Metal surfaces decomposition

Although in the dry state carbon tetrachloride may be stored indefinitely in contact with some metal surfaces, its decomposition upon contact with water or on heating in air makes it desirable, if not always necessary, to add a smaH quantity of stabHizer to the commercial product. A number of compounds have been claimed to be effective stabHizers for carbon tetrachloride, eg, alkyl cyanamides such as diethyl cyanamide (39), 0.34—1% diphenylamine (40), ethyl acetate to protect copper (41), up to 1% ethyl cyanide (42), fatty acid derivatives to protect aluminum (43), hexamethylenetetramine (44), resins and amines (45), thiocarbamide (46), and a ureide, ie, guanidine (47). 

Corrosion products and deposits. All sulfate reducers produce metal sulfides as corrosion products. Sulfide usually lines pits or is entrapped in material just above the pit surface. When freshly corroded surfaces are exposed to hydrochloric acid, the rotten-egg odor of hydrogen sulfide is easily detected. Rapid, spontaneous decomposition of metal sulfides occurs after sample removal, as water vapor in the air adsorbs onto metal surfaces and reacts with the metal sulfide. The metal sulfides are slowly converted to hydrogen sulfide gas, eventually removing all traces of sulfide (Fig. 6.11). Therefore, only freshly corroded surfaces contain appreciable sulfide. More sensitive spot tests using sodium azide are often successful at detecting metal sulfides at very low concentrations on surfaces. 

Water used in any testing procedure should be free of corrodents and of substances whose decomposition products may be corrosive. Metal surfaces should be dried after testing and kept dry prior to service. 

To a stirred solution of 5.70g (21.1 mmol) of 4,4.5,5-tetramethyl-2[(5)-(A)-3-(trimethylsilyloxy)-l-bulenyl]-l, 3,2-dioxaborolane in 130 mL of petroleum ether (bp 40-60 C) are added ca. 20 mg of cobalt(II) nitrate hexahydrale followed immediately by 2.75 g (23.1 mmol) of freshly distilled thionyl chloride. Slow evolution of sulfur dioxide ceases after 4h. The mixture is then filtered and concentrated in vacuo at r.t. to give crude 15, which is taken up in 50 mL of petroleum ether and washed with 30-mL portions of buffer (pH 5) until the pH is constant. 100 mL of brine are added to the organic phase and the pH is adjusted to 7 by addition of sat. aq NaHCO,. The pH should not exceed 7, otherwise decomposition ensues. The phases are separated and the organic phase is dried with MgS04 and concentrated at r.t. to give 15 yield 4.39 g (96%) Contact of 15 with metal surfaces should be avoided. 

At high alkali coverages (near monolayer coverage), when the adsorbed alkali overlayer shows a metal-like character, alkali-methoxy species are formed. As shown by TPD experiments in the system K/Ru(001) these alkali-methoxy species are more stable than the methoxy species on clean Ru(001) and adsorbed methanol on 0.1K/Ru(001). On metal surfaces inactive for methanol decomposition, e.g. Cu(lll), these alkali-methoxy species are formed even at low alkali coverages, due to the weaker interaction of the alkali atoms with the metal surface. The formation of these species stabilizes the methoxy species on the metal surface and enhances the activity of the metal surface for methanol decomposition. 

On the contrary, on oxygen-modified metal surfaces where secondary reactions between the adsorbed oxygen and ethylene decomposition products can easily occur, the effect of oxygen on the adsorptive capacity of the... 

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

The influence of the presence of sulfur adatoms on the adsorption and decomposition of methanol and other alcohols on metal surfaces is in general twofold. It involves reduction of the adsorption rate and the adsorptive capacity of the surface as well as significant modification of the decomposition reaction path. For example, on Ni(100) methanol is adsorbed dissociatively at temperatures as low as -100K and decomposes to CO and hydrogen at temperatures higher than 300 K. As shown in Fig. 2.38 preadsorption of sulfur on Ni(100) inhibits the complete decomposition of adsorbed methanol and favors the production of HCHO in a narrow range of sulfur coverage (between 0.2 and 0.5). 

On photolyzing CoziCOg in the matrix (20), a number of photoproducts could be observed. The results of these experiments are summarized in Scheme 4, which illustrates the various species formed. Of particular interest is the formation of Co2(CO)7 on irradiation of Co2(CO)g in CO (254 nm), as this species had not been characterized in the metal-atom study of Hanlan et al. (129). Passage of Co2(CO)g over an active, cobalt-metal surface before matrix isolation causes complete decomposition. On using a less active catalyst, the IR spectrum of Co(CO)4 could be observed. An absorption due to a second decomposition product, possibly Co2(CO)g, was also noted. 

Although the hazardous properties of di-tert-butyl diazomalo-nate are not known with certainty, it is reasonable to assume that they are similar to those of diazoacetic esters, which are considered to be moderate explosion hazards when heated. Contact with rough or metallic surfaces should be avoided. The submitter has routinely distilled 10-g. quantities of di-ferf-butyl diazomalonate under argon with no sign of decomposition. 

The direction chosen for the equilibrium reaction Is determined by convenience. A scientist interested in producing ammonia from N2 and H2 would use f. On the other hand, someone studying the decomposition of ammonia on a metal surface would use eq,r Either choice works as long as the products of the net reaction appear in the numerator of the equilibrium constant expression and the reactants appear in the denominator. Example applies this reasoning to the iodine-triiodide reaction. 

Molecular Adsorption and Decomposition on Clean and Sulfiir-Mod ed Metal Surfaces... 

Coke formation on these catalysts occurs mainly via methane decomposition. Deactivation as a function of coke content (see Fig. 3 for Pt/ y-AljO,) seems to involve two processes, i e, a slow initial one caused by coke formed from methane on Pt that is non reactive towards CO2 (see Table 3) In parallel, carbon also accumulates on the support and given the ratio between the support surface and metal surface area at a certain level begins to physically block Pt deactivating the catalyst rapidly. The coke deposited on the support very close to the Pt- support interface could be playing an important role in this process. 

Controlled decomposition of organometallics leads to nanocatalysts which exhibit a very clean and truly zerovalent metallic surface (cf. [339]). As indicated in Figure 15 in Section 3.10, the organoaluminium shell of the one-shell Pti3 cluster (size 0.75+0.1 nm) obtained via the decomposition of [(COD)Pt(CH3)2] in the presence of excess trioctylaluminium [352] is transferred to an AI2O3... 

The formation of Grignard reagents takes place at the metal surface. Reaction commences with an electron transfer to the halide and decomposition of the radical ion, followed by rapid combination of the organic group with a magnesium ion.1 It... 

A definite decomposition voltage occurs for the following reason. As soon as there is a potential difference between the electrodes, H+ ions move to the cathode and Cl ions to the anode. The ions are discharged, forming layers of adsorbed gas on the inert metal surfaces. This essentially amounts to having a hydrogen electrode and a chlorine electrode in place of the two platinum electrodes. The outcome is a typical chemical cell ... 

Interest in the adsorption of sulfur-containing molecules at metal surfaces been stimulated by a desire to elucidate the decomposition mechanisms of thiols during the catalytic removal of sulfur from feedstocks and the position of thiols as the favoured head groups for adsorbates used to construct self-assembled monolayers. We shall not survey the extensive self-assembled film literature but restrict our discussion to the simpler thiols. 

Copper metal surface area was determined by nitrous oxide decomposition. A sample of catalyst (0.2 g) was reduced by heating to 563 K under a flow of 10 % H2/N2 (50 cm min"1) at a heating rate of 3 deg.min 1. The catalyst was then held at this temperature for 1 h before the gas flow was switched to helium. After 0.5 h the catalyst was cooled in to 333 K and a flow of 5 %N20/He (50 cm3mirr ) passed over the sample for 0.25 h to surface oxidise the copper. At the end of this period the flow was switched to 10 % H2/N2 (50 entitlin 1) and the sample heated at a heating rate of 3 deg.min"1. The hydrogen up-take was quantified, from this a... 

Ultrasound enhanced the rate of dissolution of zinc metal in alkaline media, 2-3 times in sonicated condition rather than the unsonicated condition, due to the cleaning of metal surface in ultrasonic field. Decomposition of Zn-dithizone complex was also observed in ultrasonic field. In sonicated samples, the peak of... 

Extensive catalyst poisoning results from the deposition of carbon in or on the catalyst surface. In technological parlance, this is usually referred to as the formation of coke. At temperatures in excess of those normally used for catalysis, hydrocarbon decomposition at metal surfaces is known... 

Some examples of carbene dimer formation resulting from diazoalkane decomposition on transition-metal surfaces have been reported. Diazomethane is decomposed to give ethylene and N2 upon passage over a C0O/M0O3 catalyst as well as on Ni, Pd, Fe, Co, Ru and Cu surfaces 367). Similarly, 2-diazopropane is readily decomposed on Raney nickel 368). At room temperature, propene and N2 were the only detectable products, but above 50 °C, the carbene dimer 2,3-dimethyl-2-butene started to appear which reached its maximum yield at 100 °C, where approximately 40 % of the carbene fragments dimerized. It is assumed 367,368), that surface carbenes are formed as intermediates from both diazomethane and 2-diazopropane which either dimerize or desorb by migration of a P-hydrogren, if available (Scheme 40). 

In the particular case of InP, protection of the surface is necessary because direct exposure of an InP sample to a rf hydrogen plasma causes a strong surface decomposition with the formation of phosphine and metallic indium islands (Thomas et al., 1988). Hydrogen diffusion into InP Zn can be successfully performed through an undoped correctly lattice matched... 

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