CO stripping experiments

Similar studies have been carried out with Pt(l 11) and stepped surfaces with (111) terraces [Angelucci et al., 2007a, b]. The voltammetric profiles of these surfaces agree qualitatively with those depicted in Fig. 6.9. For the stepped surfaces, the potentials Ex and 2 depend linearly on the step density for terraces wider than 5 atoms. This hnear dependence is a consequence of the dependence of the oxidation rate on the step density, as was observed in the chronoamperometric CO stripping experiments. In H2SO4... 

Figure 7.11 CO stripping experiment for a Pt(l 11) electrode with = 0.14 (a) and recovery...

Figure 7.12 CO stripping experiment on a Pt(775) electrode decorated with different adatoms, as labelled, in 0.1 M HCIO4. The blank voltammogram is also included for comparison.

At potentials higher than 0.6 V vs RHE, the dissociative adsorption of water occurs on platinum, providing -OH adsorbed species, able to oxidize further the adsorption residues of ethanol. Then, oxidation of adsorbed CO species occurs as was shown by FTIR reflectance spectroscopy and CO stripping experiments [36] ... 

On Pt-Sn, assuming that ethanol adsorbs only on platinum sites, the first step can be the same as for platinum alone. However, as was shown by SNIFTIRS experiments [37], the dissociative adsorption of ethanol on a PtSn catalyst to form adsorbed CO species takes place at lower potentials than on a Pt catalyst, between 0.1 and 0.3 V vs RHE, whereas on a Pt catalyst the dissociative adsorption of ethanol takes place at potentials between 0.3 and 0.4 V vs RHE. Hence it can be stated that the same reactions occur at lower potentials and with relatively rapid kinetics. Once intermediate species such as Pt-(COCH3)adsand Pt-(CO)ads are formed, they can be oxidized at potentials close to 0.3 V vs RHE, as confirmed by CO stripping experiments, because OH species are formed on tin at lower potentials [39, 40] ... 

It can also be observed from this figure that Sn-containing catalyst is a more effective catalyst for the oxidation of CO than that containing Ru, as a lower onset potential of the oxidation wave is obtained with the former catalyst. It has also to be noted that PtSn catalysts are less active towards methanol electrooxidation than PtRu catalysts (see Section IV. 1). ° However, adsorbed CO species are proposed as reaction intermediates of methanol electro-oxidation, which seems to lead to a paradoxical behavior of PtSn based catalysts. In CO stripping experiments, a negative shift of the onset potential for the oxidation of adsorbed CO on PtSn also occurs. " On the basis of in situ infrared spectroscopy studies coupled with electrochemical measurements, Mo-... 

In order to substantiate this interpretation, experiments were carried out [98] on Pt(311)-Ru . This surface has a very high density of (100) steps. CO stripping experiments (not shown) on the Pt(311)-Ru surface were characterized by a dominance of anodic charge in the hrst of the two stripping peaks, i.e., that associated with the linear array of Pt-Ru ensembles. The charge associated with oxidation of CO adsorbed at terrace sites was very low, as one would expect for the low concentration of terrace sites on Pt(311). 

Fig. 2.19 Stepped potential CO-stripping experiment, lOOmVs" in 1 M H2SO4, argon-purged solution. The sweep window was increased from 0.75V vs. NHE (continuous line) to 0.9V after the first two cycles (dashed line). The dotted line is the complete CV going directly to 0.9V without the pre-step at 0.75 V.

At potentials higher than 0.6 V vs RHE, the dissociative adsorption of water occurring on a platinum surface according to Eq. 9.49 provides OH adsorbed species, allowing the catalyst to further oxidize the adsorption residues of ethanol. Then, oxidation of adsorbed CO species (Eq. 9.50) occurs, which is in agreement with in situ Fourier transform infrared (FTIR) measurements where CO2 starts to be detected from ca. 0.65 V vs RHE and CO stripping experiments at a platinum surface, where CO is removed from ca. 0.6 V vs RHE. Acetaldehyde can also be oxidized following Eq. 9.51 ... 

As shown in Fig. 12.1, the three-electrode cell has an inlet and an outlet for gas purging. For surface CV measurement, N2 gas is used to purge the electrolyte solution for 30 min to remove dissolved O2. For ORR measurement, pure O2 gas or air is used to purge the electrolyte solution and introduce dissolved O2 into the solution. For CO-stripping experiments, a CO/N2 mixed gas is used to purge the solution and introduce dissolved CO into the solution. In addition, there is a port for a thermometer to monitor the temperature of the electrolyte solution. To control the temperature, the whole cell is emerged in a thermal bath in which the temperature of the liquid can be adjusted to the desired level. 

CO Stripping Chronoamperometiy Before discussing experimental results, let us examine what the LH mechanism predicts for the chronoamperometric response of an experiment where we start at a potential at which the CO adlayer is stable and we step to a final potential E where the CO adlayer will be oxidized. We will also assume that the so-called mean field approximation applies, i.e., CO and OH are well mixed on the surface and the reaction rate can be expressed in terms of their average coverages dco and qh- The differential equation for the rate of change of dco with time is... 

A similar inhibition was found also for electrochemical CO oxidation. In COad stripping experiments, numerous potential cycles up to IV were necessary to remove all COad from a smooth Ru(OOOl) surface [Zei and Ertl, 2000 Lin et al., 2000 Wang et al., 2001]. CO bulk oxidation experiments under enforced mass transport conditions on polycrystalline Ru [Gasteiger et al., 1995] and on carbon-supported Ru nanoparticle catalysts [Jusys et al., 2002] led to similar results. Hence, COad can coexist with nonreactive OHad or Oad species on Ru(OOOl) at lower potentials (E < 0.55 V) [El-Aziz and Ribler, 2002]. 

Fig. 4.3 Example of sections taken from a CBCA(CO)NH experiment of the transcription factor CDC5. Strips are shown for the same set of amino acids as in Fig. 4.2. The two peaks per strip indicate the 13C-frequencies of the a- and the...

Fig. 7.14 (left) Strips from the quantitative- /-HN(CO)CACal1 experiment showing reference and cross peaks as explained in the caption of... 

Fig. 7.13. The assignment of side-chain protons is indicated in the strips from the CC-TOCSY (CO)NH experiment (right).

Differential Electrochemical Mass Spectrometry (OEMS) was also used for methanol stripping experiments, which can give some information on the electrode coverage by species coming from the adsorption and oxidation of methanol. First, it can be seen from the CVs and the MSCVs recorded on a coreduced PtogRuo2/C catalyst as an example (Fig. 19) that the coverage of the electrode is much lower from methanol adsorption (curves 2) than that from CO adsorption (curves 1). 

In the case of the co-deposited catalyst (non-alloyed), the number of electron from the methanol stripping experiment is close to 2, indicating that almost only CO is formed at the electrode surface. The number of electrons for the oxidation of bulk methanol is close to 6 (6.6), which indicates that only a small amount of formic acid is formed, and that the main reaction path leads to the formation of CO2, according to the following steps ... 

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