Manganese dioxide (MnO) has been widely used as an active material for high-performance supercapacitors due to its high theoretical capacitance, high cycling stability, low cost, and environmental friendliness. However, the effect of its crystallographic phase on charge storage performances and mechanisms is not yet clear. Herein, MnO-based supercapacitors with different structures including nanospheres, nanorods, nanotubes, and nanosheets have been fabricated and investigated. Among such structures, δ-MnO nanosheets exhibit the highest specific capacitance of 194.3 F g at 1 A g when compared with other phases and shapes. The maximum specific energy of the δ-MnO nanosheet supercapacitor is 23.4 W h kg at 971.6 W kg and the maximum specific power is 4009.2 W kg at 15.9 W h kg with a capacity retention of 97% over 15 000 cycles. The δ-MnO nanosheet mainly stores charges via a diffusion-controlled mechanism at the scan rates of 10-100 mV s, whilst the α-MnO with different morphologies including nanospheres, nanorods, and nanotubes store charges via a non-faradaic or non-diffusion controlled process especially at fast scan rates (50-100 mV s). Understanding the charge storage performance and mechanism of the MnO nanostructures with different crystallographic phases and morphologies may lead to the further development of supercapacitors.

The initial stage of glycerol conversion over H-ZSM-5 zeolite has been investigated using density functional theory (DFT) calculations on an embedded cluster model consisting of 128 tetrahedrally coordinated atoms. It is found that glycerol dehydration to acrolein and acetol proceeds favourably via a stepwise mechanism. The formation of an alkoxide species upon the first dehydration requires the highest activation energy (42.5 kcal mol(-1)) and can be considered as the rate determining step of the reaction. The intrinsic activation energies for the first dehydration are virtually the same for both acrolein and acetol formation, respectively, suggesting the competitive removal of the primary and secondary OH groups. A high selectivity to acrolein at moderate temperatures can be attributed to the selective activation of the stronger adsorption mode of glycerol through the secondary OH group and the kinetically favoured subsequent consecutive steps. In addition, the less reactive nature of acrolein relative to acetol precludes it from being converted to other products upon conversion to glycerol. In accordance with typical endothermic reactions, the forward rate constant for glycerol dehydration significantly increases with increasing reaction temperature.

Activation of methane has attracted a great deal of interest in laboratory chemical synthesis and in large-scale industrial processes. We performed density functional theory studies to investigate the C–H bond breaking of methane on Au+ and Au2 + ions in vacuum and inside different types of zeolites. The density functional M06-L and the 6-31G(d,p) basis set were employed as this level of theory had already been shown to be reasonably accurate and affordable for transition metal systems. We investigated four industrially important catalysts, ZSM-5, FAU, FER, and MCM-22, each with a particular framework topology, with respect to their performance for methane activation. The bicoordinated character of the cationic site in the ZSM-5 structure provides a higher activity than the FAU structure with a 3-fold coordination of its cationic site. The activation energy of the reaction catalyzed by Au-ZSM-5 is lower than the one with the bare Au+ cation (13.2 vs 21.3 kcal/mol) because of the structural constraint imposed by the zeolite that leads to an earlier transition state with a high charge difference of the C–H atoms where the bond is broken. It is also found that the activity of Au n + decreases already with n = 2, due to the shared positive charge. For the zeolites with large pores, Au-MCM-22 provides a higher activity due to the spacious framework of this particular type of zeolite is perfect for stabilizing the transition state structure but not the corresponding adsorption complex. The small and medium pore-sized zeolites, Au-FER and Au-ZSM-5 stabilize both the adsorption complex and the transition states, thus causing the activation energy to remain the same.