Showing papers in "Catalysis Letters in 1992"

DRS UV-Vis and EPR spectroscopy of hydroperoxo and superoxo complexes in titanium silicalite

Abstract: The most important spectroscopic features in the UV-Vis of framework and extraframework Ti(IV) in titanium silicalite are briefly summarized. The spectroscopic manifestations of the complexes formed by framework Ti(IV) in presence of H2O2 are reported. The formation of EPR active species is also considered.

Redox chemistry in excessively ion-exchanged Cu/Na-ZSM-5

Abstract: TPR/TPD and FTIR are used to characterize “excessively ion-exchanged” Cu/Na-ZSM-5. After calcination in O2 at 773 K at least two copper-oxygen species are present in addition to Cu2+ ions; these have been identified as CuO and [Cu-O-Cu]2+. Reduction in H2 transforms all these into Cu0 below 773 K. [Cu-O-Cu]2+ is autoreduced to Cu+ during outgassing. Reoxidation of Cu0 by zeolite protons to Cu+ is observed above 723 K in He or Ar; in the presence of CO this process is considerably enhanced and observed at much lower temperature, because CO is strongly adsorbed on Cu+. At 293 K CO adsorption causes reversible changes in the FTIR spectra.

Role of oxygen in selective reduction of nitrogen monoxide by propane over zeolite and alumina-based catalysts

Abstract: The role of coexisting oxygen in the selective reduction of nitrogen monoxide by propane on H-zeolites, alumina, Cu-ZSM-5 zeolite and Pt/Al2O3 catalysts was investigated. In the case of H-zeolites and alumina, oxidation of NO to NO2 played an important role for the overall selective reduction of NO. On the other hand, the initial reaction step was considered to be partial oxidation of propane over Cu-ZSM-5 and Pt/Al2O3.

Well defined Cu I (NO), Cu I (NO) 2 and Cu II (NO)X (X = O − and/or NO 2 − ) complexes in Cu I -ZSMS prepared by interaction of H-ZSM5 with gaseous CuCl

Abstract: In this note an exchange procedure of the acidic protons of H-ZSM5 by CuI ions through reaction with CuCl in the gas phase is described. In the so obtained CuI-ZSM5 exchanged zeolite the CuI ions are in well defined configuration and form with NO mono and di-nitrosyl complexes of high structural and spectroscopic quality. The CuI(NO)2 species are transformed at RT into CuII(NO)X (X=O− and/or NO 2 − ) species which could represent an intermediate in NO decomposition.

On the mechanism of nitric oxide decomposition over Cu-ZSM-5

Abstract: Cu-ZSM-5, a copper-containing zeolite, catalytically decomposes NO at temperatures below those of other catalysts. A mechanism is proposed which is based on active sites consisting of coordinatively unsaturated cupric (Cu2+) ions in a square planar configuration. These sites are posited to chemisorb NO molecules in the gem-dinitrosyl form. The pair of adsorbed NO molecules desorbs as N2 and O2. This mechanism accounts for the experimental behavior in chemisorption and decomposition without invoking a cyclical oxyreduction of the surface sites.

Surface interaction model ofγ-alumina-supported metal oxides

Abstract: An incorporation model has been proposed and used to discuss the interaction between various metal oxides and γ-alumina. The dispersion capacities of various metal oxides are predicted and the surface hydroxyl group density on γ-alumina estimated, based on the assumption that the (110) plane is preferentially exposed on the surface of γ-alumina. These results are in good agreement with the available experimental data.

Understanding the trends in the hydrodesulfurization activity of the transition metal sulfides

Abstract: It is shown that the trends in the hydrodesulfurization activity of the transition metal sulfides can be understood from the variation in the binding energy of sulfur to the transition metals. A model based on the ab initio calculations of Gelatt, Moruzzi, and Williams is used to describe the binding energy trends and to single out the important parameters determining them. One important aspect of the model is, that it predicts that the trends in the catalytic activity are given to a large extend by the coupling between the sulfur chemisorption energy and the sulfur-induced distortions of the metal lattice. The simple model also appears to describe promotional effects.

Mechanism of NO decomposition over Cu-ZSM-5

Abstract: In the preceding Letter Shelef [1] has proposed a mechanism for NO decomposition involving coordinatively unsaturated Cu2+ sites on which NO molecules are chemisorbed in the gem-dinitrosyl form. At reaction temperature this complex is supposed to decompose into N2 and O2 without involving a redox process. That such a process is feasible has been pointed out by Moser [2]. Shelef cited several reasons in support of this view and others that have led him to think that a cyclic redox mechanism is not operative. These arguments are countered herein and some new data are presented showing the infrared spectra of surface species recorded under in situ reaction conditions.

Nature of the metal-support interface in supported metal catalysts: results from x-ray absorption spectroscopy

Abstract: X-ray absorption spectra characterizing the metal-support interface in supported metal complexes and supported metal catalysts are summarized and evaluated. Single-metal-atom transition metal complexes on non-reducible metal oxide supports are bonded with metal-oxygen bonds with metal-oxygen distances of approximately 2.15 A; the bonding distance is only weakly sensitive to the oxidation state of the metal. Nearly this same metal-oxygen distance is characteristic of the metal-support interface in metal-oxide-supported metal clusters following high temperature reduction in H2 (HTR:T > 450 °C). The metals at the interface may be polarized sufficiently that they bond with the oxygen of the support much as the cations in mononuclear complexes bond with it. When the supported metals are treated in H2 at low temperatures (LTR:T < 350 °C) or are prepared under He with partially hydroxylated supports, a longer metal-support oxygen distance is observed, typically 2.5–2.7 A. This distance is suggested to characterize interactions between zero-valent metals and support oxygen. Changes in the performance of supported metal catalysts resulting from differences in the temperature of pretreatment in H2 are attributed to changes in the electronic properties and/or morphology of the metal clusters, which are suggested to be related to the concomitant changes in the structure of the metal-support interface.

Low temperature oxidative conversion of methane to syngas over NiO-CaO catalyst

Abstract: Catalytic partial oxidation of methane over NiO-CaO (with or without its prereduction by H2) at low temperatures (⩽ 973 K) under non-equilibrium conditions yields syngas (H2/CO ratio ≈ 2.0) with high conversion/selectivity and extremely high productivity. If required, the H2/CO ratio can be increased by adding water vapour to the feed. Product selectivity is controlled by the process kinetics and also the reaction path is different from that observed for the high-temperature non-catalytic and catalytic processes operating at equilibrium.

Analysis of XANES for identification of highly dispersed transition metal oxides on supports

Abstract: XANES of vanadium and niobium oxide on silica or alumina have been analyzed quantitatively by a deconvolution technique. Based on the results for reference compounds, local structures of supported vanadium and niobium species were identified. The composition was estimated from difference spectra for the samples which consisted of two kinds of species.

Iron ion-exchanged zeolite: the most active catalyst at 473 K for selective reduction of nitrogen monoxide by ethene in oxidizing atmosphere

Abstract: Among various metal ion-exchanged zeolites, metal loading alumina, and oxides, iron ion-exchanged mordenite was the most active for the selective reduction of nitrogen monoxide to nitrogen by ethene in the presence of oxygen at the temperature as low as 473 K. The catalytic activities of iron ion-exchanged zeolites depended on the zeolite structure and the iron ion exchange level. The effects of the zeolite structure are in the order of MOR > FER > MFI > Y > LTL at 473 K. The activity of iron ion-exchanged mordenite increased with the increment in the exchange level and levelled off above about 60%.

A comparative study on the activation and reactions of CH4 on supported metals

Abstract: The catalytic effects of silica-supported Pt metals were tested and compared in the decomposition of methane and its conversion into higher hydrocarbons. The dissociation of methane is readily measurable at 473–673 K. The rate of initial decomposition at 523 K was the highest on Rh, but it dropped to a low value within a short contact time. The gaseous products were hydrogen and ethane in small, variable amounts. Hydrogen of the carbonaceous species formed in the decomposition led to the production of aliphatic hydrocarbons.

Temperature-programmed desorption of H2 as a tool to determine metal surface areas of Cu catalysts

Abstract: Temperature-programmed desorption of H2 (H2 TPD) is shown to be a useful new tool for the determination of Cu metal surface areas. The technique has been employed in on-line characterization of binary Cu/Al2O3 and ternary Cu/ZnO/Al2O3 catalysts in a combined TPD-microreactor setup. The catalysts were studied both after reduction and after water gas shift activity tests. A main H2 TPD peak is observed around 300 K which can be assigned to desorption from Cu metal surface sites. The H2 TPD method is compared with the N2O frontal chromatography method which has been extensively used in previous studies for the determination of Cu surface areas. It is found that the dissociative adsorption of N2O may induce significant changes in the catalyst structure leading to errors in the surface area determination. With the H2 TPD procedure such irreversible changes can be avoided. A further advantage of the H2 TPD method is the possibility of providing insight into the nature of the exposed Cu metal surfaces.

Reduction of NO over impregnated Cu/ZSM-5 in the presence of O2

Abstract: Cu/ZSM-5 catalysts prepared by impregnation of Cu acetate are active for NO reduction. In mixtures containing both NO and O2, reductants such as CO or CH4 preferentially react with O2, but propane reacts preferentially with NO. Small amounts of O2 actually increase the reduction of NO by C3H8. The N2 yield reaches a maximum near an O2 /C3H8 ratio=5, i.e. the stoichiometry of total C3H8 oxidation. At much higher than stoichiometric O2 contents for total C3H8 oxidation the N2 yield with C3H8 (54%) is still substantially higher than, for instance, with CO of the same or larger concentration. The hypothesis that an intermediate of C3H8 oxidation is responsible for the enhanced NO reduction is discussed. Temperature-programmed reduction shows that after reaction with NO+C3H8 and sub-stoichiometric amounts of O2 Cu in the catalyst is mainly metallic, but CuO particles appear to be formed in the presence of an excess of O2.

Microcalorimetric studies of zeolite acidity

Abstract: The acidity characteristics of H-ZSM-5, H-Mordenite and H-Y zeolite have been studied by microcalorimetric and gravimetric measurements of pyridine adsorption. H-ZSM-5 and H-Mordenite have Bronsted acid sites of primarily homogenous strength, with H-Mordenite having the stronger sites, whereas H-Y zeolite had Bronsted sites of varying strength. The effects of Na exchange level in H-Y zeolite and high temperature calcination for H-Mordenite have also been examined.

13C CP/MAS and2H NMR study of tert-butyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of tert-butyl cation and tert-butyl silyl ether intermediates

Abstract: The dehydration reaction of tert-butyl alcohol, selectively labelled with13C in CH3 or C-O groups (t-BuOH[2−13C2] andt−BuOH[1-13C]), as well as selectively deuterated in methyl groups (t-BuOH[2-2H9]), was studied on H-ZSM-5 zeolite simultaneously with13 C CP/MAS and2H solid state NMR. When adsorbed and dehydrated on zeolite at 296 K,t-BuOH[2−13C1] andt-BuOH[1−13C] give rise to identical13C CP/MAS NMR spectra of oligomeric aliphatic products. This is explained in terms of the fast isomerization of the tert-butyl hydrocarbon skeleton via the formation of tert-butyl cation as the key reaction intermediate. An alkoxide species, most probably tert-butyl silyl ether (t-BuSE), was also detected as the “side” reaction intermediate. This intermediate was stable within the temperature range 296–373 K and decomposed at 448 K to produce additional amounts of final reaction products, i.e. butene oligomers. NMR data point to the existence of equilibria between the initial tert-butyl alcohol, tert-butyl cation and butene that is formed from the intermediate carbocation.

Low temperature oxidative conversion of methane to synthesis gas over Co/rare earth oxide catalysts

Abstract: CoO-rare earth oxide catalysts (particularly CoO-Yb2O3) show high activity and selectivity in the oxidative conversion of methane to CO and H2 with very high productivity at low temperatures (⩽ 700 °C as low as 300 °C).

An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

Abstract: In situ FT-IR spectroscopy allows the methanol synthesis reaction to be investigated under actual industrial conditions of 503 K and 10 MPa On Cu/SiO2 catalyst formate species were initially formed which were subsequently hydrogenated to methanol During the reaction a steady state concentration of formate species persisted on the copper Additionally, a small quantity of gaseous methane was produced In contrast, the reaction of CO2 and H2 on ZnO/SiO2 catalyst only resulted in the formation of zinc formate species: no methanol was detected The interaction of CO2 and H2 with Cu/ZnO/SiO2 catalyst gave formate species on both copper and zinc oxide Methanol was again formed by the hydrogenation of copper formate species Steady-state concentrations of copper formate existed under actual industrial reaction conditions, and copper formate is the pivotal intermediate for methanol synthesis Collation of these results with previous data on copper-based methanol synthesis catalysts allowed the formulation of a reaction mechanism

Ship-in-Bottle synthesis of Pt9-Pt15 carbonyl clusters inside NaY and NaX zeolites, in-situ FTIR and EXAFS characterization and the catalytic behaviors in13CO exchange reaction and NO reduction by CO

Abstract: [Pt9(CO)18]2−/NaY (orange-brown, 2056 and 1798 cm−1), [Pt12(CO)24]2−/NaY (dark-green, 2080 and 1824 cm−1 and [Pt15(CO)30]2−/NaX (yellow-green, 2100 and 1865 cm−1) were stoichiometrically synthesized by the reductive carbonylation of [Pt(NH3)4]2+/NaY, Pt2+/NaY and Pt2+/NaX, respectively. The IR bands characteristic of their linear carbonyls shift to higher frequencies whereas the bridging CO bands to lower frequencies, compared with those on the external zeolites and in solution. In-situ FTIR studies suggested that the subcarbonyl species such as PtO(CO) and “Pt3(CO)3(μ2 −CO)3” are formed as the proposed intermediates towards [Pt12(CO)24]2−/NaY in the reductive carbonylation of Pt2+/NaY.13CO exchange reaction preceded with the different intrazeolite Pt carbonyl species in the following order of activity at 298–343 K: “Pt3(CO)3(μ2 –CO)3”/NaY ≫ PtO(CO)/NaY>[Pt9(CO)18]2−/NaY >[Pt12(CO)24]2−/NaY. Pt-L3-edge EXAFS measurment for these synthesized samples demonstrated that they are consistent with the Pt carbonyl clusters having trigonal prismatic Pt9 and Pt12 frameworks infered to a series of the Chini complexes such as [NEt4]2[Pt3(CO)6] n ( n = 3–5). The intrazeolite Pt9 and Pt12 carbonyl clusters exhibited higher cataytic activity in NO reduction by CO towards N2 and N2O at 473 K, compared with those on the conventional Pt/Al2O3 catalysts. The mechanism of intrazeolite Pt9-Pt15 carbonyl cluster formation are discussed in terms of the intrazeolite basicity and acidity.

Influence of zirconium salt precursors on the crystal structures of zirconia

Abstract: Procedures have been developed for preparing zirconia to yield either tetragonal or monoclinic phases following low (400–600 °C) temperature calcination. The data in this note emphasize the need for paying attention to the precursor zirconium salt in preparing the monoclinic or tetragonal phases. A salt precursor obtained from a supplier at different times may produce different results. Generally, it is more difficult to obtain a precursor that will produce the monoclinic phase than the one that will produce the tetragonal form.

A scanning tunneling microscope that operates at high pressures and high temperatures (430 K) and during catalytic reactions

Abstract: We describe the construction and operation of a scanning tunneling microscope (STM) designed in our laboratory that is contained in a reaction cell and allows operation throughout a wide range of pressures (ultra-high vacuum-atmospheric) and temperatures (300–425 K). This thermally compensated, double piezo tube design is entirely mechanically clamped. Samples are inertially translated and can be easily transferred in and out of the STM. With this microscope, we have investigated the stability of Pt(110) as a function of oxygen and hydrogen pressure and temperature.

Activity of alumina-supported molybdenum nitride for carbazole hydrodenitrogenation

Abstract: The alumina-supported molybdenum nitride catalyst was extremely active in the hydrodenitrogenation of carbazole at 553–633 K and 10.1 MPa total pressure when compared with the sulfided and reduced catalysts.

On the effects of the sol-gel synthesis parameters on textural and structural characteristics of TiO2

Abstract: The effects of alcohol/alkoxide, water/alkoxide molar ratios, acid addition, type of solvent, reaction temperature and calcination temperature on textural, structural and morphological characteristics of TiO2 were studied. The results show that surface area, pore size distribution and crystalline phases were very sensitive to preparation conditions.

Oscillatory behaviour of the reduction of NO by H2 over Rh

Abstract: The NO reduction by H2 on Rh has been studied by field emission microscopy (FEM). It has been observed that this reduction shows oscillatory behaviour at 460 K andPNO = −1.5 ×10−1 Torr andPH2 =1×10-6 Torr, Unique features of FEM are the very high spatial resolution and the presence of, in principle, an indefinite number of different crystal planes. The oscillatory behaviour is reflected by periodic changes in the emission current and in the images observed. The communication between different surfaces present on the field emitter is shown on a fluorescent screen. Diffusion and gas phase coupling seem to play a role. Many of the features reported earlier for the oscillatory behaviour of the NO-H2 and NO-NH3 reactions over Pt(100) are observed on Rh as well, including the surface explosion. The vacancy model proposed earlier for the oscillations over Pt(100), can be applied to the reactions described in this paper as well.

Condensation of alcohol over solid-base catalyst to form higher alcohols

Abstract: Condensations of various primary alcohols (C2-C5) with methanol were carried out at atmospheric pressure over various metal oxides having a solid-base property. The reactions gave one or two carbon higher alcohol than the reacted primary alcohol. MgO catalyst was most active for the reaction and yielded the alcohol products in high selectivity (> 80%). Based on the results of the exchange reaction between methyl hydrogen of methanol over MgO surface, it is concluded that a rapid hydride transfer between adsorbed alcohol and adsorbed carbonyl is responsible for the selective formation of alcohols.

Decomposition of nitric oxide on metallosilicates under a large excess oxygen condition with coexistence of a low concentration cetane

Abstract: Catalytic removal of NO was conducted under the condition of large excess O2 by using different kinds of metallosilicates having MFI structure. With coexistence of a low concentration of cetane, H-Fe-silicate, which had both acidity and redox property, exhibited the maximum performance among the catalysts tested at a fairly lower temperature range, around 350 °C, and the NO was totally converted. The temperature dependence of NO conversion was consistently similar to that of cetane combustion.

Reducibility and CO hydrogenation over Pt and Pt-Co bimetallic catalysts encaged in NaY-zeolite

Abstract: Temperature programmed techniques (TPR, TPD) and X-ray diffraction (XRD) have been used to study ion migration and location as well as reducibility of platinum and cobalt ions encapsulated in Pt/NaY, Co/NaY and Pt-Co/NaY zeolites prepared by ion exchange. The temperature required to reduce Co2+ in NaY was significantly lowered by the presence of Pt and dependent upon the relative locations of Pt and Co ions in zeolite cages. The exact location was controlled by the calcination condition and the metal contents. For bimetallic catalyst with low Pt content (0.5 wt% Pt and 0.9 wt% Co), the TPR results indicated that reduction of Co2+ ions in the vicinity of Pt shifted toward lower temperature, while that of Co2+ staying alone was not affected. With high Pt loading (4.5 wt% Pt, 0.7 and 2.6 wt% Co), however, most of the Co2+ ions were reduced by means of Pt at temperature below 723 K after calcination at 573 K. The temperature for Pt reduction in bimetallic catalysts was somewhat higher than Pt/NaY and increased with Co atomic fraction, indicating that mixed oxide, PtCoxOy, might be formed during calcination. After reduction in hydrogen at 723 K, highly dispersed metal particles were formed. These fine particles were most probably confined inside zeolite cages as indicated by the absence of XRD peak for all samples after calcination and reduction. Surface composition of the bimetallic particles may be different for catalysts with similar Pt content but different Co loading. Accordingly, H/Pt ratios of 1.0 and 0.72 for catalysts with low and high Co content, respectively, were shown by hydrogen chemisorption. It was further supported by the increase in TPD peak intensity with Co loading in the high temperature range, which was related to the reoxidation of Co in bimetallic particles by surface hydroxyl groups. Preliminary results on CO hydrogenation demonstrated that activity and methanol selectivity were higher on Pt-Co bimetallic catalysts than either over monometallic Pt or Co catalyst, which was consistent with the Pt enhanced Co reduction and formation of Pt-Co bimetallic particles.

Methanol synthesis from carbon dioxide at atmospheric pressure over Cu/ZnO catalyst. Role of methoxide species formed on ZnO support

Abstract: Methanol synthesis from CO2 and H2 was carried out over a Cu/ZnO catalyst (Cu/Zn = 3/7) at atmospheric pressure, and the surface species formed were analyzed by diffuse reflectance FT-IR spectroscopy and temperature programmed desorption method. Two types of formate species and zinc methoxide were formed in the course of the reaction. Zinc methoxide was readily hydrolyzed to methanol. H2O formed through the reverse water gas shift reaction was suggested to be involved in the hydrolysis of zinc methoxide.

The preparation of silica supported Pd catalysts: the effect of pretreatment variables on particle size

Abstract: The effect of pretreatment on the dispersion of Pd catalysts supported on silica has been studied. The catalysts were prepared from [Pd(NH3)4] (NO3)2 as the metal precursor at a pH = 9. The resulting catalysts were characterized using both CO and H2 chemisorption, transmission electron microscopy, and in-situ UV reflectance spectroscopy. Pretreatment in H2 resulted in poor Pd dispersions while pretreatment in He or Ar resulted in very high dispersions. Pretreatment in O2 resulted in moderate dispersions. The results are explained by considering the chemical structure of the adsorbed surface complex under different pretreatment conditions. The chemistry of the decomposition process is considered in detail.