University of South Carolina Bimetallic Ru-Pt/C catalysts prepared by strong electrostatic adsorption and electroless deposition for direct methanol fuel cell application John Meynard M. Tengco, Bahareh Alsadat Tavakoli Mehrabadi, Weijian Diao, Taylor R. Garrick, John W. Weidner, John R. Monnier, and John R. Regalbuto University of South Carolina NAM 25 Denver, CO 5 June 2017 Good morning! Thank you for the introduction. Today I will be presenting our center’s work in integrating our specialties in catalyst synthesis in the development of platinum based bimetallic catalysts for fuel cell applications.

INTRODUCTION SUPPORTED METAL CATALYSTS - more efficient metal utilization - greater amount of active surface Large Particles atoms inside are not utilized Small Particles more metal atoms exposed (higher dispersion) In heterogeneous catalysis, the synthesis of supported metal nanoparticles is usually aimed at producing smaller particles. This is to efficiently utilize the metal, exposing more atoms, optimizing the amount of active sites for a desired reaction. Conventional methods of catalyst synthesis often use simple impregnation of the support with a solution of the metal precursor. This usually doesn’t produce well dispersed particles and results in a wide particle size distribution. Our method of Strong Electrostatic Adsorption or SEA, and its pore filling analog, Charge Enhanced Dry Impregnation, or CEDI involves adjusting the pH of the precursor solution (NEXT SLIDE)

metal uptake (per support area) INTRODUCTION: STRONG ELECTROSTATIC ADSORPTION (SEA) - Inducing surface charge on support by adjusting pH of impregnating solution - SEA at incipient wetness is also called Charge Enhanced Dry Impregnation (CEDI) metal uptake (per support area) @ PZC pH > PZC pH < PZC cation uptake anion uptake support OH OH2+ O- pH > PZC pH @ PZC pH < PZC [PtCl6]2- anionic complex [Pt(NH3)4]2+ cationic complex By varying the pH of the solution in contact with support, the interaction between precursor and support is promoted. The acidity or basicity of the solution can charge the surface of the support by protonation/deprotonation and a metal precursor complex of the opposite charge can then be electrostatically adsorbed on that charged support surface. By optimizing the condition to obtain the strongest interaction between support and precursor, where maximum uptake of precursor occurs, the migration of metal is lessened during thermal treatment, which results in smaller catalysts. Increasing catalyst activity by enhancing dispersion would only be effective to a certain extent. Size dependence of reactions can be encountered. By adding a second metal component, a bimetallic catalyst can be made (NEXT SLIDE) support [PtCl6]2- H2O - resulting close packed monolayer of ionic complex (retaining hydration sheaths) with strong interaction with support - decreased mobility of metal atoms result in smaller catalyst particles (compared to simple impregnation) reduction treatment Pt0

vs INTRODUCTION: BIMETALLIC CATALYSTS Addition of another metal can enhance catalytic activity Bimetallic Effects Bifunctional Electronic Ensemble Usual method of co-impregnation does not ensure interaction between component metals vs Bimetallic interactions, such as bifunctional, electronic or ensemble effects can further enhance catalytic performance. For bimetallic catalysts, intimate contact of the component metals should be present, and simple impregnation methods do not ensure this. The method of Electroless Deposition, ED, has been shown to produce bimetallic catalysts with high degree of component metal interaction. (NEXT SLIDE) Using a method that synthesizes a bimetallic catalyst with the required high degree of metal 1 – metal 2 interaction, such as Electroless Deposition can result in better catalysts

INTRODUCTION: ELECTROLESS DEPOSITION (ED) FOR BIMETALLIC CATALYSTS Targeted deposition of secondary metal on the surface of primary/seed catalyst Here’s a simplified illustration of the synthesis of bimetallic catalysts using ED. First, the base, or seed, catalyst with the primary metal is immersed in a solution containing the precursor of the secondary metal and a reducing agent. The reducing agent is activated on the surface of the primary metal and this reduces the secondary metal from the precursor, causing it to deposit on the surface of the particle. The deposition of the secondary metal can proceed catalytically to cover up the primary metal, or auto-catalytically where the secondary metal can also activate the reducing agent causing more of the secondary metal to deposit onto itself. Conditions of ED are fine tuned such that there is targeted deposition of the secondary metal on the primary metal and not on the surface of the support. (NEXT SLIDE) Immersion of seed catalyst in ED bath Activation of reducing agent (RA) on the surface of seed catalyst Reduction and deposition of secondary metal Catalytic deposition Auto-catalytic deposition - Necessary to have proper combination of reducing agent, metal precursor, and ED conditions

Pt based catalysts for Fuel Cells DMFC: Anode Reaction: CH3OH+ H2O → CO2 + 6H+ + 6e- Cathode Reaction: 3/2O2 + 6H+ + 6e-  → 3H2O Cell Reaction: CH3OH+ 3/2O2 → CO2 + 2H2O For this study, we looked at the viability of using the prepared platinum based catalysts on fuel cells as there is significantly large quantities of the noble metal used in this application. This is due to the need for high conductivity and also because of the low dispersion of the commercially available catalysts thus the need for a higher loading to compensate for the low amount of active sites. Significantly large quantities of Pt used in fuel cells Poor dispersion gives low S.A., thus the need to increase metal loading

20nm COMMERCIALLY AVAILABLE CARBON SUPPORTED PLATINUM CATALYST Commercial 20% Pt/Carbon (Vulcan XC72) 20nm Here is a representative dark field electron micrograph of a common, commercially available carbon supported platinum catalyst used for fuel cells. As you could see, the particles for this sample are fairly large, thus there is poor dispersion of the platinum metal. The size distribution of the particles is also wide, ranging from 1 nm to about 8nm. We don’t have information on the exact preparation method used for this catalyst, although it is likely done by simple dry impregnation. [NEXT SLIDE] dN = 3.1nm dXRD = 2.5nm - large particles, wide distribution

Development of Electroless Developer Bath for Ru on Pt/C Reduction of secondary metal on the surface Activation of reducing agent (RA) on base catalyst Further deposition of secondary metal We have carried out experiments to target the deposition of ruthenium onto the platinum surface on the commercial catalyst. We were able to produce an electroless developer bath where acidic conditions were required, with formic acid employed as the reducing agent and a cationic ruthenium precursor, hexaamineruthenium chloride, was used. Studies were done to demonstrate how effective ED of ruthenium on platinum was with this bath, using the commercially available 20% platinum on carbon. (NEXT SLIDE) Reducing Agent: Formic Acid, HCOOH Ru Precursor: Hexaammineruthenium(III) chloride, Ru(NH3)6Cl3 pH condition: Acidic, below PZC of Carbon Support (8.5), to prevent adsorption of [Ru(NH3)6]3+ on carbon surface T. R. Garrick, W. Diao, J. M. Tengco, J. R. Monnier and J. W. Weidner, Elec. Acta., 2016, 195, 106 R. P. Galhenage, K. Xie, W. Diao, J. M. M. Tengco, G. S. Seuser, J. R. Monnier and D. A. Chen, Phys. Chem. Chem. Phys., 2015, 17, 28354 W. Diao, J. M. M. Tengco, J. R. Regalbuto, and J. R. Monnier, ACS Catal., 2015, 5, 5123

Deposition Profiles for Ru ED on Commercial Pt/C Shown here are representative deposition profiles of ruthenium on platinum. The amount of ruthenium in the bath was limited to an equivalent one monolayer theoretical coverage of ruthenium on the platinum surface. The bath was tested for thermal stability to determine if conditions won’t allow for spontaneous reduction of the ruthenium without activation of platinum surface. Upon addition of the base catalyst in the bath, deposition of ruthenium occurred, as the monitoring shows a drop in ruthenium concentration. However, the deposition stops after about 75 minutes. We considered this to be due to the poisoning of the platinum surface by carbon monoxide produced from the decomposition of formic acid. This was resolved by increasing the bath temperature to effectively desorb the carbon monoxide, freeing up platinum surface for ruthenium to reduce onto. [next slide] - High temperature needed to completely deposit 1 ML (theoretical) coverage of Ru on Pt

Theoretical monodisperse coverage BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – on commercial 20% Pt/C catalyst Catalysts Pt wt loading (%) Ru wt loading (%) Theoretical monodisperse coverage θRu on Pt Ru-Pt 1 20.0 0.35 0.16 Ru-Pt 2 0.67 0.30 Ru-Pt 3 1.03 0.46 Ru-Pt 4 1.14 0.51 Ru-Pt 5 1.49 0.68 Ru-Pt 6 1.83 0.83 Ru-Pt 7 2.11 0.96 Once we have determined the ED parameters, we then proceeded to prepare a range of theoretical monodisperse coverages of ruthenium on platinum surface of the commercial catalyst. Characterization of these catalysts showed that the ruthenium deposited did not form distinct particles. [next slide]

XRD does not show alloy formation or large Ru phase BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – on commercial 20% Pt/C catalyst Representative electron micrographs and elemental maps of selected spots of 0.96 ML Ru on Pt/C HAADF-STEM XEDS Maps Platinum Ruthenium Pt@Ru/C (A) (B) (C) (D) 5nm 4nm This slide shows a representative dark field micrograph of a nearly one monolayer theoretically monodisperse ruthenium coverage on platinum as well as corresponding elemental maps of platinum and ruthenium acquired using XEDS (X-ray energy dispersive spectroscopy). As you could see the platinum and ruthenium maps correspond well which confirms targeted deposition of ruthenium on platinum. Also, note that the ruthenium maps show wider areas of ruthenium, evidence that the ruthenium is outside, or on the surface of the particles. XRD, which I won’t be showing, does not show formation of significantly large ruthenium phases nor alloy formation supporting the hypothesis that ruthenium is deposited as a thin overlayer on platinum. In addition, this high degree of association between the component metals have been confirmed with temperature programmed reduction and x-ray photoelectron spectroscopy. (NEXT SLIDE) Ru deposited is in good association with Pt on the surface (Ru and overlaid maps show a “shell”) XRD does not show alloy formation or large Ru phase TPR and XPS also confirm excellent association between component metals

SEA 6.3% Pt/Carbon (Vulcan XC72R) BASE CATALYST PREPARED BY SEA SEA 6.3% Pt/Carbon (Vulcan XC72R) Extending that prior study, we then prepared a more well dispersed base catalyst using the SEA method. This was prepared by adsorption of hexachloroplatinate ion (which is anionic) on the surface of carbon (XC72R) in acidic condition (pH3, surface is positive). The dried powder was reduced under mild ramping to 200C. Here in this slide we were able to produce about 2nm average particle size (NEXT SLIDE) STEM Number Average Size 1.93 nm X-Ray Diffraction Size 1.5 nm

COMPARISON OF BASE CATALYST PREPARED BY SEA WITH COMMERCIAL CATALYST SEA 6.3% Pt/Carbon (Vulcan XC72R) vs. Commercial 20% Pt/Carbon (Vulcan XC72) 20nm 20nm Average particle size and size distribution comparison show that the base catalyst prepared by SEA is much better, however the loading is lower due to the uptake limitation for a single SEA cycle. (NEXT SLIDE) dN = 1.9nm dXRD ~ 1.5nm dN = 3.1nm dXRD ~ 2.5nm

Theoretical monodisperse coverage BIMETALLIC Ru-Pt/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – on SEA prepared 6.3% Pt/C catalyst Catalysts Pt wt loading (%) Ru wt loading (%) Theoretical monodisperse coverage θRu on Pt 0.8 Ru-PtSEA 6.2 1.3 0.82 0.7 Ru-PtSEA 1.1 0.69 0.5 Ru-PtSEA 0.8 0.50 0.4 Ru-PtSEA 6.3 0.64 0.40 0.2 Ru-PtSEA 0.32 0.20 PtSEA Ru-Pt/C Using the same bath formulation for depositing ruthenium on platinum, we prepared another series on bimetallic catalysts using the SEA prepared carbon supported platinum as base catalyst. [next slide]

BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – on SEA prepared 6.3% Pt/C catalyst Representative electron micrographs and elemental maps of selected spots of 0.50 ML Ru on Pt/C Pt Ru And as shown in the elemental maps of a representative sample, we still get the same results of effective targeted deposition, resulting in intimate contact between the component metals. There is however some sintering of the catalyst resulting from the immersion in the aqueous reducing bath and exposure to higher temperature. (NEXT SLIDE)

Theoretical monodisperse coverage ELECTROCHEMICAL EVALUATION OF BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) Cyclic voltammetry Methanol electrooxidation (0.5 M H2SO4 with 1 M methanol electrolyte, Hg/HgSO4 reference and Pt counter electrodes) Repeated 10 times for peak current reproducibility Catalysts Pt wt loading (%) Ru wt loading (%) Theoretical monodisperse coverage θRu on Pt Commercial catalysts for Comparison: PtCOMM 20.0 (Ru-Pt)COMM 13.2 6.8 0.50 The series of catalysts prepared by ED were then evaluated for methanol electrooxidation performance through cyclic voltammetry experiments. The commercial base catalyst, and a commercially available “bimetallic” catalyst were also included for comparison.

Mass activities for DMFC ELECTROCHEMICAL EVALUATION OF BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – on commercial 20% Pt/C catalyst Mass activities for DMFC Evaluation of these prepared catalysts for direct methanol fuel cell performance has shown higher mass activities, with respect to platinum, compared to commercially available “bimetallic catalyst” which most probably has separate phases of the component metals that are not in good contact. There is an optimal peak at around 50% coverage of ruthenium on the platinum surface, a one is to one correspondence of the component metals. Based on literature, this is attributed to the formation of ruthenium hydroxide species which effectively diminishes the poisoning of the platinum surface with carbon monoxide. (NEXT SLIDE) Peak activity at 50% theoretical surface coverage (Ru/Pt = 1:1)

Mass activities for DMFC ELECTROCHEMICAL EVALUATION OF BIMETALLIC Pt@Ru/XC72R PREPARED BY ELECTROLESS DEPOSITION (ED) – comparison of Commercial vs SEA prepared base catalyst Mass activities for DMFC Evaluating the catalysts prepared using SEA and ED combined, varying the ruthenium coverage, we get pretty much the same results with a peak around 50% coverage of ruthenium on the platinum surface. This is however, slightly lower than the one using ED on commercially available platinum base catalyst. It may be possible that the low loading of platinum, which is only 6.3%, compared to 20% for the commercial sample, likely affected diffusion due to greater amount (thickness) of the catalyst deposited on the electrode as well as conductivity of the electrode resulting in lower performance. Note however, that the activity of the SEA prepared catalyst without ruthenium is significantly higher than the commercial samples. [next slide] Peak activity, same, at 50% theoretical surface coverage (Ru/Pt = 1:1)

Additional Catalyst Synthesis Higher loading of platinum on carbon by cycling SEA/CEDI - Successive charge enhanced dry impregnation cycles done to produce 20% Pt/C (Darco G-60) - Small particles based on XRD (~1.5 nm) Pt (fcc)

Additional Catalyst Synthesis Higher loading of platinum on carbon by cycling SEA/CEDI

SUMMARY and CONCLUSIONS Smaller, well dispersed, bimetallic nanoparticles of Ru and Pt were made by Electroless Deposition of Ru on Pt/C base catalyst — commercial and prepared by Strong Electrostatic Adsorption. ED prepared Ru-Pt catalysts show enhanced activity for methanol electrooxidation Electroless Deposition is a simple and viable method for the preparation of true bimetallic catalysts. Coupled with Strong Electrostatic Adsorption, well dispersed bimetallic catalysts can be made.

ELECTRON MICROSCOPY CENTER ACKNOWLEDGEMENTS ASPIRE ELECTRON MICROSCOPY CENTER Thank you

SUPPLEMENTARY INFORMATION

TPR of O-precovered Ru-Pt Bimetallic Particles 2.1% Ru corresponds to 1.0 monodisperse layer of Ru on Pt surface. B. Absence of Ru-O reduction peak confirms Ru-Pt bimetallic particles.

XRD: 6.3% Pt/XC72R SEA dxrd ≤ 1.5 nm

XRD: 20% Pt/XC72 Premetek dxrd = 2.3 nm