Novel Demonstration of Heterogeneous Redox Catalysis Using a Rotating Pencil Lead

Heat-treated, pencil lead, in powder or rod form, is used as a heterogeneous redox catalyst for the oxidation of chloride to chlorine by Ce(IV) ions in 0.5 M H2SO4 and 2 M NaCl solution. When the lead is used in ground up powder form, the resulting decay of the Ce(IV) is first order over 3 half-lives and gives a chlorine yield of 79%. When a rotating 2 mm diameter, heat-treated form of the pencil lead is used in the same test system, the kinetics of Ce(IV) yields a series of different first order rate constants, k1, as a function of rod rotation speed, ω. A plot of k1 versus ω0.7 yields a good straight line, which accords with the hydrodynamics of the system and the assumption that the rate of Ce(IV) reduction, by chloride, depends upon the rate of mass transport of the Ce(IV) ions to the surface of the pencil lead rod. The gradient of the plot allows a value for the diffusion coefficient for Ce(IV) ions in 0.5 M H2SO4 and 2 M NaCl solution to be calculated, namely: (2.7 ± 0.1)x10-6 cm2 s -1 ) which compares favourably with those reported previously.