Results from Chemisorption Studies

The ultra-high vacuum studies show that during NH3 synthesis N is formed [414]. As the desorption temperature for N is well above normal synthesis temperature, the desorption of N as N2(g) will be slow at synthesis conditions [414]. If N were not consumed by the reaction, it would soon inhibit the synthesis due to blockage of active sites [414]. These observations are consistent with N2(g) + 2 = 2N being the rate limiting step for NH3 synthesis [414]. 

Temperature programmed desorption of N2 from the industrial catalyst resembles temperature programmed desorption of N2 in the range 85 to 220 K 

Chemisorption of N2 [286, 632-640] and H2 [634] on Fe in relation to NH3 synthesis has been the subject of quantum-mechanical calculations. The dynamics of N2 chemisorption has been simulated using a semiclassical wave packet technique [641]. The simulations agree with the molecular beam experiments in the conclusion that vibrational excitation is of some importance. 

---

Oxygen chemisorption methods were used to titrate surface vanadium sites in these studies. Raman, X-ray diffraction and isotopic labeling were done to support the dispersion results from chemisorption. A further conclusion was that as the % V increased for ethane oxidation reactions that the catalytic activity and selectivity was similar to that of unsupported vanadia. 

Theoty, for the most part, has been used to provide a conceptual understanding of adsorption and reactivity. Much of the previous literature has focused on complementing surface science and organometallic chemistry in the analysis of adsorbates on model clusters and surfaces. The results from these studies have helped to establish the fundamental electronic factors that control surface chemisorption and reactivity. While theory has helped provide a wealth of information on the basic adsorbate-adsorbate and adsorbate-surface interactions, little however, has been achieved in terms of quantitative analyses. For a more indepth analysis of the quantum chemical... 

It should be noted that minerals, and oxide minerals in particular, have different types of OH groups, depending on the coordination of the O atoms, as revealed by spectroscopic studies. Goethite (a-FeOOH) has four types of surface hydroxyls whose reactivities are a function of the coordination environment of the O in the FeOH group (Sposito 1984 Sparks 2002). The FeOH groups are A-, B-, or C-type sites, depending on whether the O is coordinated with 1,3, or 2 adjacent Fe(III) ions. The fourth type of site is a Lewis acid-type site, which results from chemisorption of a water molecule on a bare Fe(III) ion. Only A-type sites are basic (can bind H+), and, on the contrary, A-type and Lewis acid sites can release a proton. The B- and C-type sites are considered unreactive. Thus, A-type sites can be either a proton acceptor or a proton donor (i.e., they are amphoteric). Other spectroscopic studies have shown that boehmite (y-AlOOH) and lepidocrocite (y-FeOOH) have two types of OH, presumably associated with different crystal faces (Lewis and Farmer 1986). 

The methods described here have been applied to the other faces of the group IV elements such as the Si(100) surface and to other semiconductor surfaces. One Important application which we have not discussed is the study of chemisorption, both on the geometry and energetics of adsorbates on surfaces. Figure 22 illustrates the result from a study of A1 on the GaAs(llO) surface. Energy surfaces for A1 atoms adsorbed on the surface were determined and comparisons made between various possible... 

Studies into the use of GSC began to take on more depth. These resulted from earlier studies into physical adsorption and chemisorption studies prior to chromatographic investigations. Gamer and Veal (24) and Garner (25) had defined... 

It is now a practice to use a variety of surface characterization techniques in the study of chemisorption and catalysis. The examples given here are illustrative most references in this section as well as throughout the chapter will contain results from several techniques. 

Previously studied possibilities for bromine storage systems are listed in Table 1. The widely known reduction of the Br2 vapor pressure by formation of adducts with various carbon materials results from strong chemisorption interactions and has Table 1. General possibilities for bromine storage... 

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

In a series of studies of carefully prepared catalysts of Pt on silica gel (7,10-12) we have shown that the Pt particles are equi-axed, (and de-finitely not cuboidal as is often assumed) that the size (or percent metal exposed) agrees with results from hydrogen chemisorption, and that the particles are free of microstrain faults or twins, except when the average size is similar to the pore size of the support. In this latter case, the particles are elongated, and there is microstrain, probably due to differ-... 

Lopes MIS, Beden B, Hahn F, Leger J-M, Lamy C. 1991. On the nature of the adsorbates resulting from the chemisorption of methanol at a platinum electrode in acid medium An EMIRS study. J Electroanal Chem 313 323-339. 

The electrodes in the direct methanol fuel cell (DMFC) (i.e. the anode for oxidising the fuel and the cathode for the reduction of oxygen) are based on finely divided Pt dispersed onto a porous carbon support, and the electro-oxidation of methanol at a polycrystalline Pt electrode as a model for the DMFC has been the subject of numerous electrochemical studies dating back to the early years ot the 20th century. In this particular section, the discussion is restricted to the identity of the species that result from the chemisorption of methanol at Pt in acid electrolyte. This is principally because (i) the identity of the catalytic poison formed during the chemisorption of methanol has been a source of controversy for many years, and (ii) the advent of in situ IR culminated in this controversy being resolved. 

The thermal stability of NH4Y zeolite in which ammonium ions have been exchanged at various levels with La3+ ions was studied. The catalytic activity of these La zeolites in isooctane cracking was measured as a function of pretreatment temperature, and an IR study of the chemisorption of pyridine was used to determine the numbers of Bronsted and Lewis sites. The structural damage resulting from high temperature calcination was examined qualitatively. 

---