Carbon cleaning surface

The mechanism Implied by these expressions Is that of a submonolayer of surface carbon which blocks n sites for NH adsorption. This suppresses NH decomposition which would occur on the clean surface (Figure 3(a)) and allows NH fragments to react with carbon to form HCN. Selectlvltles of HCN production exceed 90% at 1 Torr, and reaction probabilities (fraction of CH flux yielding HCN) approach 0.01. 

The EDX spectrum (Fig. 11.8) shows the main surface scale impurity peaks of silica, aluminium, sodium, chloride and iron. If this EDX is compared to that of a new, clean membrane surface (Fig. 11.9), the clean surface shows sulphur, carbon and oxygen, which is typical of a porous polysulphone support. It was concluded that the scale is amorphous, composed of aluminosilicate and silicate. These compounds are normally found in trace amounts in brine solutions. Analysis showed that the surface could be cleaned with hydrochloric acid and analysis of the dissolved scale was similar to the EDX spectrum analysis. Review of the plant operation determined that the precipitation was the result of high pH in combination with high silica concentrations in the brine. 

In most applications, the electrochemical compounds are usually oxidized, yielding one or more electrons per molecule reacted. The oxidized form is usually unstable and reacts further to form a stable compound that flows past the carbon electrode surface. Unfortunately, this is not always the case, with the stable oxidized form occasionally building up at the surfaces of the carbon electrode. This creates sensitivity problems and decreases the efficiency of the detector. However, the problem is usually overcome by regularly cleaning the carbon electrode surfaces, removing any oxidizable products. Eluents for EC detection must be electrochemically conductive, which is achieved by the addition of inert electrolytes (to maintain a baseline current) such as phosphate or acetate. All solvents and buffers used in preparation of an eluent must be relatively pure and selected so as to not undergo electrochemical changes at the applied electrode potentials. 

Fig. 8. XPS C(ls) core level spectra for CO adsorbed on polycrystalline iron and Fe(lOO). (a) Clean surface (b) saturation CO coverage at 20°C (c) warmed to 100°C. Lines I and II indicate the C(ls) positions for atomic carbon and carbon in molecular CO, respectively (43).

Fig. 18. AES for the HCOOH decomposition on W(IOO) (96). (a) Clean surface (b) clean surface exposed to HCOOH at 300 K and heated to 800 K (c) heated further to 1500 K to desorb carbon and oxygen atoms. The carbon and oxygen peaks in (b) indicate dissociation of the HCOOH.

Adsorption and Reaction of CO and CO2. The clean and oxygen-precovered Au(lll) surface does not chemisorb CO at temperatures down to 100 K. This was established by CO uptake experiments utilizing XPS and TPD. C(ls) and 0(ls) XPS spectra taken after a 10 L CO exposure to Au(lll) with O = 0 0/ 0.5 and 1.0 at 100 K showed no evidence for a new carbon-containing surface species, i.e., no adsorbed CO, CO2, or carbonate species is stable on the Au(lll) surface under these conditions. The area of the 0(ls) peak after CO exposure was slightly smaller for both oxygen precoverages, but this is attributed to background CO oxidation and the formation of CO2 which desorbs from the surface at 100 K (vide infra). 

The clean surface of a Au(lll) crystal at 100 K and a surface covered with o = 0.12, 0.25, 0.75, and 1.0 was dosed with 10 and 50 L CO2. XPS studies of the C(ls) and 0(ls) regions did not reveal any significant new peaks after CO2 exposure. Appreciable CO2 chemisorption does not occur on the clean or oxygen-dosed Au(lll) surface, nor does a stable surface carbonate form under these conditions. 

A further serious factor determining the apparent gap between model studies with well-defined clean surfaces and the conditions encountered with real catalysts is the fact that the latter are never prepared in a way that the degree of surface contamination is well controlled. Elements such as sulfur or carbon might always be present in considerable concentrations on the surface so that the chemical nature of the catalytically active surface could possibly be quite different. 

Similar processes presumably occur if carbon atoms that may also react with oxygen to form CO are present on the surface. These results suggest that under catalytic steady-state conditions in a C0/02 mixture the main impurities may be reacted off to volatile compounds so that the reaction proceeds, in fact, on an essentially clean surface even without the need for an ultrahigh vacuum and stringent cleaning procedures of the surface. 

UPS spectra of clean Ar+ sputtered and in Vacuo carbon-contaminated surfaces are shown in Figure 4. On the clean, sputtered surface a filled state due to Ti3+ lies 0.6 eV below the conduction band.(22) Carbon-induced filled states lie in a broad peak with considerable intensity between the valence band edge and the Ti + peak. Frank et al.lQ) reported evidence that a state lying about 1.2 eV above the valence band mediates electron transfer from Ti02 electrodes. Although these carbon states are as of now poorly defined and have not been directly implicated in any aqueous photochemistry, their nearly ubiquitous presence should be considered in discussions of charge transfer at real oxide surfaces. 

The idea that a metal atom in the zero oxidation state is both a soft acid and soft base can be used to explain surface reactions of metals. Soft bases such as carbon monoxide and olefins are strongly adsorbed on surfaces of the transition metals. Bases containing P, As, Sb, Se, and Te in low oxidation states are strongly adsorbed, blocking the active sites (Pearson, 1966). The clean surfaces are incomplete solids, in that the surface atoms have no nearest neighbors in one of the three-dimensional coordinate system. This means that there are atomic orbitals, both filled and empty, which are not being used to form surface orbitals. 

As a consequence of the identical crystallographic structure and the very similar ionic radii, MgO and NiO or CoO form uniform solid solutions Mg(i X)MxO (0 < x < 1), where M is Ni or Co (25). Real crystals are often covered by strongly adsorbed water and carbon dioxide, and thermal treatments in vacuo are needed to clean the surfaces and create the surface coordinative unsaturation mentioned previously (in particular the fourfold and threefold coordinated sites are cleaned only at very high temperatures). In this connection, it is evident that the surface chemistry of clean surfaces is primarily determined by the presence of these... 

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