Written by: Chen Shaoqing
Column: Yanzhi Chengli Catalysis Club
Foreword
When the research on single atom catalysis is hot, we share a paper on diatomic cluster catalysis (title: Bottom - up precise synthesis of stable platinum dimers on graphene , DOI: 10.1038/s41467-017-01259-z).
Research background and significance
Heterogeneous catalyst is widely used in chemical industry, energy, environment, biomedicine and other fields. In recent years, research on supported metal single atom catalysts has been carried out in full swing, which simplifies the reaction conditions required for traditional catalysts and continuously refreshes the conversion efficiency of single metal atoms to catalytic substrates. Due to the continuous attention and investment of many researchers, the current synthesis methods, characterization techniques, catalytic mechanisms, theoretical studies, etc. of single-atom catalysts have been quite in-depth research. Although there are still many challenges, its basic structure has tended to be perfected. On the other hand, , a metal nanoscale cluster containing only a few atoms, has a discrete energy band structure. Changing one atom of an ultrasmall cluster is likely to drastically alter the electronic structure of the entire cluster and significantly alter its catalytic performance. Research on metal cluster model catalysis has confirmed this atom-dependent catalytic phenomenon. However, its preparation requires harsh environments such as ultra-high vacuum, making it difficult to prepare high specific surface area substrate-supported metal cluster catalysts required for practical catalytic applications. .
Therefore, although many studies have prepared single-atom catalysts, the synthesis of few-atom metal cluster catalysts is difficult to precisely control at the atomic level, such as the synthesis of diatomic and three-atom cluster catalysts with definite structures. The key reason why is difficult to synthesize few-atom cluster catalysts is the lack of means to precisely control the agglomeration process. We know that in the process of preparing single-atom catalysts, it is usually necessary to anchor metal single atoms with defect structures on the surface of the carrier or groups such as amino groups, and finally obtain single-atom catalysts. However, in the process of preparing few-atom clusters, it is necessary to further deposit and deposit only one other metal atom on the existing single atom surface. This not only ensures that each new atom is deposited on the first atom, but also prevents multiple atoms reunite at the same location. For example, this process is basically equivalent to shooting ten arrows on the target first, and then shooting ten arrows on or near the first ten arrows (~ 0.3 nm), and there cannot be three or more arrows. arrows are close to each other.
In this article, we share a research paper recently published in Nature Communications in collaboration with the research groups of Professor Wei Shiqiang and Professor Lu Junling from the University of Science and Technology of China. In this work, ALD technology was used to deposit Pt2 diatomic clusters on the surface of graphene with a large specific surface. The structure of the catalyst was determined using spherical aberration-corrected transmission electron microscopy, X-ray absorption fine structure spectroscopy, and theoretical calculations. Subsequently, the performance of the catalyst in the hydrodehydrogenation reaction of ammonia borane, the reaction mechanism, and the structural stability of the diatomic cluster were studied.
Synthesis Regulation
First pretreat graphene to create phenol and phenol-carbonyl pairs on the surface to serve as nucleation sites for Pt species. The surface of graphene after treatment becomes defect-rich, with a specific surface area as high as 570 m2/g. Then the first Pt ALD process is carried out. By controlling the deposition conditions and benefiting from the self-limiting reaction characteristics of ALD technology, a single MeCpPtMe3 molecule is nucleated at a single phenol-related site on the carrier surface, and the surface organic groups are treated with O2 to obtain Pt single atom catalyst, the sample is named Pt1/Graphene. In the second step of the deposition process, also based on the self-limiting characteristics of ALD technology, the reaction conditions can be controlled so that a single MeCpPtMe3 molecule in the gas phase can only be adsorbed on the Pt1 single atom in the previous process. Finally, the surface ligands are removed through strong oxidant treatment to obtain Pt2 diatomic, the catalyst is named Pt2/Graphene. (There is a lot of work on the control of reaction conditions and structural regulation in the article, and the data is very rich).
structural characterization
single atom catalyst structural characterization mainly uses: 1) atomic level imaging technology to determine the position and size of the target, that is, spherical aberration correction transmission electron microscopy technology; 2) fine spectroscopy technology to detect the coordination environment of the target atom to distinguish single atoms The state is still a cluster state, while characterizing the interaction between the target atom and the carrier, that is, X-ray absorption fine structure spectroscopy; 3) Calculation and simulation of the existence state of a single atom and its interaction with the carrier, that is, density functional theory calculation (DFT) ). This article also uses these characterization methods and explains some special structural phenomena of diatomic clusters.
From the spherical aberration corrected electron microscope images, Pt1 single atoms with an average spacing of more than 2 nm (Pt1/Graphene) and Pt2 diatomic clusters with an atomic spacing of about 0.3 nm (Pt2/Graphene) can be clearly seen in different samples. In addition, ICP-AES data of multiple synthesis samples show that the Pt atomic ratio before and after the second ALD process is approximately 1:2, which statistically indicates that the gas phase MeCpPtMe molecules are at the Pt1 single atom site during the second deposition process. Nucleation (Although it is indirect evidence, it confirms the reliability of the data from a macro perspective and makes up for the shortcomings of micro-area observation by electron microscopy, which shows the rigor of the research).
The comparison and fitting data of the . This characterization result strengthens the observation results of transmission electron microscopy and obtains the existence state of Pt species in the catalyst. Subsequently, DFT calculations simulated the nucleation process and the final diatomic structural state during the catalyst preparation process, and corresponded to the synthesis process and XAS data.
Catalytic application
In this article, a diatomic catalyst is applied to the hydrolysis of NH3BH3 (AB) for hydrogen production. Comparative studies of multiple catalysts with the same Pt content show that the catalyst of diatomic cluster Pt 2 / Graphene can completely hydrolyze NH3BH3 within 0.9 min and produce the same amount of hydrogen production as the theoretical value. As a comparison, the Pt 1 /Graphene catalyst only decomposed ~42% of the substrate in 10.8 min; the nanoparticle-supported Pt/SiO2 catalyst required 6.8 min to completely hydrolyze NH3BH3. Calculation of the specific rates of each catalyst shows that the specific rate of the Pt2 diatomic catalyst is as high as 2800 mol H2 molPt−1min−1 at room temperature, which is 17 times and 45 times that of graphene-supported Pt1 single atoms and Pt nanoparticles, respectively.
Catalytic Mechanism Research
The paper uses DFT to conduct in-depth research on the catalytic mechanism, especially comparing the differences between the single-atom catalyst Pt1/Graphene and the di-atom catalyst Pt2/Graphene. Considering the strong reducibility of AB hydrolysis, the authors set the catalysts to partially reduced states when modeling, namely Pt1/Graphene-R and Pt2/Graphene-R. Calculations show that when AB is adsorbed on the second Pt atom of the Pt2/Graphene-R catalyst, the adsorption energy is -2.81 eV, which is weaker than the adsorption energy on the Pt1/Graphene-R catalyst (-3.20 eV), while AB is on Pt ( 111) The surface adsorption energy (-3.97 eV) is much stronger than that of single-atom or di-atom catalysts. On the other hand, the adsorption energy of AB hydrolysis product H2 on Pt2/Graphene-R (-1.29 eV) is also weaker than that on Pt1/Graphene-R (-2.42 eV). What’s more interesting is that calculations found that H2 molecules have chemical adsorption dissociation on Pt1/Graphene-R, but can only exist in the form of H2 molecules on Pt2/Graphene-R. In summary, the adsorption of AB and H2 by the top Pt atom of Pt2/Graphene-R is weaker than that of Pt1 single atom. These factors give it excellent catalytic performance.
stability characterization The stability of
catalyst includes the correspondence between stable performance and stable structure. In the paper, the catalyst was tested for cycle stability, and compared with the comparison sample diatomic catalyst, it had the highest catalytic stability. Spherical aberration electron microscopy testing showed that no Pt nanoparticles appeared in the catalyst after the catalytic test, and the diatomic cluster structure still dominated. After roasting at 300 oC or 400 oC, the Pt species in the catalyst are in the state of coexistence of nanoparticles, diatomic clusters, and single atoms, and the catalytic performance decreases significantly, but the specific rate is still as high as 1037 molH2 molPt−1 min−1.
Conclusion and impact
The paper is based on the self-limiting reaction characteristics of ALD technology and synthesizes a stable Pt2 diatomic composite graphene structure catalyst through precise control of the Bottom-up path. In the paper, there are in-depth studies on the regulation of the synthesis process, characterization of the structure, catalytic performance and mechanism.
