| dc.description.abstract | This study investigates the adsorption behavior of pyridazine (PD), pyrimidine (PM), and pyrazine (PZ) isomers on a gold (111) electrode, as well as their influence on formic acid catalysis. The results indicate that the adsorption of PD molecules on a gold electrode serves as a model system for understanding the structure of organics on metal electrodes. Considering the pKa value of PD is 2.2, PD can exist in both protonated and non-protonated forms, and its adsorption was examined in sulfate and perchlorate media at pH 1 and pH 3. Additionally, with a large dipole moment of 4.22 D, PD can arrange differently on the gold electrode depending on the applied potential. At positive potentials, the adsorption of anions and PD on the gold electrode is coupled. Molecular-resolution STM imaging allows direct observation of the adsorption of PD molecules on an ordered Au(111) electrode as the potential, pH, and anions vary. In pH 1 media, PD adsorbed at less positive potentials is likely protonated (PDH+), forming extended molecular chains through intermolecular hydrogen bonds. As the potential shifts to positive values, PD chains dissociate into individual molecules. Before transitioning to an ordered arrangement and vertical orientation, PD adsorbs in a disordered state at a horizontal or tilted orientation on the electrode at 0.7 V (vs. Ag/AgCl). This transition is evident as a distinct current peak in the voltammogram obtained in 0.1 M H2SO4 with 1 mM PD. The intermolecular interactions among adsorbed PD molecules determine their azimuthal orientation and spatial arrangement on the gold electrode. In both H2SO4 and HClO4, PD coverage increases with positive potential until anions replace PD adsorbed molecules on the gold electrode at 1 V. This finding suggests that at positive potentials, the much more concentrated anions become more critical than PD, besides the adsorption strength.
The adsorption behavior of PM on the gold (111) electrode shows that adsorption begins after 0.5 V, but no distinct characteristic peaks are observed, possibly due to the disordered structure of adsorbed PM molecules. During the negative scan, a reduction peak appears at 0.2 V, indicating PM molecules desorbing from the gold surface and confirming the adsorption and desorption characteristic peaks of PM. Experimental results with the Nernst equation show a negative shift of about 120 mV, confirming that this reaction involves a single electron transfer process.
The adsorption behavior of PZ on the gold (111) electrode shows that at positive potentials, the CV curve is similar to the background current, indicating only trace adsorption. At negative potentials, PZ shows significant current peaks (A1/C1), which are identified as diffusion waves, suggesting that the reaction primarily occurs in the solution. When the pH is raised to 3, characteristic peaks appear at more negative potentials, with experimental results showing a difference of 220 mV, indicating that other factors influence the reaction, causing the actual measured potential difference to be greater than the theoretical value predicted by the Nernst equation.
Regarding formic acid catalysis, among the three molecules PD, PM, and PZ in a pH 1 environment, PZ shows the highest catalytic efficiency, while in a pH 3 environment, the catalytic efficiency order is PD > PM > PZ. In a pH 3 environment, all three molecules predominantly exist in the non-protonated state. PD exhibits the highest catalytic efficiency due to its stronger adsorption ability on Au(111). The catalytic efficiency of PM is intermediate, mainly due to its moderate adsorption strength and active sites. PZ is least affected by protonation, so changing the pH value does not significantly impact its catalytic efficiency. This indicates that protonation is a key factor affecting catalytic efficiency, providing important insights for understanding and optimizing molecular design and reaction conditions in electrocatalytic reactions. | en_US |