Chemisorption dioxide catalyst

Carbon dioxide chemisorptions were carried out on a pulse-flow microreactor system with on-line gas chromatography using a thermal conductivity detector. The catalyst (0.4 g) was heated in flowing helium (40 cm3min ) to 723 K at 10 Kmin"1. The samples were held at this temperature for 2 hours before being cooled to room temperature and maintained in a helium flow. Pulses of gas (—1.53 x 10"5 moles) were introduced to the carrier gas from the sample loop. After passage through the catalyst bed the total contents of the pulse were analysed by GC and mass spectroscopy (ESS MS). 

It was found that chemisorption equilibrium is rapidly attained in most reacting systems through rapid desorption and readsorption. With a few exceptions, chemisorbed molecules can be regarded as immobile since statistical-mechanical calculations of the chemisorption equilibrium agree well with the experiment if two-dimensional translations and rotations of the chemisorbed molecules are assumed to be nonexistent. The chemisorbed state of di- or triatomic molecules can be molecular or atomic, depending on the nature of the adsorbent. For example, the carbon dioxide molecule is chemisorbed with complete dissociation into its three atoms on metallic surfaces, while on oxidic catalysts it is chemisorbed with only partial dissociation. 

The chemisorption measurements of carbon dioxide and hydrogen were obtained at 298 K using the chemisorption apparatus (Micromeritics ASAP 2000). The samples (1.0-1.2 g) used in the chemisorption studies were reduced for 12 hr at 723 K in a flow of hydrogen, evacuated at the reduction temperature and then cooled to the adsorption temperature. The difference between the adsorption isotherms obtained by the repeated use of gas dosing and degassing system gives the amount of chemisorbed species on the catalysts. 

An intensive review (F. S. Stone) examines experimental work and interpretation of interactions where the reactants (A, B, etc.) are some of the simple gases, oxygen, hydrogen, carbon monoxide, and carbon dioxide, and the catalyst (X) is one of a few selected oxide solids. We are carried to vivid realization of the importance of electronic phenomena by the experiences of photon-influenced chemisorption and catalysis on these solids. 

These catalysts are extremely sensitive to catalyst poisons, which reduce chemisorption of hydrogen and nitrogen on the active surfaces of the catalyst and thereby reduce its activity. Gaseous oxygen-, sulfur-, phosphorus-and chlorine compounds, such as water, carbon monoxide, carbon dioxide, the latter being reduced to water under ammonia synthesis conditions, are particularly troublesome in this regard. Catalyst poisoned with oxide compounds can be reactivated by reduction with pure synthesis gas. 

Boreskov et al. (4, 5) were the first to complete a systematic investigation of the relationship between particle size and catalytic activity, after their development of a technique for measuring the surface area of platinum catalysts by means of the selective chemisorption of hydrogen (6). They showed that the specific activity of platinum in the oxidation of sulfur dioxide (4) and of hydrogen (5) varied by less than one order of magnitude for catalyst samples differing in platinum surface area by four orders of magnitude. A few years later, Kobosev s ideas were further... 

C) but did not return to the initial value (Section A) within 40 min. The addition of 15% water vapor (Section D) further decreased the sulfur dioxide removal efficiency. Curve b in Figure 2 depicts the analyses of carbon dioxide when the bauxite catalyst was subjected to the water treatment. The mirror image resemblance of curves b and a in Figure 2 suggests that the reaction stoichiometry is closely represented by Equation 1 and that the poisoning effect of water is essentially caused by its competition for chemisorption on the alumina Lewis acid sites with the sulfur precursor of the intermediate (9) reductant carbonyl sulfide. 

The chemisorption of simple gases on semiconductors can be relatively simply understood in terms of the chemical reaction of the adsorbate with the catalyst Reducing gases like hydrogen and CO are strongly and irreversibly adsorbed. On heating, only water and carbon dioxide are detectable. On adsorption, H2 mainly undergoes heterolytic dissociation (Eq. 5-47) ... 

K. Stoppek-Langner, J. Goldwasser, M. HouaUa and D.M. Hercules, Infrared and carbon dioxide chemisorption study of Mo/ZrO, catalysts, Catal. Lett., 32(3—4), 263-271, 1995. 

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