Analysis of the fluorescence of mechanically processed defect-laden hexagonal boron nitride and the role of oxygen in catalyst deactivation

Abstract

The use of hexagonal boron nitride (h-BN) as a non-metal heterogeneous catalyst has been a popular subject in research since the discovery of its catalytic properties in 2016. Previous work found that an activation step was necessary for producing an effective catalyst. Density functional theory (DFT) calculations indicate defect sites, such as nitrogen (V N ) and boron (V B ) vacancies, bind favourably to olefins, hydrogen, and oxygen. In particular, the visible fluorescence intensity of processed h-BN increased with the length of exposure to air. The fluorescence behaviour of dh-BN powders when exposed to air after exposure to species such as argon, propene, and carbon dioxide is presented. Density of state calculations for molecular and atomic oxygen bound to V N and V B show that this increase in fluorescence may be due to atomic oxygen binding to V N . The fluorescence emission behaviour observed in dh-BN powders and its relationship to DOS of oxygen species bound to catalytically active defect sites provides a better understanding of potential deactivation modes for catalysts based on dh-BN.

Mechanically Enhanced Catalytic Reduction of Carbon Dioxide over Defect Hexagonal Boron Nitride

“…A previous study of the effects of oxygen on dh -BN showed strong binding of molecular O 2 in defect sites V N and V B of −4.28 and −1.38 eV, respectively . Nevertheless, TPD produced no oxygen desorption signals.…”

“…A previous study of the effects of oxygen on dh-BN showed strong binding of molecular O 2 in defect sites V N and V B of −4.28 and −1.38 eV, respectively. 36 Nevertheless, TPD produced no oxygen desorption signals. The binding energies of dissociated oxygen in V N and V B defect sites are strong: − 9.32 and −2.86 eV, respectively.…”

“…Nevertheless, TPD produced no oxygen desorption signals. The binding energies of dissociated oxygen in V N and V B defect sites are strong: – 9.32 and −2.86 eV, respectively . This strong binding energy suggests any atomic oxygen trapped in nitrogen vacancy (thus, bound to B atom) will surely remain trapped.…”

“…The binding energies of dissociated oxygen in V N and V B defect sites are strong: − 9.32 and −2.86 eV, respectively. 36 This strong binding energy suggests any atomic oxygen trapped in nitrogen vacancy (thus, bound to B atom) will surely remain trapped. However, atomic oxygen trapped in a boron vacancy (bound to N) has a possibility for desorption as NO gas at elevated temperatures.…”

“…It is important to note that the presence of oxygen on defect sites inhibits hydrogenation. 36 The anoxic conditions required for dh-BN activity are ideal for CO 2 reduction as the presence of oxygen in a CO 2 stream would reduce overall reaction efficiency, as combustion of hydrogen would be favored over reduction of CO 2 .…”