Foreword
Many alloys tend to undergo an orderly transition from random solid solutions to fully ordered intermetallic compounds. The driving force of this ordered process is the chemical force between atoms of the same type and atoms of different kinds, i.e., the energy of different types of atoms occupying adjacent lattice positions is more favorable than that of atoms of the same type. The following figure takes the FEPT alloy unit cell as an example, and it can be seen that compared with the disordered FEPT alloy, the Fe and Pt atoms in the ordered Fept unit cell occupy the fixed lattice point, forming an ordered superlattice structure.
Due to the unique atomic order structure of the obtained intermetallic compounds, they usually exhibit more superior physical and chemical properties than disordered alloys, such as magnetism, catalytic activity, and chemical stability, according to the structure-activity relationship. For example, in 2021, Professor Liang Haiwei's research group at the University of Science and Technology of China and Professor Shui Jianglan's research group at Beihang University of Aeronautics and Astronautics collaborated to develop a high-temperature sulfur-anchored synthesis methodology, realizing the universal synthesis of small-sized intermetallic compound particles. The research results were published in the internationally renowned journal Science with the title of sulfur-anchoring synthesis of platinum intermetallic nanoparticle catalysts for fuel cells.
Recently, some studies claim that "boron" has more advantages than "sulfur" in helping the synthesis of small and diverse intermetallic catalysts!
Brief introduction of the results
Supported metal catalysts undergo rapid degradation under harsh conditions due to material failure and weak metal-supported interactions. Recently,Associate Professor Wang Bin and Professor Yang Shengchun of Xi'an Jiaotong University, and Professor Yao Yonggang of Huazhong University of Science and Technologyet al. proposed the use of reducing hydroborene in situ synthesis of small size (25 nm), high dispersion (up to 80 wt%pt), and good stability of the PT BC catalyst, which theoretically forms PT-B bonds that are about 5 times stronger than PT-C. Based on the PTB C support, a series (about 18) carbon loads with a size of less than 4 nm were synthesized.
Two-, three-, four- and five-membered PT intermetallic nanocatalysts. Due to the stable intermetallic compounds and strong metal-support interactions, annealing at 1000 °C does not result in sintering of the nanoparticles. At the same time, these intermetallic catalysts also exhibit better activity and stability in electrocatalytic oxygen reduction (ORR). Therefore, by introducing boron chemistry, intermetallic catalysts with small size, high loading, stable anchoring, and flexible composition can be efficiently synthesized, which provides the possibility for their application in other electrochemical reactions.
Related work is based on ".hydrogenated borophene enabled synthesis of multielement intermetallic catalysts" as the title in ".nature communications**.
**Reading guide
Figure 1Synthesis of PTB C with different loads
Hb is produced at room temperature by exfoliation and ion exchange between protons and magnesium cations in magnesium diboride (MgB2). It is assumed that the structure of Hb consists of a sp2-bonded boron plane to form a hexagonal boron network bridged by hydrogen atoms without long procedures, as shown in Figure 1a. Through a series of controlled experiments, it was confirmed that the hydrogen in HB has significant reducibility. This property enables the in-situ reduction of PGM ions into the form of small metal nanoparticles. The authors propose a two-step synthesis process to prepare catalysts, as shown in Figure 1a. First, the prepared HB is mixed with activated carbon to form the HB C support. Subsequently, a solution containing H2PTCl6 is introduced, in which the Pt4+ ions are reduced by the bridging hydrogen atoms in Hb. The growth of PT nanoparticles is based on PT clusters formed in situ that have been anchored to the C surface by the B layer. In general, increasing the loading of PT reduces the distance between the particles, thus promoting severe sintering through particle coalescence and Ostwald maturation.
However, in the present study, the PT nanoparticles remained uniform and small in size even though the loading percentage of the metal PT on carbon (specifically Ketjen Black, EPC-600JD, KB) increased from 10% to up to 80% (Figure 1B-F). Compared with other reported and commercialized PT C catalysts, the PT B C catalyst exhibits extremely small size and uniform high-density distribution with no significant aggregation at the same loading, suggesting that there is a strong interaction between the metal and the boron fragments formed in situ on the carbon surface, effectively inhibiting the sintering process between nanoparticles. In addition, this synthesis strategy is highly versatile, and metal nanoparticles can be easily loaded onto various supports, such as carbon nanotubes, graphene, Al2O3, TiO2, and CEO2.
Figure 2Spectroscopy analysis of PT-B interactions
Figure 2A shows the XPS spectrum of the PT BC sample, from which the different PT4F and B1S signals can be resolved. The high-resolution XPS spectra of PT 4F show that the binding energy (BE) of PT 4F shifts to a higher value relative to the metal PT, indicating that there is an electron transfer from PT to B. The B1S spectra obtained from the Pt Bc and Hb C samples are shown in Figures 2C, D. The B1S peak position in the Pt Bc sample is approximately 1Positive displacement of 1 EV. The results showed that after UV irradiation induced H2 release from HB, there was a similar positive shift in the position of the B1S peak. This suggests that the depletion of active H in Hb by Pt4+ also leads to a decrease in the electron density around the B atom, resulting in the observed positive shift of the peak.
In order to further verify the PT-B interaction in PT BC, the electronic structure and local coordination environment of PT atoms were investigated by PTl3 edge Xanes and Exafs on PTB C. Figure 2e shows the normalized spectra of PT L3 edge Xanes for different samples. The intensity of the white line in the spectrum as a function of the unoccupied PT 5d state reflects the oxidation state of PT in the sample. It can be seen that the white line intensity of PT B C is slightly higher than that of PT foil, indicating that the electron density of PT decreases, indicating that electrons are transferred from PT to B. The Exafs data are shown in Figure 2F, and the fitting results further confirm the presence of PT-B bonds in PT BC.
Figure 3Synthesis of PT-based IMCs based on PT BC
The strong interaction between PT-B bonds and PT-B reveals excellent resistance to sintering**. This enabled the authors to prepare a series of multi-component IMCS nanoparticles by high-temperature annealing. As shown in Figure 3a, a metal precursor of a specific molar ratio was first impregnated into the prepared 20 wt% PTPTBC. After drying, the powder precursor was annealed at different temperatures in a 5 vol% H2 AR gas mixture to induce the structural evolution of PT-based multi-element IMCs. High-temperature annealing provides the necessary activation energy for alloying and structural ordering, but often leads to severe particle aggregation under conventional conditions. In this experiment, the particles are stably anchored to the B c and remain small in size after high-temperature annealing. In this paper, binary (PTCO B C, PTFE B C, PTCU B C, PTCO3 B C, PTFE3 B C, PTCU3 B C), ternary (PT2FECO B C, Pt2feni B C, Pt2feCu B C, Pt2Coni B C, Pt2cocu B C, Pt2nicu B C), Quaternary (Pt3feconi B C, Pt3fecocu B C, Pt3Conicu B C), Five-element (pt4feconicu b c). The XRD spectra in Figure 3B show that all multi-component IMCs exhibit superlattice diffraction peaks with an ordered FCT structure.
The HAADF-STEM images obtained from the above samples confirm that the binary, ternary, quaternary, and quintary IMCS nanoparticles are uniformly distributed on carbon supports with a narrow size distribution (Figure 3C-F). The average size calculated at the nanoscale is concentrated around 4 nm, which is smaller than most carbon-loaded intermetallic PTMs reported in the literature (Figure 3G). The successful preparation of small-size multi-element IMCs can still be attributed to the formation of PT-B bonds, which effectively anchor and stabilize the nanoparticles even at high temperatures. This is also confirmed by the ExAFS test results at the edge of the PT L3, which demonstrate the good stability of the PT-B bond in a 1000 annealed PTCO BC sample in a 5% H2 AR atmosphere (Figure 3H). In addition, ExAFS analysis confirmed that the lattice shrinkage of the particles in PTCO BC.
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Figure 4HAADF-STEM image and EDS element mapping based on PT-based IMCS
Atomic-resolution HAADF-STEM images (Figure 4) show the analysis of IMC at the atomic scale. Since the intensity of the HAADF-STEM image is proportional to the atomic number Z, the pt atomic column is brighter than the transition metal atomic column. Periodic square structures are observed in all IMCs. In the case of PTCO, the lattice spacing of the nucleus is 037 nm, corresponding to the superlattice (001) surface of the L10 ordered structure PTCO. The lattice spacing of the shell is 023 nm, corresponding to the (111) side of PT. A similar ordered quadrilateral structure belonging to space **4 mmm was observed in other samples, as shown in Figure 4b-d and its inset. As can be seen, all samples form a nucleus containing alternating layers of PT and transition metals, and the surface is a thin PT shell consisting of 2-3 atomic layers. The formation of this thin PT shell can be attributed to acid treatment and post-annealing, in which PT with lower surface energy emerges as a shell.
Figure 5Electrocatalytic performance of PT-based IMCs
To evaluate the performance of PT-based multi-component intermetallic PTM B C electrocatalysts, cyclic voltammetry (CV) and linear scanning voltammetry (LSV) were used at room temperature (about 25) at 0The performance of electrocatalytic oxygen reduction (ORR) was tested in 1 M HCO4. Measurements were carried out using RDE. For comparison, the ORR performance of commercial PT C (JM, 20% PT) electrocatalysts was tested under the same conditions. The Hupd was used to determine the electrochemically active surface area (ECSA). The ECSAs for PTCO, PT2FECU, PT3CONICU, and PT4FECONICU are respectively. 1 and 371 m2 gpt-1。Although these values are smaller than those of commercial PTC (68.).5 m2 GPT-1), but they are still larger than most PT-based IMCs nanoparticles, due to the high sintering resistance, which allows small NPs to be well retained during high-temperature annealing. As shown in Figures 5A and B, the MA and Sa values of PT-based IMCs catalysts are significantly higher than those of commercial PT C (0.).22 a mgpt-1/0.27 ma cmpt-2)。
It is worth noting that the MA and SA values of PT4FEConicu are 1., respectively0 A mgpt-1 and 28 mA CMPT-2, which is 4 of commercial PT C5 times and 104 times, it has great application potential in proton exchange membrane fuel cells (PEMFC). In addition to the electrocatalytic activity, the electrochemical stability of the ORR process is critical from a practical application perspective. All samples underwent an accelerated endurance test (ADT) at a scan rate of 100 mV S-1 at O2-saturated at 01 M HCO4, at 06 and 10 V potential.
As can be seen, the two typical IMCs catalysts maintain good activity after ADT, while the commercial PT C decreases dramatically after the 20K cycle, as shown in Figure 5C. Specifically, the PTCO BC catalyst exhibits only a slight negative shift of 4 mV in E1 2. In particular, the MA and SA of the binary PTCO BC catalyst decreased by 16% and 18%, respectively. In contrast, commercial PT C undergoes a large negative displacement of about 17 mV for E1 2 under the same conditions, while attenuating by 37% and 19% in MA and SA, indicating poor durability. In addition, the change in Hupd on the CV curve was negligible for all IMCS catalysts, while the HupD value of commercial PT C decreased significantly (Fig. 5D-F).
Bibliographic information
hydrogenated borophene enabled synthesis of multielement intermetallic catalysts，nature communications，2023.