Preoxidation

Zirconium is a highly active metal which, like aluminum, seems quite passive because of its stable, cohesive, protective oxide film which is always present in air or water. Massive zirconium does not bum in air, but oxidizes rapidly above 600°C in air. Clean zirconium plate ignites spontaneously in oxygen of ca 2 MPa (300 psi) the autoignition pressure drops as the metal thickness decreases. Zirconium powder ignites quite easily. Powder (<44 fim or—325 mesh) prepared in an inert atmosphere by the hydride—dehydride process ignites spontaneously upon contact with air unless its surface has been conditioned, ie, preoxidized by slow addition of air to the inert atmosphere. Heated zirconium is readily oxidized by carbon dioxide, sulfur dioxide, or water vapor. 

Fig. 4. EPR redox titration of ZJ. vulgaris Fepr protein at pH 7.5 of S = J components with dithionite and ferricyanide in the presence of mediators, [from (ZZ)]. ( , ) The Fepr protein-fingerprint signal (the 3+ state) monitored at g = 1.825 (O, ) signal with aU < 2 (the 5+ state) monitored atg = 1.898 ( , ) Titration in two directions starting from the isolated protein, which corresponds approximately to the top of the bell-shaped curve. ( , O) A titration starting from the fully preoxidized state. EPR conditions microwave frequency, 9.33 GHz microwave power, 13 mW modulation amplitude, 0.63 mT temperature, 15 K.

The second is a neat idea coming from Johnson Ma tthey. They invented the so-called continuously regenerating trap (CRT) consisting of a monolithic preoxidizer and a particulate trap, see Figure 9.3 [24]. The first monolith (containing Pt) oxidizes hydrocarbons and CO to CO2 and NO into NO2, which is very reactive... 

To make the seal one of the tubes is clamped vertically and the preoxidized disc is placed centrally upon the flare of this tube. Working round the tube with a hand torch the glass is softened and the disc fused into place. With the seal still hot, the tube is removed from the clamp and, rotating by hand in a bench flame, the second glass flare... 

Such a possibility has been recognized by early workers,9 but in spite of this intriguing possibility, only recently has such a metal surface been created. Chiral kink sites were created on Ag single crystal surfaces to produce the enantiomeric surfaces Ag(643)s and Ag(643)R however, no differences between (R)- and (S)-2-butanol were observed for either the temperature-programmed desorption from the clean surfaces or the dehydrogenation (to 2-butanone) from preoxidized surfaces.10 Unfortunately, Ag exhibits few catalytic properties, so only a limited array of test reactions is available to probe enantioselectivity over this metal. It would be good if this technique were applied to a more catalytically active metal such as Pt. 

A question which has occupied many catalytic scientists is whether the active site in methanol synthesis consists exclusively of reduced copper atoms or contains copper ions [57,58]. The results of Szanyi and Goodman suggest that ions may be involved, as the preoxidized surface is more active than the initially reduced one. However, the activity of these single crystal surfaces expressed in turn over frequencies (i.e. the activity per Cu atom at the surface) is a few orders of magnitude lower than those of the commercial Cu/ZnO/ALO catalyst, indicating that support-induced effects play a role. Stabilization of ionic copper sites is a likely possibility. Returning to Auger spectroscopy, Fig. 3.26 illustrates how many surface scientists use the technique in a qualitative way to monitor the surface composition. 

Unsteady-state oxidation experiments were carried out by employing the step change in CO concentration over the preoxidized catalyst [62], Figure 7.14 represents the CO and C02 responses after a step change from He to 1 vol% CO/He over the fully oxidized Cu0 j Ce0 902 > nanostructured catalyst. At low temperatures, CO breakthrough is delayed for a few seconds as can be seen from Figure 7.14a. At a temperature of 250°C, however, 20 sec is needed for the first traces of CO exit... 

Perhaps the most important application of redox chemicals in the modern laboratory is in oxidation or reduction reactions that are required as part of a preparation scheme. Such preoxidation or prereduction is also frequently required for certain instrumental procedures for which a specific oxidation state is essential in order to measure whatever property is measured by the instrument. An example in this textbook can be found in Experiment 19 (the hydroxylamine hydrochloride keeps the iron in the +2 state). Also in wastewater treatment plants, it is important to measure dissolved oxygen (DO). In this procedure, Mn(OH)2 reacts with the oxygen in basic solution to form Mn(OH)3. When acidified and in the presence of KI, iodine is liberated and titrated. This method is called the Winkler method. 

There are several examples of one-pot reactions with bifunctional catalysts. Thus, using a bifunctional Ru/HY catalyst, water solutions of corn starch (25 wt.%) have been hydrolyzed on acidic sites of the Y-type zeolite, and glucose formed transiently was hydrogenated on ruthenium to a mixture of sorbitol (96%), mannitol (1%), and xylitol (2%) [68]. Similarly a one-pot process for the hydrolysis and hydrogenation of inulin to sorbitol and mannitol has been achieved with Ru/C catalysts where the carbon support was preoxidized to generate acidic sites [69]. Ribeiro and Schuchardt [70] have succeeded in converting fructose into furan-2,5-dicarboxylic acid with 99% selectivity at 72% conversion in a one-pot reaction... 

Preoxidation treatment of the metal surfaces of the TRBPs and the GPCR reactors is strongly recommended to significantly enhance the high-temperature corrosion resistance. 

An application of an electrochemical quartz crystal microbalance (EQCM) in the study of the A11/HCIO4 system shows that even at a potential about 0.5 V more negative than the onset of AuO formation (the so-called preoxide region), the resonant frequency of the Au-covered quartz crystal decreases as that of the surface mass increases. A comparison of a voltammogram with the potential dependence of the micro-balance frequency for an Au electrode is shown in Figs. 6a and 6b. 

This increase of the mass was ascribed earlier to the adsorption of perchlorate ions, °° a conclusion that found no confirmation in work published later. It turns out that other weakly adsorbing anions of different masses (NOJ, CFySOf) give the same values of frequency decrease as was observed for C104. Ultimately, the increase of the electrode mass in the preoxide region was explained in terms of the three-dimensional hydration of AuOH, which is present in small amounts at the gold surface. The mass increase was consistent with the surface hydration for a cluster of about 32 water molecules per one AuOH site. ... 

The TPRS spectrum produced following adsorption of EtOD to near saturation at 180 K on a preoxidized Ag(l 10) showed two distinct temperature regimes for product formation, as shown in Fig. 26 139). Upon adsorption D2 was displaced from the surface. At 200-230 K evolution of... 

A single N(ls) peak at 402 eV is observed on preoxidized nickel [with clean nickel at 80 K the N(ls) peak is at 399.5 eVj. The intensity of the 402-eV peak is both temperature and pressure dependent, increasing in intensity with decreasing temperature and increasing pressure (46). 

Fig. 11. Comparison of N(ls) spectra observed after NO adsorption 1, clean nickel exposed to NO at 290 K 2, nickel preoxidized at 290 K, NO adsorbed at 80 K 3, nickel with chemisorbed oxygen layer present (formed at 80 K), NO also adsorbed at 80 K 4, clean nickel exposed to NO at 80 K.

Figure 22. Maximum power density for a Cu-Ce02-YSZ anode SOFC as a function of the conversion of fuel entering the anode compartment. The data for preoxidation of 77-butane by ceria ( ) and 1 wt % Pd-ceria (a) are shown. The maximum power densities obtained when /7-butane was diluted with He to a concentration equivalent to that obtained by total oxidation are shown by (O). (Reprinted with permission from ref 177. Copyright 2003 The Electrochemical Society, Inc.)...

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