Mechanism for Catalytic Stability Enhancement of Fe-III[Co-III(CN)(6)] by Doping Divalent Ions for Organophosphate Hydrolysis

Catalytic activity and stability of iron hexacyanocobaltate (FeIII[CoIII(CN)6],Fe-Co) for organophosphate hydrolysis are improved by doping of divalent metal ions (MII=MnII,NiII,orCuII). The catalytic activity of a series of (FeIII0.8MII0.3)[CoIII(CN)6](FeM-Cocompounds) was examined in a mixed solution of water and ethanol [9:1 (v/v)] containing disodiump-nitrophenyl phosphate (p-NPP) at 60 degrees C.FeM-Cocompoundsexhibited high conversions (>= 60%, 25 h), which are about four times that of Fe-Co(16%). Repetitive catalytic examinations indicated that theconversion at the second runs in the systems usingFeM-Cocompoundsmaintained over 90% of those at thefirst runs. The low catalytic activity ofFe-Coin the mixed solution resulted from decomposition due to leaching of FeIIIions by ligand exchange from the CN ligand top-NPP as evidenced by IR and XPS measurements for recovered precipitatesafter catalytic examination. Thus, the catalytic stability enhancement observed inFeM-Cocompounds resulted from weakerinteraction betweenp-NPP and FeIIIions, which can be evidenced by reduced surface acidity evaluated from temperature-programmed desorption of pyridine on the surfaces. The heats of pyridine adsorption onFeM-Cocompound surfaces were around60-80 kJ mol-1, which were less than half that ofFe-Co(160 kJ mol-1). Based on these results, a mechanism is proposed for securing Fe-NC bonds and enhancing the catalytic stability by doping of divalent metal ions to Fe-Co.