Water Is Not Universally Promotional: Size‐Dependent Inversion of Water Effects in Methanol Steam Reforming

Water is widely assumed to be universally promotional in methanol steam reforming. Here, we show that this assumption fails when metal nuclearity and support hydroxyl chemistry are considered. Using a well‐defined set of Pt single‐atom and nanoparticle catalysts supported on hydroxyl‐rich CeO 2 and hydroxyl‐poor Al 2 O 3 , we uncover a striking, size‐dependent inversion of water effects. On CeO 2 ‐supported Pt nanoparticles, water activation generates surface hydroxyls that withdraw electron density from Pt, weaken CO adsorption and alleviate CO poisoning, thereby enhancing activity. In contrast, the same hydroxyl species inhibit methanol conversion on single‐atom Pt by competing for adsorption sites and stabilizing kinetically trapped formate intermediates. These opposing effects vanish on Al 2 O 3 , where water activation is limited. This size‐dependent inversion extends to other single‐atom catalysts and reducible supports (e.g., Pd 1 /CeO 2 , Rh 1 /CeO 2 ), confirming its generality and establishing a predictive framework for understanding and controlling water effects in reforming catalysis.