Water splitting into H2 and O2 under visible light was achieved using simple organic dyes such as coumarin and carbazole as photosensitizers on an n-type semiconductor for H2 evolution, a tungsten(VI) oxide (WO3) photocatalyst for O2 evolution, and a triiodide/iodide (I3(-)/I(-)) redox couple as a shuttle electron mediator between them. The results on electrochemical measurements revealed that the oxidized states of the dye molecules having an oligothiophene moiety (two or more thiophene rings) in their structures are relatively stable even in water and possess sufficiently long lifetimes to exhibit reversible oxidation-reduction cycles, while the carbazole system required more thiophene rings than the coumarin one to be substantially stabilized. The long lifetimes of the oxidized states enabled these dye molecules to be regenerated to the original states by accepting an electron from the I(-) electron donor even in an aqueous solution, achieving sustained H2 and I3(-) production from an aqueous KI solution under visible light irradiation when they were combined with an appropriate n-type semiconductor, ion-exchangeable layered niobate H4Nb6O17. The use of H4Nb6O17 loaded with Pt cocatalyst inside the interlayer allowed the water reduction to proceed preferentially with a steady rate even in the presence of a considerable amount of I3(-) in the solution, due to the inhibited access of I3(-) to the reduction site, Pt particles inside, by the electrostatic repulsion between the I3(-) anions and the negatively charged (Nb6O17)(4-) layers. It was also revealed that the WO3 particles coloaded with Pt and IrO2 catalysts exhibited higher rates of O2 evolution than the WO3 particles loaded only with Pt in aqueous solutions containing a considerable amount of I(-), which competitively consumes the holes and lowers the rate of O2 evolution on WO3 photocatalysts. The enhanced O2 evolution is certainly due to the improved selectivity of holes toward water oxidation on IrO2 cocatalyst, instead of undesirable oxidation of I(-). Simultaneous evolution of H2 and O2 under visible light was then achieved by combining the Pt/H4Nb6O17 semiconductor sensitized with the dye molecules having an oligothiophene moiety, which can stably generate H2 and I3(-) from an aqueous KI solution, with the IrO2-Pt-loaded WO3 photocatalyst that can reduce the I3(-) back to I(-) and oxidize water to O2.
Photocatalytic splitting of water into H2 and O2 under visible light irradiation is achieved using a coumarin dye-adsorbed lamellar niobium oxide for hydrogen evolution
We aim to achieve
resource recycling by capturing and using CO
2
generated
in a chemical production and disposal process.
We focused on CO
2
conversion to CO by the reverse water
gas shift–chemical looping (RWGS-CL) reaction. This reaction
proceeds in two steps (H
2
+ MO
x
⇆ H
2
O + MO
x
–1
; CO
2
+ MO
x
–1
⇆
CO + MO
x
) via a metal oxide that acts
as an oxygen carrier. High CO
2
conversion can be achieved
owing to a low H
2
O concentration in the second step, which
causes an unwanted back reaction (H
2
+ CO
2
⇆
CO + H
2
O). However, the RWGS-CL process is difficult to
control because of repeated thermochemical redox cycling, and the
CO
2
and H
2
conversion extents vary depending
on the metal oxide composition and experimental conditions. In this
study, we developed metal oxides and simultaneously optimized experimental
conditions to satisfy target CO
2
and H
2
conversion
extents by using machine learning and Bayesian optimization. We used
transfer learning to improve the prediction accuracy of the mathematical
models by incorporating a data set and knowledge of oxygen vacancy
formation energy. Furthermore, we analyzed the RWGS-CL reaction based
on the prediction accuracy of each variable and the feature importance
of the random forest regression model.
Mn and Li promoted Rh catalysts supported on SiO 2 with a thin TiO 2 layer were synthesized by stepwise incipient wetness impregnation approach. The thin TiO 2 layer on the surface of SiO 2 was proved to stabilize those small Rh nanoparticles and hinder their agglomeration. The reducibility of Rh on these catalysts depends on Rh particle size as well as the position of manganese oxide, and large Rh nanoparticles with MnO on Rh nanoparticles can be only reduced at an elevated temperature. Catalyst with large Rh particles exhibits a higher CO conversion and higher products selectivity towards long chain hydrocarbons and C2-oxygenates at the expense of decreasing methane formation than a similar catalyst with smaller Rh particles. This was attributed to the synergistic effect of Mn and Li promotion and molar ratio between Rh 0 and Rh δ+ sites on the surface of Rh nanoparticles. Moreover, Rh nanoparticles on MnO are proved to be more efficient in promoting hydrogenation of acetaldehyde to ethanol than its counterpart with MnO on Rh nanoparticles. Finally, in order to target high C2-oxygenates selectivity, low reaction temperature together with a low H 2 /CO ratio in the feed is recommended.
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