Search Results

You are looking at 21 - 30 of 90 items for :

  • Refine by Access: All Content x
Clear All

Abstract  

Temperature programmed desorption mass spectrometry (TPD-MS) and thermal gravimetry (TG) were utilized for the study of the thermal dehydration and decomposition of copper selenate pentahydrate (CuSeO4·5H2O). From the two techniques we suggest that the dehydration is a 3-step process reaching completion at 300 °C. The decomposition process however, is far more complicated consisting of several successive steps occurring between 480 and 900 °C. Initiated with the emission of oxygen, the decomposition of the anhydrous salt continues with SeO2 emission via several unstable intermediates up to the conversion of the remaining copper monoxide into dicopper monoxide accompanied by oxygen emission.

Restricted access

Abstract  

A commercial Pd/activated carbon catalyst (10%) was treated using several redox processes: reduction with gaseous hydrogen at 140 °C, reduction by negative electrochemical polarization in acidic and basic environments, oxidation with aqueous hydrogen peroxide, and positive electrochemical polarization in acidic and basic environments. To establish the electrochemical reduction/oxidation conditions, the potentials of hydrogen and oxygen evolution at Pd/AC powder electrodes were determined from cyclic voltammetric (CV) measurements. The samples were examined for the presence of palladium oxide phases on dispersed metal particles using XRD, TPR, and TPD. The metal oxide phase disappeared following hydrogen and electrochemical reduction. Oxidative treatment of the commercial catalyst differentiated the palladium oxide layers on the metal particle surface. Changes in the surface chemistry of the Pd/PdO/AC system were confirmed by the electrochemical behavior of electrodes prepared from the carbon samples.

Restricted access

A graphic method is proposed to determine all of the kinetic parameters in Wigner-Polanyi equation of desorption. A desorption rate curve from a single temperature-programmed desorption experiment is required by this method to determine the order of reaction (n), the activation energy (E d) and the pre-exponential factor (v) of the equation. The proposed method has been applied to the oxygen desorption from PdO/Al2O3 samples prepared by impregnating γ-Al2O3 with H2PdCl4 solution used as examples. From the graphic method, the values ofn=2, andv=1.37±0.80×109 s−1 were successfully determined for the desorption. The value ofE d depended on the dispersion of palladium (D) on PdO/Al2O3 samples, and was expressed by the equation:E d=175+174D kJ·mol−1. This graphic method is a direct and time-saving technique, on comparing with other methods suggested in the literature, for analysis of data from temperature-programmed desorption of simple desorption processes.

Restricted access

On the examples of the temperature programmed desorption of water from a NaA and a 0.45 NiNaX zeolite, it is shown that, through the use of non-linear temperature programmes with increasing heating rate, the individual desorption steps at higher temperatures of a complex desorption process are better recognizable, or can be distinguished almost completely from desorption at lower temperatures. This type of temperature processing therefore offers a suitable means of improving the finding on complex desorption processes relating to porous catalysts and other systems.

Restricted access
Restricted access

from the isotherms at P/P 0 = 0.99. The acidity measurement of Al-HMS materials was evaluated by TPD of ammonia in a TPD/TPR 2900 analyzer (Micromeritics). Samples were activated at 500 °C for 1 h in a flow of helium; subsequently, ammonia was

Restricted access

saturated N-containing heterocycles among the modified catalysts. The roles of Cr and La were investigated by XRD, XPS, and NH 3 -TPD and the reaction parameters of different aminoalcohols to the corresponding amines were optimized in a continuous fixed

Restricted access