| dc.description.abstract | In situ scanning tunneling microscopy (STM) and Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS) have been used to investigate the catalytic activity of a Pt thin supported by Pd(111) (denoted by Pt@Pd(111)) toward the oxidation of fuel molecules such as methanol, formic acid, and formaldehyde. Annealing a Pd bead with a hydrogen flame results in absorption hydrogen in the Pd bulk and possibly producing Pd hydride. This hydrogen-containing Pd(111) electrode can reduce PtCl62- to metal Pt, producing Pt islands on the Pd(111) electrode. The amount of Pt deposit varied with immersion time and [PtCl62-]. The effect of immersion time on the morphology of the Pt deposit and electrocatalytic activity was investigated. It is consistent with the positive shift of oxidation peak toward methanol oxidation, because the form of Pt islands will change the position of oxidation peak.
Pt@Pd(111) electrode resulted in 20, 10, 2 times higher oxidation currents than Pt(111) in 0.1 M H2SO4 containing 1 M methanol, formic acid, and formaldehyde, respectively. The reason behind these promotions can be traced to the modification of the electronic structure of the Pt adlayer by the Pd substrate. This issue was addressed by x-ray photoelectron spectroscopy (XPS), which shows a downshift of the 4f5/2 and 4f7/2 levels. This change likely reduces the bonding energy of CO molecule, the common poisonous species generated in the oxidation of methanol. On the other hand, the electrode with opposite structure, Pt@Pd(111), is passivated toward oxidation of small organic fuel molecule. The downshift in the Pt 4f binding energy means downshift of its d-band center and lower density of state at the Fermi level, which ultimately weakens the chemisorption bonds. This view is supported by IR results showing C-O vibration shifted from higher wavenumber to the lower wavenumber, implying low C-O vibration. Meanwhile, the intensity of C-O absorption band seen with was lower than that of Pt(111), suggesting less adsorbed CO molecules on the Pd@Pt(111) electrode. This result suggests that the oxidation of these organic molecules might bypass the route to generate CO molecules or the bond strength between CO and Pt is weaker.
The improvement of catalytic activity is twofold. In addition to electronic effect, the roughness and geometric structure of the electrode also contribute. Cyclic voltammetry shows that the catalytic activity would increase with immersing time. As the reduction charge of OH species increases for samples prepared by immersion time longer than 60 s in the dosing Pt salt, the overall area of the electrode increases also. The onset potential of CO oxidation shifts negatively. Thus, roughness of electrode was also important to the catalytic activity.
The durability of this Pt@Pd(111) and Pt(111) electrodes toward oxidation reaction of was studied by using chronoamperometry in methanol containing sulfuric acid. The oxidation current observed at both electrodes decreased with time, revealing poisonous phenomena attributed to the generation of CO species. The extent of poisoning is less at the Pt@Pd(11) electrode than the Pt(111), which results from different oxidation mechanism and the extent of poisonous effect. The reason is when formaldehyde oxidation on ragged surface, it is easily to form CO molecules. | en_US |