Water splitting in low-temperature ac plasmas at atmospheric pressure

Abstract

Plasma-induced water splitting at atmospheric pressure has been studied with a novel fan-type Pt reactor and several tubular-type reactors: an all-quartz reactor, a glass reactor, and three metal reactors with Pt. Ni, and Fe as electrodes. Reaction products have been analyzed by using GC (gas chromatography) and Q-MS (quadrupole mass spectrometry). Optical emission spectroscopic studies of the process have been carried out by employing a CCD (charge-coupled device) detector. Water splitting with tubular quartz and glass reactors is probably non-catalytic. However, a heterogeneous catalytic function of surface of metal electrodes has been observed. The variation of hydrogen yield (YH) and energy efficiency (Ee) with operational parameters such as input voltages (Uin), flow rates of carrier gas (FHe), and concentrations of water (CW) has been examined. Plasma-induced water splitting can be described with a kinetic equation of-dCw/dt = kCW 0.2. The rate constants at 3.25 kV are 2.8 × 10−4, 3.5 × 10−3, and 3.4 × 10−2 mol0.8L−0.8 min−1 for tubular glass reactor, a tubular Pt reactor, and a fan-type Pt reactor, respectively. A CSTR (continuous-stirred tank reactor) and PFR (piston-flow reactor) model have been applied to a fan-type reactor and tubular reactor, respectively. A mechanism on the basis of optical emission spectroscopic data has been obtained comprising the energy transfer from excited carrier gas species to water molecules, which split via radicals of HO·, O·, and H· to form H2 and O2. The fan-type Pt reactors exhibit highest activity and energy efficiency among the reactors tested. Higher yields of hydrogen are achieved at higher input voltages, low flow rates, and low concentrations of water (YH = 78 % at Uin of 3.75 kV, FHe of 20 mL/min, and CW of 0.86 %). The energy efficiency exhibits an opposite trend (Ee = 6.1 % at Uin of 1.25 kV, FHe of 60 mL/min and CW of 3.1 %).