Benzaldehyde‐Promoted (Auto)Photocatalysis under Visible Light: Pitfalls and Opportunities in Photocatalytic H 2 O 2 Production**

Abstract Selective photooxidation of aromatic alcohols to corresponding aldehydes is a widely used model reaction for evaluation of performance of heterogeneous photocatalysts. A top example is the photocatalytic production of hydrogen peroxide via reduction of dioxygen with concomitant photooxidation of benzyl alcohol to benzaldehyde. Herein we demonstrate the ability of benzaldehyde to promote the auto‐photocatalytic oxidation of benzyl alcohol and produce large quantities of H 2 O 2 in solvent‐free (no water) or biphasic (with water) systems even under nominally visible (>420 nm cut‐off filter) light irradiation. These results shed critical light on the plethora of research reports on photocatalytic H 2 O 2 production in which benzyl alcohol was employed as electron donor. Since the autocatalytic pathway based on the photocatalytic activity of benzaldehyde formed during photocatalysis under such conditions cannot be neglected, the interpretations of photocatalytic performance are likely contentious and distorted in such reports. The use of benzyl alcohol as a model electron donor in photocatalytic studies should be therefore definitely discouraged. On the other hand, as we found that benzaldehyde‐mediated photocatalysis of H 2 O 2 production proceeds also using alternative electron donors ( e. g ., ethanol), our results highlight the possible prospects for a more general use of benzaldehyde and other aromatic aldehydes in visible light‐driven photocatalysis.