The Formation of Isocyanates on Noble Metals and Supported Noble Metal Catalysts

The formation of support-NCO and noble metal-NCO groups on oxide supported noble metal catalysts and high area evaporated metal films during the NO/CO reaction was studied by FTIR spectroscopy. With SiO2 and Al2O3 as supports the results reported in the literature were essentially reproduced. With TiO2 and ZrO2 absorption bands in the region between 2000 and 2100 cm-1 were detected, which had not been described before. On evaporated metals NCO-groups could be detected in situ. They are located at 2185 cm-1 for Pd and at 2170 cm-1 for Pt. Based on kinetic measurements the spillover of NCO-groups from the noble metal to the support is proposed as a mechanism for support-NCO formation. For this spillover a connection between support and noble metal is necessary, which is formed during hydrogen treatment at elevated temperatures. This connecting species could be a noble metal/support intermetallic phase, as Pd3Ti resp. Pt3