Can hydrogen be generated by UV- photodegradation of biomass residues in water media?

The photoinduced processes can be promising routes for hydrogen generation. This study explores hydrogen production through the non-catalyzed photodegradation of various biomass materials, a process that remains largely understudied compared to the catalyzed ones, i.e., photoreforming. Using as precursor almond shell (AS), a lignocellulosic biomass residue, various solid and liquid materials obtained from it were tested as substrates. These materials were obtained through different pretreatment methods including grinding, milling, pyrolysis, and hydrothermal carbonization (HTC), and compared with milled cellulose (MC). Photodegradation tests, conducted in aqueous media under UV light, revealed that hydrogen production strongly depends on the structural and compositional features of the substrates. Among the solid samples, ground almond shell (GAS) and milled cellulose (MC) showed promising hydrogen yields. However, the liquid residue from the HTC process using diluted phosphoric acid (HMAS-L2), which is rich in simple organic acids, stood out, delivering the highest hydrogen production across all the substrates, and reaching an impressive value of 105 μmol of H₂ in 5 h of reaction. Attention was also given to the production of other gases, particularly carbon dioxide and methane, as a result of the photodegradation. CO₂ production occurred for all the substrates. PMAS (the pyrolyzed milled almond shell) and, specifically, HMAS-L2 generated detectable amounts of CH₄ (5 and 22 μmol, respectively).

The H₂/CO₂ ratios reached 0.66 for MC and 0.44 for HMAS-L2, highlighting the interest in evaluating the non-catalyzed biomass photodegradation as a preliminary step for future photoreforming studies. These findings enhance our understanding of biomass-based hydrogen generation and open new avenues for exploring non-catalyzed photoinduced processes.