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SummaryThe hybridization of polyoxometalates (POMs) through an organic–inorganic association offers several processing advantages in the design of heterogeneous catalysts. A clear understanding of the organization of these hybrid materials on solid surfaces is necessary to optimise their properties. Herein, we report for the first time the organization of Keggin phosphotungstic [PW12O40]3− and Wells–Dawson (WD) phosphomolybdic [P2Mo18O62]6− anions deposited on mica (hydrophilic), and highly oriented pyrolytic graphite (HOPG) (hydrophobic) surfaces. Next, the supramolecular organization of the organic–inorganic hybrid materials formed from the association of POM anions and dimethyldioctadecylammonium bromide (DODA) is investigated as a function of the hydrophilic or hydrophobic nature of the surfaces. The height of the Keggin-POM anions, measured with tapping mode (TM-AFM) is always in good agreement with the molecular dimension of symmetric Keggin-POM anions (ca. 1 nm). However, the asymmetric WD-POM anions form monolayer assemblies on the surfaces with the orientation of their long molecular axis (ca. 1.6 nm) depending on the hydrophilic or hydrophobic properties of the substrate. Namely, the long axis is parallel on mica, and perpendicular on HOPG. When hybridized with DODA, the organization of the hybrid material is dictated by the interaction of the alkyl side chains of DODA with the substrate surface. On HOPG, the DODA–POM hybrid forms small domains of epitaxially arranged straight nanorod structures with their orientation parallel to each other. Conversely, randomly distributed nanospheres are formed when the hybrid material is deposited on freshly cleaved mica. Finally, a UV–ozone treatment of the hybrid material allows one to obtain highly dispersed isolated POM entities on both hydrophilic and hydrophobic surfaces. The hybridization strategy to prevent the clustering of POMs on various supports would enable to develop highly dispersed POM-based heterogeneous catalysts with enhanced functionalities.

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The main objective of this study has been twofold: (i) to investigate the effect of the nature of the slurry used to structurate a catalyst on monoliths; (ii) to study the influence of the nature of the monoliths on the physicochemical properties and on the catalytic activities in the combustion of methane. The slurry used in this work was made with an acidified solution of Pd(2 wt %)/Fe−Al oxide catalyst. This catalyst was prepared by the citrate method and deposited on ceramic and metallic monoliths according to a dip coating or an orbital stirring procedure. Both types of structured catalysts were studied using the following physicochemical techniques: specific surface area measurements (BET), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The Pd(2 wt %)/Fe−Al oxide catalyst used for monolith coating does not undergo any textural or crystallinity modification during the deposition procedure. Catalytic activities obtained for powder and monolithic catalysts are compared. The surface of the support plays an important role in the structuration of catalysts. The amount of deposited catalyst has an influence on the catalytic activity. The amount of deposited catalysts depends on the nature of the monolith, on the coating technique, and on the nature of the slurry. Comparisons between structured Pd(2 wt %)/Fe−Al oxide and structured LaMnO3 on ceramic and metallic monoliths (previous work) were made to clarify the influence of the nature of the slurry on the amount of deposited catalysts. The amount of deposited catalysts on ceramic monolith using Pd(2 wt %)/Fe−Al oxide catalyst is lower than that deposited using LaMnO3. In the case of metallic monoliths, the difference is less significant. The influence of a diffusion limitation of the reaction rate in structured catalysts with Pd(2 wt %)/Fe−Al oxide seems to be less important than with LaMnO3. Higher catalytic activities were observed for Pd(2 wt %)/Fe−Al oxide on the ceramic monoliths at high temperature. Coating on the metallic monoliths led to a better activity at low temperature compared to the corresponding powder catalyst.