Some experimental aspects of the determination of kinetic parameters by means of temperature-programmed desorption

Abstract

The process of desorption of a gas from a catalyst surface, especially when temperatureprogrammed, can be affected by the occurrence of other chemical or physical processes.On the basis of a simple method of analysis, which makes possible the examination of definite fractional areas of a peak, and therefore the examination of definite ranges of surface coverage during desorption, this paper examines two experimental cases of temperature-programmed desorption, relating to propylene metathesis on the practical catalysts, WO3/SiO 2 and Re20-//AI20 3.Some particular methods and procedures for collecting TPD data and obtaining reliable kinetic results in the presence of multiple peaks, and therefore in the case of the overlapping of other physical phenomena, are developed: the results on the adsorptive and desorptive properties of the two catalysts are compared with the results reported in the literature, obtained from kinetic studies concerning propylene metathesis on the same catalysts.Among the techniques for investigating and characterizing practical catalysts, temperature-programmed desorption (TPD) has become widespread in recent years. However, the obtaining of information on the nature of the interactions between adsorbed species and a catalyst surface requires the determination of reliable kinetic parameters and, in spite of the simplicity of the method, TPD spectra can be affected by many experimental factors (surface heterogeneity, diffusional resistances, desorption with readsorption or with reaction) in addition to those problems which derive from the use of a non-isothermal technique.Many desorption processes are very complex and require a careful choice of the experimental conditions in order to obtain meaningful results: TPD is a typical nonisothermal method and has retained the "philosophy" of flash desorption, from which it was derived. However, much attention must be paid to the heating rate, the sample mass, the adsorption temperature and the carrier gas flow rate, which not only influence the occurrence of heat and mass transfer limitations and readsorption, mass transfer limitations and readsorption, but also determine the possibility of overlapping or separation of different desorption processes.Many methods have been suggested [1][2][3], as far as the analysis procedure of the TPD spectra is concerned, to overcome these difficulties and to relate the kinetic parameters unambiguously and reliably to the rate-limiting process.

Catalyst Activation and Kinetics for Propylene Metathesis by Supported WOx/SiO2 Catalysts

“…4,5 Spinicci et al performed propylene temperature-programmed desorption (TPD) and observed that propylene desorption follows second-order kinetics from supported WO x /SiO 2 catalysts. 6 This TPD study, however, monitored only desorption of propylene, and no information about the desorption of 2-butene or ethylene metathesis reaction products was provided.…”

“…The current literature of olefin metathesis by supported WO x /SiO 2 catalysts has taken a global view of this reaction and has also reported on phenomenological observations. Most of the reported research has focused on fitting overall reaction rates with kinetic models. − Begley et al fitted their kinetic data for a supported 9% WO 3 /SiO 2 catalyst with both second-order Langmuir–Hinshelwood and first-order Eley–Rideal reaction mechanisms and found that the Eley–Rideal model adequately correlated their data . They also concluded that the catalyst surface is saturated at high pressures (300–900 psia and 300–500 °C) and that the reaction rate is controlled by the rate of adsorption of olefin on the surface sites .…”

“…They also concluded that the catalyst surface is saturated at high pressures (300–900 psia and 300–500 °C) and that the reaction rate is controlled by the rate of adsorption of olefin on the surface sites . From kinetic analysis, Luckner et al claimed that the olefin metathesis reaction requires dual surface sites and in a subsequent paper reported that the reaction is first-order in propylene partial pressure (400–450 °C and 1–8 atm) with a supported 10% WO 3 /SiO 2 catalyst. , Spinicci et al performed propylene temperature-programmed desorption (TPD) and observed that propylene desorption follows second-order kinetics from supported WO x /SiO 2 catalysts . This TPD study, however, monitored only desorption of propylene, and no information about the desorption of 2-butene or ethylene metathesis reaction products was provided.…”

Reaction Mechanism and Kinetics of Olefin Metathesis by Supported ReOx/Al2O3 Catalysts

“…9 Spinicci et al concluded that desorption of propylene from supported ReO x /Al 2 O 3 catalysts follows second-order kinetics with temperature-programmed desorption (TPD) studies. 10 Some kinetic studies suggested that olefin metathesis proceeds via the Langmuir−Hinshelwood kinetic model, 8,11,12 but Kapteijn et al considered the L-H model inadequate and developed a first-order kinetic model based on intermediate carbene species (ReCH 2 and ReCHCH 3 ) with product desorption being the rate-determining step. 13 This mechanism is more in line with Chauvin's metallacyclobutane mechanism that was proposed for homogeneous and supported organometallic catalysts.…”

“…In the presence of gasphase C This bimolecular reaction involving recombination of surface intermediates was earlier proposed by Spinicci et al from TPD studies of propylene desorption. 10 The bimolecular recombination of surface intermediates cannot be dominant at elevated temperatures, since it would give a reaction order of 1 with respect to propylene partial pressure, which is much lower than the measured reaction order of ∼1.8. The steady-state apparent reaction order of slightly less than 2, ∼1.8, may reflect the partial contribution of this surface bimolecular reaction pathway at elevated temperatures.…”