Isocyanate species, spillover

Isocamphane, 20 281 Isocyanate reactions, 13 393 species, spillover of, 34 41 2 formation in catalytic exhaust-gas reaction, 34 41... 

This species may spill over from Pt to Si02 by a mere heating in vacuum of the previous system. Indeed, the band at 2310 cm -1 (-NCO on Si02) started to appear at room temperature and was maximum at 200°C, whereas the band at 2180 cm -1 (-NCO on Pt) decreased simultaneously. This research is one example of the use of IR spectroscopy to follow the spillover of adsorbed species. In the same work (158) it was reported that chemisorbed oxygen increased not only the stability of -NCO on Pt but also the extent of spillover of -NCO onto Si02. Other metals like Rh or Pd supported on SiOz also favor the formation of metal-NCO by interaction with HNCO at — 83°C. The isocyanate species spills over onto Si02 at 100 to 150°C but simultaneously it is partially decomposed into CO, which remains adsorbed on the metal (159). It is of interest to note that Si-NCO, formed on Rh/SiOz catalyst by the interaction of NO + CO, was very stable, as shown by Hecker and Bell (160). It is not decomposed at around 330°C by 100% of H2,2% of 02, 2% of NO, 3% of CO, or even 3% of HzO. In the presence of air at 25°C Si-NCO disappears after 40 h. 

Si-NCO and Si-CN species are formed. The formation of isocyanate species is a common feature of the CO-NO interaction on various supported metals, as demonstrated by Solymosi and co-workers " and by others. In all cases the interaction takes place on the metals, but the isocyanate species migrate to the carrier surface through a spillover process. 

A comparison of TPR results on Rh/Si02 and Ce-Rh/Si02 shows (i) Ce decreases the temperature for 99% conversion, (ii) Ce decreases the spillover of isocyanate from Rh to Si02> (iii) Ce stabilizes tiie formation of bidentate nitrato species at temperatures above 473 K, (iv) Ce decreases the formation of N2O as indicated by a decrease in intensity of 2240 cm-l nd decreases the formation of NO2 as indicated by the MS response over Ce-Rh/Si02 catalyst, and (v) Ce decreases the formation of Rh (CO) which exhibits a shoulder band at 2105 cm-l. 

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