Hydrogenation of Nitroxides on Pt/SiO2

Article abstract of DOI:10.1021/ja00375a001

The hydrogenation of the nitroxide 2,2,6,6-tetramethylpiperidinyl-N-oxy (TMPNO) in ethylcyclohexane and other solvents has been studied from 0.6 to 35 deg C on Pt/SiO2.Hydrogenations were sandwiched between hydrogenations of cyclopentene using the techniques of Hussey et al.Results were shown to be uninfluenced by mass transfer of hydrogen between gas and liquid phases and in the catalyst pores.Significant levels of poisons in the TMPNO appeared to have been absent.Rates of hydrogenation of TMPNO to the hydroxylamine were very fast.At 20 deg C, turnover frequencies per se cond per surface atom of Pt for the loss of unsaturated molecules were ca. 130 for TMPNO and ca. 13 for cyclopentene, and E_a's were 21 and 34 kJ mol^-1, respectively.The rates were zero order in TMPNO and about first order in hydrogen.In competitive hydrogenations of equimolar mixtures, TMPNO hydrogenated 3 times faster than cyclopentene.It appears that both TMPNO and cyclopentene absorb strongly enough on Pt to form an essentially saturated adsorbed layer and that the rate of hydrogenation is that of the dissociative adsorption of hydrogen at gaps in the adsorbed layer of the unsaturated molecule.The relative rates of hydrogenation of such molecules are then proportional to the product (effective area of gaps)x(sticking coefficient of hydrogen).