Surface bidentate

The v(OD) peak at 2706 cm 1 on an OD-covered ZnO surface attributable to linear OD groups on two-coordinated Zn ions, decreased by reaction with CO at 473 K and accompanied with the appearance of uLS(OCO) and vs(OCO) peaks for surface bidentate formats (DCOO ) at 1586 and 1342 cm1, respectively, suggesting that the OD groups react with CO to produce the bidentate formates. The formates (DCOO-) react with the D atoms of bridge (2682 cm ) or threefold-hollow (2669 cm-1) OD groups at 573 K as monitored by FT-IR, evolving D2, COz, D20, and CO in the gas phase. 

It was found that the rate constant of the forward decomposition of the surface bidentate formate (DCOO ) to produce D2 and C02 increased from 0.34X10 4 sec-1 under vacuum to 5.3 X10-4 sec-1 under ambient water. Electron donors such as NH3, CH3OH, pyridine, and THF also increased the decomposition rate the rate constants of the forward decomposition of the surface formates at 553 K were determined to be 28.0X10 4, 7.7X10 4, 8.1X10-4, and 6.0X10 4 sec-1 under NH3, methanol, pyridine, and THF vapors (0.4 kPa), respectively. It is likely that the driving force for the forward decomposition of the formate is electron donation of the adsorbed molecule to the Zn ion on which the bidentate formate adsorbs. The reactant-promoted mechanism for the catalytic WGS reaction on ZnO is illustrated in Scheme 8.2. 

The FTIR data reported in Figure 6.2b showed that only nitrate species were formed upon N02 adsorption, mainly of the ionic type (bands at 1320, 1420-1440 cm-1, vaSymN03 split for the partial removal of the degeneracy 1035-1020 cm-1, vsymN03) and in minor amounts of bidentate type (1560 cm-1, vasymN02 mode expected around 1300 cur1 obscured by the modes of ionic nitrates). Notably, the adsorbed nitrates were related to the Ba component as the surface of the alumina support was almost completely covered by Ba, as pointed out by FTIR data [25], which showed the disappearance of OH groups of the alumina support. 

The FTIR data recorded upon N0/02 adsorption in the presence of C02 showed that bidentate carbonates were immediately formed along with nitrite species. By increasing the time of contact, carbonates were partially displaced, while nitrites evolved to nitrate species [52], Notably, the amounts of surface nitrites present at each contact time are lower in the presence of C02 than in its absence. This indicates that C02 competes for the surface oxygen sites able to give nitrites at the beginning of the adsorption process in fact, after several minutes of exposure to the N0/02/C02 mixture, the amount of nitrates stored was comparable to that obtained in the absence of C02. In conclusion, while in the presence of C02 the nitrite route is inhibited to some extent due to the competition between NO and C02 for the surface oxygen sites of the Ba phase, the nitrate route is only marginally affected by the C02 presence, if any. 

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