Electrochemical In-situ Surface-Enhanced Raman Spectroscopic Study On The Adsorption And Oxidation Of Carbon Monoxide At Noble Metal Electrode Surfaces

Fuel cells are expected to play important roles for the sustainable society. However, most of the fuel cells working at low temperature, such as proton exchange membrance fuel cell (PEMFC) and direct methanol fuel cell (DMFC), meet the same serious problem, i.e, deactivation of anode catalyst induced by adsorption of carbon monoxide (CO) from the fuel. Hence, a fundamental understanding of the adsorption and oxidation of CO at noble metal electrodes such as platinum (Pt) and palladinum (Pd), which is one of the most important components in the anode electrocatalysts, will be of great help to alleviate the CO poisoning problem.In this Ph.D. thesis, the adsorption and oxidation of CO molecules at core-shell nanoparticle electrodes of Au@M are investigated systematically [where Au and M (M=Pt, Pd, Rh, Ru) represent core and shell materials, respectively] by in-situ surface-enhanced Raman spectroscopy (SERS) measurements. These results are briefly summarized below.1. The effects of CO coverage, CO partial pressure, and solution pH on the vibrational properties of Pt—CO and C—O stretching modes of CO adlayer at Au@Pt core-shell nanoparticle electrodes. With the increase in COad coverage a decrease (increase) in the Pt—COL (COL) peak frequency together with the increase of all the band intensities are observed. The chemical effects are considered to be responsible for the frequency change of Pt—COL stretching band, and both the chemical and the dipole-dipole coupling effects within adsorbed CO molecules are the main causes for frequency change of C—OL.With the solution switch from CO-saturated 0.5 M H2SO4 solution to CO-free 0.5 M H2SO4 solution at 0.06 V, the peak intensities of Pt—COL and C—OLincrease ca.6% and ca.30%, respectively, together with a slight change in the peak frequency. The intensity change of Pt—COL is mainly attributed to the CO orientation change, while the dipole coupling effects, chemical effects and the CO orientation change are responsible for the intensity and frequency changes of the C—OL stretching vibration. The SERS spectra recorded upon changes in the electrolyte between H2SO4, Na2SO4 and NaOH at constant potentials reveal that a substantial change (up to 50%) in the band intensities of Pt—CO and CO stretching vibrations. In addition, the C—OL (Pt—COL) peak frequency is ca.7 cm-1 higher (2 cm-1 lower) in H2SO4 than that in Na2SO4, while no differences in the peak frequencies have been discerned when switching between in Na2SO4 and NaOH. Small lateral shift of COad from atop edges to exactly atop positions upon the the co-adsorption of H atoms in H2SO4 has been proposed to explain such frequency change, while the band intensity changes are identified to be due to the changes in the chemical enhancement factor upon the electrolyte switch.2. Potential dependence of CO adsorption at Au@Pt nanoparticle electrodes in a wide potential window. Throughout the potential regime examined, the Stark slopes of dvC—O/dE are always positive while that of dvPt—CO/dE are negative. The Stark slopes of dvPt—CO/dE are almost constant while that of dvC—O/dE increase toward negative potentials. Qualitatively, all the potential-dependent spectral behavior can be rationalized by the delicate changes in the counterweighing effects of the chemical bonding (d(Pt)→2π*(CO) back-donation and 5σ(CO)→sp(Pt) donation) and the dipole—dipole coupling interaction, the latter originates from potential induced changes in the fractional surface coverage of COL and COB species. Based on the fact that the influence of the dipole—dipole coupling interaction on the Pt—CO vibration is'negligible, the measured dvpt-co/dE can serve as indicator to evaluate the potential-induced changes in bonding strength and bond length of Pt-CO. From which we estimate the potential induced changes in the binding energies and bonding lengths of COL (COB) of ca. 0.20 (0.37) eV/V and 0.005 (0.01) A/V, respectively. The larger△Eb/△E of COB than COL reveals that the chemical bonding of COB is more sensitive to the changes in the interfacial electric field than that of COL. Peoridic DFT calculations have been performed to estimate the field-depdendent vibrational frequencies and Stark slopes for the saturated CO adlayer. The DFT calculations demonstrate that the Stark slopes of dvpt-co/dE and dvC—O/dE are much smaller than the present observation. The predicted order of the Stark slope of metal-adsorbate stretching vibrations, i.e.,|dvPt—COL/dE|>|dvPt-COB/dE| as predicted by peoridic DFT calculations, contradicts our experimental observation as well as that from DFT calculations using the cluster model.3. The origin of pre-peak in bulk CO oxidation at Au@Pt electrode and the mechanism elucidation of methanol electro-oxidation in electrolyte solutions with different pH. A pre-peak has been reported between 0.4～0.8 V for the oxidation of CO pre-adsorbed in the hydrogen adsorption potential region in a CO saturated 0.5 M H2SO4 solution; The pre-peak disappear if CO pre-adsorbed in the double layer region. However, the essential nature for the pre-peak is still in controversy. The in-situ SERS spectra of CO pre-adsorbed at 0.06 and 0.35 V have been determined and no distinguished differences are observed in spectra between the two potentials. It is proposed that the pre-peak is not caused by the CO adsorption structure, but due to the adlayer structure change on the Pt electrode surface. The mechanisms of methanol oxidation in acid, neutral, and basic solutions are found to be similar from the in-situ SERS measurements. CO from methanol dehydrogenation can adsorb at atop and bridge site on Pt surface. The ratios of COL and COB significantly change with solution pH, which may be related with the electric field and co-adsorbed species.4. SERS characterization and DFT calculation of CO adsorption at Au@Pd electrodes in the solutions with different pH. DFT calculations reveal that hollow site is the most stable adsorption site for COad on Pd(111) surface and CO binding energy for CO adsorption on Pd(111) surface increase with the negative shift of electrode potential. In the electric field region of -1～0.5 V/A, the vibrational frequencies of Pd—COL,M and C—OL,M are linear with the external electric field. EC-SERS studies reveal that the Stark slopes of both Pd—CO and C—O stretching vibrations can be divided into three distinct region:dvC-OM/dE decreases from 185～207 cm-1/V (from -1.5 to -1.2 V) to 83～84 cm-1/V (-1.2 to-0.15 V) and then to 43 cm-1/V (-0.2 to 0.55 V); on the other hand, dvPd-COM/dE changes from-8～-10 cm-1/V (from -1.5 to -1.2 V) to-31～-30 cm-1/V (-1.2 to-0.15 V) and then to-15 cm-1/V (-0.2 to 0.55 V). The simultaneously recorded cyclic voltammetry reveals that at E<-1.2 V hydrogen evolution reaction (HER) occurs. Based on the results of cyclic voltammetry and periodic DFT calculations, the unusual high dvC—OM/dE and the small dvPd—COM/dE in HER region are explained by the conversion of COad from bridge-to hollow-sites induced by co-adsorbed hydrogen atoms formed from dissociated water at negative potentials.5. The adsorption and electrochemical oxidation of CO at Au@Ru and Au@Rh nanoparticle electrodes. It is found that the Au@Ru core-shell nanoparticles are unstable and contain pinhole on the surface in acidic solution. The Au@Ru core-shell nanoparticles are relatively stable in neutral solution and have been evaluated. It is found that the bands of Ru—CO and C—O vibration modes are observed in the potential region (-1.2～-0.6 V vs. NHE). These band intensities greatly changes with potential. The bands of Ru—CO and C—O vibrations for CO bound to reduced and oxidized Ru surface sites are obtained in the more positive potential region (>-0.6 V vs. NHE). The C—O band intensity of COad on oxidized surface sites is much larger than that on reduced Ru surface sites, which probably originates from the larger Raman scattering cross-section of C—O vibration of COad on oxidized surface sites than that on reduced Ru surface sites. In-situ SERS experiments of CO adsorption on Au@Rh core-shell nanoparticle electrode reveal that Au@Rh nanoparticle electrode is only stable in basic solution. The mechanism of CO oxidation on Au@Rh nanoparticle electrode in CO saturated basic solution was found to be of the Langmuir-Hinshelwood type.Based on the present in-situ SERS and DFT calcualtions, we have a more comprehensive understanding of adsorption and electrochemical oxidation of CO on Pt, Pd, Rh, and Ru electrode surfaces, which could provide the general guidance for the research and development of high active anode catalysts with good CO-tolerant ability for the fuel cells working at the low temperatures.