Adsorption on alloys

Liu P, Nprskov JK. 2001b. Ligand and ensemble effects in adsorption on alloy smfaces. Phys Chem Chem Phys 3 3814. 

In this section we are going to present three examples demonstrating the main factors which determine the strength of adsorption on alloy surfaces. We will demonstrate that knowledge of the surface composition and chemical ordering is essential for understanding adsorption. In some cases it is necessary to study both the alloy with chemical contrast and the adsorbate on the atomic scale. 

It can be expected that the electronic structure changes would be reflected by the heats of adsorption of suitable chosen molecules. Indeed, Shek et al (17) report that one maximum in the thermal desorption profile of CO shifts to lower temperatures when the Cu content of alloys increases. If the variations in the entropy changes upon adsorption can be neglected (probably - they can) this would indicate a lower heat of adsorption of CO on alloys than on Pt from abt. 33 Kcal/mol on pure Pt,to 26 Kcal/mol for an alloy with abt. 20% Cu. 

However, before we accept this conclusion as the definitive one, a word of caution is necessary. Due to the CO-CO interactions the heats of adsorption depend on the coverage 0. Shek et al (17) compared Pt and alloys at a constant dosage and this could mean that the coverage of Pt was (at the same dosage) lower on Pt than on alloys, and consequently - the heat of adsorption higher. Shek et al report the above mentioned data for the (111) faces of Pt and alloys. They studied also the (110) faces, but there the effect of alloying is masked by the reconstruction of the surface upon CO-adsorption (18). 

The different kinds of adsorbed CO were observed by in situ infrared reflectance spectroscopy. The results showed that using bulk Pt-Ru alloys, the adsorbed CO species formed by dissociation of methanol, or from dissolved CO on the surface of the electrode, are different on R and on Ru. The adsorption of CO occurs on pure Pt and Ru and on alloys of different compositions, but a shift detected in the wave number of the... 

Atli A, Ahon M, Beccat P, BertoUni JC, Tardy B. 1994. Carbon monoxide adsorption on a PtgoFe2o(lll) single-crystal alloy. Surf Sci 302 121-125. 

Ge Q, Desai S, Neurock M, Kourtakis K. 2001. CO adsorption on Pt-Ru surface alloys and on the surface of Pt-Ru bulk alloy. J Phys Chem B 106 9533-9536. 

Koper MTM, Shubina TE, van Santen RA. 2002. Periodic density functional study of CO and OH adsorption on Pt-Ru alloy surfaces Implications for CO-tolerant fuel cell catalysts. J Phys Chem 106 686. 

Et4N]2[Fe2lr2(CO)i2] cluster precursor, which exhibit a high activity in the synthesis of methanol from CO and H2, were studied by Ir and Fe Mossbauer spectroscopy. The study extends from the precursors via the fresh to the aged catalysts. The presence of iridium in the metallic state as well as the presence of trivalent, divalent and alloyed iron is detected. Representative Ir and Fe Mossbauer spectra are shown in Fig. 7.69. Information about the adsorption on the surface of MgO... 

Information on the chemical state of iridium on going from the molecular precursors, and its adsorption on the surface of the support can be obtained by Ir Mossbauer spectroscopy. It allows to estimate the composition of the Ir-containing alloys that are possibly formed during the activation treatment of supported bimetallic systems. The main results obtained in the application of Ir Mossbauer spectroscopy to characterize two Ir-containing bimetallic supported nanoparticles, i.e., Pt-Ir on amorphous silica and Fe-Ir on magnesia are presented and discussed... 

Zinc dialkyldithiophosphates (ZDDPs), which act as antiwear additives in lubricating oils and were postulated to exist in various molecular forms (monomer, dimer or neutral form, and basic form), were studied by multi-edge (Zn K-, P K- and S K-) XAS for structural assessment [311]. Grazing incidence absorption spectroscopy measurements have provided evidence for breakdown of the ZDDP molecule following its adsorption on to a steel substrate surface [312]. XANES and CEMS were used to study the interaction of per-fluoropolyalkyl ether (PFPAE) additives with Fe-based alloys [313],... 

Evidence for a marked difference between the surface and bulk compositions of dilute copper-nickel alloys has been reported recently by a number of investigators (82, 87-90). Much of the experimental evidence comes from hydrogen adsorption data (74, 82, 87, 90). The conclusions of van der Plank and Sachtler were based on the premise that nickel chemisorbs hydrogen while copper does not (82, 87). The total adsorption of hydrogen at room temperature was taken as a measure of the amount of nickel in the surface. However, in hydrogen adsorption studies on the catalysts used to obtain the catalytic results in Fig. 6, the amount of adsorption on the copper catalyst, while small compared to the adsorption on nickel, is not negligible (74) However, the amount of strongly adsorbed... 

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