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1.
J Am Chem Soc ; 146(7): 4993-5004, 2024 Feb 21.
Artículo en Inglés | MEDLINE | ID: mdl-38333965

RESUMEN

Alkyne hydrogenation on palladium-based catalysts modified with silver is currently used in industry to eliminate trace amounts of alkynes in alkenes produced from steam cracking and alkane dehydrogenation processes. Intensive efforts have been devoted to designing an alternative catalyst for improvement, especially in terms of selectivity and catalyst cost, which is still far away from that as expected. Here, we describe an atomic design of a high-performance Ni-based intermetallic catalyst aided by active machine learning combined with density functional theory calculations. The engineered NiIn catalyst exhibits >97% selectivity to ethylene and propylene at the full conversion of acetylene and propyne at mild temperature, outperforming the reported Ni-based catalysts and even noble Pd-based ones. Detailed mechanistic studies using theoretical calculations and advanced characterizations elucidate that the atomic-level defined coordination environment of Ni sites and well-designed hybridization of Ni 3d with In 5p orbital determine the semihydrogenation pathway.

2.
Angew Chem Int Ed Engl ; 63(1): e202314288, 2024 Jan 02.
Artículo en Inglés | MEDLINE | ID: mdl-37988201

RESUMEN

Methanol serves as a versatile building-block for various commodity chemicals, and the development of industrially promising strategies for its conversion remains the ultimate goal in methanol chemistry. In this study, we design a dual Cu-Cs catalytic system that enables a one-step direct conversion of methanol and methyl acetate/ethanol into high value-added esters/aldehydes, with customized chain length and saturation by leveraging the proximity and distribution of Cu-Cs sites. Cu-Cs at a millimeter-scale intimacy triggers methanol dehydrogenation and condensation, involving proton transfer, aldol formation, and aldol condensation, to obtain unsaturated esters and aldehydes with selectivities of 76.3 % and 31.1 %, respectively. Cu-Cs at a micrometer-scale intimacy significantly promotes mass transfer of intermediates across catalyst interfaces and their subsequent hydrogenation to saturated esters and aldehydes with selectivities of 67.6 % and 93.1 %, respectively. Conversely, Cu-Cs at a nanometer-scale intimacy alters reaction pathway with a similar energy barrier for the rate-determining step, but blocks the acidic-basic sites and diverts the reaction to byproducts. More importantly, an unprecedented quadruple tandem catalytic production of methyl methacrylate (MMA) is achieved by further tailoring Cu and Cs distribution across the reaction bed in the configuration of Cu-Cs||Cs, outperforming the existing industrial processes and saving at least 15 % of production costs.

3.
Angew Chem Int Ed Engl ; : e202410979, 2024 Jul 05.
Artículo en Inglés | MEDLINE | ID: mdl-38967363

RESUMEN

Catalytic removal of alkynes is essential in industry for producing polymer-grade alkenes from steam cracking processes. Non-noble Ni-based catalysts hold promise as effective alternatives to industrial Pd-based catalysts but suffer from low activity. Here we report embedding of single-atom Pd onto the NiGa intermetallic surface with replacing Ga atoms via a well-defined synthesis strategy to design Pd1-NiGa catalyst for alkyne semi-hydrogenation. The fabricated Pd1Ni2Ga1 ensemble sites deliver remarkably higher specific mass activity under superb alkene selectivity of >96% than the state-of-the-art catalysts under industry-relevant conditions. Integrated experimental and computational studies reveal that the single-atom Pd located synergizes with the neighbouring Ni sites to facilitate the σ-adsorption of alkyne and dissociation of hydrogen while suppress the alkene adsorption. Such synergistic effects confer the single-atom Pd on the NiGa intermetallic with a Midas touch for alkyne semi-hydrogenation, providing an effective strategy for stimulating low active Ni-based catalysts for other selective hydrogenations in industry.

4.
Small ; : e2307924, 2023 Dec 10.
Artículo en Inglés | MEDLINE | ID: mdl-38072771

RESUMEN

A rational crystallization strategy is essential to obtain high-quality protein crystals, yet the established methods suffer from different limitations arising from the single regulation on either nucleation or supersaturation. Herein, a nucleation-supersaturation dual-driven crystallization (DDC) strategy that realizes synergistic regulation of heterogeneous nucleation sites and solution supersaturation based on dual surface and confinement effects for efficient protein crystallization is reported. This strategy relies on a p(PEGDA-co-DMAA) hydrogel template with pre-filled NaCl under designed concentrations. Once dropping hen egg white lysozyme (HEWL) protein solution on the hydrogel, the wrinkled surface provides numerous nucleation sites, while the internal structure regulates the solution supersaturation in the crystallization region through diffusion. Finally, DDC strategy can create high-quality HEWL crystals with large sizes (100-300 µm), well-defined morphologies (hexagon and tetragon), and a significantly accelerated nucleation time (9-12 times faster than that achieved using the conventional hanging drop method). It also performs well at wider protein concentrations (10-50 mg mL-1 ) and categories (e.g., achieving fast crystallization and large-size crystals of trypsin), therefore demonstrating clear advantages and great potential for efficiently fabricating protein crystals desirable for diverse applications.

5.
Acc Chem Res ; 55(22): 3230-3241, 2022 11 15.
Artículo en Inglés | MEDLINE | ID: mdl-36321554

RESUMEN

Heterogeneous catalysis is the workhorse of the chemical industry, and a heterogeneous catalyst possesses numerous active sites working together to drive the conversion of reactants to desirable products. Over the decades, much focus has been placed on identifying the factors affecting the active sites to gain deep insights into the structure-performance relationship, which in turn guides the design and preparation of more active, selective, and stable catalysts. However, the molecular-level interplay between active sites and catalytic function still remains qualitative or semiquantitative, ascribed to the difficulty and uncertainty in elucidating the nature of active sites for its controllable manipulation. Hence, bridging the microscopic properties of active sites and the macroscopic catalytic performance, that is, microscopic-to-macroscopic transition, to afford a quantitative description is intriguing yet challenging, and progress toward this promises to revolutionize catalyst design and preparation.In this Account, we propose mesokinetics modeling, for the first time enabling a quantitative description of active site characteristics and the related mechanistic information, as a versatile tool to guide rational catalyst design. Exemplified by a pseudo-zero-order reaction, the kinetics derivation from the Pt particle size-sensitive catalytic activity and size-insensitive activation energy suggests only one type of surface site as the dominant active site, in which the Pt(111) with almost unchanged turnover frequency (TOF111) is further identified as the dominating active site. Such a method has been extended to identify and quantify the number (Ni) of active sites for various thermo-, electro-, and photocatalysts in chemical synthesis, hydrogen generation, environment application, etc. Then, the kinetics derivation from the kinetic compensation effects suggests a thermodynamic balance between the activation entropy and enthalpy, which exhibit linear dependences on Pt charge. Accordingly, the Pt charge can serve as a catalytic descriptor for its quantitative determination of TOFi. This strategy has been further applied to Pt-catalyzed CO oxidation with nonzero-order reaction characteristic by taking the site coverages of surface species into consideration.Hence, substituting the above statistical correlations of Ni and TOFi into the rate equation R = ∑Ni × TOFi offers the mesokinetics model, which can precisely predict catalytic function and screen catalysts. Finally, based on the disentanglement of the factors underlying Pt electronic structures, a de novo strategy, from the interfacial charge distribution to reaction mechanism, kinetics, and thermodynamics parameters of the rate-determining step, and ultimately catalytic performance, is developed to map the unified mechanistic and kinetics picture of reaction. Overall, the mesokinetics not only demonstrates much potential to elucidate the quantitative interplay between active sites and catalytic activity but also provides a new research direction in kinetics analysis to rationalize catalyst design.


Asunto(s)
Catálisis , Cinética , Tamaño de la Partícula , Termodinámica , Oxidación-Reducción
6.
Angew Chem Int Ed Engl ; 61(16): e202200190, 2022 Apr 11.
Artículo en Inglés | MEDLINE | ID: mdl-35132761

RESUMEN

Carbon monoxide (CO) is notorious for its strong adsorption to poison platinum group metal catalysts in the chemical industry. Here, we conceptually distinguish and quantify the effects of the occupancy and energy of d electrons, emerging as the two vital factors in d-band theory, for CO poisoning of Pt nanocatalysts. The stepwise defunctionalization of carbon support is adopted to fine-tune the 5d electronic structure of supported Pt nanoparticles. Excluding other promotional mechanisms, the increase of Pt 5d band energy strengthens the competitive adsorption of hydrogen against CO for the preferential oxidation of CO, affording the scaling relationship between Pt 5d band energy and CO/H2 adsorption energy difference. The decrease of Pt 5d band occupancy lowers CO site coverage to promote its association with oxygen for the total oxidation of CO, giving the scaling relationship between Pt 5d occupancy and activation energy. The above insights outline a molecular-level understanding of CO poisoning.

7.
Angew Chem Int Ed Engl ; 61(51): e202215225, 2022 Dec 19.
Artículo en Inglés | MEDLINE | ID: mdl-36269685

RESUMEN

Precisely tailoring the distance between adjacent metal sites to match adsorption configurations of key species for the targeted reaction pathway is a great challenge in heterogeneous catalysis. Here, we report a proof-of-concept study on the atomically sites-tailored pathway in Pd-catalyzed acetylene hydrogenation, i.e., increasing the distance of adjacent Pd atoms (dPd-a-Pd ) for configuration matching in acetylene semi-hydrogenation against coupling. dPd-a-Pd is identified as a structural descriptor for describing the competitiveness for reaction pathways, and the increased dPd-a-Pd prefers the semi-hydrogenation pathway due to simultaneously promoted C2 H4 desorption and the destabilized transition state of the C2 H3 * coupling. Spectroscopic, kinetics and electronic structure studies reveal that increasing dPd-a-Pd to 3.31 Šdelivers superior selectivity and stability due to energy matching and appropriate hybridization of Pd 4d with In 2s and, especially, 2p orbitals.

8.
Angew Chem Int Ed Engl ; 60(26): 14394-14398, 2021 Jun 21.
Artículo en Inglés | MEDLINE | ID: mdl-33856709

RESUMEN

We have developed a generalizable strategy to quantify the effect of surface barriers on zeolite catalysis. Isomerization of n-pentane, catalyzed by Pt/Beta, is taken as a model reaction system. Firstly, the surface modification by chemical liquid deposition of SiO2 was carried out to control the surface barriers on zeolite Beta crystals. The deposition of SiO2 leads to a very slight change in the physical properties of Beta crystals, but an obvious reduction in Brønsted acid sites. Diffusion measurements by the zero-length column (ZLC) method show that the apparent diffusivity of n-pentane can be more than doubled after SiO2 deposition, indicating that the surface barriers have been weakened. Catalytic performance was tested in a fixed-bed reactor, showing that the apparent catalytic activity improved by 51-131 % after SiO2 deposition. These results provide direct proof that reducing surface barriers can be an effective route to improve zeolite catalyst performance deteriorated by transport limitations.

9.
Phys Chem Chem Phys ; 21(24): 12859-12871, 2019 Jun 28.
Artículo en Inglés | MEDLINE | ID: mdl-31184338

RESUMEN

The exposed termination of transition-metal oxide surfaces plays a major role in determining the catalyst performance in redox reactions. In this contribution, the surface phase diagrams of LaMO3(001) (M = Sc-Fe) and LaMO3(110) (M = Co-Cu) are constructed by using the DFT+U method. The stabilities of six terminations derived from the stoichiometric MO2 and LaO surfaces are determined over a wide range of temperatures and oxygen partial pressures. The surface phase diagrams are calculated towards the O-rich limit in which the chemical potential of oxygen anions of perovskites equals that of gas-phase oxygen while the chemical potential of M cations is limited by thermodynamic boundary conditions. It is found that the surface phase diagrams are closely related to the reducibility of M cations, which is reflected in the oxygen adsorption energy and oxygen vacancy formation energy on the MO2- and LaO-terminated surfaces and can be measured by the third ionization energies of the M2+ cations. According to the surface phase diagrams, the most stable surface termination is predicted to be of MO2 type for LaMO3 (M = Sc-Fe) and LaO type for LaMO3 (M = Co-Cu) under solid oxide fuel cell operating conditions. Because the M cations become more readily reduced on going from left to right across the period, LaCoO3 may form an oxygen-deficient crystal structure at high temperatures and LaNiO3 and LaCuO3 would be decomposed into oxides containing the transition metals in a lower oxidation state.

10.
J Am Chem Soc ; 136(48): 16736-9, 2014 Dec 03.
Artículo en Inglés | MEDLINE | ID: mdl-25405630

RESUMEN

We report a size-dependent activity in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane. Kinetic study and model calculations revealed that Pt(111) facet is the dominating catalytically active surface. There is an optimized Pt particle size of ca. 1.8 nm. Meanwhile, the catalyst durability was found to be highly sensitive to the Pt particle size. The smaller Pt particles appear to have lower durability, which could be related to more significant adsorption of B-containing species on Pt surfaces as well as easier changes in Pt particle size and shape. The insights reported here may pave the way for the rational design of highly active and durable Pt catalysts for hydrogen generation.

11.
J Colloid Interface Sci ; 660: 574-584, 2024 Apr 15.
Artículo en Inglés | MEDLINE | ID: mdl-38266339

RESUMEN

Due to their biocompatibility and adjustable chemical structure and morphology, hydrogels have great potential in many applications, and can be used to enhance protein crystal quality and crystallization efficiency, contributing to biomedicine manufacturing. Monodispersed PEGDA hydrogel microspheres (HMSs) were synthesized using a Lego-inspired microfluidic device. The generated droplets were then UV polymerized, partially hydrolyzed with 0.1 M NaOH solution to improve their absorption capacity, and soaked in a buffer solution containing 0, 0.5, 1, 2, and 4 M NaCl. Salt-loaded HMSs were used as the medium for the enhanced crystallization of hen egg white lysozyme from aqueous solutions. Different supersaturations were achieved in the protein solutions by releasing NaCl of different concentrations from HMSs, as confirmed by electrical conductivity measurements. HMSs with or without NaCl can both provide heterogeneous nucleation sites due to their nano-porous structure and wrinkled surface. The addition of NaCl-loaded HMSs to the protein solution can also increase or decrease the supersaturation in the whole solution or locally near the HMS, leading to controllable nucleation time and crystal size distribution dependent on the NaCl concentration loaded into HMSs.


Asunto(s)
Hidrogeles , Cloruro de Sodio , Hidrogeles/química , Cristalización , Microesferas , Proteínas/química
12.
Nat Commun ; 15(1): 3249, 2024 Apr 16.
Artículo en Inglés | MEDLINE | ID: mdl-38627484

RESUMEN

The development of sustainable and anti-poisoning single-atom catalysts (SACs) is essential for advancing their research from laboratory to industry. Here, we present a proof-of-concept study on the poisoning of Au SACs, and the antidote of Au nanoparticles (NPs), with trace addition shown to reinforce and sustain propylene epoxidation. Multiple characterizations, kinetics investigations, and multiscale simulations reveal that Au SACs display remarkable epoxidation activity at a low propylene coverage, but become poisoned at higher coverages. Interestingly, Au NPs can synergistically cooperate with Au SACs by providing distinct active sites required for H2/O2 and C3H6 activations, as well as hydroperoxyl radical to restore poisoned SACs. The difference in reaction order between C3H6 and H2 (nC3H6-nH2) is identified as the descriptor for establishing the volcano curves, which can be fine-tuned by the intimacy and composition of SACs and NPs to achieve a rate-matching scenario for the formation, transfer, and consumption of hydroperoxyl. Consequently, only trace addition of Au NPs antidote (0.3% ratio of SACs) stimulates significant improvements in propylene oxide formation rate, selectivity, and H2 efficiency compared to SACs alone, offering a 56-fold, 3-fold, and 22-fold increase, respectively, whose performances can be maintained for 150 h.

13.
Nanotechnology ; 24(4): 045609, 2013 Feb 01.
Artículo en Inglés | MEDLINE | ID: mdl-23299661

RESUMEN

An eco-friendly, facile and scalable hydrothermal approach, in which the reduction and functionalization of graphite oxide (GO) are completed in one pot, is proposed for the synthesis of monolayer 3-aminopropyltriethoxysilane (APTES)-functionalized graphenes (A-FGs). Atomic force microscopy, transmission electron microscopy and x-ray diffraction analyses indicate that the as-synthesized A-FGs consist of only one or a few layered graphenes, while x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis reveal that APTES is bonded to graphene by the dehydration reaction between the Si-OH (produced by APTES hydration) and the -OH on the GO surface. As a result, free amino groups are left on the A-FGs. Moreover, A-FGs are highly dispersible in dimethylsulfoxide, APTES and ethylene glycol, and their solubilities are up to 0.89, 4.03 and 0.90 mg ml(-1), respectively.


Asunto(s)
Aminas/química , Coloides/química , Cristalización/métodos , Grafito/química , Membranas Artificiales , Nanoestructuras/química , Nanoestructuras/ultraestructura , Sustancias Macromoleculares/química , Ensayo de Materiales , Conformación Molecular , Tamaño de la Partícula , Propiedades de Superficie
14.
Eur J Pharm Biopharm ; 179: 37-46, 2022 Oct.
Artículo en Inglés | MEDLINE | ID: mdl-36041596

RESUMEN

Pharmaceutical cocrystals have matured into an effective technique for tuning the physicochemical and mechanical properties of drugs in solid form simultaneously. Herein, in order to provide a novel cocrystal form of oral medicine metformin hydrochloride (MH), citric acid (CA) was selected as an efficient ligand after screening a variety of inorganic and organic acids. Thus, based on the principle of crystal engineering, we report a novel cocrystal: metformin hydrochloride - citric acid (MHCA) after the systematic screening, which was experimentally proved to be constituted with 1:1 stoichiometry. Compared with pure MH, MHCA has been proved higher solubility in water, methanol, and ethanol from 283.15 to 313.15 K. Through single-crystal X-ray diffraction (SC-XRD), the particular molecular structure of MHCA has been determined as the orthorhombic system and Pbca space group. Besides, the binding model of MH-CA was built for investigating the binding energy and stability between two components at 278, 298, and 318 K, which were found to be essential for the prediction and analysis of cocrystals. The contribution of different intermolecular interactions and the strength of molecular packing in the cocrystal also have been investigated by Hirshfeld surface analysis. It was found that the cocrystal structure was mainly stabilized by intermolecular hydrogen bonds existing as N-H···O between components, which indicated that the diffusion-combination trend of molecules enhanced the regular array of cocrystal. The results revealed that the molecules of MH and CA formed supramolecular cocrystals mainly induced by hydrogen bonds after passive contacts, such as co-crystallization or grind.


Asunto(s)
Ácido Cítrico , Metformina , Etanol , Ligandos , Metanol , Preparaciones Farmacéuticas , Solubilidad , Agua
15.
Nat Commun ; 13(1): 5534, 2022 Sep 21.
Artículo en Inglés | MEDLINE | ID: mdl-36131070

RESUMEN

Mechanism driven catalyst design with atomically uniform ensemble sites is an important yet challenging issue in heterogeneous catalysis associated with breaking the activity-selectivity trade-off. Herein, a trimer Ni1Sb2 site in NiSb intermetallic featuring superior selectivity is elaborated for acetylene semi-hydrogenation via a theoretical guidance with a precise synthesis strategy. The trimer Ni1Sb2 site in NiSb intermetallic is predicted to endow acetylene reactant with an adequately but not excessively strong σ-adsorption mode while ethylene product with a weak π-adsorption one, where such compromise delivers higher ethylene formation rate. An in-situ trapping of molten Sb by Ni strategy is developed to realize the construction of Ni1Sb2 site in the intermetallic P63/mmc NiSb catalysts. Such catalyst exhibits ethylene selectivity up to 93.2% at 100% of acetylene conversion, significantly prevailing over the referred Ni catalyst. These insights shed new lights on rational catalyst design by taming active sites to energetically match targeted reaction pathway.

16.
Polymers (Basel) ; 13(16)2021 Aug 09.
Artículo en Inglés | MEDLINE | ID: mdl-34451185

RESUMEN

Novel thermoplastic polyamide elastomers (TPAEs) consisting of long-chain semicrystalline polyamide 1212 (PA1212) and amorphous polyetheramine were synthesized via one-pot melt polycondensation. The method provides accessible routes to prepare TPAEs with a high tolerance of compatibility between polyamide and polyether oligomers compared with the traditional two-step method. These TPAEs with 10 wt % to 76 wt % of soft content were obtained by reaction of dodecanedioic acid, 1,12-dodecanediamine, and poly(propylene glycol) (PPG) diamine. The structure-property relationships of TPAEs were systematically studied. The chemical structure and the morphologic analyses have revealed that microphase separation occurs in the amorphous region. The TPAEs that have long-chain PPG segments consist of a crystalline polyamide domain, amorphous polyamide-rich domain, and amorphous polyetheramine-rich domain, while the ones containing short-chain PPG segments comprise of a crystalline polyamide domain and miscible amorphous polyamide phase and amorphous polyetheramine phase due to the compatibility between short-chain polyetheramine and amorphous polyamide. These novel TPAEs show good damping performance at low temperature, especially the TPAEs that incorporated 76 wt % and 62 wt % of PPG diamine. The TPAEs exhibit high elastic properties and low residual strain at room temperature. They are lightweight with density between 1.01 and 1.03 g/cm3. The long-chain TPAEs have well-balanced properties of low density, high elastic return, and high shock-absorbing ability. This work provides a route to expand TPAEs to damping materials with special application for sports equipment used in extremely cold conditions such as ski boots.

17.
Gut Pathog ; 13(1): 68, 2021 Nov 17.
Artículo en Inglés | MEDLINE | ID: mdl-34789342

RESUMEN

BACKGROUND: Poultry necrotic enteritis (NE) is an economically important disease caused by C. perfringens. The disease causing ability of this bacterium is linked with the production of a wide variety of toxins. Among them, necrotic enteritis B-like (NetB) toxin is reported to be involved in the pathogenesis of NE; in addition there is some circumstantial evidence that tpeL toxin may enhance virulence, but this is yet to be definitely shown. The situation becomes more complicated in the presence of a number of predisposing factors like co-infection with coccidia, type of diet and use of high protein diet. These co-factors alter the intestinal environment, thereby favoring the production of more toxins, leading to a more severe disease. The objective of this study was to develop a successful animal model that would induce clinical signs and lesions of NE using C. perfringens type G strains obtained from field outbreaks. A separate trial was simultaneously considered to establish the role of dietary factor with coccidial co-infection in NE. RESULTS: The results have shown that use of net-B positive C. perfringens without predisposing factors induce moderate to severe NE (Av. Lesion score 1.79 ± 1.50). In a separate trial, addition of fish meal to a feed of C. perfringens challenged birds produced higher number of NE cases (Av. Lesion score 2.17 ± 1.28). However, use of less virulent E. necatrix strain along with fish meal in conjunction with net-B positive strain did not alter the severity of NE lesions in specific pathogen free chicken (Av. Lesion score 2.21 ± 1.13). CONCLUSIONS: This study suggests that virulent C. perfringens type G strains can induce NE lesions in the absence of other predisposing factors. Birds in the clostridia challenged group showed moderate to severe NE lesions. Use of less virulent coccidia strain contributed to a lesser extent in increasing the severity of disease. Maize based diet along with fishmeal (1:1) increased the severity of lesions but statistically it was non-significant. The NE lesions in all experimental groups were found to be present more frequently in the duodenum. In this way, this study provided an effective model for in vivo production of NE in poultry birds.

18.
Nat Commun ; 12(1): 6888, 2021 Nov 25.
Artículo en Inglés | MEDLINE | ID: mdl-34824271

RESUMEN

A molecular-level understanding of how the electronic structure of metal center tunes the catalytic behaviors remains a grand challenge in heterogeneous catalysis. Herein, we report an unconventional kinetics strategy for bridging the microscopic metal electronic structure and the macroscopic steady-state rate for CO oxidation over Pt catalysts. X-ray absorption and photoelectron spectroscopy as well as electron paramagnetic resonance investigations unambiguously reveal the tunable Pt electronic structures with well-designed carbon support surface chemistry. Diminishing the electron density of Pt consolidates the CO-assisted O2 dissociation pathway via the O*-O-C*-O intermediate directly observed by isotopic labeling studies and rationalized by density-functional theory calculations. A combined steady-state isotopic transient kinetic and in situ electronic analyses identifies Pt charge as the kinetics indicators by being closely related to the frequency factor, site coverage, and activation energy. Further incorporation of catalyst structural parameters yields a novel model for quantifying the electronic effects and predicting the catalytic performance. These could serve as a benchmark of catalyst design by a comprehensive kinetics study at the molecular level.

19.
J Am Chem Soc ; 132(30): 10398-406, 2010 Aug 04.
Artículo en Inglés | MEDLINE | ID: mdl-20662517

RESUMEN

Low-temperature CO oxidation over a compositional series of Pd-Au nanoalloy catalysts supported on silica fume was studied. Except for the pure metals, these materials invariably showed biphasic separation into palladium- and gold-rich components. Performance was optimal for a catalyst of bulk composition Pd(4)Au(1), a mixture of Pd(90)Au(10) (72.5 at. %) and Pd(31)Au(69) (27.5 at. %), that was remarkably active at 300 K and more stable than a pure Au catalyst. For bulk materials dominated by Pd (Pd:Au = 16:1; 8:1; 4:1), the palladium-rich alloy fraction frequently adopted hollow sphere or annular morphology, while the gold-rich crystals were often multiply twinned. Quantitative powder X-ray diffraction (XRD) showed that under the synthesis conditions used, the Au solubility limit in Pd crystals was approximately 12 at. %, while Pd was more soluble in Au (approximately 31 at. %). This was consistent with X-ray photoelectron spectroscopy (XPS), which revealed that the surfaces of Pd-rich alloys were enriched in gold relative to the bulk composition. In situ Fourier transform infrared spectra collected during CO oxidation contained a new band at 2114 cm(-1) (attributed to linear CO-Au/Au-Pd bonds) and reduced intensity of a band at 2090 cm(-1) (arising from a linear CO-Pd bond) with escalating Au content, indicating that the Pd sites became increasingly obscured by Au. High-resolution electron micrographs (HRTEM) of the Pd-rich alloys revealed atomic scale surface defects consistent with this interpretation. These results demonstrate that gold-containing biphasic Pd nanoalloys may be highly durable alternatives for a range of catalytic reactions.

20.
Phys Chem Chem Phys ; 12(9): 2170-6, 2010 Mar 07.
Artículo en Inglés | MEDLINE | ID: mdl-20165765

RESUMEN

Hydrogen peroxide (H(2)O(2)) synthesis directly from dioxygen and dihydrogen was carried out using a continuous flow reactor with a Pd catalyst. The effects of ionic liquids on the selectivity to H(2)O(2) were studied on a Pd/SiO(2) catalyst. It was found that the ionic liquid [BMIM][BF(4)] in water or ethanol is quite beneficial to the selectivity to H(2)O(2). Ca. 95% selectivity after 1 h in both solvents and a relatively high selectivity i.e. (about 50% in ethanol and 40% in water) after 5 h reaction have been achieved. On the other hand, a plausible mechanism for the effects of ion liquids on this reaction system was suggested on the basis of the preliminary results.

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