Scalable Reactor Design for Electrocatalytic Nitrite Reduction with Minimal Mass Transfer Limitations

A parallel-plate thin-layer (PPTL) flow reactor with potential control and custom-made cathode of Pd-In modified activated carbon cloth was developed for electrocatalytic removal of nitrite from water; the effect of applied potential and flow rate were investigated. Compared to other reactors in the literature, rapid nitrite reduction (first-order rate constant is 0.38 L g(Pd)(-1) min(-1)), high current efficiency (CE, 51%), and low ammonium selectivity (5.4%) were observed at an applied potential of -0.60 V vs the Ag/AgCl reference electrode (RE) and a flow rate of 40 mL min(-1) in a phosphate buffer solution of pH 6.5. Slightly faster kinetics were observed at more negative potentials (0.57 L g(Pd)(-1) min(-1) at -1.0 V/RE), but then ammonium production (88%), H-2 gas evolution (E-0 = -0.61 V/RE), and current loss (CE < 10%) became problematic. Nitrite reduction was measured in the PPTL flow reactor for almost 50 2 h cycles over six months, with little apparent loss (30%) in activity. A reactive transport model was developed and used to simulate the kinetic data. The fitted intrinsic rate constant (k(w)) was 5.2 x 10(-6) m s(-1), and ratios of dimensionless Nusselt numbers to reaction rate constant values support the reactor being more reaction than mass transfer limited. Application of the parametrized model demonstrated how the PPTL reactor could be scaled (e.g., cathode dimensions, flow channel thickness), operated (i.e., flow rate), or modified (i.e., greater intrinsic catalyst activity) to most efficiently remove nitrite from larger flow streams.