Effect of pH on the Nitrite Hydrogenation Mechanism over Pd/Al2O3 and Pt/Al2O3: Details Obtained with ATR-IR Spectroscopy

It is well-known that activity and selectivity to N2 during nitrite hydrogenation over noble metal catalysts in water depend on the pH of the solution, but mechanistic understanding is lacking. Attenuated total reflection infrared (ATR-IR) spectroscopy is an ideal tool to perform detailed studies on catalytic surfaces in water. In this paper, the influence of pH was studied on adsorption and subsequent hydrogenation of nitrite in water between pH 5 and 9 over Pd/Al2O3 and Pt/Al2O3, using ATR-IR spectroscopy. On both catalysts, pH clearly influenced the surface coverage and reaction rates of intermediates. For Pt/Al2O3, lowering the pH induced the increasing surface coverage of key reaction intermediates like NOsteps1620 cm−1 and “HNO”(ads)1540 cm−1, as well as increased hydrogenation rates, explaining the higher TOF at lower pH as reported in the literature. For Pd/Al2O3, the effect of pH on selectivity is controlled by the rate constants of the formation and hydrogenation of the most stable reaction intermediates to N2 (NO(ads)1720 cm−1) and NH4+ (NH2(ads)1510 cm−1)

Engineering hybrid nanotube wires for high-power biofuel cells

Poor electron transfer and slow mass transport of substrates are significant rate-limiting steps in electrochemical systems. It is especially true in biological media, in which the concentrations and diffusion coeffi cients of substrates are low, hindering the development of power systems for miniaturized biomedical devices. In this study, we show that the newly engineered porous microwires comprised of assembled and oriented carbon nanotubes (CNTs) overcome the limitations of small dimensions and large specific surface area. Their improved performances are shown by comparing the electroreduction of oxygen to water in saline buffer on carbon and CNT fi bres. Under air, and after several hours of operation, we show that CNT microwires exhibit more than tenfold higher performances than conventional carbon fi bres. Consequently, under physiological conditions, the maximum power density of a miniature membraneless glucose / oxygen CNT biofuel cell exceeds by far the power density obtained for the current state of art carbon fi bre biofuel cells