Selenium modification of Rh(111) surfaces

Abstract

Apart from expensive metals, metal chalcogenides are considered as catalyst because they facilitate the ORR and tolerate organics. Existing studies in this area concentrate on nanoparticles, consequently offering little knowledge of the role of the atomic surface structure and composition in the promotion of selective ORR catalysis. Additionally, research has not yet resolved the stability issues of the adsorbed chalcogenide layer. Therefore, it is vital to study cathode surfaces in low temperature fuel cells to improve the efficiency of energy conversion and to furnish environmental benefits. <br /> The selenium modified model catalysts were examined with the scanning probe microscope in order to visualize the adlattices of adsorbed atoms, ions or molecules at atomic level. The adlayer structure of SO₄^2- on Rh(111) was also studied. Based on the images obtained, the orientation of the Rh(111) substrate was determined and a mathematical procedure was developed to determine the drift, to correct for it and to calibrate the STM scanner (appendix A). The Se adlayer structure was atomically resolved by both STM and AFM. A 2x√3 structure was obtained by STM with a surface coverage of ϑ = 0.25, coinciding well with the deposited charge. A similar structure was also observed in friction images (atomic stick slip). These images could only be obtained at coverages close to but just below those at which the roughening of the surface starts. This roughening is most probably due to the place exchange between Rh and Se atoms similar to the well known roughening of Pt surfaces in the oxygen adsorption region.<br /> The in situ STM studies of the Se/Rh(111) system showed that with Se surface coverage of around ϑ = 0.25, a roughening starts at the steps and then extends to terraces; within a short time the whole surface becomes modified. It was observed that the surface roughening occurs around 0.55 V in SeO₃^2- containing solution. Moreover, around ϑ = 0.1 atomically smooth isolated domains of Se were observed which merged together at more negative potentials in Se free solution. A minimum surface coverage of ϑ = 0.2 was required to observe this roughening in SeO_3^2- free solution, which, however occurred at more negative potentials than observed in SeO_3^2- containing solution.<br /> Additionally, the stability of the adsorbed selenium on model metal fcc surfaces such as Rh(111), Pt(111) and Au(111) was explored. The focus was on the electrochemical desorption of the selenide from selenium modified model surfaces through differential electrochemical mass spectrometry under controlled flow through conditions. <br /> Through the experiments with DEMS, not only was qualitative data obtained but also a quantitative study of the amounts of reductively dissolved selenium as selenide at low potentials could be made. At these potentials close to hydrogen evolution, and in the absence of selenium containing solution, the formation of the H₂Se species involved the transfer of two electrons only. However, in the presence of selenious acid H₂Se species could also proceed via six electrons. The SPM technique, especially AFM, also confirmed the formation of Se particles at a lower potential of around 0 V vs. RHE. <br /> The ORR activity of Rhodium single crystalline surface (Rh(111)) and its electrochemical modification with Selenium (Se) submonolayer loadings was investigated in acidic media. ORR activity of these model Se/Rh(111) surfaces was carried out in the modified dual thin layer flow through cell (DTLFC) developed in our group for differential electrochemical mass spectrometric (DEMS) studies. <br /> The potential of zero charge was found to be ~0.2 V vs RHE from CO charge displacement experiment at various fixed potentials. The oxygen reduction proceeds mainly via four electrons with small amount of H₂O₂ generation on Rh(111) and Se modified Rh(111). Electrochemical modification of Rh(111) by Se submonolayers resulted in a better ORR activity. A Se coverage of ϑ_Se = 0.1 leads to the best ORR catalytic activity with an anodic shift of the onset potential by ~ 40 mV that of pure Rh(111). However, a surface coverage of about 0.18 would completely block the Rh(111) sites and would hamper its ORR activity. Astonishingly, one Se atoms block five Rh sites from both adsorbing hydrogen and CO. A large tolerance of such an ORR catalyst surface with respect to all kinds of organic fuel can thus be expected.<br /> I compared the surface morphologies of inductively heated selenium modified Rh(111) surfaces under controlled environment with that of simple selenium modified surfaces using scanning tunnelling microscopy (STM). Additionally, the surface morphologies of heat treated surfaces resulted in atomically smooth deposits, which remained stable with in the potential window used for ORR studies. A higher oxidation potentials, some Se stripped off the surface which redeposited at negative potentials. Surface areas of (Se/Rh(111) surfaces modified and heat treated at different temperatures were also determined using CO stripping experiments. Oxygen reduction activity of these heat treated modified surfaces was monitored using dual thin layer flow cell under controlled conditions. The heat treated Se/Rh(111) showed better ORR activity than that of pure Rh(111). <br /> Also friction was studied on a selenium modified fcc(111) surfaces (namely Rh, Pt and Au) for various coverages. Investigations at three surfaces were performed under electrochemical conditions which offered some advantages in comparison to those performed under ultra high vacuum (UHV) conditions. Furthermore, friction was studied in the presence of pyridine. Pyridine adsorbed on Au (111) was used as a model system to understand the effect of organic adsorbates. <br /> In the third part of these AFM experiments the surface morphology of the Nafion^®/Pt(100) interface was studied. The structure and molecular processes occurring at the Nafion^® membrane/electrode interface are least understood. To have a better understanding the nano-scale properties of the Pt(100)/ Nafion^® interface were investigated by using STM and AFM. In STM, the surface morphology of the thin Nafion^® film appeared smooth on the atomic scale; monoatomic steps of the substrate were clearly visible. The thickness of the Nafion^® membrane was determined using AFM tip penetration experiments and also by scratching the Nafion^® layer from the electrode surface. Force deflection ("approach") curves demonstrated that the tip penetrated in the Nafion^® layer at a normal force greater than 5 nN.