Thermal desorption catalysts

Figure 4.45. Thermal desorption spectra (bottom) and corresponding catalyst potential variation (top) after electrochemical O2 supply to Ag/YSZ at 260-320°C at various initial potentials Uwr Each curve corresponds to different adsorption temperature and current, thus different values of Uwr, in order to achieve nearly constant initial oxygen coverage.31 Reprinted with permission from Academic Press.

We have undertaken a series of experiments Involving thin film models of such powdered transition metal catalysts (13,14). In this paper we present a brief review of the results we have obtained to date Involving platinum and rhodium deposited on thin films of tltanla, the latter prepared by oxidation of a tltanliua single crystal. These systems are prepared and characterized under well-controlled conditions. We have used thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and static secondary Ion mass spectrometry (SSIMS). Our results Illustrate the power of SSIMS In understanding the processes that take place during thermal treatment of these thin films. Thermal desorption spectroscopy Is used to characterize the adsorption and desorption of small molecules, In particular, carbon monoxide. AES confirms the SSIMS results and was used to verify the surface cleanliness of the films as they were prepared. 

The temperature of DeNOx reaction (function 3) comparing Figure 5.5a (NO TPD, Cat nr) to Figure 5.5b - TPSR in the presence of a three-function catalyst (CoPd/HMordenite, Cat I ), in a complete flowing feed N0/HC(CH4)/02 (excess) -the temperature of DcNOx is that of the NO thermal desorption. According to the model, the catalyst will have to produce C H O . (CH3OH, HCHO) (function 2) to proceed to the DcNOx process, as discussed in Section 4.2. 

Then the reduction of stored NO with hydrogen was addressed. The stability/reactivity of the NO adsorbed species was analysed under different atmospheres (inert and reducing) both at constant temperature and under temperature programming. The bulk of data pointed out that in the absence of significant thermal effects in the catalyst bed, the reduction of stored nitrates occurs through a Pt-catalysed surface reaction that does not involve the thermal desorption of the stored nitrates as a preliminary step. A specific role of a Pt-Ba interaction was suggested, which plays a role in the NO storage phase as well. 

The book has been written as an introductory text, not as an exhaustive review. It is meant for students at the start of their Ph.D. projects and for anyone else who needs a concise introduction to catalyst characterization. Each chapter describes the physical background and principles of a technique, a few recent applications to illustrate the type of information that can be obtained, and an evaluation of possibilities and limitations. A chapter on case studies highlights a few important catalyst systems and illustrates how powerful combinations of techniques are. The appendix on the surface theory of metals and on chemical bonding at surfaces is included to provide better insight in the results of photoemission, vibrational spectroscopy and thermal desorption. 

Temperature programmed desorption (TPD) or thermal desorption spectroscopy (TDS), as it is also called, can be used on technical catalysts, but is particularly useful in surface science, where one studies the desorption of gases from single crystals and polycrystalline foils into vacuum [2]. Figure 2.9 shows a set of desorption spectra of CO from two rhodium surfaces [14]. Because TDS offers interesting opportunities to interpret desorption in terms of reaction kinetic theories, such as the transition state formalism, we will discuss TDS in somewhat more detail than would be justified from the point of view of practical catalyst characterization alone. 

CO oxidation, 38 236 differential heat of adsorption, 38 217 Biphasic systems, catalysis see Multiphase homogeneous catalysis BiPMo catalysts, 34 39 in formamide to nitrile reaction, 34 39 Bi-postdosing thermal desorption spectroscopy cyclohexene, 42 240... 

Mercury Recovery Services, Inc. (MRS), has developed the Mercury Removal/Recovery Process (MRRP) to treat media contaminated with mercury. The ex situ process uses medium-temperature thermal desorption to remove the mercury from contaminated wastes. Process wastes are heated in a two-step process to recover metallic mercury in a 99% pure form. MRS claims MRRP can be applied to soils, activated carbon, mixed waste, catalysts, electrical equipment, batteries, lamps, fluorescent bulbs, mercurous and mercuric compounds, mercury-contaminated waste liquids, and debris. 

The GEiM-lOOO low-temperature thermal desorption unit is an ex situ technology that treats soils contaminated with volatile organic compounds (VOCs). This process involves a countercurrent drum, pulse-jet baghouse, and a catalytic oxidizer mounted on a single portable trailer. As the soil is heated in the GEM-1000 unit, contaminants are vaporized. The contaminants are then directed to the system s catalytic oxidizer, which is designed to convert virtually all of the VOCs to carbon dioxide and water vapor. The oxidizer contains approximately 4.9 ft of noble metal catalyst and can destroy between 95 and 99% of the hydrocarbons when operating between 600 and 1250°F. 

Other temperature-programmed techniques include Temperature Programmed Oxidation and Sulfidation (TPO and TPS) for investigating oxidation and sulfidation behaviour, and Temperature Programmed Desorption (TPD) (also called Thermal Desorption Spectroscopy [TDS]), which analyses gases desorbed from the surface of a solid or a catalyst on heating. 

Having identified the existence of separate oxidation sites, it is desirable to determine the densities of sites which contribute to the different selectivities among different catalysts. Since the products of the thermal desorption experiments described above are directly correlated with the active sites, it is possible to measure the number of active sites by measuring the quantities of desorbed products, provided that each and every active site produces only one molecule of reaction product. To achieve this condition, it must be established that the adsorbate is fully equilibrated with the surface, that there is no multiple reaction per site during equilibration, that there is no readsorption and reaction of desorbed products, and that all reaction products are quantitatively determined. 

C. After purging out the gas-phase butene, the catalyst was heated to various temperatures indicated, and the desorbed species were purged out of the reactor by He. Then a pulse of oxygen (or NzO) was passed over the catalyst at various temperatures. After the pulse had completely left the reactor, thermal desorption was resumed, and the desorption products were collected and analyzed. 

Catalyst desorption temperature (°C) Gas pulse Pulsing temperature (°C) Pulse size (IO17 molecules) Thermal desorption products (1017 molecules m 2) ... 

In this chapter, we discuss TPR and reduction theory in some detail, and show how TPR provides insight into the mechanism of reduction processes. Next, we present examples of TPO, TP sulfidation (TPS) and TPRS applied on supported catalysts. In the final section we describe how thermal desorption spectroscopy reveals adsorption energies of adsorbates from well-defined surfaces in vacuum. A short treatment of the transition state theory of reaction rates is included to provide the reader with a feeling for what a pre-exponential factor of desorption tells about a desorption mechanism. The chapter is completed with an example of TPRS applied in ultra-high vacuum (UHV), in order to illustrate how this method assists in unraveling complex reaction mechanisms. 

The poisoning of Pt oxidation catalysts by phosphorus was also studied by Hegedus and Gumbleton107 and by Angele and co-workers.108-110 In a work by Dulcey and co-workers for the first time the thermal desorption of the PO radical was observed in the catalytic decomposition of DMMP on a polycrystalline Pt surface.111 Catalytic decomposition of Sarin using a Pt catalyst results initially in stoichiometric amounts of the oxidation products C02, HF, H20, and H3P04.106 The application of quantum-chemical methods to platinum and palladium surfaces has been quite limited because of the difficulty to treat large clusters of platinum and palladium atoms. 

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