Redhead method

Particularly popular among surface scientists is the Redhead method [26], in which the activation energy of desorption is given by ... 

Thermodynamic data regarding the adsorption of CO on Au/Ti02 catalysts with varying Au cluster sizes have been acquired with TPD using the well-known Redhead method " and with IRAS using the Clausius-Clapeyron relationship. Results for these measurements are displayed in Fig. 6. CO adsorption on Au clusters larger... 

One of the standard surface science methods for assessing the concentration and stability of a chemisorbed species is thermal desorption spectroscopy (TDS). An early paper by Redhead ( 7) developed the conceptual framework for certain cases. Many papers since then have expanded the applicability of this method. Recent work of Madix Q8) > Weinberg (9) and Schmidt CIO) is particularly noteworthy. Most of this work focuses on the desorption of a single molecular species and not on reactions in desorbing systems. However, qualitative features of the temperature dependence of reactions can be assessed using this method. Figures 1 and 2 taken from the... 

Although quantitative calculation of the accurate PESs remains a difficult task (see Section 20.1.2), the two-state-model describes the essential reaction dynamic process and is useful for a qualitative understanding. When the reaction coordinate is set to the adsorbate-surface distance (one-dimension), the two-state-model is called the Menzel-Gomer-Redhead [49] and/or Antoniewicz [50] model. We refer to them as the MGR models. The MGR models are often used successfully to analyze photodesorption on metal surfaces by assuming a short residential time on the excited PES. There are several methods to simulate the quantum dynamics of the MGR models, for example, stochastic wavepacket [51], open density matrix methods [52], and so on. 

The shortcoming of Redhead s method is chat it is basd on che assumption that all the desorption parameters are coverage-independent. This is often not true in practice. 

The second technique, initially adopted by Redhead (22) uses a much slower desorption rate [35°K/sec or less (23)]. This method enables a greater resolution of the individual peaks of the spectrum (Fig. 5), and more accurate estimates of the long-term pumping effects of the system to be made. The adsorption-desorption rates for CO on the walls of the apparatus are allowed to come to equilibrium by leaking CO... 

In 1961, two researchers published reports of systematic studies of CO on poly crystalline tungsten using flash desorption experiments. Ehrlich (27) used a technique involving the very rapid flash, and Redhead (22) the slower desorption method. The results obtained from these two studies, and from later reports by Ehrlich (16, 28), were in good agreement, bearing in mind the limitations of the technique and the undefined nature of the poly crystalline substrates. These results and more recent studies will be discussed in the next two sections. 

The method of determining these desorption energies differs between authors. The approach adopted by Redhead (22) is to use the Polanyi-Wigner equation for desorption, that is... 

Despite its wide use, the Redhead model has inherent difficulties such as the heating rate problems mentioned above. An alternative analysis method has been proposed and is outlined below. 

Given such postulated, but not general, rate equations, a variety of methods is available for evaluating the rate parameters. We will consider the uniqueness or otherwise of the rate parameters subsequently, but first we will describe briefly the analytical procedures with specific reference to their application to metal—semiconductor systems. The simplest, and in many cases most accurate, is the peak temperature method first described by Redhead [63]. AE can be directly obtained by measuring the temperature, Tp, at which the desorption rate is a maximum, then by setting d2n/dt2 = 0, an expression for AE can be derived in terms of v, Tp and the known heating rate 3 v is usually assumed to be 1013 s-1. 

Method (ii). Redhead [63] derived equations for desorption rate maxima for various orders of reaction, but the expressions are rather insensitive to order so, again, evaluation of the pre-exponential term does not necessarily provide physically useful information. Because Tp can be obtained accurately, however, the energy term can also be precisely estimated and where the adsorbate—surface interaction dominates the adsorbate—adsorbate interaction, it provides a reliable value for the... 

Table 3.7-1 The values of n and V , from the Redhead s method (Redhead, Langmuir, 1996 ...

The monolayer coverages obtained by the BET method are also included in the table, and these values are very comparable to those obtained by the Redhead s method. 

To extract quantitative information, mathematical considerations are necessary. Most simply, the peak position and the shape of the temperature programmed desorption spectrum are analyzed on the basis of the method proposed by Redhead. Here a sufficiently high pumping speed is assumed. Since the detected signal is proportional to the amount of the desorbed species under the experimental conditions, the desorption rate during a temperature rise is given... 

Apart from this approach which implies the evidence of Tmax, there is another which includes the value of peak width in the analysis. Also, many authors rely on the application of other, even more simplified methods that enable the calculation of kinetic parameters. Particularly popular among surface scientists are the Redhead s and Kissinger s methods. 

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