Characterization TPRS experiments

Temperature-programmed reduction (TPR) is normally used in the characterization of catalysts [18,91-93], In general, to carry out a TPR experiment, a reducing gas mixture, typically 5% hydrogen in nitrogen, flows continuously over the sample [92], The gas flow rate can be varied precisely using either built-in controls or an optional mass flow controller accessory. 

Bruckner and Kondratenko (2006) used a similar approach to characterize VOx/Ti02 catalysts. In a separate TPR experiment carried out with a quartz reactor equipped with a UV-vis fiber optical probe, the relationship between the "absorbance" at 800 nm and the degree of reduction as determined from H2 consumption via mass spectrometry was established. The absorbance at 800 nm increased with increasing reduction of the vanadium, but not linearly. During the catalytic reaction experiment, the absorbance at 800 nm was then used to determine the average valence of vanadium. Because contributions of reduced titanium species in the analyzed spectral range could not be excluded, only a lower limit of the vanadium oxidation state could be determined, which was 4.86 at 523 K and C3H8/02 = 1 1. 

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 noted Ni/MgO (mol.%Ni = 5 10) and 5%Ni-5%M/MgO (M= Fe or Cu) were prepared by the impregnation method. The obtained solids were dried and calcined in the same conditions as the catalysts prepared by coprecipitation. All catalysts were characterized by chemical analysis, specific surface area, XRD, TPR, SEM and TEM-EDX. The XRD diflractograms were recorded in a SIEMENS D-5000 diffractometer using the CuKa (X=0.154nm) radiation. TPR experiments were performed in the temperatures range 25-900°C... 

The two zeolite catalysts were characterized by De La Torre et al. [68]. Total acidity is higher over BEA but ZSM-5 shows a higher number of strong acid sites desorbing ammonia beyond 220 °C. In the optimized catalysts (1.4 % Cu-ZSM-5 and 2.1 % Cu-BEA), all the copper remains in the Cu " state. Increasing Cu loading leads to H2/CU < 1 in TPR experiments, which confirms the formation of Cu" and may be Cu° species. Reduced species of copper appear to be less active and less selective to N2 (higher formation of N2O). 

The prepared catalysts and the chemical compositions measured by atomic absorption, are listed in Table 1. Complementary characterization experiments such as hydrogen chemisorption in a pulse apparatus and temperature-programmed reduction (TPR) were performed using experimental systems and methods described in detail elsewhere [10]. 

Physico-chemical characterizations of the monoliths were performed on the powder of the crushed monoliths (TPR) or directly on the honeycomb substrates opportunely sectioned ((BET, XPS, SEM/EDAX). Details on the characterization techniques are reported elsewhere [4-6]. The experimental apparatus used to perform the catalytic tests is described elsewhere [9]. Before the catalytic tests the samples were pre-treated in situ under flowing O2 (10 vol.% in N2) at 400 °C for 30 min, then cooled to room temperature in nitrogen. CO oxidation only tests were carried out over the two prepared monoliths (mono 1 and 2) by feeding the mixture CO 1750 ppm, O2 10 vol.%, N2 balance. Combined light-off experiments were performed over the Pd-Pt promoted substrate Pd-Pt/mono 2) by feeding the reaction mixture CO 5000 ppm, CH4 400 ppm, O2 1.0 vol.%, N2 balance. In all the experiments, the GHSV was set at a realistic value, i.e. 100,000 h ... 

Second, the well known chemisorption behavior of ethene is characterized with the same combination of EES and TPD on surfaces and serves as a future comparison for the study of the chemisorption behavior of ethene on size-selected Pt clusters by means of EES. The reactivity of ethene towards the hydrogenation reaction is probed by TPR and also further preliminary experiments (AES and IRRAS) are shown in order to investigate the mechanism of the ethene hydrogenation reaction on size-selected clusters. 

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