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1.
Artigo em Inglês | MEDLINE | ID: mdl-34232625

RESUMO

Metallic nanocrystals (NCs) can be synthesized with tailored nonequilibrium shapes to enhance desired properties, e.g., octahedral fcc metal NCs optimize catalytic activity associated with {111} facets. However, maintenance of optimized properties requires stability against thermal reshaping. Thus, we analyze the reshaping of truncated fcc metal octahedra mediated by surface diffusion using a stochastic atomistic-level model with energetic input parameters for Pd. The model describes NC thermodynamics by an effective nearest-neighbor interaction and includes a realistic treatment of diffusive hopping for undercoordinated surface atoms. Kinetic Monte Carlo simulation reveals that the effective barrier, Eeff, for the initial stage of reshaping is strongly tied to the degree of truncation of the vertices in the synthesized initial octahedral shapes. This feature is elucidated via exact analytic determination of the energy variation along the optimal reshaping pathway at low-temperature (T), which involves transfer of atoms from truncated {100} vertex facets to form new layers on {111} side facets. Deviations from predictions of the low-T analysis due to entropic effects are more prominent for higher T and larger NC sizes.

2.
Chem Soc Rev ; 50(11): 6483-6506, 2021 Jun 08.
Artigo em Inglês | MEDLINE | ID: mdl-34100033

RESUMO

This review covers recent progress in using single molecule fluorescence microscopy imaging to understand the nanoconfinement in porous materials. The single molecule approach unveils the static and dynamic heterogeneities from seemingly equal molecules by removing the ensemble averaging effect. Physicochemical processes including mass transport, surface adsorption/desorption, and chemical conversions within the confined space inside porous materials have been studied at nanometer spatial resolution, at the single nanopore level, with millisecond temporal resolution, and under real chemical reaction conditions. Understanding these physicochemical processes provides the ability to quantitatively measure the inhomogeneities of nanoconfinement effects from the confining properties, including morphologies, spatial arrangement, and trapping domains. Prospects and limitations of current single molecule imaging studies on nanoconfinement are also discussed.

3.
Semin Nephrol ; 41(2): 156-167, 2021 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-34140094

RESUMO

Prolactin levels are increased in chronic kidney disease (CKD) as a result of reduced clearance and increased secretion. Hyperprolactinemia manifests as galactorrhea and hypogonadism. Treatment of hyperprolactinemia should focus on improving bothersome galactorrhea or hypogonadism by using dopamine agonists and/or replacement of sex hormone(s). Changes in the hypothalamic-pituitary-adrenal axis in CKD are characterized by increases in adrenocorticotropic hormone (ACTH) and cortisol levels, largely preserved circadian rhythms of ACTH and cortisol, and a normal response of cortisol to ACTH, metyrapone, and insulin-induced hypoglycemia. However, the hypothalamic-pituitary-adrenal axis is less inhibited by 1 mg dexamethasone but retains normal suppression by higher-dose dexamethasone. Diagnosis of adrenal insufficiency in CKD patients, as in normal subjects, usually is made by finding a subnormal cortisol response to ACTH. The mainstay of treatment of adrenal insufficiency is to replace glucocorticoid hormone. Cushing's disease in CKD is difficult to diagnose and relies on the dexamethasone suppression test and the midnight salivary cortisol test because the 24-hour urine free cortisol test is not useful because it is increased already in CKD. Treatment of Cushing's disease involves surgery, complemented by radiation and/or medical therapy if necessary. Growth hormone levels are increased and insulin-like growth factor 1 levels are normal in patients with CKD. In a normal patient with CKD, as in one with acromegaly, there can be a paradoxic increase in growth hormone after an oral glucose load. Therefore, diagnosis of acromegaly in renal insufficiency is challenging. The treatment of choice for acromegaly is surgery, although data for medical treatment for acromegaly in CKD are rare. In patients with renal impairment, arginine vasopressin levels are increased as a result of decreased clearance, and there also is impairment of arginine vasopressin signaling in renal tubules. Diabetes insipidus can be masked in advanced kidney disease until kidney transplantation. Diagnosis of the syndrome of inappropriate antidiuretic hormone is similar in mild or moderate kidney disease as in normal subjects, but is challenging in patients with advanced kidney disease owing to the impairment in urine dilution.

4.
Artigo em Inglês | MEDLINE | ID: mdl-34114306

RESUMO

Effective control on chemoselectivity in the catalytic hydrogenation of C=O over C=C bonds is uncommon with Pd-based catalysts because of the favored adsorption of C=C bonds on Pd surface. Here we report a unique orthorhombic PdSn intermetallic phase with unprecedented chemoselectivity toward C=O hydrogenation. We observed the formation and metastability of this PdSn phase in situ. During a natural cooling process, the PdSn nanoparticles readily revert to the favored Pd3 Sn2 phase. Instead, using a thermal quenching method, we prepared a pure-phase PdSn nanocatalyst. PdSn shows an >96 % selectivity toward hydrogenating C=O bonds of various α,ß-unsaturated aldehydes, highest in reported Pd-based catalysts. Further study suggests that efficient quenching prevents the reversion from PdSn- to Pd3 Sn2 -structured surface, the key to the desired catalytic performance. Density functional theory calculations and analysis of reaction kinetics provide an explanation for the observed high selectivity.

5.
Chem Commun (Camb) ; 57(44): 5454-5457, 2021 Jun 04.
Artigo em Inglês | MEDLINE | ID: mdl-33954323

RESUMO

The mechanism of the deactivation and regeneration of PtSn intermetallic compound nanoparticle (iNP) catalysts was studied by in situ TEM investigation. Our study reveals the reversible dynamic structural transition of the iNPs during deactivation and regeneration, which provides a direct correlation between the atomic structure and the catalytic activity of the iNPs.

6.
Sci Total Environ ; 773: 144765, 2021 Jun 15.
Artigo em Inglês | MEDLINE | ID: mdl-33940703

RESUMO

Knowledge on the occurrence and distributions of organic compounds, especially PAHs, POPs and ECs, in karstic river basins is limited. This study aims to determine the depositional history and sources of PAHs, PCBs, OCPs, antibiotics, EDCs and phenolic compounds and the ecological risk they have in the Panyang River Basin, an area with a typical karstic landscape and a high-longevity population. Sediment core analysis was adopted, correlation and principal component analyses were conducted to analyze pollution sources, and lead isotope technology was implemented for dating analysis. The sediment core covered 108 years. PCBs were detected with concentrations ranging from 3.80 to 16.18 µg/kg in the core with two concentration peaks in 1950 and 2005 that were related to anthropogenic effects. Eight of the 20 targeted phenolic compounds were detected, with concentrations ranging from 0.42 to 1.10 mg/kg. All PAHs were detected in the cores, with concentrations from 12.91 to 37.80 µg/kg. They were mainly related to natural diagenetic processes and domestic and agricultural sources. The concentrations of different OCP compounds ranged from undetected to 213.43 µg/kg and were mainly related to agricultural activities and long-range transportation. These key findings can assist environmental planning and management in this river basin.

7.
J Am Chem Soc ; 143(13): 5182-5190, 2021 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-33779171

RESUMO

Applying metal-organic frameworks (MOFs) on the surface of other materials to form multifunctional materials has recently attracted great attention; however, directing the MOF overgrowth is challenging due to the orders of magnitude differences in structural dimensions. In this work, we developed a universal strategy to mediate MOF growth on the surface of metal nanoparticles (NPs), by taking advantage of the dynamic nature of weakly adsorbed capping agents. During this colloidal process, the capping agents gradually dissociate from the metal surface, replaced in situ by the MOF. The MOF grows to generate a well-defined NP-MOF interface without a trapped capping agent, resulting in a uniform core-shell structure of one NP encapsulated in one single-crystalline MOF nanocrystal with specific facet alignment. The concept was demonstrated by coating ZIF-8 and UiO-66-type MOFs on shaped metal NPs capped by cetyltrimethylammonium surfactants, and the formation of the well-defined NP-MOF interface was monitored by spectroscopies. The defined interface outperforms ill-defined ones generated via conventional methods, displaying a high selectivity to unsaturated alcohols for the hydrogenation of an α,ß-unsaturated aldehyde. This strategy opens a new route to create aligned interfaces between materials with vastly different structural dimensions.

8.
J Am Chem Soc ; 2021 Mar 15.
Artigo em Inglês | MEDLINE | ID: mdl-33719436

RESUMO

The pursuit of two-dimensional (2D) borides, MBenes, has proven to be challenging, not the least because of the lack of a suitable precursor prone to the deintercalation. Here, we studied room-temperature topochemical deintercalation of lithium from the layered polymorphs of the LiNiB compound with a considerable amount of Li stored in between [NiB] layers (33 at. % Li). Deintercalation of Li leads to novel metastable borides (Li∼0.5NiB) with unique crystal structures. Partial removal of Li is accomplished by exposing the parent phases to air, water, or dilute HCl under ambient conditions. Scanning transmission electron microscopy and solid-state 7Li and 11B NMR spectroscopy, combined with X-ray pair distribution function (PDF) analysis and DFT calculations, were utilized to elucidate the novel structures of Li∼0.5NiB and the mechanism of Li-deintercalation. We have shown that the deintercalation of Li proceeds via a "zip-lock" mechanism, leading to the condensation of single [NiB] layers into double or triple layers bound via covalent bonds, resulting in structural fragments with Li[NiB]2 and Li[NiB]3 compositions. The crystal structure of Li∼0.5NiB is best described as an intergrowth of the ordered single [NiB], double [NiB]2, or triple [NiB]3 layers alternating with single Li layers; this explains its structural complexity. The formation of double or triple [NiB] layers induces a change in the magnetic behavior from temperature-independent paramagnets in the parent LiNiB compounds to the spin-glassiness in the deintercalated Li∼0.5NiB counterparts. LiNiB compounds showcase the potential to access a plethora of unique materials, including 2D MBenes (NiB).

9.
J Chem Phys ; 154(9): 094710, 2021 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-33685166

RESUMO

Heterogeneous single-metal-site catalyst or single-atom catalyst research has grown rapidly due to the accessibility of modern characterization techniques that can provide invaluable information at the atomic-scale. Herein, we study the structural evolution of isolated single Pt sites incorporated in a metal-organic framework containing bipyridine functional groups using in situ diffuse reflectance infrared Fourier transform spectroscopy with CO as the probe molecule. The structure and electronic properties of the isolated Pt sites are further corroborated by x-ray photoelectron spectroscopy and aberration-corrected scanning transmission electron microscopy. We find the prerequisite of high temperature He treatment for Pt activation and CO insertion and inquire into the structural transformation of Pt site process by dynamic nuclear polarization-enhanced solid-state nuclear magnetic resonance spectroscopy.

10.
BMC Pregnancy Childbirth ; 21(1): 216, 2021 Mar 17.
Artigo em Inglês | MEDLINE | ID: mdl-33731035

RESUMO

BACKGROUND: Gestational diabetes mellitus (GDM) and excessive body weight are two key risk factors for adverse perinatal outcomes. However, it is not clear whether restricted gestational weight gain (GWG) is favorable to reduce the risk for adverse pregnancy and neonatal outcomes in women with GDM. Therefore, this study aimed to assess the association of GWG after an oral glucose tolerance test with maternal and neonatal outcomes. METHODS: This prospective cohort study assessed the association of GWG after an oral glucose tolerance test (OGTT) with pregnancy and neonatal outcomes in 3126 women with GDM, adjusted for age, pre-pregnancy body mass index, height, gravidity, parity, adverse history of pregnancy, GWG before OGTT, blood glucose level at OGTT and late pregnancy. The outcomes included the prevalence of pregnancy-induced hypertension (PIH) and preeclampsia, large for gestational age (LGA), small for gestational age, macrosomia, low birth weight, preterm birth, and birth by cesarean section. GDM was diagnosed according to the criteria established by the International Association of Diabetes and Pregnancy Study Groups. RESULTS: GWG after OGTT was positively associated with risk for overall adverse pregnancy outcomes (adjusted odds ratio [aOR] = 1.72, 95% confidence interval [CI] = 1.50-1.97), LGA (aOR = 1.29, 95%CI = 1.13-1.47), macrosomia (aOR = 1.24, 95%CI = 1.06-1.46) and birth by cesarean section (aOR = 1.91, 95%CI = 1.67-2.19) in women with GDM. Further analyses revealed that a combination of excessive GWG before OGTT and after OGTT increased the risk of PIH and preeclampsia, LGA, macrosomia, and birth by cesarean section compared with adequate GWG throughout pregnancy. In contrast, GWG below the Institute of Medicine guideline after OGTT did not increase the risk of adverse perinatal outcomes despite GWG before OGTT. CONCLUSION: Excessive GWG after OGTT was associated with an elevated risk of adverse pregnancy outcomes, while insufficient GWG after OGTT did not increase the risk of LBW. Restricting GWG after diagnosis of GDM in women with excessive GWG in the first half of pregnancy may be beneficial to prevent PIH and preeclampsia, LGA, macrosomia, and birth by cesarean section.


Assuntos
Cesárea/estatística & dados numéricos , Diabetes Gestacional , Hipertensão Induzida pela Gravidez , Pré-Eclâmpsia , Adulto , Índice de Massa Corporal , Manutenção do Peso Corporal , China/epidemiologia , Diabetes Gestacional/diagnóstico , Diabetes Gestacional/epidemiologia , Feminino , Macrossomia Fetal/epidemiologia , Ganho de Peso na Gestação , Teste de Tolerância a Glucose/métodos , Humanos , Hipertensão Induzida pela Gravidez/epidemiologia , Hipertensão Induzida pela Gravidez/prevenção & controle , Recém-Nascido Pequeno para a Idade Gestacional , Pré-Eclâmpsia/epidemiologia , Pré-Eclâmpsia/prevenção & controle , Gravidez , Resultado da Gravidez/epidemiologia , Nascimento Prematuro/epidemiologia , Medição de Risco , Fatores de Risco
11.
Obes Facts ; : 1-9, 2021 Feb 03.
Artigo em Inglês | MEDLINE | ID: mdl-33535214

RESUMO

OBJECTIVE: To establish suggested gestational weight gain (GWG) using several distinct methods in a Chinese population. METHODS: This study analyzed data from the medical records of singleton pregnancy women during 2011-2017 in Beijing, China. Suggested GWG was calculated using four distinct methods. In method 1, suggested GWG was identified by the interquartile method. Subsequently, risk models for small for gestational age (SGA) and large for gestational age (LGA) with respect to GWG were constructed. GWG was treated as a continuous variable in method 2, and as a categorized variable in methods 3 and 4. RESULTS: An average GWG of 15.78 kg with a prevalence of LGA at 19.34% and SGA at 2.12% was observed among the 34,470 participants. Methods 1 and 2 did not yield clinically applicable results. The suggested GWGs were 11-17/11-16 kg, 9-19/9-15 kg, 4-12/4-10 kg, and 0-12/0-6 kg by method 3/method 4 for underweight, normal-weight, overweight, and obese women, respectively. The GWG range suggested by method 3 resulted in a larger proportion of participants (62.03%) within range, while the suggested GWG range by method 4 was associated with a lower risk of LGA compared to that conferred by the Institute of Medicine (IOM) criteria. CONCLUSION: This study suggests a modest GWG goal compared to IOM recommendations based on a large Chinese cohort.

12.
J Magn Reson ; 321: 106869, 2020 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-33197680

RESUMO

Nuclear spin hyperpolarization derived from parahydrogen can enable nuclear magnetic resonance spectroscopy and imaging with sensitivity enhancements exceeding four orders of magnitude. The NMR signal enhancement is proportional to 4xp-1, where xp is the parahydrogen mole fraction. For convenience, many labs elect to carry out the ortho-para conversion at 77 K where 50% enrichment is obtained. In theory, enrichment to 100% yields an automatic three-fold increase in the NMR signal enhancement. Herein, construction and testing of a simple and inexpensive continuous-flow converter for high para-enrichment is described. During operation, the converter is immersed in liquid helium contained in a transport dewar of the type commonly found in NMR labs for filling superconducting magnets. A maximum enrichment of 97.3±1.9% at 30 K was observed at 4.5 bar and 300 mL/min flow rate. The theoretically predicted 2.9-fold increase in the signal enhancement factor was confirmed in the heterogeneous hydrogenation of propene to propane over a PdIn/SBA-15 catalyst. The relatively low-cost to construct and operate this system could make high parahydrogen enrichment, and the associated increase in the parahydrogen-derived NMR signals, more widely accessible.

13.
Phys Chem Chem Phys ; 22(36): 20815-20828, 2020 Sep 23.
Artigo em Inglês | MEDLINE | ID: mdl-32914158

RESUMO

Heteronuclear correlation (HETCOR) spectroscopy is one of the key tools in the arsenal of the solid-state NMR spectroscopist to probe chemical and spatial proximities between two different nuclei and enhance spectral resolution. Dipolar heteronuclear multiple-quantum coherence (D-HMQC) is a powerful technique that can be potentially utilized to obtain 1H detected 2D HETCOR solid-state NMR spectra of any NMR active nucleus. A long-standing problem in 1H detected D-HMQC solid-state NMR experiments is the presence of t1-noise which reduces sensitivity and impedes spectral interpretation. In this contribution, we describe novel pulse sequences, termed t1-noise eliminated (TONE) D-HMQC, that minimize t1-noise and can provide higher sensitivity and resolution than conventional D-HMQC. Monte-Carlo and numerical simulations confirm that t1-noise in conventional D-HMQC primarily occurs because random MAS frequency fluctuations cause variations in the NMR signal amplitude from scan to scan, leading to imperfect cancellation of uncorrelated signals by phase cycling. The TONE D-HMQC sequence uses 1H π-pulses to refocus the evolution of 1H CSA across each SR421 recoupling block, improving the stability of the pulse sequence to random MAS frequency fluctuations. The 1H refocusing pulses also restore the orthogonality of in-phase and anti-phase magnetization for all crystallite orientations at the end of each recoupling block, enabling the use of 90° flip-back or LG spin-lock trim pulses to reduce the intensity of uncorrelated signals. We demonstrate the application of these methods to acquire 1H detected 2D 1H{35Cl} and 1H{13C} HETCOR spectra of histidine·HCl·H2O with reduced t1-noise. To show generality, we also apply these methods to obtain 2D 1H{17O} spectra of 20%-17O fmoc-alanine and for the first time at natural abundance, 2D 1H{25Mg} HETCOR spectra of magnesium hydroxide. The TONE D-HMQC sequences are also used to probe 1H-25Mg and 1H-27Al proximities in Mg-Al layered double hydroxides and confirm the even mixing of Mg and Al in these materials.

14.
Proc Natl Acad Sci U S A ; 117(42): 26151-26157, 2020 10 20.
Artigo em Inglês | MEDLINE | ID: mdl-32989148

RESUMO

Emerging evidence suggests a resurgence of COVID-19 in the coming years. It is thus critical to optimize emergency response planning from a broad, integrated perspective. We developed a mathematical model incorporating climate-driven variation in community transmissions and movement-modulated spatial diffusions of COVID-19 into various intervention scenarios. We find that an intensive 8-wk intervention targeting the reduction of local transmissibility and international travel is efficient and effective. Practically, we suggest a tiered implementation of this strategy where interventions are first implemented at locations in what we call the Global Intervention Hub, followed by timely interventions in secondary high-risk locations. We argue that thinking globally, categorizing locations in a hub-and-spoke intervention network, and acting locally, applying interventions at high-risk areas, is a functional strategy to avert the tremendous burden that would otherwise be placed on public health and society.


Assuntos
Controle de Doenças Transmissíveis/métodos , Doenças Transmissíveis Emergentes/prevenção & controle , Infecções por Coronavirus/prevenção & controle , Transmissão de Doença Infecciosa/prevenção & controle , Saúde Global/tendências , Pandemias/prevenção & controle , Pneumonia Viral/prevenção & controle , Betacoronavirus , COVID-19 , Clima , Doenças Transmissíveis Emergentes/epidemiologia , Doenças Transmissíveis Emergentes/transmissão , Infecções por Coronavirus/epidemiologia , Infecções por Coronavirus/transmissão , Previsões , Humanos , Cooperação Internacional , Modelos Teóricos , Pneumonia Viral/epidemiologia , Pneumonia Viral/transmissão , SARS-CoV-2 , Viagem
15.
Nanoscale ; 12(36): 18545-18562, 2020 Sep 28.
Artigo em Inglês | MEDLINE | ID: mdl-32970090

RESUMO

Intermetallic alloy nanocrystals have emerged as a promising next generation of nanocatalyst, largely due to their promise of surface tunability. Atomic control of the geometric and electronic structure of the nanoparticle surface offers a precise command of the catalytic surface, with the potential for creating homogeneous active sites that extend over the entire nanoparticle. Realizing this promise, however, has been limited by synthetic difficulties, imparted by differences in parent metal crystal structure, reduction potential, and atomic size. Further, little attention has been paid to the impact of synthetic method on catalytic application. In this review, we seek to connect the two, organizing the current synthesis methods and catalytic scope of intermetallic nanoparticles and suggesting areas where more work is needed. Such analysis should help to guide future intermetallic nanoparticle development, with the ultimate goal of generating precisely controlled nanocatalysts tailored to catalysis.

16.
Int J Mol Sci ; 21(18)2020 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-32961983

RESUMO

Reprogramming of cellular energy metabolism, such as lipid metabolism, is a hallmark of squamous cell carcinoma of the head and neck (SCCHN). However, whether protein expression related to fatty acid oxidation (FAO) affects survival in SCCHN remains unclear. We aimed to investigate FAO-related enzyme expression and determine its correlation with clinicopathological variables in SCCHN patients. Immunohistochemical analysis (IHC) of FAO-related protein expression, including carnitine palmitoyltransferase 1 (CPT1), the acyl-CoA dehydrogenase family, and fatty acid synthase (FAS), was performed using tissue microarrays from 102 resected SCCHN tumors. Expressions were categorized according to IHC scores, and the statistical association with clinicopathological factors was determined. Moderate-to-high expression of long-chain acyl-CoA dehydrogenase (LCAD) had a protective role against cancer-related death (adjusted hazard ratio (HR), 0.2; 95% confidence interval (CI), 0.05-0.87) after covariate adjustment. Age and clinical stage remained independent predictors of survival (adjusted HR, 1.75; 95% CI, 1.22-2.49 for age; adjusted HR, 14.33; 95% CI, 1.89-108.60 for stage III/IV disease). Overexpression of medium-chain acyl-CoA dehydrogenase and FAS correlated with advanced tumor stage (T3/T4); however, none of these factors were independent predictors of survival. Several FAO-related enzymes were upregulated and LCAD overexpression had a protective effect on overall survival in advanced SCCHN patients. FAO-related-enzyme expression might have a prognostic impact on survival outcomes in SCCHN.


Assuntos
Acil-CoA Desidrogenase de Cadeia Longa/metabolismo , Ácidos Graxos/metabolismo , Neoplasias de Cabeça e Pescoço/metabolismo , Carcinoma de Células Escamosas de Cabeça e Pescoço/metabolismo , Adulto , Idoso , Idoso de 80 Anos ou mais , Carnitina O-Palmitoiltransferase/metabolismo , Ácido Graxo Sintases/metabolismo , Feminino , Neoplasias de Cabeça e Pescoço/enzimologia , Neoplasias de Cabeça e Pescoço/patologia , Humanos , Imuno-Histoquímica , Masculino , Pessoa de Meia-Idade , Estadiamento de Neoplasias , Prognóstico , Modelos de Riscos Proporcionais , Carcinoma de Células Escamosas de Cabeça e Pescoço/enzimologia , Carcinoma de Células Escamosas de Cabeça e Pescoço/mortalidade , Análise Serial de Tecidos , Regulação para Cima
17.
J Am Chem Soc ; 142(36): 15276-15281, 2020 09 09.
Artigo em Inglês | MEDLINE | ID: mdl-32815720

RESUMO

A highly branch- and enantioselective 1,4-enynes synthesis from readily available terminal alkynes and racemic allylic carbonates by Sonogashira type synergistic Rh and Cu catalysis under neutral conditions has been developed. Aliphatic and aromatic terminal alkynes with various functional groups could be used directly. An inner-sphere reductive elimination C(sp)-C(sp3) bond formation mechanism is supported by the stoichiometric reaction.

18.
Nat Commun ; 11(1): 4091, 2020 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-32796938

RESUMO

Catalytic cleavage of strong bonds including hydrogen-hydrogen, carbon-oxygen, and carbon-hydrogen bonds is a highly desired yet challenging fundamental transformation for the production of chemicals and fuels. Transition metal-containing catalysts are employed, although accompanied with poor selectivity in hydrotreatment. Here we report metal-free nitrogen-assembly carbons (NACs) with closely-placed graphitic nitrogen as active sites, achieving dihydrogen dissociation and subsequent transformation of oxygenates. NACs exhibit high selectivity towards alkylarenes for hydrogenolysis of aryl ethers as model bio-oxygenates without over-hydrogeneration of arenes. Activities originate from cooperating graphitic nitrogen dopants induced by the diamine precursors, as demonstrated in mechanistic and computational studies. We further show that the NAC catalyst is versatile for dehydrogenation of ethylbenzene and tetrahydroquinoline as well as for hydrogenation of common unsaturated functionalities, including ketone, alkene, alkyne, and nitro groups. The discovery of nitrogen assembly as active sites can open up broad opportunities for rational design of new metal-free catalysts for challenging chemical reactions.

19.
J Am Chem Soc ; 142(31): 13305-13309, 2020 08 05.
Artigo em Inglês | MEDLINE | ID: mdl-32687344

RESUMO

Nanoconfinement imposes physical constraints and chemical effects on reactivity in nanoporous catalyst systems. In the present study, we lay the groundwork for quantitative single-molecule measurements of the effects of chemical environment on heterogeneous catalysis in nanoconfinement. Choosing hydrophobicity as an exemplary chemical environmental factor, we compared a range of essential parameters for an oxidation reaction on platinum nanoparticles (NPs) confined in hydrophilic and hydrophobic nanopores. Single-molecule experimental measurements at the single particle level showed higher catalytic activity, stronger adsorption strength, and higher activation energy in hydrophobic nanopores than those in hydrophilic nanopores. Interestingly, different dissociation kinetic behaviors of the product molecules in the two types of nanopores were deduced from the single-molecule imaging data.

20.
ACS Nano ; 14(7): 8551-8561, 2020 Jul 28.
Artigo em Inglês | MEDLINE | ID: mdl-32639718

RESUMO

Stability against reshaping of metallic fcc nanocrystals synthesized with tailored far-from-equilibrium shapes is key to maintaining optimal properties for applications such as catalysis. Yet Arrhenius analysis of experimental reshaping kinetics, and appropriate theory and simulation, is lacking. Thus, we use TEM to monitor the reshaping of Pd nanocubes of ∼25 nm side length between 410 °C (over ∼4.5 h) and 440 °C (over ∼0.25 h), extracting a high effective energy barrier of Eeff ≈ 4.6 eV. We also provide an analytic determination of the energy variation along the optimal pathway for reshaping that involves transfer of atoms across the nanocube surface from edges or corners to form new layers on side {100} facets. The effective barrier from this analysis is shown to increase strongly with the degree of truncation of edges and corners in the synthesized nanocube. Theory matches experiment for the appropriate degree of truncation. In addition, we perform simulations of a stochastic atomistic-level model incorporating a realistic description of diffusive hopping for undercoordinated surface atoms, thereby providing a visualization of the initial reshaping process.

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