The structure sensitivity of metal catalysts and rational catalyst design are always hot topics in heterogeneous catalysis. In recent years, the single-site catalyst (usually called single-atom catalyst, SAC) has attracted intense interest because of its ultimate metal utilization efficiency and coordination structure controllability. However, in more heterogeneous catalysis processes, the breaking and formation of specific chemical bonds require limited but continuous metal-metal bonding sites on the surface of catalysts. At the same time, the specific electronic properties of active metal centers and the arrangement of the metal-metal sites with multiple atoms (such as the C7 site of iron catalyst and the B5 site of ruthenium catalyst in ammonia synthesis), have a definite influence during the specific catalytic process, which is known as the “ensemble effect”. In the previous perspective article (ACS Cent. Sci. 2021, 7, 262), the authors proposed the concept of the fully-exposed cluster catalyst (FECC), which is expected to achieve both the high metal utilization efficiency and the ensemble effect with limited but continuous multi-sites. And the FECC could provide a theoretical basis and research paradigm for the catalyst structure design in specific reaction processes.
Fig. 1 Pd FECC using in the dehydrogenation of 12H-N-ethylcarbazole, exhibits excellent activity due to the high metal utilization efficiency and ideal metal electronic properties (Nat. Catal. doi: 10.1038/s41929-022-00769-4)
Hydrogenation utilization by means of in situ catalytic dehydrogenations of liquid organic hydrogen carriers (LOHC) serves a great purpose of hydrogen transfer more efficiently and safely. Among the reported LOHC candidates, N-ethylcarbazole/dodecahydro-N-ethylcarbazole (NEC/DNEC) constitute a promising medium for reversible hydrogen storage and utilization due to the mild dehydrogenation temperature and many other physicochemical advantages. Supported Pd catalysts have been shown to possess excellent intrinsic reactivity in the dehydrogenation of DNEC as compared to other group-VIII metals. However, as limited by the structural analysis tools available, the intrinsic reactivities of different supported Pd species (for example, Pd single atoms, Pd clusters with different coordination numbers, as well as different sized Pd particles) remain ambiguous. Consequently, screening the optimal Pd structures, from the single-atom level to the nanometer-scale in a reliable way, for the different stages of dehydrogenation of DNEC would be of great significance. To meet this end, Ding Ma’s group works closely with collaborators from University of Chinese Academy of Sciences, Southern University of Science and Technology, and Institute of Metal Research. By utilizing defect-rich nanodiamond (ND) as support material, the authors managed the synthesis of a series of catalysts from Pd single atoms to Pd ensembles with few atoms, atomic-layered clusters, and 2–10 nm NPs. With combined single-atom sensitive electron microscopy, single-site sensitive optical spectroscopy, and bulk sensitive X-ray absorption spectroscopy, it is able to quantitatively determine the statistical site distributions of different Pd species within different Pd/ND catalysts. The authors further establish the correlation between the average Pd-Pd coordination numbers (C.N.Pd-Pd) of Pd species and the site-specific TOFs. The authors show that the optimal site is the fully-exposed Pd cluster with an average Pd-Pd coordination number around 4.4, favoring both the activation of reactants and desorption of products, whereas the Pd single-atoms are almost inactive (Fig. 1). The study highlights that for certain catalytic reactions, the construction of fully-exposed metal cluster without the presence of spectators (i.e., Pd single-atoms in this work), could help to maximize the reactivity and the atomic efficiency of noble metals, which is extremely important for the fabrication of highly-efficient hydrogen production catalyst for next generation.
Fig. 2 FECC has the best activity in cyclohexane dehydrogenation process due to the ensemble effect of Pt catalysts (J. Am. Chem. Soc. 2022, 144, 8, 3535–3542)
The ensemble effect in heterogeneous catalysis was also studied with Pt catalysts in cyclohexane dehydrogenation reaction. Although the Pt-SAC has the highest metal atom utilization efficiency, it remains inactive at 553 K. In contrast, both Pt cluster and nanoparticles could catalyze the dehydrogenation of cyclohexane, but the larger particles have lower activity. Notably, the average Pt-Pt coordination number of FECC is about 2–3, showing the best catalytic performance (Figure 2). The ensemble requirement on the C―H bonds activation process and the product toxicity were considered as the primary reasons combined with the experiments and theoretical calculations. The Pt FECC could achieve an optimal balance between the two factors, which proved the importance of Pt FECC consisting of few Pt atoms in cyclohexane catalytic dehydrogenation.
Fig. 3 The effect of structural heterogeneity of Rh catalysts on cyclohexanol dehydrogenation and different structural requirements of multistep reactions (J. Am. Chem. Soc. 2022, 144, 11, 5108–5115)
The authors discovered that the supported rhodium catalyst, with isolated Rh1 sites and ensembled Rh sites (Rhe, including Rh cluster, Rhn, and Rh nanoparticles, Rhp), has a much higher reactivity for the reaction of cyclohexanol dehydrogenation to phenol than single-atom (Rh1) or nanoparticle catalysts (Rhp). It was also demonstrated that the isolated Rh species (Rh1) is extremely active in the first step of dehydrogenation, the conversion of cyclohexanol to cyclohexanone, but it cannot catalyze the subsequent reaction step, the conversion of cyclohexanone to phenol. In contrast, the Rh ensemble sites (Rhe) are extremely efficient at forming phenol. Only through the coexistence of Rh1 and Rhe could optimal reaction performance be achieved (Fig. 3).
These works were supported by the Ministry of Science and Technology, National Natural Science Foundation of China, and Beijing National Laboratory for Molecular Sciences.
The first authors of the series works include Meng Wang (associate professor, School of Chemistry and Molecular Engineering, Peking University), and Chunyang Dong, Jie Zhang, Yu Guo (postdoctoral fellow, School of Chemistry and Molecular Engineering, Peking University), Mi Peng, Yuchen Deng (Ph.D., School of Chemistry and Molecular Engineering, Peking University), Zirui Gao (Ph.D. candidate, School of Chemistry and Molecular Engineering, Peking University); Zhimin Jia (Ph.D. candidate, Institute of Metal Research, Chinese Academy of Sciences); Jincheng Liu, (Ph.D., Tsinghua University); Yinlong Li (Ph.D. candidate, Southern University of Science and Technology).
Original link for the papers: https://www.nature.com/articles/s41929-022-00769-4
https://pubs.acs.org/doi/full/10.1021/jacs.1c12261
https://pubs.acs.org/doi/full/10.1021/jacs.2c00202
