Dihydrogen chemisorption

Reaction between carbon monoxide and dihydrogen. The catalysts used were the Pd/Si02 samples described earlier in this paper. The steady-state reaction was first studied at atmospheric pressure in a flow system (Table II). Under the conditions of this work, selectivity was 100% to methane with all catalysts. The site time yield for methanation, STY, is defined as the number of CH molecules produced per second per site where the total number of sites is measured by dihydrogen chemisorption at RT before use, assuming H/Pd = 1. The values of STY increased almost three times as the particle size decreased. The data obtained by Vannice et al. (11,12) are included in Table II and we can see that the methanation reaction on palladium is structure-sensitive. It must also be noted that no increase of STY occurred by adding methanol to the feed stream which indicates that methane did not come from methanol. 

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... 

Autocatalytic decomposition of [PtMe2(COD)] on platinum black, under dihydrogen has already been pointed out in Uquid solution [44]. Indeed, the platinum atoms present in the starting complex are incorporated into the surface of the solid platinum catalyst, thus becoming the reactive sites for further cycles of chemisorption and reaction (Scheme 1). 

E. E. Donath History of Catalysis in Coal Liquefac-tioa - G. K Boreskov Catalytic Activation of Dioxygen. - M. A. Wannice Catalytic Activation of Carbon Monoxide on Metal Surfaces. - S.R. Morrison Chemisorption on Nonmetallic Surfaces. - Z. Knor Chemisorption of Dihydrogen. - P.N.lfylander Catalytic Processes in Organic Conversions. 

Selective chemisorption uses a probe molecule that does not interact significantly with the support material but forms a strong chemical bond to the surface metal atoms of the supported crystallites. Chemisorption will be discussed in more detail in Section 5.2. Dihydrogen is perhaps the most common probe molecule to measure the fraction of exposed metal atoms. An example of H2 chemisorption on Pt is shown below ... 

For the hydrogenation of propionaldehyde (CH3CH2CHO) to propanol (CH3CH2CH2OH) over a supported nickel catalyst, assume that the rate-limiting step is the reversible chemisorption of propionaldehyde and that dihydrogen adsorbs dissociatively on the nickel surface. 

Another way to change concentration of active material is to modify the catalyst loading on an inert support. For example, the number of supported transition metal particles on a microporous support like alumina or silica can easily be varied during catalyst preparation. As discussed in the previous chapter, selective chemisorption of small molecules like dihydrogen, dioxygen, or carbon monoxide can be used to measure the fraction of exposed metal atoms, or dispersion. If the turnover frequency is independent of metal loading on catalysts with identical metal dispersion, then the observed rate is free of artifacts from transport limitations. The metal particles on the support need to be the same size on the different catalysts to ensure that any observed differences in rate are attributable to transport phenomena instead of structure sensitivity of the reaction. 

Physisorption involves only a weak attraction between the substrate and the adsorbent but in chemisorption a chemical reaction takes place between the adsorbent and atoms on the catalyst surface. As a result, chemisorbed species are attached to the surface with chemical bonds and are more difficult to remove. If the adsorption of hydrogen on nickel is considered as an example, the reaction involves the breaking of an H-H bond and the formation of two Ni-H bonds on the surface. As shown in Fig. 2.3, this adsorption occurs by way of an initially adsorbed dihydrogen molecule. It proceeds via a electron donation and back bonding to the a orbitals of the hydrogen molecule with the final formation of the two surface M-H species. 

As shown in Figure 16-4, a dihydrogen molecule is weakly attracted to the surface of the catalyst (physisorption). For chemisorption to occur, the adsorbate chemically bonds to the surface of the substrate. This causes the bond between the hydrogens to break, making them free to undergo reactions with other nearby chemicals. When the final product is created (the free hydrogen atoms binds to another reactant), the final product undergoes desorption. Traditionally, this flows in the gas stream for collection at the end. 

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