Methanol formation, activation barriers

DPT calculations modeling ZSM-5 as a ((SiH3)4A104M) (M = Fe, Co) cluster show that in the presence of water, the rate of methanol formation increases. For both clusters, with and without water, the step which has the highest activation barrier is the methanol formation reaction from the hydroxy complex formed on the clusters. The Co-ZSM-5 cluster has a lower activation harrier when compared to that of the Fe-ZSM-5 cluster (49 kcal/mol vs 53 kcal/mol). Water decreases the activation harrier for methanol formation. Activation barrier values decrease to 48 and 39 kcal/mol, for Fe- and Co-ZSM-5 clusters respectively, in the presence of a water molecule adsorbed after the formation of a hydroxyl group. ... 

The reason for the lack of methanol decomposition on Pt(lll) has been explained using DFT results. On a 1/4 ML covered surface, the activation barriers for the formation of hydroxymethyl (CH2OH) and methoxy (CH30) have been calculated to be 0.96 eV and 1.47 eV, respectively.92 For a 1/9 ML covered surface, the barriers have been calculated as 0.63 eV and 0.81 eV, respectively.96 These barriers are significantly larger than the desorption barriers. Thus, adsorbed methanol will desorb prior to decomposition in UHV. 

After discussing the dehydration of methanol and formation of DME, we are able to illustrate a number of key theoretical concepts. The first is that carbocation fragments are found in transition states, rather than as stable intermediates. Furthermore, the nature of these species is different from what is predicted from gas-phase studies, experimental or theoretical. The cluster, i.e., the zeolite, controls the stabilization of this carbocationic fragment. Second, we see that each different reaction requires a different transition state, rather than formation of a transition state that can be converted in a number of possible reactions. (This latter view received some support as a result of different processes possessing very similar activation barriers.)... 

In our laboratory, we have studied the coupling of two methanol molecules at the acid site of chabazite (see Fig. 1) (Giurumescu and Trout, 2001). This is hypothesized to be an important elementary step in the formation of the first carbon-carbon bond in methanol-to-olefins processes. Because this step has a significant activation barrier, we have chosen to use the method of constraints, with the constraint being the carbon-carbon distance. Simulations were performed at 673 K for 1.5 ps at each point. Our free energy profile is shown in Fig. 15. 

The comparison of the CO activation and CO hydrogenation barriers over Pd strongly indicates that CO hydrogenation to methanol is much more prevalent than CO activation subsequently to form CH4. This discussion also illustrates why on a transition-metal surface (in the absence of any steric constraints) there can be a preference for a particular reaction channel. On Pd(lll) the overall reaction barrier for methanol formation is significantly lower than that for the CO dissociation reaction. 

To further the mechanistic understanding, computational studies have been conducted by Bock and co-workers, who analysed a range of possible transition states in the formation of the B-O bond. In these studies the reaction was studied on the basis that a molecule of water was eliminated in each step, trigonal geometry was therefore restored to boron in each step. The binding of methanol, 1,2-ethanediol and o-glucose at boron were examined. Where the reactions were simulated in vacuo or in acetonitrile the activation barriers were significant. On the other hand, if water, ammonia or NaOH were used in... 

Another study on the preparation of supported oxides illustrates how SIMS can be used to follow the decomposition of catalyst precursors during calcination. We discuss the formation of zirconium dioxide from zirconium ethoxide on a silica support [15], Zr02 is catalytically active for a number of reactions such as isosynthesis, methanol synthesis, and catalytic cracking, but is also of considerable interest as a barrier against diffusion of catalytically active metals such as rhodium or cobalt into alumina supports at elevated temperatures. 

Two adsorption complexes were found for the interaction of the neutral water and methanol molecules with the methoxy-zeolite lattice, as shown in Fig. 15. The water molecule essentially is a spectator during the formation of DME, although it does stabilize the DME once it is formed. The largest activation energy of the elementary steps is that for methoxy formation (160 kJ/mol). This is similar to the barrier to DME formation via the... 

Presently, the oxidation of methanol on pure platinum has more academic interest than practical application once DMFC universally employs platinum based materials having two or more metals as an anodic catalyst In absence of methanoUc inteimediate readsorption, the maximum reactiOTi rate for CO oxidation is 100-fold smaller than maximum reaction rate for CO adsorption from methanol dehydrogenation steps [11]. Indeed, the mechanism of methanol oxidation on platinum is expected to be equal to that on its alloys despite different kinetics which would result in a selection of pathway. In terms of complex activation theory, alloyed Pt is intend to lower the Ea barrier for CO adsorption, thus driving methanol oxidation to completion. As previously established [3], there are several factors that affect the calculated activation energy for the MOR at a given potential, such as coverage of methanoUc intermediates and anion adsorption from the electrolyte as well as pH and oxide formation processes. 

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