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

Abstract

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 × 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.

Powering the Remediation of the Nitrogen Cycle: Progress and Perspectives of Electrochemical Nitrate Reduction

“…Generally, transition metals exhibit higher selectivity toward NH 3 in acidic solutions due to high proton concentrations. 78 The precious metals (e.g., Pd, Pt, Ru, and Ir) have a high affinity toward hydrogen, which leads to the competitive adsorption between N-species and hydrogen. 48 It was reported that the hydrogen atoms occupy more active sites at more negative potentials, suppressing the adsorption of NO 3 − and N-intermediates and decreasing the NO 3 RR efficiency.…”

“…An adsorbed NO can be reduced by five adsorbed hydrogen atoms consecutively to produce NH 3 (eqs –). Adsorbed hydrogen atoms on precious metals such as Pt and Pd facilitate the formation of N–H bonds in acidic solutions. , Generally, transition metals exhibit higher selectivity toward NH 3 in acidic solutions due to high proton concentrations . The precious metals (e.g., Pd, Pt, Ru, and Ir) have a high affinity toward hydrogen, which leads to the competitive adsorption between N-species and hydrogen .…”

Factors Impeding Replacement of Ion Exchange with (Electro)Catalytic Treatment for Nitrate Removal from Drinking Water

“…For electrocatalytic treatment, electricity costs required to generate hydrogen in situ were determined from our recent work assuming ∼50% hydrogen use efficiency for NO 3 − reduction to N 2 . 49 Pd costs were determined assuming reaction kinetics from a trickle bed reactor, and either a 20-, 5-, or 1-year Pd life span. Faster kinetics have been realized for electrocatalytic reactors, and these will be discussed in a subsequent section.…”

“…139 In the latter case, Yan et al showed that the production of molecular hydrogen can be eliminated when nitrite reduction is promoted above −0.61 vs Ag/AgCl reference electrode in a pH 6.5 electrolyte. 49 This approach favors complete hydrogen utilization because no off-gassing occurs, and it favors N 2 production by limiting hydrogen coverage on the catalyst surface.…”

“…For catalytic treatment, hydrogen gas costs were determined from the literature assuming either steam methane reforming (least expensive) or electrochemical generation. For electrocatalytic treatment, electricity costs required to generate hydrogen in situ were determined from our recent work assuming ∼50% hydrogen use efficiency for NO 3 – reduction to N 2 . Pd costs were determined assuming reaction kinetics from a trickle bed reactor, and either a 20-, 5-, or 1-year Pd life span.…”