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Supported platinum catalysts exhibit versatile applications in heterogeneous catalysis while their active sites are identified difficultly due to the structural complexity of solid catalysts. Herein, via a comprehensive microscopic and spectroscopic study using several PtOx/CeO2 catalysts with well-controlled CeO2 morphology, we found that, besides the CeO2 morphology, the structure of supported platinum species over PtOx/CeO2 catalysts is determined by the reaction atmosphere, dominantly forming components of the platinum-ceria solid solution and fine Pt nanoparticles during propane oxidation and preferential oxidation of CO in hydrogen (CO-PROX) reactions, respectively. Catalytic performance of the platinum-ceria catalysts in propane oxidation is tightly associated with the content of metallic Pt sites which is affected by reducibility of platinum-ceria solid solution, whereas the Pt(II) species arising from the Pt-CeO2 interaction coupling with the surface oxygen vacancy concentration of ceria contribute to the CO reactivity in CO-PROX reaction. There results provide insights into the active sites of platinum-ceria catalysts in both reactions and broadens the concept of morphology-dependent catalysis of oxide-based nanocrystal catalysts that varies with the reactions catalyzed. [ABSTRACT FROM AUTHOR]

[Display omitted] • CeO 2 -supported single Pd atoms are the most intrinsically active in CO oxidation. • The CO reaction rate on isolated Pd sites is exclusively promoted by H 2. • The oxidation of CO proceeds through a Langmuir-Hinshelwood mechanism. • The decomposition of formate species contribution dominantly in CO oxidation. • A stronger H-spillover effect was observed on isolated Pd sites. • Bridged-OH contributes to the consumptions of bicarbonate and formate species. Supported metal catalysts are the most widely used in industrial processes and the metal particle size plays a crucial factor in determining the catalytic performance. Herein, CeO 2 -supported Pd catalysts with different Pd size regimes ranging from single atoms, to nanoclusters (1–2 nm), and to nanoparticles (>2 nm) were used for both CO oxidation and preferential oxidation of CO in H 2 (CO-PROX). Compared to Pd nanoclusters and nanoparticles, CeO 2 -supported single Pd atoms (Pd SA /CeO 2) are the most intrinsically active in CO oxidation, with an apparent activation energy of ca. 40 kJ mol−1. Results of kinetic investigations and in situ diffuse reflectance infrared Fourier transformed spectroscopy demonstrate the CO oxidation proceeding through a Langmuir-Hinshelwood mechanism with the decomposition of formate species acting dominantly as the rate-determining step. The CO reaction rate is exclusively promoted on Pd SA /CeO 2 catalysts for the CO-PROX reaction, which could be ascribed to a stronger H-spillover effect on isolated Pd sites to produce bridged-OH on CeO 2 surface, simultaneously facilitating the consumptions of bicarbonate and formate species. There results greatly deepen the fundamental understanding of the Pd size regimes over Pd/CeO 2 catalysts for the oxidation of CO. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pd/TiO 2 {100} is highly active and stable for CO 2 hydrogenation into formate. • The activity is benefit from the moderate basic site and metallic Pd species. • Higher density of moderate basicity over Pd/TiO 2 {100} favors for CO 2 activation. • More metallic Pd species over Pd/TiO 2 {100} contributes to H 2 activation. • The stability is benefit from strong Pd-TiO 2 interactions. • The oxygen vacancy in the TiO 2 {100} promote the Pd-TiO 2 interactions. Pd supported on different anatase TiO 2 nanocrystals predominantly exposing either {1 0 0}, {1 0 1}, or {0 0 1} facets were tested for CO 2 hydrogenation into formate. Remarkable morphology-dependent catalysis was observed. Compared to 2%Pd/TiO 2 {1 0 1} and 2%Pd/TiO 2 {0 0 1} catalysts, 2%Pd/TiO 2 {1 0 0} is highly active and stable, affording an unprecedented turnover frequency of ca. 1369 h−1 and keeping stable after 6 cycles at 313 K. This can be associated with, on the one hand, higher density of moderate basic site and relatively more Pd(0) species over 2%Pd/TiO 2 {1 0 0} contribute to the activations of CO 2 and H 2 , respectively, favorable for the activity; On the other hand, higher oxygen vacancy concentrations in the TiO 2 {1 0 0} promote the Pd-TiO 2 interactions and result in the formation and stability of flat Pd particles over 2%Pd/TiO 2 {1 0 0}, beneficial to the stability. These results highlight the importance of oxide morphology in formate formation and open up possibilities of oxide morphology engineering for developing efficient Pd-based catalysts for CO 2 hydrogenation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The CO oxidation catalyzed by various CuO-CeO 2 interfacial sites involves a typical Mars-van Krevelen mechanism. • The CuO-CeO 2 interfaces in the CeO 2 -CuO/c-Cu 2 O (cubes) nanocomposites are more intrinsically active at CeO 2 loadings no less than 0.75 wt%. • The higher activity is relevant to lower coordinated oxygen ions and thus better CO reactivity for the CeO 2 -CuO/c-Cu 2 O nanocomposites. • The active oxygen species on CuO-CeO 2 interface in catalyzing CO oxidation should come from CuO rather than CeO 2. • A highly active 13.2 %CeO 2 -CuO/c-Cu 2 O(s) (small) catalyst with high density of active sites for CO oxidation is realized on fine Cu 2 O cubes. Several CeO 2 -CuO/Cu 2 O nanocomposites with different CuO structures were used to identify the structure sensitivity of CuO in the CeO 2 -CuO/Cu 2 O catalyzed CO oxidation. The CO oxidation catalyzed by various CuO-CeO 2 interfacial sites involves a typical Mars-van Krevelen mechanism, in which the CuO-CeO 2 interfaces in the CeO 2 -CuO/c-Cu 2 O (cubes) nanocomposites are more intrinsically active, exhibiting ca. 15 kJ mol−1 lower activation energy than those in the CeO 2 -CuO/o-Cu 2 O (octahedra) and CeO 2 -CuO/d-Cu 2 O (rhombic dodecahedra) nanocomposites at CeO 2 loadings no less than 0.75 wt%. The higher activity is relevant to lower coordinated oxygen ions on CuO/c-Cu 2 O surface and thus better CO reactivity for the CeO 2 -CuO/c-Cu 2 O nanocomposites, which therefore indicates that the active oxygen species on CuO-CeO 2 interface should come from CuO rather than CeO 2. Moreover, a highly active 13.2 %CeO 2 -CuO/c-Cu 2 O(s) catalyst for CO oxidation is realized on fine Cu 2 O cubes, which thus has high density of active sites. [ABSTRACT FROM AUTHOR]