Thermal programmed desorption

Several metal oxides (either acidic or alkaline) have also been investigated for urea alcoholysis [228, 229], with PG finding PC product yields in excess of 90% for ZnO, PbO, and MgO. In such studies, the results obtained coupled with the results of thermal programmed desorption (TPD) and Fourier transform infrared (FTIR) analyses, indicated that catalysts with appropriate acid and base properties were required for the synthesis of CCs. These results confirmed the reports of Aresta et al. [94] and Ball et al. [39], who previously had investigated the reaction of primary and secondary alcohols with urea to form carbonate. These authors found the reaction to proceed in two steps, with a combination of a weak Lewis acid and a Lewis base improving the carbonate formation. 

Figure 10.28 (a) TEM images of magnesium nanoparticles on carbon support, left bright field and right dark field [140], (b) Thermal programmed desorption (TPD) heating with... 

Truong CM, Wu M-C, Goodman DW (1993) Adsorption of formaldehyde on nickel oxide studied by thermal programmed desorption and high-resolution electron energy loss spectroscopy. J Am Chem Soc 115 3647... 

The dissociative adsorptions of H2 at the 0%-Mg3c and Osc-Mgtc sites are energetically fevourable the exothermic effects of the reactions are 12 and 14 kcal/mol, respectively. Our energies calculations conform to the trend of the site activity suggested by an experimental TPD (thermal programmed desorption) study [16] ... 

The stability of the thiol SAMs is extremely important for their applications. Thermal stability is restricted up to 100° Increasing the temperature above this value results in thiol desorption mainly as disulfides and finally as thiol molecules. Recent results from thermal-programmed desorption have shown that disulfides are desorbed at around 400 These disulfides arise from the thiol... 

Some aspects related to catalysts characteristic and behaviour will be treated such as determination of metal surface area and dispersion, spillover effect and synterisation. A detailed description of the available techniques will follow, taking in consideration some aspects of the gas-solid interactions mechanisms (associative/dissociative adsorption, acid-base interactions, etc.). Every technique will be treated starting from a general description of the related sample pretreatment, due to the fundamental importance of this step prior to catalysts characterisation. The analytical theories will be described in relation to static and dynamic chemisorption, thermal programmed desorption and reduction/oxidation reactions. Part of the paper will be dedicated to the presentation of the experimental aspects of chemisorption, desorption and surface reaction techniques, and the relevant calculation models to evaluate metal surface area and dispersion, energy distribution of active sites, activation energy and heat of adsorption. 

In general, in case of associative mechanism, the chemisorption process follows the first order in the other case it is of the second order. As already reported (Table 3, par. 3.1), the adsorption mechanism depends upon the energetic of the process and it is different for each pair of metal-gas. The knowledge of the chemisorption mechanism permits to calculate the activation energies. Thermal programmed desorption is commonly used to estimate activation energy ... 

A way to overcome this difficulty is to operate under a nonisothermal condition, imposing a temperature ramp on the system. Typical ramps employed in thermal programmed desorption are the linear ramp, T = Tg + cct, or the inverse-linear ramp, IfT = 1/T — t/a , with a and a the corresponding rates. The progressive increase of temperature allows that sites with lower desorption energies respond to the temperature ramp sooner than the sites with higher energy. This fact allows a kind of thermal desorption spectroscopy of the surface. 

DSC can also be used for the Thermal Programmed Desorption (TPD) determination coupled with a gas analyzer [11, 34]... 

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