HOCO intermediate

However, a variety of studies since the mid-1970s has established that it is not, in fact, a simple bimolecular reaction as implied by reaction (14) but rather involves the formation of an excited HOCO intermediate (e.g., see Fulle et al., 1996 Golden et al., 1998 and references therein) ... 

FIGURE 5.3 Typical potential energy diagram proposed for reaction of OH with CO (adapted from Mozurkewich et. al f984). Note the well corresponding to formation of the (HOCO) intermediate. 

Reaction Probability versus Collision Energy. From the above energy disposal measurements, we conclude that product excitations are nonstatistical at high collision energies, even if the HOCO intermediate ensures statistical behavior at lower energies. Next, we turn to the overall cross-... 

A stable HCO intermediate species must precede the formation of H2 and CO2. It has been proposed that the key intermediate species for the WGS reaction is either a formate (HCOO) or a carbonate (COs). Recent theoretical calculations also suggest the possibility of a carboxylate (HOCO) intermediate." These species have different coordination modes and different lifetimes on the surface of the catalyst. Adsorption of HCOOH and COg was used to create HCOO and CO3 groups on CeOx/Au(lll) surfaces. HCOO ds appeared to have greater stability than COg d with desorption temperatures up to 600 K while CO3 only survived on the surface up to 300 K. Both species could be valid intermediates for the WGS because the reaction temperatures in Fig. 6.11 are elevated... 

K). On the CeOx/Au(lll) catalysts, the presence of Ce+ led to the dissociation of HgO to give OH groups. The adsorption of CO on these systems led to the formation of carbonates and formates but no clear signal was found for a carboxylate (HOCO) intermediate. Thus, the carboxylate intermediate seen in DFT calculations is experimentally elusive and difficult to detect. The low thermal stability of the carboxylate makes it an ideal transient species for the and it may only be present under steady... 

The elementary reaction OH -h CO CO2 + H is the primary mechanism for formation of CO2 in combustion. This reaction, as well as its reverse, have been studied extensively using a variety of methods, including the crossed molecular beams method. The reaction is thought to involve the participation of long-lived HOCO intermediates, as illustrated in Fig. 14. Note that although OH -h CO may form the intermediate complex with at most only a small potential energy barrier, a substantial barrier exists for H-atom loss forming CO2. 

In Chap. 3, Hua Guo et al. stress the importance of tunneling in both unimolecular and bimolecular reactions. They have calculated the thermal rate constant, k T), for the exothermic HO + CO H + CO2 reaction [190]. In combustion, this reaction is the main source of heat as the last step of hydrocarbon oxidation. They explain that it proceeds via the HOCO intermediate. In the last step of the reaction, the system has to surmount a transition state. There the reaction coordinate is essentially the H-O stretch and tunneling can dominate. This gives rise to a strong non-Arrhenius... 

This observation is consistent with experimental evidence for this reaction [59, 61], as well as our earlier QCT work [101] and previous QM calculations on the LTSH PES [98, 99]. Interestingly, the vibrational excitation in the reactant has Uttle impact on the capture probabilities. This follows that the energy imparted in the OH vibration helps to surmount TS2 as the reaction coordinate at the saddle point is essentially the O-H stretch. However, this is possible only when the HOCO intermediate is relatively short-lived, rendering incomplete randomization of energy before surmounting TS2, which retains energy in the O-H bond. A short-lived HOCO intermediate is consistent with experimental observations [52,53,109,110] and our QCT studies of this system on this new PES [101,102]. 

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