ABSTRACT
Spectroscopic methods enabling real-time monitoring of dynamic surface processes are a prerequisite for identifying how a catalyst triggers a chemical reaction. We present an in situ photoluminescence spectroscopy approach for probing the thermocatalytic 2-propanol oxidation over mesostructured Co3O4 nanowires. Under oxidative conditions, a distinct blue emission at â¼420 nm is detected that increases with temperature up to 280 °C, with an intermediate maximum at 150 °C. Catalytic data gained under comparable conditions show that this course of photoluminescence intensity precisely follows the conversion of 2-propanol and the production of acetone. The blue emission is assigned to the radiative recombination of unbound acetone molecules, the n â π* transition of which is selectively excited by a wavelength of 270 nm. These findings open a pathway for studying thermocatalytic processes via in situ photoluminescence spectroscopy, thereby gaining information about the performance of the catalyst and the formation of intermediate products.
ABSTRACT
In this work the photoluminescence (PL) of Co x Fe3-x O4 spinel oxide nanoparticles under pulsed UV laser irradiation (λ exc = 270 nm) is investigated for varying Co/Fe ratios (x = 0.4â¯2.5). A broad emission in the green spectral range is observed, exhibiting two maxima at around 506 nm, which is dominant for Fe-rich nanoparticles (x = 0.4, 0.9), and at around 530 nm, that is more pronounced for Co-rich nanoparticles (x > 1.6). As examinations in different atmospheres show that the observed emission reacts sensitively to the presence of water, it is proposed that the emission is mainly caused by OH groups with terminal or bridging metal-O bonds on the Co x Fe3-x O4 surface. Raman spectroscopy supports that the emission maximum at 506 nm corresponds to terminal OH groups bound to metal cations on tetrahedral sites (i.e., Fe3+), while the maximum around 530 nm corresponds to terminal OH groups bound to metal cations on octahedral sites (i.e., Co3+). Photoinduced dehydroxylation shows that OH groups can be removed on Fe-rich nanoparticles more easily, leading to a conversion process and the formation of new OH groups with different bonds to the surface. As such behavior is not observed for Co x Fe3-x O4 with x > 1.6, we conclude that the OH groups are more stable against dehydroxylation on Co-rich nanoparticles. The higher OH stability is expected to lead to a higher catalytic activity of Co-rich cobalt ferrites in the electrochemical generation of oxygen.
ABSTRACT
We report on the time-dependent influence of atmospheric species on the electrical properties of functionalized graphene sheets (FGSs). When exposed to laboratory air, FGSs exhibit a significant, irreversible decrease in electrical conductance with time, strongly depending on the oxygen content of the FGSs. To separate the roles of charge carrier density and mobility in this aging process, we performed electron transport measurements using a back-gate field-effect transistor architecture. Investigating the position of the Dirac point under different atmospheres, we found that adsorbed atmospheric species result in pronounced p-doping, which - on a short time scale - can be reversed under nitrogen atmosphere. However, on a time scale of several days, the resistance increases irreversibly, while the Dirac point voltage remains constant. From these experiments, we conclude that the aging of FGSs is related to the chemisorption of atmospheric species leading to enhanced carrier scattering due to an increasing amount of sp3- regions and thus to a reduced charge carrier mobility.
