Reactivity of Surface Species in Heterogeneous Catalysts Probed by In Situ X-ray Absorption Techniques

2.1.3.Cr/S iO 2 Phillips catalyst.The Cr/SiO 2 catalysts were obtained by impregnating a SiO 2 support (400 m 2 /g) with H 2 CrO 4 , following the procedure described elsewhere. 64,[113][14][115][116] A Cr loading of 4 wt % was adopted to have an acceptable signal-to-noise ratio in transmission XAFS experiments. 117A Cr loading of 0.5 wt % was used for XAFS experiments in fluorescence mode. 118 In both cases, samples were then activated according to the following procedure: (i) activation at 773 K; (ii) calcination in O 2 at the same temperature for 1h (oxidized sample, light orange).After the activation and oxidation steps two different reduction procedures have been followed: (i) a reduction step at 623 K followed by CO removal at the same temperature, thus obtaining the hereafter called CO-reduced system; (ii) a reduction step in C 2 H 4 at 523 K followed by evacuation at the same temperature, thus obtaining the hereafter called C 2 H 4 -reduced catalyst.2.1.4.CuCl 2 /Al 2 O 3 .CuCl 2 /-Al 2 O 3 catalyst has been prepared with a Cu loading of 5.0 wt% by impregnation of -alumina (Condea Puralox SCCa 30/170, surface area: 168 m 2 g -1 , pore volume: 0.50 cm 3 g -1 ) with an aqueous solution of CuCl 2 /H 2 O following the incipient wetness method, as described elsewhere.119 2.1.5.Metal supported catalysts.Pt-supported catalysts were prepared by impregnation of supports with an aqueous solution of Pt(NH 3 ) 4 (NO 3 ) 2 (Aldrich 27,872-6) as described elsewhere. 120 .Supported Pd samples have been prepared in the Chimet laboratories on -Al 2 O 3 (surface area = 121 m 2 g -1 ; pore volume = 0.43 cm 3 g -1 ) and on two different activated carbons, from wood (hereafter Cw: surface area = 980 m 2 g -1 ; pore volume = 0,62 cm 3 g -1 ) and peat (hereafter Cp: surface area = 980 m 2 g -1 ; pore volume = 0.47 mm 3 g -1 ) origin, following the deposition-precipitation method 121,122 with Na 2 PdCl 4 as palladium precursor and Na 2 CO 3 as basic agent. Techniques and Experiential set-upsThis Section counts for seven subsections.Sections 2.2.1 and 2.2.1 are devoted to briefly describe how the X-ray beam is "shaped" in the optic hutch to the requests of the XAFS experiment.Sections 2.2.3.and 2.2.4 reports the set-ups for X-ray absorption and emission spectroscopies, respectively.The latter set-up allows to perform high-energy resolution fluorescence detected