Bisulfate adsorption

Figure 9.5 Anion adsorption on Pt in several electiolytes bisulfates and TFMSA as determined by radiotracers (RT), and OH and H as reflected in A/r, the ampUtiide of the XANES peak. The effect of 6 M TFMSA suppressing OH formation can be seen. (Reproduced with permission from Teliska et al. [2007].)...

The specific adsorption of bisulfate anions is observed in H2SO4 in both EXAFS and XANES data and, in agreement with voltammetry, is seen to impede oxygen adsorption. Significant specific anion adsorption was found in 6 M TFMSA, but not in 1 M TFMSA [Teliska et al., 2007]. As mentioned above, this specific anion adsorption suppresses OH adsorption (particularly the formation of subsurface O), causes the Pt nanoparticle to become more round, and weakens the Pt-Pt bonding at the smface. The specific anion adsorption becomes site-specific only after lateral interactions from other chemisorbed species such as OH force the anions to adsorb into specific sites. 

The anion electronic effect in a recent analysis of the inhibition effect of bisulfate and comparison with the effect of OH adsorption (Pt-OH formation), Wang and co-workers found that the electronic effect must also be operative [Wang et al., 2004]. 

Faguy PW, Markovic N, Adzic RR, Fierro C, Yeager E. 1990. A study of bisulfate adsorption on Pt(lll) single crystal electrodes using in-situ Fourier transform infrared spectroscopy. J Electroanal Chem 289 245 -262. 

In this contribution, we present new results of a cychc voltanunetiy study of the influence of (CeHglads on Hypo and on anion adsorption at the Pt(l 11) electrode in aqueous HCIO4 and H2SO4, and relate them to analogous studies at Pt(110) and Pt(lOO) electrodes in aqueous H2SO4. Perchlorate anions are known to be less strongly adsorbed on Pt than bisulfate or sulfate anions thus, the cychc voltanunetiy behavior in the two electrolytes is expected to reveal important differences. 

An in situ EUR study of bisulfate and sulfate adsorption indicated that the coverage of adsorbed sulfate increased in the presence of Cu underpotential deposition on polycrystalline Au. ... 

Wifckowski and coworkers [37] have reported adsorption of bisulfate anions on Au(lll), Pt(lll), and Rh(lll) electrodes in sulfuric acid solution using electrochemical and several nonelectrochemical techniques. It was concluded from the low-energy electron-diffraction data that the structure of bisulfate on gold is different from that on Pt(lll) and Rh(lll). Adsorption of bisulfate on Au(lll) is associated with a charge transfer from the electrode to the adsorbate. However, the formation of this particular bond does... 

Inukai et al. [512] have used STM to study Hg UPD on Au(lll) in sulfuric and perchloric acid solutions. For sulfuric acid, the influence of adsorption of bisulfate was indicated. It has been found that after the formation of the first UPD adlayer, two different structures were simultaneously formed on the same terrace. For perchloric acid, only a single structure was found. These results reflected a significant influence of the supporting electrolyte anions on the UPD structure. Recently, Abaci et al. [513] have presented the temperature-dependent studies on the influence of counteranions on Hg UPD on Au(lll). 

Fig. 6.93. A typical equilibrium configuration for a model of bisulfate adsorption on Rh(111), generated by Monte Carlo methods in the ordered (-

Fig. 6.94. Comparison of adsorption properties of different electrode surfaces. Bisulfate adsorption as a function of electrode potential on different platinum planes (110), (111), (100) and on polycrystalline platinum. Data obtained by the radiotracer technique. (Re-printed from Y.-E. Sung, A. Thomas, M. Gamboa-Aldeco, K. Franaszczuk and A. Wieckowski, J. Electroanal. Chem. 378 131, copyright 1994, Figs. 14 and 15, with permission from Elsevier Science.)...

What does Eq. (6.246) mean This equation represents the adsorption process of ions on metallic surfaces. It includes several conditions that are characteristic of the adsorption process of ionic species, namely, surface heterogeneity, solvent displacement, charge transfer, lateral interactions, and ion size. However, is this equation capable of describing the adsorption process of ions In other words, what is the success of the isotherm described in Eq. (6.246) Figure 6.104 shows a comparison of data obtained experimentally for the adsorption of two ions—chloride and bisulfate—on polycrystalline platinum, with that obtained applying Eq. (6.246). The plots indicate that the theory is able to reproduce the experimental results quite satisfactorily. The isotherm may be considered a success in the theory of ionic adsorption. 

What information can be obtained from A/ ds One important parameter involved in the enthalpy of the reaction is the ion-metal bond. However, A7 ds includes all the different interactions involved in the adsorption process, e.g., the breaking of the hydration sheet of the electrode and the ion, lateral interactions, and heterogeneity of the surface. One can subtract all these energy terms from AH ds and obtain in this way the energy (or strength) of the ion-metal bond. For the example of the adsorption of bisulfate ions onpolycrystalline platinum (Fig. 6.105), the ion-metal bond was found to be -214 60 kJ mol-1. 

Table 6.13 shows the calculated values of entropy of adsorption for different models of the adsorption reaction of bisulfate on polycrystalline platinum electrodes, as well as its comparison with the experimental value. The conditions that best describe the experiments are those shown on line 4. This means that in solution the bisulfate... 

Bisulfate ions in solution belong to the C3V symmetry group. Adsorption through three oxygen atoms does not change the symmetry. But if the ion is one-or two-fold coordinated upon adsorption, a change to a symmetry should be observed. Table 5 indicates four IR-active bands for this symmetry of sulfate ions in solution. 

Only one adsorption band is observed at pH 0.23 (Fig. 48). Considering first the solution modes, according to Table 4 we expect two loss bands for bisulfate, which is the main solution species at this pH, at 1043 and 1195 cm". The first is very weak and the second is missing in the spectra of Fig. 48. Only one strong band is observed between 1220 and 1280 cm". A possible explanation for the missing solution feature at 1200 cm" could be that it is superimposed on an adsorbate feature, and both cancel out. 

In conclusion, the possibility of HSO adsorption to a large extent at the Pt(l 11) electrode surface can be ruled out. This interpretation contrasts with that in the literature, where the adsorbed species at Pt(lll) are considered to be bisulfate ions [141]. 

The distance between two adjacent Pt atoms, 2.77 A, fits well with the distance between two oxygen atoms in the sulfate ion, c. 2.5 A (value given for bisulfate in [36]). So, from geometric considerations, a symmetry is very probable. A 2 symmetry has been proposed for the adsorption of sulfate on well-ordered Pt(lOO) electrodes (see below) where the threefold coordination is not possible due to mismatch between surface structure and threefold coordination. Two well-separated strong bands (1200 and 1100 cm ) are observed, which account for the Qv symmetry. There is definitely a difference in the behavior of the adsorbate on both single... 

The adsorption and structure of anions such as bromide, cyanide, sulfate/bisulfate, and iodide on metal electrodes have been extensively studied by in-situ STM in electrolyte solutions. Figure 41a displays a cyclic voltammogram for an Au(lll) electrode in 1-mM KI solution. The anodic/cathodic peaks below 0 V versus Ag/AgI are associated with adsorption/desorption of iodine at the surface. The smaller peaks at 0.5V are due to a phase transition in the adsorbed iodine layer, as can be observed by STM images taken at various electrode potentials. STM images shown in Figure 41b taken at a potential of —0.2 V show a periodic structure with perfect... 

The voltammetry curve for the Ru(ioio) surface in 0.05 M H2SO4 solution (Fig. 4a) reveals a remarkable difference between the oxidation processes forRu(OOOl) andRu(10l 0). The oxidation of this face is more facile than that of Ru(OOOl), as indicated by the onset of the reaction at lower potentials and by increase of the charge with each potential cycle. This difference most likely is the consequence of the more open structure of the Ru(1010). A pair of peaks at 0.12 and 0.3 V is reminiscent of hydrogen adsorption on Pt metals. However, CO displacement showed a negative charge of -354 pC cm . Thus, the peaks probably represent partial Ru oxidation to RuOH, wherein OH is the predominant adsorbed species, perhaps with some co-adsorption of bisulfate. 

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