Platinum Surface Modification And Electrocatalytic Reaction And Enhanced Infrared Spectroscopy

Surface electrochemistry is the foreland crossed by electrochemistry and surface chemistry, with an emphasis on the studies of electrocatalysis on metals including many fuel cell related reactions such as electrocatalytic oxidation of CO, anodic oxidation of fuel and cathodic reduction of oxygen. Catalyst design is the key to achieve higher catalytic performance, as a result, it is important to characterize the property of the catalysts upon the surface modification and to explore the mechanism of the catalysis process. Platinum is a widely used catalyst in fuel cells, therefore, it will be of great significance in both scientific and application aspects to investigate and analyze the adsorption and reaction on the modified Pt surfaces on molecular level.Electrochemical surface enhanced infrared absorption spectroscopy with Kretschmann ATR configuration (EC-ATR-SEIRAS) possesses merits of high surface sensitivity, strong surface signal and unrestricted mass transport, and facilitates the detection of coadsorbed water which may be included in the interfacial reactions. Specially, it is quite suitable for tracing the irreversible reaction of small molecules with high polarity. Furthermore, the EC-ATR-SEIRAS can effectively distinguish the intermediate during the electrocatalytic process and deduce the orientation and bonding of the molecules on the electrode surface, which provides important evidence to the electrocatalytic mechanism on molecular level and helps to design new modified catalysts with high catalytic activity.In the present thesis, investigations which are focused on the surface modification on Pt and its influence to the electrocatalysis have been carried out with ATR-SEIRAS. Firstly, the effect of the presence of specifically adsorbed chloride or iodide on the CO oxidation has been detailedly studied. Secondly, a versatile mimetic underpotential deposition (MUPD) approach to controlled modification of Sb on various Pt surface (Pt-Sb) towards efficient electrocatalysis of formic acid has been proposed and extended to the surface modification of Pb (Pt-Pb). Thirdly, the combined ATR-SEIRAS and DFT calculations has been used for the mechanistic study of formic acid electrooxidation at the Sb modified Pt electrodes. At last, the Pt-Sb and Pt-Pb electrodes prepared by MUPD have been applied to the primary investigation of ethanol electrooxidation. The main results and conclusions are summarized as follows: 1. Effect of specifically adsorbed anions on the CO electro-oxidation at Pt electrodeAs an important catalytic process, the electro-oxidation of CO has been extensively investigated on various single crystalline and polycrystalline Pt electrodes. On the other hand, high surface area Pt nanoparticles synthesized from PtCl62-ions-containing solutions, are mainly used in fuel cell-related practical catalysts in the pursuit of a better utilization of Pt. As a result, specifically adsorbed Cl- ions are likely brought into the system during the synthesis of the Pt-based catalysts and hardly removed from the Pt surfaces. Along this line, it is of fundamental and practical interest to understand the effect of Cl- on the CO electro-oxidation at Pt electrodes. Meanwhile, it is controversial that when and where the Cl" adsorbs on the surface and how it affects the CO oxidation. In this section, the electro-oxidation of CO adlayer on Pt electrode in Cl--containing 0.1 M HClO4 has been investigated with in situ attenuated-total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). Two potentials were selected for predosing CO on the Pt electrode: one is in the H-UPD region, i.e.,0.1 V (vs. RHE) and the other is the double-layer region, i.e.,0.45 V (vs. RHE). The broadening of the prewave and the main peak for the CO oxidation is observed, in addition to the positively shifted oxidation potentials. The spectroelectrochemical data suggest the specific adsorption of Cl- starts at a potential as negative as 0.1 V which may compete with the adsorption of OH at CO-unoccupied sites (including but not limited to defect sites) and/or hinder the diffusion of CO to OH adsorption sites on Pt electrode, slowing down the CO oxidation. This competitive Cl- adsorption at lower potentials disrupts the interfacial free H2O molecules on the top of CO adlayer, signaled by a reduced OH stretching band intensity. Furthermore, the oxidation of CO is more interfered by the adsorption of iodides, which not only occupy the free sites but also replace partial CO molecules on the surface, and the CO main oxidation occurs only after the oxidation of iodides. On the other hand, the dynamics of CO oxidation is not obviously changed by the hydrogen evolution, indicating the structure of CO adlayer and is rather stable. 2. A new versatile approach to modify Sb or Pb on Pt towards efficient oxidation of formic acidTraditional surface modification of Sb could be achieved by either irreversible adsorption (IRA) or underpotential deposition (UPD) with subsequently external potential control to desorb partial adatoms. It is not suitable for the large scale synthesis or improvement of carbon supported catalysts in practice. In this section, a new facile approach towards developing superior Pt-based catalysts for HCOOH electrooxidation has been proposed, which is exemplified with a mimetic underpotential deposition (MUPD) of Sb on Pt surfaces to attain a favorable coverage. Suitable Sb modification was achieved simply through immersing a bulk Pt electrode or dispersing Pt/C powders in a Sb(III) solution mixed with ascorbic acid (AA). AA serves as the mild reducing agent to ensure freshly reduced Pt surfaces for Sb modification, as demonstrated by the negatively shifted open circuit potential. The catalytic activity towards HCOOH electrooxidation on the above Sb-modified Pt/C catalyst far exceeds that on commercial Pt-Ru/C or Sb-modified Pt/C through traditional irreversible adsorption. This electroless approach is generally applicable to all types of Pt surfaces, in particular suited for upgrading Pt/C for practical anode catalysts of direct formic acid fuel cells. Such an approach has also been extended to the modification of Pb on the Pt surface towards HCOOH electrooxidation.3. Combined ATR-SEIRAS and first-principles study on electrooxidation of formic acid at Sb modified Pt electrodesSo far it is still controversial about the mechanism of HCOOH oxidation on Pt surface and the effect of the formate intermediate during the process of HCOOH oxidation. The so-called "dual pathway" and "triple pathway" were proposed in which the formate was considered to be the active intermediate of'direct pathway" and the "spectator" during HCOOH oxidation, respectively. Note that the formate is also supposed to be helpful for the HCOOH oxidation on the basis of DFT calculations. However, no literatures of HCOOH oxidation on Sb-modified Pt (Pt-Sb) could be found yet. In this section, in situ electrochemical surface-enhanced infrared absorption spectroscopy (EC-SEIRAS) together with a periodic density functional theory (DFT) calculation has been initially applied to investigate the mechanism of formic acid electro-oxidation on Pt-Sb electrode. EC-SEIRAS measurement reveals that the main formic acid oxidation current on Pt-Sb electrode is ca 10-fold enhanced as compared to that on clean Pt electrode, mirrored by nearly synchronous decrease of the CO and formate surface species, suggesting a "non-formate" oxidation as the main pathway on the Pt-Sb electrode. Based on the calculations from periodic density functional theory (DFT), the catalytic role of Sb adatoms can be rationalized as a promoter for the adsorption of the CH-down configuration but an inhibitor for the adsorption of the O-down configuration of formic acid, kinetically facilitating the complete oxidation of HCOOH into CO2. In addition, Sb modification lowers the CO adsorption energy on Pt, helps to mitigate the CO poisoning effect on Pt.4. Primary investigation of the ethanol oxidation on Pt-M (M=Sb, Pb) electrodes with SEIRASEthanol is the very promising fuel in the low temperature fuel cells, and it has been intensively investigated in the last decade. The key of ethanol oxidation is how to break up the C-C bonding of ethanol molecule, and thus promotes the complete oxidation of ethanol. In this section, the primary investigation of ethanol oxidation on Pt-Sb or Pt-Pb electrodes prepared by MUPD has been carried out with SEIRAS. The results show that the Pt-M (M=Sb, Pb) not only negatively shift the peak potential but also enhance the peak current of ethanol oxidation. In the SEIRAS spectra, bands of CO and free water are clearly observed on Pt-M electrodes, which implies that it is easier for ethanol to break the C-C bonding and form CO on Pt-M electrodes than that on clean Pt. Besides that, bands at ca.1238 and 1398 cm-1 which are probably concerned to the vibration of CH3COOH, could be observed at low potentials while bands of C=O stretching of CH3CHO are hardly observed. The results imply that ethanol may be oxidized to acetic acid directly on the Pt-M electrodes.