A series of CuO
catalysts dispersed on SiO2−Al2O3 support with a copper content from 0.2 to 12 wt %
corresponding to 0.04−3.7 atomCu nm-2 was prepared by chemisorption-hydrolysis method from copper
solutions. The catalysts were characterized in their bulk (XRD, redox cycles with H2 and O2) and surface (N2
adsorption, SEM, XPS, and DRS) properties. Copper species were found to be uniformly spread on the SiO2−Al2O3 support as small aggregates both in the low and high copper loaded samples. Spectroscopic evidence
agrees with Cu2+ presence in an axially distorted octahedral environment of O-containing ligands. At high
copper loading, the existence of copper centers in closer interaction occurred forming structures of oxocations-like type. Temperature programmed reduction (TPR) experiments confirmed the presence of dispersed copper
species which underwent complete reduction. Comparing the position and shape of two successive TPR profiles,
carried out interposing an oxidation run (temperature programmed oxidation), it was found that the smallest
centers (<1 atomCu nm-2) are characterized by low stability and high mobility. Increasing the copper
content diminished the mobility of the CuO
species, as larger CuO
aggregates were formed. The selective
catalytic reduction of NO
with ethene in the presence of excess oxygen was studied in a flow apparatus at
fixed reactant concentration (1500 ppm of NO
and C2H4 and 15000 ppm of O2) and contact time (8 g s
mmol-1), with an online FT-IR analytical device. Catalysts containing up to about 1 atomCu nm-2 displayed
very little activity, while catalysts with higher copper content were active and selective. Maximum activity
was associated with samples containing 1.5−2 atomCu nm-2, while samples with higher copper concentration
were less active. Results point to the need to individuate relationships between structure and catalyst properties
and activity to optimize the preparation of suitable tailored copper-containing catalysts.
Magnesium−aluminum double oxides derived from the thermal treatment of layered hydroxides (hydrotalcites) have been tested for CO2 adsorption. The effects of various preparation parameters, such as the incorporated cation (K or Na), the mode of addition of magnesium and aluminum precursors, the presence of sonication, and the calcination temperature, on the adsorption capacity under mild conditions were studied using thermogravimetry and calorimetry. Calorimetric and FTIR data were used to explain the adsorption mechanisms leading to the undesirable irreversible adsorption. This adsorption was related to the formation of unidentate CO2-adsorbent species with the strongest adsorption sites, whereas bidentate and surface bicarbonates lead to highly reversible adsorption. In conclusion, preparation procedures that lead to an increase in the strength of basic sites do not lead to significant increases in the adsorption capacity, but rather lead to more difficult regeneration of the saturated support.
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