XAS/DRIFTS/MS spectroscopy for time-resolved operando investigations at high temperature.

The combination of complementary techniques in the characterization of catalysts under working conditions is a very powerful tool for an accurate and in-depth comprehension of the system investigated. In particular, X-ray absorption spectroscopy (XAS) coupled with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectroscopy (MS) is a powerful combination since XAS characterizes the main elements of the catalytic system (selecting the absorption edge) and DRIFTS monitors surface adsorbates while MS enables product identification and quantification. In the present manuscript, a new reactor cell and an experimental setup optimized to perform time-resolved experiments on heterogeneous catalysts under working conditions are reported. A key feature of this setup is the possibility to work at high temperature and pressure, with a small cell dead volume. To demonstrate these capabilities, performance tests with and without X-rays are performed. The effective temperature at the sample surface, the speed to purge the gas volume inside the cell and catalytic activity have been evaluated to demonstrate the reliability and usefulness of the cell. The setup capability of combining XAS, DRIFTS and MS spectroscopies is demonstrated in a time-resolved experiment, following the reduction of NO by Rh nanoparticles supported on alumina.

A structural and surface approach to size and shape control of sulfur-modified undoped and Fe-doped TiO2 anatase nano-materials.

In situ synchrotron X-ray diffraction and diffuse reflectance infrared spectroscopy (DRIFTS) are combined to study the influence of sulfur on the crystallization of pure and Fe-doped titania nano-materials. Using these two tools we have investigated the effect of sulfur on the nucleation and growth processes of the anatase polymorph from amorphous powders and show how the addition of sulfur controls the primary particle size and shape of the materials. As well known, sulfur leads to the stabilization of the oxide particle size against sintering during thermal treatments and here we interpret the physico-chemical basis of such behaviour as an exclusive effect on grain growth kinetics, in turn linked to the dehydration of the surface layers of the materials. In addition this work shows that the presence of sulfur also affects the shape of the anatase particles, favouring the existence of (101)-type surfaces and elongated (along the c crystallographic axis) particles. This combined analysis of how sulfur influences morphological aspects of the anatase phase as it grows provides a basis for understanding of surface and chemical properties of anatase nano-powders that are highly dependent upon particle morphology.

Operando DRIFTS study of the redox and catalytic properties of CuO/Ce(1-x)Tb(x)O(2-δ) (x = 0-0.5) catalysts: evidence of an induction step during CO oxidation.

Catalysts of 1 wt% copper oxide supported on cerium oxide or cerium-terbium mixed oxides are comparatively examined with respect to their redox and catalytic properties for CO oxidation. Characterization of the catalysts had shown that they contain highly dispersed CuO-type entities on the corresponding nanostructured fluorite supports with copper dispersion increasing with increasing amounts of terbium in the support. In contrast, the CO oxidation catalytic activity decreases with increasing amounts of terbium in the support. On the basis of operando-DRIFTS experiments, one of the factors that could explain such behaviour is related to the greater difficulty in generating reduced copper sites active for the reaction in the presence of terbium, which in turn is evidenced to constitute an induction stage. Analysis of the redox properties is complemented by XPS which confirms the greater resistance to copper reduction in the presence of terbium.

Redox behaviour of Pd-based TWCs under dynamic conditions: analysis using dispersive XAS and mass spectrometry.

The redox behaviour of Pd-based TWCs is studied under dynamic, cycling conditions (e.g. lambda oscillations) on a 50 millisecond scale. Pd temporal response to gas inlet mixture changes is governed by metal-promoter interface properties.