Activation energy of CO oxidation

As was previously mentioned, PtRu alloys exhibit improved performance over pure Pt alloys.117,118 This is primarily a result of the ability of Ru to dissociate H20 for reaction with CO adsorbed on Pt sites.115,116 That CO oxidation on pure Ru is unfavorable indicates that on the bimetallic surface, CO is oxidized only on the Pt sites.119 Thus, CO is oxidized on Pt sites adjacent to Ru sites, where water is activated.120,121 This is known as a bifunctional mechanism. In addition, the presence of Ru atoms reduces the adsorption energy of CO on neighboring Pt atoms, lowering the activation energy of CO oxidation.122 This effect is purely electronic and is less significant than the bifunctional effect of Ru.123 One significant limitation of PtRu is the weak adsorption of methanol on Ru, particularly at room temperature.117,124 The weak adsorption severely hinders methanol decomposition, which is evident in Fig. 7 by the drop in current density for PtRu electrodes with high Ru composition.125... 

Fig. 9. Reduced Arrhenius plots (k/ko against 1/T) of NiO and NiO/Ag. Comparison of measured activation energies of CO oxidation (top) with the conductivity of NiO (centre) and the Arrhenius plot obtained by impurity level analysis (bottom). Solid lines, dark dotted lines, light dash-dotted, conductivity...

A special study of CO conversion under the same conditions showed that, although the conversion increases with temperature, it remains much lower than the observed CO2 yield. The activation energy of CO oxidation on a quartz surface was foimd to be 20.4 kj/mol, while... 

The current experimental setup, with its excellent temporal resolution afforded by rapid-scan data collection, allows us to perform kinetic measurements of catalysts in parallel. An example of this can be seen in the measurements of reaction order and apparent activation energies during CO oxidation over supported rhodium... 

Since adsorption of O2 is essentially nonactivated, the apparent activation energy for CO oxidation is simply the negative of the enthalpy of CO adsorption on Pd. This result has been experimentally observed [M. Boudart, J. Mol. Catal. A Chem., 120 (1997) 271]. 

Fig. 2.21 (a) Typical shape of operating temperature influence on response of metal oxide gas sensors and adsorption/ desorption parameters controlling this dependence, (b) Simulation of the influence of adsorption/desorption parameters on temperature dependence of SnO gas response to CO. Here is the activation energy of CO adsorption, and... 

Table 53 Comparison of activation energies for CO oxidation on Pt with values from the literature...

DFT calculation results in Fig. 20.5 show the polarization of the valence electrons of gold solid clusters and hollow cages and the activation energies for CO oxidation [49]. The lower the effective atomic CN is, the stronger the polarization and the lower the activation energy will be for CO oxidation of the gold catalyst, which is in accordance with observations for Pt catalysts shown in Fig. 20.3. Bond contraction, core electron entrapment, and valence charge polarization happen only to the outermost two atomic layers [49-51]. 

Fig. 20.5 a Shell-resolved LDOS of Aujs and AU55 clusters b size-resolved DOS of Aui3 55 i47 solid clusters and Ag12.42.134 hollow cages and c activation energies for CO oxidation pertained to different geometric structures of gold [49]... 

In a study of oxidation resistance over the range 1200—1500°C an activation energy of 276 kj/mol (66 kcal/mol) was determined (60). The rate law is of the form 6 = kT + C the rate-controlling step is probably the diffusion of oxygen inward to the SiC—Si02 interface while CO diffuses outwards. 

The NO reduction over Cu-Ni-Fe alloys has been studied recently by Lamb and Tollefson. They tested copper wires, stainless steel turnings, and metal alloys from 378 to 500°C, at space velocities of 42,000-54,000 hr-1. The kinetics is found to be first order with respect to hydrogen between 400 and 55,000 ppm, and zero order with respect to NO between 600 and 6800 ppm 104). The activation energies of these reactions are found to be 12.0-18.2 kcal/mole. Hydrogen will reduce both oxygen and NO when they are simultaneously present. CO reduction kinetics were also studied over monel metals by Lunt et al. 43) and by Fedor et al. 105). Lunt speculated that the mechanism begins by oxidant attack on the metal surface... 

With respect to CO oxidation an activity order similar to that described above for CH4 combustion has been obtained. A specific activity enhancement is observed for Lai Co 1-973 that has provided a 10% conversion of CO already at 393 K, 60 K below the temperature required by LalMnl-973. This behavior is in line with literature reports on CO oxidation over lanthanum metallates with perovskite structures [17] indicating LaCoOs as the most active system. As in the case of CH4 combustion, calcination at 1373 K of LalMnl has resulted in a significant decrease of the catalytic activity. Indeed the activity of LalMnl-1373 is similar to those of Mn-substituted hexaaluminates calcined at 1573 K. Dififerently from the results of CH4 combustion tests no stability problems have been evidenced under reaction conditions for LalMnl-1373 possibly due to the low temperature range of CO oxidation experiments. Similar apparent activation energies have been calculated for all the investigated systems, ranging from 13 to 15 Kcal/mole, i.e almost 10 Kcal/mole lower than those calculated for CH4 oxidation. 

Figure 8. Rate of carbon monoxide oxidation on calcined Pt cube monolayer as a function of temperature [27]. The square root of the SFG intensity as a function of time was fit with a first-order decay function to determine the rate of CO oxidation. Inset is an Arrhenius plot for the determination of the apparent activation energy by both SFG and gas chromatography. Reaction conditions were preadsorbed and 76 Torr O2 (flowing). (Reprinted from Ref. [27], 2006, with permission from American Chemical Society.)...

It should be noted that the choice of Fe0 950 (wustite) rather than FeO in the preceding reactions is not arbitrary [10]. The steam-iron reaction would produce very little hydrogen at these temperatures if magnetite were reduced to FeO instead of Fe095O. Hacker et al. [55] determined that the activation energy of magnetite reduction with H2 and CO is equal to 95 and 98 kj/mol, respectively. The energy of activation of wustite oxidation with steam was found to be 29 kj/mol. 

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