Chemisorption behavior

A comparison of the qualitative features of the FRC spectra for the catalyst studied show a clear distinction between Rh/S102 and Rh/T102, In terms of their reversible H2-chemlsorptlon. Suprlslngly, little difference was observed between normal and SMSI-Rh/T102. "Normal" Rh/T102 behaved quite differently from Rh/S102, In spite of their similarities In total, l.e., static, chemisorption behavior. 

The reversible H2 chemisorption behavior of Rh/S102 Is different from that of Rh/T102 ... 

In Its H2 chemisorption behavior, Rh/T102 In the "normal" state Is more like Rh/T102 In the SMSI state than Rh/S102>... 

However, it will at any rate be clear now that the palladium, nickel, and iridium catalysts used in our experiments differ widely in surface characteristics, as is evident from the variations in chemisorptive behavior. An obvious question that may be asked now is whether the catalysts differ also in catalytic behavior. This induced us to study the reaction of benzene with deuterium on the nickel and iridium catalysts. 

It has also to be remembered that the band model is a theory of the bulk properties of the metal (magnetism, electrical conductivity, specific heat, etc.), whereas chemisorption and catalysis depend upon the formation of bonds between surface metal atoms and the adsorbed species. Hence, modern theories of chemisorption have tended to concentrate on the formation of bonds with localized orbitals on surface metal atoms. Recently, the directional properties of the orbitals emerging at the surface, as discussed by Dowden (102) and Bond (103) on the basis of the Good-enough model, have been used to interpret the chemisorption behavior of different crystal faces (104, 105). A more elaborate theoretical treatment of the chemisorption process by Grimley (106) envisages the formation of a surface compound with localized metal orbitals, and in this case a weak interaction is allowed with the electrons in the metal. 

That CO chemisorption is perturbed on strained-layer Ni is not surprising in view of CO chemisorption behavior on other metal overlayer systems. For example, on Cu/Ru it has been proposed that charge transfer from Cu to Ru results in decreased occupancy of the Cu 4s level. This electronic modification makes Cu more nickel-like , and results in an increase in the binding energy... 

The chemisorption of over 25 hydrocarbons has been studied by LEED on four different stepped-crystal faces of platinum (5), the Pt(S)-[9(l 11) x (100)], Pt(S)-[6(l 11) x (100)], Pt(S)-[7(lll) x (310)], and Pt(S)-[4(l 11 x (100)] structures. These surface structures are shown in Fig. 7. The chemisorption of hydrocarbons produces carbonaceous deposits with characteristics that depend on the substrate structure, the type of hydrocarbon chemisorbed, the rate of adsorption, and the surface temperature. Thus, in contrast with the chemisorption behavior on low Miller index surfaces, breaking of C-H and C-C bonds can readily take place at stepped surfaces of platinum even at 300 K and at low adsorbate pressures (10 9-10-6 Torr). Hydrocarbons on the [9(100) x (100)] and [6(111) x (100)] crystal faces form mostly ordered, partially dehydrogenated carbonaceous deposits, while disordered carbonaceous layers are formed on the [7(111) x (310)] surface, which has a high concentration of kinks in the steps. The distinctly different chemisorption characteristics of these stepped-platinum surfaces can be explained by... 

The intentional design of model systems can be envisioned, as for instance binary or multiple assemblies (clusters) of active components and poisons, for the examination of their activity in chemisorption, or specific reactions. The results can then be compared with respective clusters containing the active species only. Perhaps, such model systems will be amenable to computational methods capable of predicting their chemisorptive behavior and their surface reactivity. Such approaches are now employed for the design of improved multicomponent catalysts and can, obviously, be used to study the reverse effect, i.e., the mutual deactivation of the cluster components. 

The aim of specific poisoning is the determination of the chemical nature of catalytically active sites and of their number. The application of the HSAB concept together with eight criteria that a suitable poison should fulfill have been recommended in the present context. On this basis, the chemisorptive behavior of a series of hard poisoning compounds on oxide surfaces has been discussed. Molecules that are usually classified as soft have not been dealt with since hard species should be bound more strongly on oxide surfaces. This selection is due to the very nature of the HSAB concept that allows only qualitative conclusions to be drawn, and it is by no means implied that compounds that have not been considered here may not be used successfully as specific poisons in certain cases. Thus, CO (145, 380-384), NO (242, 381, 385-392, 398), and sulfur-containing molecules (393-398) have been used as probe molecules and as specific poisons in reactions involving only soft reactants and products (32, 364, 368). 

The stepped nickel 9(lll)x(lll) and stepped-kinked Ni 7(lll)x(310) surfaces displayed a benzene coordination chemistry that was quantitatively and qualitatively identical with that of the Ni(lll) surface except that not all the benzene was displaced by trimethylphosphine indicating that either a small percentage ( 10%) of the benzene on these surfaces either was present in different environments or was dissociatively (9) chemisorbed see later discussion of stereochemistry. Benzene chemisorption behavior on Ni(110) was similar to that on Ni(lll) except that the thermal desorption maximum was lower, vl00°C, and that trimethylphosphine did not quantitatively displace benzene from the Ni(110)-C H surface. In these experiments, no H-D exchange was observed with CgHs + C D mixtures. 

MVD of ruthenium on Pt(llO) has been shown to provide an ideal system for the study of the promotion of electrocatalytic reactions on a well-characterized Pt-Ru alloy surface [85,86]. In transfer studies, XPS, LEISS, and LEED have been used to characterize the Pt(110)-Ru alloy system, and TPD and stripping voltammetry used to investigate the chemisorption behavior of CO, and the promotion of CO electro-oxidation as a function of incorporated ruthenium. The facile incorporation of ruthenium in the relatively open-packed Pt(110)-(1 x 2) surface provided an ideal model for the alloy system. It is also interesting to note also that the clean Pt(llO) surface exhibits the highest hydrogen oxidation currents of the three basal planes of platinum [108]. 

Unfortunately, only relatively little studies have been carried out so far where the influence of the electronic structure of a glassy metal substrate on the chemisorption behavior of probe molecules such as CO and N2 has been investigated. However, in view of the high flexibility in the chemical composition... 

The data on the catalyst containing rhenium alone indicate signficant chemisorption of carbon monoxide, but no chemisorption of hydrogen. As expected, the platinum catalyst chemisorbs both carbon monoxide and hydrogen, and the values of CO/M and H/M are nearly equal. The platinum-rhenium catalyst exhibits a value of CO/M about twice as high as the value of H/M. This result approximates what one would expect if hydrogen chemisorbed on only the platinum component of the catalyst. While this chemisorption behavior is consistent with the possibility that the platinum and rhenium are present as two separate entities in the catalyst, they do not rule out the possibility that bimetallic clusters of platinum and rhenium are present. 

C, and then reduced in CO at temperatures ranging from 300 to 800 °C. The more extreme CO treatments, in terms of temperature and time, tended to lower the average catalytic activity. This behavior parallels the chemisorption behavior shown in Figure 15. 

Growth of surface alloy films and chemisorption behavior of CO on Sm/Ru(001) XPS and TPD studies... 

Halogen species can also be important bonding modifiers, because they are powerful electron acceptors. Indeed, they are used as promoters in several catalytic processes (for example, ethylene oxidation to ethylene oxide over silver, or during partial oxidation of methane). Nevertheless, their molecular and atomic chemisorption behavior has been studied less and therefore is not as well understood as the role of coadsorbed alkali-metal ions. 

Besides these apparent similarities, there are, of course, also substantial differences. Long range effects on metal surfaces are completely absent in those cluster models (and clusters) which contain only a few surface atoms. Some characteristics of the cluster model which have a large impact on the chemisorption behavior (e.g. the ionization potential, the electron affinity, the position of the Fermi level, the static polarizability, etc.) may be considerably different than the corresponding bulk values. These differences obviously reflect the truncated nature of the cluster and the atypical coordination of its constituent atoms. The so-called bond preparation approach has been proposed to overcome, at least... 

Chemisorption behavior of simple moleeules provides important information on eatalyst surfaees. First, it ean quantify the number of sites exposed on a given eatalyst if the stoiehiometry of the ehemisorption and its loeation are known. The turnover rate, the number of catalj ie eyeles per reaetion site per time, is based on this number of surfaee sites titrated by the selective chemisorption. Second, energetics and modes of chemisorption probed by temperature-programmed desorption (TPD), calorimetry, or vibrational spectroscopies reveal the nature of adsorbate-adsorbent interactions and electronic state of the surface. Most frequently employed probe molecules are CO, H2, NO, and O2, and simple acid and base molecules are used to probe the acid-base properties of the surface. 

Figure4.7 shows MIE/UP spectra of different amounts of CO on / t(lll), to further study the chemisorption behavior. The MIE spectra of CO show the superior surface sensitivity of MIES compared to UPS. With MIES for 0.1 CO/SA ( 1 /4 ML) first CO features can be seen, whereas with UPS only with increasing coverages small feamres are visible, that are dominated by those from the support. Consequently, in the following argumentation only the MIES data set is further described and discussed (IP energies are used for description unless noted differently). The MIE spectrum at 0.03 COjSA coverage shows no clear peaks from CO, further the spectrum shape differs in comparison to those seen for other adsorbates on Pt(l 11) in later sections. For 0.05 CO/SA and 0.07 CO/SA a peak at 18.3eV and a small peak at 15.8eV is...

Second, the well known chemisorption behavior of ethene is characterized with the same combination of EES and TPD on surfaces and serves as a future comparison for the study of the chemisorption behavior of ethene on size-selected Pt clusters by means of EES. The reactivity of ethene towards the hydrogenation reaction is probed by TPR and also further preliminary experiments (AES and IRRAS) are shown in order to investigate the mechanism of the ethene hydrogenation reaction on size-selected clusters. 

Despite the failure of the activation of TCE on clusters observed in TPD, EES experiments of deposited selected clusters of similar sizes were performed. The exclusion of additional molecules and the chemisorption behavior for selected clusters, reduces the origin of possible shifts in peak energy positions in EE spectra to the effect of physisorption induced relaxation shifts. Thus, it allows for uncomplicated comparison of the differences in physisorption on the surfaces and supported clusters (if omitting compensating effects of potential energy and electron relaxation, as discussed in Sect. 2.2.4). 

Other connections to surface potential and atomic chemisorption behavior have been given by Lang (1981), and Lang and Williams (1978). 

Studies of chemisorption of gaseous molecules on oxides may provide new insights into the various processes involved in the formation, aging, and degradation of passive films. We will restrict ourselves to a brief description of the main features of the chemisorption behavior on oxides. We will focus on results obtained on well-defined surfaces of single crystals with O2 and H2O as adsorbates, hr the past 10 years, precise data on the structure and the electronic properties of surfaces of single crystals of oxide have been obtained by LEED and surface-sensitive spectroscopies (UPS, XPS, HEELS). Excellent introductions to this subject can be found in Refs. Ill and 112. A large part of the data presented here is extracted from these reviews. 

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