Palladium is a silver-white transition metal that is soft, has good ductility and plasticity, and can be forged, rolled and drawn. Bulk metal palladium can absorb a large amount of hydrogen, causing its volume to expand significantly, become brittle and even break into pieces. Palladium powder with micron-sized particles is available in flake, amorphous and spherical shapes. Palladium catalyst is a variety of catalysts made with metal palladium as the main active component. It is a catalyst often used in chemical and chemical engineering reaction processes. It has high catalytic activity, strong selectivity, easy preparation of the catalyst, and low usage. Performance can be optimized through changes and improvements in manufacturing methods and compounding with other metal or cocatalyst active components.
Studying the interfacial species and electrooxidation processes of various catalytic materials is crucial in basic research. This analytical approach provides valuable insights into surface phenomena such as adsorption, desorption, and redox reactions, revealing the fundamental mechanisms and dynamics of electrochemical processes. In addition, it can controllably adjust catalytic activity, selectivity, and stability, thereby improving electrocatalytic performance and increasing energy conversion efficiency. Therefore, understanding the electrochemical oxidation mechanism on the catalyst surface is of great significance for determining the real active sites of the catalyst and rationally designing and developing the catalyst.
Some researchers constructed a Pd(111)/Au heterogeneous single crystal with controllable number of layers based on the underpotential deposition (UPD) method, and further used surface-enhanced Raman spectroscopy to monitor the electrooxidation process on the surface of the Pd single crystal in situ. , through direct spectral evidence of reaction intermediate species, anion effects and catalyst composition evolution, combined with DFT theoretical calculations, the electrooxidation reaction mechanism at different stages on the Pd surface was revealed.
During the initial oxidation process (low potential range), there will be the formation of *OH intermediate species during the conversion of Pd into PdOx. ClO4- is adsorbed on the electrode surface and will form hydrogen bonds with *OH, thus promoting the deprotonation process of *OH (that is, accelerating the conversion of *OH to *O) and accelerating the oxidation of Pd. The deprotonation process was studied by using DFT to calculate the dehydrogenation free energy of *OH with/without ClO4-adsorption on the Pd(111)/Au surface. The dehydrogenation free energy decreases from 1.05 eV (OH-OH) to 0.7 eV (OH-ClO4-), indicating that the dehydrogenation process is easier because the formation of *OH...O(ClO3-) weakens the O-H of the *OH species key.
Due to the different electronic structures of Pd(111)/Au after deposition of different Pd atomic layers, Pd(111)/Au-1, 2 and 3ML Pd were also constructed in this research work for in-situ Raman characterization to elucidate the electronic Effect of structure on electrooxidation process. The Raman characteristic peaks of *O and *OH species adsorbed on the surface of 1ML Pd appear at 624 cm-1 and 1072 cm-1 respectively. When the number of Pd layers increases to 2ML and 3ML, these two peaks shift to 622 cm-1, 1078 cm-1 and 621 cm-1, 1079 cm-1 respectively. Combining DFT calculations of the d band center and *OH Raman frequency and adsorption energy, it is revealed that the thicker the Pd layer, the higher the Raman frequency of *OH rotational vibration, and the stronger the adsorption of *OH. In addition, as the number of Pd layers increases, the coverage of ClO4- increases, accelerating the conversion of *OH to *O, thereby causing the oxidation potential to advance relatively early.
During the deep oxidation process (high potential range), the oxygen atoms in PdOx exhibit dynamic behavior rather than remaining in a fixed state. Through heavy oxygen water isotope experiments (obtaining 16O-18O species), it is revealed that the oxygen atoms in PdOx will participate in the formation of peroxygen species (*OO), eventually forming oxygen.
This work not only elucidates the mechanism of the electrooxidation process at different stages on the Pd surface through direct spectroscopic evidence of key reaction intermediate species, but also establishes the relationship between electrochemical behavior and electronic effects.
