Temperature-programmed sulphiding

Temperature-programmed sulphiding (TPS) is another member of a family of techniques in which catalysts are characterised by subjecting them to a temperature-programme under specified conditions. The applicability of TPS is highlighted by an example in this chapter. 

EXAMPLE 2 TEMPERATURE-PROGRAMMED SULPHIDING OF M0O3/AI2O3... 

Carburization was carried out in a fixed-b i stainlras strel reactor (ID = 6 mm). Typically about 0.5 g of metal sulphide was sandwiched between 2 layers of quartz wool placed centrally in a temperature-programmed furnace. iTie sample was heated at 10 Kmin" to the... 

With only a few exceptions, the CdS is deposited from a standard ammo-nia/thioureabath at ca. 70°C, with variations in the concentrations of reactants, the use of temperature programming, and some variation in pH (using an ammonium chloride buffer). It is notable that, in spite of many attempts to substitute CdS by another CD material (driven by the desire for a environmentally friendlier material), CdS remains the best material to date for this purpose, both for CIS and CdTe cells. Other materials deposited by CD include various Zn(OH,S), Zn(OH,Se), and ln(OH,S) compounds and ln(OH)3. The three first materials appear to be incompletely sulphided or selenided hydroxides, and it is not clear whether they are a mixed or a single phase. Also, it is usually unclear whether oxide or hydroxide also occurs [although one XPS study of ln(OH,S) has demonstrated the absence of either ln(OH)3 or ln2S3 in the film]. While some of these buffer layers approach CdS in terms of cell efficiency, they are invariably inferior. 

Kinetic studies focus on the selection of an adequate rate expression and determination of the unknown rate parameters it contains (eq 1). Generally, the rate is not measured directly but is derived from a measured quantity, conversion or concentration, at given operating conditions such as catalyst amount and feed rate. Apart from kinetic studies to determine the rate equation, other purposes of measuring rates are comparison of various catalyst formulations in screening of new catalysts, the time-dependent behavior of the catalyst actvity to predict its long term performance and to characterize catalysts such as in temperature programmed reduction (TPR) or sulphidation (TPS) studies. 

Temperature programmed reduction (TPR), oxidation (TPO), desorption (TPD), sulphidation (TPS) and reaction spectroscopy (TPRS) form a class of techniques in which a chemical reaction is monitored while the temperature increases linearly in time. These techniques are applicable to real catalysts and single crystals and have the advantage that they are experimentally simple and inexpensive in comparison to many other spectroscopies. Interpretation on a qualitative base is rather straightforward [13]. 

A direct application is the study and optimization of catalyst pretreatment. In industrial practice, catalysts are frequently pretreated using a temperature-programmed technique. Examples are reduction in the fat hardening catalysis and sulphiding in the hydrotreatment of oil fractions in the refinery. 

Chemical methods of determining the nature and surface concentrations of active components have also been attempted Mo oxide by butene adsorption and, in reduced catalysts, by oxygen chemisorptionM0S2 and/or active site concentrations by pyridine adsorption and thiophen adsorption and temperature-programmed desorption of thiopen cf. p. 200), surface acidity by NH3 adsorption cf. p. 199). Sulphides in a Ni-Mo/ AI2O3 catalyst have been characterized by differential thermal analysis of the catalyst in oxygen. ... 

An example where changes in pretreatment steps are very drastic is in hydrotreatment. The most applied catalysts are based on a combination of molybdenum and cobalt sulphides. They are manufactured in the oxidic state, and under reaction conditions these oxides are not stable and are transformed into the sulphidic state. In practice, the oxidic catalyst is presulphided, either in the reactor (by adding H2S or a compound that is easily converted into H2S), or ex situ by the manufacturer. These presulphiding procedures often involve temperature-programmed reactions. 

---