Low-temperature oxidation of CO, perhaps the most extensively studied reaction in the history of heterogeneous catalysis, is becoming increasingly important in the context of cleaning air and lowering automotive emissions. Hopcalite catalysts (mixtures of manganese and copper oxides) were originally developed for purifying air in submarines, but they are not especially active at ambient temperatures and are also deactivated by the presence of moisture. Noble metal catalysts, on the other hand, are water tolerant but usually require temperatures above 100 degrees C for efficient operation. Gold exhibits high activity at low temperatures and superior stability under moisture, but only when deposited in nanoparticulate form on base transition-metal oxides. The development of active and stable catalysts without noble metals for low-temperature CO oxidation under an ambient atmosphere remains a significant challenge. Here we report that tricobalt tetraoxide nanorods not only catalyse CO oxidation at temperatures as low as -77 degrees C but also remain stable in a moist stream of normal feed gas. High-resolution transmission electron microscopy demonstrates that the Co(3)O(4) nanorods predominantly expose their {110} planes, favouring the presence of active Co(3+) species at the surface. Kinetic analyses reveal that the turnover frequency associated with individual Co(3+) sites on the nanorods is similar to that of the conventional nanoparticles of this material, indicating that the significantly higher reaction rate that we have obtained with a nanorod morphology is probably due to the surface richness of active Co(3+) sites. These results show the importance of morphology control in the preparation of base transition-metal oxides as highly efficient oxidation catalysts.

Cu/CeO2 catalysts are highly active for the low-temperature water-gas shift-a core reaction in syngas chemistry for tuning H2/CO/CO2 proportions in feed-streams-but direct identification and a quantitative description of the active sites remains challenging. Here, we report that the active copper clusters consist of a bottom layer of mainly Cu + atoms bonded on the oxygen vacancies of ceria, in a form of Cu +-Ov-Ce 3+ , and a top layer of Cu 0 atoms coordinated with the underlying Cu + atoms. This atomic structure model is based on directly observing copper clusters dispersed on ceria by a combination of scanning transmission electron microscopy and electron energy loss spectroscopy, in situ probing the interfacial copper-ceria bonding environment by infrared spectroscopy, and rationalization by density functional theory calculations. These results, together with reaction kinetics, reveal that the reaction occurs at the copper-ceria interfacial perimeter via a site cooperation mechanism: the Cu + site chemically adsorbs CO while the neighboring-Ov-Ce 3+ site dissociatively activates H2O. Copper nanoparticles, dispersed on ceria, constitute a highly efficient catalyst system for reactions in syngas (a mixture of H2, CO, and CO2) chemistry, such as the low-temperature water-gas shift (WGS) reaction 1-7 and CO/CO2 hydrogenation yielding methanol 8-13. In these technologically highly relevant Cu/CeO2 catalysts, copper is commonly viewed as the active component, while the ceria support, with a prominent redox behavior, tunes the dispersion and chemical state of the copper nanoparticles via strong metal-support interactions 14-16. In the case of the low-temperature WGS, a crucial reaction for regulating the H2/CO/CO2 proportions in feed gases for the downstream industrial applications, the active sites have been presumably proposed to locate at the copper-ceria interface. This hypothesis is based on intensive experimental studies on both real Cu/CeO2 catalysts 2-6 and model CeO2/Cu systems 17,18 as well as theoretical simulations of copper-ceria interactions 19-23. A direct experimental verification of the geometric and electronic structures of the copper-ceria interface at atomic scale, however, together with a quantitative description of the active sites for the activation of CO and H2O molecules during the low-temperature WGS reaction on the Cu/CeO2 catalysts, has not yet been obtained.

Au/CeO(2) catalysts are highly active for low-temperature CO oxidation and water-gas shift reaction, but they deactivate rapidly because of sintering of gold nanoparticles, linked to the collapse or restructuring of the gold-ceria interfacial perimeters. To date, a detailed atomic-level insight into the restructuring of the active gold-ceria interfaces is still lacking. Here, we report that gold particles of 2-4 nm size, strongly anchored onto rod-shaped CeO(2), are not only highly active but also distinctively stable under realistic reaction conditions. Environmental transmission electron microscopy analyses identified that the gold nanoparticles, in response to alternating oxidizing and reducing atmospheres, changed their shapes but did not sinter at temperatures up to 573 K. This finding offers a new strategy to stabilize gold nanoparticles on ceria by engineering the gold-ceria interfacial structure, which could be extended to other oxide-supported metal nanocatalysts.

Nanocatalysts are characterised by the unique nanoscale properties that originate from their highly reduced dimensions. Extensive studies over the past few decades have demonstrated that the size and shape of a catalyst particle on the nanometre scale profoundly affect its reaction performance. In particular, controlling the catalyst particle morphology allows a selective exposure of a larger fraction of the reactive facets on which the active sites can be enriched and tuned. This desirable surface coordination of catalytically active atoms or domains substantially improves catalytic activity, selectivity, and stability. This phenomenon is called morphology-dependent nanocatalysts: catalyst particles with anisotropic morphologies on the nanometre scale greatly affect the reaction performance by selectively exposing the desired facets. In this review, we highlight important progress in morphology-dependent nanocatalysts based on the use of rod-shaped metal oxides with characteristic redox and acid-base features. The correlation between the catalytic properties and the exposed facets verifies the chemical nature of the morphology effect. Moreover, we provide an overview of the interactions between the rod-shaped oxides and the metal nanoparticles in metal-oxide catalyst systems, involving crystal-facet-selective deposition of metal particles onto different crystal facets in the oxide supports. A fundamental understanding of active sites in morphologically tuneable oxides enclosed by the desired reactive facets is expected to direct the development of highly efficient nanocatalysts.

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Heterogeneous catalysis performs on specific sites of a catalyst surface even if specific sites of many catalysts during catalysis could not be identified readily. Design of a catalyst by managing catalytic sites on an atomic scale is significant for tuning catalytic performance and offering high activity and selectivity at a relatively low temperature. Here, we report a synergy effect of two sets of single-atom sites (Ni1 and Ru1) anchored on the surface of a CeO2 nanorod, Ce0.95Ni0.025Ru0.025O2. The surface of this catalyst, Ce0.95Ni0.025Ru0.025O2, consists of two sets of single-atom sites which are highly active for reforming CH4 using CO2 with a turnover rate of producing 73.6 H2 molecules on each site per second at 560 °C. Selectivity for producing H2 at this temperature is 98.5%. The single-atom sites Ni1 and Ru1 anchored on the CeO2 surface of Ce0.95Ni0.025Ru0.025O2 remain singly dispersed and in a cationic state during catalysis up to 600 °C. The two sets of single-atom sites play a synergistic role, evidenced by lower apparent activation barrier and higher turnover rate for production of H2 and CO on Ce0.95Ni0.025Ru0.025O2 in contrast to Ce0.95Ni0.05O2 with only Ni1 single-atom sites and Ce0.95Ru0.05O2 with only Ru1 single-atom sites. Computational studies suggest a molecular mechanism for the observed synergy effects, which originate at (1) the different roles of Ni1 and Ru1 sites in terms of activations of CH4 to form CO on a Ni1 site and dissociation of CO2 to CO on a Ru1 site, respectively and (2) the sequential role in terms of first forming H atoms through activation of CH4 on a Ni1 site and then coupling of H atoms to form H2 on a Ru1 site. These synergistic effects of the two sets of single-atom sites on the same surface demonstrated a new method for designing a catalyst with high activity and selectivity at a relatively low temperature.

Nanorust: Fe2O3 nanomaterials with controllable crystal phase and morphology have been successfully fabricated. The γ‐Fe2O3 nanorods that are enclosed by the reactive {110} and {100} facets are highly active and distinctively stable for selective catalytic reduction of NO with NH3. The picture shows the shape and surfaces and the surface atomic configurations of the preferentially exposed {110} and {001} planes.

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