(203b) The Solar Thermochemical Reduction of CO2 to CO Via the Heterogeneous Oxidation of Zn(g)

The
heterogeneous oxidation of Zn(g) is a promising means of sustainably reducing
CO₂ to CO in the two-step Zn/ZnO solar thermochemical cycle. Prior efforts
on CO₂ reduction in the cycle have focused on the oxidation of solid
or liquid Zn, and elucidate the need for faster kinetics. The rate of the
oxidation of solid and liquid Zn is limited by solid-state diffusion through
the ZnO layer that forms on the Zn surface. To eliminate the solid-state
diffusion barrier, we propose reducing CO₂ to CO
via the heterogeneous oxidation of Zn(g). The Zn(g) oxidation pathway promises
complete and raid conversion of Zn, which ultimately increase
cycle thermal efficiency. Cycle efficiencies of 30% are possible even when
additional energy must be supplied to vaporize zinc, and the cycle efficiency
could be higher if heat recuperation was implemented.

Experiments were
conducted in a tubular flow reactor maintained at atmospheric pressure to elucidate the kinetics of Zn(g) oxidtion. The rate of the heterogeneous oxidation of Zn(g) was measured gravimetrically. Temperatures
were varied between 800 and 1150 K, and Zn(g) concentrations up to 57 mol% were
achieved, extending the range previously considered in the literature. A range
of reacting gas compositions were considered with CO₂-to-Zn molar
ratios ranging between 1 and 60. CO production was monitored with a Raman laser
gas analyzer. A numerical model decouples the surface kinetics and the
transport of reacting species in the gas phase from the global reaction rate.

Figure 1 shows
the measured and predicted rates of the heterogeneous oxidation of Zn(g) along the
axis of the tube flow reactor for temperatures between 800 and 1150 K. The oxidation
of Zn(g) by CO₂ is rapid, on the order of 10^-8?10^-5
mol cm^-2 s^-1, representing at least an order of magnitude
increase over the rates typically observed in the interface-controlled regime
of Zn(s) particle oxidation. The best fit between the reaction rate data and
the numerical model is obtained using second-order kinetics, with Zn(g) and CO₂
both behaving as first-order reactants. The activation energy for the Arrhenius
reaction rate constant is 44±3 kJ/mol and the preexponential factor is (92±6)×10^-3
mol cm^-2 s^-1 atm^-2. As a result of the rapid
rate of oxidation of Zn(g), less than one second is required to convert more than
85% of the Zn to ZnO.

The present work
confirms that substantial improvements in the thermal efficiency of the Zn/ZnO
thermochemical cycle for reducing CO₂ to CO are possible if the critical
Zn oxidation step were to utilize the heterogeneous oxidation of Zn(g). Additionally,
the kinetic expression obtained for the heterogeneous oxidation of Zn(g) is suitable
for reactor design and scale-up since it reflects the intrinsic surface
kinetics, and is not influenced by mass transfer.