No abstract

Highly porous gallium oxide was synthesized by reconstructing its surface and body with mesopores and macropores. For the first time, the efficient photocatalytic conversion of CO 2 into a high energy carrier, CH 4 , using the porous gallium oxide was realized without any co-particle or sacrificial reagent. The enhanced photocatalytic activity is mainly attributed to the 300% higher CO 2 adsorption capacity, as well as the 200% increased surface area, compared to the bulk nanoparticles. Furthermore, we propose the new reaction pathway based on the result that the carbon dioxide was converted directly into methane without going through carbon monoxide intermediates.Due to the fact that the earth's current primary energy source is obtained through the combustion of fossil fuels, resulting in pollution and a climate change, the idea of artificial photosynthesis that uses carbon dioxide (CO 2 ) to produce hydrocarbon fuels would offer an alternative durable source of energy. 1-5 Among these hydrocarbon fuels, a promising candidate fuel is methane (CH 4 ), since it carries a high amount of energy per mass (55.7 kJ g À1 ). In order to convert CO 2 into CH 4 , it is important to develop a novel photocatalyst that uses solar energy and has a high affinity for CO 2 . [6][7][8] Grimes et al. 9 studied a photocatalytic conversion of CO 2 using TiO 2 nanotubes, enabling charge carriers to readily reach surface species. Meanwhile, the photoactivity on the titania system was limited by its insufficient reduction potential 5 and high recombination rate of photo-generated electron-hole pairs. 10,11 Therefore, new hetero-structures combining with noble metals 12 such as Pt 13 and Ru 14 have been suggested to be used for the separation of electron-hole pairs, but there is a problem that novel metals are expensive. However, metal oxide photocatalysts with d 10 configurations (In 3+ , Ga 3+ , Ge 4+ , Sn 4+ , Sb 5+ ) 15,16 are of great attention because hybridization between the s and p orbital of metals in the conduction band could enhance the mobility of photogenerated electrons, thus, producing high photocatalytic activity. 17 Among them, gallium oxide (Ga 2 O 3 ) is a promising CO 2 reduction photocatalyst due to its high reduction potential for CO 2 . 18-21 Tanaka et al. 20 carried out the photoreduction of CO 2 using H 2 as a reductant over the bulk crystal of b-Ga 2 O 3 , but found that it gave only CO as the product. Consequently, the further breakthrough on gallium oxide for the conversion of CO 2 into CH 4 has remained unsolved.Herein, we report a novel porous gallium oxide with meso-pores and macropores. The small amount of photocatalytic reaction sites, which is the biggest problem 22 with working with gallium oxide, was solved by reconstructing its surface and body with mesopores and macropores, combining the template method with hydrolysis of gallium nitrate. The synthetic procedures of the new porous Ga 2 O 3 and its CO 2 conversion processes are illustrated in Scheme 1. The efficient photocatalytic conversion of CO 2 ...

Electrochemical alcohol oxidation is considered a promising alternative to the oxygen evolution reaction due to the production of high-value products and early onset potential. Herein, we analyze the different reactivities of NiOOH and Cu(OH) 2 toward the electrochemical oxidation of alcohol and aldehyde on the furan ring and utilize their characteristics synergistically to enhance the performance of 5-hydroxymethylfurfural (HMF) to 2,5furandicarboxylic acid (FDCA) conversion. We discovered that Cu(OH) 2 has higher reactivity for the oxidation of aldehyde to carboxylic acid than NiOOH, while NiOOH exhibited excellent reactivity toward the oxidation of alcohol to aldehyde. Furthermore, NiOOH−Cu(OH) 2 mixed electrodes showed higher activity and faster conversion of HMF to FDCA than individual NiOOH or Cu(OH) 2 electrodes. The alcohol oxidation of HMF is initiated by NiOOH, and Cu(OH) 2 quickly converts the remaining aldehydes to carboxylic acids at the NiOOH/Cu(OH) 2 interface. Further enhancement of the HMF oxidation kinetics of NiOOH/Cu(OH) 2 was achieved by preparing a nanofoam structure comprising nanoscale pores and nanodendritic frames, showing instantaneous conversion to FDCA without producing unreacted intermediates.