Chromium temperature-programmed

On the basis of temperature programmed reduction and oxidation (TPR-TPO) measurements, it was proposed that the calcination of chromia used as a catalyst for the selective fluorination of CF3CH2C1 into CF3CH2F depends strongly on the gas used to calcine the precursor [50], Furthermore, the fluorination of CF3CH2C1 depends directly on the number of reversibly oxidizible chromium atoms in the catalysts. The oxidation/reduction properties are related closely to the atmosphere of pre-treatment and a linear relationship between the catalytic activity and the hydrogen uptake during the second reduction step has been found [51],... 

In the present work the causes oi the deactivation of alumocopperchroniLum and aiumoniagnesiuci chromium catalysts for fuel combustion have been investigated by a variety of instrumental methods IR-spectroscopy electron diffuse reflectance spectroscopy (EDRS), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and a pulse microcatalytic method. 

Often, visible or infrared (IR) spectra of calcined catalysts were obtained and compared with those of bulk Cr(VI) compounds [35,83, 93,95-100]. However, the lack of suitable reference data made exact interpretations difficult [63,101]. In recent years, many other techniques, such as diffuse reflectance IR, Raman, EPR, EXAFS-XANES, and even temperature-programmed reduction have also been applied to the problem [92,102-127], Most measurements were made with silica-supported catalysts, but alumina and other oxides and mixed oxides were also investigated as supports, and with a wide range of chromium loadings. An excellent summary of all this work has been published by Weckhuysen et al. [76]. 

It was found that the benzene, toluene, mesitylene, durene and hexamethylbenzene tricarbonyl chromium could best be separated and quantitatively determined in mixtures by use of temperature programming with the flame ionization detector. Figure 204 illustrates the separation achieved. It can be seen that the resolution is excellent, and that the retention times of the early components are far enough removed from the tail of the solvent peak that measurements from the original base line are possible in each case. Under isothermal conditions (135°C) using flame ionization, the benzene tricarbonyl--chromium and toluene tricarbonylchromium peaks eluted on the tail of the benzene peaks and the retention time of the hexamethylbenzene tricarbonylchromium was 32.6 minutes. This peak was also considerably broader and flatter when eluted isothermally. For this determination therefore, programmed temperature operation is the most efficient means of analysis not only is the resolution improved, but also the total time of analysis is reduced to 15 minutes. 

Bmnet, S., Requieme, B., Matonba, E., Barrault, J., and Blanchard, M. Characterization by temperature-programmed reduction and by temperature-programmed oxidation (TPR-TPO) of chromium (IH) oxide-based catalysts correlation with the catalytic activity for hydrofluoro alkane synthesis. J. Catal 1995,152, 70-74. 

Concerning the activation by ethylene monomer, Liu et al. reported an extensive investigation on reduction of the hexavalent chromate by means of XPS, temperature-programmed desorption-mass spectrometry (TPD-MS) methods and found that surface chromium species might exist in three oxidation states (1) surface chromate Cr(VI)Ox,surf species, (2) surface-stabihzed trivalent Cr(III) species, and (3) surface-stabilized Cr(II) species (Liu et al., 2002). Some short alkenes including propylene and butylene as well as the reduction by-product of formaldehyde were confirmed based on TPD-MS characterizations (Liu et al., 2003). 

A thermodynamic analysis was conducted for corrosion of iron alloys in supercritical water. A general method was used for calculation of chemical potentials at elevated conditions. The calculation procedure was used to develop a computer program for display of pH-potential diagrams (Pourbaix diagrams). A thermodynamic analysis of the iron/water system indicates that hematite (Fe203> is stable in water at its critical pressure and temperature. At the same conditions, the analysis indicates that the passivation effect of chromium is lost. For experimental evaluations of the predictions, see the next paper in the symposium proceedings. 

A computer program was developed for electrochemical equilibria calculations and graphical pH-potential diagram presentation of a one-metal/one-nonmetal/water system. The program can be used for temperatures and pressures exceeding the supercritical point of water. The calculations show that hematite (Fe203) is the oxidation product of iron in supercritical water, and the oxidation product of chromium in supercritical water is an ionic species, Cr04 . Passivation effect of chromium is lost in supercritical water. 

The specific heat of chromium rich Cr-Ni and Cr-Fe-Mo alloys was measured by [1971Bau] in the temperature range 1.3-4.2 K. Measurements were made for compositions of 20 mass% Mo and 0-20 mass% Fe. This experimental program was intended primarily for the determination of the electronic band stmcture of 3d transition elements. It is known that Cr-Fe alloys exhibit unusual electron specific heat coefficients and an abnormally low Debye temperature, which is attributed to a complex magnetic stmcture of these alloys. The addition of 20 at.% Mo may avoid such complications, as the alloy becomes paramagnetic at liquid He temperatures. The results of the measurements are shown in Table 4. The temperature dependence of the specific heat is expressed as C = 234 R (770) + where R is the gas constant and 0 is the apparent Debye temperature. The authors concluded that the values of the specific heat coefficients and the Debye temperature are rather unusual (a low value of the apparent Debye temperature). They cannot be caused by... 

In order to select the alloy best suited to this application, an extensive program of corrosion tests was carried out by ORNL (USA) on the available commercial nickel-base alloys and austenitic stainless steels [22,30—34]. These tests were performed in a temperature gradient system with various fluoride media and different temperatures (maximum temperature and temperature gradient). Chromium, which is added to most alloys for high-temperature oxidation resistance, is quite soluble in molten fluoride salts. Metallurgical examination of the surveillance specimens showed the corrosion to be associated with outward diffusion of Cr through the alloy. It was concluded that the chromium content shall be maintained as low as reasonably possible to keep appropriate air oxidation properties. The corrosion rate is marked by initial rapid attack associated with dissolution of Cr and is largely driven by the impurities in the salt [22,30—34]. This is followed by a period of slower linear corrosion rate... 

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