Spectroscopic Identification of Hydrogen Spillover Species in Ruthenium-Modified High Surface Area Carbons by Diffuse Reflectance Infrared Fourier Transform Spectroscopy

In recent years, carbon-based sorbents have been recognized for their potential application within vehicular hydrogen storage applications. One method by which sorbents have been reported to store appreciable hydrogen at room temperature is via a spillover process: where molecular hydrogen is first dissociated by metal nanoparticle catalysts and atomic hydrogen subsequently migrates onto the carbon substrate. Many reports have invoked the spillover mechanism to explain enhancements in reversible room temperature hydrogen uptake for metal-decorated sorbents. However, there is a lack of experimental evidence for the proposed chemical species formed as well as several differing theoretical explanations describing the process. In this report, we utilize diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to identify the various chemical species formed upon room temperature H₂ charging of ruthenium-decorated high surface area carbons. Room temperature H₂ loading of a control sample with no ruthenium nanoparticles (Ru NPs) leads to broad reversible peaks in the DRIFTS spectrum that correspond to the vibration-rotation transitions of weakly bound physisorbed hydrogen molecules. In contrast, the sample modified with Ru NPs shows a variety of reversible and irreversible peaks in addition to the physisorbed H₂ peaks. Rigorous experimental and theoretical analysis enables the assignment of the peaks to ruthenium-mediated formation of water, surface hydroxyl groups (R-OH, where R = carbon or ruthenium), and C-H bonds. The low-energy DRIFTS peaks assigned to spillover C-H bonds were additionally confirmed using inelastic neutron spectroscopy. Reversible vibrational peaks consistent with ruthenium-mediated formation of C-H bonds provide much-needed spectroscopic evidence for the spillover process. The results demonstrated here should facilitate future mechanistic investigations of hydrogen sorption on transition metal nanoparticles and high surface area activated carbons.