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Title: ESR studies of surface adsorption and catalysis under ultra-high vacuum conditions. Progress report, March 1, 1982-February 28, 1983

Abstract

A study was conducted of the new phenomenon of surface-suppressed ESR (SSESR), in which stable paramagnetic radicals adsorbed on clean Ag or Cu surfaces have their ESR signal suppressed until several layers of molecules insulate subsequent layers from the metal surface. This study combined UHV-ESR, CREMSEE, and thermal desorption. A tentative model based upon transmission of the effects of the surface conduction band of the metal by the weak exchange forces between the paramagnetic species was suggested. Experiments on a model system by means of Laser Time Resolved ESE spectroscopy were performed. A photo-induced transient electron was produced from Rb ions in nonpolar medium. The spin relaxation of the transient electron was studied vs temperature and alkali metal concentration. The transient electron is initially spin-polarized, and the degree of spin-polarization is affected by the wavelength. Such an effect would be expected from a long-lived geminate radical-pair mechanism (Rb/sup +/ + e/sup -/) causing the spin-polarization, with the initial separation distance affected by the energy of the photon absorbed. An extended Hueckel MO analysis of our previous O/sub 2//sup -//Ti-vycor study was completed; a new and improved photo-CREMSEE apparatus was completed and utilized; motional dynamics of NO/sub 2/ on zeolites wasmore » analyzed; and work was conducted on a photo-induced source of H atoms for Laser Time Resolved ESE experiments were carried out.« less

Authors:
Publication Date:
Research Org.:
Cornell Univ., Ithaca, NY (USA). Dept. of Chemistry
Sponsoring Org.:
USDOE
OSTI Identifier:
6721428
Alternate Identifier(s):
OSTI ID: 6721428; Legacy ID: DE83000995
Report Number(s):
DOE/ER/04991-4; COO-4991-4
ON: DE83000995
DOE Contract Number:
AC02-78ER04991
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; COPPER; SORPTIVE PROPERTIES; HYDROGEN; PHOTOLYSIS; NITROGEN DIOXIDE; ADSORPTION; ORGANIC NITROGEN COMPOUNDS; OXYGEN; RUBIDIUM; PHOTOIONIZATION; SILVER; SURFACES; ELECTRON SPIN RESONANCE; ZEOLITES; AMINES; ATOMS; CATALYSIS; MOLECULAR ORBITAL METHOD; RADICALS; RESEARCH PROGRAMS; SOLUTIONS; SOLVATED ELECTRONS; SURFACE TREATMENTS; TITANIUM; VYCOR; WATER VAPOR; WORK FUNCTIONS; ALKALI METALS; CHALCOGENIDES; CHEMICAL REACTIONS; DECOMPOSITION; DISPERSIONS; ELECTRONS; ELEMENTARY PARTICLES; ELEMENTS; FERMIONS; FLUIDS; FUNCTIONS; GASES; INORGANIC ION EXCHANGERS; ION EXCHANGE MATERIALS; IONIZATION; LEPTONS; MAGNETIC RESONANCE; MATERIALS; METALS; MINERALS; MIXTURES; NITROGEN COMPOUNDS; NITROGEN OXIDES; NONMETALS; ORGANIC COMPOUNDS; OXIDES; OXYGEN COMPOUNDS; PHOTOCHEMICAL REACTIONS; RESONANCE; SORPTION; SURFACE PROPERTIES; TRANSITION ELEMENTS; VAPORS 400201* -- Chemical & Physicochemical Properties; 400500 -- Photochemistry

Citation Formats

Freed, J. H.. ESR studies of surface adsorption and catalysis under ultra-high vacuum conditions. Progress report, March 1, 1982-February 28, 1983. United States: N. p., 1982. Web. doi:10.2172/6721428.
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Freed, J. H.. ESR studies of surface adsorption and catalysis under ultra-high vacuum conditions. Progress report, March 1, 1982-February 28, 1983. United States. doi:10.2172/6721428.
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Freed, J. H.. Wed . "ESR studies of surface adsorption and catalysis under ultra-high vacuum conditions. Progress report, March 1, 1982-February 28, 1983". United States. doi:10.2172/6721428. https://www.osti.gov/servlets/purl/6721428.
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@article{osti_6721428,
title = {ESR studies of surface adsorption and catalysis under ultra-high vacuum conditions. Progress report, March 1, 1982-February 28, 1983},
author = {Freed, J. H.},
abstractNote = {A study was conducted of the new phenomenon of surface-suppressed ESR (SSESR), in which stable paramagnetic radicals adsorbed on clean Ag or Cu surfaces have their ESR signal suppressed until several layers of molecules insulate subsequent layers from the metal surface. This study combined UHV-ESR, CREMSEE, and thermal desorption. A tentative model based upon transmission of the effects of the surface conduction band of the metal by the weak exchange forces between the paramagnetic species was suggested. Experiments on a model system by means of Laser Time Resolved ESE spectroscopy were performed. A photo-induced transient electron was produced from Rb ions in nonpolar medium. The spin relaxation of the transient electron was studied vs temperature and alkali metal concentration. The transient electron is initially spin-polarized, and the degree of spin-polarization is affected by the wavelength. Such an effect would be expected from a long-lived geminate radical-pair mechanism (Rb/sup +/ + e/sup -/) causing the spin-polarization, with the initial separation distance affected by the energy of the photon absorbed. An extended Hueckel MO analysis of our previous O/sub 2//sup -//Ti-vycor study was completed; a new and improved photo-CREMSEE apparatus was completed and utilized; motional dynamics of NO/sub 2/ on zeolites was analyzed; and work was conducted on a photo-induced source of H atoms for Laser Time Resolved ESE experiments were carried out.},
doi = {10.2172/6721428},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Sep 01 00:00:00 EDT 1982},
month = {Wed Sep 01 00:00:00 EDT 1982}
}
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Technical Report:
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DOI:  10.2172/6721428
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  • A combined uhv-ESR and CREMSEE study was completed on the adsorption of the stable paramagnetic nitroxide DTBN (ditertiary butyl nitroxide) on clean and air oxidized Cu and Ag surfaces. On the oxidized surface good temperature-dependent ESR signals are obtained from chemisorbed DTBN corresponding to about 20 to 30% of a monolayer coverage. On the other hand, no ESR signals are observed from adsorbed DTBN above -100/sup 0/C, although large reductions in CREMSEE threshold power as a function of surface dosage with DTBN clearly demonstrate that the DTBN is chemisorbing. When CREMSEE is operated at a microwave power level just belowmore » threshold for self-sustaining signals, it may be used as a sensitive electron-amplifier and detector of photo-electrons. This photo-CREMSEE method was used to measure the work function of air-oxidized Ag on the interior surface of an uhv-microwave cavity, yielding 4.06 +- 0.05 eV or several tenths of an eV below that for a clean Ag surface. ESR experiments were performed to study the motional dynamics of NO/sub 2/ on a variety of zeolite surfaces and crushed vycor. The motional dynamics of NO/sub 2/ on NaY was similar to those observed on vycor except that the planar-rotation observed at low temperatures (below 77/sup 0/K) is much faster in the case of NO/sub 2//NaY. Successful, time resolved photo-induced electron-spin-echo experiments were performed on a simple model system: Rb/tetrahydrofuran which yields a photo-induced transient solvated electron with a large initial chemically induced spin polarization. Spin relaxation times as short as 50 ns were measured. Both the signal decay time and the phase memory time were measured as a function of temperature and of amount of crown, which complexes the alkali cation. Both these times are composites of the reaction lifetime of the solvated electron and of ordinary spin relaxation properties.« less

  • This project is directed to developing ESR for the study of surface adsorption and catalysis on clean well-characterized metallic and oxide surfaces under ultra-high vacuum (UHV) conditions and to compare the results with real surfaces, which are important in actual catalysis. Results with ESR-UHV system on the NO/sub 2//Cu system showed that only in the presence of H/sub 2/O can the surface process(es) proceed that lead to stable Cu/sup 2 +/ complexes which are observable by ESR. By monitoring the CREMSEE (cyclotron-resonance from microwave-induced secondary electron emissions) threshold, definite evidence for the oxidation of the Cu surface by the cleanmore » NO/sub 2/ has been obtained. In real systems, as opposed to the clean surfaces prepared under UHV conditions, small amounts of H/sub 2/O are usually present. The UHV studies permitted a demonstration of the reactivity requirements. Detailed studies of the oxidation process of NO/sub 2/ on Cu have been performed. A newer ESR-UHV system with better expected performance has been built. An extensive study of motional dynamics of free radicals on supported surfaces has been completed. These include the O/sub 2//sup -/ radical absorbed on Ti supported on crushed vycor and NO/sub 2/ adsorbed on crushed vycor. The O/sub 2//sup -/ is found to exhibit planar rotation about an axis perpendicular to the internuclear axis of the O/sub 2//sup -/ and parallel to the normal to the plane. The higher temperature motion may be fit with a small activation energy of 0.5Kcal/mole, while the lower temperature motion shows anomalies. The NO/sub 2/ exhibits planar-rotation at the lower temperature, but above 77/sup 0/K it becomes more nearly isotropic presumably due to translational diffusion. Preliminary experiments imply that addition of acetylene increases the rate of rotational motion of the O/sub 2//sup -/. Preliminary studies leading to time-resolved UHV-ESR experiments of photo-induced catalysis have been performed with the new excimer laser.« less

  • In this first year since March, we have completed construction of the ESR-uhv system, and we have undertaken preliminary experiments with it, that have demonstrated the feasibility of the technique. With our newer ESR-uhv cavity we have achieved a sensitivity of 10/sup 12/ spins per Gauss linewidth or close to theoretical. We have also developed in-situ techniques for ESR studies of surface adsorption and reactions of gases on (1) metals and oxides supported on crushed Vycor (ca. 1 torr); (2) metal wire and turnings and their oxides (ca. 1 torr); (3) metal powders (ca 10/sup -7/ to 10/sup -8/ torr).more » With NO/sub 2//Vycor, temperature-dependent adsorbed NO/sub 2/ spectra were observed which are amenable to a slow-tumbling analysis. A comparison of /sup 63/Cu hyperfine parameters from NO/sub 2/ on Cu/Vycor vs pure Cu has lead to the tentative conclusion that on the pure metal adjacent sites become paramagnetic, but the Cu/sup 2 +/ is forming on surfaces that are microscopically crystalline, so that adjacent Cu/sup 2 +/ ions have identical orientations of their g-tensor, although the macroscopic sample is polycrystalline. Related studies on the uhv-ESR cavity have been performed on the NO/sub 2/-Cu/Vycor system. An intense cyclotron resonance signal is observed in our uhv-ESR cavity at pressures less than 10/sup -2/ torr. It has a g-factor of 2.000 and is attributed to microwave stimulation of exo-electron emissions. It may also be suppressed by the use of reduced microwave power.We have completed preliminary studies of oxygen ion radical (O/sub 2//sup -/ and O/sup -/) formation on partially reduced Ti on Vycor support. The process is greatly assisted by uv irradiation. Different types of O/sub 2//sup -/ and O/sup -/ ESR signals are obtained and are correlated with differences in preparation and previous work with other supports.« less

  • This project was directed to developing ESR for the study of surface adsorption and catalysis on clean and well-characterized metallic and oxide surfaces under uhv conditions to compare the results of such studies with ''real'' surfaces, which are important in actual catalysis. Time resolution of the order of nanoseconds was to be introduced in the study of catalytic processes by means of Laser Time Resolved Electron-Spin Echo (ESE) Spectroscopy. We conducted an extensive study of the new phenomenon of surface-suppressed ESR (SSESR); in which stable paramagnetic radicals adsorbed on clean Ag or Cu surfaces have their ESR signal suppressed untilmore » several layers of molecules effectively insulate subsequent layers from the metal surface. This study employed a combination of uhv-ESR, CREMSEE, and thermal desorption techniques. A tentative model based upon transmission of the effects of the surface conduction band of the metal by the weak exchange forces between the paramagnetic species was suggested. In other work: an extended Hueckel MO analysis of our previous O/sub 2//Ti-vycor study was completed; a new and improved photo-CREMSEE apparatus was completed and utilized; analysis of our motional dynamics study of NO/sub 2/ on zeolites was carried out; and preliminary efforts directed to developing a photo-induced source of H atoms for Laser Time Resolved ESE experiments were carried out. 54 refs., 30 figs., 6 tabs.« less

  • ; ; ; ...
    During this period, the developed Supercritical-Fluid Chromatography (SFC) instrumentation was applied to the analysis of a vacuum still bottoms. Small diameter (50 to 100 ..mu..m i.d.) capillary columns were used to resolve mixture components, and scanning fluorescence detection provided selective detection and identification of many separated components. This report describes the initial attempts to combine the developments in column technology and SFC instrumentation which have been made during the past 18 months to the analysis of an actual high-molecular-weight coal-derived product. In addition, an evaluation of the stability of stationary phases resulting from crosslinking procedures has been made. The commonlymore » used peroxides for free-radical generation also promote oxidation of the tolyl crosslinking groups which ultimately lead to decomposition of the phase and limited resistance to the solvating power of the supercritical fluids used in chromatography. Azo-compounds have been found to be effective in crosslinking, and do not promote oxidation. 23 figures, 2 tables.« less