On April 21, 2022, Beijing time, Professor Martin of Peking University, Professor Zhou Wu of the University of Chinese Academy of Sciences, Liu Hongyang, researcher of the Institute of Metal Research of the Chinese Academy of Sciences, and Associate Professor Wang Yanggang of southern University of Science and Technology co-published an article entitled “Fully-exposed palladium cluster catalysts enable hydrogen production from nitrogen” on Nature Catalysis heterocycles” research results.
The researchers found that the atomic-scale dispersion of fully exposed palladium clusters, due to their ultra-high metal utilization and ideal metal electronic properties, and meet the multi-site “ensemble requirement” favorable for multi-step dehydrogenation reactions, make them exhibit excellent catalytic activity and cycle stability in the catalytic dehydrogenation of organic hydrogen-carrier molecules (12H-N-ethylcarbazole).
Dong Chunyang, a postdoctoral fellow at Peking University, Gao Zirui, a doctoral candidate, Peng Mi, a postdoctoral fellow, and Li Yinlong, a doctoral student at Southern University of Science and Technology, are the co-first authors of the paper.
The structural sensitivity of metal catalysts has always been a hot topic in the field of heterogeneous catalysis. In the past decade, in pursuit of higher utilization of metal atoms, metal catalysts with unit points (often referred to as single-atom catalysts, or single-atom catalysts — SACs) have received widespread attention in the field of catalysis. Although SACs theoretically have 100% metal atomic utilization and a relatively controllable coordination structure and exhibit excellent reaction performance in partial catalytic reactions, in more heterogeneous catalytic reactions, the rupture or formation of specific chemical bonds requires the surface of the metal catalyst to provide a limited but continuous metal-metal bonding site; at the same time, the specific electronic properties of the metal eigengen. The arrangement of metal-metal loci including multiple atoms, such as the C7 center of the iron-based catalyst in the synthesis of ammonia and the B5 center of the ruthenium-based catalyst, are critical to obtaining excellent catalyst performance. In a previous perspective article (ACS Cent. Sci. 2021, 7, 262), the research team proposed that the fully-exposed metal cluster catalyst (FECC) is expected to meet both the high atomic utilization of the metal catalyst and the limited but continuous multi-point “group requirements” that favor the catalytic reaction, thereby achieving “simultaneous acquisition of fish and bear paws”. But for specific catalytic reactions, quantitative studies of metal structure sensitivity from the nanoscale to the atomic scale are critical.
The use of liquid organic hydrogen storage medium (LOHC) for efficient catalytic hydrogen production is of great significance to the safe and efficient transportation of hydrogen, an important link in hydrogen energy utilization. As the most representative liquid organic hydrogen storage medium, 12H-N-ethylcarbazole / N-ethyl carbazole (DNEC / NEC) has attracted wide attention due to its many excellent physicochemical properties (Figure 1) and low dehydrogenation temperature. Studies have shown that Pd-group heterogeneous catalysts have higher reactivity in the dehydrogenation of DNEC compared with other VIII group metals. However, due to the lack of effective structural analysis and statistical methods, the intrinsic reaction performance of different Pd species on the catalyst (such as Pd single atoms, Pd nanoparticles, and Pd clusters with different coordination numbers) has not been accurately determined. Therefore, how to establish an effective method to investigate the structure-activity relationship of cross-scale Pd species and screen out the optimal catalytic activity site required for the reaction is the key to developing a cost-effective LOHC dehydrogenation catalyst.
Figure 1: Physicochemical property advantages of 12H-N-ethylcarbazole / N-ethylcarbazole as an organic liquid hydrogen storage medium.
In view of the above problems, the research team selected the surface-rich nanodiamond (ND) as the support to achieve the accurate construction of the Pd species structure on the ND surface, and obtained a catalyst containing a series of main structural units, in order: Pd single atom (Pd1/ND), low-number fully exposed Pd cluster (Pdn1/ND), fully exposed Pd cluster (Pdn2/ND), and 2-10 nm Pd nanoparticles (Pdp/ND) (Figure 2. a-f）。 It should be noted that there are also a large number of Pd single atoms in Pd catalysts (Pdn1/ND, Pdn2/ND and Pdp/ND) with fully exposed clusters and nanoparticles as the main structural units, which increases the difficulty of confirming the intrinsic reactivity of different sites.
Based on the differences in CO adsorption patterns of different Pd species, the research team quantitatively depicted the dispersion state of Pd on the surface of different catalysts by infrared spectroscopy, and the proportion of Pd species with different structures in the catalyst could be estimated (Figure 2. g）。 The results showed that in addition to the differences in the structure of the main Pd species, the proportion of Pd single atoms on the surface gradually decreased as the dispersion of the catalyst decreased. This quantitative analysis method of different Pd species on the surface of the carrier lays the foundation for establishing the relationship between the average coordination number (C.N.Pd-Pd) and site-specific TOF of Pd species at different scales.
Figure 2: HAADF-STEM plot (a-f) and CO-probe infrared spectrogram (g) of different Pd/ND catalysts.
Figure 3: Coordination structure and electron structure analysis of different Pd/ND catalysts.
In the step-by-step dehydrogenation reaction of DNEC, based on clear structural properties and site distribution statistics, the research team established the relationship between different Pd species and their site-specific TOFs using the Pd-Pd mean coordination number (C.N.Pd-Pd) of Pd species as a structural descriptor (Figure 4.a). In the first and final steps of the dehydrogenation reaction, the fully exposed Pd cluster site (its C.N.Pd-Pd is 4.4) compared with the larger Pd particles (C.N.Pd-Pd 6.9), while ensuring extremely high atomic utilization, exhibits a higher site-specific reactivity (site-specific TOF), and its fully exposed nature, so that the normalization to the activity of all metals (Pd-normalized activity) is currently more than 6 times the best catalyst reported. In contrast, the Pd single atom exhibits extremely low (near zero) catalytic activity, which makes it not substantially contribute to the catalytic reaction when coexisting with the Pd cluster in the Pd catalyst, but consumes a precious metal load, exhibiting a “bystander” nature. Theoretical calculations further show that the Pd single-atom catalyst cannot be efficiently activated due to the absence of adjacent metal bonding sites, and the strong adsorption between the product (*NEC) and the metal crystal plane representing the large size Pd nanoparticle makes it difficult for the product to desorb from the Pd nanoparticle. The fully exposed Pd cluster sites, which combine high metal utilization and ideal metal electronic properties, and meet the limited but continuous multi-point “group requirements” that are beneficial to the reaction, can effectively catalyze the activation of the reactants and the desorption of the products, resulting in optimal catalytic dehydrogenation activity (Figure 4. b, c）。
Figure 4: (a) Structure-activity diagram of different Pd species in the initial dehydrogenation reaction of DNEC with C.N. Pd as the structural descriptor. (b) Dehydrogenation reaction pathways and barrier trends of DNEC on Pd1/G, Pd13/G and Pd(111) surfaces. The corresponding transition state model during dehydrogenation is shown in (c).
The research work highlights how to balance the two important parameters of metal atomic utilization and ensemble effect in the design of precious metal catalysts. At the same time, the paper also pointed out that the structure of the metal activity site required for different reactions is not the same, and the precise structural control of the catalyst is very critical to the maximum utilization of precious metal atoms. For the DNEC dehydrogenation reaction that this work is concerned about, it is particularly important to accurately synthesize fully exposed Pd clusters with specific coordination numbers with uniform structures to avoid wasting precious metals on onlookers such as Pd single-atom species.
Figure 5: Group effect of Pt catalyst in cyclohexane dehydrogenation process, FECC has optimal activity (J. Am. Chem. Soc. 2022, 144, 8, 3535–3542）
The authors also investigated the catalyst structure requirements for dehydrogenation of cyclohexane, a reactant with a smaller size, on a Pt-based catalyst. For cyclohexane dehydrogenation reactions, although Pt-SAC has the highest metal atomic utilization efficiency, it is still inactive even at 553 K. In contrast, both Pt cluster and nanoparticle catalysts can catalyze this reaction, but larger particles are less intrinsically active. Notably, the average Pt–Pt coordination of fully exposed Pt FECC is about 2–3, showing optimal catalytic performance. Combining experimental results and theoretical calculations, the difference in activity of this series of catalysts is believed to come from the group requirements during the activation of the C-H bond and the toxicization of the product, and FECC can achieve an optimal balance between these two factors, demonstrating the importance of FECC composed of a small number of platinum atoms to catalyze the cyclohexane dehydrogenation reaction (Figure 5).
Figure 6: The influence of structural heterogeneity of Rh catalyst on cyclohexanol dehydrogenation process, and the tandem reaction has different requirements for structure (J. Am. Chem. Soc. 2022, 144, 11, 5108–5115）
In the cyclohexanol dehydrogenation reaction, the authors found that the supported Rh catalyst prepared by the traditional method of combining the single atom Rh1 site and the Rh collection site (Rhe, including the cluster Rh, nanoparticle Rhep) has better catalytic activity than the catalyst with only a single atom Rh1 site or only a Rh pool site. Further studies have found that due to the relative ease of activation of the first C-H bond of cyclohexanol, the single-atom Rh1 site exhibits much higher activity than the Rhe catalyst in the first step of dehydrogenation (cyclohexanol to cyclohexanone), but is extremely low in the subsequent reaction step (cyclohexanone to phenol), while the Rhe collection site exhibits excellent catalytic activity in the cyclohexanone to phenol reaction step. The optimal cyclohexanol dehydrogenation activity can only be obtained if both Rh1 and Rhe are present in the catalyst (Figure 6). This phenomenon illustrates the important role of species heterogeneity of active metals in catalytic reactions containing multi-step catalytic steps, and points out the direction for metal catalyst design. (Source: Science Network)
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