My friend and colleague John Yates was a joy to share life with. His perpetual enthusiasm, imagination, and sincere interest in whatever happened to be the topic of the moment whether a complex issue in surface science, the fate of an astronomically distant galaxy, the program for Lorin Maazel’s next Pittsburgh Symphony concert, the well-being of his scientific “children” and real family, or speculation on what’s likely to be tonight’s specialties on the menu of our favorite Il Pizzico Restaurant near NIST, was infectious. John’s retirement in 2006 from his chair at the University of Pittsburgh [which he went to from NBS/NIST in 1982] brought him to an emeritus-like guest faculty desk at the University of Virginia. Soon upon settling in at the university, his “desk” became an office became a full-fledged surface science lab with an active group of students and post docs supported by research grants that totaled among the highest within UVA chemistry, all of which was built up from scratch after John “retired”. He was the only person I know who confessed that he really liked writing research proposals. He lived and executed his retirement with more excitement, energy, and enthusiasm than a young, tenure-seeking assistant professor; the energizer bunny personified.

I will present a number of anecdotal/amusing illustrative vignettes drawn from my experiences with “the energizer bunny” and discuss why his seminal JCP papers on thermal broadening in ESD [51,1264(1969)] and on resonance ESD [90,5793(1989)] were so influential in my own conceptualization and realizations in surface dynamics.

Finally, John and I had the privilege of being co-participants in two NIST Oral History interviews in 2014 and 15, once as a “target” and then as an interviewer [of each other]. This was a great way for us to summarize our nearly half a century of being a part of each other’s life, a privilege that I feel very fortunate and honored to have had. [transcripts available upon request: gadzuk@nist.gov [mailto:gadzuk@nist.gov] ]

In the 1970's John Yates spent a sabbatical year at Caltech, where he touched the lives of many students and postdocs working in the group of Prof. Henry Weinberg. My story starts as a young student, fascinated with symmetry and statistical mechanics, who was privileged to work in the laboratory with John, in those early days of surface science when it seemed that every observation opened new horizons.

I will provide highlights of some of the exciting research that followed from those early days of surface science, discussing the impact of scanning tunneling microscopy, the linkages of surface science and nano-electronics, and the practical impacts of many years of fundamental studies from the perspectives of industry and ARPA-E.

Model systems are useful to establish structure/morphology reactivity relations in heterogeneous catalysis. Here we use a Metal-Insulator-Metal (MIM) structure to advice CO₂.

We had shown before that ultra-thin oxide films could be used as effective barriers to charge Au islands with electrons from the metal below the support. The idea is to use the electrons stored in the islands to activate molecules by electron transfer. Such a molecule is the energy economy relevant carbon-dioxide for example. We demonstrate adsorption of CO₂ at gold islands and the reversible formation of CO₂ anions and oxalates based on information from microscopy and spectroscopy.

We show how to transfer those ideas developed for thin film systems to bulk materials, where we use appropriate dopants within the support material to provide the electron source. The electron transfer to Au islands as well as the activation of oxygen will be demonstrated.

The first paper on thermal desorption John Yates published with Ted Madey was entitled: Nitrogen Desorbs with Complex Kinetics. Thermal Desorption Spectroscopy, in more and more sophisticated experimental set-ups including flow reactors, became then a prominent techniques in John´s Laboratory to study simple and complex surface chemical reactions. In this short report, we look further into “complex kinetics” and find that the theoretical framework of a thermally activated process can not be applied to the detachment of particles in a micro-fluidic shear-flow experiment.

The theoretical description of the desorption process treats bond breaking as a thermally activated process, which can be described by the Arrhenius equation. The activation energy and the pre-exponential factor can be determined from a series of TPD experiments with either varying coverage or at constant coverage, using different heating rates. Here we experimentally and theoretically analyze the detachment of microscopic polystyrene beads from different self-assembled monolayer (SAM) surfaces in a shear flow to develop a mechanistic model for the removal of cells from surfaces. The detachment of the beads from the surface is treated, as in thermal desorption experiments, as a thermally activated process to determine activation barrier and attempt frequency of the rate determing step in bead removal. The statistical analysis of the experimental shear detachment data, obtained in phosphate-buffered saline solution, results in an activation energy of detachment around 20 kJ/mol. This value is orders of magnitude lower than the adhesion energy measured by atomic force microscopy (AFM). The same order of magnitude for the adhesion energy measured by AFM is derived from ab initio calculations of the van der Waals interaction energy between the polystyrene beads and the SAM-covered gold surface. We hence conclude, that the rate determing step for detachment of the beads is the initiation of rolling on the surface (overcoming static friction), and not physical detachment as would be the case in a gas desorption experiment.

A constant theme throughout the long scientific career of John T. Yates, Jr. was the use of infrared spectroscopy to probe the properties of surfaces and of adsorbed molecules. In his very first publication in 1961, based on his PhD thesis from MIT, he used transmission infrared spectroscopy to study the interaction of carbon monoxide with alumina-supported nickel surfaces. He continued to use innovative methods of transmission infrared spectroscopy in his laboratories at the National Bureau of Standards, the University of Pittsburgh, and the University of Virginia. The final publication of his career, which appeared in 2016, featured transmission IR spectra of CO interacting with the surfaces of titania-supported gold. In addition to transmission IR spectroscopy of high-area powdered samples, he was also a leader in the use of reflection absorption infrared spectroscopy (RAIRS), also known as infrared reflection absorption spectroscopy (IRAS), a method that allows IR spectra to be obtained on small-area metal single crystals. RAIRS is one of the few surface science techniques that can be used both under UHV conditions as well as in the presence of an ambient pressure of gas. John Yates was a pioneer in bridging the so-called pressure gap by using RAIRS to probe gas-surface interactions in the presence of elevated gas pressures. Recent examples from other research groups will be presented to highlight the continuing impact John Yates has had in the use of infrared spectroscopy in surface science.

In this presentation I will give a brief overview of how my post-doc with John T. Yates Jr. influenced my way of working and how that philosophy could be used too go from fundamental investigations of reactions on surfaces to actually understand and make new heterogeneous catalysts. First we shall discuss how mass-selected nanoparticles of CuZn alloys to elucidate the dynamics of the methanol synthesis catalysts. The produced nanoparticles will be compared to the conventional CuZnAl at 1 bar for synthesizing methanol from CO₂ and H₂ [1, 2, 3]. The methanol synthesis on CuZn will also be discussed with respect to our very recent findings of using alloys of NiGa for methanol synthesis [4]. The use of mass-selected nanoparticles will be further demonstrated for electrochemical Oxygen Reduction Reaction, which is really the limiting reaction in Proton Exchange Membrane Fuel Cells. Here we have found entirely new classes of electro-catalysts by alloying Pt with early transition metals [5] or the lanthanides [6]. We have also shown that it is possible to make mass-selected nanoparticles of these alloys with very good activities [7] and PtGd alloys [8].

1. S. Kuld, C. Conradsen, P.G. Moses, I. Chorkendorff and J. Sehested, Angew. Chemie 53 (2014) 1.

2. C. Holse, C. F. Elkjær A. Nierhoff, J. Sehested, I. Chorkendorff, S. Helveg, J. H. Nielsen, J. Chem. Phys. 119 (2015) 2804-2812.

3. S. Kuld, M. Thorhauge, H. Falsig, C. Elkjær, S. Helveg, I. Chorkendorff, J. Sehested, Accepted SCIENCE (2016).

4. F. Studt, F. Abild-Pedersen, I. Sharafutdinov, C. F. Elkær, J. S. Hummelshøj, S. Dahl, I. Chorkendorff, and J. K. Nørskov, Nature Chemistry. 6 (2014) 320.

5. J. Greeley, I.E.L. Stephens, A.S. Bondarenko, T. P. Johansson, H. A. Hansen, T. F. Jaramillo, J. Rossmeisl, I. Chorkendorff, J. K. Nørskov, , Nature Chem. 1 (2009) 522.

6. M. Escudero-Escribano, P. Malacrida, M- Hansen, U. G. Vej-Hansen, A. Velázquez-Palenzuela V. Tripkovic, J. Schøitz, J. Rossmeisl, I. E.L. Stephens, I. Chorkendorff, SCIENCE 352 (2016) 73-76.

7. P. Hernandez-Fernandez, F. Masini, D. N. McCarthy, C. E. Strebel, D. Friebel, D. Deiana, P. Malacrida, A. Nierhoff, A. Bodin, A. M. Wise, J. H. Nielsen, T. W. Hansen, A. Nilsson, I. E.L. Stephens, I. Chorkendorff, Nature Chemistry 6 (2014) 732-8.

8. A. Velázquez-Palenzuela, F. Masini, A. F. Pedersen, M. Escudero-Escribano, D. Deiana, P. Malacrida, T. W. Hansen, D. Friebel, A. Nilsson, I. E.L. Stephens, I. Chorkendorff, J. Catal. 328 (2015) 297-307.

Converting CO₂ into valuable chemicals and fuels is one of the most practical routes for reducing CO₂ emissions while fossil fuels continue to dominate the energy sector. The catalytic reduction of CO₂ by H₂ can lead to the formation of three types of products: CO through the reverse water-gas shift (RWGS) reaction [1], methanol via selective hydrogenation [2], and hydrocarbons through combination of CO₂ reduction with Fischer-Tropsch (FT) reactions. In the current talk we will discuss some of our recent results in CO₂ conversion [3]. Our research approaches involve the combination of DFT calculations and surface science studies over single crystal surfaces, catalytic evaluations over supported catalysts, and in-situ characterization under reaction conditions. We will also discuss challenges and opportunities in this important research field [4].

References:

[1] M.D. Porosoff, X. Yang, J.A. Boscoboinik, and J.G. Chen, “Molybdenum carbide as alternative catalysts to precious metals for highly selective reduction of CO₂ to CO”, Angewandte Chemie International Edition, 53 ( 2014) 6705-6709.

[2] X. Yang, S. Kattel, S.D. Senanayake, J.A. Boscoboinik, X. Nie, J. Graciani, J.A. Rodriguez, P. Liu, D.J. Stacchiola and J.G. Chen, “Low pressure CO₂ hydrogenation to methanol over gold nanoparticles activated on a CeOx/TiO₂ interface”,Journal of the American Chemical Society, 137 (2015) 10104-10107.

[3] M.D. Porosoff, M. Myint, S. Kattel, Z. Xie, E. Gomez, P. Liu and J.G. Chen, “Identifying different types of catalysts for CO₂ reduction by ethane through dry reforming and oxidative dehydrogenation”, Angewandte Chemie International Edition, 54 (2015) 15501-15505.

[4] M.D. Porosoff, B. Yan and J.G. Chen, “Catalytic reduction of CO₂ by H₂ for synthesis of CO, methanol and hydrocarbons: Challenges and opportunities”, Energy & Environmental Science, 9 (2016) 62-73.

Bulk molecular ionic solids exhibit a fascinating diversity of electronic ground states, including unconventional superconductivity. The electronic properties of these systems have historically avoided surface analytical studies due to the intrinsic difficulty with surface preparation. We are therefore pursuing epitaxial growth of charge-transfer compounds, toward understanding their fundamental properties and creating new kinds of metal-organic or organic-organic interfaces [1,2]. A primary challenge is to assure compatibility of supported epilayers with the redox processes so as to enable, enhance but not eliminate significant charge transfer and electron correlations with supported structures.

In this talk, I will discuss 2D molecular structures of TTF and TCNQ molecules supported on metal and graphitic surfaces. We established that these molecules self-organize into a "zoo" well-ordered structure with a wide-range of TTF:TCNQ ratios [3] - a marked deviation from a single stable 1:1 ratio in the bulk. We propose that the formation these structures is governed by a surface phase diagram that exhibits at least four distinct stable compositions. The diagrammatic picture explains many of the properties of such systems that often seem incidental: the morphology of epilayers, governed by nucleation and growth; coexistence and abundance of various phases, and the distinct molecular structure of phase boundaries. Using coupled image and computational analysis, we conclude that the observed shapes are dictated by the preferential formation of a well-defined “quad”-motif involving TTF(TCNQ) molecules coordinated by at least four of its matching neighbors. Thus enabled deterministic control is beneficial to both electronic properties of surface phases (which can develop local magnetic moments) and the formation of quasi-2D TTF-TCNQ solids, which develop decidedly non-bulk Mott-insulating state despite having 1:1 bulk ratio. We anticipate that these properties are quite general for multicomponent molecular compounds, providing new opportunities for self-organized and electronically interesting molecular systems.

This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

[1] G. A. Rojas et. al, and P. Maksymovych, "Ionic Disproportionation of Charge Transfer Salt Driven by Surface Epitaxy", J. Phys. Chem. C, 117 (2013) 19402.

[2] C. Park et. al, and P. Maksymovych, "Weak competing interactions control assembly of strongly bonded TCNQ ionic acceptor molecules on silver surfaces", Phys. Rev. B 90 (2014) 125432.

[3] S. Jeon et. al and P. Maksymovych, ACS Nano in review (2016).