Cerium dioxide (CeO2, ceria) is becoming an ubiquitous constituent in catalytic systems for a variety of applications. 2016 sees the 40th anniversary since ceria was first employed by Ford Motor Company as an oxygen storage component in car converters, to become in the years since its inception an irreplaceable component in three-way catalysts (TWCs). Apart from this well-established use, ceria is looming as a catalyst component for a wide range of catalytic applications. For some of these, such as fuel cells, CeO2-based materials have almost reached the market stage, while for some other catalytic reactions, such as reforming processes, photocatalysis, water-gas shift reaction, thermochemical water splitting, and organic reactions, ceria is emerging as a unique material, holding great promise for future market breakthroughs. While much knowledge about the fundamental characteristics of CeO2-based materials has already been acquired, new characterization techniques and powerful theoretical methods are dee...

Fundamentals and Catalytic Applications of CeO2-Based Materials

Carbon-based nanomaterials have been widely used as catalysts or catalyst supports in the chemical industry or for energy or environmental applications due to their fascinating properties. High surface areas, tunable porosity, and functionalization are considered to be crucial to enhance the catalytic performance of carbon-based materials. Recently, the newly emerging metal–organic frameworks (MOFs) built from metal ions and polyfunctional organic ligands have proved to be promising self-sacrificing templates and precursors for preparing various carbon-based nanomaterials, benefiting from their high BET surface areas, abundant metal/organic species, large pore volumes, and extraordinary tunability of structures and compositions. In comparison with other carbon-based catalysts, MOF-derived carbon-based nanomaterials have great advantages in terms of tailorable morphologies and hierarchical porosity and easy functionalization with other heteroatoms and metal/metal oxides, which make them highly efficient as...

Development of MOF-Derived Carbon-Based Nanomaterials for Efficient Catalysis

Materials for hydrogen-based energy storage – past, recent progress and future outlook

Copper–ceria as one of the very active catalysts for oxidation reactions has been widely investigated in heterogeneous catalysis. In this work, copper oxide (1 wt % Cu loading) deposited on both ceria nanospheres with a {111}/{100}-terminated surface (1CuCe-NS) and with nanorod exposed {110}/{100} faces (1CuCe-NR) have been prepared for the investigation of crystal plane effects on CO oxidation. Various structural characterizations, especially including aberration-corrected scanning transmission electron microscopy (Cs-STEM), X-ray absorption fine structure (XAFS) technique, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), were used to precisely determine the structure and status of the catalysts. It is found that the copper oxides were formed as subnanometer clusters and were uniformly dispersed on the surface of the ceria support. The results from XAFS combined with the temperature-programmed reduction technique (H2-TPR) reveal that more reducible CuOx clusters w...

Crystal Plane Effect of Ceria on Supported Copper Oxide Cluster Catalyst for CO Oxidation: Importance of Metal–Support Interaction

Catalytic decomposition of ammonia with complete conversion for generating hydrogen at low temperature (

Low-temperature ammonia decomposition catalysts for hydrogen generation

Copper-ceria is one of the very active catalysts for the preferential oxidation of carbon monoxide (CO-PROX) reaction, which is also a typical system in which the complexity of copper chemistry is clearly exhibited. In the present manuscript, copper–ceria catalysts with different Cu contents up to 20 wt % supported on CeO2 nanorods were synthesized by a deposition–precipitation (DP) method. The as-prepared samples were characterized by various structural and textural detections including X-ray diffraction (XRD), Vis-Raman, transmission electron microscopy (TEM), ex situ/in situ X-ray absorption fine structure (XAFS), and temperature-programmed reduction by hydrogen (H2-TPR). It has been confirmed that the highly dispersed copper oxide (CuOx) clusters, as well as the strong interaction of Cu-[Ox]-Ce structure, were the main copper species deposited onto the ceria surface. No separated copper phase was detected for both preoxidized and prereduced samples with the Cu contents up to 10 wt %. The fresh copper–...

Highly Dispersed Copper Oxide Clusters as Active Species in Copper-Ceria Catalyst for Preferential Oxidation of Carbon Monoxide

Copper–ceria catalysts with the Cu contents up to 40 at.% were prepared by the coprecipitation method. Ammonium carbonate was used as the leaching agent on the as-calcined samples to remove the weakly bound copper species. The parent and leached catalysts were tested for the preferential oxidation of carbon monoxide (CO-PROX) reaction from 40 to 200 °C in 1%CO/1%O2/50%H2/N2 with a space velocity of 60,000 mL h−1 gcat−1. When the Cu doping amount reached 30 at.%, the stable and constant CO-PROX reactivity, including CO conversion and O2 selectivity, was presented for both parent and ammonium carbonate leached catalysts. The fresh and used samples have been carefully characterized by various techniques such as power X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), transmission electron microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and temperature-programmed reduction by hydrogen (H2-TPR). It was confirmed that there are two different types of copper species, i.e. weakly bound CuOx clusters and strongly bound Cu–[Ox]–Ce structure, in both parent and leached catalysts. Furthermore, we have demonstrated that the weakly bound CuOx clusters removed by ammonium carbonate can be recovered by the migration of Cu–[Ox]–Ce species under the reduction or reaction conditions. The strongly bound copper species in CeO2 have been identified to be the reservoir for the active Cu sites for CO-PROX.

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Effect of strongly bound copper species in copper–ceria catalyst for preferential oxidation of carbon monoxide

Uniform Au nanoparticles (∼2 nm) with narrow size-distribution (standard deviation: 0.5–0.6 nm) supported on both hydroxylated (Fe_OH) and dehydrated iron oxide (Fe_O) have been prepared by either deposition-precipitation (DP) or colloidal-deposition (CD) methods. Different structural and textural characterizations were applied to the dried, calcined and used gold-iron oxide samples. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) showed high homogeneity in the supported Au nanoparticles. The ex situ and in situ X-ray absorption fine structure (XAFS) characterization monitored the electronic and short-range local structure of active gold species. The synchrotron-based in situ X-ray diffraction (XRD), together with the corresponding temperature-programmed reduction by hydrogen (H2-TPR), indicated a structural evolution of the iron-oxide supports, correlating to their reducibility. An inverse order of catalytic activity between DP (Au/Fe_OH Au/Fe_O) was observed. Effective gold-support interaction results in a high activity for gold nanoparticles, locally generated by the sintering of dispersed Au atoms on the oxide support in the DP synthesis, while a hydroxylated surface favors the reactivity of externally introduced Au nanoparticles on Fe_OH support for the CD approach. This work reveals why differences in the synthetic protocol translate to differences in the catalytic performance of Au/FeOx catalysts with very similar structural characteristics in CO oxidation.

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Uniform 2 nm gold nanoparticles supported on iron oxides as active catalysts for CO oxidation reaction: structure-activity relationship.

Transition metal (Fe, Co, and Ni) nanoparticles dispersed in an alumina matrix as catalysts for NH3 decomposition have been synthesized by a facile co-precipitation method. The fresh and used catalysts were characterized by various techniques including powder X-ray diffraction (XRD), N2 adsorption/desorption, and transmission electron microscopy (TEM). Also, temperature-programmed reduction by hydrogen (H2-TPR) combining in situ XRD was performed to investigate the reducibility of the studied catalysts. For the ammonia decomposition reaction, 88% conversion of ammonia can be realized at the reaction temperature as low as 600 °C using a space velocity of 72 000 cm3 gcat−1 h−1 NH3 during a long term (72 h) catalysis test without any observable deactivation. The small amount of alumina (low to 10 at%) can act as the matrix in which the catalytically active transition metal nanoparticles were stabilized. Thus, the agglomeration of active transition metals under reaction conditions was significantly suppressed and the high activity of catalysts was maintained.

Transition metal nanoparticles dispersed in an alumina matrix as active and stable catalysts for COx-free hydrogen production from ammonia

Nanostructured mixed metal oxides with porous hollow-interior structure hold great promise in environmental-related catalysis, owing to their excellent catalytic properties. However, facile fabrication of such mesoporous architecture is still challenging. Here, by using a transient aerosol-assisted self-assembly (AASA) method, we synthesized the Co3O4-Al2O3 mesoporous hollow spheres (MHS) composed of thermally stable Co3O4 nanoparticles partially anchored to amorphous interfacial Al2O3. The as-prepared Co3O4-Al2O3 nanocomposites displayed distinct features of large pore volume and stable assembled morphology, and thus showed significant advantages in mass transfer and redox behavior. For Fischer-Tropsch synthesis, a very important reaction in fuel production, the Co3O4-Al2O3 MHS exhibited significant catalytic performance in conversion, selectivity and stability for the desired gasoline products. Therefore, we provide a facile and controlled approach towards the preparation of mesoporous hollow materials to achieve novel mixed metal oxide nanocatalysts those are good candidates in energy-production application.

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Qi-Sheng Song

Papers

Highly Dispersed Copper Oxide Clusters as Active Species in Copper-Ceria Catalyst for Preferential Oxidation of Carbon Monoxide

Effect of strongly bound copper species in copper–ceria catalyst for preferential oxidation of carbon monoxide

Uniform 2 nm gold nanoparticles supported on iron oxides as active catalysts for CO oxidation reaction: structure-activity relationship.

Transition metal nanoparticles dispersed in an alumina matrix as active and stable catalysts for COx-free hydrogen production from ammonia

Co3O4-Al2O3 mesoporous hollow spheres as efficient catalyst for Fischer-Tropsch synthesis