Non‐oxidative dehydroaromatization of methane (MDA) is a promising catalytic process for direct valorization of natural gas to liquid hydrocarbons. The application of this reaction in practical technology is hindered by a lack of understanding about the mechanism and nature of the active sites in benchmark zeolite‐based Mo/ZSM‐5 catalysts, which precludes the solution of problems such as rapid catalyst deactivation. By applying spectroscopy and microscopy, it is shown that the active centers in Mo/ZSM‐5 are partially reduced single‐atom Mo sites stabilized by the zeolite framework. By combining a pulse reaction technique with isotope labeling of methane, MDA is shown to be governed by a hydrocarbon pool mechanism in which benzene is derived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation.

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The active site requirements for methane dehydroaromatization by Mo/HZSM-5 were investigated by employing as catalysts physical mixtures of Mo-bearing supports (HZSM-5, SiO 2 , γ-Al 2 O 3 , and activated carbon) and HZSM-5. Separation of the two catalyst components after activation or reaction was possible by using two different sieve fractions. Our comparison demonstrates that migration of volatile Mo oxides into the micropores of HZSM-5 is at the origin of the observed catalytic synergy in methane dehydroaromatization for physical mixtures. The propensity of Mo migration depends on the activation method and the Mo−support interaction. Migration is most pronounced for Mo/SiO 2 . Prolonged exposure of HZSM-5 zeolite to Mo oxide vapors results in partial destruction of the zeolite framework. Mo carbide dispersed on nonzeolitic supports afforded predominantly coke with only very small amounts of benzene. The main function of the zeolite is to provide a shape-selective environment for the conversion of methane to benzene. A comparison of Mo/HZSM-5 and Mo/Silicalite-1 demonstrates that aromatization of methane is an intrinsic ability of molybdenum carbides dispersed in the 10-membered-ring micropores of MFI zeolite. Thus, one important role of the Brønsted acid sites is to promote the dispersion of the Mo oxide precursor and, accordingly, the active Mo carbide phase in the micropores of HZSM-5.

Surface carbon (coke, carbonaceous deposits) is an integral aspect of methane dehydroaromatization catalyzed by Mo/zeolites. We investigated the evolution of surface carbon species from the beginning of the induction period until the complete catalyst deactivation by the pulse reaction technique, TGA, 13C NMR, TEM, and XPS. Isotope labeling was performed to confirm the catalytic role of confined carbon species during MDA. It was found that “hard” and “soft” coke distinction is mainly related to the location of coke species inside the pores and on the external surface, respectively. In addition, MoO3 species act as an active oxidation catalyst, reducing the combustion temperature of a certain fraction of coke. Furthermore, after dissolving the zeolite framework by HF, we found that coke formed during the MDA reaction inside the zeolite pores is essentially a zeolite-templated carbon material. The possibility of preparing zeolite-templated carbons from the most available hydrocarbon feedstock is important for the development of these interesting materials.

In organic solar cells, photoexcitation of the donor or acceptor phase can result in different efficiencies for charge generation. We investigate this difference for four different 2-pyridyl diketopyrrolopyrrole (DPP) polymer-fullerene solar cells. By comparing the external quantum efficiency spectra of the polymer solar cells fabricated with either [60]PCBM or [70]PCBM fullerene derivatives as acceptor, the efficiency of charge generation via donor excitation and acceptor excitation can both be quantified. Surprisingly, we find that to make charge transfer efficient, the offset in energy between the HOMO levels of donor and acceptor that govern charge transfer after excitation of the acceptor must be larger by ∼0.3 eV than the offset between the corresponding two LUMO levels when the donor is excited. As a consequence, the driving force required for efficient charge generation is significantly higher for excitation of the acceptor than for excitation of the donor. By comparing charge generation for a total of 16 different DPP polymers, we confirm that the minimal driving force, expressed as the photon energy loss, differs by about 0.3 eV for exciting the donor and exciting the acceptor. Marcus theory may explain the dichotomous role of exciting the donor or the acceptor on charge generation in these solar cells.