TEMPO‐Oxidized Cellulose Nanofibers as Pseudocatalysts for in Situ and on‐Demand Hydrogen Generation via Aluminum Powder/Pure Water Reactions at a Temperature below 50 °C

Abstract

Cellulose nanofibers (CNFs) prepared via 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO)‐mediated oxidation of the C6 primary hydroxyls of native cellulose to carboxylates are used as pseudocatalysts for enhancing the aluminum powder/pure water reactions. The Al powder/pure water reaction is a stepwise reaction. It starts from hydration of the outmost native Al2O3 thin layer and then the reaction of the inner metallic Al with water. At lower temperatures (<50 °C), OH− and Al3+ ions are the preliminary products of the native Al2O3 thin layer hydration. Once Al powders are mixed with pure water containing 0.1–0.5 wt% TEMPO‐CNFs, condensed networks consisting of TEMPO‐CNFs self‐establish over the native Al2O3 thin layer. Al3+ ions are captured by TEMPO‐CNFs via the formation of insoluble Al3+/TEMPO‐CNFs complexing nanostructures and the conjugated OH− ions are being restricted nearby the native Al2O3‐based thin layer via electrostatic repulsion. A highly alkaline condition (pH > 11) is dynamically generated, and as a result, the native Al2O3 thin layer dissolves rapidly via the reaction with OH− ions. The OH− ions function also as catalysts, accelerating the reaction of metallic Al with water. Al powders (2–200 μm) react promptly and a nearly 100% Al/H2 conversion is obtained at the reaction temperature below 50 °C.