Electrocatalytic activity of Pd-loaded Ti/TiO2 nanotubes cathode for TCE reduction in groundwater

Abstract

A novel cathode, Pd loaded Ti/TiO2 nanotubes (Pd-Ti/TiO2NTs), is synthesized for the electrocatalytic reduction of trichloroethylene (TCE) in groundwater. Pd nanoparticles are successfully loaded on TiO2 nanotubes which grow on Ti plate via anodization. Using Pd-Ti/TiO2NTs as the cathode in an undivided electrolytic cell, TCE is efficiently and quantitatively transformed to ethane. Under conditions of 100 mA and pH 7, the removal efficiency of TCE (21 mg/L) is up to 91% within 120 min, following pseudo-first-order kinetics with the rate constant of 0.019 min(-1). Reduction rates increase from 0.007 to 0.019 min(-1) with increasing the current from 20 to 100 mA, slightly decrease in the presence of 10 mM chloride or bicarbonate, and decline with increasing the concentrations of sulfite or sulfide. O2 generated at the anode slightly influences TCE reduction. At low currents, TCE is mainly reduced by direct electron transfer on the Pd-Ti/TiO2NT cathode. However, the contribution of Pd-catalytic hydrodechlorination, an indirect reduction mechanism, becomes significant with increasing the current. Compared with other common cathodes, i.e., Ti-based mixed metal oxides, graphite and Pd/Ti, Pd-Ti/TiO2NTs cathode shows superior performance for TCE reduction.

Fig. 1 –

(a) FSEM for a Ti/TiO₂NTs slice, (b) FSEM of PdeTi/TiO₂NTs, (c) EDX of PdeTi/TiO₂NTs, (d) XPS spectra of PdeTi/TiO₂NTs, and (e) XRD patterns of Ti/TiO₂NTs and Pd-Ti/TiO₂NTs.

Fig. 2 –

(a) CV spectrums of TCE reduction on Ti/TiO₂NTs and PdeTi/TiO₂NTs. The scanning conditions were based on 2.28 mM TCE, 100 mV/s scanning rate, pH 7, 500 rpm and 40 mM Na₂SO₄ as electrolyte. (b) Profiles of TCE reduction on the Pd-Ti/TiO₂NTs cathode. The reaction conditions were based on 100 mA, pH 7, 163 µM TCE and 10 mM Na₂SO₄ background electrolyte. A pseudo first-order model is used to fit the decay of TCE with time, which is expressed as Ln(C₀/C ) [ kt D b. where C₀ is the initial concentration of the reactant, C is the concentration of TCE at time t, t is the reaction time, k is the reaction rate constant (min⁻¹) and b is a constant.

Fig. 3 –

(a) TCE reduction on the Pd-Ti/TiO2NTs cathode at different currents and (b) the corresponding transformation efficiencies. The reaction conditions were based on 163 µM TCE, pH 7 and 10 mM Na2SO4 background Electrolyte.

Fig. 4 –

Effects of (a) anionic ions and (b) different concentrations of RSCs on TCE reduction on the Pd-Ti/TiO₂NTs cathode. Reaction conditions were based on 163 µM TCE, 100 mA current and pH 7. Ions concentration was 10 mM unless otherwise specified.

Fig. 5 –

Repeated reduction of TCE on the Pd-Ti/TiO₂NTs cathode. Reaction conditions were based on 163 µM TCE, pH 7, 100 mA and 10 mM Na₂SO₄ background electrolyte. The same cathode was used for ten successively repeated applications.