CO₂ is one of the primary causes of global warming; therefore, it is vital to control the emissions. Even when long-term CO₂ storage (sequestration) is possible, it is more desirable to use catalysts to convert CO₂ into value-added chemicals and fuels. One of the most interesting products that can be obtained from CO₂ is methanol, as this can be used as (1) fuel for DMFC, (2) a hydrogen storage medium, and (3) a convenient chemical feedstock to produce countless products. However, the hydrogenation of CO₂ to methanol is not straightforward since it competes with the methanation reaction. Thus, it is necessary to use catalysts that can adsorb and activate CO₂ without breaking both C-O bonds in the molecule. Cu-ceria interfaces have been reported to be very active in the hydrogenation of CO₂ to methanol due to the formation of an unstable carboxylate intermediate that is readily converted. However, the number of active sites has been limited due to the large size of the CeO₂ nanoparticles deposited onto the copper surface. To address this limitation, we developed a series of inverse CeO_x-Cu/SiO₂ catalysts with different CeO_x loadings to generate a high number of interfacial sites for an improved hydrogenation rate.

First, a monometallic Cu/SiO₂ catalyst was synthesized by strong electrostatic adsorption (SEA). Then, after the addition of CeO_x by controlled surface reactions (CSR), these inverse catalysts were characterized by temperature-programmed reduction, inductively coupled plasma optical emission spectroscopy, N₂O chemisorption, X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM). While the XPS results showed 100% Ce(III) at low CeO_x loadings, which is attributed to the small size of the CeO_x nanoislands, this value decreased at higher CeO_x contents. The larger size of the CeO_x nanoclusters at higher CeO_x loadings was also confirmed by STEM, which revealed the presence of ceria nanorods due to the non-selective deposition of CeO_x onto the Cu nanoparticles. Unexpectedly, when studied in the hydrogenation of CO₂, these catalysts provided not only methanol but also higher alcohols.

STEM images of CeO<sub>x</sub>-Cu/SiO<sub>2</sub> (Cu:Ce at. ratio= 1:0.31)

STEM images of CeO_x-Cu/SiO₂ (Cu:Ce at. ratio= 1:0.31)

Ibeh Omodolor

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