Kulathuiyer Seshan

ELEMENTARY STEPS OF NOX ADSORPTION AND SURFACE REACTION ON A COMMERCIAL STORAGE–REDUCTION CATALYST

The surface species formed during adsorption of NOx on a commercial NSR catalyst (containing barium oxide, Pt, and alumina as the main components) were investigated by in situ IR spectroscopy. During adsorption of NO, mainly linear and bridged bonded nitrites of Ba- O- N- O- Ba type were formed on Al and Ba oxide components. Nitrites were detected during the initial phase of the NO/O2 and NO2 adsorption, whereas with further exposure nitrates were the dominant surface species. Using the different surface species and reaction intermediates identified by IR spectroscopy a series of sequential reaction steps during the sorption of NOx on a NSR catalyst was derived. Initially, NO is stored in the form of nitrites on the storage component (Ba oxide). NO2 formed by oxidation on the noble metal component (Pt) sorbs either molecularly by forming nitrate species or dissociatively by forming nitrites. After a certain concentration of NOx is adsorbed, the transformation and further oxidation of the surface nitrites into surface nitrates by NO2 occur. The stability of the NOx surface species was found to increase in the order Al nitrites < Ba nitrites < Al nitrates < Ba nitrates.

Roles of supports, Pt loading and Pt dispersion in the oxidation of NO to NO2 and of SO2 to SO3

Three types of platinum catalysts, Pt/SiO2, Pt/?-Al2O3 and Pt/ZrO2 were examined for the oxidation of NO to NO2 of SO2 to SO3. The activity order for both oxidation reactions was found to be: Pt/SiO2 > Pt/?-Al2O3 > Pt/ZrO2. The effect of Pt loading and Pt dispersion on the catalytic activity and selectivity was examined. Over Pt/SiO2, the specific activities were found to be strongly size-dependent, the larger Pt particles exhibiting higher specific activity than the smaller ones. Over the Pt/?-Al2O3 catalysts, however, the size-dependence appeared to be much less significant than that of the Pt/SiO2 catalysts. The results for Pt/ZrO2 catalysts, however, showed that the specific activities for both reactions were most probably size-independent. The catalytic selectivity for the oxidation of NO relative to that of SO2 appeared to be size-independent for all the catalysts. The interaction of NO, NO2 and SO2 with the catalysts and the effect of the support on the interaction were investigated using TPD technique.

Synthesis, characterization and catalytic activity of the pillared molecular sieve MCM-36

MCM-36 materials were prepared by swelling the layered MCM-22 precursors with large organic molecules and then pillaring the resulting material with polymeric silica. A mesopore region with 0.25–0.3 nm thickness between the microporous layers was identified. The BET surface area obtained for MCM-36 was 2.5 to 3 times higher than that of MCM-22. The sorption characteristics of linear alkanes are similar for MCM-22 and MCM-36 materials, indicating that sorption is dominated by the 10-membered ring microporous channel system and that the pore structure of the MCM-22 layers stays intact during the swelling and pillaring processes. Swelling and pillaring, however, decreased the concentration of Bronsted acid sites compared with the starting material. The largest fraction of strong Bronsted acid sites is located in this zeolite layer. Adsorption of 2,2,4-trimethylpentane indicates that only about 10% of the bridging hydroxyl groups are located in the mesoporous region. The superior catalytic performance of MCM-36 compared with MCM-22 for alkylation of isobutane with n-butene indicates that the open mesoporous structure can be successfully utilized to make acid sites of the layers accessible to large molecules.

Studies on the deactivation of NOx storage-reduction catalysts by sulfur dioxide

The interaction of sulfur dioxide with a commercial NOx storage-reduction catalyst (NSR) has been investigated using in situ IR and X-ray absorption spectroscopy. Two pathways of catalyst deactivation by SO2 were identified. Under lean conditions (exposure to SO2 and O2) at 350 °C the storage component forms barium sulfates, which transform from surface to hardly reducible bulk sulfate species. The irreversible blocking of the Ba sites led to a decrease in NOx storage capacity. Under fuel rich conditions (SO2/C3H6) at 350–500 °C evidence for the formation of sulfides on the oxidation/reduction component (Pt) of the catalyst was found, which blocks the metal surface and thus hinders the further reduction of the sulfides.

On the magnetic properties of ultra-fine zinc ferrites

Zinc ferrite belongs to the class of normal spinels where it is assumed to have a cation distribution of Zn2+(Fe3+)2(O2−)4, and it is purported to be showing zero net magnetisation. However, there have been recent reports suggesting that zinc ferrite exhibits anomaly in its magnetisation. Zinc ferrite samples have been prepared by two different routes and have been analysed using low energy ion scattering, Mossbauer spectroscopy and magnetic measurements. The results indicate that zinc occupies octahedral sites, contrary to the earlier belief that zinc occupies only the tetrahedral sites in a normal spinel. The amount of zinc on the B site increases with decrease in particle size. The LEIS results together with the Mossbauer results and the magnetic measurements lead to the conclusion that zinc occupies the B site and the magnetisation exhibited by ultrafine particles of zinc is due to short range ordering.

Oxidative dehydrogenation of propane over niobia supported vanadium oxide catalysts

Oxidative dehydrogenation (ODH) of propane is examined over a series of catalysts, which include Nb2O5 supported monolayer V2O5 catalysts, bulk vanadia-niobia with different vanadium oxide loadings and prepared by four different methods, V2O5and Nb2O5. The intrinsic activity (TOF) of the samples studied indicates that vanadium containing active sites are indispensable for catalytic oxidative dehydrogenation of propane. Variations in the chemical environment of the vanadium ion do not cause significant changes in activity per site and, hence, all samples show similar TOF when the rates are normalised to the concentration of V on the surface. Selectivity to propene on the other hand strongly depends on the nature of the catalyst because readsorption and interaction of propene with the acid sites leads to total oxidation. Optimization of the weak sorption of propene is, therefore, concluded to be the key factor for the design of selective oxidative dehydrogenation catalysts.

The role of the oxidic support on the deactivation of Pt catalysts during the CO2 reforming of methane

Pt supported on ?-Al2O3, TiO2 and ZrO2 are active catalysts for the CO2 reforming of methane to synthesis gas. The stability of the catalysts increased in the order Pt/?-A12O3 < Pt/TiO2 < Pt/ZrO2. For all catalysts, the decrease in activity with time on stream is caused by carbon formation, which blocks the active metal sites for reaction. With Pt/TiO2 and Pt/ZrO2, deactivation started immediately after the start of the reaction, while the Pt/?-A12O3 catalyst showed an induction period during which carbon was accumulated without affecting the catalytic activity.

Selective reduction of NO to N2 in the presence of oxygen over supported silver catalysts

Selective reduction of NO (0.1%) with propylene (0.1%) in the presence of oxygen (5%) with He balance was carried out over Ag supported on Al2O3, H-ZSM5 and H-Y catalysts. Although their activities were different, all three catalysts showed high selectivity to N2 (around 95%). Through several characterizations, it was concluded that silver was present in the form of Ag2O clusters, but not in metallic form for the three catalysts. High selectivity to N2 on all the catalysts is attributed to the same active phase of silver oxide. The difference in catalytic activity is considered to be due to the amounts and the size of Ag2O clusters. Normalized activity per Ag site (TOF) was not the same (Ag/Al2O3>Ag-ZSM5>Ag-HY), which is explained by the carbon depositions and facile transformation of Ag species under redox condition over zeolite-based catalysts.

Oxidative conversion of propane over lithium-promoted magnesia catalyst. I. Kinetics and mechanism

Oxidative conversion of lower alkanes over lithium-promoted magnesia catalysts offers a viable alternative for propene and ethene production. The catalytic performance of propane oxidative dehydrogenation and cracking shows yields up to 50% of olefin (propene and ethene). The reaction kinetics were studied by means of variation of the partial pressures of the reactants as well as by addition of product species to the reaction mixture. The observations can be qualitatively explained with a mechanism including activation of propane on the catalyst generating propyl radicals that undergo a radical-chain mechanism in the gas phase. Alkane activation is rate determining. Oxygen has two functions in the mechanism. First, the presence of small amounts of oxygen influences the radical gas-phase chemistry significantly because the type and concentration of chain propagator radicals are greatly increased. At higher oxygen partial pressures the radical chemistry is only slightly influenced by the increasing oxygen concentration. The second function of oxygen is to facilitate the removal of hydrogen from the surface OH? species that are formed during the activation of propane on the catalyst. Carbon dioxide has a strong inhibiting effect on the reaction without changing the product distribution, due to strong adsorption on the site that activates propane.

In situ catalytic pyrolysis of lignocellulose using alkali-modified amorphous silica alumina

Canadian pinewood was pyrolyzed at 450 °C in an Infrared oven and the pyrolysis vapors were converted by passing through a catalyst bed at 450 °C. The catalysts studied were amorphous silica alumina (ASA) containing alkali metal or alkaline earth metal species including Na, K, Cs, Mg and Ca. The catalysts effectiveness to reduce the bio-oil oxygen content, to enhance the bio-oil energy density and to change the liquid and gas product distribution were evaluated using different techniques including gravimetric analysis, elemental analysis, Karl-Fischer titration, GC/MS and micro-GC analysis. According to the results K/ASA found to be the most effective catalysts for conversion of hollocellulose (hemicellulose and cellulose)-derived vapors of pinewood while Cs/ASA catalyst was the most effective catalyst for conversion of lignin-derived vapors and production of hydrocarbons.