Abstract
Constructing single atom catalysts with fine-tuned coordination environments can be a promising strategy to achieve satisfactory catalytic performance. Herein, via a simple calcination ...temperature-control strategy, CeO
2
supported Pt single atom catalysts with precisely controlled coordination environments are successfully fabricated. The joint experimental and theoretical analysis reveals that the Pt single atoms on Pt
1
/CeO
2
prepared at 550 °C (Pt/CeO
2
-550) are mainly located at the edge sites of CeO
2
with a Pt–O coordination number of
ca
. 5, while those prepared at 800 °C (Pt/CeO
2
-800) are predominantly located at distorted Ce substitution sites on CeO
2
terrace with a Pt–O coordination number of
ca
. 4. Pt/CeO
2
-550 and Pt/CeO
2
-800 with different Pt
1
-CeO
2
coordination environments exhibit a reversal of activity trend in CO oxidation and NH
3
oxidation due to their different privileges in reactants activation and H
2
O desorption, suggesting that the catalytic performance of Pt single atom catalysts in different target reactions can be maximized by optimizing their local coordination structures.
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•Many Rh catalysts were tested for N2O decomposition under diesel conditions.•Rh/CeO2 is the only catalyst with good activity in a wet feed after aging.•Aging does not deactivate ...Rh/CeO2 but significantly increases its activity.•Removal O2 from the catalyst has been demonstrated to be the rate limiting step.
Numerous Rh catalysts were evaluated for N2O decomposition for automotive applications. Some Rh-containing spinel materials exhibit excellent fresh activities in the absence of H2O but become inactive after hydrothermal aging or when tested in a wet feed. Rh catalysts supported on zeolites can be very active in a dry feed even after aging but are extremely sensitive to H2O. Rh/CeO2 is an exceptional catalyst for this reaction in the presence of both H2O and O2. Hydrothermal aging (750 °C/20 h) significantly increases its activity. A similar activity enhancement was found by calcining the support before Rh impregnation. XPS results show a surface enrichment of Rh species on the aged Rh/CeO2 catalyst relative to the fresh catalyst. Aberration corrected STEM images reveal that Rh is buried in the bulk on the fresh catalyst and pulled out onto the surface of the support after thermal treatments. All catalysts are inhibited by H2O with the zeolite-based Rh catalysts being the worst. The aged Rh/CeO2 catalyst is less sensitive to H2O relative to others. DRIFTS data show that H2O sensitivity is related to catalyst hydrophilicity; a high coverage of OH groups on a catalyst reduces its N2O decomposition activity. H2-TPR results show that a Rh/CeO2 catalyst can be readily reduced at < 100 °C. On a reduced Rh/CeO2 catalyst, near complete N2O conversion can be obtained with a lean feed at 250 °C for a duration equivalent to its oxygen storage capacity. The N2O-DRIFTS experiments over a pre-reduced Rh/CeO2 catalyst show that Ce3+ sites are quickly oxidized to Ce4+ upon contacting N2O at room temperature, resulting N2 and adsorbed O, with the latter being an efficient oxidizer. Based on these results, a N2O decomposition mechanism is proposed for the Rh/CeO2 catalyst.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Supported precious metal single-atom catalysts have shown interesting activity and selectivity in recent studies. However, agglomeration of these highly mobile mononuclear surface species can ...eliminate their unique catalytic properties. Here we study a strategy for synthesizing thin film stabilized single-atom Pd1 catalysts using atomic layer deposition (ALD). The thermal stability of the Pd1 catalysts is significantly enhanced by creating a nanocavity thin film structure. In situ infrared spectroscopy and Pd K-edge X-ray absorption spectroscopy (XAS) revealed that the Pd1 was anchored on the surface through chlorine sites. The thin film stabilized Pd1 catalysts were thermally stable under both oxidation and reduction conditions. The catalytic performance in the methanol decomposition reaction is found to depend on the thickness of protecting layers. While Pd1 catalysts showed promising activity at low temperature in a methanol decomposition reaction, 14 cycle TiO2 protected Pd1 was less active at high temperature. Pd L3 edge XAS indicated that the low reactivity compared with Pd nanoparticles is due to the strong adsorption of carbon monoxide even at 250 °C. These results clearly show that the ALD nanocavities provide a basis for future design of single-atom catalysts that are highly efficient and stable.
Multi-metallic nanoparticles constitute a new class of materials offering the opportunity to tune the properties via the composition, atomic ordering and size. In particular, supported bimetallic ...nanoparticles have generated intense interest in catalysis and electrocatalysis. However, traditional synthesis methods often lack precise control, yielding a mixture of monometallic and bimetallic particles with various compositions. Here we report a general strategy for synthesizing supported bimetallic nanoparticles by atomic layer deposition, where monometallic nanoparticle formation is avoided by selectively growing the secondary metal on the primary metal nanoparticle but not on the support; meanwhile, the size, composition and structure of the bimetallic nanoparticles are precisely controlled by tailoring the precursor pulse sequence. Such exquisite control is clearly demonstrated through in situ Fourier transform infrared spectroscopy of CO chemisorption by mapping the gradual atomic-scale evolution in the surface composition, and further confirmed using aberration-corrected scanning transmission electron microscopy.
Phase transformationa universal phenomenon in materialsplays a key role in determining their properties. Resolving complex phase domains in materials is critical to fostering a new fundamental ...understanding that facilitates new material development. So far, although conventional classification strategies such as order-parameter methods have been developed to distinguish remarkably disparate phases, highly accurate and efficient phase segmentation for material systems composed of multiphases remains unavailable. Here, by coupling hard-attention-enhanced U-Net network and geometry simulation with atomic-resolution transmission electron microscopy, we successfully developed a deep-learning tool enabling automated atom-by-atom phase segmentation of intertwined phase domains in technologically important cathode materials for lithium-ion batteries. The new strategy outperforms traditional methods and quantitatively elucidates the correlation between the multiple phases formed during battery operation. Our work demonstrates how deep learning can be employed to foster an in-depth understanding of phase transformation-related key issues in complex materials.
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IJS, KILJ, NUK, PNG, UL, UM
Engineering surface defects on metal oxide supports could help promote the dispersion of active sites and catalytic performance of supported catalysts. Herein, a strategy of ZrO2 doping was proposed ...to create rich surface defects on CeO2 (CZO) and, with these defects, to improve Pt dispersion and enhance its affinity as single sites to the CZO support (Pt/CZO). The strongly anchored Pt single sites on CZO support were initially not efficient for catalytic oxidation of CO/C3H6. However, after a simple activation by H2 reduction, the catalytic oxidation performance over Pt/CZO catalyst was significantly boosted and better than Pt/CeO2. Pt/CZO catalyst also exhibited much higher thermal stability. The structural evolution of Pt active sites by H2 treatment was systematically investigated on aged Pt/CZO and Pt/CeO2 catalysts. With H2 reduction, ionic Pt single sites were transformed into active Pt clusters. Much smaller Pt clusters were created on CZO (ca. 1.2 nm) than on CeO2 (ca. 1.8 nm) due to stronger Pt-CeO2 interaction on aged Pt/CZO. Consequently, more exposed active Pt sites were obtained on the smaller clusters surrounded by more oxygen defects and Ce3+ species, which directly translated to the higher catalytic oxidation performance of activated Pt/CZO catalyst in vehicle emission control applications.
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IJS, KILJ, NUK, PNG, UL, UM
Al2O3-supported Pt/Pd bimetallic catalysts were studied using in situ atmospheric pressure and ex situ transmission electron microscopy. Real-time observation during separate oxidation and reduction ...processes provides nanometer-scale structural details – both morphology and chemistry – of supported Pt/Pd particles at intermediate states not observable through typical ex situ experiments. Significant metal vaporization was observed at temperatures above 600 °C, both in pure oxygen and in air. This behavior implies that material transport through the vapor during typical catalyst aging processes for oxidation can play a more significant role in catalyst structural evolution than previously thought. Concomitantly, Pd diffusion away from metallic nanoparticles on the surface of Al2O3 can also contribute to the disappearance of metal particles. Electron micrographs from in situ oxidation experiments were mined for data, including particle number, size, and aspect ratio using machine learning image segmentation. Under oxidizing conditions, we observe not only a decrease in the number of metal particles but also a decrease in the surface area to volume ratio. Some of the metal that diffuses away from particles on the oxide support can be regenerated and reappears back on the catalyst support surface under reducing conditions. These observations provide insight on how rapid cycling between oxidative and reductive catalytic operating conditions affects catalyst structure.
Compared with Pt/Al2O3, sintering-resistant Pt nanoparticle catalysts promoted by ZnO significantly improved the reactivity and selectivity toward hydrogen formation in the aqueous phase reforming ...(APR) of 1-propanol. The improved performance was found to benefit from both the electronic and geometric effects of ZnO thin films. In situ small-angle X-ray scattering and scanning transmission electron microscopy showed that ZnO-promoted Pt possessed promising thermal stability under APR reaction conditions. In situ X-ray absorption spectroscopy showed clear charge transfer between ZnO and Pt nanoparticles. The improved reactivity and selectivity seemed to benefit from having both Pt-ZnO and Pt-Al2O3 interfaces.
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IJS, KILJ, NUK, PNG, UL, UM
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