Role of defects in carbon materials during metal-free formic acid dehydrogenation
Commercial graphite (GP), graphite oxide (GO), and two carbon nanofibers (CNF-PR24-PS and CNF-PR24-LHT) were used as catalysts for the metal-free dehydrogenation reaction of formic acid (FA) in liquid phase. Raman and XPS spectroscopies demonstrated that the activity is directly correlated with the defectiveness of the carbon material (GO> CNF-PR24-PS> CNF-PR24-LHT> GP). Strong deactivation phenomena were observed for all the catalysts after 5 minutes of reaction. Density functional theory (DFT) calculations demonstrated that the single vacancies present on the graphitic layers are the only active sites for FA dehydrogenation while other defects such as double vacancies and Stone Wales (SW) defects, rarely adsorb FA molecule. Two different reaction pathways were found, one passing through a carboxyl species and the other through an hydroxymethylene intermediate. In both mechanisms, the active sites were poisoned by an intermediate species such as CO and atomic hydrogen, explaining the catalyst deactivation observed in the experimental results.