François Bozon-Verduraz

Preparation and characterization of highly dispersed silica-supported ceria

Abstract Silica-supported ceria samples of various cerium content ( 0.008 to 11 % Ce ) were prepared either through anchoring cerium acetylacetonate in organic medium or by capillary aqueous impregnation of cerium nitrate. After calcination, ceria particles as small as 2 nm were obtained by the anchoring process while impregnation leads to larger particles (5–9 nm). These samples show significant spectroscopic differences with pure ceria: (i) increase of the X-ray photoelectron spectroscopy binding energies of the Ce 3d and of the O 1s lines, (ii) blue shift of the band gap measured by diffuse reflectance, (iii) observation of narrow UV bands at low cerium content, (iv) increase of the CO stretching vibration of CO-Ce4+ terminal species. These results are ascribed to the coordinative unsaturation of surface cerium and oxygen ions.

Nanodisks of Au and Ag produced by laser ablation in liquid environment

Abstract Ablation of Au and Ag targets in water by a Cu vapor laser generates Au and Ag sols. The metal nanoparticles obtained after evaporation are disk-shaped (diameter in the 20–60 nm range, thickness of few nanometers). Their formation is observed at laser fluence between 10 and 20 J / cm 2 . Both aqueous sols are characterized by well-resolved plasmon bands around 400 nm (Ag) and 520 nm (Au).

Infrared studies on SnO2 and Pd/SnO2

Abstract The peculiarities of the IR studies of adsorption on semiconducting oxides are reviewed with special attention to SnO2. Absorption by free electrons is very strong in reducing atmospheres and well-defined conditions are required to get vibrational information on the nature of surface species. CO adsorption on SnO2 at room temperature (r.t.) gives rise to CO–Sn2+ and CO–Sn4+ end-on species and to various carbonate entities. NO adsorption leads to the formation of mononitrosyls, nitrite and nitrate species. Pd/SnO2 catalysts are prepared according to original procedures: photodeposition and grafting of a molecular complex; the grafting method leads to highly dispersed PdO which is reduced to well-dispersed Pd upon CO adsorption at r.t., whereas NO gives rise to nitrosyl species mainly adsorbed on ionic Pd. The specificity of SnO2 is evidenced upon interaction of a stoichiometric (CO+NO) mixture at r.t.: formation of N2O on SnO2 alone, generation of N2O and isocyanate on PdO/SnO2. These results are explained by the presence of oxygen vacancies and may be related to the low-temperature catalytic activity of SnO2-supported palladium in deNOx reactions reported elsewhere.

Niobium pentoxide prepared by soft chemical routes: morphology, structure, defects and quantum size effect

Niobium pentoxide (Nb2O5) is prepared by soft chemical routes (digestion either in ammonia or in hydrazine solutions) and is compared to a commercial sample. According to various characterization methods (XRD, HRTEM, DTA-TG, UV-visible diffuse reflectance, Raman and EPR spectroscopies), Nb2O5 shows particular bulk and surface properties. The phase transformation temperatures (amorphous → pseudo-hexagonal, pseudo-hexagonal → orthorhombic and orthorhombic → monoclinic) for the synthetic Nb2O5 are about 100–150 °C higher than for the commercial sample. The textural properties depend strongly on the preparation method. After calcination at 400 °C, the sample prepared in ammonia has a larger pore volume (0.22 cm3 g−1) and a better resistance to sintering at 600 °C. The preparation in hydrazine gives the following advantages only if the gel is sonically redispersed in ethanol: low particle size, narrow size distribution, and higher resistance to sintering (140 m2 g−1 at 600 °C). Preparation in ammonia or in hydrazine favors the formation of defects, characterized by a significant absorption in the visible range adjacent to the interband transition (3.4 eV, 360 nm). These defects, which disappear upon heating in oxygen, are identified as Nb4+ species and ionized oxygen vacancies as confirmed by EPR measurements. Finally, the nanoparticles obtained by sonication in ethanol (average particle size 4.5 nm) shows a significant band gap increase (from 3.4 eV to 4.2 eV) which is assigned to a quantum size effect.

One-step construction of silver nanowires in hexagonal mesoporous silica using the polyol process

An original one-step preparation of single-crystal silver nanowires in hexagonal mesoporous silica is presented. The silver precursor, silver nitrate, is reduced in ethylene glycol (EG). This procedure avoids thermal treatments which can lead to phase segregation. The absorbance spectrum of the resulting hybrid material exhibits a transverse resonance plasmon peak near 360 nm whereas the longitudinal oscillation is shifted to the near-infrared region at about 1500 nm.

Characterization of active sites on AgHf2(PO4)3 in butan-2-ol conversion

Abstract The silver–hafnium phosphate (AgHf 2 (PO 4 ) 3 ), belonging to the Nasicon-type structure, was synthesized by a sol–gel method and characterized by several techniques. The Ag + cations were found to be, as in zeolite frameworks, easy to reduce to metallic silver. This reduction was investigated by STEM/EDX which showed that, under an intense electron beam, the Ag + ions diffuse toward the surface of the sample and form metallic particles of sizes varying from 3 to 15 nm. The X-ray diffraction patterns confirmed that the reduction does not damage the phosphate structure. The Ag + ions were replaced in the structure by protons giving rise to (PO–H) acid groups. The catalytic behavior of AgHf 2 (PO 4 ) 3 was studied in butan-2-ol conversion. In the absence of O 2 , the reaction leads to an abrupt decrease of the dehydrogenation activity and a dehydration reaction that reaches a pseudo-stationary state. Concomittantly, the Ag + cations are reduced to Ag 0 . In the presence of O 2 , the dehydrogenation reaction undergoes a complex transitory state which can be decomposed into two steps. During the first one, the activity decreases in concert with the reduction of Ag + ions. In the second step it increases as the amount of oxidized silver species (Ag x O y ), which are the active sites for the alcohol dehydrogenation, increases. UV–VIS characterization of AgHf 2 (PO 4 ) 3 in dynamic conditions similar to those used in the catalytic tests confirmed that butan-2-ol reduces Ag + ions to metallic silver which, in the presence of O 2 in the reaction mixture, are oxidized to (Ag x O y ).

Nanosized tin dioxide: Spectroscopic (UV–VIS, NIR, EPR) and electrical conductivity studies

Nanosized tin dioxide samples (mean particle size: 3 nm, surface area: 170 m2 g−1) have been prepared and studied by UV–VIS–NIR diffuse reflectance spectroscopy, transmission electron microscopy, EPR and electrical conductivity measurements. The absorption threshold due to the SnO2 interband transition appears near 340 nm (3.6 eV) compared to about 1770 nm (0.7 eV) for orthorhombic SnO. When treated in reducing conditions (vacuum, hydrazine), SnO2 shows also a broad absorption in the visible range (350–600 nm) which disappears upon heating in oxygen. This broad absorption is also recorded upon direct heating in oxygen. This spectral feature is ascribed to the formation of oxygen vacancies, which depends on both oxygen pressure and temperature. The variation of electrical conductivity with oxygen pressure P, expressed by σ = k P−1/4, shows that singly ionized anionic vacancies are the main structure defects, in agreement with EPR measurements. An original method of preparation of colloidal SnO2 is also presented; the samples obtained show a still larger surface area (250 m2 g−1), a very narrow particle size distribution (1.1–2.4 nm), a high resistance to sintering and a low propensity to nonstoichiometry.

SnO2‐supported palladium catalysts: activity in deNOx at low temperature

High surface area tin dioxide (174 m2/g) has been synthetised and characterised by XRD, TEM and UV‐visible DRS. DRS gives evidence for the formation of oxygen vacancies (donor levels) under reducing conditions. CO adsorption gives rise to terminal carbonyl species linked to Sn4+ and Sn2+. Palladium–tin oxide catalysts have been prepared from various precursors (Pd(acac)2 and Pd(NO3)2) and by different preparation methods (grafting, photodeposition); they are active in deNOx reactions at low temperature (180 °C) in the presence of stoichiometric CO–NO–O2 mixtures. A mechanism involving palladium and oxygen vacancies is proposed.

Preparation, characterization and reactivity of Pd/Nb2O5 catalysts in hexa-1,5-diene hydrogenation

Abstract Palladium/niobia catalysts are prepared by various methods involving either gas or liquid phase reduction. Although giving rise to average or low dispersion, the reduction of palladium precursors in a liquid medium (hydrazine or ethylene–glycol) appears to be a promising method since a low dispersion favors the activity in the hydrogenation of hexa-1,5-diene in liquid phase. The substitution of alumina by niobia improves the fractional selectivity and the yield of hex-1-ene in all cases. A very good global selectivity is also observed.

Application of electroless procedures to the preparation of palladium catalysts

The influence of various (laser, UV lamp and thermal) activation treatments on the diffuse reflectance and ESR spectra of CeO2 are examined. Ceria-supported palladium catalysts are prepared by electroless deposition of the metal from palladium chloride and hydrazine hydrate solutions. The atomic defects induced in ceria by the activation procedures appear to initiate the palladium deposition.