Christiane Leclercq

Chemical, physical and catalytic properties of ZSM-5 and ZSM-11 zeolites : a study by electron microscopy, EDX-STEM and XPS

Abstract In order to ascertain the relative importance of shape-selectivity due to channel size limitations and of zeolitic grain size in the conversion of methanol to higher hydrocarbons on Pentasil-type zeolites, several catalysts were prepared using different synthesis and acidification procedures. Acidity of the samples was determined by microcalorimetry of the differential heat of NH3 adsorption. Crystallite size and morphology were determined by high resolution electron microscopy while the Al distribution across the crystallites was monitored by high resolution EDX-STEM and XPS. The ZSM-11 samples were composed of spherical aggregates having diameter of 0.6 ± 0.2 μ m for one batch and 6 ± 2 μm for another batch. The former sample was composed of aggregates of crystals of 5 to 10 nm in diameter and gave an XRD pattern with broad peaks. However, infrared vibrational spectra and microdiffraction were consistent with ZSM-5 or -11 material. The latter sample was composed of aggregate cores with crystalline needles of 200 to 400 nm in size emerging from the surface. EDX-STEM technique shows that within aggregate crystals the Al distribution is rather homogeneous while a great heterogeneity exists between grains. XPS data show that the outermost layers are in average richer in Al than the inner layers. In the sample with needles the AI content within the needles is homogeneous but varies from one grain to another one. However this Al content is 1.5 to 4 times lower than in aggregate core of each grain. XPS technique also shows that the outer layers of the needles are very poor in Al (Si:Al = 70 ± 10). ZSM-5 samples were shown to be composed of parallelipipeds about 0.5 to 1 μm in size with a narrow size distribution. The Al content was shown to be approximately homogeneous within the grain but very different from one grain to another (factor 2 to 3). Acidification by HCl was also shown by XPS to eliminate lattice Al from the outerlayers. It appears from the selectivities in aromatic hydrocarbons in the reaction of methanol conversion to a mixture of hydrocarbons that H-ZSM-5 yields preferentially A8 aromatics whereas H-ZSM-11 gives more A9 aromatics. This holds true for both acidification procedures and for different crystallite sizes. This was attributed to different physical constraints resulting from the difference in the free void space at the channel intersections between ZSM-5 and ZSM-11 zeolites. Large crystallites of ZSM-11 (second sample) gave more light aliphatics (C1 + C2) and less C6+ aliphatics and A9+ aromatics than the small crystallites (first sample) and resemble the ZSM-5 zeolite of about the same crystallite size for this particular aspect. The acid strength of active sites, as characterized by microcalorimetry of NH3 adsorption, and the acid site distribution were observed not to play an important role in methanol conversion reaction, as long as sufficient acid sites are present. Particular emphasis is placed upon misleading conclusions on the particle size effect which may stem from inaccurate SEM characterization and on the oversimplification of steric limitations due to the size of the channels.

Titania-supported bimetallic catalyst synthesis by photocatalytic codeposition at ambient temperature : preparation and characterization of Pt-Rh, Ag-Rh, and Pt-Pd couples

Abstract We have examined the possibilities of preparing some bimetallic catalysts supported on TiO2 (Degussa P-25) by UV illumination (λ > 300 run, radiant flux, 60 MW cm−2), at room temperature for 24 h, of aqueous suspensions containing 500 mg of this oxide, and cations of the desired metals in amounts corresponding to metal loadings between 0.5 and 2.5 wt% if complete reduction were achieved. In the case of the H2PtC16RhCl3 couple, Rh deposition almost did not take place. For the mixture AgNO3RhC13 at pH 2.6, Rh deposition was substantial but still incomplete. In contrast, Rh deposition was complete in aqueous ammonia (pH 11). TEM and STEM-EDX analysis of the solid recovered in this latter case showed that Rh formed 1- to 3-nn particles and was also found in 8- to 30-nm particles containing ca. 80 wt% Ag (with respect to Rh) presumably as a result of its deposition on more rapidly crystallized Ag. Solids prepared from H2PtC16 and Pd(NO3)2 were studied in more detail because both metals were completely deposited, at least at levels ≤1 wt%. They were heterogeneous, since Pd and Pt formed ca. 1- or 3-nm crystallites, respectively; however, most of the crystallites were found to contain both metals with a high and varied proportion of Pd. An XPS Ar+-sputtering depth profile of the I wt% Pt-1 wt% Pd/TiO2 sample showed a similar behavior for Pt and Pd, which might indicate the formation of a solid solution in some particles. As this formation is attributed to similar reduction rates for both metallic salts, it is inferred that alloys are obtained only in cases where the salts and conditions can be chosen to fulfill this requirement. This limits the capabilities of the photoassisted codeposition method for preparing effective bimetallic supported catalysts.

Location and structure of coke deposits on alumina-supported platinum catalysts by EELS associated with electron microscopy

Abstract The location and structure of carbonaceous deposits formed upon coking Pt Al 2 O 3 catalysts with cyclopentane have been studied by conventional TEM and by electron energy loss spectroscopy (EELS) associated with high resolution scanning transmission electron microscopy. The TEM investigation shows patches of amorphous coke covering the support surrounding each metal particle. This is substantiated by EELS showing that the coke coverage can extend as far as 20 nm from a given particle. From the absence of fine structure on the EELS peak of carbon and from the comparison with reference compounds (graphite, coronene, and pentacene), it is concluded that the local structure of coke is not graphitic or pregraphitic but may consist in a disordered arrangement of polyaromatic molecules. After partial combustion at 563 K, the coke covering the metal particles is removed but the coke on the support is still present. This confirms previous temperature-programmed Oxidation results.

Study by electron microscopy and electron diffraction of formation of nickel epitaxially grown catalysts

Abstract In order to study the influence of the crystallographic arrangement at the surface of a metal-supported catalyst on its catalytic properties, catalysts have been prepared with well faceted nickel crystallites presenting particular faces at their surface. Catalysts were obtained by reduction by hydrogen of nickel antigorite evacuated at different temperatures. The morphology and structure of nickel antigorite, the parent of the catalyst, has been determined by electron microscopy and electron diffraction at different temperatures. Rolled or flat sheets were observed. The texture of the catalysts obtained after reduction was determined. The crystallites of nickel are hexagonal platelets with either their (111) or their (110) plane parallel to the sheet support, depending on the conditions of preparation and reduction. Study of the orientations of (111) type or (110) type was done.

Subnanometer nickel clusters in complex reducing agents

Abstract Nickel-based, complex reducing agent (NiCRA) prepared by reacting Ni(OAc)2, NaH and t-BuONa in THF were studied by transmission electron microscopy. Nickel is in the form of 1 nm-large clusters. In presence of bipyridyl ligands particles as small as 0.5 nm are detected. The formation and stability of nickel clusters is attributed to the strong reducing potential of the NaH—t-BuONa mixture complexing the nickel clusters. Bipyridyl ligands exert an additional stabilization so that clusters containing less than 10 atoms could be present.

Hydrogen titration, scanning electron microscopy, and associated X-ray emission studies of PtRuAl2O3 catalysts

Abstract Two series of platinum-ruthenium catalysts deposited on γ-Al 2 O 3 pellets and equivalent to 2 wt% Pt in number of (Pt + Ru) atoms were investigated. The first series (I) was prepared by successive impregnation of H 2 RuCl 6 and of H 2 PtCl 6 in water, and the second (II) by coimpregnation of H 2 PtCl 6 and of H 2 RuCl 6 in ethanol (75%) plus water. The chemisorption (O 2 , H 2 ) and titration (mainly hydrogen temperature-programmed titration of chemisorbed oxygen) data showed catalysts (I) to be composed of mixtures of Pt and Ru particles, and catalysts (II) to be composed of alloy particles. This conclusion was on the whole corroborated by single particle X-ray emission analysis using X-ray emission in a scanning transmission electron microscope. This last technique further showed that the platinum particles in catalysts (I) contained a small amount of Ru. The reasons for the difference of state of the metallic particles in catalysts (I) and (II) are not totally elucidated but a tentative interpretation is given.

Photoassisted deposition of rhodium on platinum/titania samples as a method of preparing bimetallic catalysts

Abstract Rh-Pt/TiO2 samples obtained by UV illumination at room temperature in the absence of oxygen of suspensions in aqueous Rh3+ solutions of particulate 2.5 wt.-% Pt/TiO2 (previously prepared by impregnation and reduction in hydrogen), were examined by X-ray photoelectron spectroscopy and transmission and scanning transmission electron microscopy. Depending on the medium (water, 2-propanol-water, ammonia solution buffered at pH 11 with ammonium chloride or nitrate), rhodium particles (ca. 1 wt.-% loading) of various mean size and degree of reduction were found. Interestingly, rhodium was deposited on titania in neutral medium and on platinum in basic medium. These results, which demonstrate that different types of bimetallic catalysts can be prepared by photoassisted deposition, are discussed in terms of the separation of photoproduced charges and surface charge.

Photo-Assisted Deposition of Noble Metals: Investigation of a New Route for Metallic and Bimetallic Catalyst Preparation

The photocatalytic deposition of noble metals on photosensitive supports is proposed to synthesize well dispersed metal catalysts in mild conditions of preparation. The effects of various parameters governing the kinetics of photo-deposition (light flux, solution, concentration, temperature, nature of ions) have been studied. Noble metals, such as Pt, Pd, Ag, Rh, Au, Ir have been deposited on titania. Other photosensitive supports such as oxides (ZnO, Nb2O5, ZrO2, ThO2, etc.) or sulfides (CdS) have also been employed. Bimetallic catalysts (Pt-Pd, Pd-Ag, Pt-Ir) on TiO2 have been prepared with a probable alloy formation according to STEM analysis. Although common insulating supports are theoretically unsuitable for photodeposition, a limited deposition of Pt was observed on alumina. A photodeposited 0.5 wt% Pt/TiO2 was used in two bi functional photocatalytic reactions. It exhibited a higher activity than a well dispersed analogue obtained by impregnation and H2 reduction, possibly because the support did not undergo a severe reduction treatment.