Interaction of D2 with Ti-adsorbed polyaniline and implication for hydrogen storage.

Interaction of D2 with Ti-adsorbed polyaniline (PANI) and TiO2 at low Ti coverages has been investigated by temperature programmed desorption, Auger electron spectroscopy, and H2-D2 exchange in a high pressure cell. In contrast to recent DFT calculations (Lee et al. Phys. Rev. Lett. 2006, 97, 56104-56107) that have shown a multiple number of molecular hydrogen chemisorption on Ti-decorated PANI, only 0.52 D2 molecules per Ti atomically adsorb on Ti-deposited PANI at 87 K at a nominal Ti coverage of 2 ML. Broad TPD spectra of D2, HD, and H2 were observed, which showed a common peak at 250 K and another isotope-dependent peak at higher temperature. Ti deposited on TiO2 forms clusters with a size distribution at 87 K, on which D2 atomically adsorbs. As the Ti coverage increases, the D2 desorption peak gradually shifts from 200 to 260 K and the number of D2 molecules adsorbed per Ti also increases from 0.26 at 0.1 ML to 0.87 at 2 ML. We attribute this to the fact that D2 adsorbs on larger clusters with a greater adsorption energy. TPD spectra of D2 desorbing from Ti (2 ML)/PANI and Ti (1 ML)/TiO2 look very similar to each other. This led us to conclude that Ti also adsorbs on PANI in clusters. We suggest that no molecular D2 adsorption on Ti/PANI at 87 K is due to a reduced electron backdonation ability of Ti upon clustering. The small number of adsorbed D2 per Ti on both substrates was ascribed to irreversible D2 adsorption only on relatively large Ti clusters, while D2 adsorbs reversibly on small clusters. This was indirectly confirmed by the observation that continuous H2-D2 isotope exchange occurs to produce HD on Ti/PANI as well as on Ti/TiO2 at 106 K in a high pressure cell at 5.7 mTorr. Desorption mechanisms involving H atom abstraction from PANI by Ti clusters are proposed to account for the complex isotope-dependent TPD spectra. Implication of the present results for hydrogen storage based on Ti-dispersed polymers is briefly addressed.