Water desorption, decomposition surfaces

The decomposition behavior of formic acid on the close-packed Ru(lOTO) surface parallels the reaction on nickel, except that the autocatalytic process was not observed (lOJ). Water was desorbed at 183 K by apparent second-order kinetics following adsorption of HCOOH at 100 K. Subsequent desorption of Hj, COj, and CO suggested the formation of the surface anhydride. The rate constant for decomposition was 2.6 x 10 sec exp —26.9 kcal/mol// r. ... 

Recently, interesting effects of NO and water on the formation and properties of a-oxygen were discovered. Even small amounts of NO facilitated desorption of Oa from the surface, strongly increasing the rate of catalytic decomposition of N20 to the... 

The survey of the investigations and results covers the release of water from salts and hydroxides, the calcination of carbonates and oxalates, the reactions of metallic oxides and carbonates with SO2, and reactions on the surface of carbon. The application of the non-isothermal method to the thermal decomposition of carboxylic acids and polymeric plastics as well as to the pyrolyses of natural substances, in particular bituminous coal, is explained. Finally, chemical reactions in a liquid phase, the desorption of gases from solids, annihilation processes in disturbed crystal lattices and the emission of exo-electrons from metallic surfaces are discussed. 

Decomposition reactions of larger aliphatic alcohols have been examined in detail on the (Oil [-faceted TiO2(001) surface [80]. Ethanol adsorbed at 300 K exhibited a low temperature desorption peaks for ethanol and water at 365 K and a high temperature desorption state for decomposition products at 588 - 595 K. Half of the ethanol adsorbed on the surface desorbed as ethanol at 365 K. Half of the remaining surface ethoxide groups desorbed as ethanol at 588 K. The... 

This coadsorption has been studied for Rh loaded onto Ce02(i 11) films. As with the systems mentioned above, the interaction between water and CO depends strongly upon the oxidation state of the ceria. The origin of most of the oxidation effects are the result of the interaction of the individual components, e.g. hydroxyls form from H O only on a reduced ceria surface. Rh catalyses the decomposition of hydroxyls, leading to a decrease in the resulting recombinative H2 peak desorption temperature from 580 K in the Rh-free surface to about 500 K when Rh is present on the surface. This effect is presumably due to H spillover onto the Rh, facilitating recombination and desorption. There are subtle interactions between the coadsorbates also. The coadsorption of CO somewhat inhibits this process of recombinative H2 desorption, but the presence of the hydroxyls slightly lowers the temperature of recombinative CO desorption. 

Interactions between water vapor and amorphous pharmaceutical solids were evaluated using isothermal microcalorimetry. " The desorption of water from theophylline monohydrate has been investigated using microcalorimetric approaches.The properties of surfactants and surface-active drugs in solution were studied by Attwood et al. " using calorimetry, while titration microcalorimetry has been utilized to elucidate the nature of specific interactions in several pharmaceutical polymer-surfactants systems. " Drug decomposition was evaluated as a function of different... 

Ammonia adsorption on Lewis sites is stronger than that on Bronsted sites [97]. In situ infrared spectroscopy has been used to monitor surface coverages by various species under reaction conditions. Temperature programmed desorption shows that no NO decomposition occurs in the temperature range 100-600 K. By means of in situ FTIR spectroscopy it was observed that the fractional surface coverages by ammonia on the Bronsted and Lewis acid sites were 0.26 and 0.39, respectively, at 573 K. No adsorption of NO was found. Moreover, it was stated that water does not block the sites for ammonia adsorption. 

The model is formulated on the premise that the decomposing hydrate particle is surrounded by a cloud of the product gas hence the driving force for the decomposition process is expressed in terms of the fugacity difference given in Eq. (1). The process of decomposition possibly involves (1) destruction of the clathrate host (water) lattice at the surface of the particle, and (2) and desorption of the guest (hydrate former) molecules from the surface. The particle size distribution was incorporated in the calculations for the determination of the intrinsic rate constants.The following Arrhenius type equation is used to represent the effect of temperature on the intrinsic rate constant ... 

Two examples in which adsorptive or catalytic properties have been affected by radiation-induced desorption or decomposition are the destruction of catalytic centers on copper and nickel (55, 60) already described (Section III,A,3,c) and the effect of radiolysis of surface water on adsorption of carbon monoxide by zinc oxide (153). 

Madey and Netzer have examined the adsorption of HjO on Ni(lll) at 80 K, as well as the effect of preadsorbed oxygen, by ESDIAD (electron stimulated desorption ion angular distribution), TPD and LEED. At room temperature, water does not adsorb on Ni(lll). Water adsorbed on Ni(lll) at 80 K desorbs at 170 K (from the first monolayer) and 150 K (from an ice multilayer) without evidence for any decomposition products. For oxygen predosed Ni(lll) surfaces there is an extra peak at 275-300K in HjO TPD spectra, which is due to the interaction between water and adsorbed oxygen. The authors suggested the formation of OH above 120 K which recombined above 200 K to desorb as water and oxygen. Similar results have also been reported on Ni(110)5, Ni(100)5 and stepped Nidll). ... 

The dehydrogenation reaction produces acetone and hydrogen, and is dominant over basic oxides ( ) The dehydration reaction produces propene and water, and is dominant over acidic oxides. It would be interesting to see if the competition between these two pathways depend on the exposed crystal planes of ZnO. We report here the results of such an investigation. 2-Propanol was decomposed on ZnO single crystal surfaces by the temperature programmed decomposition technique. To assist the interpretation of data, the temperature programmed desorption of propene and acetone were also studied. 

These, together with the observation that acetone and propene were always evolved at the same temperature suggest that acetone and propene are formed from a common intermediate on the different surfaces, the formation or decomposition of which is the rate limiting step. The evolution of water, however, was at the same temperature as the desorption of adsorbed water. Thus the process is desorption limited. 

From the temperature at which the decomposition products evolved, it would seem that the 0-polar surface should be the most active in 2-propanol decomposition. However, a close examination of the temperatures in Table I shows that on the 0-polar surface, the desorption temperature of the minor product water was actually rather high - higher than any of the products from the nonpolar surface. Thus in a steady state reaction at temperatures below about 100 C, the 0-polar surface could be easily poisoned by adsorbed water, leaving only the nonpolar surface active. 

In conclusion, the chemical properties of ZnO depend on the particular surface plane that is exposed. This surface specificity has now been demonstrated for the decomposition of 2-propanol, methanol, formaldehyde and formic acid, and adsorption and desorption of acetone, propene, water, CO, and CO2. These data have made possible better understanding of the results using ZnO powder. It will be intersting to se<5 how different are the catalytic properties of these surfaces. 

The literature pertaining to the catalytic properties of magnetite focuses primarily on the water-gas shift reaction. A number of reaction kinetics studies have been reported in which WGS reaction pathways have been proposed (1,2,7-18), In short, two types of mechanisms have been put forward, these being the adsorptive and regenerative mechanisms. In the adsorptive pathway, reactants adsorb on the surface where they react to form surface intermediates, followed by decomposition to products and desorption from the surface (12-18), Support for this adsorptive mechanism has been provided by tracer studies and apparent stoichiometric number analyses. Two such adsorptive mechanisms consistent with experimental observations are shown below. 

Applications of FAB have been succesfully performed in the characterization of a wide range of compounds (dyes, surfactants, polymers...) but little attention has been devoted to the capabilities of this technique to solve environmental concerns, such as organic pollutants identification in water. The widespread use of surfactants in the environment has required the emplo yment of both sensitive and specific methods for their determination at trace levels. GC/MS and HPLC procedures has been used for the determination of anionic (LAB s) and non ionic surfactants (NPEO) in water (1-4). Levsen et al (5) identified cationic and anionic sirrfactants in surface water by combined field desorption/ collisionally activated decomposition mass spectrometry (FD/CAD), whereas FAB mass spectrometry has been used for the characterization of pine industrial surfactants (6-8). 

Steps 2C and 3C show the reaction of adsorbed alcohol with pre-dosed oxygen D280 is the only water product evolved and is desorbed at low temperatures leaving two methoxy species for every pre-dosed oxygen atom. The products evolved at 350 K are then desorbed in decomposition-limited peaks from the break-up of the methoxy. The work of Wachs and Madix [331] showed further reaction to produce C02 desorption from the surface at 480 K, but such a strongly bound species could not be observed by Bowker and Madix [331] or by Sexton [332] and so some impurity adsorption must be inferred in the earlier work. 

Recalling that the reduction of the sample is carried out in deuterium we would expect little H2 to be evolved, but the majority of hydrogen evolved from the surface is H2 (Figure 3). This result suggests that the higher temperature H2 desorption peaks are due to the decomposition of water liberated from the support, as this is the only possible source of the... 

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