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Developing highly active cluster catalysts for the bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is significant for future renewable energy technology. Here, we employ first-principles calculations combined with a genetic algorithm to explore the activity trends of transition metal clusters supported on C2N. Our results indicate that the supported clusters, as bifunctional catalysts for the OER and the ORR, may outperform single-atom catalysts. In particular, the C2N-supported Ag6 cluster exhibits outstanding bifunctional activity with low overpotentials. Mechanistic analysis indicates that the activity of the cluster is related to the number of atoms in the active site as well as the interaction between the intermediate and the cluster. Accordingly, we identify a descriptor that links the intrinsic properties of the clusters with the activity of both the OER and the ORR. This work provides guidelines and strategies for the rational design of highly efficient bifunctional cluster catalysts.

Cobalt oxidation is a relevant deactivation pathway of titania-supported cobalt catalysts used in Fischer-Tropsch synthesis (FTS). To work towards more stable catalysts, we studied the effect of the surface area of the titania support and noble metal promotion on cobalt oxidation under simulated high conversion conditions. Mössbauer spectroscopy was used to follow the evolution of cobalt during reduction and FTS operation as a function of the steam pressure. The reduction of the oxidic cobalt precursor becomes more difficult due to stronger metal-support interactions when the titania surface area is increased. The reducibility was so low for cobalt on GP350 titania (surface area 283 m2/g) that the catalytical activity was negligible. Although cobalt was more difficult to reduce on P90 titania (94 m2/g) than on commonly used P25 titania (50 m2/g), the Co/P90 catalyst showed increased resistance against cobalt sintering and higher FTS performance than Co/P25. The addition of platinum to Co/P90 led to a higher reduction degree of cobalt and a higher cobalt dispersion, representing a catalyst with promising performance at relatively low steam pressure. Nevertheless, the stronger cobalt-titania interactions result in more extensive deactivation at high steam pressure due to oxidation., RST/Fundamental Aspects of Materials and Energy, RID/TS/Instrumenten groep

The mechanism of n-hexadecane (n-C16) hydroisomerization/hydrocracking was studied over bifunctional catalysts employing Pd and Pt as (de)hydrogenation components and large-pore zeolites and (ordered) mesoporous materials as acidic supports. Zeolite Y and Beta supports were compared to amorphous silica-alumina as well as aluminium-containing ordered mesoporous MCM-41, MCM-48 and SBA-15 materials to cover a wide range of pore sizes and topologies. Products were analyzed in terms of mono- and di-branched C16 isomers and cracked products as a function of the n-C16 conversion. All samples followed a similar mechanism in which, at low conversion, di-branched isomers with methyl groups toward the ends of the tetradecane chain were observed. At higher conversion, the products shifted to isomers with methyl groups closer to the center at higher n-C16 conversion. Consistent with these differences, the typical ‘M’ shape distribution of cracked products was obtained at low conversion, which evolved into ideal symmetric cracking patterns at higher conversion. The unusual di-branched isomer distribution at low conversion and the corresponding ‘M’ shaped cracked products distribution have earlier been associated with pore mouth catalysis induced by restricted diffusion in medium-pore zeolites. Here we show that such reaction pathways also occur in catalysts with large enough pores to exclude diffusion restrictions.

The role of Lewis and Brønsted acid sites in the Diels-Alder cycloaddition (DAC) of ethylene to 2,5-dimethylfuran (2,5-DMF) to p-xylene was investigated. Amorphous silica catalysts containing Al3+ (ASA), Ga3+ (ASG), and In3+ (ASI) were prepared via homogeneous deposition-precipitation. Silica modified with Zr4+ (ASZ) was prepared by impregnation. Their acidic properties were characterized by various IR and NMR spectroscopic techniques. Measurements using pyridine as a probe molecule highlighted the presence of mostly Lewis acid sites (LAS) in all materials. Using CO as a probe, in contrast, demonstrated the existence of Brønsted acid sites (BAS) in ASA and ASG, which were nearly absent in ASI and ASZ. Differences in basic strength can explain the contrast in results observed between the two probe molecules. The highest p-xylene yield (~20%) in the DAC reaction, could be achieved with ASA and ASG. The lack of BAS in ASI and ASZ resulted in inferior performance in the DAC, with p-xylene yields below 5%. These results indicate the importance of BAS for the DAC reaction. Several other heterogeneous and homogeneous catalysts were explored for the DAC reaction to show the generality of our conclusion that BAS play a critical role in obtaining p-xylene from 2,5-DMF and ethylene.

Hierarchical zeolites can offer substantial benefits over bulk zeolites in catalysis. A drawback towards practical implementation is their lengthy synthesis, often requiring complex organic templates. This work describes an accelerated synthesis of nanolayered MWW zeolite based on the combination of interzeolite transformation (IZT) with a dual-templating strategy. FAU zeolite, hexamethyleneimine (HMI), and cetyltrimethylammonium bromide (CTAB) were respectively employed as Al source and primary zeolite, structure directing agent, and exfoliating agent. This approach allowed to reduce the synthesis of nanolayered MWW to 48 h, which is a considerable advance over the state of the art. Tracking structural, textural, morphological, and chemical properties during crystallization showed that 4-membered-ring (4MR) units derived from the FAU precursor are involved in the faster formation of MWW in comparison to a synthesis procedure from amorphous precursor. CTAB restricts the growth of the zeolite in the c-direction, resulting in nanolayered MWW. Moreover, we show that this approach can speed up the synthesis of nanolayered FER. The merits of nanolayered MWW zeolites are demonstrated in terms of improved catalytic performance in the Diels-Alder cycloaddition of 2,5-dimethylfuran and ethylene to p-xylene compared to bulk reference MWW sample.