Yuxian Gao

Morphology-dependent surface chemistry and catalysis of CeO2 nanocrystals

Surface chemistry and catalysis over oxide nanocrystals with well-defined morphologies are emerging as a hot topic, and the relevant progress on CeO2 nanocrystals is critically reviewed. CeO2 nanocrystals expose morphology-dependent crystal planes on their surfaces, and CeO2 nanocrystal and metal/CeO2 nanocrystal catalysts exhibit morphology-dependent surface chemistry and catalysis. A comprehensive analysis of the published literature nicely correlates the surface chemistry and catalysis of CeO2 nanocrystal and metal/CeO2 nanocrystal catalysts with the surface composition/structure and oxygen vacancy structure of exposed CeO2 crystal planes. A concept of morphology engineering of oxide nanocrystals not only for the optimization of the catalytic performance but also for the fundamental understanding of the structure–activity relation and catalytic reaction mechanism is discussed.

Morphology Effect of CeO2 Support in the Preparation, Metal–Support Interaction, and Catalytic Performance of Pt/CeO2 Catalysts

Pt/CeO2 catalysts with various Pt loadings were prepared by a conventional incipient wetness impregnation method that employed CeO2 cubes (c‐CeO2), rods (r‐CeO2), and octahedra (o‐CeO2) as the support and Pt(NH3)4(NO3)2 as the metal precursor. Their structures and catalytic activities in CO oxidation in excess O2 and the preferential oxidation of CO in a H2‐rich gas (CO‐PROX) were studied, and strong morphology effects were observed. The impregnated Pt precursor interacts more strongly with CeO2 rods and cubes than with CeO2 octahedra, and the reduction/decomposition of the Pt precursor impregnated on CeO2 octahedra is easier than that on CeO2 rods and cubes. With the same Pt loading, the Pt/o‐CeO2 catalyst contains the largest fraction of metallic Pt, whereas the Pt/c‐CeO2 catalyst contains the largest fraction of Pt2+ species. The reducibility of pure CeO2 and CeO2 in the Pt/CeO2 catalysts follows the order r‐CeO2>c‐CeO2>o‐CeO2, and the reducibility of CeO2 depends on the Pt loading for the Pt/c‐CeO2 catalysts but not much for the Pt/r‐CeO2 and Pt/o‐CeO2 catalysts. The catalytic performance of Pt/CeO2 catalysts in both CO oxidation and the CO‐PROX reaction follows the order Pt/r‐CeO2>Pt/c‐CeO2> Pt/o‐CeO2. The Pt0‐CeO2 ensemble is more active than the Pt2+‐CeO2 ensemble in the catalysis of CO oxidation in excess O2. H2‐assisted CO oxidation catalyzed by the Pt/CeO2 catalysts was observed in the CO‐PROX reaction, and the Pt2+ species and CeO2 with a large concentration of oxygen vacancies constitute the active structure of the Pt/CeO2 catalyst for the CO‐PROX reaction. The effect of the morphology of the CeO2 support in the preparation, metal–support interaction, and catalytic performance of Pt/CeO2 catalysts can be correlated the exposed crystal planes and surface composition/structure of the CeO2 support with different morphologies. These results not only demonstrate that the structure and catalytic performance of oxide‐supported catalysts can be tuned by controlling the morphology of the oxide support but also deepens the fundamental understanding of CO oxidation reactions catalyzed by Pt/CeO2 catalysts.

Structural features and catalytic performance in CO preferential oxidation of CuO–CeO2 supported on multi-walled carbon nanotubes

Multi-walled carbon nanotube (MWCNT) supported CuO–CeO2 catalysts were prepared using an impregnation method aided by an ultrasonication treatment. The structural features of the catalysts were extensively characterized by XRD, TGA, H2-TPR, XPS, TEM and Raman spectroscopy. The catalytic performance in the preferential oxidation of CO in a H2-rich stream was extensively investigated with respect to the influences of Cu/Ce ratios, the different support materials and the presence of CO2 and H2O in the feed stream. In contrast to those supported on other materials, the catalytic performance was greatly enhanced for the MWCNT-supported CuO–CeO2 catalysts, which was discussed in terms of the synergistic interactions between Cu–Ce mixed oxides and MWCNT. The optimized catalyst with a Cu/Ce ratio of 5/5 can achieve complete CO conversion and high selectivity toward CO2 in a wider temperature range as well as favorable performance under the simulated reformatted gas mixture. The deactivation of CuO–CeO2 catalysts supported on MWCNT at elevated temperatures was also discussed in relation to the temperature-dependent in situ XAFS spectra during CO-PROX reaction.

Identification of different oxygen species in oxide nanostructures with 17O solid-state NMR spectroscopy

Nanostructured oxides find multiple uses in a diverse range of applications including catalysis, energy storage, and environmental management, their higher surface areas, and, in some cases, electronic properties resulting in different physical properties from their bulk counterparts. Developing structure-property relations for these materials requires a determination of surface and subsurface structure. Although microscopy plays a critical role owing to the fact that the volumes sampled by such techniques may not be representative of the whole sample, complementary characterization methods are urgently required. We develop a simple nuclear magnetic resonance (NMR) strategy to detect the first few layers of a nanomaterial, demonstrating the approach with technologically relevant ceria nanoparticles. We show that the 17O resonances arising from the first to third surface layer oxygen ions, hydroxyl sites, and oxygen species near vacancies can be distinguished from the oxygen ions in the bulk, with higher-frequency 17O chemical shifts being observed for the lower coordinated surface sites. H217O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. 17O NMR spectra of thermally treated nanosized ceria clearly show how different oxygen species interconvert at elevated temperature. Density functional theory calculations confirm the assignments and reveal a strong dependence of chemical shift on the nature of the surface. These results open up new strategies for characterizing nanostructured oxides and their applications.

Selective CO Methanation over Ru Catalysts Supported on Nanostructured TiO2 with Different Crystalline Phases and Morphology

Nanostructured titanium dioxides were synthesized via various post-treatments of titanate nanofibers obtained from titanium precursors by hydrothermal reactions. The microstructures of TiO2 and supported Ru/TiO2 catalysts were characterized with X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray analysis, and nitrogen adsorption isotherms. The phase structure, particle size, morphology, and specific surface area were determined. The supported Ru catalysts were applied for the selective methanation of CO in a hydrogen-rich stream. The results indicated that the Ru catalyst supported on rutile and TiO2-B exhibited higher catalytic performance than the counterpart supported on anatase, which suggested the distinct interaction between Ru nanoparticles and TiO2 resulting from different crystalline phases and morphology.

The most active Cu facet for low-temperature water gas shift reaction

Identification of the active site is important in developing rational design strategies for solid catalysts but is seriously blocked by their structural complexity. Here, we use uniform Cu nanocrystals synthesized by a morphology-preserved reduction of corresponding uniform Cu2O nanocrystals in order to identify the most active Cu facet for low-temperature water gas shift (WGS) reaction. Cu cubes enclosed with {100} facets are very active in catalyzing the WGS reaction up to 548 K while Cu octahedra enclosed with {111} facets are inactive. The Cu–Cu suboxide (CuxO, x ≥ 10) interface of Cu(100) surface is the active site on which all elementary surface reactions within the catalytic cycle proceed smoothly. However, the formate intermediate was found stable at the Cu–CuxO interface of Cu(111) surface with consequent accumulation and poisoning of the surface at low temperatures. Thereafter, Cu cubes-supported ZnO catalysts are successfully developed with extremely high activity in low-temperature WGS reaction.Nanocrystals display a variety of facets with different catalytic activity. Here the authors identify the most active facet of copper nanocrystals relevant to the low-temperature water gas shift reaction and further design zinc oxide-copper nanocubes with exceptionally high catalytic activity.

Cu-Co Composite Oxides Supported on Multi-walled Carbon Nanotubes for Catalytic Removal of CO in a H2-rich Stream

Multi-walled carbon nanotubes (MWCNT) supported Cu-Co composite oxides catalysts were prepared by an ultrasonication treatment-aided impregnation method. The structure properties of the catalysts were characterized by XRD, TEM, H2-TPR, XPS and Raman spectra, indicating the strong interactions between Cu and Co mixed oxides as well as between metal oxides and MWCNT support. The catalytic performance of CO removal in a H2-rich stream was examined. In contrast to the single Cu and Co catalyst, the unique performance was observed for Cu-Co composite catalysts, which features an unusual reaction pathway through the combination of CO preferential oxidation and CO methanation especially at high reaction temperature. The optimal catalyst with Cu/Co ratio of 1/8 can achieve the complete CO conversion in a wider temperature range of 150–250 °C under the space velocity as high as 120 L/(h·g), which demonstrates a promising catalyst for the effective CO removal in a H2-rich stream.