Kinetic study and flow reactor modeling of electrochemical hydrogen evolution and glucose reduction of Raney nickel

The kinetics of electrochemical H{dollar}sb2{dollar} evolution and glucose reduction (to sorbitol) on powdered Raney nickel catalyst was investigated in a batch slurry reactor. Electro-generation of hydrogen on the nickel catalyst was found to proceed via a Volmer-Heyrovsky mechanism. The H{dollar}sb2{dollar} evolution kinetic parameters were determined as a function of electrolyte pH (7.0-14.7) and temperature (298-328 K) by performing potential sweep experiments in the slurry reactor. During glucose electro-hydrogenation, the Volmer and Heyrovsky steps for H{dollar}sb2{dollar} evolution were found to occur in conjunction with a chemical catalytic reaction between adsorbed atomic H and organic substrate in solution. A kinetic model was formulated to predict electric current and glucose reduction rate at a given cathodic overpotential. The model included rate equations for the individual reaction steps and Langmuir adsorption isotherm for glucose along with equations for the Raney Ni open circuit potential shift due to glucose adsorption, steady-state atomic H balance and charge balance at the electrode/electrolyte interface. The theoretical predictions were in excellent agreement with experimental potential sweep data (with an average error of 8-10%) and the computed sorbitol production rates were within 8-15% of constant potential electrolyses in the batch reactor.; A computer model was developed to simulate electrocatalytic hydrogenation of glucose in a flow reactor containing a packed bed Raney nickel powder cathode. The mathematical formulation included the relevant equations describing the transport of reactants and products, conservation of species electroneutrality, and Ohm's law in solution. The model also contained the kinetic rate equations for H{dollar}sb2{dollar} evolution and glucose reduction which were obtained from the batch slurry reactor studies. The flow reactor model was solved in one-dimension with no adjustable parameters to predict sorbitol production rates and current efficiencies. The simulation results reproduced accurately (with an average error of 9%) constant current electrolyses data in the literature at a reactor temperature of 333 K with glucose feed concentration ranging from 0.4 M to 1.6 M and applied current density in the range 0.005-0.021 A/cm{dollar}sp2.{dollar} The model predictions indicated that sorbitol production rates and current efficiencies can be maximized by operating the flow reactor at the highest glucose feed concentration and temperature possible and by pulsing the applied current (in either ON-OFF or ON-ON mode) to the reactor.