The once reported Ag-modified CaTiO3 photocatalyst was reexamined by optimizing the Ag loading amount and using a conventional photochemical reactor. This revealed that the Ag-modified CaTiO3 photocatalyst actually showed both high production rate of CO (54 µmol h −1) and excellent selectivity toward CO formation (94%) by suppressing the H2 production via water splitting. It is suggested that the high photocatalytic performance originates from not only the optimized amount of cocatalyst and the high irradiation light intensity but also the high concentration of dissolved CO2 that was achieved by a bubbling flow of CO2 at the lower reaction temperature. These reaction conditions provided ca. 40 times higher CO formation rate. It was proposed that the deposited small Ag nanoparticles are the selective active sites for CO formation and the CaTiO3 crystal surface produces H2 preferably.
To obtain more efficient photocatalyst for photocatalytic reduction of CO2 with H2O and figure out the reason for nonstoichiometric O2 evolution, silver-loaded sodium titanate photocatalysts were further studied in the improved reaction conditions. After preliminary tests for two kinds of sodium titanate samples with different ratio of sodium to titanium (Na2Ti6O13 and Na2Ti3O7), several sodium hexatitanate (Na2Ti6O13) photocatalysts were further prepared in the flux method by changing the parameters such as the flux, the loading amount of Ag cocatalyst, and the loading method of the Ag cocatalyst. As a result, a Ag/Na2Ti6O13 sample prepared in a sodium chloride flux, with 1.0 wt% of Ag cocatalyst loaded by a photodeposition method, exhibited the highest production rate (4.6 μmol h −1 ) and the highest selectivity (74%) to carbon monoxide among the examined samples, which are more than 29 times higher production rate and 2.7 times higher selectivity to carbon monoxide than those in the previous report. Furthermore, although required oxygen production rate equivalent to the formation rates of the reduced products was not observed in the previous study, it was found that the developed Ag/Na2Ti6O13(NaCl) photocatalyst produced enough amount of oxygen after a long induction period of 50 h in the present condition. The reasons for the insufficient oxygen formation in the initial period were also investigated and clarified, i.e., the chloride residues and the photoadsorption of O2 on the surface are responsible for the insufficient O2 evolution at the initial period.
Carbon
monoxide (CO) is an important feedstock for the chemical
industry, and hydrogen peroxide (H2O2) is also
an important chemical with versatile applications. Here, a photocatalytic
system was discovered for the simultaneous production of CO and H2O2 from CO2 and H2O with
both high activity and selectivity without any externally applied
voltage and any consumption of chemical compounds, which was promoted
by using a calcium titanate (CaTiO3) photocatalyst modified
with a silver–manganese oxide (Ag–MnO
x
) dual cocatalyst. Although a Ag/CaTiO3(CTO) photocatalyst
was reported to reduce CO2 with water to form CO and O2, the coexistence of MnO
x
species
with silver nanoparticles (NPs) not only improved the CO formation
rate more than 2 times with a selectivity of 76% but also changed
the selectivity of the oxidative reaction and forms H2O2 instead of O2 with a high selectivity of 99%.
The Ag NPs promoted the CO2 reduction to CO by the photoexcited
electrons, and the Mn(III) oxide species deposited on the CaTiO3 surface contributed to the water oxidation to H2O2 by the positive holes in the aid of HCO3
– as a reaction mediator. The produced CO is easily
separated from the aqueous solution to the gas phase, and the H2O2 is stably stored in the aqueous HCO3
– solution. This photocatalytic system can utilize
the stable and ubiquitous molecules (CO2 and H2O) to produce reactive and useful molecules (CO and H2O2), concurrently, meaning that it can convert photoenergy
to storable chemical energy to increase sustainability.
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