Photocatalytic Production of H₂ from Biomass

Laboratory Setup and Methods

Photocatalytic hydrogen evolution experiments were carried out in a 16 mL sealed quartz cuvette under irradiation with a 405 nm LED. The reaction suspension contained the carbon nitride–based photocatalyst and a selected biomass-derived sacrificial electron donor in an alkaline aqueous solution of 10 mL. Prior to illumination, the system was purged with argon to remove hydrogen.

Hydrogen production was quantified by measuring the hydrogen partial pressure in the cuvette headspace. For this purpose, a Clark-type hydrogen sensor in a needle for piercing was used. After irradiation, the cuvette was thoroughly shaken to equilibrate hydrogen between the liquid phase and headspace, and the sensor was inserted into the headspace for the measurement. The total amount of hydrogen produced in the cuvette was calculated from the hydrogen partial pressure using the ideal gas law and Henry’s law.

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Calculations

By thoroughly shaking the cuvette prior to the headspace measurement, equilibrium between the liquid and gas phases was established, resulting in identical hydrogen partial pressures in both phases. The amount of hydrogen in the 6 mL headspace was calculated using the ideal gas law. An example calculation using an arbitrary hydrogen partial pressure of 1500 Pa is shown here.

The hydrogen present in the gas and liquid phases can be summed to determine the total amount of hydrogen in the cuvette. With 3.69 μmol of hydrogen in the 6 mL gas phase and 0.12 μmol in the 10 mL liquid phase, the liquid-phase hydrogen accounts for approximately 3.25% of the total hydrogen, highlighting the low solubility of hydrogen in water.

Hydrogen production from the carbon nitride-based photocatalyst in the presence of a biomass-derived sacrificial electron donor in alkaline aqueous solution was quantified via gas-phase measurements. During irradiation, equilibration between the liquid and gas phases was not performed continuously, resulting in a modest increase in the measured hydrogen partial pressure in the headspace (Fig. 2).

Gas-phase measurements obtained after equilibration showed higher hydrogen partial pressures, as hydrogen dissolved in the liquid phase redistributed into the headspace. These end-point measurements were used to calculate the total amount of hydrogen produced for the different photocatalyst compositions.

Carbon nitride photocatalysts loaded with varying amounts of cobalt as a cocatalyst were evaluated for hydrogen production. Photocatalysts containing 0.3% and 0.6% (wt.%) cobalt showed the highest hydrogen yields, reaching nearly three times the hydrogen production of the cobalt-free control (Fig. 3). At higher cobalt loadings, hydrogen production decreased, which is attributed to partial blocking of the limited active sites on the carbon nitride surface and reduced light absorption.

The sensor-based approach enabled sensitive, real-time detection of hydrogen evolution in the sealed cuvette. In this study, gas-phase measurements were used to calculate the total hydrogen produced; however, the same sensor approach can be applied to measure hydrogen directly in both the liquid and gas phases simultaneously, enabling validation of phase equilibration and further improving measurement accuracy.