Zeolites temperature-programmed oxidation

The regeneration of Y-zeolite catalysts used in isobutane alkylation with C4 olefins was studied. The coke formed on these catalysts during this reaction needs temperatures higher than 500°C to be burnt off with air. Ozone was used in this study to eliminate most of the coke at a much lower temperature. After a treatment at 125 C with ozone, the small amount of coke remaining on the catalyst can be removed with air at 250°C. The ozone not only eliminates coke from the catalyst, but also modifies its burning characteristics as measured by Temperature Programmed Oxidation, shifting the peak to lower temperatures. This allows a combined treatment with ozone at 125°C followed by air at 250°C to restore the activity and stability of Y-zeolite catalysts for isobutane alkylation. 

This section summarizes the chemistry of the SC isobutane regeneration process. To understand the nature of the hydrocarbons that remain adsorbed on the surface of the USY zeolite catalyst both before and after SC isobutane regeneration, a series of ex-situ temperature-programmed oxidation (TPO), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and ultraviolet-visible (UV-vis) analyses was performed on samples submitted to different TOS 10 under isobutane/butene reaction conditions. 

Temperature-programmed oxidation (TP0) of coke on ZSM-5 was carried out in a 1.0% oxygen-1n-argon mixture. Coked samples were dehydrated at 300°C for 30 minutes in flowing argon before TP0 commenced. Samples (ca. 10 mg) were heated at 10°C min-1 from 300°C to 750°C. Products evolved from the zeolite were analysed in a continously scanning Extranuclear SpectrEL mass spectrometer (model no. 275-50). Flow of the gas mixture was controlled by a fixed capillary leak (10 cm min"1). 

Discrimination between ruthenium metal inside the zeolite pores or at the external surface was made using a combination of temperature programmed oxidation and x-ray line broadening (9). 

Fig. 21.21 During temperature programmed oxidation (TPO), HC storage resulted in significant exotherms over beta zeolite SCR catalysts (Catalysts A, B and C) compared to the Cu/CHA SCR catalyst (Catalyst D) [19]. During TPO, the space velocity was 15,000 h and there were no HCs, 10 % O2, 5 % H2O, 5 % CO2, and balance N2. Copyright SAE International. Reprinted with permission from 2008-01-0767...

The title Spectroscopy in Catalysis is attractively compact but not quite precise. The book also introduces microscopy, diffraction and temperature programmed reaction methods, as these are important tools in the characterization of catalysts. As to applications, I have limited myself to supported metals, oxides, sulfides and metal single crystals. Zeolites, as well as techniques such as nuclear magnetic resonance and electron spin resonance have been left out, mainly because the author has little personal experience with these subjects. Catalysis in the year 2000 would not be what it is without surface science. Hence, techniques that are applicable to study the surfaces of single crystals or metal foils used to model catalytic surfaces, have been included. 

With simple probe molecules, such as H2, information about the number of surface metal atoms is readily obtained by using adsorption measurements. However, even with such simple probe molecules further information about the heterogeneity of a surface may be obtained by performing temperature-programmed desorption measurements. With probe molecules which are chemically more specific (e.g., NH3 and organic amines, H2S and organic sulfides) it may be possible to obtain information about the number and nature of specific types of surface sites, for example, the number and strength of Lewis or Bronsted acid sites on oxides, zeolites or sulfides. 

Temperature-programmed reduction and desorption (TPR, TPD) have been applied to study the stability of Pt-Co bimetallic particles entrapped in NaY zeolite cages upon O2 oxidation and reaction with surface protons generated during the reduction of transition metal cations. Oxidation of Pt/NaY catalyst with O2 at 573 K causes shift of TPR peak to lower temperature due to formation of partially oxidized Pt particles. Similar treatment for Pt-Co/NaY bimetallic catalysts results in complete isolation of Pt and Co in Pt-Co particles, leaving Pt and Co in supercages and sodalite cages, respectively. 

Dealuminated M-Y zeolites (Si/Al = 4.22 M NH4, Li, Na, K, Cs) were prepared using the dealumination method developed by Skeels and Breck and the conventional ion exchange technique. These materials were characterised by infrared spectroscopy (IR) with and without pyridine adsorption, temperature-programmed desorption (t.p.d.) of ammonia. X-ray difiracto-metry (XRD) and differential thermoanalysis (DTA). They were used for encapsulation of Mo(CO)5. Subsequent decarbonylation and ammonia decomposition was monitored by mass spectrometry (MS) as a function of temperature. The oxidation numbers of entrapped molybdenum as well as the ability for ammonia decomposition were correlated to the overall acidity of the materials. It was found that the oxidation number decreased with the overall acidity (density and/or strength of Bronsted and Lewis acidity). Reduced acidity facilitated ammonia decomposition. 

We have recently reported that cerium-exchanged mordenite (CeNa-MOR) is a higjily active and selective catalyst for NO reduction witli ammonia in oxygen-rich conditions [23]. We have furtlier found tliat it oxidizes SO2 to a negligible extent [24, 25], and tliat it appears to flilfill the aforementioned requirements for an ideal NH3-SCR catalyst. In the present paper, we will report on these aspeets of eerium-exchanged zeolite catalysts. Furthermore, its surface interaction with two reactants, NO and NH3, is examined with the temperature programmed desorption (TPD) technique. 

The development of new catalysts for the storage and reduction of NO, from lean-burn engines is a challenge to meet the demands of the Euro IV norm in 2005. The new catalyst systems should also be able to adsorb NO, already at comparable low temperature and desorb it at high temperature. Cu and Ba exchanged zeolites of various types (Y, MOR, MFI, L, CHA) and their ability to adsorb NOx isothermally from a dry feed and to desorb it under temperature-programmed conditions (55-600°C) are described. Their NOx adsorption /desorption properties under wet conditions are compared. Furthermore, a zeolite material modified by manganese oxide is developed which is able to adsorb NOx from a wet feed already at temperatures as low as 120 °C and desorb it above 350°C. 

Composites containing different types of guests (metal or alloy particles, oxides, sulfides, complexes, polymers) in the cavities of zeolite hosts are prepared for various appHcations in materials research and catalysis. Except for quality assessment by detection of extra-zeolite material after synthesis or thermal treatments, photoemission plays a largely auxiliary role in this area, cooperating with bulk techniques such as X-ray absorption, UV-Vis, IR of probe molecules, and temperature-programmed reduction. The attention drawn to the significance of intra-zeolite potentials by XPS studies [12] has, however, contributed to the elaboration of a new theory of metal-support interactions [18,19]. 

Figure 6.28. Temperature-programmed mass spectrometry of the decomposition of Pt +(NHs)4 ion exchanged in zeolite nZEM-Sl " . The corresponding reaction steps are indicated. Three peaks can be distinguished in the TPD spectrum. Oxygen is consumed in only two of the N2 formation peaks. The mechanism that explains the occurrence of these three peaks is consistent with proposals made earlier on the homogeneous oxidation of Ru-amine complexes in basic solutionl and the reaction of NO with Ru or Os complexes ].

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