Chemisorbed Hydrogen

Variants on this procedure are possible. Carbon monoxide has been used in place of hydrogen,but although this eliminates some problems, it often creates others. Chemisorbed hydrogen has been titrated with alkenes these reactions may be followed using frontal elution and gas chromatography to detect products, or pulse-wise. Chemisorbed hydrogen also reacts almost instantly with excess deuterium  

It might have been logical to start a disquisition on the interaction of hydrogen with supported metals with a paragraph or two on the rates of chemisorption, as was the case with unsupported metals (Section 3.22), but the quantitative measurement of rates of adsorption (and of desorption) on highly porous materials presents formidable difficulties, and the general opinion seems to be that the reward does not justify the effort needed. It is true that some years ago it was discovered that the 

Use has also been made of the NMR behaviour of the Pt nucleus in following the consequences of hydrogen chemisorption on platinum catalysts.  

IINS is less widely used because it requires a source of thermal neutrons, but its advantage lies in hydrogen s very high incoherent inelastic cross-section it has however been mainly applied to metal powders (Raney Pt and Pd blacks), but the adsorption of hydrogen on Ru/C and Pt/C catalysts has been studied. Adsorption sites were determined by comparison with known structures (e.g. hydridocarbonyls ). HREELS cannot be used with supported metals. 

The strength of the metal-hydrogen bond mh as derived from the heat of adsorption by the equation 

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F1 NMR of chemisorbed hydrogen can also be used for the study of alloys. For example, in mixed Pt-Pd nanoparticles in NaY zeolite comparaison of the results of hydrogen chemisorption and F1 NMR with the formation energy of the alloy indicates that the alloy with platinum concentration of 40% has the most stable metal-metal bonds. The highest stability of the particles and a lowest reactivity of the metal surface are due to a strong alloying effect. 

In addition to actual synthesis tests, fresh and used catalysts were investigated extensively in order to determine the effect of steam on catalyst activity and catalyst stability. This was done by measurement of surface areas. Whereas the Brunauer-Emmett-Teller (BET) area (4) is a measure of the total surface area, the volume of chemisorbed hydrogen is a measure only of the exposed metallic nickel area and therefore should be a truer measure of the catalytically active area. The H2 chemisorption measurement data are summarized in Table III. For fresh reduced catalyst, activity was equivalent to 11.2 ml/g. When this reduced catalyst was treated with a mixture of hydrogen and steam, it lost 27% of its activity. This activity loss is definitely caused by steam since a... 

The electrocatalytic hydrogenation (ECH) of an unsaturated organic substrate (Y=Z) in aqueous or mixed aqueous-organic media (eqs [2] to [4] where M represents an adsorption site of the catalyst, and M(H) and M(Y=Z), chemisorbed hydrogen and the adsorbed substrate respectively) involves the same hydrogenation steps as those of classical catalytic hydrogenation (CH) (steps [2] to [4] stoichiometry for adsorbed species only, not for surface M) (1) ... 

The results of Table 1 show clearly that electronation of nitrocumene (39) (Scheme 15) does compete with reaction with chemisorbed hydrogen, M(H), at some stage in the electrohydrogenation process. The simplest interpretation is a direct competition between electronation of the nitro compound (eq. [7]) and reaction of the adsorbed nitro compound with chemisorbed hydrogen, M(H) (eq. [13]). However, it is quite possible that the electronation of the adsorbed nitro compound (eq. [20]) could be faster than its reaction with M(H) (eq. [13]) and the competition would then be between the cleavage of the adsorbed radical anion (eq. [21]) and its reaction with M(H) (eq. [22]). 

The surface reaction of the chemisorbed hydrogen and tetraethyltin (a controlled surface reaction) was used to produce bimetallic surface species with direct Sn-Pt interaction, which were decomposed in a hydrogen atmosphere in a subsequent step (Scheme 7.21).308... 

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... 

The great majority of experimental data (see Section III.A) indicate that the hydrogen-deuterium exchange reaction belongs to the class of acceptor reactions (i.e., reactions that are accelerated by electrons and decelerated by holes). This means that the experimenter, as a rule, remains on the acceptor branch of the thick curve in Fig. 8a, on which the chemisorbed hydrogen and deuterium atoms act as donors. Here a donor impurity must enhance the catalytic activity, while an acceptor impurity must decrease it. This is what actually occurs, as we have already seen (see Section III.A). 

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