E. S. Shpiro

Infrared spectroscopy studies of the mechanism of the selective reduction of NOx over Cu-ZSM-5 catalysts

The adsorption and subsequent reaction of nitric oxide, propene, molecular oxygen and combinations of these gases has been studied on samples of Cu-ZSM-5 catalysts of high activity in the selective reduction of NO by hydrocarbons, using infrared spectroscopy. When only propene is adsorbed, it first forms an adsorbed allyl and then undergoes stepwise oxidation to adsorbed acrolein, car☐ylic acid species and carbon oxides. Since no gas phase oxygen is present, these oxidations involve extra-lattice oxygen from the zeolite. These steps highlight the importance of small, oxygen containing copper clusters in the reaction. Exposure of a fresh catalyst to NO and propene results in the formation of an organic nitro compound, as well as the species observed for propene alone. No co-ordinated NO or dimeric species, of the sort that are important in the absence of hydrocarbon, are observed. Continued exposure of the nitro compound to propene results in the formation of nitrile species, which we have reported previously. Almost identical results are observed for the full gas mixture of propene, NO and oxygen, in helium. To probe the possible role of nitro species in the reaction pathway, the interaction of nitromethane with the catalyst has been studied. Nitromethane decomposes selectively, liberating nitrogen in the presence of oxygen over both Cu-ZSM-5 and H-ZSM-5 catalysts. We propose that organic nitro compounds are important intermediates in the reaction, and speculate on a decomposition pathway to nitrogen, based on known organic chemistry, which passes through azoxy or dinitroso species. Nitriles are proposed to form under reducing conditions, but these may also decompose on exposure to oxygen, liberating nitrogen.

Reduction behavior and metathesis activity of WO3Al2O3 catalysts: I. An XPS investigation of WO3Al2O3 catalysts

The reduction of WO3Al2O3 catalysts with 7–23 wt% WO3 and of pure tungsten compounds was studied by XPS. In the pure compounds, W(VI) is reduced to the metal via W(V) and W(IV). An additional intermediate observed only in WO3 may be W(II) or W(IV) engaged in a particular interaction. In the reduction of Al2(WO4)3 a stabilization of W(VI) and W(IV) relative to Al-free samples is observed at high reduction temperatures. On the Al2O3 support the reduction of W(VI) starts at substantially higher temperatures than that in pure compounds. The stabilization is highest at low WO3 content, where the severe conditions required for W(VI) reduction lead to the direct formation of metallic W. At WO3 loadings near the theoretical monolayer capacity of the Al2O3 support, the lower temperatures of reduction permit the detection of W(IV) as an intermediate. Metallic W on Al2O3 exhibits a negative XPS shift relative to the bulk metal.

The state of platinum in high-silica zeolites and its catalytic activity in ethane and propane aromatization

Abstract The catalytic properties of Pt-containing high-silica zeolites in the aromatization of lower alkanes and the XPS spectra of these catalysts have been investigated. Both the electronic states of Pt and catalytic activity depended strongly on the pre-treatment conditions and changed in the course of the reaction. The development of the catalytic activity observed during successive catalytic runs in the pulse mode was accompanied by an increase in positive charge on highly dispersed metallic clusters located in the vicinity of acidic OH groups of zeolite. The Ptγ + centres in combination with acidic sites are likely to be responsible for ethane and propane transformations into aromatics over Pt-containing-high silica zeolites.

Reduction and metathesis activity of MoO3/Al2O3 catalysts: I. An XPS investigation of MoO3/AI2O3 catalysts

Abstract MoO 3 /Al 2 O 3 catalysts (2–13 wt% MoO 3 ) were investigated by XPS in the oxidized form, after thermal treatment in flowing Ar (973 K), and after reduction in H 2 (673–973 K), which are conditions typically employed in the activation of these catalysts for the metathesis reaction. A new assignment of Mo 3d binding energies to Mo oxidation states was applied in the analysis of the reduced samples. During the thermal treatment in flowing Ar, part of the hexavalent Mo present in the initial samples underwent reduction to Mo(V), which could also be detected by EPR. The reduction of alumina-supported Mo(VI) in H 2 was found to produce surfaces, on which Mo(VI), Mo(V), Mo(IV), Mo(II), and, at reduction temperatures above 900 K, Mo(O), coexist. For reduction temperatures of about 800 K, distributions of these Mo states, which differ from those reported in the literature by a Mo(V) contribution not exceeding 10% of the total Mo and by the presence of Mo(II), were derived. When reduced MoO 3 /Al 2 O 3 catalysts were subsequently treated in flowing inert gas at 973 K a partial reoxidation of the surface was observed.

Reduction behavior and metathesis activity of WO3Al2O3 catalysts: III. The activation of WO3Al2O3 catalysts

The activation of WO{sub 3}/Al{sub 2}O{sub 3} catalysts for the metathesis of propene (573 K, 0.1 MPa, flow reactor) was investigated. For catalysts containing {le}12 wt% WO{sub 3} the best activation procedure is a thermal treatment in flowing inert gas at a temperature depending on the WO{sub 3} content (up to 1143 K at 0.5-4 wt% WO{sub 3}). The specific activity of W after these procedures and the reducibility of W(VI) show opposite trends. Hydrogen as an activation medium is inferior to Ar at WO{sub 3} contents {le}12 wt% but with a 23-wt% WO{sub 3}/Al{sub 2}O{sub 3} catalyst conditions were found where activation in H{sub 2} yields higher activities than an inert treatment at the same temperature. The propene metathesis was found to be first order in propene at 573 K and 0.1 MPa. The apparent activation energy decreases from 40 to 25 kJ/mole with increasing WO{sub 3} content. The results imply that the active sites are formed exclusively from W(VI) precursors when the WO{sub 3} content is {le}12 wt%. At high WO{sub 3} content prereduction under appropriate conditions leads to surfaces with coexisting W(IV) and W(VI) precursors.

Reduction and aromatization activity of chromia-alumina catalysts: II. An XPS investigation of chromia-alumina catalysts

Abstract Chromia-alumina catalysts with 1.8 to 20 wt% Cr 2 O 3 , including samples containing additional elements as K, Pt, and Re, were reduced in H 2 in the 773–923 K temperature range at atmospheric pressure and investigated by X-ray photoelectron spectroscopy. In the Cr 2 p spectra of oxidized samples, Cr 6+ and Cr 3+ were detected. While Cr 6+ was rapidly transformed to Cr 3+ in the initial reduction phase a new chromium signal emerged after longer exposure times. This signal representing the product of Cr 3+ reduction was ascribed to highly dispersed zerovalent chromium with metallic band structure not formed, subject to a stabilization by the support. Signals that could be attributed to Cr 2+ and Cr 5+ were not found in this study. With regard to catalytic experiments reported earlier, the aromatization activity of chromia-alumina catalysts was ascribed to Cr 3+ coordinatively unsaturated sites.

X.p.s. investigation of methylene blue incorporated into faujasites and AIPO family molecular sieves

Abstract The distribution of methylene blue (MB) between shell and core domains of molecular sieve crystalsand the extent of aggregation of the molecules in the molecular sieve hosts is studied using X-ray photoelectron spectroscopy. It follows (i) from differences of the binding energies of the N1s electrons of solid MB and of molecular sieve-supported MB and (ii) from N/Si atomic ratios that MB is encapsulated inside molecular sieve cages and channels as isolated singular species or small aggregate if added as dissolved dye to the batch composition for the hydrothermal crystallization of molecular sieves (faujasite Y, SAPO-5, SAPO-34, AIPO-5). Ion-exchange of the MB cation into zeolites results also in high dispersions of the dye, but can produce a local destruction of the zeolite framework at the crystal surface. The structure-directing role of the dye was recognized for MB-loaded SAPO-5 and SAPO-34, exhibiting an improved phase purity.

Structural studies of platinum/ZSM-5 catalysts

Abstract Pt/ZSM-5 catalysts have been prepared, characterised by extended X-ray absorption fine structure (EXAFS), and electron microscopy, and their activity in ethane hydrogenolysis has been measured. Calcination temperature is important in determining the size of the platinum particles formed. For a 0.5% Pt/H-ZSM-5 catalyst calcined at 720 K and reduced at 620 K or 790 K, small particles within the zeolite framework are observed, with a nearest neighbour coordination number of ca. 6 and an average diameter of about 8 A. Calcination of a similar catalyst at 790 K resulted in larger metal particles, with the average diameter in the range 12–15 A, estimates from EXAFS and electron microscopy being in agreement within experimental error. Contractions of up to 4% in the average PtPt interatomic distance were observed, but there is clear evidence that the larger particles at least retain the face-centred cubic structure. EXAFS indicates the presence of Pt0 bonding as a result of coordination to the zeolite framework, with bond lengths of 1.92−2.03 A. These catalysts have higher specific activity in ethane hydrogenolysis and alkane aromatization than materials where the platinum is in the form of large particles, ( d > 100 A), at the external surface of the zeolite, and reasons for this are discussed.

Platinum promoting effects in Pt/Ga zeolite Catalysts of lower alkane aromatization. I. Ga and Pt electronic states, dispersion and distribution in zeolite crystals in dependence of preparation techniques. Dynamic effects caused by reaction mixture

Abstract X-ray photoelectron spectroscopy (XPS), infrared (IR), electron microscopy (EM) and energy dispersive X-ray (EDAX) studies of in various ways prepared Ga/H-ZSM-5, Pt-Ga/H-ZSM-5 as well as Ga-silicate and Pt/Ga-silicate have been performed. The external surface of the zeolite crystal was originally strongly enriched with a Ga2O3 phase the dispersion of which depends on the preparation as well. Ga reduction, evidenced by XPS Ga 3d line shifts, occurred during reduction and/or the reaction mixture treatment and it was accompanied by migration of GaI cations into the zeolite channels. The migration of unreduced GaIII species via solid-state reactions is less pronounced. The gallium reduction and migration is dramatically promoted by platinum introduced into both Ga/H-ZSM-5 and Ga-silicate; the reduction degree in the last sample is limited by the concentration of the extra-framework GaIII ions. Platinum was shown to be finely dispersed in highly acidic Pt-Ga/H-ZSM-5 while most of the platinum formed aggregates (30–50A˚) on the external surface of Ga-silicate. IR of adsorbed hydrogen and pyridine confirmed that reduction results in the formation of new strong Lewis acidic sites. Along with GaI-HZ and GaIIHZ structures mixed Pt-Ga particles located within the zeolite structure are most likely to be responsible for the dramatic enhancement of the aromatic selectivity found with Pt-Ga/zeolite catalysts (see Part II).

FTIR evidence of the formation of platinum carbonyls from Pt metal clusters encaged in KL zeolite

Reactivity of Pt metal clusters supported on KL zeolite toward CO was studied by FTIR spectroscopy. Investigation of the CO adsorption was performed within a wide CO pressure range (1–500 mbar). IR data on the CO adsorption at high pressure (500 mbar) suggest the transformation of the finest Pt particles into neutral Pt carbonyls ((Zeol-O:)mPtx(CO)y) stabilized by the basic oxygen atoms of the KL framework. The transformation is found to be readily reversible upon CO adsorption-desorption at room temperature (RT).