Multitechnique synchrotron-based approach in heterogeneous catalysis: from preparation to reaction

Abstract

The evolution of synchrotron-based experiments, covering a multipurpose surface and bulk characterization of catalytic solids, during their preparation process and under reaction conditions, is presented. Particular emphasis is given to recent studies that attempt to unveil contemporary unresolved questions concerning the control of physico-chemical properties of nano-solids as well as the evolution of such solid materials under reactive atmospheres. To reach this goal, we first focus on the analysis of photo-active TiO2-based materials. As well known, morphological control of highly active anatase materials appears as a critical issue (1). A XRD-PDF analysis joined with surface-sensitive vibrational techniques provide evidence that size control in anatase-type materials is critically connected with the preparation step and, particularly, with the middle range order (3 to 6 Å) present in the solid amorphous precursor of anatase materials (2). Concerning primary particle size, a thermodynamic, rather than the previously presumed kinetic, control of the primary particle size is thus demonstrated (3). We will also address the more complex physical origin of surface or shape control in nanostructured anatase and how the two main morphology parameters, e.g. primary particle size and shape, affect photo-activity in bare and cation-doped anatase-type materials (2-4). As a second example we focus on the time-resolved XAFS/XRD/DRIFTS analysis of three-way catalysts. The performance of such system involves the close relationship between two phases of different nature, an active metal and a promoter oxide, supported on an alumina carrier. The real conditions under which these systems operate consists of the so-called lambda cycle; a time-dependent oscillation of the reactant mixture (exhaust gases coming from the motor) around the stoichiometric point, e.g. point at which equal molar amounts of oxidant ¿NO, O2- and reductive ¿HC, CO- gases contact the catalyst surface at a rate of ca. 1-3 Hz (2). We will show that the chemistry observed in cyclic conditions (e.g. upon time modification of the inlet gas phase mixture seen by the catalyst, which goes from neatly oxidizing to reducing mixtures and vice versa several times per second) is far from that observed under average, ¿static¿ stoichiometric ones (2). More specifically, the work provides evidence that the catalytic behaviour is grounded in a unique redox/phase and morphology interplay occurring at the noble metal component together with oxygen handling aspects related to the metal-promoter interface. All these phenomena are significant during cyclic modification of the gas atmosphere and will be analyzed as a function of the primary particle size of both components, the metal and promoter (2,5).