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The influence of adsorbed sulfide (S2−ads) on CO monolayer electro-oxidation reaction (CMOR with M for monolayer to emphasize that oxidation always starts with a saturated CO coverage) on carbon-supported Pt nanoparticles (Pt/C NPs) was investigated using electrochemical and in situ ATR-SEIRAS (attenuated total reflection-surface enhanced IR absorption spectroscopy) techniques. An enhanced kinetics of CMOR on the S2−ads-Pt/C was observed by potentiodynamic CO stripping and potentiostatic CO oxidation analyses. Electrochemical impedance measurements revealed a weakened bonding of oxygen-containing species with the S2−ads-Pt/C surface as compared with the clean Pt/C surface, which we believe was the main reason for the observed enhanced CMOR by the S2−ads. The in situ SEIRAS data showed the existence of both linear- and bridge-bonded CO (COL and COB respectively) in all samples and that the sulfide adsorption increased substantially the fraction of the former on the available Pt sites and facilitated the conversion between them. Our results also indicate that the frequently observed pre-CMOR current arises most likely from oxidation of weakly-bonded CO rather than from reactions taking place at defect sites.

Pt-based core–shell (M@Pt where M stands for core element) nanoparticles (NPs) have recently been under increasing scrutiny in the fields of fuel cell and lithium air battery electrocatalysis due to their promising prospects in optimizing catalytic activity, reducing Pt loading and consequently lowering its cost. To achieve the latter, delineating spatially resolved local (surface) elemental distribution and associated variations in electronic properties under working condition (i.e., in situ) is arguably a prerequisite of fundamental importance in investigating electrocatalysis but unfortunately is still sorely missing. In this regard, in situ195Pt electrochemical NMR (EC-NMR) of Pt-based NPs is unique in terms of accessing such information, particularly the spatially resolved partition between the s- and d-like Fermi level local density of states (Ef-LDOS) modified by the core elements. In this paper, we report a comparative in situ195Pt EC-NMR investigation of Ru@Pt vs Au@Pt NPs which was complemented ...

In-situ electrochemical spectroscopic methods, such as solid-state nuclear magnetic resonance (NMR), attenuated-total-reflection surface-enhanced IR reflection adsorption spectroscopy (ATR-SEIRAS) and surface-enhanced Raman scattering spectroscopy (SERS), offer complementary atomic and molecular scale information on the electronic, structural, and molecular properties of nanoparticle electrocatalysts and mechanism(s) of surface reactions. In situ electrochemical (EC) 195Pt and 13CO NMR enables measurements of s- and d-like metal surface local density of states at the Fermi level (E f -LDOS) and 5σ- and 2π*-like E f -LDOS at the probing CO. Such quantitative, electronic-orbital-specific (EOS) information makes it possible to relate electronic properties of the electrocatalysts with surface reaction mechanism(s) that can be monitored by vibrational spectroscopy, i.e. ATR-SEIRAS and SERS. In this chapter, we discuss integrated in situ EC-NMR, -SEIRAS and -SERS studies of CO and methanol oxidation reactions on core-shell Ru@Pt and Au@Pt nanoparicles (NPs) through which we intend to demonstrate that the integrated approach makes “the whole is greater than the sum of its parts”.

The effect of adsorbed sulfide (S2–ads) on methanol (MeOH) electro-oxidation reaction (MOR) taking place on carbon-supported Pt nanoparticles (NPs) was investigated using electrochemical and attenuated-total-reflection (ATR)-FTIR spectroscopic techniques. The observed substantial enhancement in MOR activity per active site was rationalized by an enhanced parallel reaction pathway by the S2– adsorption that did not lead to poisonous CO. Moreover, there were delayed/suppressed adsorption of electrolyte anions and oxygen-containing species and increased amount of isolated free water monomers caused by the sulfide adsorption-induced more negatively charged Pt sites, which led to more available active sites, weakened bonding interaction between the Pt NPs and oxygen-containing species, and more active water species for MOR along the parallel reaction pathway.

This paper reports a detailed electrochemical in situ surface-enhanced infrared reflection absorption spectroscopic (SEIRAS) investigation of two different core−shell, Ru@Pt and Au@Pt/C, metal nanoparticles (NPs). We were able to identify the most active sites and surface water species involved in the carbon monoxide oxidation reaction (COR) and methanol oxidation reaction (MOR) on these NPs. We discovered that exposing the as-synthesized Ru@Pt NPs to air could turn them into largely surface-ruthenated NPs whose structure was rather stable under multiple potential cyclings between −0.2 and 0.7 V (vs Ag/AgCl, 3 M) and reduction at −0.3 V but could be annealed by the COR. The SEIRAS data enabled the identification of the Ru-coordinated-to-Ru, Ru-coordinated-to-Pt, and Pt-islands-on-Ru-core sites on the COR-annealed Ru@Pt NPs among which the most active sites were the Pt-islands-on-Ru-core sites for the COR and MOR, as evidenced by an onset potential as low as −0.1 V for the COR. For the Au@Pt/C NPs, the SEI...

The chemical state of adsorbed sulfur (S) on a Pt surface was identified and investigated by a combination of detailed electrochemical (EC) measurements, in situ surface enhanced Raman scattering (SERS) and ab initio density functional theory (DFT) calculations. The SERS data of the adsorbed S coupled with the DFT calculations provide the first convincing evidence showing that the adsorbed sulfur is in a sulfide (S2-) state.

Introduction Conventional carbon supports suffer from catastrophic failure during start-up/shut-down events, which result in high cathode interfacial potentials, which, in turn, lead to carbon corrosion and catalyst degradation. Carbon corrosion is a major problem in fuel cells because it leads to loss of catalyst activity and collapse of electrode structure. Nissan, as an automotive OEM, has always approached the issue of catalyst support durability at the materials level, rather than at the systems-level, since system-level mitigation is an added cost to the balance of plant (BOP), and presents one additional factor that can fail during operation. NTCNA and IIT have been working on the development of high-surface-area, corrosion-resistant non-carbon supports, and we have previously demonstrated a class of highly stable non-carbon supports, e.g. ruthenium-titanium oxide (RTO) (1). In this work, we present the results of our work on a second class of non-carbon supports - tin-doped indium oxide (ITO). Materials and Methods RDE Testing – RDE tests were performed in a 3-compartment electrochemical cell containing 0.1 M HClO4 electrolyte. A glassy carbon disk (0.196 cm2) uniformly covered with Pt/C or Pt/ITO catalyst served as the working electrode. The counter and reference electrodes were Pt foil and RHE, respectively. Membrane Electrode Assembly (MEA) Fabrication - Catalyst inks were prepared by mixing Pt/C or Pt/ITO with water, n-propanol, and a Nafion ionomer dispersion. The mass-based ionomer/support (I/S) ratio in the ink was kept constant at 0.9. The cathode catalyst layers were formed on GDLs using an automated robotic spray system. MEAs (25 cm2) were prepared by hot pressing cathode GDEs (0.35 mgPt/cm2) and commercial anode GDEs (0.4 mgPt/cm2) onto Nafion® NR211 membranes. Results and Discussion Figure 1 shows the RDE performance of Pt/ITO vs. a commercially available Pt/C benchmark catalyst of comparable Pt particle size. The ECA obtained for Pt/ITO (22 m2/gPt) was lower than Pt/C (45 m2/gPt) due to the low surface area of ITO which causes Pt particle agglomeration. Even with a lower ECA, Pt/ITO shows promising performance since its mass activity of ~150 mA/mgPt matches that of the Pt/C benchmark catalyst. However, we have repeatedly observed that Pt/ITO performs very poorly in MEA testing. Figure 2 shows a typical H2/O2 polarization curve obtained for Pt/ITO at a high Pt loading of 0.35 mg/cm2. The only difference between the two MEAs is the cathode catalyst. Other than the dramatic difference between the two polarization curves, it can also be seen that Pt/ITO shows a very high HFR (high frequency resistance) of ~220 mΩ·cm2. The HFR represents the ohmic resistance due to proton transport in the membrane and the electronic contact resistances in the cell. The high HFR is attributed to the poor electronic conductivity of the ITO support since the two MEAs in Figure 2 have the same membrane, cell hardware, and cell components. It is hypothesized that hydroxylated species form on the ITO surface under fuel cell operating conditions. Hydroxide and oxy-hydroxide species may readily form on the surface of ITO due to hydrolysis reactions. Hydroxylated species such as In(OH)3 have low solubilities, hence they may remain adsorbed on the ITO surface, forming a passivating layer that will increase ohmic resistance (2). Furthermore, the Pourbaix diagram for indium shows that under fuel cell conditions, the formation of In3+ is favorable, and it is possible that In3+ can act as a poison that covers the Pt active sites. This is supported by the changes observed in the CV profile in MEA (loss of Hupd features and resistive behavior), suggesting some changes in the chemical properties of ITO and Pt poisoning. The promising RDE results did not translate to good performance in MEA, and this is hypothesized to be caused by cross-over hydrogen in MEA. H2 crossing over from anode to cathode may accelerate the formation of hydroxylated species. Experiments are underway to verify this. References 1. J. Parrondo, T. Han, E. Niangar, C. Wang, N. Dale, K. Adjemian and V. Ramani, Proceedings of the National Academy of Sciences, 111 (2014) 45-50. 2. B. Michael, P. A. Veneman, F. S. Marrikar, T. Schulmeyer, A. Simmonds, W. Xia, P. Lee, and N. R. Armstrong. Langmuir, 23(22), 11089-11099 (2007). Figure 1

In this presentation, we will discuss a combined in situ SEIRAS (surface enhanced IR reflection absorption spectroscopy) and NMR (nuclear magnetic resonance) study of Ru core–Pt shell (Ru@Pt) electrocatalysts for methanol oxidation reaction (MOR) [1. 2]. The Ru@Pt electrocatalysts were synthesized with commercial Ru black (~ 3 nm, Johnson-Matthey) as starting material. Ethylene glycol (EG) was used as both reaction medium and reducing agent in the synthesis [3]. First, the Ru black was reduced at 140 °C. Then a given volume of H2PtCl6·6H2O solution was added to the EG suspension of the pre-reduced Ru black and Pt was deposited onto Ru black at 120 °C. We have carried out in situ SEIRAS investigation of the Ru@Pt nanoparticles (NPs) of different Pt packing densities (PDs). We were able to identify the most active sites and surface water species involved in the carbon monoxide oxidation reaction (COR) and MOR on these NPs. We discovered that exposing the as- synthesized Ru@Pt NPs to air could turn them into largely surface-ruthenated NPs whose structure was rather stable under multiple potential cyclings between -0.2 and 0.7 V (vs Ag/ AgCl, 3 M) and reduction at -0.3 V but could be annealed by the COR. The SEIRAS data enabled the identification of the Ru-coordinated-to-Ru, Ru-coordinated-to-Pt, and Pt-islands- on-Ru-core sites on the COR-annealed Ru@Pt NPs among which the most active sites were the Pt-islands-on-Ru-core sites for the COR and MOR, as evidenced by an onset potential as low as -0.1 V for the COR. Among the three different surface water species, namely the water monomer, the weakly hydrogen-bonded water, and the strongly hydrogen-bonded water, the SEIRAS data pointed to the weakly hydrogen-bonded water as the dominant source that provided oxygen for the COR and MOR. Point-by-point 195Pt NMR spectra of the as- received and EC-cleaned samples of Ru@Pt NPs in comparison with that of pure Pt/C NPs [4], all recorded in a supporting electrolyte (0.1 M HClO4) at 80 K. The vertical dashed line indicates the peak position (1.100 G/kHz, G stands for magnetic field). The as-received Ru@Pt NPs were freshly synthesized with a shelf life of only a few hours. With the protection of ethylene glycol, little surface oxidation would be expected in such a short time. Indeed, no significant surface oxidation was observed in the corresponding 195Pt NMR spectrum. The clean-surface peak now appeared at 1.1053 G/kHz, corresponding to a 4795 ppm negative shift with respect to the clean-surface peak position (1.1000 G/kHz) of the pure Pt NPs, revealing a strong electronic effect of Ru on Pt, which is opposite to that of Au. A Gaussian deconvolution of the surface peak gave a surface atoms fraction of ≈74%, indicating a dominant Ru core Pt shell structure. Results of T2 and T1 measurements at different spectral positions of the surface fall onto their respective single relaxation curves, indicating the same highly similar surface structural and electronic properties. 13C NMR spectra of 13CO on the Ru@Pt sample were also obtained. The peaks at ≈300 and ≈219 ppm can be assigned to CO on Pt and on Ru sites, respectively, based on literature values. The results of temperature-dependent T 1 measurements show the pass-through-the-origin straight lines that are the hallmark of the Korringa relaxation behavior [5] which indicates that the adsorbed CO molecules on three different sites all acquired metallic characteristics through surface bonding. For CO on Ru sites, the shift (219 ppm) and Korringa constant T1T = 138 s·K (where T is the absolute temperature at which T1 is measured) are very close to those observed on pure Ru [6] and on Ru deposited on Pt NPs [7], which lends strong support to our peak assignment that is also consistent with the expected exposure of Ru core for a Pt PD of 40%. On the other hand, the Korringa constants measured at the same spectral position (333 ppm) were different for CO on surface Pt atoms of the Au@Pt and on those of the Ru@Pt The above discussed combined in situ SEIRAS and NMR investigation enabled us to gain deeper insights into how Ru core influences the electronic properties of he Pt core by which highly active sites were identified. Acknowledgments The authors thank financial support from DOE (DE-FG02-07ER15895) and NSF (CHE-0923910). References [1] Atienza, D. O.; Allison, T. C.; Tong, Y. Y. J. J. Phys. Chem. C 2012, 116, 26480−26486. [2] Chen, D.; Hofstead-Duffy, A.; Park, I.; Atienza, D. O.; Susut, C.; Sun, S.-G.; Tong, Y. Y. J. J. Phys. Chem. C 2011, 115, 8735–8743. [3] Du, B.; Rabb, S. A.; Zangmeister, C.; Tong, Y. Y. Phys. Chem. Chem. Phys 2009, 11, 8231–8239. [4] Tong; Rice, C.; Godbout, N.; Wieckowski, A.; Oldfield, E. J Am Chem Soc 1999, 121, 2996–3003. [5] Korringa, J. Phys. 1950, XVI, 601. [6] Wang, P. K.; Ansermet, J. P.; Rudaz, S. L.; Wang, Z.; Shore, S.; Slichter, C. P.; Sinfelt, J. H. Science 1986, 234, 35.. [7] Tong, Y. Y.; Kim, H. S.; Babu, P. K.; Waszczuk, P.; Wieckowski, A.; Oldfield, E. J. Am. Chem. Soc. 2001, 124, 468.