Tuning Supported Catalyst Reactivity with Dendrimer-Templated Pt-Cu Nanoparticles

The effects of particle composition on heterogeneous catalysis were studied using dendrimer-encapsulated nanoparticles (DENs) as precursors to supported Pt-Cu catalysts. Bimetallic Pt-Cu DENs with varying Pt/Cu ratios were prepared in an anaerobic aqueous solution and deposited onto a high-purity commercial alumina support. The dendrimer template was then thermally removed to yield supported nanoparticle catalysts, which were studied with toluene hydrogenation and CO oxidation catalysis as well as infrared spectroscopy of adsorbed CO. Incorporating Cu into Pt nanoparticles had opposite effects on the two test reactions. Cu acted as a mild promoter for CO oxidation catalysis, and the promoting effect was independent of the amount of Cu present. Conversely, Cu acted as a strong poison for toluene hydrogenation catalysis, and the normalized rate tracked inversely with Cu content. Infrared spectroscopy of the supported nanoparticles indicated that electronic effects (electron donation from Cu to Pt) were minimal for these materials. Consequently, the catalysis results are interpreted in terms of potential structural differences as a function of Cu incorporation and reaction conditions

Preparation and Characterization of 3 nm Magnetic NiAu Nanoparticles

Using PAMAM dendrimers as nanoparticle templates, a synthetic route to prepare 3 nm magnetic NiAu nanoparticles was developed. Aqueous solutions of hydroxyl-terminated generation 5 PAMAM dendrimers in 25 mM NaClO4 were shown to bind aqueous NiII. Coreduction of NiII and AuIII salts yielded bimetallic dendrimer stabilized nanoparticles, which were extracted into toluene with decanethiol. Characterization of the resulting monolayer protected clusters (MPCs) with transmission electron microscopy and UV-visible, atomic absorption, and X-ray photoelectron spectroscopies suggested that the MPCs had substantial surface enrichment in Au. Superconducting quantum interference device (SQUID) measurements at 5 K show the bimetallic MPCs to have low coercivity and saturation magnetization relative to bulk Ni. Solution nuclear magnetic resonance (NMR) studies using the Evans method showed the bimetallic nanoparticles retain magnetic properties at ambient temperatures

Tuning Supported Catalyst Reactivity with Dendrimer-Templated Pt-Cu Nanoparticles

The effects of particle composition on heterogeneous catalysis were studied using dendrimer-encapsulated nanoparticles (DENs) as precursors to supported Pt-Cu catalysts. Bimetallic Pt-Cu DENs with varying Pt/Cu ratios were prepared in an anaerobic aqueous solution and deposited onto a high-purity commercial alumina support. The dendrimer template was then thermally removed to yield supported nanoparticle catalysts, which were studied with toluene hydrogenation and CO oxidation catalysis as well as infrared spectroscopy of adsorbed CO. Incorporating Cu into Pt nanoparticles had opposite effects on the two test reactions. Cu acted as a mild promoter for CO oxidation catalysis, and the promoting effect was independent of the amount of Cu present. Conversely, Cu acted as a strong poison for toluene hydrogenation catalysis, and the normalized rate tracked inversely with Cu content. Infrared spectroscopy of the supported nanoparticles indicated that electronic effects (electron donation from Cu to Pt) were minimal for these materials. Consequently, the catalysis results are interpreted in terms of potential structural differences as a function of Cu incorporation and reaction conditions. Introduction Highly dispersed supported metal nanoparticle catalysts are an important class of industrial materials, as one-third of material U. S. gross national product involves a catalytic process somewhere in the production chain. 1 State of the art heterogeneous catalysts often contain dopant metals, which are incorporated to promote a desired reaction, prevent undesirable side reactions, or enhance catalyst longevity. Recent advances in computational methods are beginning to shed light onto the fundamental properties of bimetallic catalyst systems, and predictive models are beginning to emerge. To address these fundamental issues, preparation methods for new model catalysts and appropriate control materials are needed. Inorganic and organometallic cluster compounds have been used to successfully prepare novel bimetallic catalysts, 5 particularly for metal combinations that are thermodynamically unstable in the bulk. Background Polyamidoamine (PAMAM) dendrimer-encapsulated nanoparticles (DENs) are attractive potential precursors for model heterogeneous catalysts. PAMAM dendrimers are versatile synthetic hosts, as they have been used to template a variety of metal and semiconductor nanoparticles on the order of 1-3 nm. In this work, we extend this general preparative route for heterogeneous catalysts to the Pt-Cu system (Scheme 1) and report the first study varying heterogeneous catalyst composition using DENs as catalyst precursors. Coincident with these goals is the desire to begin learning how to tune nanoparticle properties and reactivity through the controlled addition of dopant metals. Additionally, we seek to begin developing methods of rationally and systematically testing theoretical models for bimetallic catalyst systems. The Pt-Cu system is a good choice for initial investigations into composition effects because the two metals are miscible through the entire composition range. 34 Additionally, the system has been well studied and characterized, and there is current interest in its reactivity for dehydrodehalogenation, 35,36 dehydrogenation, Metal Uptake by G5-OH Dendrimers. UV-vis spectra during dendrimer-metal complexation were collected using an Ocean Optics, Inc. high-resolution fiber optic spectrometer (HR2000CG UV-NIR). For Pt 2+ uptake by G5-OH PAMAM dendrimer, 10 mL of 0.01 mM dendrimer solution was mixed in a round-bottom flask with sufficient 5 mM K 2 PtCl 4 to prepare the following Pt/dendrimer ratios: 30:1, 23:1, and 15:1. A portion of the solution was transferred to a cuvette and analyzed at various time intervals using 0.01 mM G5-OH PAMAM dendrimer solution as a reference. After 4 days the pH was increased to 7. For Cu uptake by G5-OH, 10 mL of 0.01 mM dendrimer solution was mixed in a round-bottom flask with sufficient 5 mM Cu(NO 3 ) 2 to result in the following Cu/dendrimer ratios: 30:1, 23:1, and 15:1. An aliquot of the solution was immediately transferred to a cuvette for UV-vis spectroscopic analysis. A spectrum was obtained every 3 min for 30 min, and the pH was increased to 7. For Cu uptake by the Pt-dendrimer complex, 10 mL of 0.01 mM dendrimer solution was mixed in a round-bottom flask with sufficient 5 mM K 2 PtCl 4 to result in the following Pt/dendrimer ratios: 30:1, 23:1, and 15:1. The solutions were stirred for 2 days, and the Cu uptake procedure was followed using sufficient 5 mM Cu(NO 3 ) 2 to result in a metal/dendrimer ratio of 45:1 for all solutions. Preparation of Supported Nanoparticles. The DENs were prepared based on techniques detailed in the literature. NaBH 4 solution (0.045 M) was degassed, and 1 mL was added to the flask as N 2 was continually bubbled through the SCHEME 1 Dendrimer-Templated Pt-Cu Catalysts J. Phys. Chem. B, Vol. 110, No. 17, 2006 8607 solution. The solution turned a champagne color as the metals were reduced after stirring for 20 min. The pH of the solution was then adjusted to 8.5 with NaOH and HNO 3 . The alumina support was prepared by degassing 700 mg of 60-80 mesh alumina in a Schlenk flask for 1 h. The DEN solution was then added to the degassed support via cannula, and the mixture was stirred overnight under N 2 . The solid gray catalyst was isolated from a colorless solution by vacuum filtration over a medium grain frit, washed with 95% ethanol, and dried at 80°C in a vacuum oven overnight. The supported DENs were activated by exposing the dried catalyst to flowing O 2 for 16 h at 300°C , as described previously. 31 Pt and Cu Analysis with Atomic Absorption Spectroscopy. Pt and Cu loadings were determined with a Varian SpectrAA 220FS atomic absorption spectrometer using an acetylene/air flame, as described previously. 32 Briefly, the sample was treated with freshly prepared aqua regia. The pH of the resulting solution was adjusted to 6.0-7.0 with ammonium hydroxide, condensed, and transferred to a 10 mL volumetric flask containing sufficient La(NO 3 ) 3 to yield a final solution of 1% La. Atomic absorption (AA) standards were prepared from Aldrich AA standard solutions and Nanopure water. FT-IR Spectroscopy. Spectra were collected using a Thermo Nicolet Nexus 470 spectrometer equipped with a deuterated triglyceride sulfate (DTGS) detector using 2 cm -1 resolution. A water-cooled stainless steel IR flow cell with NaCl windows was used to hold the pressed catalyst wafer (18-20 mg). The optical bench and beam path (outside the flow cell) were continuously purged with N 2 . A heating element wrapped around the cell with a thermocouple placed in close proximity to the catalyst sample allowed for in situ heating of samples and collection spectra at different temperatures. All spectra were collected with gases flowing at 60 mL/min; the gas composition was manipulated using the external manifold. Previously oxidized catalyst samples underwent in situ reduction (20% H 2 balance He at 300°C for 3 h) followed by flushing with He for 1 h at 300°C. The samples were then cooled under He flow, and a background spectrum was collected at room temperature. A 5% CO/He mixture was then flowed over the sample for 15 min, followed by pure He. IR spectra of CO adsorbed on the surface were collected once the gas-phase CO had been removed from the cell. CO desorption was accomplished by heating the sample under He flow. CO Oxidation Catalysis. The feed and reactor effluent composition was monitored with an SRI 8610C gas chromatograph with a thermal conductivity detector. Toluene Hydrogenation Catalysis. The oxidized catalyst was diluted 20:1 by mass with R-Al 2 O 3 . Approximately 300 mg of the diluted catalyst was loaded into a glass U-tube and reduced in flowing H 2 . The temperature was ramped 5°C/min to 200°C and soaked for 1 h. The temperature was then ramped 5°C/min to 300°C and held at that temperature for an additional 1.5 h. The reactor was then cooled to 60°C under flowing H 2 . A 3.2% toluene in H 2 feed was prepared using a two-stage bubbler. The first stage was held at approximately 40°C, and the second stage was maintained at 23°C. . Peak areas were standardized against external calibration solutions of toluene in hexanes and methylcyclohexane in toluene. by monitoring conversion as a function of flow rate. Flow rates were measured with a bubble flow meter immediately prior to each injection. Plots of conversion versus inverse space velocity were linear and had y-intercepts of 0.02% or lower. Results and Discussion Nanoparticle and Catalyst Preparation. Because PtCl 4 2-uptake is slow (1-2 days) relative to Cu 2+ (seconds), we used sequential binding of PtCl 4 2-followed by Cu 2+ to load generation 5 PAMAM dendrimers (G5-OH) with metal precursors (Scheme 1). Most previous studies with these metaldendrimer systems used G4-OH dendrimers, so the use of G5-OH dendrimers in the present study may have moderate effects on the kinetics of metal uptake. Additionally, metal-dendrimer interactions with one precursor may affect the binding of the second when preparing bimetallic nanoparticles. Accordingly, the individual metal binding steps in Scheme 1 were monitored with UV-vis spectroscopy. PtCl 4 2-uptake by G5-OH, monitored by following the disappearance of the PtCl charge transfer band at 380 nm, was complete in approximately 24 h and did not change after adjusting the solution pH to 7 (spectra in the Supporting Information). Changes in the 380 nm band were concurrent with changes in a dendrimerbased UV band at 320 nm indicating uptake into the dendrimer interior. 8608 J. Phys. Chem. B, Vol. 110, No. 17, 2006 Hoover et al. The consistent relative excess of Cu in the final materials also indicates that complexation of Pt(II) species to the dendrimer interior may not be complete in 48 h or that Cu 2+ may displace some of the Pt(II) species within the dendrimer. Our study is consistent with studies from other labs in that UVvis spectra of mixtures of PtCl 4 2-and PAMAM dendrimers cease to change after approximately 2 days. Infrared Spectroscopy of Adsorbed CO. Infrared spectra of CO adsorbed on the activated catalysts, shown in Evaluations of structural and electronic effects and their potential impact on catalytic activity are of fundamental interest and importance. CO adsorption studies are an important tool in evaluating these effects, as both can impact observed CO stretching frequencies. The absence of Cu-CO stretching frequencies provides important indirect information regarding the state of Cu in the catalysts. CO is relatively unstable and easily removed from fully reduced Cu surfaces (i.e., during the He flush required to remove gas-phase CO) yet is stable at ambient temperatures when adsorbed on cationic Cu species. The shifts in Pt-CO stretching frequencies are attributable to a combination of electronic and dipole-dipole coupling effects. Electronic effects arise from a net (partial) electron donation from Cu to the more electronegative Pt. This enhances population of Pt d-bands (relative to a pure Pt material), increasing CO back-bonding and resulting in a lower CO stretching frequency. Dipole-dipole coupling, which is thoughspace interaction between adsorbed dipoles vibrating at the same frequency (Pt-CO adsorbates), also plays an important role in CO stretching frequencies on supported catalysts. Increased dipole coupling, which occurs when vibrating dipoles are close to one another, causes a blue shift in the observed CO stretching frequency. Consequently, as one metal dilutes another, dipole coupling effects are diminished, and a resulting red shift is observed in the CO stretching frequency. Dendrimer-Templated Pt-Cu Catalysts J. Phys. Chem. B, Vol. 110, No. 17, 2006 8609 To evaluate the relative importance of dipole coupling effects, low-coverage CO stretching frequencies Catalytic Test Reactions. Catalytic activity of the supported nanoparticles was evaluated with the toluene hydrogenation and CO oxidation test reactions. Toluene hydrogenation activities, collected in The data in CO oxidation activity data for the Pt-containing catalysts is shown in Structural and Electronic Effects on Catalytic Activity. It is appropriate and important to reconcile the divergent effects of Cu incorporation on the two test reactions, particularly since they are opposite in both direction and magnitude (mild promotion of CO oxidation that is independent of Cu content versus strong inhibition of toluene hydrogenation that is Cudependent). Structural and electronic factors can affect the catalytic activity for these oxidation-reduction reactions. Pt is 8610 J. Phys. Chem. B, Vol. 110, No. 17, 2006 Hoover et al. slightly more electronegative than Cu, so it is possible that the more active metal is enriched in electron density upon incorporation of Cu. However, the electronegativity difference is small (2.2 vs 1.9), and the IR spectroscopy data indicate that any electronic differences between the mono-and bimetallic catalysts are correspondingly mild. The low-coverage Pt-CO stretching frequencies show only a weak trend with Cu incorporation, but the shifts are fairly small and are in the range of singleton Pt-CO stretching frequencies for monometallic Pt catalysts. Although these small potential electronic effects might be a reasonable explanation for the copper's promotion of CO oxidation catalysis (vide infra), we do not believe that they are sufficient to explain the 100-fold decrease in Pt activity for toluene hydrogenation. The trend in hydrogenation activity tracks directly with the metal stoichiometry, which is set and controlled in the initial nanoparticle synthesis. This reduction in catalytic activity with increasing Cu content is likely due to surface enrichment in copper under a reducing atmosphere. Cu surface enrichment in this system been postulated for a number of traditionally prepared supported Pt-Cu catalysts, particularly under reducing atmospheres. The simplest structural explanation is that surface Cu atoms provide a new type of surface site that binds CO more weakly than monometallic Pt sites. The E app values for the bimetallic nanoparticles, which strongly depend on heats of adsorption, are 30% lower than that of the monometallic Pt catalyst. This explanation is also attractive in light of models for this reaction over Pt, which implicates desorption of a key weakly bound CO-based intermediate as the rate-determining step. Oxygen is also more competitive for binding on Cu than Pt (Cu-CO bonds are weak, vide supra), so Cu incorporation may simply increase the amount of surface oxygen available on the particle. Because CO binding appears to enrich the surface in Pt, the surface stoichiometry under the reaction conditions is unclear. It is also possible that, in the presence of CO, the bimetallic particles result in Pt-on-Cu subsurface alloys that have been predicted by some computational models. 6-9 These subsurface alloys can be activated through electronic influences or structurally through the epitaxial compression of surface Pt atoms. This explanation is consistent with a mild electronic effect on Pt observed in the infrared spectra. At the same time, correlations between IR spectra and catalytic activity must be drawn with extreme care, since there is no guarantee that the strongly bound Pt-CO species observed in the IR are the active species in CO oxidation catalysis. Additionally, the expected influence of the electronic effect (enhanced metal-ligand back-bonding inferred from a slightly weaker CO bond) would predict stronger Pt-C bonds and is inconsistent with a model involving desorption in the slow step. 47 Although we cannot rule out an electronic nature of the activity enhancement, we favor structural explanations

Dendrimer Templates for Heterogeneous Catalysts: Bimetallic Pt-Au Nanoparticles on Oxide Supports

Polyamidoamine (PAMAM) dendrimers were used to template Pt, Au, and bimetallic Pt–Au dendrimer encapsulated nanoparticles (DENs) in solution. Adjusting the solution pH allowed for slow, spontaneous adsorption of the nanoparticles onto silica, alumina, and titania. After dendrimer removal, the catalysts were characterized with infrared spectroscopy of adsorbed CO and tested with CO oxidation catalysis. Infrared spectroscopy of the monometallic Pt catalysts showed a slight shift in the CO stretching frequency for the different supports. For the bimetallic catalysts, infrared spectra showed CO adsorbed on both Pt and on Au sites. Spectra collected during CO desorption showed substantial interactions between the two bands, confirming the presence of bimetallic particles on all the supports. The bimetallic catalysts were found to be more active than the monometallic catalysts and had lower apparent activation energies. The titania supported Pt–Au catalyst was resistant to deactivation during an extended treatment at 300 °C. Correlations between IR spectra and catalytic activity showed differences between the mono- and bimetallic materials and implicated a bimetallic Pt–Au ensemble at the catalytic active site. This is the first study to show that DENs are appropriate precursors for studying support effects on catalysis by metal nanoparticles, although the magnitude of the effects were small

Blood Myo-Inositol Concentrations in Preterm and Term Infants

© 2020, The Author(s), under exclusive licence to Springer Nature America, Inc. Objective: To describe relationship between cord blood (representing fetal) myo-inositol concentrations and gestational age (GA) and to determine trends of blood concentrations in enterally and parenterally fed infants from birth to 70 days of age. Design/Methods: Samples were collected in 281 fed or unfed infants born in 2005 and 2006. Myo-inositol concentrations were displayed in scatter plots and analyzed with linear regression models of natural log-transformed values. Results: In 441 samples obtained from 281 infants, myo-inositol concentrations varied from nondetectable to 1494 μmol/L. Cord myo-inositol concentrations decreased an estimated 11.9% per week increase in GA. Postnatal myo-inositol concentrations decreased an estimated 14.3% per week increase in postmenstrual age (PMA) and were higher for enterally fed infants compared to unfed infants (51% increase for fed vs. unfed infants). Conclusions: Fetal myo-inositol concentrations decreased with increasing GA. Postnatal concentrations decreased with increasing PMA and were higher among enterally fed than unfed infants

Cortisol awakening response and developmental outcomes at 6–7 years in children born extremely preterm

BackgroundExtremely preterm (EPT) birth has been related to dysregulation of stress responses and behavioral/learning problems at school age. Early adverse experiences can blunt HPA axis reactivity. We hypothesized that an attenuated cortisol awakening response would be associated with developmental and behavioral problems at school age in EPT children.MethodsThis secondary analysis of a sub-cohort of the SUPPORT study included children born between 24 and 27 weeks, evaluated at 6-7 years with a neurodevelopmental battery and cortisol measures. Differences were tested between EPT and a term-born group. Relationships of cortisol awakening response to test scores were analyzed.ResultsCortisol was measured in 110 EPT and 29 term-born 6-7 year olds. Unadjusted WISC-IV and NEPSY-II scores were significantly worse among EPT children only. Conners Parent Rating Scale behavior scores were significantly worse among EPT children. After adjusting for covariates, blunted cortisol awakening responses were found to be associated with poorer scores on memory tests and greater problems with inattention for the EPT group (p < 0.05) only.ConclusionsAmong children born EPT, we identified an association of blunted cortisol awakening response with memory and inattention problems. This may have implications related to stress reactivity and its relationship to learning problems in children born EPT.ClinicaltrialsGov idExtended Follow-up at School Age for the SUPPORT Neuroimaging and Neurodevelopmental Outcomes (NEURO) Cohort: NCT00233324.ImpactIn children born EPT, stress reactivity may have a relationship to learning problems. Cortisol awakening response should be a component for follow-up in EPT born children. Components of executive function, such as memory and attention, are related to stress reactivity

Admission Temperature and Associated Mortality and Morbidity among Moderately and Extremely Preterm Infants

ObjectiveTo evaluate the temperature distribution among moderately preterm (MPT, 29-33 weeks) and extremely preterm (EPT, <29 weeks) infants upon neonatal intensive care unit (NICU) admission in 2012-2013, the change in admission temperature distribution for EPT infants between 2002-2003 and 2012-2013, and associations between admission temperature and mortality and morbidity for both MPT and EPT infants.Study designProspectively collected data from 18 centers in the Eunice Kennedy Shriver National Institute of Child Health and Human Development Neonatal Research Network were used to examine NICU admission temperature of inborn MPT and EPT infants. Associations between admission temperature and mortality and morbidity were determined by multivariable logistic regression. EPT infants from 2002-2003 and 2012-2013 were compared.ResultsMPT and EPT cohorts consisted of 5818 and 3213 infants, respectively. The distribution of admission temperatures differed between the MPT vs EPT (P < .01), including the percentage <36.5°C (38.6% vs 40.9%), 36.5°C-37.5°C (57.3% vs 52.9%), and >37.5°C (4.2% vs 6.2%). For EPT infants in 2012-2013 compared with 2002-2003, the percentage of temperatures between 36.5°C and 37.5°C more than doubled and the percentage of temperatures >37.5°C more than tripled. Admission temperature was inversely associated with in-hospital mortality.ConclusionsLow and high admission temperatures are more frequent among EPT than MPT infants. Compared with a decade earlier, fewer EPT infants experience low admission temperatures but more have elevated temperatures. In spite of a change in distribution of NICU admission temperature, an inverse association between temperature and mortality risk persists

Markers of Successful Extubation in Extremely Preterm Infants, and Morbidity After Failed Extubation

To identify variables associated with successful elective extubation, and to determine neonatal morbidities associated with extubation failure in extremely preterm neonates. This study was a secondary analysis of the National Institute of Child Health and Human Development Neonatal Research Network's Surfactant, Positive Pressure, and Oxygenation Randomized Trial that included extremely preterm infants born at 240/7 to 276/7 weeks' gestation. Patients were randomized either to a permissive ventilatory strategy (continuous positive airway pressure group) or intubation followed by early surfactant (surfactant group). There were prespecified intubation and extubation criteria. Extubation failure was defined as reintubation within 5 days of extubation. Of 1316 infants in the trial, 1071 were eligible; 926 infants had data available on extubation status; 538 were successful and 388 failed extubation. The rate of successful extubation was 50% (188/374) in the continuous positive airway pressure group and 63% (350/552) in the surfactant group. Successful extubation was associated with higher 5-minute Apgar score, and pH prior to extubation, lower peak fraction of inspired oxygen within the first 24 hours of age and prior to extubation, lower partial pressure of carbon dioxide prior to extubation, and non-small for gestational age status after adjustment for the randomization group assignment. Infants who failed extubation had higher adjusted rates of mortality (OR 2.89), bronchopulmonary dysplasia (OR 3.06), and death/ bronchopulmonary dysplasia (OR 3.27). Higher 5-minute Apgar score, and pH prior to extubation, lower peak fraction of inspired oxygen within first 24 hours of age, lower partial pressure of carbon dioxide and fraction of inspired oxygen prior to extubation, and nonsmall for gestational age status were associated with successful extubation. Failed extubation was associated with significantly higher likelihood of mortality and morbidities. ClinicalTrials.gov: NCT00233324