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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

First-Principles Investigation of the Formation of Pt Nanorafts on the MoC Support and Their Catalytic Activity for Oxygen Reduction Reaction 2

Chethana Bhadravathi Krishnamurthy, Oran Lori, Lior Elbaz, and Ilya Grinberg J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00949 • Publication Date (Web): 12 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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The Journal of Physical Chemistry Letters

First-Principles Investigation of the Formation of Pt Nanorafts on the Mo2C Support and Their Catalytic Activity for Oxygen Reduction Reaction Chethana B. Krishnamurthy, Oran Lori, Lior Elbaz, Ilya Grinberg* Department of Chemistry, Bar-Ilan University, Ramat Gan, Israel 52900

ABSTRACT. We use first-principles calculations to study the formation of Pt nanorafts and their ORR catalytic activity on Mo2C. Due to the high Pt binding energy on C atoms, Pt forms sheetlike structures on the Mo2C surface instead of agglomerating into particles. We find that the disordered Mo2C surface carbon arrangement limits the Pt sheet growth, leading to the formation of 4-6 atom Pt nanorafts. The O-O repulsion between the O atoms on the Mo2C and O adsorbate enhances the ORR activity by weakening the O adsorption energy. We find a significant change from the usual scaling of the energies of the intermediates in the ORR pathway and a strong interaction between the nanoraft and water that lead to a high activity of the Pt nanorafts. Fundamentally, our work demonstrates that the activity of metal catalysts can be strongly affected by manipulation of the atomic arrangement of the supporting carbide surface.

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Low-temperature polymer electrolyte membrane fuel cells (PEMFC) enable clean and efficient energy conversion with high energy density and have therefore attracted attention of many researchers in the field of sustainable energy. Oxygen reduction reaction (ORR) at the cathode is considered as the bottleneck reaction in PEMFCs, motivating extensive research for the development of efficient and cost-effective improved electrocatalyst to enhance its sluggish kinetics.1–8 Platinum is currently the most efficient commercial electrocatalyst for ORR, but even Pt catalysts lack sufficient activity because of sluggish ORR kinetics9 and the considerable amount of Pt required for catalysis leads to a high cost of PEMFCs. Thus, the development of more cost-effective catalysts by eliminating or decreasing the amount of Pt in the catalysts is a major objective of current research.1,10–12 Here, the reduction in the size of the Pt nanoparticles is key for the reduction of Pt amount because only the Pt atoms at the nanoparticle surface act as catalytic sites. Deposition of Pt over transition metal carbide (TMC) has been investigated to meet the challenge of reducing the Pt amount without compromising the stability and catalytic activity for ORR.13 In particular, molybdenum carbide (Mo2C) has been found to be an excellent catalyst support for Pt and exhibited relative durability when compared to carbon supports.14–20 Recently, Elbaz and co-workers21 further reduced the Pt loading and synthesized unique rafts of platinum consisting of 6 atoms or less, on the Mo2C surface that showed enhanced catalytic ORR activity. Most importantly, the nanorafts showed better stability and durability of Pt/Mo2C catalyst in comparison with Pt/C after 5000 potential cycles. To the best of our knowledge, no studies have been conducted to explore the mechanism of formation of Pt nanorafts on the Mo2C substrate and the origin of the enhanced ORR activity on the Pt nanoraft. 2 ACS Paragon Plus Environment

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In this work, we used density functional theory (DFT) calculations to gain insights into the kinetics and mechanism involved in the formation of Pt nanorafts on the Mo2C surfaces and their catalytic ORR activity by considering the adsorption energy of the atomic oxygen. To examine the thermodynamics of Pt nanoraft formation on the Mo2C surface, we compare the binding energies of Pt atoms of the growing nanoraft with the bulk cohesive energy of Pt (-5.95 eV in our calculations). Loading of Pt on Mo2C surface can lead to three different arrangement of Pt atoms, namely as either a bulk-like particle or a thin film or as a nanoparticle. If the binding energy of the Pt atoms on the Mo2C surface is smaller than the bulk cohesive energy, Pt atoms will prefer to bind to each other and form large bulk-like nanoparticles. If the Pt binding energy is consistently larger than the bulk cohesive energy of Pt until a full coverage of Pt on Mo2C is reached, Pt will cover the Mo2C surface and form a continuous film (Figure 1). For stable formation of small nanorafts, the binding energy of Pt on Mo2C must on the one hand be larger than the Ecoh of Pt, disfavoring Pt agglomeration and on the other hand must decrease strongly at some critical size of the nanoraft, disfavoring the formation of a thin Pt film.

Figure 1. Graphical representation of formation of Pt nanorafts on Mo2C and prevention of agglomeration of Pt particles.



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Investigation of all possible adsorption sites for the Pt binding on ordered Mo2C (Mo and C) surface found that Pt prefers to adsorb on the C atom of the Mo2C surface. This is due to the significant electronic metal-support interactions22,23 between Pt and C atom of Mo2C, in agreement with the experimental observations21 and previous DFT calculations.13,24–26 We also find that as the number of Pt atoms on the orthorhombic ordered Mo2C surface increases, there are two possible adsorption modes, namely inside and outside of the channel formed by the C atoms of Mo2C. Examination of the Pt adsorption energies and geometries presented in Figure 2 shows that Pt atoms are deposited in a channel with increasing binding energy until the deposition of the 4th Pt atom in the channel increases the binding energy to -6.76 eV and form a one-Pt-atom-thick atomic-wire. Thus, we find that the zig-zag arrangement of C atoms in a channel (Fig. S1), along with the strong interaction of Pt with the C atom promotes the uninterrupted growth of Pt as atomic-wire in the channel. After the adsorption of 3 Pt atoms in the channel, an adsorption site is possible outside the channel where the adsorbed Pt atoms is bonded both to the Mo2C surface and the neighbor Pt atoms (Fig. 2B). In this case, the binding energy of the Pt is reduced to -6.18 eV but is still significantly higher than the cohesive energy of the bulk Pt and the 5th Pt atom is once again adsorbed in the channel with a high adsorption energy. However, the binding of 6th and 7th Pt atoms is much weaker with the binding for the 7th atom on top of the first layer Pt (Ebind=-5.34 eV), weaker than the binding in the bulk Pt. Therefore, instead of deposition on another Pt atom, the 7th Pt atom will prefer to interact with neighboring C atom on the Mo2C surface and initiates the formation of new Pt nanorafts. Thus, for the ordered Mo2C surface, Pt will either form atomic-wires or nanorafts with the size of less than 6 atoms, with the atomic-wires strongly preferred. 4 ACS Paragon Plus Environment

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(A)

(B)

Figure 2. (A) Binding energies of Pt atoms on the ordered Mo2C surface. (B) Formation of Pt-nanorafts on the ordered Mo2C surface (i) to (iv) Pt deposited growing in a channel; (v) to (viii) Pt atoms deposited outside the channel with increase in Pt-loading. Purple, blue and yellow spheres represent C, Mo and Pt atoms, respectively.

To understand the effect of carbon arrangements on the mechanism of Pt nanoraft formation and for a better comparison with the disordered Mo2C surface used in previous experiments, we investigated Pt adsorption on two different disordered Mo2C surfaces. As seen in Figure S1, the random arrangements of carbon atoms on the Mo2C surface eliminate the continuous zig-zag channel on the Mo2C surface, with only a short channel available for Pt adsorption. As shown in Figure 3A, the binding for the first three atoms that are adsorbed in the channel is quite strong and is similar to that for the ordered Mo2C surface. For the 4th atom, the lack of the continuous channel due to the disordered C arrangement forces the Pt atoms to adsorb either as an isolated Pt atom in the neighboring channel (Fig. 3B(ii)) or on a site outside the channel attached to the previously adsorbed Pt atoms (Fig. 3B(iii)). In both cases, the binding is stronger than that of bulk Pt. Further growth of the adsorbed Pt cluster is favored for the 5th atom but is unfavorable for the 6th atom (Figs. 3A and 3B(iv)). (As shown in figure 3B(iv), the deposited 5th Pt atom binds strongly to the C atom in the nearest neighbor channel and weakens 5 ACS Paragon Plus Environment


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the bonding between 3rd and 4th Pt atoms and therefore, terminates the further growth of Pt nanoraft). Thus, our results clearly show that the disordered Mo2C surface restricts the growth of Pt nanoraft in a channel, indicating that arrangements of carbon atoms in Mo2C surface plays a vital role in the formation of Pt nanorafts.

(B)

(A)

(D)

(C)

Figure 3. (A) Binding energies of the Pt atoms on the disordered Mo2C surface. (B) Growth of Pt nanorafts on first disordered Mo C surface (i) and (ii) Pt atoms deposited inside the channel; (iii) and (iv) Pt atoms 2

deposited outside the channel. (C) Binding energies of the Pt atoms on the second disordered Mo2C surface. (D) Formation of Pt-nanorafts on the second disordered Mo2C surface (i) to (iii) Pt atoms deposited in a channel; (iv) and (v) Pt atoms deposited outside the channel with increase in Pt-loading. Purple, blue and 6 yellow spheres represent C, Mo and Pt atoms, respectively. ACS Paragon Plus Environment

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Investigation of the second disordered Mo2C surface confirms the crucial role of carbon arrangement in Pt nanoraft formation. As shown in Figure 3C and 3D, the first three Pt atoms form a small line of 3 Pt atoms in a channel with stronger binding energy. The disordered C arrangement breaks up the continuous channel and the energy for adsorption of Pt in the cluster becomes smaller than the bulk Pt energy for the 4th Pt atom so that Pt prefers to bind as an isolated atom rather than continue the growth of the Pt cluster on the Mo2C surface. As a result, Pt prefers to form a raft-like structure on the Mo2C surface instead of agglomerating into a particle or wetting the surface as a thin film. Overall, our calculation reveals that (i) the carbon arrangements on the Mo2C surface control the formation of Pt nanorafts and prevent agglomeration into bulk-like particles (ii) the ordered arrangement of carbon atoms prefers to form uninterrupted Pt atomic-wire in a channel and a small Pt nanoraft with less than six atoms outside the channel (iii) disordered arrangement of carbon atom restricts the growth of Pt nanorafts to 3 atoms inside and outside the channel, respectively. The Pt nanorafts observed in our calculations (Figure S2) closely resemble the experimentally observed nanorafts as imaged by STEM (Figure S3).21 To evaluate the catalytic activity of Pt nanorafts for the ORR, we calculate the O adsorption energies on the Pt/Mo2C nanorafts and compare the obtained values to the value for the Pt(111) surface. Previous studies have shown that Eads,O is a good descriptor for the ORR activity with the Eads (relative to H2O ) in the 1.5-1.9 eV range indicating high catalytic activity,27 with Eads,O for Pt(111) of 1.48 eV and the peak of the volcano curve located at Eads,O of 1.75 eV.



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Because oxygen has a strong affinity towards Mo and can oxidize the Mo2C surface to MoO328,29 (binding energy of -2.39 eV relative to H2O in our calculations), we performed Eads,O calculations on the Pt/Mo2C surface with oxygen covering the Mo2C surface. We examined half and full coverage of O on the Mo atoms of Mo2C and evaluated the energy of adsorption of additional oxygen atoms on the three possible O adsorption sites on the supported Pt atoms of the nanoraft (Figure S4). Examination of atomic oxygen binding energies listed in Table 1 shows that the Eads,O differs significantly between different Mo2C surfaces and are also affected by the O coverage on the Mo2C surface. For the disordered Mo2C surface, we find that the Eads,O on the most favored site is in the desired 1.5-1.9 eV range for both O coverages, with Eads,O of 1.58 obtained for full O coverage and Eads,O of 1.8 eV (essentially at the top of the volcano curve) obtained for the half O coverage (Figure 4A, see SI for details regarding the construction of the volcano plot). These O binding energies are greater that Eads,O for the Pt(111) surface and are closer to the peak of the volcano curve, indicating that the nanoraft Pt bind O more weakly, which should be more favorable for ORR. This is consistent with the higher ORR activity observed experimentally for the nanorafts. By contrast, the ordered Mo2C surface shows Eads,O on the bridge site of the nanoraft that is smaller than that of Pt(111), indicating stronger binding. This suggests that the ordered orthorhombic Mo2C phase is not likely to lead to good ORR performance. We now investigate the origin of the observed differences in Eads,O values of the different nanorafts. We first calculate the d-band center values for the different Pt atoms of the nanorafts (Table 1, Figures S5-S7). A plot of Eads,O versus d-band center shows large scatter, indicating that the d-band center alone cannot account for the observed variation in Eads (Figure S11a). We therefore investigate the effect of the repulsion between the O atom adsorbed on the nanoraft and 8 ACS Paragon Plus Environment

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the O atoms adsorbed on the Mo2C as another possible contribution to the observed Eads,O variations. Assuming that all O atoms carry roughly the same negative charge, we estimate the O-O repulsion energy Erep by the sum of rj-1 where rj is the distance between the O atom on the nanoraft to the jth O atom adsorbed on Mo2C. Using a rather simplistic predictor constructed by the combination of equally weighed contributions of the Erep and d-band center, we obtain a qualitatively linear trend between the predictor and the calculated Eads values (Fig. 4B). Thus, for the nanoraft, in addition to the usual d-band center dependence, the O-O repulsion exerts a strong effect on Eads,O and must be taken into account. For the adsorption of two O atoms on the Pt nanoraft, corresponding to the dissociative ORR pathway, we found that the Eads,O per O atom is 1.5 eV on the 6-atom Pt nanoraft on the ordered Mo2C surface and 1.25 eV on the 4-atom Pt nanoraft on the disordered surface. Following the now standard approach pioneered by Norskov et al.,30–32 we investigated the dissociative and associative ORR pathways on the nanoraft of the first disordered Mo2C surface by obtaining the adsorption energies of the *OOH, *O and *OH intermediates with and without the inclusion of the effect of the water solvent. The obtained gas phase adsorption energies are Eads,OOH=3.1 eV, Eads,O =1.58 eV, and Eads,OH =-0.03 eV. While the calculated Eads values for *OOH and *OH follow the scaling relation Eads,OOH = Eads,OH +3.18 eV, the relationship between Eads,OH and Eads,O is very different from that found in previous work30 (Eads,OH =0.5 Eads,O+0.02 eV). The Eads,OOH and Eads,OH are lower than those predicted by the usual scaling, suggesting that the second bond made by the adsorbate O and the nanoraft is very weak compared to the single nanoraft-O bonds of *OOH and *OH. The very low Eads,OH means that even with no applied potential the final step of the ORR pathway is uphill, which will certainly give rise to very poor catalytic performance at higher potentials (Figure S14). By 9 ACS Paragon Plus Environment


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contrast, the inclusion of an H2O monolayer shifts the energies of all ORR intermediates up by approximately 1 eV. Examination of the DFT-relaxed structures shows that this is due to the lower energy of the H2O on the bare nanoraft, where a spontaneous dissociation of one H2O molecule into OH bonded to Pt and H3O is obtained (Figure S13). In other words, an OOH, O or OH adsorbate must replace the OH already bound to Pt and force it to recombine with the H3O to produce 2H2O molecules, increasing the adsorption energies of all intermediates. Examination of the reaction pathways with inclusion of H2O shows that for the dissociative pathway the *O intermediate energy of 2.25-2.6 eV is very close to or higher than the starting energy of 1/2O2 of 2.45 eV. This is much higher than the 1.5 eV energy of *O for Pt(111) and will lead to a high barrier for the dissociation of O2 into *O. Therefore, the dissociative pathway is unfavorable. By contrast, for the associative pathway (Fig. 4C), the energy of the first intermediate (OOH) is only ≈4.3 eV, comparable to the 4.15 eV energy of the Pt(111), and predicting U=0.6 as the potential at which the OOH intermediate is equal in energy to the reactants, compared to U=0.75 for Pt(111). This suggests that the nanorafts will show catalytic activity similar to Pt(111), in agreement with experimental results.


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Table 1. Oxygen adsorption energies on different Pt/Mo2C surfaces. Structure of Pt/Mo2C

Oxygen adsorption energy (eV) Saturated oxygen coverage

Partial oxygen coverage

Outside channel

Outside the channel

Inside the Channel

Ordered

Bridge site 1.05

the Inside the Channel Terminal Channel site Terminal 1.70 2.25

Bridge site 1.01

Terminal site 1.51

Channel Terminal -0.47

Mo2C

(-2.13)

(-2.05)

(-2.55)

(-2.17)

(-2.13)

(-2.51)

1.15

1.62

1.57

0.98

0.84

0.72

Disordered 2.0

1.58

1.40

1.81

1.78

0.15

Mo2C

(-2.19)

(-1.82)

(-2.68)

(-2.33)

(-2.41)

(-2.36)

1.86

1.81

2.40

1.13

1.27

1.30

NA

NA

1.59

NA

NA

-0.80

Second disordered

(-2.47)

(-2.39)

Mo2C

1.50

0.92

NA: Pt nanoraft is restricted to only 3 Pt atoms in a channel and Pt atom adsorption outside the channel is not favorable as shown in Figure 3C. The values in the parenthesis are the d-band center values (in eV) of Pt nanoraft atoms involved in the bonding with the O adsorbate prior to adsorption (see SI). The values in bold are the Erep values calculated as described in the text and SI.



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(B)

(A)

(C)

Figure 4. (A) Volcano plot of atomic O adsorption energies. ∆E values used in plot (A) are the O

minimum O adsorption values on different Mo C surfaces along with the values from ref [27]. (B) Plot 2

of O-binding energy on Pt nanoraft vs the predictor combining the effects of d-band center and of O-O repulsion between the O-Pt and O-Mo2C. (C) Associative pathway at U = 0 and U = 1.23 for the Pt nanoraft on the first disordered Mo2C system and for Pt(111) (data for Pt(111) are taken from ref. [ 31]), in the presence of H2O.

In summary, using first-principles calculations, we have shown that the C arrangement in the Mo2C support controls the morphology of the Pt deposited on the Mo2C surface. Agglomeration of large Pt nanoparticles at low Pt loading is unfavorable due to very strong interactions between the C atoms on the Mo2C and the adsorbed Pt atoms. It is also found that deposited Pt prefers to grow in 1D lines on the ordered Mo2C surface while formation of a thin film wetting the entire Mo2C surface is unfavorable due to weak Pt-Mo interactions. For the disordered Mo2C surface, the disorder in the C arrangement limits 1D lines to 3 atoms and leads to the formation of small 4-atom nanorafts.

Examination of O adsorption on the nanorafts

shows that the effects of the Mo2C support and the repulsion between the surface O atoms and the O adsorbed on the Pt lead to weaker binding of O atoms. Our calculations also show that the O-binding energy of the Pt/Mo2C system is controlled by a combination of d-band center and the 12 ACS Paragon Plus Environment

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O-O repulsion effects. Examination of the ORR pathway shows unusual deviation from the usual O/OH scaling relation and a strong impact of a water monolayer and predicts that the catalytic activity of the nanoraft via the associative pathway will be comparable to that of Pt(111). Our results are in agreement with experimental results of Elbaz et al.21 and demonstrate that catalytic performance can be strongly affected by the manipulation of the atomic arrangement of the supporting carbide surface.

ASSOCIATED CONTENT Supporting information Computational details, geometry of formation of Pt nanorafts on different Mo2C, adsorption site of oxygen on Pt nanorafts, density of states, ORR pathways. AUTHOR INFORMATION Corresponding author *Email: [email protected]. Tel: 972-3-738-4345. Fax: 972-3-738-4053. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors wish to thank the Israel Fuel Cell Consortium (IFCC) of the Israel National Research Center for Electrochemical Propulsion (INREP) for funding this work under contract by the Israeli Committee for Higher Education and the Israel Prime Minister’s Offices’ Fuel Choices and Smart Mobility Initiative. LE would also like to thank the Israeli Ministry of Energy for partially funding this work (grant no. 216-11-046).

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