Chemisorbed phase

As LEED studies have shown, the stmcture of a chemisorbed phase can change with 6. In terms of transition state theory, we can write A = (I/tq) and a common observation is that while E may change with a phase change, AS will tend to change also, and similarly. The result, again known as a compensation effect, is that the product remains relatively constant... 

The physical and chemical properties of sufficiently small particles, in particular their magnetism, must depend on their size, whence the idea of using NMR to study them. Because of the theoretical and technical difficulties associated with the NMR detection of metals, and specially since only a few can be detected, NMR work has been mainly devoted to the indirect study of metallic properties, by the intenuediary of easily detected chemisorbed phases ( F1, C) or of physically adsorbed probes ( Xe). We will develop the cases of Pt and Pd. 

Fig. 12. Vertical ionisation energies, Fermi (E.) and vacuum levels (V. L.) for gaseous (1), condensed and chemisorbed phases of (a) benzene, (b) acetylene and (c) ethylene, all plotted relative to cr-orbital ionisation potentials (I. P.) for the gas phase. Relaxation shifts are given by the vacuum level shifts while bonding shifts are given by relevant jr-orbital shifts. [Reproduced with permission from J. E. Demuth and D. E. Eastman, Phys. Rev. Letters 32, 1123 (1974)]...

At 300K, and in the absence of hydrogen, the characterisation of butene chemisorbed phase is made impossible since it undergoes transformations such as dehydrogenation on Pd(lll) and probably formation of butylidyne on Pt(lll). By extrapolation of the data to lower temperature, i.e under associative chemisorption conditions, we can however speculate that butene, with a single C=C bond, is n adsorbed on Pd(lll) and di-CT adsorbed on Pt(lll). These results are corroborated by theoretical calculations [32] whose mains results are reported in Table 2. 

Shumbera RB, Kan HH, Weaver JF (2006) Adsorption of gas-phase oxygen atoms on Pt(lOO)-hex-RO.7 degrees Evidence of a metastable chemisorbed phase. Surf Sci 600 2928... 

In Region III, Fig. 7-4, there is a transition between the so-ealled physisorbed and the chemisorbed phases, which is characterised by a large current flow associated with a partial charge transfer. In addition pH dependent studies have revealed that this process involves the abstraction of one proton The deprotonation of both uracil and thymine could occur at... 

Domains need not cover a surface completely with their own structure they can be islands surrounded by a different structure. At an early stage of adsorption, for example, islands are sometimes formed surrounded by regions of surface that are free of chemisorbed atoms. At a later stage, at least in principle, surrounding structure might be a separate chemisorbed phase of different coverage. In fact, simultaneous presence of two different surface phases is not at all unusual. 

ML s 4-fold hollow + Bridged 1 A above Mo Chemisorbed Phase... 

An extension of this approximation is the so-called "precursor" model [32,33], which is illustrated in Fig. 1.7. This model assumes a finite lifetime of particles in a second layer on the top of the chemisorbed phase ("extrinsic preciu-sor") during... 

The formation of bulk phases is most common in the case of oxygen. Transformation of the chemisorbed phase into oxide proceeds generally through a nucleation and growth mechanism. As an example. Fig. 2.26a shows an STM image from a Ru(0 001) surface that had been exposed to O2 at elevated temperature [41 ]. The right part is still the Ru(0 001) surface covered by a 1 x 1 O adlayer, while the left part had been transformed into a thin... 

MOLECULAR MODELING OF CHEMISORBED PHASES UNDER COMPETING... 

Information about the structural, electronic, energetic, or kinetic properties of chemisorbed phases formed at single-crystal surfaces can be obtained by means of a large variety of techniques, which are mainly based on the interaction of slow electrons, ions, or neutral particles with matter and the high surface sensitivity of these probes. Description of these tools can be found in the literature. In the following sections we will summarize some of the results obtained using these methods with systems which are of relevance for ammonia synthesis. 

The resonance energy has shifted from hoj= 32 eV in free CO (Fig. 3) to hw = 36 eV in CO on Ni(100). The same shift (+ 1 eV) is observed on all other surfaces where CO adsorbs molecularly and for which experiments have been performed to date (Cu(IOO), PtdlD, Ni(111), Ni(HO), Co(0001), etc.). This shift is even more dramatic if one considers that the binding energy of the 4a level is lower in the chemisorbed phase, due to improved hole screening from the metal electrons. Ep shifts actually from 12 eV in free CO to 20 eV in the chemisorbed case. Within the fairly crude estimates possible, there are no indications that the d-spacing has changed sufficiently to solely account for this effect. 

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