Oxidation temperature-programmed

The temperature programmed oxidation is the inverse of the programmed reduction and serves to oxidize the metal and to transform the metal oxide in the lower oxidation state to the highest state. It is used in carbon combustion, coke and diesel particulates. As examples we have the oxidatirai of metals, oxides and carbon as follows  

Transformations occur as shown in the scheme below, indicating the oxygen consumption with the increase of temperature. 

Thermodynamically these transformations are possible, given that the free energy change is negative (AG° 0). The main reactions are known and highly exothermic, such as 

The temperature programmed oxidation is done using the same experimental multipurpose unit, coupled to a quadrupole mass spectrometer or a thermal conductivity detector (TCD), as shown in scheme Fig. 6.9. In addition we can use the thermo-differential method (TGA-DTA), as shown in Fig. 6.8. 

Different TPO analysis procedures can be made, depending on the objectives, i.e., metal oxidation and combustion (carbon, coke, or diesel). 

The TPO analysis can be performed in different ways, using different detection methods  

The detection of CO2 with a TCD requires that the gases coming out of the cell be fully oxidized, in order to transform CO and other hydrocarbons released from the catalysts into CO2. Since the technique involves the separation of gases in a GC column, its disadvantages are that it is a noncontinous analysis and that it misses the fine structure, the peak height and its temperature. Besides, the 

2 Determination of Coke Location. The TPO technique allows the determination of the coke location on supported metal catalysts, such as naphtha reforming. Since the metal, typically platinum promoted with rhenium, iridium, tin, or germanium, has a catalytic effect for coke burning, the TPO profile displays two main peaks. The low temperature peak is due to the oxidation of the coke directly deposited on the metal particle, or in its vicinity . In this way, it is possible to study the effect of catalyst formulation and operational conditions on the formation of coke on the metal and on the support. 

Platinum and palladium were used to facilitate the regeneration of zeolite catalysts used in isobutane alkylation. The TPO profiles in these cases display a characteristic peak, very sharp, that is not usually found in any other system, due to the presence of a large amount of hydrocarbonaceous deposits located very close to the metallic particles .  

3 Determination of Oxidation Kinetics. The reaction rate between oxygen and coke depends upon the oxygen partial pressure, the composition, morphology and location of coke, and the catalytic effect of catalyst components and impurities. Therefore, the study of the kinetics of coke oxidation provides information regarding coke characteristics. Additionally, it is necessary to 

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TEM observation and elemental analysis of the catalysts were performed by means of a transmission electron microscope (JEOL, JEM-201 OF) with energy dispersion spectrometer (EDS). The surface property of catalysts was analyzed by an X-ray photoelectron spectrometer (JEOL, JPS-90SX) using an A1 Ka radiation (1486.6 eV, 120 W). Carbon Is peak at binding energy of 284.6 eV due to adventitious carbon was used as an internal reference. Temperature programmed oxidation (TPO) with 5 vol.% 02/He was also performed on the catalyst after reaction, and the consumption of O2 was detected by thermal conductivity detector. The temperature was ramped at 10 K min to 1273 K. 

After a purge under Ar at 773 K a second temperature-programmed oxidation leads also to a consumption of O2 in excess with respect to the Pd amount. 

In the absence of palladium, the Zr02 support (after treatment under H2 at 773 K in order to remove possible impurities) does not absorb O2 or H2 during successive temperature-programmed oxidations and reductions until 773 K. Moreover, with the Pd/Al203 solid the O2 or H2 consumption never exceed the amounts corresponding to Pd. 

COj temperature-programmed oxidation evolution profiles, 39 191-192 Coupled heterogeneous catalytic reactions, kinetics of, 24 1-49 see also Kinetics coupling through catalytic surface, 24 9-13 experimental studies, 24 22-49 apparatus and procedure, 24 25, 26 catalysts, 24 26-28... 

Figure 9. Temperature programmed oxidation (TPO) data showing CO2 evolution m/e = 44) of thermally deposited carbon from a Cu-Ce02-YSZ SOFC anode material after exposure to n-butane for 30 min (solid line) and a graphite powder sample (dashed line). (Reprinted with permission from ref 172. Copyright 2003 The Electrochemical Society, Inc.)...

Coke Characterization by Temperature-Programmed Oxidation of Spent FCC Catalysts That Process Heavy Feedstock... 

The catalytic coke produced by the activity of the catalyst and simultaneous reactions of cracking, isomerization, hydrogen transfer, polymerization, and condensation of complex aromatic structures of high molecular weight. This type of coke is more abundant and constitutes around 35-65% of the total deposited coke on the catalyst surface. This coke determines the shape of temperature programmed oxidation (TPO) spectra. The higher the catalyst activity the higher will be the production of such coke [1],... 

Bayraktar, O., and Kugler, E. Characterization of Coke on Equilibrium Fluid Catalytic Cracking Catalysts by Temperature-Programmed Oxidation. Applied Catalysis A General 233 (2002) 197-213. 

Other temperature-programmed techniques include Temperature Programmed Oxidation and Sulfidation (TPO and TPS) for investigating oxidation and sulfidation behaviour, and Temperature Programmed Desorption (TPD) (also called Thermal Desorption Spectroscopy [TDS]), which analyses gases desorbed from the surface of a solid or a catalyst on heating. 

Swann et al. [9] used isotopic labeling and temperature-programmed oxidation (TPO) (ex situ) approaches to show that the carbon which had formed during the C02 reforming of methane may have derived from both CH4 and C02. 

Catalytic superactivity of electron-deficient Pd for neopentane conversion was recently verified for Pd/NaHY (157, 170). The reaction rate was positively correlated with the proton content of the catalyst. Samples that contained all the protons generated during H2 reduction of the catalysts were two orders of magnitude more active than silica-supported Pd. Samples prepared by reduction of Pd(NH3)2+NaY displayed on intermediate activity. It was suggested that Pd-proton adducts are highly active sites in neopentane conversion. With methylcyclopentane as a catalytic probe, all Pd/NaY samples deactivated rapidly and coke was deposited. Two types of coke were found (by temperature-programmed oxidation), one of... 

Mx and M2 refer to the metals in the precursor metal carbonyl and in the oxide support, respectively.) Temperature-programmed-oxidation and -reduction experiments provide complementary information. 

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