The kinetics and mechanism of CeIV oxidation of the water oxidation catalyst [(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+ (1, RuIIIORuIII) have been investigated by UV−visible measurements with application
of global analysis. The reaction proceeds by stepwise oxidation of RuIIIORuIII to RuVORuV with oxidation of
RuIVORuIII the slow step. RuVORuV has been identified as an intermediate by the appearance of its ClO4
- salt
as a black suspension in concentrated solutions at 5 °C. It is the key intermediate in water oxidation. The
mechanism may involve a bimolecular step and formation of a peroxo-bridged intermediate. Catalytic water
oxidation is greatly retarded after just a few catalytic cycles because of anation induced by O2 evolution.
Silica gel, under various hydration conditions, was investigated using a novel calorimetric adsorption (CalAd) method. This method combines data from calorimetric and adsorption titrations of a solid surface using a small probe molecule in a noninteractive solvent. The Cal-Ad method proved to be more sensitive than other characterization techniques (e.g. temperature-programmed desorption) and calorimetric techniques. Previously, it was believed that the silica surface consisted of one type of hydrogen-bonding site. The Cal-Ad method has elucidated three hydrogenbonding sites of different strengths using pyridine as the basic probe molecule. The equilibrium constant of binding, enthalpy of binding, and number of each of these sites have been determined.
Ce(IV) oxidation of
the ruthenium blue dimer leads to catalytic oxidation of water.
Kinetic and spectroscopic studies have revealed evidence for
RuVORuV as the active catalyst. Redox
cycling occurs between RuIIIORuIII and
RuVORuV via a complex sequence of coupled
steps involving oxidations by Ce(IV) and second-order cross
reactions of the dimer. The oxygen-evolving step is not rate
determining in the catalytic
cycle.
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