Chemisorption precursor

To account for the electronic field, the authors modeled die O2 + iT(H20)3 + e /Pt(l 11) system. The study observed that at first the proton transfer intermediate was formed rapidly, similar to results reported by Jinnouchi et al. [87], flien end-on chemisorption and electron transfer proceeded. The formation of die end-on chemisorption precursor H-O-O-Pt had an energy barrier of about 0.4 eV. They suggested that the mechanism for die first eleetron transfer involved 1) proton transfer 2) electron transfer 3) dissoeiation and hydroxyl adsorption (Figure 5.11) ... 

We begin this section with a description of the valence band, 4f, and 4d photoelectron spectra of what is believed to be stoichiometric Ce02. We then conclude with a review of the oxidation of metallic Ce. As we will see, all oxidation studies have been carried out on polycrystalline, evaporated films in spite of the fact that chemisorption, precursor states, and oxidation rates depend on the crystallographic plane of the surface and the presence of grain boundaries or lowerdimensional defects. There remains, therefore, a great deal of work to be done with these complex systems brfore they are fully understood. 

Note that the van der Waals forces tliat hold a physisorbed molecule to a surface exist for all atoms and molecules interacting with a surface. The physisorption energy is usually insignificant if the particle is attached to the surface by a much stronger chemisorption bond, as discussed below. Often, however, just before a molecule fonus a strong chemical bond to a surface, it exists in a physisorbed precursor state for a short period of time, as discussed below in section AL7.3.3. 

Note that chemisorption often begins with physisorption into a weakly bound precursor state. While in this... 

The chemisorption of a molecule is often a precursor [31] to fiirther reactions such as dissociation (see section A3,9.5.2). that is, the molecule must reside in the precursor state exploring many configurations until finding that leading to a reaction. Where there is more than one distinct chemisorption state, one can act as a precursor to the other [32], The physisorption state can also act as a precursor to chemisorption, as is observed for the 02/Ag(l 10) system [33],... 

Rettner C T, Schweizer E K and Stein H 1990 Dynamics of chemisorption of N2 on W(100) Precursor-mediated and activated dissociation J. Chem. Phys. 93 1442... 

J Rettner C T and Mullins C B 1991 Dynamics of the chemisorption of O2 on Pt(111) Dissociation via direct population of a molecularly chemisorbed precursor at high incidence kinetic energy J. Chem. Phys. 94 1626... 

U ] Rettner C T and Stein H 1987 Effect of the translational energy on the chemisorption of N2 on Fe(111) activated dissociation via a precursor state Phys. Rev. Lett. 59 2768... 

A MgO-supported W—Pt catalyst has been prepared from IWsPttCOIotNCPh) (i -C5H5)2l (Fig. 70), reduced under a Hs stream at 400 C, and characterized by IR, EXAFS, TEM and chemisorption of Hs, CO, and O2. Activity in toluene hydrogenation at 1 atm and 60 C was more than an order of magnitude less for the bimetallic cluster-derived catalyst, than for a catalyst prepared from the two monometallic precursors. 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

Physisorption, originating from Van der Waals interaction between reactant and surface. This weakly exothermic process is reversible and does not result in any new chemical bonds being formed. In general physisorption does not lead to catalytic activity but may be a precursor to chemisorption. 

The interaction of hydrogen (deuterium) molecules with a transition metal surface c an be conveniently described in terms of a Lennard--Jones potential energy diagram (Pig. 1). It cxxislsts of a shallcw molecular precursor well followed by a deep atomic chemisorption potential. Depending on their relative depths and positions the wells m or may not be separated by an activation energy barrier E as schematically Indicated by the dotted cur e in Fig. 1. 

When the metal nanoparticles are inserted into zeolite supercages, the size of the metal particles is confined according to the size of the supercage. However, after reduction of the precursor metal ions in a stream of hydrogen, the protons replacing the metal ions in the cation exchange position also interfere with the metal particles, influencing thereby their chemisorption and catalytic properties. 

The observed distribution can be readily explained upon assuming that the only part of polymer framework accessible to the metal precursor was the layer of swollen polymer beneath the pore surface. UCP 118 was meta-lated with a solution of [Pd(AcO)2] in THF/water (2/1) and palladium(II) was subsequently reduced with a solution of NaBH4 in ethanol. In the chemisorption experiment, saturation of the metal surface was achieved at a CO/Pd molar ratio as low as 0.02. For sake of comparison, a Pd/Si02 material (1.2% w/w) was exposed to CO under the same conditions and saturation was achieved at a CO/Pd molar ratio around 0.5. These observations clearly demonstrate that whereas palladium(II) is accessible to the reactant under solid-liquid conditions, when a swollen polymer layer forms beneath the pore surface, this is not true for palladium metal under gas-solid conditions, when swelling of the pore walls does not occur. In spite of this, it was reported that the treatment of dry resins containing immobilized metal precursors [92,85] with dihydrogen gas is an effective way to produce pol-5mer-supported metal nanoclusters. This could be the consequence of the small size of H2 molecules, which... 

SCHEME 1 Modification of double-stranded DNA with organosulfur precursor and immobilization on An surface via chemisorption. 

Dissociative chemisorption was considered to be either direct, when the incoming diatomic molecule has sufficient energy to surmount the barrier without being trapped into the molecular state, or indirect, when it passes via the molecular (precursor) state into the dissociated state. If the dissociated state is not immediately equilibrated with the lattice, the fragments will move across... 

Highly mesoporous carbon supported Pd catalysts were prepared using sodium formate and hydrogen for the reduction of the catalyst precursors. These catalysts were tested in the enantioselective hydrogenation of isophorone and of 2-benzylidene-l-benzosuberone. The support and the catalysts were characterized by different methods such as nitrogen adsorption, hydrogen chemisorption, SEM, XPS and TPD. 

The anchoring and the reduction methods of precious metal precursors influence the particle size, the dispersion and the chemical composition of the catalyst. The results of SEM and H2 chemisorption measurements are summarised in Table 3. The XPS measurements indicate that the catalysts have only metallic Pd phase on their surface. The reduction of catalyst precursor with sodium formate resulted in a catalyst with lower dispersion than the one prepared by hydrogen reduction. The mesoporous carbon supported catalysts were prepared without anchoring agent, this explains why they have much lower dispersion than the commercial catalyst which was prepared in the presence of a spacing and anchoring agent (15). 

Relevant to the synthesis of ammonia over iron catalysts is the observation of Ertl et al. (54) that potassium preadsorbed by an iron catalyst ((7 = 0.1) increased the rate of synthesis at 430 K by a factor of about 300. This effect the authors attributed to an enhancement of the heat of adsorption of molecular nitrogen due to transfer of electronic charge from potassium to the surface of the iron catalyst. This would be entirely in keeping with the precursor model proposed for nitrogen chemisorption (55). 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

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