Chemisorption hydrogenation

The metal accesibiMty of the catalysts was determined by gas chemisorption (hydrogen, oxygen) at room temperature in a conventional volumetric equipment. The widely admitted adsorption stoichiometries are H/Pt = 0/Pt = 1. No chemisorption of hydrogen and oxygen on gold was observed at this conditions (14). The carvone hydrogenation reaction, in... 

One cannot fault Faraday for not inventing dissociative chemisorption. Hydrogen cind oxygen were not known to be diatomic in 1834. Anyway, Faraday did not believe in atoms cind assigned such ideas to metaphysics. 

The electronegativity system may be used. For example, in the chemisorption (with dissociation) of hydrogen on tungsten. 

We consider first some experimental observations. In general, the initial heats of adsorption on metals tend to follow a common pattern, similar for such common adsorbates as hydrogen, nitrogen, ammonia, carbon monoxide, and ethylene. The usual order of decreasing Q values is Ta > W > Cr > Fe > Ni > Rh > Cu > Au a traditional illustration may be found in Refs. 81, 84, and 165. It appears, first, that transition metals are the most active ones in chemisorption and, second, that the activity correlates with the percent of d character in the metallic bond. What appears to be involved is the ability of a metal to use d orbitals in forming an adsorption bond. An old but still illustrative example is shown in Fig. XVIII-17, for the case of ethylene hydrogenation. 

Figure Bl.26.1. Sorption isothemi for chemisorption of hydrogen on palladium film at 273 K (Stephens S J 19597. Phys. Chem.. 63 188-94).

Roohefort A, Andzelm J, Russo N and Salahub D R 1990 Chemisorption and diffusion of atomlo hydrogen In and on oluster models of Pd, Rh, and bimetalllo PdSn, RhSn, and RhZn oatalysts J. Am. Chem. Soo. 112 8239-47... 

Madhavan P and Whitten J L 1982 Theoretical studies of the chemisorption of hydrogen on copper J. Chem. Phys. 77 2673-83... 

Cremaschi P and Whitten J L 1987 The effect of hydrogen chemisorption on titanium surface bonding Theor. Chim. Acta. 72 485-96... 

Ravenek W and Geurts EMM 1986 Hartree-Fock-Slater-LCAO implementation of the moderately large-embedded-cluster approach to chemisorption. Calculations for hydrogen on lithium (100) J. Chem. Phys. 84 1613-23... 

The use of silver fluoroborate as a catalyst or reagent often depends on the precipitation of a silver haUde. Thus the silver ion abstracts a CU from a rhodium chloride complex, ((CgH )2As)2(CO)RhCl, yielding the cationic rhodium fluoroborate [30935-54-7] hydrogenation catalyst (99). The complexing tendency of olefins for AgBF has led to the development of chemisorption methods for ethylene separation (100,101). Copper(I) fluoroborate [14708-11-3] also forms complexes with olefins hydrocarbon separations are effected by similar means (102). 

Hydrogen gas chemisorbs on the surface of many metals in an important step for many catalytic reactions. A method for estimating the heat of hydrogen chemisorption on transition metals has been developed (67). These values and metal—hydrogen bond energies for 21 transition metals are available (67). 

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. 

The previous volume measurement was done by methane because this does not react and does not even adsorb on the catalyst. If it did, the additional adsorbed quantity would make the volume look larger. This is the basis for measurement of chemisorption. In this experiment pure methane flow is replaced (at t = 0) with methane that contains C = Co hydrogen. The hydrogen content of the reactor volume—and with it the discharge hydrogen concentration— increases over time. At time t - t2 the hydrogen concentration is C = C2. The calculation used before will apply here, but the total calculated volume now includes the chemisorbed quantity. 

As this field is very wide, we will discuss first the gases that can be used to study metal dispersion by selective chemisorption, and then some specific examples of their application. The choice of gases, is, of course, restricted to those that will strongly chemisorb on the metal, but will not physically adsorb on the support. Prior to determining the chemisorption isotherm, the metal must be reduced in flowing hydrogen details are given elsewhere. The isotherm measurement is identical to that used in physical adsorption. 

To give an idea of the wide rai e of catalytic systems that have been investigated where chemisorption data were essential to interpret the results, some of the author s papers will be discussed. Measurements were reported on the surface areas of a very wide range of metals that catalyze the hydrogenation of ethane. In the earliest paper, on nickel, the specific catalytic activity of a supported metal was accurately measured for the first time it was shown also that the reaction rate was direcdy proportional to the nickel surface area. Studies on the same reaction... 

Standard Test Methodfor Surface Area of Catalysts. (D3663—78) Standard Test Method for Hydrogen Chemisorption on Supported Platinum on Alumina Catalysts. (D3908-80) American Society for Testing and Materials (ASTM), Philadelphia, PA. 

One of us (RES) suggested over two years ago[6] that the resolution to this question lies in the electric field inherent to the arc plasma. As argued then, neither thermal nor concentration gradients are close to the magnitudes required to influence tip annealing, and trace impurities such as hydrogen, which might keep the tip open, should have almost no chemisorption residence time at 3000°C. The fact that w ell-formed nanotubes are found only in the cathode deposit, where... 

Table III. Hydrogen Chemisorption as an Indicator of Catalytically Active Area...

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... 

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