Single-atom catalysts (SACs) of non-precious transition metals (TMs) often show unique electrochemical performance, including the electrochemical carbon dioxide reduction reaction (CO 2 RR). However, the inhomogeneity in their structures makes it difficult to directly compare SACs of different TM for their CO 2 RR activity, selectivity, and reaction mechanisms. In this study, the comparison of isolated TMs (Fe, Co, Ni, Cu, and Zn) is systematically investigated using a series of crystalline molecular catalysts, namely TM-coordinated phthalocyanines (TM-Pcs), to directly compare the intrinsic role of the TMs with identical local coordination environments on the CO 2 RR performance. The combined experimental measurements, in situ characterization, and density functional theory calculations of TM-Pc catalysts reveal a TMdependent CO 2 RR activity and selectivity, with the free energy difference of ΔG(*HOCO) − ΔG(*CO) being identified as a descriptor for predicting the CO 2 RR performance.

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Ultrafine crystalline TiO2 powders were prepared by just heating and stirring aqueous TiOCl2 solution with a Ti+4 concentration of 0.5 mol/l at room temperature to 100°C under one atmosphere. The crystallinity, the phase transformation and the particle shape of ultrafine TiO2 powders obtained by this simple precipitation method were analyzed using an X-ray diffractometer (XRD), transmission electron microscopy (TEM), differential thermal analyzer (DTA), and scanning electron microscopy (SEM). TiO2 crystalline precipitates with a pure rutile phase were formed below 65°C, then TiO2 crystalline precipitates with an anatase phase started forming at temperatures higher than 65°C, which ends with the pure anatase phase at 100°C. The direct formation of TiO2 crystalline precipitates from an aqueous TiOCl2 solution is due to the existence of the OH- ions in distilled water which cause the crystallization of TiOCl2 to TiO2 without hydrolyzation to Ti(OH)4. Conventionally, rutile-phase TiO2 is obtained at much higher temperatures. However, in this study a stable rutile-phase TiO2 was obtained by a simple method at close to room temperature.

Zinc oxide (ZnO) powders were synthesized by the modified glycine‐nitrate process (MGNP) with various oxidants and fuels. Single‐phase ZnO powders were easily obtained regardless of oxidants and fuels. The particle size and shape of ZnO powders were dependent on the types of fuels. The ZnO powder synthesized using Zn(OH)2 and glycine as an oxidant and a fuel, at a fuel/oxidant ratio of 0.8, showed the best powder characteristics, such as an average grain size of 30 nm and the specific surface area of 120 m2/g. The removal of silver ions from the waste‐development solution was tried to confirm photocatalytic activities of the synthesized ZnO powder. The silver ions were completely removed within 15 min. This silver recovery rate is three times higher than that of commercial state‐of‐the‐art TiO2. The photoluminescence (PL) measurement also showed the PL intensity at ultraviolet (UV) of the synthesized ZnO powder is almost three times higher than that of commercial state‐of‐the‐art TiO2. The synthesized ZnO nanopowder absorbed more UV than any other powders, including commercial state‐of‐the‐art TiO2 and ZnO powders. This means the high UV absorption efficiency leads to the generation of more electrons that are involved in the reduction of silver ions.

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ZnO nanopowders were prepared by solution combustion method (SCM). The ZnO nanopowders synthesized using Zn(OH) 2 and glycine as an oxidant and a fuel (with fuel/oxidant ratio, F/O=0.8), showed excellent crystalline and photocatalytic characteristics. In order to evaluate the photocatalytic reactivity of the prepared ZnO nanopowder, it was tried to decompose total organic carbon (TOC) from aqueous phenol solution. Several kinds of TiO 2 nanopowders were also tried to compare the photocatalytic reactivity. Surprisingly, SCM ZnO nanopowder shows 1.6 fold higher destruction rates of the organic pollutant than P-25 TiO 2 nanopowder that is known as a kind of standard photocatalyst.

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