Temperature-programmed reduction, calcined

One way in which cobalt dispersion can be increased is the addition of an organic compound to the cobalt nitrate prior to calcination. Previous work in this area is summarized in Table 1.1. The data are complex, but there are a number of factors that affect the nature of the catalyst prepared. One of these is the cobalt loading. Preparation of catalysts containing low levels of cobalt tends to lead to high concentrations of cobalt-support compounds. For example, Mochizuki et al. [37] used x-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) to identify cobalt silicate-like species in their 5% Co/Si02 catalysts modified with nitrilotriacetic acid (NTA). The nature of the support also has... 

Temperature-programmed reduction (TPR) profiles of fresh catalyst samples were obtained using a Zeton Altamira AMI-200 unit. Calcined fresh samples were first heated and purged in flowing argon to remove traces of water. TPR was performed using 30 cc/min 10% H2/Ar mixture referenced to argon. The ramp was 5°C/min from 50 to 1,100°C, and the sample was held at 1,100°C for 30 min. 

FIGURE 8.3 Temperature-programmed reduction profiles of (top) 25% Co/Si02 and (bottom) 15% Co/Si02, including catalysts calcined using (bold) 5% NO in N2 and (light)... 

Temperature Programmed Reduction (TPR) Studies. In the TPR studies, a gas mixture of 2% H2 in Ar is passed over powdered samples of the calcined catalysts. The catalysts are held in the middle of a 5-mm diameter, 0.4-m long quartz reactor with... 

Figure 6.18 illustrates the technique with a study on a proprietary cobalt on alumina Tropsch catalyst for Fischer-Tropsch synthesis (the reaction of synthesis gas, CO + Fl2, to hydrocarbon fuels) [55]. Trace amounts of platinum help to obtain an appreciable degree of reduction for the cobalt (similarly as in the temperature-programmed reduction of bimetallic Fe-Rh catalysts in Fig. 2.4). The left part of Figure 6.18 shows Co K-edge XANES of metal and oxide reference compounds, and illustrates the strong intensity of the white line region for ionic cobalt compounds. The XANES spectrum of the calcined CoPt/A Ch catalyst re-...

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],... 

Temperature programmed reduction of calcined clay samples revealed that their reducibility was sensitive both to the thermal pretreatment conditions as well as to Al/Al+Si ratio (Figure 3). Table III summarizes the degree of reduction to Co, as measured from the H2 uptake to 450 C. On samples previously calcined to 600 C, reduction to form metallic Co was confirmed by both XPS and SAD. 

Calcination and Reduction of Co/Al Hydroxycarbonates. As found with the Co/Al clays, temperature programmed reduction studies revealed that pre-calcination in air was necessary to produce a metallic cobalt-containing catalyst at reasonable reduction temperatures ... 

In this work the performance of Cr-Co spinels as catalysts for total combustion of methane is studied. The spinels were prepared from nitrate precursors. The effect of temperature and time of calcining was studied using x-ray diffraction (XRD) in order to check the crystalline structure and the absence of other phases, N2 physisorption (BET) in order to study the porous structure of these solids and Temperature Programmed Reduction (TPR) in order to determine the presence of different metallic species. 

Fig. 3. Temperature-programmed reduction profiles of (a) Pd/NaY and (b) Pd/MgY after calcination at 500°C (152).

Fig. 14. Temperature-programmed reduction profiles of Pd/NaY after reoxidation to different temperatures following initial calcination to 500°C and reduction at 500°C. The peak at about 0°C is attributed to the reduction of Pd oxide the peak at about 180°C is attributed to the reduction of redispersed Pd ions in sodalite cages (76).

Activity and selectivity of Pt/KL catalysts depend crucially on the method of their preparation. There seems to be a consensus that Pt/KL samples prepared by incipient wetness impregnation display a higher Pt dispersion and a higher aromatization yield than samples prepared by ion exchange. Temperature-programmed reduction shows that in samples prepared by impregnation, followed by calcination, a considerable fraction of the Pt is present as Pf ions, whereas Pt prevails in ion-exchanged samples after calcination (53). 

The molybdenum dispersion also depends on the phosphorus content of the catalyst. Atanasova et al. (68, 87) reported that the dispersion of molybdenum and nickel, measured by X-ray photoelectron spectroscopy (XPS), shows a steep increase due to the presence of phosphorus at low loadings. The dispersion of molybdenum in NiMoP/Al catalysts increases further as a result of calcination, whereas that of nickel decreases. In contrast, Sajkowski et al. (83) reported, on the basis of an extended X-ray absorption fine structure (EXAFS) investigation, that phosphorus does not affect the size of the polymolybdate species, Mangnus et al. (31) inferred that the stacking of molybdates does not increase as a result of the addition of phosphorus since the height of a temperature-programmed reduction (TPR) peak at 400°C due to the reduction of deposited multilayered molybdenum oxo-species was found to be independent of the phosphorus content. However, Chadwick el al. (60) concluded from XPS measurements that the dispersion of molybdenum decreases upon addition of phosphorus. 

Figure 1,14 Temperature programmed reduction profiles for a supported monometallic catalyst Pd/Al203, for two calcination temperatures.

The precursor and the calcined catalyst were characterized by various techniques such as nitrogen adsorption, mercury porosimetry, X-ray diffraction (XRD), atomic emission spectrometry by inductively coupled plasma (ICP), thermogravimetric analysis, and temperature-programmed reduction (TPR). More details about the catalyst preparation and characterization can be found in a previous work (22). 

Temperature programmed reduction CTPK) and hydrogen adsorption TFK was earned out in conventional dynamic equipment After calcination, catalytic samples were exposed to (9 1) flowing at 0.1S mol h l. Temperature was raised linearly at a rate of 10 K min-l from 298 K to 1250 K. 

Temperature programmed reduction (TPR) of the calcined samples were performed in a RIG-100 In Situ Research Instruments catalyst characterization apparatus. The TPR experiments were performed in a quartz gas flow reactor, from room temperature to 1000°C, with a heating rate of 7.5 °C.min under a stream of 5% v/v H2 in argon (total flow rate 25 ml.min ) at atmospheric pressure. 

The catalysts and their precursors were characterized using various techniques, viz. TGA (TGA/SDTA 851 from Mettler TOLEDO), CO chemisorpiton, nitrogen physisorption (Quantachrome Autosorb-6B), Temperature Programmed Reduction (TPR, at 10 K/min in a flow of 7.6% H2 in Ar), elemental analysis (ICP-OES) and High Resolution Transmission ElectroMicroscopy (HRTEM), to optimise the procedures of preparation, calcination and pretreatment. 

Temperature programmed reduction experiments were performed using an apparatus described by Robertson et al. [9]. The reduction was carried out with a purified hydrogen-argon mixture (10 vol.% hydrogen) at a heating rate P = 10 K min up to 1048 K. The TPR reactor was charged with 0.25 g of calcined (fresh) catalyst. Before the reduction, the catalyst was oxidized in a O2 flow to 1048 K for 1 h, and then cooled down to 300 K in Ar. This reduction-oxidation cycle was repeated several times. 

For Temperature Programmed Reduction Studies (TPR) the heating rate used was 0.2°C/s. The H2 and Ar flow rates used were 0.48 ml/min and 7.1 ml/min respectively. Where calcined catalysts were used, the sample size was approximately 40 mg, whereas the TPR profiles of activated catalysts were obtained using 0.25 g samples. The activation procedure used was as described for FT screening. The temperature range was from room temperature to 800°C. 

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]. 

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