future energy systems have been extensively investigated to replace conventional fossil fuel-based power generation sources.1, 2 Of possible new energy sources, fuel cells have been extensively studied as non-polluting power sources owing to their high energy densities and wide operating range of temperatures.3, 4 In particular, fuel cell systems of direct methanol fuel cell (DMFC) and proton exchange membrane fuel cell (PEMFC) operating at low temperatures ( Pt/Ru (–1.91 eV) > Pt (–1.89 eV) > Pt/Ir (–1.63 eV). The Pt/Ir 20 ACS Paragon Plus Environment

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(111) has a lower CO adsorption energy and a longer bond length of Ir-CCO (1.44 Å) than those of other surfaces, which indicates that CO adsorption on the bimetallic surface is unfavorable when compared with that on other surfaces. In fact, based on its low CO adsorptive affinity, the CO tolerance of Pt/Ir (111) could be considered comparable to that of representative bimetallic Pt/Ru (111).

Figure 6. Geometric configuration of CO interaction on (a) Pt(111), (b) Ir(111), (c) Pt/Ir (111), and (d) Pt/Ru(111).

The optimized geometry of the dimer binding of Pt-CO shows a high correlative of the orbital sharing conformation. In order to understand the mechanism governing the CO tolerance of the bimetallic alloys, we examined the energy tendency in accordance with the molecular orbitals that are required for the binding of CO to electrocatalysts. The electron delocalization of the desired system leads to an alteration to the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) band gap. Since the degree of delocalization is electrocatalyst21 ACS Paragon Plus Environment


dependent, different electrocatalysts exhibit differing changes in the molecular orbitals for CO adsorptive performance. Furthermore, bimetallic alloying influences the adsorption characteristics and surface chemistry because the metals (i.e., Pt and Ir) have different lattice parameter and internal bonding properties.29 Therefore, introducing Ir atoms are strained as Pt and Ir atoms produce new bonding to fit into the parent Pt atoms. This, in turn, alters to the conformation of molecular orbitals for all metals. Since large band gap of CO-Pt/Ir (111) system represents the requirement of a relatively high energy to excite the molecules, CO adsorption on Pt/Ir (111) (0.79 eV) is readily inhibited compared with those of Pt (111) (0.64 eV) and Ir (111) (0.39 eV) systems. In addition, narrow band gap of Pt/Ru (111) (0.45 eV) is indicative of the high CO adsorption energy, which shows that Pt/Ir (111) is potentially effective alloys. From the results of the HOMOLUMO band gap and binding interaction of the CO-Pr/Ir (111) system, we confirm that CO poisoning on Pr/Ir (111) is diminished after the final dehydrogenation of CH3OH. The molecular orbital theory stipulates that the strength and the Metal-C σ bond formation contributes to the degree of CO chemisorption via the Blyholder mechanism. This mechanism governs the donation of the electron density from the 5σ orbital of CO to the metal, and back-donation of the electron density from the surface into the 2π* orbital of CO.42, 43 Moreover, the bimetallic alloying of Pt with Ir metal results in the destabilization of the metal-CO* π-back bond because Ir promotes the back-donation into 2π* LUMO. This is confirmed by the lower LUMO energy level (–2.88 eV) of the Pt/Ir (111) alloy compared to that of pure Pt (111) (–2.27 eV). The DFT calculation examined the electronic properties of the binding nature of the CO affinity. The results indicate that the molecular alignment of Pt/Ir (111) should reduce CO poisoning, stemming from the low anodic potential of CO in the anode of DMFC.44


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OH- adsorption on the electrocatalysts. Aqueous phases in the DMFC system contain a plenty of hydroxyl (OH-) groups. Therefore, adsorbed CO could be removed from the electrocatalyst surface through the OH- oxidation of CO to CO2 and H+, thereby minimizing CO poisoning. We optimized the structural geometry of the OH- adsorbed on the electrocatalysts in order to understand the binding natures of the OH- affinity to various electrocatalysts, which could directly lead to CO oxidation (Figure 7).

Figure 7. Geometric configuration of OH- binding on (a) Pt(111), (b) Ir(111), (c) Pt/Ir (111), and (d) Pt/Ru(111).

The Pt/Ru (111) electrocatalysts, which conventionally represented a high OH- affinity for the bifunctional effect, was also included in this analysis. The strong OH- binding (Table S3) of Pt/Ru (–2.94 eV) > Ir (–2.18 eV) > Pt/Ir (–1.91 eV) > Pt (–1.67 eV), indicates chemical binding formation between OH- and electrocatalysts. This chemisorption results in the monodentate formation of OHPt and the bidentate formation of OH-Pt/Ir, Ir, and Pt/Ru with a short bond length (< 2.1 Å) when the oxygen atom of OH- approaches electrocatalysts. To determine the affinity of adsorptive 23 ACS Paragon Plus Environment


mechanism of OH-, we again investigated the energy alteration of the molecular orbitals in OHelectrocatalyst system. For comparison with representative bimetallic alloys, Pt/Ru (111), which exhibits a high affinity for OH- adsorption, we used the change in the OH- adsorption energy with the size of the bandgap to compare the OH- affinity of Pt/Ru (111) and Pt/Ir (111). The Pt/Ru (111) provides an extremely high adsorption energy with the narrow band-gap of the OH- molecule. The Pt/Ir (111) also exhibits an enhanced adsorption energy with reducing the HOMO/LUMO band-gap owing to changes in the energy levels of Pt and Ir due to overlapping p orbitals.29 Finally, this leads to an increased electron delocalization for bimetal systems. Furthermore, narrow band-gap of bimetallic Pt/Ir(111) provides a preferable reaction affinity to OH-, resulting from the lower LUMO energy level (–2.59 eV) of Pt/Ir (111) compared to that of OH-Pr (111) (–2.42 eV). These data indicate that OH- is adsorbed more readily to the Pt/Ir (111) surface than to the Pt (111) surface. The adsorption characteristics of CO and OH- molecules reveal that alloying Pt with Ir results in a high energetic preference for OH- as well as a reduced CO binding on Ir sites, which implies that Pt/Ir can readily oxidize CO to CO2. Bi-functional effect of OH-promoted CO oxidation. At last for CO oxidation reactions, we determined the reaction pathways for sequential CO oxidation by OH- on Pt (111), Ir (111), and Pt/Ru (111) to determine the role of bimetals in reducing CO poisoning and producing H+, simultaneously. Figure 8 shows the calculated potential energy profile along with the refined geometry of intermediates for the CO oxidation pathway in sequence.


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Figure 8. Potential energy profiles of CO oxidation by OH- on different electrocatalysts with optimized geometry of their intermediates at stationary points.

This represents the reaction profile between CO and OH- for the formation of CO2 and H+ and their chemisorption and subsequent decomposition on the electrocatalysts. Note that we employed the most stable configurations of CO and OH- adsorption as the initial structures. Initially, the CO and OH- molecules are chemically adsorbed on the electrocatalysts, leading to the reaction between CO and OH-. CO oxidation by OH- occurs readily for all of the structures, and the resulting COOH* is an important intermediate in the oxidative removal of CO. Moreover, compared to pure Pt (111), COOH forms more readily on the surface of the bimetallic Pt/Ir (111), Pr/Ru (111), and Ir (111) electrocatalysts. In the following step, electrocatalysts should lead to the dehydrogenation of the adsorbed COOH*, thereby forming COO* and H*. The energies along with the corresponding energy profiles are all positive, which implies that a high energy input is needed for 25 ACS Paragon Plus Environment


the next step of the reaction. This result suggests that the dehydrogenation of the adsorbed COOH* is a rate-determining step owing to the difficult dissociation of O-H bond. Relatively, Pt/Ir (111) exhibits the most stable formation (the lowest energy: 0.18 eV) of all of the structures. Fortunately, the oxygen of the adsorbed COOH acts as a promoter, i.e., as a BrФnsted base that initiates the decomposition of the O-H bond of OH-, which allows CO2 formation. Thus, in the final step, the adsorbed CO2 can transform to the gas-phase CO2. With respect to the energetic preference for CO oxidation, the dissociative chemisorption of H+ occurs readily in all systems, but bimetallic electrocatalysts result in the strongly oxidative removal of CO. In particular, Pt/Ir (111) (–0.39 eV) shows a preferable CO oxidation similar to that of Pt/Ru (111) (–0.31 eV), which indicates that alloying Pt with Ir can improve CO tolerance. Taken together, these results support the assertion that the bi-functional effect of bimetal alloys results in an affinity for CO oxidation that is comparable to that of Pt/Ru alloys.

4. Conclusion In this study, we introduced bimetallic Pt-Ir alloys as electrocatalysts (Pt-Ir/C) to enhance MOR activities, and the proposed approach is attributed to an improved CO tolerance from the perspective of the extension for new comparative species. From both experimental findings and DFT calculations, the results fundamentally clarify the systematic characteristics of methanol decomposition on Pt-Ir alloys, and possibly other bimetallic electrocatalysts, such as Pt- and Irbased alloys. The well-distributed Pt-Ir nanoparticles represented the enhanced current density and the electrochemical stability of the anode, facilitating methanol oxidation and high CO oxidation. Concomitant with experimental studies, the theoretical properties of Pt-Ir alloys for methanol


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oxidation and CO tolerance were gradationally investigated by performing DFT calculations. For the methanol oxidation, Pt-Ir alloys provided a more favorable activity than pure Pt, which implies that the bimetal alloys could be potential electrocatalysts of high electro-activity. With respect to the effect of the CO tolerance, Pt-Ir alloys exhibits a lower CO binding energy (electronic effect) and more favorable CO oxidation by OH- (bi-functional effect) than the conventional catalyst (Pt/Ru), and this is attributed to the broad electronic band-gap weakening binding energy and the lowest energy level for the oxidative CO removal, respectively. Our theoretical results indicated that the Pt-Ir alloys played multiple roles of both the electronic and bi-functional effect on CO tolerance, including the favorable pathway for the methanol dehydrogenation. This can serve as a basis for further studies on methanol oxidation and the CO resistance mechanism of various bimetallic electrocatalysts, in particular, Pt- and Ir-based metal alloys, in order to obtain highly electrocatalytic active and stable electrodes in DMFCs. We expect that from the integrated theoretical and experimental approach, the suggested methodology can be extended to various Ptbased bimetals or non-Pt electrocatalysts for catalytic process, which assists in the design and development of highly efficient electrocatalysts in fuel cell system.

ASSOCIATED CONTENT Supporting Information. The theoretical characteristics, which are obtained from density functional theory calculations, are included. This material is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.

AUTHOR INFORMATION



Corresponding Author Seung geol Lee: [email protected] Min Cho: [email protected]

Funding Sources This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1C1B2014633 and nano-material fundamental technology development2016M3A7B4909370).

Notes Competing financial interests: The authors declare no competing financial interests.

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Graphical abstract


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Active Methanol Oxidation Reaction by enhanced CO Tolerance on Bi

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