N2 as a Quantitative FTIR Probe Molecule for Reliable Assessment of the Dispersion of Ru Supported on Hydroxyapatite

Accurate assessment of the metal dispersion is known to be important in the field of catalysis, as these data are used to calculate the turnover frequency of a given reaction on a given metal. This study shows that N2 can be used as an FTIR probe molecule not only qualitatively but also quantitatively. For the first time, it is found that the integrated molar absorption coefficient of N2 adsorbed on the HAp supports (epsilonN2-HAp = 0.092±0.008 cm/µmol) does not depend on the chemical composition of HAp, while that of N2 adsorbed on the HAp-supported Ru0 nanoparticles of about 2.2 nm (epsilonN2-Ru0: 0.5-1.03 cm/µmol) is sensitive to the chemical composition of HAp. This study also shows, for the first time, that epsilonN2-Ru0 increases with increasing electron density of the Ru0 nanoparticles, as monitored by the position of the maximum of the absorption band of the N2 stretching vibration at about 2200 cm-1 (nuN2-Ru0), with a remarkable correlation between epsilonN2-Ru0 and nuN2-Ru0. The epsilonN2-Ru0 data are two orders of magnitude smaller than the previously reported integrated molar absorption coefficients of CO (epsilonCO-Ru0). Finally, epsilonN2-HAp and epsilonN2-Ru0 allow us to estimate the Ru dispersion, which is in remarkable agreement with that determined by TEM.