Field emission, initiation

In this chapter, we have chosen from the scientific literature accounts of symposia published at intervals during the period 1920 1990. They are personal choices illustrating what we believe reflect significant developments in experimental techniques and concepts during this time. Initially there was a dependence on gas-phase pressure measurements and the construction of adsorption isotherms, followed by the development of mass spectrometry for gas analysis, surface spectroscopies with infrared spectroscopy dominant, but soon to be followed by Auger and photoelectron spectroscopy, field emission, field ionisation and diffraction methods. 

The heat of chemisorption is, of course, the energy difference between the chemical bonds formed and those broken. One of the strongest bonds to be broken in dissociative chemisorption on metals is the N-N bond of N2. This chemisorption is known to be rate limiting in ammonia synthesis. Brill et al. reported in 1967 field emission results indicating that N2 adsorption on Fe is strongest on the (111) face." Then-suspicion that this might be the initial step in ammonia synthesis over Fe catalysts... 

Most of the subnanometer-resolution studies used field emission gun cryomicroscopes because of the advantages mentioned above (Table I). When the LaBg gun is used, the spatial coherence of the instrument is relatively poor. Therefore, a high-resolution image must be recorded with a small defocus value to minimize the dampening due to the partial coherence envelope function (Zhou and Chiu, 1993). Consequently, these images have low contrasts for their low-resolution features, making it difficult to locate the particles and to determine their initial orientation... 

The NMR spectra were recorded on Bruker AC200 (200 MHz) and AC300 (300 MHz) spectrometers at ambient temperature in NMR solvents obtained from ISOTEC Inc.. G.C. analysis were performed on Unicam PU4600 and PU610 apparatus with 30 m J W Scientific DB-1, DB-17 and AT-SILAR capillary columns and flame ionization detectors. Product yields were determined by peak area analysis response factors for selected substrates and products were foimd to be virtually identical. Internal standards were used in the initial stage of this study, but were found to influence the catalyst characteristics. G.C.M.S. was performed on a Unicam Automass apparatus combined with 610 series G.C. apparatus equipped with 30 m J W Scientific DB-1 and DB-17 columns. TEM-EDAX was performed on a Phillips CM 200 microscope equipped with a field emission gun. TEM-EDAX samples were prepared by application of a few droplets of a suspension of the catalyst in ethanol onto a holey carbon film which was supported by a nickel grid after which the ethanol was allowed to evaporate. 

Armstrong 78) studied, in the same system, the work function changes caused by CO adsorption on two tungsten single crystals, one exposing the W(IOO) surface, the other the W(112) surface. These experiments confirmed the results obtained by field emission 44) that the W(112) plane is more active in CO adsorption than the W(IOO) plane. Armstrong measured an initial sticking probability for CO at 300°K of about... 

The FN plot therefore yields the f power of the intensity average (45, 60) of (jS< 3 2). When adsorption occurs, each individual adatom acts just like an area of different emitting properties. If emission is lowered by adsorption, those spots on which adsorption occurs will not contribute to the work function, which may therefore remain initially unaffected. The only change noticeable may be in the total emitting area. Conversely, if adsorption raises emission, then the areas on which adsorption occurs may dominate, even on a single, well-defined plane. In field emission the work function therefore need not be linearly related to surface concentration. 

P. D. Cobden, V. V. Gorodetskii, B. E. Nieuwenhuys, Field emission microscope study of the initial behaviour of the palladium-hydrogen system at low temperatures, Sutf. Sci. 1999, 432, OIOS. 

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