TPR/TPO experiments

Meanwhile, as shown in Figure 2, the conversion of H2S on V2O5 for the reactant composition B at 225 C was 17% lower than that for the reactant composition A. As we thought that the deterioration of activity of V2O5 under the influence of water vapor was not entirely due to the thermodynamic equilibrium, we conducted TPR/TPO experiments by varying the water contents of reactant flow. 

The redox properties of ceria-zirconia mixed oxides are interesting, because these materials find applications as electrolytes for solid oxide fuel cells, supports for catalysts for H2 production, and components in three-way automobile exhaust conversion catalysts. The group of Kaspar and Fornasiero (Montini et al., 2004, 2005) used TPR/TPO-Raman spectroscopy to identify the structural features of more easily reducible zirconia-ceria oxides and the best method for their preparation by suitable treatments. TPR/TPO experiments and Raman spectra recorded during redox cycles demonstrated that a pyrochlore-type cation ordering in Ce2Zr2Og facilitates low temperature reduction. 

To explain the different behavior of oxidized and reduced Rh, Ru and Ir on US-Ex during the interaction with CO, we performed TPR-TPO experiments. Figure 4. shows the reduction profiles for the calcined samples. Rh is reduced at temperatures much lower than those necessary to reduce Ru and Ir. As the formation of well defined surface carbonyls by interaction of the oxidized samples with CO needs primarily a reduction of the metal ions by CO itself, it seems to be logically to expect higher temperatures for the formation of the surface carbonyls in the order Rh < Ru < Ir. The other point is the oxidative disruption in the case of Rh and Ru but not Ir to form well defined surface carbonyls with the reduced samples. Subsequent TPO-TPR experiments showed that after a reoxidation to 473 K ca. 60% of Eh and 90% of Ru had been oxidized, whereas only 10% of reduced Ir/US-Ex had been oxidized by TPO at 548 K. These results indicate the difficulty to oxidize Ir on US-Ex as support and may explain that we could... 

From calorimetric TPR-TPO experiments heats of reduction and reoxidation of eationic or metal species can be determined. Data obtained from these experiments may provide kinetie data of theoretical significance as well as an insight into the mechanism of the reduetion processes. 

The organization of one TPR or TPO experiment is somehow different in comparison with that one applied for temperature-programmed desorption. For example, the sample has to be purged with inactive gas, before exposure to active (reductive or oxidative) gas. These differences can be seen in Table4.1, which presents the organisation of both TPD and TPR/TPO experiments. 

During the TPR/TPO experiments, several products as H2O, CO, or CO2 can be formed. It is important to remove all the undesired gas molecules that can interfere in the signal output. Besides a correct pre-treatment procedure of the sample, use of suitable traps to stop secondary products are necessary and they have to be present in the apparatus configuration. 

The choice of the analytical parameters for a TPR/TPO experiment, in particular sample volume/mass, temperature increasing rate, and flow concentration and flow rate, is fundamental to obtain significant reaction profiles. 

Concentration of hydrogen/oxygen in TPR/TPO experiment affects the sensitivity of the detector and the Tmax values vary. For example, an increase on H2 concentration from 3% (1.23/xmol/cm ) to 15% (6.15/rmol/cm ) produced a decrease in Tmax from 330 to 290 °C for the reduction process of NiO [8] (Fig. 5.7). This indicates that the sensitivity is inversely proportional to the H2 concentration in the reactant mixture. 

The variation of the total flow rate influences the results obtained in a TPR/TPO experiment, too. In the case of hydrogen reduction of Cu-exchanged zeolite, an increase in total flow rate from 10 to 20 cm /min (with 4% H2 concentration) lowered the value of Tmax by 15-20 °C [9]. Recalling the flow reactor theory, it is expected that an increase in total flow rate for a reactant consumed by a first-order process results in a lowering of the degree of conversion and then an increase of reactant concentration in the reactor. The observed increase of H2 concentration in the reactor and decrease of Tmax value of the reduction event with total flow rate are in agreement with theory. 

Fig. 10.5. TPR/TPO of silica-supported Rh, Fe, and Fe-Rh catalysts. The left TPR curves have been measured from freshly impregnated catalysts, the right ones after complete oxidation in the TPO experiment (from van t Blik and Niemantsverdriet [14]).

Temperature-programmed experiments (TPR-TPO) involved the following cycles ... 

The changes in the CNs in the second TPO experiment are summarized in Figure 22.7 (2nd). The profile of the second TPO was different from that of the first TPO in that the oxidation of Pd clusters progressed without an increase in the CN (Pd—Pd). As a result, the removal of the Pd clusters from the zeolite framework and their oxidation took place simultaneously in the second TPO. The curve-fitting analysis of the EXAFS data obtained in the third TPR run is included in Figure 22.6 (3rd). The changes in the CNs were very similar to those of the second run. In other words, the formation of Pd4 clusters was observed after the disappearance of the... 

TPO experiments were performed for the reduced catalysts in order to investigate the stability of these systems towards gas phase oxygen (Fig. 3). The perovskites were completely reduced in a TPR experiment with a stream of 10% H, in Ar heating from room temperature to 923 K at a rate of 10 K/min. The temperature was then rapidly decreased to avoid sintering of the cobalt metal and TPO analysis performed. TPO profiles showed that the oxidation takes place in two steps. The first one at near 473 K (peak I) has an oxygen consumption which suggests that the cobalt is oxidised to two Co and one Co 7 rherefore, this oxidation probably corresponds to the formation of the cobalt oxide spinel according to the reaction ... 

Figure 5. Goldschmidt s tolerance factor t versus (a) reduction and (b) oxidation temperatures obtained by TPR and TPO experiments for the perovskites LnCoO,.

Non-Steady-State Reactors for Testing Fixed-Bed Catalysts In non-steady-state reactors, reaction conditions such as temperature or reactant concentrations are changed temporarily [103-105]. Temperatnre-programmed snrface reaction (TPSR) experiments, temperatnre-programmed desorption (TPD), and temperature-programmed reduction and oxidation (TPR, TPO) [106,107] are established methods dealing with non-steady-state reactor operation. Among these methods, TPSR is a technique that can be applied directly under reaction conditions relevant for catalytic processes. 

The X-ray diffractions performed in a Guinier chamber (Ka Fe) have shown that, in all cases, TPO experiments performed after a TPR reproduced the initial compound. Only one oxidation peak was observed at 625 K for LaCoOs and LaCoOs / C03O4 and at 725 K for C03O4 no significant peak was observed forBa2CoW06. 

The experimental device used in Temperature Programmed Reduction/Oxidation (TPR/TPO) studies was similar to that described in ref [9]. The quadrupole mass spectrometer used to analyse the evolved gases was a VG Spectralab SX-200 instrument interfaced to a PC-type microcomputer. The experiments were run under the following conditions Flow rate of H2 (O2) ... 

Taufiq-Yap and co-workers (211) surmised the same mechanism from temperature-programmed reaction (TPR) and TPD experiments on n-butane, 1-butene, and 1,3-butadiene. Temperature-programmed oxidation (TPO) experiments suggest that the active oxygen species for selective oxidation is lattice oxygen and that the replenishment of the surface oxygen from the bulk is the rate-determining step. 

In TPD experiment, gaseous molecules (atoms) of interest are adsorbed at the surface, at constant temperature. The adsorption is very often performed at ambient temperature, but can be sometimes done at sub-ambient or at elevated temperature. In the modifications of technique such as TPO or TPR, gaseous species are consumed while temperature is increased in a programmed manner. In the case of TPD procedure, desorption of adsorbate is monitored while increasing the solid sample temperature in a controlled fashion while in the case of TPR/TPO, the consumption of active gas is monitored during temperature increase, as explained later in more details (see Sect.4.4). 

Similarly to the case of TPD, the data obtained from TPR or TPO experiments are presented as the variation of detector signal intensity as a function of time (or temperature). Generally, the data that can be obtained from temperature-programmed reduction or oxidation are ... 

Abstract The redox properties of the metal oxides impart them peculiar catalytic activity which is exploited in reactions of oxidation and reduction of high applicative importance. It is possible to measure the extent of oxidation/reduction of given metal oxide by thermal methods which are become very popular TPR and TPO analyses. By successive experiments of reduction and oxidation (TPR-TPO cycles) it is possible to control the reversible redox ability of a given oxide in view of its use as catalyst. The two methods are here presented with explanation on some possibility of exploitation of kinetic study to derive quantitative information on the reduction/oxidation of the oxide. Examples of selected metal oxides with well-established redox properties which have been used in catalytic processes are shown. 

To obtain meaningful TPR/TPO profiles, certain restrictions on the choice of combinations of operating parameters are self-evident. Of particular importance in the case of a TPR experiment, is that the hydrogen feed rate should be equal to the maximum reduction rate, otherwise distortion of the reduction profile will occur. Certain combinations of total flow rate, reactant concentration, sample volume (mass), and heating rate are allowed while other ones are not. 

When supported metal oxide phases are concerned, the kind of surface species that can be present at the surface depends on the support nature, being active a strong or weak metal-support interaction. CuO is an important catalytic phase, easy to disperse on acid supports that can interact with it with strong metal-support bond this interaction can influence the redox properties of CuO. Modified silicas with amount of alumina (SA), titania (ST), and zirconia (SZ) in 12-14 wt.% concentration were used to support CuO (8-9 wt.%) [20] a commercial silica-alumina support was comparatively studied (SAG). On the different supports, the CuO redox properties were controlled by combining TPR and successive TPO experiments. 

Rieck and Bell/Mitchell and Vannice—Ce addition promotes WGS during methanation. Rieck and Bell351 examined 2%Pd/Si02 catalysts promoted with various lanthanides (La, Ce, Pr, Nd, and Sm) in the range 4.4-4.7%, among them, ceria, and carried out TPR (H2) and TPO (02) and H2 and CO TPD (He flow) experiments, along with TPSR experiments (H2 used instead of He). The catalysts were ramped at a rate of 1 °C per sec in a flow of 75 ccm H2 and 25 ccm CO. The authors observed that in addition to methanation, C02 was produced, and that the... 

Over the years, TAP systems have been used in catalyst industry research ranging from determining selectivity and activity of well-defined surfaces to distinguishing between sequential or parallel reaction paths. The capabilities of the TAP system are varied and can perform applications such as TPD, TPSR, TPO and TPR. In pulse experiments, the adsorption of reactants provides detailed information on the thermal stability of adsorbed intermediates. Fig. 4 shows the general parameters of the TAP system. 

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