For high-temperature catalytic reaction, it is of significant importance and challenge to construct stable active sites in catalysts. Herein, we report the construction of sufficient and stable ...copper clusters in the copper‒ceria catalyst with high Cu loading (15 wt.%) for the high-temperature reverse water gas shift (RWGS) reaction. Under very harsh working conditions, the ceria nanorods suffered a partial sintering, on which the 2D and 3D copper clusters were formed. This partially sintered catalyst exhibits unmatched activity and excellent durability at high temperature. The interaction between the copper and ceria ensures the copper clusters stably anchored on the surface of ceria. Abundant in situ generated and consumed surface oxygen vacancies form synergistic effect with adjacent copper clusters to promote the reaction process. This work investigates the structure-function relation of the catalyst with sintered and inhomogeneous structure and explores the potential application of the sintered catalyst in C1 chemistry.
Abstract
As the water-gas shift (WGS) reaction serves as a crucial industrial process, strategies for developing robust WGS catalysts are highly desiderated. Here we report the construction of ...stabilized bulk-nano interfaces to fabricate highly efficient copper-ceria catalyst for the WGS reaction. With an in-situ structural transformation, small CeO
2
nanoparticles (2–3 nm) are stabilized on bulk Cu to form abundant CeO
2
-Cu interfaces, which maintain well-dispersed under reaction conditions. This inverse CeO
2
/Cu catalyst shows excellent WGS performances, of which the activity is 5 times higher than other reported Cu catalysts. Long-term stability is also very solid under harsh conditions. Mechanistic study illustrates that for the inverse CeO
2
/Cu catalyst, superb capability of H
2
O dissociation and CO oxidation facilitates WGS process via the combination of associative and redox mechanisms. This work paves a way to fabricate robust catalysts by combining the advantages of bulk and nano-sized catalysts. Catalysts with such inverse configurations show great potential in practical WGS applications.
The crucial role of the metal–oxide interface in the catalysts of the water–gas shift (WGS) reaction has been recognized, while the precise illustration of the intrinsic reaction at the interfacial ...site has scarcely been presented. Here, two kinds of gold–ceria catalysts with totally distinct gold species, <2 nm clusters and 3 to 4 nm particles, were synthesized as catalysts for the WGS reaction. We found that the gold cluster catalyst exhibited a superiority in reactivity compared to gold nanoparticles. With the aid of comprehensive in situ characterization techniques, the bridged −OH groups that formed on the surface oxygen vacancies of the ceria support are directly determined to be the sole active configuration among various surface hydroxyls in the gold–ceria catalysts. The isotopic tracing results further proved that the reaction between bridged surface −OH groups and CO molecules adsorbed on interfacial Au atoms contributes dominantly to the WGS reactivity. Thus, the abundant interfacial sites in gold clusters on the ceria surface induced superior reactivity compared to that of supported gold nanoparticles in catalyzing the WGS reaction. On the basis of direct and solid experimental evidence, we have obtained a very clear image of the surface reaction for the WGS reaction catalyzed by the gold–ceria catalyst.
The construction of stable active site in nanocatalysts is of great importance but is a challenge in heterogeneous catalysis. Unexpectedly, coordination-unsaturated and atomically dispersed copper ...species were constructed and stabilized in a sintered copper–ceria catalyst through air-calcination at 800 °C. This sintered copper–ceria catalyst showed a very high activity for CO oxidation with a CO consumption rate of 6100 μmolCO·gCu –1·s–1 at 120 °C, which was at least 20 times that of other reported copper catalysts. Additionally, the excellent long-term stability was unbroken under the harsh cycled reaction conditions. Based on a comprehensive structural characterization and mechanistic study, the copper atoms with unsaturated coordination in the form of Cu1O3 were identified to be the sole active site, at which both CO and O2 molecules were activated, thus inducing remarkable CO oxidation activity with a very low copper loading (1 wt %).
Small-size (<5 nm) gold nanostructures supported on reducible metal oxides have been widely investigated because of the unique catalytic properties they exhibit in diverse redox reactions. However, ...arguments about the nature of the gold active site have continued for two decades, due to the lack of comparable catalyst systems with specific gold species, as well as the scarcity of direct experimental evidence for the reaction mechanism under realistic working conditions. Here we report the determination of the contribution of single atoms, clusters and particles to the oxidation of carbon monoxide at room temperature, by the aid of in situ X-ray absorption fine structure analysis and in situ diffuse reflectance infrared Fourier transform spectroscopy. We find that the metallic gold component in clusters or particles plays a much more critical role as the active site than the cationic single-atom gold species for the room-temperature carbon monoxide oxidation reaction.
Two comparable models of BiOI/BiOCl heterojuctions with different interface structures (crystal surface orientation and crystal surface combination), denoted as BiOI(001)/BiOCl(001) and ...BiOI(001)/BiOCl(010), have been prepared via integrating heterojuncton nanostructure construction with crystal facet engineering. BiOI(001)/BiOCl(010) had a greater degree of lattice mismatch and displayed higher visible-light photocatalytic activity than BiOI(001)/BiOCl(001). In general, the activity of a photocatalyst (ηPC) has a positive correlation with light harvesting (ηLH), charge separation (ηCS), and charge injection (ηCI). On the basis of the experimental results, we considered that the higher ηCI value of BiOI(001)/BiOCl(010) was the main reason for its better visible-light photocatalytic performance. In combination with theoretical calculations, we found that the higher ηCI value of BiOI(001)/BiOCl(010) was the result of a shorter photogenerated electron diffusion distance, assisted by the self-induced internal electric fields of the BiOCl slabs. This indicated that the crystal facet combination is the key to enhancing the photocatalytic activity of BiOI/BiOCl. Our work offers an archetype for the further design of heterojunction photocatalysts with a fine tuning of the interface structures in order to reach optimized charge injection and enhanced photocatalytic activity.
Effective separation and migration of photogenerated electron-hole pairs are two key factors to determine the performance of photocatalysts. It has been widely accepted that photocatalysts with ...heterojunctions usually exhibit excellent charge separation. However, the migration process of separated charges in the heterojunction structures has not been fully investigated. Herein, photocatalysts with heterojunctions are constructed by loading g-C3N4 nanoparticles onto BiOCl nanosheets with different exposed facets (BOC-001 and BOC-010). The g-C3N4 nanoparticles with decreasing size and increasing zeta potential could induce stronger coupling and scattering in the heterojunction. The relationship between the crystal facet orientation in the BiOCl nanosheets and charge separation/effective migration behaviours of the materials is investigated. The visible light photocatalytic activity of the composites is evaluated by methyl orange (MO) and phenol degradation experiments, and the results show that ng-CN/BOC-010 composites exhibit higher photocatalytic performance than that of ng-CN/BOC-001 composites. Both photoelectrochemical and fluorescence emission measurements indicate that the different exposed facets in ng-CN/BiOCl composites could induce the migration of the photogenerated electrons in different ways, but do not significantly alter the separation efficiencies. The separated electrons in ng-CN/BOC-010 undergo a shorter transport distance than that of ng-CN/BOC-001 to reach the surface reactive sites. The study may suggest that the crystal facet orientation in polar semiconductors is a critical factor for designing highly efficient heterojunction photocatalysts.
Recent advances in the synthesis of collidal metal nanoparticles of controlled sizes and shapes that are relevant for catalyst design are reviewed. Three main methods, based on colloid chemistry ...techniques in solution,
i.e.
, chemical reduction of metal salt precursors, electrochemical synthesis, and controlled decomposition of organometallic compounds and metal-surfactant complexes, are used to synthesize metal nanoparticles. Their catalytic activity and selectivity depend on the shape, size and composition of the metal nanoparticles, and the support effect, as shown for many reactions in quasi-homogeneous and heterogeneous catalysis. A specially designed type of thermally stable catalysts-"embedded" metal catalysts, in which metal nanoparticles are isolated by porous support shells so that metal sintering is effectively avoided at high temperatures, are also introduced. The ultilization of pre-prepared colloidal metal nanoparticles with tuned size, shape and composition as components of designed catalysts opens up new field in catalysis.
The use of pre-prepared colloidal metal nanoparticles with tuned size, shape and composition as components of designed catalysts opens up a new field in catalysis.
•We review the fundamentals of applying CFD to precooling of fresh produce.•We summarize the parameters used to analyze packaging performance.•We review recent studies that focus on optimizing the ...design of fresh produce packaging.•We discuss various challenging issues.
Optimizing the design of fresh produce packaging is vital for ensuring that future food cold chains are more energy efficient and for improving produce quality by avoiding chilling injuries due to nonuniform cooling. Computational fluid dynamics models are thus increasingly used to study the airflow patterns and heat transfer inside ventilated packaging during precooling. This review discusses detailed and comprehensive mathematical modeling procedures for simulating the airflow, heat transfer, and mass transfer that occurs during forced-air precooling of fresh produce. These models serve to optimize packaging design and cooling efficiency. We summarize the most commonly used parameters for performance, which allows us to directly compare the cooling performance of various packaging designs.
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 CuO x clusters with only Cu–O coordination structure exclusively dominated in the surface of 1CuCe-NS, while the Cu species in 1CuCe-NR existed in both CuO x clusters and strongly interacting Cu-O x -Ce. In situ DRIFTS results demonstrate that the CeO2-{110} face induced a strongly bound Cu-O x -Ce structure in 1CuCe-NR which was adverse to the formation of reduced Cu(I) active sites, resulting in low reactivity in CO oxidation (r CO = 1.8 × 10–6 molCO gcat –1 s–1 at 118 °C); in contrast, CuO x clusters on the CeO2-{111} face were easily reduced to Cu(I) species when they were subjected to interaction with CO, which greatly enhanced the catalytic reactivity (r CO = 5.7 × 10–6 molCO gcat –1 s–1 at 104 °C). Thus, for copper–ceria catalyst, in comparison with the well-known reactive {110}CeO2 plane, {111}CeO2, the most inert plane, exhibits great superiority to induce more catalytically active sites of CuO x clusters. The difference in strength of the interaction between copper oxides and different exposed faces of ceria is intrinsically relevant to the different redox and catalytic properties.
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