AVS2016 Session HC+SS-WeM: Bridging Gaps in Heterogeneously-catalyzed Reactions

Motivated mainly by catalysis, gas-surface interaction between single crystal surfaces and molecules has been studied for decades. Most of these studies have been performed in well-controlled environments, and has been instrumental for the present day understanding of catalysis. We have in recent years explored the possibilities to perform experiments at conditions closer to those of a technical catalyst, in particular at increased pressures. In this contribution, results from catalytic CO oxidation over Pd single crystal surfaces using High Pressure X-ray Photo emission Spectroscopy (HPXPS), Planar Laser Induced Fluorescence (PLIF), and High Energy Surface X-Ray Diffraction (HESXRD) will be presented.

Armed with structural knowledge from ultra-high vacuum experiments, the presence of adsorbed molecules and gas-phase induced structures can be identified, and related to changes in the reactivity and/or to reaction induced gas-flow limitations. The strength and weaknesses of the experimental techniques will be discussed.

A primary motivation in studying the assembly of molecules at an interface is to determine the underlying principles which drive the spontaneous formation of supramolecular structures under specific environmental conditions, with the ultimate goal being an understanding which allows control over such processes. Recently we have postulated the importance of solution-phase cluster formation during the pulse-deposition, in vacuum, of small organic and organometallic molecules which contain strong hydrogen bonding components, namely carboxylic acid functional groups, in addition to weak hydrogen bonding donors. Specifically, we have studied the cluster formation of 1,1’-ferrocenedicarboxylic acid, Fc(COOH)₂, which has been pulse deposited on Au(111)-on-mica substrates. We employed low temperature scanning tunneling microscopy (LT-STM) to observe which molecular clusters persist at the interface following pulse-deposition from solution. We subsequently performed annealing experiments to determine which of these species represent stable conformations, and which are metastable species formed in the solution droplet during deposition. LT-STM images of Fc(COOH)₂ show a coexistence between dimer, chiral hexamer, and square tetramer clusters. Since the bulk crystal structures for Fc(COOH)₂ are all comprised entirely of molecular dimers, the most stable 2D supramolecular structure likely is some array of dimers, whereas the other species present are metastable. We attribute the initial strong presence of metastable species to the formation of clusters in solution, followed by their precipitation as predicted by Ostwald’s rule, then their adsorption on to the substrate in a kinetically-trapped conformation. Electrospray ionization mass spectrometry (ESI-MS) of Fc(COOH)₂ solutions supports the presence of hexamer clusters in solvent droplets, which confirms the possibility that the chiral hexamers observed after deposition precipitate from the solution droplet. Sequential mild annealing stepsof this surface results in the formation of linear rows of square tetramers, chiral dimer domains, and eventually tilted dimer arrays. The prevalence of each of these species is related to the surface’s thermal history, and this is characteristic of a system evolving under kinetic control. We propose that injection of solution into a vacuum environment can be exploited to produce supramolecular 2D-structures not observed in the bulk crystal, and that the frequency of nature of the metastable species present should be dependent on variables such as the solution concentration, solvent, and droplet size.

The chemical activities of bimetallic Pt-Re clusters supported on TiO₂(110) and single-crystal Pt-Re alloy surfaces are investigated as model systems for understanding Pt-Re catalysts in the water gas shift (WGS) reaction. The activities of these Pt-Re bimetallic surfaces are studied in a microreactor coupled to an ultrahigh vacuum chamber so that the surfaces can be characterized by X-ray photoelectron spectroscopy (XPS) before and after reaction. Bimetallic clusters consisting of a Re core covered by a Pt shell have turnover frequencies that are almost twice as high as that of the pure Pt clusters at 160 °C. Furthermore, the Re in the active bimetallic clusters remains in its metallic state because Re is not readily oxidized when it remains subsurface; there is no evidence that ReO_x is active in promoting the WGS reaction. Pure Re clusters are not active for the WGS reaction, and bimetallic clusters with significant Re at the surface are less active than pure Pt clusters. Surface Re is oxidized under reaction conditions and sublimes as Re₂O₇. Post-reaction infrared spectroscopy studies show that CO and hydroxyls are detected on the surface.

A long-standing challenge in the study of heterogeneously catalyzed reactions on silver surfaces has been the determination of what surface phases are of greatest chemical importance. This is due to the coexistence of several different surface phases on oxidized silver surfaces. A further complication is subsurface oxygen (O_sub). O_sub are O atoms dissolved into the near surface of a metal, and are expected to alter the surface in terms of chemistry and structure, but these effects have yet to be well characterized. We studied oxidized Ag(111) surfaces after exposure to gas-phase O atoms with a combination of surface science techniques to determine the resultant surface structure; we observed that once 0.1 ML of O_sub has formed, the surface dramatically, and uniformly, reconstructs to a striped phase at the expense of all other surface phases. Furthermore, O_sub formation is hindered at temperatures above 500 K. We also observed a coexistence of several surface oxides at intermediate deposition temperatures, and the predominance of the p(4x5√3) surface reconstruction at elevated temperatures.