TY - JOUR

T1 - Mechanisms of Photocatalytic Molecular Hydrogen and Molecular Oxygen Evolution over La-Doped NaTaO3 Particles: Effect of Different Cocatalysts and Their Specific Activity

AU - Ivanova, I.

AU - Kandiel, T.A.

AU - Cho, Y.-J.

AU - Choi, W.

AU - Bahnemann, D.

N1 - Funding information: We gratefully acknowledge financial support by the BMBF (Bundesministerium für Bildung und Forschung), initiative Hycats (No. 033RC1012A), and the Global Research Laboratory (GRL) Program (No. NRF-2014K1A1A2041044) funded by the Korea Government (MSIP) through the National Research Foundation (NRF). We thank H. C. Starck for providing the La-doped NaTaO3 materials for the current study. The authors thank Dr. Ralf Dillert and Dr. Amer Hakki for stimulating discussions and for help in the preparation of the manuscript. We gratefully acknowledge financial support by the BMBF (Bundesministerium fu.r Bildung und Forschung), initiative Hycats (No. 033RC1012A), and the Global Research Laboratory (GRL) Program (No. NRF-2014K1A1A2041044) funded by the Korea Government (MSIP) through the National Research Foundation (NRF).

PY - 2018/3/2

Y1 - 2018/3/2

N2 - A better understanding of the mechanisms of H 2 and O 2 evolution over cocatalyst-loaded photocatalysts is an essential step in constructing efficient artificial systems for the overall water splitting. In this paper, La-doped NaTaO 3 particles loaded with different cocatalysts (i.e., noble metals and metal oxides) have been synthesized and used as model photocatalysts to study the mechanisms of photocatalytic H 2 and/or O 2 evolution from pure water, aqueous methanol solution, and aqueous silver nitrate solution. It was found that the photocatalytic activity and selectivity toward H 2 and/or O 2 evolution strongly depend on the nature of the cocatalyst and the investigated system. For pure water and aqueous silver nitrate systems, the affinity of the cocatalyst nanoparticles to react with the photogenerated charge carriers (electrons or holes) was found to be the main reason for the observed selective behavior for H 2 and O 2 evolution. The creation of active sites and subsequent decrease in activation energy is thought to play a secondary role. In the presence of methanol, when the dark injection of an electron into the conduction band of the photocatalyst is possible, the catalytic roles of the investigated cocatalysts toward the formation of H 2 gas were found to be decisive, in addition to the charge separation and interfacial electron transfer processes. No overall water splitting into H 2 and O 2 can be achieved utilizing La-doped NaTaO 3 loaded with only one cocatalyst; however, it was found that the loading of La-doped NaTaO 3 with two different cocatalysts, i.e. RuO 2 and CoO, enables the simultaneous formation of H 2 and O 2 from pure water. The modification of photocatalyst with two different cocatalysts seems to be essential for enhancing the efficiency of overall photocatalytic splitting. The interfacial electron transfer on the cocatalyst-loaded La-doped NaTaO 3 was determined by measuring the cathodic and anodic photocurrents in the presence of Fe 2+/Fe 3+ electron shuttle. Methanol and bromate were used as electron donors and electron acceptors during the cathodic and anodic photocurrent measurements, respectively. By correlation of the photocurrent with the activity of the investigated cocatalysts, it was concluded that the creation of active sites and subsequent decrease in activation energy for H 2 evolution is the main requirement for efficient H 2 evolution from the aqueous methanol system, whereas the interaction with the photogenerated holes and the formation of intermediates allowing a multielectron transfer process seems to be an essential step for the water oxidation and O 2 evolution. This information appears to be crucial for a rational design of a highly active photocatalyst for overall water splitting under UV-vis illumination.

AB - A better understanding of the mechanisms of H 2 and O 2 evolution over cocatalyst-loaded photocatalysts is an essential step in constructing efficient artificial systems for the overall water splitting. In this paper, La-doped NaTaO 3 particles loaded with different cocatalysts (i.e., noble metals and metal oxides) have been synthesized and used as model photocatalysts to study the mechanisms of photocatalytic H 2 and/or O 2 evolution from pure water, aqueous methanol solution, and aqueous silver nitrate solution. It was found that the photocatalytic activity and selectivity toward H 2 and/or O 2 evolution strongly depend on the nature of the cocatalyst and the investigated system. For pure water and aqueous silver nitrate systems, the affinity of the cocatalyst nanoparticles to react with the photogenerated charge carriers (electrons or holes) was found to be the main reason for the observed selective behavior for H 2 and O 2 evolution. The creation of active sites and subsequent decrease in activation energy is thought to play a secondary role. In the presence of methanol, when the dark injection of an electron into the conduction band of the photocatalyst is possible, the catalytic roles of the investigated cocatalysts toward the formation of H 2 gas were found to be decisive, in addition to the charge separation and interfacial electron transfer processes. No overall water splitting into H 2 and O 2 can be achieved utilizing La-doped NaTaO 3 loaded with only one cocatalyst; however, it was found that the loading of La-doped NaTaO 3 with two different cocatalysts, i.e. RuO 2 and CoO, enables the simultaneous formation of H 2 and O 2 from pure water. The modification of photocatalyst with two different cocatalysts seems to be essential for enhancing the efficiency of overall photocatalytic splitting. The interfacial electron transfer on the cocatalyst-loaded La-doped NaTaO 3 was determined by measuring the cathodic and anodic photocurrents in the presence of Fe 2+/Fe 3+ electron shuttle. Methanol and bromate were used as electron donors and electron acceptors during the cathodic and anodic photocurrent measurements, respectively. By correlation of the photocurrent with the activity of the investigated cocatalysts, it was concluded that the creation of active sites and subsequent decrease in activation energy for H 2 evolution is the main requirement for efficient H 2 evolution from the aqueous methanol system, whereas the interaction with the photogenerated holes and the formation of intermediates allowing a multielectron transfer process seems to be an essential step for the water oxidation and O 2 evolution. This information appears to be crucial for a rational design of a highly active photocatalyst for overall water splitting under UV-vis illumination.

KW - La-doped NaTaO

KW - cocatalysts

KW - hydrogen evolution

KW - photocatalysis

KW - sacrificial systems

KW - water splitting

UR - http://www.scopus.com/inward/record.url?scp=85042908175&partnerID=8YFLogxK

U2 - 10.1021/acscatal.7b04326

DO - 10.1021/acscatal.7b04326

M3 - Article

VL - 8

SP - 2313

EP - 2325

JO - ACS catalysis

JF - ACS catalysis

SN - 2155-5435

IS - 3

ER -