Mechanism of Zn Particle Oxidation by H₂O and CO₂ in the Presence of ZnO

Abstract

In this work we investigate the mechanism of Zn oxidation with CO2 and/or H2O to produce solar derived fuels (CO and/or H2) as part of the Zn/ZnO thermochemical redox cycle. It has been observed that the ZnO contamination of Zn produced by solar thermal reduction of ZnO (solar Zn) facilitates oxidation of the metallic Zn by CO2 and H2O, allowing for nearly complete conversion at temperatures as low as 350 °C. Reaching the same reaction extent starting with pure Zn requires considerably higher temperatures which imposes use of unconventional hard-to-operate reaction configurations utilizing Zn as vapor. The mechanism of this enhancement is investigated by studying the oxidation of solid Zn diluted with ZnO or Al2O3 at 350–400 °C utilizing thermogravimetry. It is found that ZnO 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 Zn sublimation competes with the growth of an impervious ZnO scale over the surface of the remaining solid Zn, the presence of the ZnO increases the reaction extent according to the magnitude of its surface area. This mechanism is supported by energy-dispersive X-ray (EDX) spectroscopy, revealing a substantial deposition of produced ZnO over the surface of the ZnO-seeded Al2O3 diluent.

A study of suppressed formation of low-conductivity phases in doped Li7La3Zr2O12 garnets by in situ neutron diffraction

“…28,41 In the starting mixture for LLZO-Zn60, the metallic Zn melts at ~420 °C, and then it is oxidized near 600 °C by emitted CO 2 or carbonates. 42 In all samples, Li 2 CO 3 does not decompose 43 until the garnets …”

Structural evaluation of percolating, self-healing polyurethane–polycaprolactone blends doped with metallic, ferromagnetic, and modified graphene fillers

“…The bonding of Zn with emerging, gaseous CO 2 and H 2 O particles corresponds well with the composition of degrading material—this process occurs with high efficiency between 350°C and 400°C, even in inert TG atmosphere. 34 Despite Zn’s weaker thermoxidative resistance, composites with it show similar change in parameters to those doped with non-passivated Al—comparable enthalpy and peak positioning regardless of ferromagnetic Fe 3 O 4 presence. The only visible change is in energy needed for physical phase transitions—it is roughly 0.4 ÷ 1.0 J g −1 .…”

“…Eventual oxidation and carbonization of Fe 3 O 4 to Fe 2 O 3 or Fe 2 (CO 3 ) 3 were ruled out during TG since inert (N 2 ) conditions were implemented, and these reactions require sufficient amount of energy at much higher temperatures-in between 600 C and 900 C. 36 The same remark relates to all Al fillers. 34 Furthermore, carbonization is also not likely to occur since material weight should increase much more during the experiment what was not observed.…”

“…All aforementioned facts were verified in experiments (Table 6) and literature. 29,34 Mass balance in TG after thermal analysis is in good correlation with the weight amount of added fillers which means that the most of polymer matrix has evaporated. However, weight loss might be bigger due to partial rGO reduction.…”

Chemical looping syngas from CO2 and H2O over manganese oxide minerals

“…To our knowledge, water splitting over oxides of mineral origin has never been reported. Volatile oxides are used in solid‐vapour thermochemical reduction and water splitting cycles, among them Zn/ZnO at 350 °C, as well as /SnO . Fe‐, Cu‐, and mixed Cu‐Fe can split water, with a maximum of 0.2 hydrogen production .…”