Cu single-atom catalysts (SACs) supported on CeO 2 −TiO 2 were prepared by a sol−gel method and tested for CO oxidation between 120 and 350 °C. Operando and in situ spectroscopic methods including diffuse reflectance infrared Fourier transform (DRIFT), electron paramagnetic resonance (EPR), and near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) combined with other basic characterizations were applied to identify active sites and to derive reliable structure−reactivity relationships. Rising the Cu content from 0.06 to 0.86 wt % resulted in a significant decrease of the Cu-mass normalized CO 2 formation rate from 690 to 310 μmol CO 2 •g Cu −1 •s −1 at 250 °C, which was attributed to the formation of the less active CuO x species. The catalysts showed high stability during time on stream for more than 1000 min with negligible agglomeration of Cu single sites. Spectroscopic results revealed that active sites are single Cu ions on the surface of highly dispersed ceria particles, shuttling between −Cu 2+ −O−Ce 4+ − and −Cu + −□−Ce 3+ − by supplying active oxygen for oxidation of CO to CO 2 . The highest concentrations of Cu single sites and O vacancies associated with Ce 3+ species correlated with the highest CO oxidation activity.
Recently, great attention has been paid to Ceria-based materials for selective catalytic reduction (SCR) with NH 3 owing to their unique redox, oxygen storage, and acid-base properties. Two series of bimetallic catalysts issued from Titania modified by Ce and Nb were prepared by the one-step sol-gel method (SG) and by the sol-gel route followed by impregnation (WI). The resulting core-shell and bulk catalysts were tested in NH 3-SCR of NO x. The impregnated Nb5/Ce40/Ti100 (WI) catalyst displayed 95% NO x conversion at 200 • C (GHSV = 60,000 mL•g −1 •h −1 , 1000 ppm NO x , 1000 ppm NH 3 , 5% O 2 /He) without forming N 2 O. The catalysts were characterized by various methods including ICP-OES, N 2-physisorption, XRD, Raman, NH 3-TPD, DRIFTS, XPS, and H 2-TPR. The results showed that the introduction of Nb decreases the surface area and strengthens the surface acidity. This behavior can be explained by the strong interaction between Ceria and Titania which generates Ce-O-Ti units, as well as a high concentration of amorphous or highly dispersed Niobia. This should be the reason for the excellent performance of the catalyst prepared by the sol-gel method followed by impregnation. Furthermore, Nb5/Ce40/Ti100 (WI) has the largest NH 3 adsorption capacity, which is helpful to promote the NH 3-SCR reaction. The long-term stability and the effect of H 2 O on the catalysts were also evaluated.
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