Temperature programmed desorption intermediates

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

Several spectroscopic, microscopic and diffraction techniques are used to investigate catalysts. As Fig. 4.2 illustrates, such techniques are based on some type of excitation (in-going arrows in Fig. 4.2) to which the catalyst responds (symbolized by the outgoing arrows). For example, irradiating a catalyst with X-ray photons generates photoelectrons, which are employed in X-ray photoelectron spectroscopy (XPS) -one of the most useful characterization tools. One can also heat a spent catalyst and look at what temperatures reaction intermediates and products desorb from the surface (temperature-programmed desorption, TPD). 

At intermediate coverages the dissociation starts between 200 and 450 K, then stops when the overall coverage becomes too high, and then restarts when new vacancies are formed due to NO desorption. The temperature-programmed desorption spectra also show a first-order-like N2 desorption peak... 

The production of butadiene from butene involves at least three surface intermediates adsorbed butene, 7t-allyl, and butadiene. One or more of these may be particularly vulnerable to attack by gas-phase oxygen on a-Fe203. From the temperature programmed desorption experiments, it was found that the products of isomerization, selective oxidation, and combustion... 

Finally, new methods of analysis have recently been developed that may allow characterization of single atoms on surfaces such as atomic force microscopy.9 In certain cases, in situ experiments can be done such as the study of electrodes, enzymes, minerals and biomolecules. It has even been shown that one atom from a tip can be selectively placed on a desired surface.10 Such processes may one day be used to prepare catalysts that may enhance selectivity. Other methods that show promise as regards detection of surface catalytic intermediates are temperature programmed desorption techniques.11 Selective poisoning of some surface intermediates with monitoring via temperature programming methods may also allow the preparation of more selective catalysts. 

In contrast to the acetaldehyde decarbonylation, reactions with ethanol over Rh (111) did not lead to formation of methane but rather to an oxametallocycle via methyl hydrogen abstraction. These data suggest that ethanol formed over supported rhodium catalysts may not be due to hydrogenation of acetaldehyde. This study shows how surface science studies of model catalysts and surfaces can be used to extract information about reaction mechanisms since the nature of surface intermediates can often be identified by methods such as temperature programmed desorption and high resolution electron energy loss spectroscopy. 

Fig. 2. Temperature-programmed desorption (TPD) spectra from 4.0 L of 2-C3H7I adsorbed on Ni(lOO) surfaces predosed with various amounts of oxygen. Three regimes are observed for this system (1) that for the clean nickel, where only the hytbogenation-dehydrogenation steps typical of transition metals are seen (left) (2) that for nickel oxide, where there is little reactivity, and where only complete oxidation is observed (right) and (3) that for an intermediate oxygen surface coverage, where some partial oxidation is manifested by the appearance of a TPD peak for acetone around 350 K (center).

A Cu0/Ti02 catalysts was characterized by NH3 temperature programmed desorption (TPD) and Fourier transform infrared (FT-IR) spectroscopy and tested for NH3 oxidation. TPD measurements showed two forms of adsorbed NH3, one of which could be removed by treatment with water vapour. FT-IR spectra showed NH3 coordinated to Lewis acid sites, which gave rise, after treatment at 150°C, to adsorbed hydrazine and nitrxyl species. In NH3 oxidation tests conversions up to 90% were observed. N2 was the main product, N2O and NO being formed to lower extents. The addition of water vapour in the feed influenced the product distribution. A reaction mechanism was proposed, involving adsorbed hydrazine, nitroxyl and amido species as intermediates for N2, N2O and NO production, respectively. 

In our discussion of the influence of structure on the turnover rate our understanding is frequently hampered by lack of information on the ratedetermining step and the most abundant surface intermediate. It would be logical to consider the structure sensitivity of the rate of an elementary step, such as the desorption of a chemisorbed gas. Results on temperature programmed desorption as a function of particle size might be simpler to interpret than those of global reactions consisting of a sequence of steps. However, few such data are available. 

The co-adsorption of reactants at below normal reaction temperature, with temperature-programmed desorption of reactants, intermediates and products, has also received increasing attention in recent years. GSC techniques again find use in this context. 

This reaction can be quite complex and can result in the formation of numerous products including acetaldehyde, ethanol, ethyl acetate, ethane, CH4 and CO. As a result, this network of reactions provided an excellent opportunity to examine the role a support can play, because with Pt/Si02 catalysts at low conversions only hydrogenolysis occurred to form CH4 and CO, whereas with Pt/Ti02 catalysts ethanol (50%), ethyl acetate (30%) and ethane (20%) were produced [33]. In situ characterization using IR spectroscopy (DRIFTS) under reaction conditions combined with TPD and TPR (Temperature Programmed Desorption and Reduction, respectively) led to the identification of acyl and acetate species on the catalyst [34] however, only the acyl species was a reactive intermediate at lower temperatures because the acetate species was too stable. This valuable information was incorporated into the reaction model. 

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