Conversions mechanisms

Generally, the most important reaction is that of tantalum with oxygen, since it tends to form oxides when heated in air. Reaction starts above 300°C and becomes rapid above 600°C . The scale is not adherent, and if the oxidised material is heated above 1000°C oxygen will diffuse into the bulk of the material and embrittle it. At 1200°C catastrophic oxidation attack takes place at a rate of about 150 mm/h Oxygen is not driven off by heating alone, but in vacuum above 2300°C it is removed as a suboxide. The first step of the conversion mechanism of tantalum into oxide was shown to occur by the nucleation and growth of small plates along the 100) planes of the BCC metaP. ... 

A possible layered precursor to the layered nanoproduct conversion mechanism is thus proposed. The silver clusters formed at the initial heating stage by the partial decomposition of AgSR serve as nuclei at further reaction stages, and their distribution naturally inherits the layered pattern of the precursor. The following growth is mainly controlled by the atom concentration and atom diffusion path, which are both constrained by the crystal structure of the precursor [9]. 

McMurray, P. H. and J. C. Wilson, Growth Laws for the Formation of Secondary Ambient Aerosols Implications for Chemical Conversion Mechanisms, J. Geophy. Res.. 88(C9) 5101-5108 (1983). 

Application of the energy gap law to the energy conversion mechanism in Scheme 1 leads to a notable conclusion with regard to the efficiency for the appearance of separated redox products following electron transfer quenching. From the scheme, the separation efficiency, sep> is given by eq. 18. Diffusion apart of the... 

In acetonitrile solution this complex displays a one-electron oxidation (E° = +0.99 V vs. SCE), that is chemically reversible on the cyclic voltammogram time scale. However, exhaustive oxidation produces the Mn3IUMnIV derivative [Mn403(02CMe)4(dbm)3], which, as previously mentioned, possesses a cuboidal geometry. The conversion mechanism is... 

As a typical example, we consider the behaviour of the chloro-hydride complex [RuHC1(PP3)], which displays a rather simple conversion mechanism. In fact, Figure 16a shows that it undergoes an irreversible two-electron oxidation that generates, on the reverse scan, a voltam-metric profile identical to that observed for the chloro monocation [RuC1(PP3)] +, Figure 16b. 

In contrast to a conventional p-n-junction-type solar cell, the mechanism of the DSSC does not involve a charge-recombination process between electrons and holes because electrons are injected from the dye photosensitizers into the semiconductor, and holes are not formed in the valence band of the semiconductor. In addition, electron transport takes place in the Ti02 film, which is separated from the photon absorption sites (i.e., the photosensitizers) thus, effective charge separation is expected. This photon-to-current conversion mechanism of the DSSC is similar to that for photosynthesis in nature, where chlorophyll functions as the photosensitizer and electron transport occurs in the membrane. 

Hydrodemetallation reactions require the diffusion of multiringed aromatic molecules into the pore structure of the catalyst prior to initiation of the sequential conversion mechanism. The observed diffusion rate may be influenced by adsorption interactions with the surface and a contribution from surface diffusion. Experiments with nickel and vanadyl porphyrins at typical hydroprocessing conditions have shown that the reaction rates are independent of particle diameter only for catalysts on the order of 100 /im and smaller (R < 50/im). Thus the kinetic-controlled regime, that is, where the diffusion rate DeU/R2 is larger than the intrinsic reaction rate k, is limited to small particles. This necessitates an understanding of the molecular diffusion process in porous material to interpret the diffusion-disguised kinetics observed with full-size (i -in.) commercial catalysts. 

For pure a-chromia, a-Cr203, TN = 308 K. At this temperature and up to about 350 K the conversion mechanism is less than 1% dissociative. But this is true only if the pretreatment consists of flowing hydrogen at... 

By using amorphous-BN it is possible to reduce the pressure to 7.0 Gpa and the temperature to 1070 K [153]. Two types of conversion mechanisms have been described ... 

Conversions of only surface compounds are considered. It is suggested that the gas-phase composition remains unchanged. If, in this case, the mechanism is linear with respect to intermediates, the conversion mechanism for these intermediates is monomolecular. 

Let us remember once again that all these limitations refer to the constants of the real mechanism also involving gas-phase substances. For the conversion mechanisms for intermediates (under the assumption of constant concentrations for gas-phase reactants), the conditions (75) cannot be used directly. Thus mechanism (b) (an irreversible linear cycle) for intermediates is possible. The simplest example is the irreversible catalytic isomerization... 

Limitations on the conversion mechanism for intermediates can result from the analysis of the mechanism involving the participation of gases. Thus for the four-step mechanism of CO oxidation on Pt the third and the fourth steps must be simultaneously either reversible or irreversible. 

As an example, let us consider the above fragment of the conversion mechanism for n-hexane [its graph is given in Fig. 3(f)]. The weights of some arcs are equal to the sums of those of individual reactions. For example, the weight of the arc from HK to K amounts to bHK K = b1 + b5 + b6. Let us write down the rate for step (3). It enters into four cycles (see Fig. 4). Cycle I (HK MCK IK HK) has the cyclic characteristics... 

Friction and the changing mechanical advantage of these motion conversion mechanisms mean the available torque may vary greatly with travel. One notable exception is vane-style rotary actuators whose offset piston pivots, giving direct rotary output. 

Of significance in the study of the conversion mechanism are the following additional points. 2,3,4,6-Tetra-O-acetyl-o-galactosone hydrate, like the D-glucose derivative, is converted in acetic anhydride-pyridine to... 

Additional tests with ammonium compounds were performed to address the effect of ammonium ion (see Fig. 8). It is clear that the catalyst inhibition was not based only on the presence of ammonium ion. Ammonium carbonate showed the largest inhibition of the glucose hydrogenation reaction, while chloride and hydroxide had lesser effects. Ammonium nitrate caused no apparent inhibition on glucose conversion. A similar lack of effect was shown with potassium nitrate. In the case of ammonium nitrate, the glucose conversion mechanism was affected, so that the sorbitol yield was reduced by about 20%, but numerous byproducts and overreaction products (lower molecular weight polyols) were evident. 

Aromatic polycarboxylates easily form 2D or 3D networks, for instance [Nd2(122)3(dmf)4]-H2O which present a 2D structure in which the 1,4-naphthalenedicarboxylate anions link Ndm ions of two adjacent double chains keeping them at a short distance of about 4.1 A (J. Yang et al., 2006). This allows up-conversion to take place, albeit with very low efficiency a blue emission is seen at 449.5 nm upon excitation at 580 nm (corresponding to the 4Gs/2 magnetic properties an energy-transfer up-conversion mechanism involving no excited state absorption is more likely. 

Up-conversion relies on sequential absorption and luminescence with intermediate steps to generate shorter wavelengths. Hence, the presence of more than one metastable excited state is required the intermediate metastable states act as excitation reservoirs. One typical example is ground-state absorption followed by inter-mediate-state excitation, excited-state absorption, and final-state excitation to give the up-conversion (the intermediate states and final states are real states) [1, 35], There are many types of up-conversion mechanisms such as excited-state absorption, energy transfer up-conversion and cooperative up-conversion. All these up-conversion processes can be differentiated by studying the energy dependence, lifetime decay curve, power dependence, and concentration dependence by experimental measurements [36-39]. 

Xie, T.Y, Hamielec, A.E., Wood P.E., Woods, D.R., Experimental investigation of vinyl chloride polymerisation at high conversion mechanism, kinetics and modeling, Polymer, 1991, 32(6), 537-557... 

M. A. Strahemechny and R. J. Hemley, New ortho-para conversion mechanism in dense solid hydrogen. Phys. Rev. Lett., 2000, 85(26), 5595-5598. 

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