Surface reactions, temperature-programmed desorption studies

Peck JW, Mahon DI, Koel BE (1998) A temperature programmed desorption study of the reaction of methylacetylene on Pt(lll) and Sn/Pt(lll) surface alloys. Surf Sci 410 200... 

By field ion mass spectroscopy of NH3 on Fe at room temperature, 4 -10"" torr, N Hy species and FeN Hy species are detected [510]. Secondary ion mass-spectroscopy of NH3/Fe(110) at 130 K shows FCnNH " with n, m = 1, 2, with n = 0,1,2, 3,4, H2 and Fe" [545]. The spectrum is interpreted as fragments of molecularily adsorbed NH3 [545]. When the temperature is increased, FeNH and FeNHl decrease smoothly, while Fe increases smoothly, illustrating the decrease in surface coverage and the reaction of NH3(g) with the surface at higher temperatures [545]. The intensity of NH3 with NH2 decreases steeply above 300 K consistent with the temperature programmed desorption studies [545]. Both ions are probably formed from adsorbed species [545]. The intensity of NH" decreases more slowly than NH2 and NH3 the reason is probably that NH" is formed from both NH3(g) and NH [545]. 

The opposite of adsorption, desorption, represents the end of the catalytic cycle. It is also the basis of temperature-programmed desorption (TPD), an important method of studying the heats of adsorption and reactions on a surface, so that the activation... 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

There is of course attenuation of the signal, as shown in Fig. 5, taken from Joyner and Roberts (28) The gas phase spectrum will also be obtained, but this usually can be separated easily from the signal of the solid. This sample cell arrangement thus permits the study of the stationary-state surface during catalysis and also its evolution in response to pulses and step functions in the gas composition. The temperature of the sample should be controlled so that the surface can be studied during temperature-programmed desorption and reaction. 

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. 

The surface structure and acid sites of alumina-supported molybdenum nitride catalysts have been studied using temperature-programed desorption (TPD), and reduction (TPR), diffuse reflectance infrared spectroscopy, and X-ray diffraction (XRD) analysis. The nitride catalysts were prepared by the temperature-programmed reaction of alumina-supported molybdenum oxide (12.5% and 97.1%) with NH3 at temperatures of 773, 973, and 1173 K. TPR and XRD analyses showed that y-Mo2N was already formed at 973 K. On the basis of NH3-TPD measurements and IR spectroscopy, it was found that Lewis acid sites were predominant over Bronsted acid sites on the surface of Mo2N/A1203. 

Another application that is enabled by the bright X-rays at a synchrotron is the measurement of XPS spectra in real time, to follow the dynamics of surface reactions, and in a recent review Baraldi et al. [52] described an impressive series of examples. Here, we discuss a study on the adsorption dynamics of CO on Rh(lll), a system that also serves as an example in the sections on temperature-programmed desorption (see Chapter 2) and infrared spectroscopy (see Chapter 8). 

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. 

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