First-Principles Study on CO Removing Mechanism on Pt-Decorated

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A First-Principles Study on CO Removing Mechanism on Pt Decorated Oxygen-Rich Anode Surfaces (Pt/o-MO(110), M = Ru and Ir) in DMFC 2

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Chi-You Liu, Chun-Chih Chang, Jia-Jen Ho, and Elise Yu-Tzu Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b13051 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017

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The Journal of Physical Chemistry

A First-Principles Study on CO Removing Mechanism on Pt Decorated Oxygen-rich Anode Surfaces (Pt2/o-MO2(110), M = Ru and Ir) in DMFC

Chi-You Liu, Chun-Chih Chang, Jia-Jen Ho, Elise Y. Li* Department of Chemistry, National Taiwan Normal University No. 88, Section 4, Tingchow Road, Taipei 116, Taiwan

* Corresponding author E-mail: [email protected] Phone: (886)-2-77346219 Fax: (886)-2-2932424 1

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Abstract Direct methanol fuel cell (DMFC) is an efficient power source. However, the DMFC anodes are easily toxified by CO or other hydrocarbons, which terminates the methanol oxidation reaction (MOR). The most commonly used high performance catalyst on DMFC anodes is Pt or bimetallic PtRu. In this work, we apply densityfunctional theory (DFT) to investigate the adsorption of CO and H2O on pristine Pt2/MO2(110) and the oxygen-rich Pt2/o-MO2(110) surfaces (M = Ru and Ir). We find that the application of the oxygen-rich surfaces significantly reduces the adsorption energies of CO and H2O molecules as well as the major reaction barrier (CO + OH → CO2) in the water-gas-shift-like (WGS-like) reactions forming CO2. Our detailed analyses on the electronic interaction between the catalysts and adsorbates indicate that Pt2/o-MO2(110) may be a promising DMFC anode material which reduces the poison problem, and that it may be the actual experimental system that is responsible for the observed efficient CO removal.

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I. Introduction Direct methanol fuel cell (DMFC) represents a promising renewable power source for portable and mobile applications.1 During the operation, methanol oxidation reaction (MOR, CH3OH + H2O → CO2 + 6H+ + 6e-) and oxygen reduction reaction (ORR, 1.5O2 + 6H+ + 6e- → 3H2O) occur at the anode and the cathode, respectively. The most common catalysts for DMFC anodes are Pt-based materials.1-8 DFT computations have confirmed that the reaction barriers of MOR are low on Pt(111) surface even without the inclusion of the solvation effect.8 Despite of their high performance in MOR, many issues persist with Pt-based catalysts, such as high cost, slow oxidation rate, and poor catalyst stability.9-11 The most severe of all is CO poisoning on the Pt-based anode that occurs during the stepwise dehydrogenation of MOR, leading to a non-reversible loss of reaction activity within several cycles.1-7, 12-14 Previous studies have shown that the adsorbed CO may be detached via

coupling

with OH species in aqueous environment through the water-gas-shift-like (WGS-like) reactions,1-3, 12, 15-21 as shown in Scheme 1. It has been shown that the application of Pt/Ru bimetallic meterials1-2 or the combination of Pt/RuO23 and Pt/IrO2,4-7 while maintaining the MOR activity, leads to an effective production of OH species on the catalyst surface. Among these, RuO2 and IrO2 surfaces have been shown to exert the highest catalytic activity for water dissociation1,

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and remain stable in an acidic

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environment.5, 23 The OH species generated on the RuO2 or IrO2 surfaces would thus couple efficiently with the CO molecules adsorbed at neighboring Pt active sites, recovering the MOR performance.1, 3, 12 In addition, RuO2 and IrO2 can act as benefit species for dispersing Pt into smaller clusters or atoms on surfaces.4-5, 12, 24-26 Previous studies have shown that the facile water splitting reaction driven by RuO2 or IrO2 surfaces results in the O or OH-covered surfaces, depending on the environment.1, 3, 12, 23-24, 27 Both experimental28-29 and theoretical30-34 studies indicated that in the presence of oxygen, the oxygen-covered (also called “oxygen-rich” or “orich”) RuO2 or IrO2 surfaces might form. Crihan et al.29 observed via STM the existence of fully occupied o-rich RuO2(110) surface at room temperature. First-Principles calculations reported energy barriers as low as 0.25 and 0.29 eV, respectively, for the dissociation of the oxygen molecule into the two coordinatively unsaturated O site (Ocus) on pristine RuO2(110)31 and IrO2(110)34 surfaces. The existence of such o-rich surfaces is often overlooked regarding the catalytic performance of RuO2 or IrO2 surfaces for MOR reactions. It is evident that a thorough examination for the catalytic reaction mechanism on these surfaces is necessary. In this study, we employ the DFT study to investigate the Pt atom dispersion as well as aggregation on pristine or o-rich metal oxide surfaces (MO2(110) and oMO2(110), M = Ru and Ir), and calculate the reaction barriers of WGS reactions on 4


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both Pt2/MO2(110) and Pt2/o-MO2(110) surfaces. We characterize the RuO2(110) and IrO2(110) surface properties and analyze the effect of the adsorbed oxygen atoms on the WGS-like reactions. Our detailed analyses on the electronic interaction between the catalysts and adsorbates indicate that the o-MO2(110) surfaces significantly increase the efficiency of CO release via the WGS-like reactions, and successfully explains the high catalytic performance of MOR observed in previous experiments.

II. Computational Details The density-functional theory (DFT) and the plane-wave method, as implemented in the Vienna ab initio simulation package (VASP),35-38 are employed to calculate the energies and structures of reactants, intermediates, transition structures and products of the reactions on RuO2(110) and IrO2(110). The projector-augmented-wave method (PAW)39-40 is used in conjunction with the generalized gradient-approximation (GGA) and Perdew−Wang 1991 (PW91)41-42 exchange-correlation functional, with a cutoff energy of 400 eV. The energy convergence criteria for the electronic and the ionic steps are 10-4 and 10-3 eV, respectively. The Monkhorst-Pack k-point grids43 are set as 4 x 2 x 1 and 4 x 6 x 1 for the (2 x 2) and (6 x 2)-MO2(110) (M = Ru and Ir) supercells, respectively. A vacuum space of over 12 Å is introduced to ensure negligible interactions between slabs. All layers are fully relaxed during optimization. 5



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The adsorption energies are calculated based on: Eads=E(slab

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+ adsorbate)-(Eslab

+

Eadsorbate), where E(slab + adsorbate), Eslab, and Eadsorbate are the computed electronic energies of the surface with adsorbed molecule, the clean surface and the free molecule, respectively. The transition state structures are located by nudged-elastic-band (NEB) method44-46, and the profiles of potential-energy surfaces (PES) are constructed accordingly. Frequency calculations are applied to verify the adsorbed intermediates and the transition states (with only one imaginary frequency). The Bader charge analysis is used in our calculation47-49. The electron density difference diagrams are plotted at the 0.003 |e|/Bohr3 isosurfaces.

III. Result and discussion Formation of Pt2/MO2(110) and Pt2/o-MO2(110) Figure 1(a)~(c) shows the structures of the pristine (2 x 2)-MO2(110) (M = Ru and Ir) surfaces, and the (2 x 2)-MO2(110) adsorbed with one or two Pt atoms. The positions of the bridging oxygen (Obr), the three-folded oxygen (O3f), the bridging metal (Mbr), and the coordinatively unsaturated metal (Mcus) sites are labelled. The optimized RuO2(110) and IrO2(110) surfaces give rise to almost identical structures as shown in Figure 1. The ELF diagram of the sliced MO2(110) surface shows that the electrons are localized at the Obr and O3f sites (red region) in both RuO2(110) and IrO2(110) systems, 6


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as shown in Figure S1. The enclosure of these atoms forms two possible hollow sites, H1 and H2, the center of Obr-O3f-O3f and Obr-Obr-O3f triangle, respectively, for Pt adsorption. Pt atoms are found to preferentially attach to H1 sites when sequentially added onto the MO2(110) surface, as shown by the optimized structures in Figure 1(b) and (c). No stable H2-adsorbed Pt structures can be located. The adsorption energies of the first Pt atom is on the range of 3.7 to 4.1 eV on MO2(110) surface, as shown in Table 1. When the second Pt atom is attached (Figure 1(c)), the two Pt atoms exert strong mutual attraction and shift considerably away from the center of the H1 site (0.09 Å and 0.15 Å per Pt atom for RuO2(110) and IrO2(110), respectively.) toward each other, signifying substantial Pt aggregation in the Pt2/MO2(110) structure. A larger (6 x 2) -MO2(110) super-cell, as shown in Figure S2, is constructed to further investigate the Pt aggregation effect. Table S1 lists the co-adsorption energies of the two Pt atoms adsorbed on combinations of H1 sites (A ~ H). The two Pt atoms exhibit maximum shifting from the H1 center when adsorbed on the nearest H1 sites (site A and B in Figure S2), and the co-adsorption energies are 0.8 ~ 0.9 eV higher than other configurations. In other words, the two adsorption events cannot be treated as independent adsorbates as the co-adsorption energies (> 8 eV) are much larger than twice of the first adsorption energies, as shown in Table 1 and Table S1. We then consider the Pt adsorption on o-rich surfaces. As previously mentioned,31, 7



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O2 molecules dissociate spontaneously with a small energy barrier (< 0.3 eV) on

RuO2(110) and IrO2(110) surfaces to form two Ocus atoms on top of Rucus and Ircus sites, as shown in Figure 1(d). Pt atoms preferentially attach to the bridging sites between the two Ocus atoms when adsorbed on the fully oxygen covered o-MO2(110) surfaces (Figure 1(e) and (f)). The adsorption energies of Pt atom on o-MO2(110) (> 5 eV) are much larger than those on pristine MO2(110) (< 4.1 eV) , as shown in Table 1. The adsorption of the second Pt atom is almost independent from the first adsorption event, as evidenced by the near identical first and second adsorption energies. The distances between the two adsorbed Pt atoms in Pt2/o-MO2(110) are over 6 Å , significantly longer than those in Pt2/MO2(110) (~2.93 Å ). In other words, Pt atoms will disperse much better on o-rich surfaces to form the Pt decorated o-rich surfaces (Ptn/o-MO2(110) surfaces). This occurrence also rationalizes previous experimental observations of better Pt dispersion with the inclusion of RuO2 and IrO2 species.4-5, 12, 24-26 The partial density of states (PDOS) analysis of the d-band states for Pt atoms in gas phase and for Pt atoms adsorbed on MO2(110) or o-MO2(110) surfaces are also performed and the results are shown in Figure 2(a)~(e). Pt2/MO2(110) shows a broad Pt d-band distribution, reflecting substantial d orbital interaction due to the short Pt-Pt distance. As mentioned previously, upon co-adsorption on MO2(110), the Pt atoms shift away from the H1 center, moving toward each other. The PDOS for the fully relaxed 8


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Pt2/MO2(110) and the fixed Pt2/MO2(110), as shown in Figure S3, indicates that this additional aggregation induces a slightly broader d-band distribution. In the contrary, the Pt2/o-MO2(110) shows a much more concentrated, gas-phase like Pt d-band distribution.

This is uncommon for metal adsorption on metal oxide surfaces

considering the large adsorption energy (> 5 eV per Pt) for this system. Adsorption of CO and H2O on Pt2/MO2(110) and Pt2/o-MO2(110) Figure 3 shows the most stable adsorption structures of CO and H2O adsorbed on Pt2/RuO2(110) and Pt2/o-RuO2(110) surfaces. The optimized structures for those on Pt2/IrO2(110) and Pt2/o-IrO2(110) surfaces are very similar. The corresponding adsorption energies and relevant bond length variations for CO and H2O are listed in Table 2 and Table S2, respectively. The CO molecule attaches onto the Pt atom and forms approximately a 45-degree angle with the surface, as shown in Figure 3(a) and (b). Surprisingly, while the C-Pt bond lengths on o-rich surfaces are slightly longer (by about 0.035 Å ) than those on pristine surfaces, the adsorption energies of CO on Pt2/oMO2(110) surfaces is decreased by over 0.75 eV than those on Pt2/MO2(110) surfaces. For the adsorption of H2O, the water molecule attaches onto the Pt atom via the oxygen atom (Ow) and forms a hydrogen bond with the nearby Obr atom (Figure 3(c) and (d)), as evidenced by the typical hydrogen bond distance between the hydrogen atom (Ha) and Obr (~1.60 Å ), and the slightly lengthened Ow-Ha bond (~1.03 Å ) (Table S2). For 9



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water adsorption, the adsorption energies are also reduced by 0.40 ~ 0.65 eV on the orich surfaces. To further investigate the adsorbate-surface interaction, we carried out the Bader charge analysis for the adsorption events on pristine and o-rich MO2(110) systems, as shown in Table S3~S6. For Pt2/MO2(110) and Pt2/o-MO2(110) surfaces, Pt atoms are found to carry more positive charges when adsorbed on o-rich surfaces (q ~ 0.92 |e|) than on pristine surfaces (q < 0.3 |e|) (Table S4). The excessive positive charge of the Pt atom on the o-rich surface is due to strong electron-withdrawing by the two neighboring under-saturated Ocus atoms, which accumulate an additional negative charge (Δq = -0.11 ~ -0.16 |e|) upon Pt adsorption. When CO is adsorbed to Pt2/MO2(110), the charge of Pt increases by about 0.2 |e|, indicating significant electron donation from Pt to CO (Table S5). For the Pt2/o-MO2(110) surfaces, however, the two active Ocus atoms continue to extract more negative charge (Δq = -0.11 ~ -0.13 |e|) upon subsequent CO adsorption and suppress the electron back-donation from Pt to CO. This unique electron distribution in o-rich surfaces results in a weaker Pt-CO bonding and thus lower CO adsorption energies. The above-mentioned interactions between CO adsorbates and surfaces can be directly visualized by the electron density difference before and after CO adsorption as shown in Figure 4(a)~(d). Upon CO adsorption, both the Pt2/MO2(110) and Pt2/o10


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MO2(110) surfaces show a net electron flow from the Pt atom (the yellow region) towards CO (the blue region), and from neighboring oxygen atoms on the surface, Obr or Ocus on MO2(110) or o-MO2(110), respectively, to the Pt atom. Nevertheless, a more careful inspection reveals that the electron accumulation near the active Pt atom is reduced (the blue region marked by the red arrow in Figure 4(c) and (d)) for CO adsorption on the Pt2/o-MO2(110) than on the Pt2/MO2(110) surface. In other words, the Ocus atoms on the o-rich surfaces are more reluctant to share electrons with the Pt atom, which is consistent with the Bader charge analysis results. The Bader charge analysis also reveals that the H2O adsorption results in a net charge accumulation on the Ocus atom opposite to the Pt-Ow bond via the trans-effect. The electron density difference plot, as shown in Figure S4, before and after water adsorption indicates clearly the formation of the hydrogen bond (Ow-Ha-Obr) between water molecule and surfaces, as a significant amount of electron density flows from the Obr to the H atom of water can be visualized. The formation of the hydrogen bond shall facilitate the Ow-Ha bond breaking to generate OH species, as previously observed experimentally1, 3, 12. WGS reactions on Pt2/MO2(110) and Pt2/o-MO2(110) surfaces We then consider the WGS reactions to convert CO on both Pt2/MO2(110) and Pt2/o-MO2(110) surfaces. The potential energy surface (PES) diagram and the detailed 11



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structures for all intermediates are shown in Figure 5 and Figure 6, respectively. First, the CO and H2O molecules are co-adsorbed on the Pt-decorated MO2 surfaces (IM1 in Figure 6). The two species are found to attach to neighboring Pt atoms in Pt2/MO2(110) system, but to the same Pt atom in Pt2/o-MO2(110) system. The adsorbed H2O then dissociates to form OH and H species (IM2), through a low reaction barrier (Ea < 0.35 eV) assisted by the hydrogen bond interaction on these surfaces. The OH species would then couple with the CO adsorbate to form OCOH intermediate on the Pt atom (IM3). This step (CO + OH → OCOH) yields the largest reaction barrier in the entire WGS reactions; the activation barriers on Pt2/RuO2(110), Pt2/IrO2(110), Pt2/o-RuO2(110) and Pt2/o-IrO2(110) surfaces are 1.93, 1.94, 1.23, and 1.46 eV, respectively. At the [OH(a) + CO(a)] intermediate state (IM2 in Figure 6), the distances between OH and CO groups (H-O…C-O) are 2.91, 2.70, 3.04, and 2.66 Å on RuO2(110), o-RuO2(110), IrO2(110), and o-IrO2(110), respectively. The OH and CO groups are a lot closer when adsorbed on the Pt2/o-MO2(110), resulting in easier combination on oxygen-rich surfaces. Note that if the CO and H2O are adsorbed on separate Pt atoms in Pt2/o-MO2(110), the reaction would be almost impossible to proceed with a distance between the adsorbates over 6 Å . The OCOH intermediate then undergoes the O-H bond scission to form CO2 and then become desorbed (IM4). To complete the catalytic cycle, the H-H coupling barriers are calculated to assess 12


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the probability of forming the hydrogen molecule products. The H-H coupling barriers on all surfaces are over 1 eV and are endothermic (IM5). The desorption energies of the hydrogen molecule, 0.66~1.49 eV, are also quite high (IM6). In this case, the CO conversion reactions may stop at the CO2 and the protonated surface (IM4) or proceed to the final CO2 + H2 products (IM6).

VI. Conclusion Our DFT investigation shows that the o-rich surfaces (o-MO2(110)) facilitate Pt dispersion more effectively than pristine surfaces. The calculation results indicate that the experimentally observed Pt dispersion may have occurred on the Pt2/o-MO2(110) surfaces rather than the pristine MO2(110), as previously assumed. The Pt decorated oxygen-rich surfaces (Pt2/o-MO2(110)) significantly decrease the CO adsorption energies by about 0.8 eV than pristine MO2(110) surfaces, via the participation of the Ocus sites on o-MO2(110). Furthermore, the activation energy barriers of WGS reactions are significantly reduced on Pt2/o-MO2(110). In summary, we believe that the formation of the o-rich surfaces is partially responsible for the observed high performance in catalytic reactions of DMFC anodes.

Supporting Information 13



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Detailed electronic analysis and structures of MO2(110) and o-MO2(110).

Acknowledgments This research is supported by the Ministry of Science and Technology (MOST) in Taiwan (MOST 105-2113-M-003-008 and MOST 102-2113-M-003-008–MY4). We thank National Center for High-performance Computing (NCHC) of Taiwan for the help on computational resources.

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Scheme 1. The WGS-like reaction mechanism. Symbol (a) refers to the adsorbed state.

Figure 1. Optimized structures of the (a) RuO2(110), (b) Pt/RuO2(110), (c) Pt2/RuO2(110), (d) o-RuO2(110), (e) Pt/o-RuO2(110), and (f) Pt2/o-RuO2(110) surface. The definitions of Mcus, O3f, Obr, Mbr, and Ocus sites are indicated. The colors of the elements Ru, O, and Pt are Persian green, red, and blue, respectively.

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Figure 2. Partial density of state (PDOS) projection of the d-band structures of two Pt atoms in (a) gas phase, (b) Pt2/RuO2(110), (c) Pt2/IrO2(110), (d) Pt2/o-RuO2(110), and (e) Pt2/o-IrO2(110), respectively.

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Figure 3. Calculated structures (side view) of CO adsorbed on (a) Pt2/RuO2(110) and (b) Pt2/o-RuO2(110) surfaces, and H2O adsorbed on (c) Pt2/RuO2(110) and (d) Pt2/oRuO2(110) surfaces. The colors of elements Ru, O, Pt, C, and H are Persian green, red, blue, gray, and white, respectively.

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Figure 4. Calculated electron density difference diagrams (Isosurface = 0.003 |e|/Bohr3) before and after CO adsorption on (a) Pt2/RuO2(110), (b) Pt2/IrO2(110), (c) Pt2/oRuO2(110), and (d) Pt2/o-IrO2(110) surfaces. Blue and yellow represent charge accumulation and depletion regions, respectively. Red arrows indicate the region with a comparatively less electron accumulation upon CO adsorption for o-rich systems.

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Figure 5. Calculated potential energy surface (PES) diagram for the WGS reaction on Pt2/RuO2(110), Pt2/IrO2(110), Pt2/o-RuO2(110) and Pt2/o-IrO2(110) surfaces. The intermediates (IMn, n=1-6) and the reaction barriers (in eV) for the transition states (TSn, n=1-5) for each reaction step are marked. The optimized structures for each intermediate are shown in Figure 6.

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Figure 6. The calculated reaction intermediates of WGS reactions on (a) Pt2/RuO2(110), (b) Pt2/IrO2(110), (c) Pt2/o-RuO2(110), and (d) Pt2/o-IrO2(110) surfaces. The colors of the elements Ru, Ir, O, Pt, C, and H are Persian green, Lavender, red, blue, gray, and white, respectively.

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Table 1. The adsorption energies (∆E, in eV) of one or two Pt atoms on RuO2(110), IrO2(110), o-RuO2(110), and o-IrO2(110) surfaces. Adsorption Energy ∆E (eV)

Adsorbate

a

RuO2(110)

IrO2(110)

Pt (H1)

-3.68

-4.09

2Pt (H1)

-8.46a

-9.40a

o-RuO2(110)

o-IrO2(110)

First Pt

-5.44

-5.98

Second Pt

-5.31

-5.87

The co-adsorption energy of two Pt atoms

Table 2. The adsorption energies (∆E, in eV) of CO and H2O molecules on Pt2/RuO2(110), Pt2/o-RuO2(110), Pt2/IrO2(110), and Pt2/o-IrO2(110) surfaces. Adsorbate

Adsorption Energy ∆E (eV) Pt2/RuO2(110)

Pt2/IrO2(110)

CO

-2.86

-2.84

H2O

-1.75

-1.67

Pt2/o-RuO2(110)

Pt2/ o-IrO2(110)

CO

-1.99

-2.07

H2O

-1.09

-1.24

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TOC Graphic The CO removal reaction catalyzed by Pt2/o-MO2(110) (M = Ru and Ir).

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First-Principles Study on CO Removing Mechanism on Pt-Decorated

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