Proposed formation mechanism and active species of hydrogen molecules generated from a novel magnesium–citric acid–hydroxypropyl cellulose coating (MgCC) material

Abstract

The presence of acids is known to accelerate the reaction (Mg + 2H2O = Mg(OH)2 + H2). We developed a novel Mg–citric acid coating (MgCC) material produced by milling Mg powder coated with hydroxypropyl cellulose (HPC); because of its H2 generation, this material could be used in antioxidant therapy and antiaging applications. After milling in the presence of citric acid, this material produced H2-rich water upon addition to cooled water. Although the reaction was considered to involve a two-electron transfer from Mg to 2H2O, the role of the acid in H2 generation remains incompletely understood. To clarify the reaction mechanism, we performed studies on the deuterium kinetic isotope effects (KIE) and electron spin resonance (ESR). We observed differences in the concentration ratios, such as H2/D2 > 1 and H2/(H2 + D2 + HD) > 1, involved in H2, D2, and (H2 + D2 + HD) production, and found that adducts with hydrogen atoms (H) were not obtained from the spin-trapping reaction between 5-(2, 2-Dimethyl-1,3-propoxy cyclophosphoryl)-5-methyl-1-pyrroline N-oxide (CYPMPO) and the MgCC material. The H2, D2, and HD produced from MgCC were identified by using a gas chromatograph connected to a mass spectrometer. The spin-trapping techniques showed that the H adducts formed by the reaction of NaBH4 with CYPMPO could not be observed from reaction of MGCC with CYPMPO in H2O. The data suggest that the rate-controlling step and proposed transition state (TS) exist in the reaction pathway of the O–H bond cleavage and H–H bond formation. A TS of a structure such as [Mg(OH2)2]∗ could be expected in the reaction pathway between Mg and 2H2O by density functional theory calculations. Also, these results show that H2 generation is accelerated in the presence of acids because the activation energy of the TS is significantly smaller than that of H2O.