Hypothesis, catalyst performance

This paper reports first results of research and development work to achieve nitrogen oxide reduction under lean diesel exhaust gas conditions. Much attention is paid to the influence of operation conditions on catalyst performance. A major part of the paper deals with the influence of the hydrocarbon component, the hydrocarbon concentration and the HC/NO ratio on the activity of a special developed platinum based catalyst. Other aspects discussed are a spectroscopic characterization and a selectivity study. A hypothesis of a "dual-site" reaction mechanism for NOx-reduction in lean diesel exhaust gas precious metal based catalyst is established. Finally, first promising results on the performance of the catalyst system in a vehicle dynamometer test are given. 

This hypothesis was tested by carrying out a kinetic study of the HKR of epichlorohydrin using Jacobsen Co(III)-salen catalyst with four different counterions, namely, acetate (OAc), tosylate (OTs), chloride (Cl) and iodide (I) (22). Approximately, 0.5 mol% loading of all the catalysts was used to perform the HKR of epichlorohydrin. As shown in Table 43.2, the ran initial rates with Co-OAc and Co-OTs salen catalysts were similar and slightly below those with Co-Cl and Co-I salen. Nevertheless, all of the catalysts were quite active initially. After conducting... 

Table 3. In order to test the hypothesis that heterogeneous catalyst is better than the homogeneous one, beeause the former allows improving the control over iron ions during the Fenton ehemistry, several consecutive degradations were performed with a fixed amount of eatalyst. Figure 19 shows the time needed for four consecutive batch degradations of 100 ml 50 mM Na2S04 (pH 2.8) + 0.06 mM RB5 + 10.7 mM H2O2 (twice of the stoiehiometrie amount required) + 0.1 g eatalyst Cl. At the end of each discoloration the solution was allowed to settle down into the reaetor and the solution was withdrawn carefully. Then, a fresh solution (100 ml 50 mM Na2S04 + 0.06 mM RB5 + 10.7 mM H2O2) was reloaded over the previous 0.1 g eatalyst Cl to start a new RB5 discoloration. The process was repeated three times.

On the basis of the observed stereoinduction trend, the addition of HCN took place over the diaminocyclohexane portion of the catalyst away from the amino acid and amide unit. The last hypothesis led to the prediction that a more sterically demanding amino acid or amide unit (Figure 6.14) could additionally favor the cyanide attack compared to the less bulky diaminocyclohexane unit and thus making the Schiff base catalyst more enantioselective in Strecker reactions of aldimines and ketimines. To evaluate this perspechve, the authors performed a model-(mechanism-) driven systematic structure optimizations by stepwise modification of the amide, the amino acid, and the (thio)urea unit of catalyst 42 and examined these derivatives of 42 (lmol% loading ) in the model Strecker reaction (toluene ... 

Asymmetric amplification is the direct consequence of species of racemic composition being stable and of low reactivity. It should be found in all reactions dealing with chiral auxiliaries. One can expect such a phenomenon in diastereo-selective reactions performed on chiral substrates (e.g., reduction of a keto steroid by a chiral catalyst) or kinetic resolution, a hypothesis that has just recently been confirmed.82-83... 

As demonstrated throughout this volume, catalyst development is much more than just plant testing. Rejecting a hypothesis because of poor plant performance and not determining whether the catalyst was improperly prepared or treated can inhibit a breakthrough. Similarly, characterizing only the catalysts that give excellent performance so as to understand the system may cause researchers to miss the critical variable. 

This increased sensitivity is believed due to the fact that the onium salt produces a much stronger acid, in this case HSbF. Another contributing factor could be participation of free radicals, formed during irradiation of the onium salt. To test this hypothesis, experiments were performed with a latent UV-deblockable sulfonic acid. This compound produces both PTSA and free radicals when it is irradiated. Although the acid produced was PTSA, the curing result was consistent with the result from the onium salt experiment. These experiments indicates it is the free radicals which are effective in crosslinking the matrix. However, it may also just be a solubility effect, e.g. catalysts II and III may be simply more soluble in the MAGME-polymers than PTSA. Further experimentation is needed to determine whether it is a solubility effect or participation of free radicals that explains the low sensitivity of PTSA. In the experiments with pure PTSA, no increase in sensitivity was observed when the PTSA concentration was increased above 0.3J w/w. 

In order to check various model assumptions a first step in the analysis requires to perform mass balance calculations involving the amount of gas oil injected and the amount of hydrocarbons obtained by pressure difference under vacuum conditions, with the key hypothesis that no hydrocarbons remain on catalyst surface at 2 psia. Given mass balance closure was within typically 5-7% error for all experiments conducted, the following was validated a) the total mass of hydrocarbons including various lumps during the reaction... 

Analysis performed with the EQCM technique enabled to investigate the possible adsorption mode of oxygen on a catalyst and then to develop a hypothesis for the ORR mechanism. However, this method is less efficient for catalysts with multiple and complex oxide formation steps, like gold or platinum catalysts, since it is difficult to observe separated adsorption steps with the micro-balance. " ... 

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