Adsorption in catalysis

Selected entropy-enthalpy values from adsorption equilibrium constants in published rate expressions. There is a surprising lack ofwell defined values of this type, making it difficult to understand or document the behaviour of adsorption in catalysis. 

Carbon Monoxide Carbon monoxide is a model adsorbate for studying molecular adsorption in catalysis and electrocatalysis. It is a major poisoning species in the oxidation of small organic... 

Structure Modification. Several types of stmctural defects or variants can occur which figure in adsorption and catalysis (/) surface defects due to termination of the crystal surface and hydrolysis of surface cations (2) stmctural defects due to imperfect stacking of the secondary units, which may result in blocked channels (J) ionic species, eg, OH , AIO 2, Na", SiO , may be left stranded in the stmcture during synthesis (4) the cation form, acting as the salt of a weak acid, hydrolyzes in aqueous suspension to produce free hydroxide and cations in solution and (5) hydroxyl groups in place of metal cations may be introduced by ammonium ion exchange, followed by thermal deammoniation. 

In tills chapter we consider systems in which a reaction between two gaseous species is carried out in die adsorbed state on die surface of a solid. The products of die reaction will be gaseous, and die solid acts to increase die rate of a reaction which, in die gaseous state only, would be considerably slower, but would normally yield die same products. This effect is known as catalysis and is typified in industty by die role of adsorption in increasing die rate of syndiesis of many organic products, and in die reduction of pollution by die catalytic converter for automobile exliaust. 

Henry Eyring s research has been original and frequently unorthodox. He woj one of the first chemists to apply quantum mechanics in chemistry. He unleashed a revolution in the treatment of reaction rates by use of detailed thermodynamic reasoning. Having formulated the idea of the activated complex, Eyring proceeded to find a myriad of fruitful applications—to viscous flow of liquids, to diffusion in liquids, to conductance, to adsorption, to catalysis. 

It is important to distinguish clearly between the surface area of a decomposing solid [i.e. aggregate external boundaries of both reactant and product(s)] measured by adsorption methods and the effective area of the active reaction interface which, in most systems, is an internal structure. The area of the contact zone is of fundamental significance in kinetic studies since its determination would allow the Arrhenius pre-exponential term to be expressed in dimensions of area"1 (as in catalysis). This parameter is, however, inaccessible to direct measurement. Estimates from microscopy cannot identify all those regions which participate in reaction or ascertain the effective roughness factor of observed interfaces. Preferential dissolution of either reactant or product in a suitable solvent prior to area measurement may result in sintering [286]. The problems of identify-... 

Upon calcination the template is removed and the zeolite s well-defined pores are available for adsorption and catalysis. Particularly challenging is the field of electrophilic aromatic substitution. Here often non-regenerable metal chlorides serve as the catalyst in present industrial practice. Zeolites are about to take over the job and in fact are doing so for aromatic alkylation. 

In order to get the pore system of zeolites available for adsorption and catalysis the template molecules have to be removed. This is generally done by calcination in air at temperatures up to 500 °C. A careful study (ref. 12) of the calcination of as-synthesized TPA-containing MFI-type single crystals by infrared spectroscopy and visible light microscopy showed that quat decomposition sets in around 350 °C. Sometimes special techniques are required, e.g. heating in an ammonia atmosphere (ref. 13) in the case of B-MFI (boron instead of aluminum present) to prevent loss of crystallinity of the zeolite during template quat removal. 

Langmuir s research on how oxygen gas deteriorated the tungsten filaments of light bulbs led to a theory of adsorption that relates the surface concentration of a gas to its pressure above the surface (1915). This, together with Taylor s concept of active sites on the surface of a catalyst, enabled Hinshelwood in around 1927 to formulate the Langmuir-Hinshelwood kinetics that we still use today to describe catalytic reactions. Indeed, research in catalysis was synonymous with kinetic analysis... 

Inhibitors are species that bind to enzymes, modifying their activity. Competitive inhibitors bind at the same site as the substrate binds this is analogous to competitive adsorption in heterogeneous catalysis. The reaction scheme becomes ... 

Characterization is an important field in catalysis. Spectroscopy, microscopy, diffraction and methods based on adsorption and desorption or bulk reactions (reduction, oxidation) all offer tools to investigate the nature of an active catalyst. With such knowledge we hope to understand catalysts better, so that we can improve them or even design new catalysts. 

The most common application of infrared spectroscopy in catalysis is to identify adsorbed species and to study the way in which these species are chemisorbed on the surface of the catalyst. Sometimes infrared spectra of adsorbed probe molecules such as CO and NO give valuable information on adsorption sites on a catalyst. We will first summarize the theory behind infrared absorption. 

Figure 4.22. The infrared spectrum of gas phase CO shows rotational fine structure, which disappears upon adsorption, as shown by the spectrum of CO adsorbed on an Ir/Si02 catalyst. [J.W. Niemantsverdriet, Spectroscopy in Catalysis, An Introduction (2000), Wiley-VCH, Weinheim.]...

The structure and geometry of a surface play a dominant role with respect to its reactivity in adsorption and catalysis. It is therefore always necessary to specify which structure we are dealing with and, hence, it is important to have a notation that describes the various surfaces in a unique manner. A crystal surface is described by a vector normal to it, given by... 

Conversely, an atom in Fig. 6.23 with an affinity level that initially is empty becomes partly occupied upon adsorption. Hence, charge is transferred from the metal to the atom. This sets up a dipole that increases the surface contribution to the work function. This is the case for adsorbed halides, which will be negatively charged at the surface. We will later see that such dipole fields can explain promotion and inhibition effects caused by various adsorbates in catalysis. 

Linear relations between the activation energies and heats of adsorption or heats of reaction have long been assumed to be valid. Such relations are called Bronsted-Evans-Polanyi relations [N. Bronsted, Chem. Rev. 5 (1928) 231 M.G. Evans and M. Polanyi, Trans. Faraday Soc. 34 (1938) 11]. In catalysis such relations have recently been found to hold for the dissociation reactions summarized in Pig. 6.42, and also for a number of reactions involving small hydrocarbon fragments such as the hydro-... 

Sokol skij, D. V., and G. D. Zakumbaeva, Adsorption and Catalysis of 8th Group Metals [in Russian], Nauka Publishers, Alma-Ata, Kazakhstan, 1973. 

It is most convenient to explain catalysis using an example. We have chosen a hydrogenation catalysed by nickel in the metallic state. According to the schematic of Fig. 3.1 the first step in the actual catalysis is adsorption . It is useful to distinguish physisorption and chemisorption . In the former case weak, physical forces and in the latter case relatively strong, chemical forces play a role. When the molecules adsorb at an active site physisorption or chemisorption can occur. In catalysis often physisorption followed by chemisorption is the start of the catalytic cycle. This can be understood from Fig. 3.2, which illustrates the adsorption of hydrogen on a nickel surface. 

V.F. Kiselev and O.V. Krylov, Electron Phenomena in Adsorption and Catalysis on Semiconductors and Dielectrics, Nauka Publ., Moscow, 1979... 

Kh. Dunken and V. Lygin, Quantum Chemistry of Adsorption on the Surface of Solids, Mir Publ., Moscow, 1980 E.E. Gutman, N.N. Savvin, I.A. Myasnikov et al.. Application of Optical Spectroscopy in Adsorption and Catalysis, Nauka Publ., Alma-Ata, 1980... 

T. Paryjczak, "Gas Chromatography in Adsorption and Catalysis", Wiley, Chichester, 1986. 

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