Due to a high oxygen looping capacity per unit mass of Zn, the Zn/ZnO thermochemical cycle is regarded as a promising path to storing solar energy into liquid fuels. In the first step of this cycle ZnO is dissociated using concentrated solar radiation into a gaseous mixture of Zn and O2. The Zn is then separated from O2 by rapid quenching that yields a solid Zn-ZnO product mixture commonly known as solar Zn. In the next step, the solar Zn is oxidized with CO2 and H2O to produce syngas (a mixture of CO and H2) and ZnO; the former is further processed into liquid transportation fuels while the latter is recycled to the solar step to continue the cycle.
The exploration of Zn oxidation with CO2 using pure Zn particles has demonstrated negligible conversions precluding efficient recovery of the solar energy stored in solid Zn. Apparently, a fast development of a thin but impervious ZnO scale over the Zn surface hinders further supply of CO2 to the reaction site. This finding had motivated some of the researchers in the field to consider utilizing Zn as vapor or liquid which not only imposes unconventional, hard-to-operate reactor concepts but it also decreases the overall cycle efficiency due to the additional energy required to melt and/or vaporize Zn.
Curiously, in contrast to the low reaction extents obtained with pure Zn particles, solar Zn was found to completely and rather quickly oxidize with CO2 even at temperatures below the melting point of Zn (420°C). This important enhancement in the oxidation kinetics of solid Zn has been thoroughly investigated via thermogravimetry using Zn-ZnO blends prepared from commercially available powders as solar Zn surrogates. A broad set of isothermal Zn conversion-versus-time data was acquired within wide ranges of CO2 concentrations, sample compositions (ZnO surface area per mole of Zn), and temperatures. The study revealed that the ZnO surface acts as the site for the oxidation of Zn originating from the vapor phase, thereby serving as a sink for Zn vapor and maintaining the driving force for sustainable Zn sublimation. As this reaction pathway competes with the formation of an impervious ZnO layer on the Zn surface via direct Zn oxidation, it enables the conversions of solid solar Zn that are substantially higher than the conversions of the Zn particles oxidized in the absence of ZnO.
To understand better the mechanism and the rate-controlling step(s) of the overall reaction, several mathematical models have been tested against the experimental results. The models consider the following serial/parallel pseudo-steady state processes: (1) direct oxidation of the Zn surface leading to the formation of the impervious ZnO scale, (2) sublimation of Zn and its subsequent gas-phase transport to the ZnO surface, and (3) reaction of CO2 and Zn(g) at the ZnO surface according to one of Langmuir-Hinshelwood or Eley-Rideal mechanisms.
The direct Zn oxidation was found to be a minor contributor to the overall conversion as most of the Zn sublimated and reacted at the ZnO surface. However, this direct oxidation still affects the conversion rate because the formation of the impervious ZnO scale decreases the Zn sublimation rate. Although the Zn conversion rate was found to increase significantly with the increase in ZnO weight fraction, this increase could not be explained solely by the increase in ZnO surface area per mole of Zn; the model suggests that an additional enhancing effect of the ZnO as diluent may be originating from a decrease in the effective size of Zn particle aggregates, thus decrease in the effective diffusional resistance for Zn gas-phase transport to the ZnO surface. As to the reaction at the ZnO surface, the model discrimination could not discern between two likely rate-limiting steps: (a) the surface reaction between adsorbed CO2 and Zn and (b) the reaction of Zn(g) with adsorbed CO2.