Temperature-programmed reduction procedure

Fig. 9.11. Left structures resulting from deposition-precipitation of Fe(II) by different procedures. Right temperature-programmed reduction of Fe oxide deposited on silica by hydrolysis of urea (U 90(1)), injection at 90°C (190(1)), and injection at 45°C. For comparison bulk Fe oxide is included.

In order to explore this hypothesis, we examined the oxidation state of the metals on the catalyst after CPS as compared to Ecats using temperature programed reduction (TPR). We found diat the metals on the CPS catalysts (both V, and to a lesser extent Ni) were in a higher oxidation state than the metals on Ecat, even for a high excess oxygen FCC regenerator. Based on this result, the standard CPS procedure was... 

Investigation of supported ruthenium phase stability was based on atomic absorption spectroscopy (AAS) measurements of the active metal loading variation in catalysts caused by different pre-treatment procedures. Ruthenium stabilisation in the presence of platinum was verified by temperature programmed reduction (TPR) experiments. Finally, the stabilising influence of Pt on Ru catalysts was evidenced using methylcyclopentane (MCP) hydrogenolysis as model reaction. 

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 (H2-TPR) measurements were performed using a linear quartz gradientless microreactor and a 6% H2/Ar mixture flowing at 60 STP cm min according to the procedure described elsewhere [6]. 

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. 

Temperature programmed reduction (TPR) is a convenient technique to characterise metal oxide catalysts. Generally, TPR provides information on the influence of support materials, pre-treatment procedures and the influence of metal additives on the catalyst reducibility. The TPR technique is intrinsically quantitative and also produces kinetic information. Hurst et al. [1] reviewed in 1982 the thermodynamics, kinetics and mechanisms of reduction thoroughly with illustrative examples dealing with the reduction of many siqrported and unsupported oxides. In literature there are two, in principle, different techniques to determine tinetic parameters from TPR experiments. One requires TPR data collected with different heating rates and utilises only one point from each TPR curve, and the oth is based on computer simulated nonlinear regression and exploits the whole experimental TPR-curve/curves. 

In order to understand better these interesting systems without complications that might arise due to different preparation procedures, we compared oxygen-treated WC and Mo2C prepared by similar reduction/ carburization procedures from their respective oxides. The effects of pretreatment conditions were also studied. An attempt was made to correlate the kinetic behavior of these catalysts in n-hexane-H2 reactions with their physical properties obtained from X-ray diffraction (XRD), CO chemisorption, temperature-programed reaction (TPR) with flowing H2 or He, temperature programed desorption (TPD) of adsorbed NH3, and X-ray photoelectron spectroscopy (XPS). 

XRD experiments can be carried out to characterize gas-solid reactions and, with some limitations, fluid-solid reactions more generally, as long as the fluid contributes little to the pathway of sight for the X-rays. Areas of recent investigation are catalytic gas-solid reactions, electrochemical processes, synthesis procedures involving precipitation and dissolution of solids, temperature-programmed reaction studies of crystallization, and oxidation and reduction of solids. This enumeration covers essentially all phases of the life of a catalyst. 

Another method of analysis makes use of a subtraction procedure and has been applied to investigate the oxidation state of a series of Ce-Zr mixed oxides during a temperature programmed process [195]. A difference spectrum between the sample at a defined state and the calcined, fully oxidized material displays a positive peak at the position of feature Bo, characteristic of Ce, and a negative peak at the position of feature C, characteristic of Ce. The overall peak-to-peak amplitude is then proportional to the average reduction degree of Ce in the sample. 

TPR experiments were conducted with an Altimira temperature-programmed system following procedures described previously [8]. Catalyst powder samples (100 to 300 mg) were heated from room temperature to 1073 K at a rate of 15o/min in a flowing stream (40 cc/min) of 8% H2 in Ar. Prior to reduction, samples were given a standard in situ preoxidation at 773 K for 1 h. TPR traces reported here are baseline corrected (using a smooth curve subtraction procedure) and nonnalized to an equivalent sample size. 

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