Efficient Overall Water Splitting under Visible-Light Irradiation on (Ga_1-xZn_x)(N_1-xO_x) Dispersed with Rh−Cr Mixed-Oxide Nanoparticles:  Effect of Reaction Conditions on Photocatalytic Activity

Abstract

The photocatalytic activity of (Ga(1-x)Zn(x))(N(1-x)O(x)) loaded with Rh-Cr mixed-oxide (Rh(2-y)Cr(y)O3) nanoparticles for overall water splitting under visible-light irradiation (lambda > 400 nm) is investigated with respect to reaction pH and gas pressure. The photocatalytic performance of the catalyst is found to be strongly dependent on the pH of the reactant solution but largely independent of gas pressure. The present photocatalyst exhibits stable and high photocatalytic activity in an aqueous solution of pH 4.5 for 72 h. The photocatalytic performance is much lower at pH 3.0 and pH 6.2, attributable to corrosion of the cocatalyst and hydrolysis of the catalyst. The dispersion of Rh(2-y)Cr(y)O3 as a cocatalyst on the (Ga(1-x)Zn(x))(N(1-x)O(x)) surface promotes hydrogen evolution, which is considered to be the rate-determining step for overall water splitting on this catalyst.

Photocatalytic Water-Splitting Reaction from Catalytic and Kinetic Perspectives

“…is useful for investigating whether a cocatalyst affects hydrogen or oxygen evolution sites. A good example is demonstrated for Rh 2-y Cr y O 3 cocatalysts on (Ga 1-x Zn x )(N 1-x O x ), as listed in Table 1 [49]. (Ga 1-x Zn x )(N 1-x O x ) without cocatalyst loading exhibited no activity for the photocatalytic HER, even in the presence of an electron donor (methanol).…”

“…This experiment indicated that Rh 2-y Cr y O 3 functions as an efficient hydrogen evolution site. Another important feature of the Rh 2-y Cr y O 3 cocatalyst is its high selectivity for HER [49]. The water-splitting rate is significantly reduced in the presence of oxygen when RuO 2 is used as a cocatalyst because the photoreduction of oxygen competes with HER on RuO 2 .…”

“…This change in the water-splitting rate cannot be explained by the apparent activation energy alone. Side reactions, such as oxygen reduction, compete with hydrogen evolution on RuO 2 , whereas Rh 2-y Cr y O 3 is selectively active for hydrogen evolution [49]. As a result, the water-splitting rate on RuO 2 /(Ga 1-x Zn x )(N 1-x O x ) decreases drastically in the presence of oxygen.…”

Hydrogen Generation through Solar Photocatalytic Processes: A Review of the Configuration and the Properties of Effective Metal-Based Semiconductor Nanomaterials

“…Even for the same base photocatalyst, the optimal pH depends on the co-catalyst due to hydrolysis and corrosion phenomena [156,160].…”

Functional Materials for Sustainable Energy Technologies: Four Case Studies

“…Several visible-light-driven photocatalyst materials loaded with cocatalysts such as Cr x Rh 2Àx O 3 /(Ga 1Àx Zn x )(N 1Àx O x ), Ru/SrTiO 3 :RhBiVO 4 , and graphitic carbon nitrides have been developed recently with a photocatalytic response at about 500 nm, but their quantum yields are still only a few percent. [34,[46][47][48] Photocatalytic hydrogen production by water splitting is an example of more general conversion processes that involve evolution of oxygen as one component and formation of chemical fuels as the other. An important example that is the focus of intense research is the photocatalytic reduction of carbon dioxide to methane, methanol, or ethanol.…”