Temperature-programmed sulfidation

To summarize, TPR is a highly useful technique, which provides a quick characterization of metallic catalysts. It gives information on the phases present after impregnation and on the eventual degree of reduction. For bimetallic catalysts, TPR patterns often indicate whether or not the two components are mixed. In favorable cases, where the catalyst particles are uniform, TPR yields activation energies for the reduction as well as information on the mechanism of reduction. 

Catalysts used for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of heavy oil fractions are largely based on alumina-supported molybdenum or tungsten to which cobalt or nickel is added as a promoter [11]. As the catalysts are active in the sulfided state, activation is carried out by treating the oxidic catalyst precursor in a mixture of H2S and H2 (or by exposing the catalyst to the sulfur-containing feed). The function of hydrogen is to prevent the decomposition of the relatively unstable H2S to elemental sulfur, which would otherwise accumulate on the surface of the 

Around 500 K, the catalyst consumes H2 in a sharp peak, while simultaneously H2S and some additional H20 are produced. Amoldy et al, assign the uptake of hydrogen and the evolution of H2S to the hydrogenation of excess sulfur formed via the decomposition of the oxysulfide species in (2-8) at low temperatures  

At higher temperatures, the catalyst continues to exchange oxygen for sulfur until all molybdenum is present as MoS2  

Moulijn and colleagues used a TPS apparatus equipped with a mass spectrometer such that the consumption or production of the gases H2S, H2 and H20 could be detected separately. In the TPS patterns of MoC 3/Al2C)3 (Fig. 2.6), the presence of negative peaks mean that the corresponding gas is consumed, whereas positive peaks mean that it is produced. Below 400 K, H2S is consumed 

Although Eqs. (2-8) to (2-11) explain the results satisfactorily, one needs to be aware that TP studies detect only those reactions in the catalyst that are accompanied by a net production or consumption of gases. Suppose, for instance, that Eq. (2-11) is the result of two consecutive steps  

(2-13) follows Eq. (2-12) instantaneously, the effect will not be noticeable in the H2 signal [12]. Despite these limitations, it is concluded that TPS with mass spectrometric detection is a highly useful technique for studying the sulfidation of hydrotreating catalysts. We return to the sulfidation of molybdenum oxides in Chapter 3 (photoemission), Chapter 4 (ion spectroscopy), and also in a case study on hydrodesulfurization catalysts (Chapter 9). 

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Figure 4.21. Temperature programmed sulfidation (TPS) of M0O3/AI2O3 catalysts in a mixture of H2S and H2, showing the consumption of these gases and the production of H2O as a function of temperature. Note that H2S evolves from the catalyst around 500 K, which is attributed to the hydrogenation of elementary sulfur. [Adapted from P. Arnoldy, J.A.M. van den Heijkant, G.D. de Bok and J.A. Moulijn, J.Coto/. 92 (1985) 35.]...

Temperature programmed sulfidation or temperature programmed reaction spectroscopy usually deal with more than one reactant or product gas. In these cases a TCD detector is inadequate and one needs a mass spectrometer for the detection of all reaction products. With such equipment one obtains a much more complete picture of the reaction process, because one measures simultaneously the consumption of reactants and the formation of products. 

Figure 9.17 Quick EXAFS measurements show the effect of temperature-programmed sulfidation on the Mo K edge in NiMo/SiOj catalysts (adapted from Cattaneo et al. [60]).

Fig. 19. Combined QEXAFS and temperature-programmed sulfiding results of a Mo/ A1203 catalyst during sulfiding in a H2S/Ar gas mixture (a) Fourier transforms of the in situ EXAFS spectra above the Mo X-edge (b) variation in the H2S concentration in the gas outlet from the in situ EXAFS cell as simultaneously recorded by a mass spectrometer (61).

Temperature-programmed sulfidation or TP reaction spectroscopy (TPRS) usually deal with more than one reactant or product gas. In these cases, a... 

The sulfidation mechanism was investigated by temperature-programmed sulfidation, as the oxidic catalyst was heated in a flow of H2S and ff2, and the consumption of IH S and ff2 and the evolution of H20 were measured continuously (13). ft was found that IH S is taken up and H20 given off, even at room temperature, indicating a sulfur-oxygen exchange reaction. This conclusion was confirmed by quick extended X-ray absorption fine structure (QEXAFS) studies (Fig. f, phase 2), which also demonstrated... 

Since hydrotreating catalysts arc usually used in the presence of H2 and H2S, it is important to understand the influence of phosphorus on the reduction and sulfidation of the supported metal-oxo-specics. It is also important to know whether the phosphates arc sensitive to such treatments. In this section, activation of the catalysts is discussed on the basis of XPS, TPR, and temperature-programmed sulfidation results. Note that the bulk of the alumina support is not chemically modified by the reduction-sulfidation treatments. However, some hydrogen-reactive species and surface SH groups have already been detected on it (31, 70). 

Zeuthen, P. Blom, P. Muegge, B. Massoth, F.E. Temperature-programmed sulfidation and oxidation of Ni-Mo/alumina catalysts and reaction with ammonia. Appl. Catal. 1991, 68 (1),... 

TPAT technologies include temperature programmed desorption (TPD), temperature programmed reduction (TPR), temperature programmed oxidation (TPO), temperature programmed sulfide (TPS), temperature programmed surface reaction (TPSR) and so on. TPD is the most extensively studied, widely applied and the most mature method. Hence, TPD will be focused in the following section. 

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