Surface reaction with water

As discussed in Chapter 7.B.3, N02 undergoes a surface reaction with water, which is perhaps enhanced at the air-water interface, forming HONO ... 

Activated Carbon With a surface of 300-2000 m g and a pore diameter of 1-5 nm, activated carbon is an outstanding adsorbent for a multitude of substances. It is produced by gentle heating of organic precursors like wood, turf or coconut peel, but also of pit-coal etc. The activation (to obtain the required porous structure) results either from hydrothermal surface reaction with water vapor or from impregnating the starting material with agents that cause oxidation and... 

White Phosphorus Oxidation. Emission of green light from the oxidation of elemental white phosphoms in moist air is one of the oldest recorded examples of chemiluminescence. Although the chemiluminescence is normally observed from sotid phosphoms, the reaction actually occurs primarily just above the surface with gas-phase phosphoms vapor. The reaction mechanism is not known, but careful spectral analyses of the reaction with water and deuterium oxide vapors indicate that the primary emitting species in the visible spectmm are excited states of (PO)2 and HPO or DPO. Ultraviolet emission from excited PO is also detected (196). 

The corrosion behavior of plutonium metal has been summarized (60,61). a-Plutonium oxidizes very slowly in dry air, typically <10 mm/yr. The rate is accelerated by water vapor. Thus, a bright metal surface tarnishes rapidly in normal environments and a powdery surface soon forms. Eventually green PUO2 [12059-95-9] covers the surface. Plutonium is similar to uranium with respect to corrosion characteristics. The stabilization of 5-Pu confers substantial corrosion resistance to Pu in the same way that stabilization of y-U yields a more corrosion-resistant metal. The reaction of Pu metal with Hquid water produces both oxides and oxide-hydrides (62). The reaction with water vapor above 100°C also produces oxides and hydride (63). 

Chemical Reactivity - Reactivity with Water Slow reaction with water, evolving hydrogen chloride (hydrochloric acid) Reactivity with Common Materials Slow evolution of hydrogen chloride from surface moisture reaction can cause slow coaosion Stability During Transport Stable Neutralizing Agents for Acids and Caustics Flush with water, rinse with sodium bicarbonate or lime solution Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Reactions with water are complicated and are affected by the presence of oxygen. With boiling water or steam, oxide is formed on the surface of the metal and H2 is liberated. Since the metals react readily with the latter, hydrides are produced which themselves react rapidly with... 

The finely divided hydride produced by pyrolysis is pyrophoric in air, while synthesis from the elements produces a substantially air-stable product [1]. That prepared by reduction of butylmagnesium bromide with lithium tetrahydroalumi-nate is pyrophoric and reacts violently with water and other protic compounds [2], The hydride produced from magnesium anthracene has a very large specific surface area and is pyrophoric [3], In the context of use of the hydride for energy storage purposes, ignition and combustion behaviour of 100-400 g portions were studied, as well as the reaction with water [4],... 

Five common desiccant materials are used to adsorb water vapor montmorillonite clay ([(Na,Cao.5)o.33(Al,Mg)2Si40io(OH)2 H20], silica gel, molecular sieves (synthetic zeolite), calcium sulfate (CaS04), and calcium oxide (CaO). These desiccants remove water by a variety of physical and chemical methods adsorption, a process whereby a layer or layers of water molecules adhere to the surface of the desiccant capillary condensation, a procedure whereby the small pores of the desiccant become filled with water and chemical action, a procedure whereby the desiccant undergoes a chemical reaction with water. 

Formic acid adsorbed with near unit sticking probability on clean Fe(lOO). The reaction product spectrum for HCOOH on Fe(lOO) is shown in Fig. 17 (95). As with Cu( 110) the reaction proceeded via two steps, one which evolved Hj at 350 K and the other which formed, CO2, and CO at 490 K. A small amount of CO was evolved at 800 K due to the reaction of residual carbon and oxygen atoms on the surface. From these results it was concluded that CO, H2, and CO2 were formed by a common rate-limiting step at 490 K. Since the H2/H2 TPD peak appears at 400 K and below, this step was determined to be a surface reaction. No water was formed. Evidently the reaction proceeded by the pathways... 

SO, (Table 4.22), that photolysis of this compound will become important at higher altitudes, sufficiently so to compete with its reaction with water to form sulfuric acid. Compare the S03 photolysis rate for an overhead sun for an altitude of 0 km to its rate of hydrolysis assuming a collision-controlled value for the effective bimolecular rate constant of 10 111 cm3 molecule-1 s-1 and 50% RH at 298 K at the earth s surface. 

Reactions at the interface. There has been increasing recognition that reactions may also occur at the interface itself. That is, species such as SOz, NH3, and organics do not simply cross the interface by physical transport but rather form unique chemical species at the interface (e.g., Donaldson et al., 1995 Allen et al., 1999 Donaldson, 1999 Donaldson and Anderson, 1999). These unique interface species can then react at the surface without actually being taken up into the bulk of the solution. Although relatively little is currently known at a molecular level about such processes, reactions in this fourth phase may prove to be very important in atmospheric processes, for example in the generation of HONO in the N02 reaction with water at surfaces (see Chapter 7.B.3b). 

HONO also undergoes deposition at surfaces in competition with its formation by the N02 heterogeneous reaction with water. For example, the mass accommodation coefficient for HONO on water has been reported to be in the range of 4 X 10 3 to 0.15 over temperatures from 278 to 297 K (e.g., Kirchner et al., 1990 Bongartz et al., 1994 Mertes and Wahner, 1995). Thus aqueous particles and surfaces having adsorbed water can also act as a sink for gaseous HONO. This is consistent with the observations of Harrison et al. (1996) on the direction of HONO fluxes from the surface at various concentrations of N02 at N02 concentrations below 10 ppb in rural areas, surfaces were observed to be a net sink of HONO (e.g., see Harrison et al., 1996 and Harrison and Peak, 1997). 

Hazardous Reactions with Water-Reactive Materials. Accdg to Refs 1,2,3 5, w reacts with a variety of elements, compds and mixts to generate at RT either an expln or a combustion reaction because of the hydrogen released and the heat of reaction. Table 1 presents an identification of the reactants, and a generalized classification of the results of these reactions into either expln or combustion . From Ref 3, a more quantitative approach yielded data in terms of temp increase and the time required to reach the max temp on a selected group of w-reactants (Table 2). Table 3 displays the results of an attempt to determine if the order of addition (effects of dilution and/or surface... 

Raney-nickel catalysts are barely sensitive to catalyst poisoning (as are Pt-activated cathodes), e.g., by iron deposition, but they deteriorate due to loss of active inner surface because of slow recrystallization—which unavoidably leads to surface losses of 50% and more over a period of 2 years. A further loss mechanism is oxidation of the highly dispersed, reactive Raney nickel by reaction with water (Ni + 2H20 — Ni(OH)2 + 02) under depolarized condition, that is, during off times in contact with the hot electrolyte after complete release of the hydrogen stored in the pores by diffusion of the dissolved gas into the electrolyte. 

Boric Oxide. Boric oxide, B203, formula wt 69.62, is the only commercially important oxide. It is also known as diboron trioxide, bone anhydride, or anhydrous boric acid. B203 is normally encountered in the vitreous state. This colodess, glassy solid has a Mohs hardness of 4 and is usually prepared by dehydration of boric acid at elevated temperatures. It is mildly hygroscopic at room temperature, and the commercially available material contains ca 1 wt % moisture as a surface layer of boric acid. The reaction with water ... 

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