Surface Reactions on an Atomic Scale

Abstract

Field Pulse Desorption Mass-Spectrometry is applied to investigate intermediates of surface reactions. The formation of metal-subcarbonyl compounds is studied at rhodium-and ruthenium field emitters. Electrostatic field effects, field-induced reactions and field fragmentation are measured. The decomposition of methanol on rhodium and ruthenium shows two temperature regimes. At lower temperatures chemisorbed CO hinders the catalytic decomposition. Above the CO-desorption temperatures the C-H-bond cleavage of surface methoxy groups is rate determining. With photon-pulses the field-induced polymerization of ethylene is studied. Adsorbed water -layers form cluster ions (H 2 O) z · H+ with 3<z<16. A photon energy of > 7 eV is needed for the formation of these cluster ions.A special version of the atom probe can be used to study dynamic processes on an atomic scale (1). In the field ion microscope section, a particular area of a tip emitter (=catalyst-)surface is selected. Depending on the tip radius, the tip to screen distance and the probe hole diameter, either single surface atoms or up to several hundred surface sites can be analyzed in the time-of-flight mass spectrometer. With its high detection sensitivity, this technique is able to identifying individual chemisorbed species at characterized crystallographic sites on low-or high-index single crystal surfaces.

Conduction and Electrostriction of Polymers Induced by High Electric Fields

“…As an example, local electric fields, generated by a sharp metal tip, can be used to preferentially dissociate certain species that one wants to deposit locally under the tip, e.g., in field-assisted chemical vapor deposition. Block [8], following earlier work by Inghram and Gomer [9] had developed a pulsed field technique in the field ion microscope that allows the investigation of the field effect on chemical reactions; similar work was reported by Beckey and Röllgen [10]; a detailed account had been given by Block [4,5]. Systems that have been studied by this technique are the formation of metal subcarbonyls, the polymerization of acetone, the reaction of sulphur on metal surfaces, the decomposition of methanol on metal surfaces, hydride formation on semiconductors, NO reactions on metals and many more.…”

“…They are thus strong enough to induce the re-arrangement of electronic orbitals of atoms and molecules leading to new phenomena that can be summarily described as field-induced chemisorption and field-induced chemistry. Several review articles have been written over the years about these topics [1][2][3][4][5]. For reference in the remainder of this article we note that a field of 1 V/Å corresponds to a force of 0.16 nN, and 300 k B /Å is 42 pN.…”

“…Using the field pulse technique Block and collaborators discovered the formation of large cluster ions of atoms and molecules such as (H 2 .O) n and S n [4,5,24,25]. They exposed a tungsten tip in a field ion microscope to a beam of atoms or molecules leading to a multilayer adsorbed film.…”

“…A typical mass spectrum from an adlayer of sulfur is reproduced in Figure 6 [24]. at high reaction field [5].…”