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

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

Clinical and virologic characteristics of the first 12 patients with coronavirus disease 2019 (COVID-19) in the United States.

Data on the detailed clinical progression of COVID-19 in conjunction with epidemiological and virological characteristics are limited. In this case series, we describe the first 12 US patients confirmed to have COVID-19 from 20 January to 5 February 2020, including 4 patients described previously1,2,3. Respiratory, stool, serum and urine specimens were submitted for SARS-CoV-2 real-time reverse-transcription polymerase chain reaction (rRT-PCR) testing, viral culture and whole genome sequencing. Median age was 53 years (range: 21–68); 8 patients were male. Common symptoms at illness onset were cough (n = 8) and fever (n = 7). Patients had mild to moderately severe illness; seven were hospitalized and demonstrated clinical or laboratory signs of worsening during the second week of illness. No patients required mechanical ventilation and all recovered. All had SARS-CoV-2 RNA detected in respiratory specimens, typically for 2–3 weeks after illness onset. Lowest real-time PCR with reverse transcription cycle threshold values in the upper respiratory tract were often detected in the first week and SARS-CoV-2 was cultured from early respiratory specimens. These data provide insight into the natural history of SARS-CoV-2. Although infectiousness is unclear, highest viral RNA levels were identified in the first week of illness. Clinicians should anticipate that some patients may worsen in the second week of illness

Researching COVID to Enhance Recovery (RECOVER) adult study protocol: Rationale, objectives, and design.

IMPORTANCE: SARS-CoV-2 infection can result in ongoing, relapsing, or new symptoms or other health effects after the acute phase of infection; termed post-acute sequelae of SARS-CoV-2 infection (PASC), or long COVID. The characteristics, prevalence, trajectory and mechanisms of PASC are ill-defined. The objectives of the Researching COVID to Enhance Recovery (RECOVER) Multi-site Observational Study of PASC in Adults (RECOVER-Adult) are to: (1) characterize PASC prevalence; (2) characterize the symptoms, organ dysfunction, natural history, and distinct phenotypes of PASC; (3) identify demographic, social and clinical risk factors for PASC onset and recovery; and (4) define the biological mechanisms underlying PASC pathogenesis. METHODS: RECOVER-Adult is a combined prospective/retrospective cohort currently planned to enroll 14,880 adults aged ≥18 years. Eligible participants either must meet WHO criteria for suspected, probable, or confirmed infection; or must have evidence of no prior infection. Recruitment occurs at 86 sites in 33 U.S. states, Washington, DC and Puerto Rico, via facility- and community-based outreach. Participants complete quarterly questionnaires about symptoms, social determinants, vaccination status, and interim SARS-CoV-2 infections. In addition, participants contribute biospecimens and undergo physical and laboratory examinations at approximately 0, 90 and 180 days from infection or negative test date, and yearly thereafter. Some participants undergo additional testing based on specific criteria or random sampling. Patient representatives provide input on all study processes. The primary study outcome is onset of PASC, measured by signs and symptoms. A paradigm for identifying PASC cases will be defined and updated using supervised and unsupervised learning approaches with cross-validation. Logistic regression and proportional hazards regression will be conducted to investigate associations between risk factors, onset, and resolution of PASC symptoms. DISCUSSION: RECOVER-Adult is the first national, prospective, longitudinal cohort of PASC among US adults. Results of this study are intended to inform public health, spur clinical trials, and expand treatment options. REGISTRATION: NCT05172024

Researching COVID to Enhance Recovery (RECOVER) adult study protocol: Rationale, objectives, and design.

ImportanceSARS-CoV-2 infection can result in ongoing, relapsing, or new symptoms or other health effects after the acute phase of infection; termed post-acute sequelae of SARS-CoV-2 infection (PASC), or long COVID. The characteristics, prevalence, trajectory and mechanisms of PASC are ill-defined. The objectives of the Researching COVID to Enhance Recovery (RECOVER) Multi-site Observational Study of PASC in Adults (RECOVER-Adult) are to: (1) characterize PASC prevalence; (2) characterize the symptoms, organ dysfunction, natural history, and distinct phenotypes of PASC; (3) identify demographic, social and clinical risk factors for PASC onset and recovery; and (4) define the biological mechanisms underlying PASC pathogenesis.MethodsRECOVER-Adult is a combined prospective/retrospective cohort currently planned to enroll 14,880 adults aged ≥18 years. Eligible participants either must meet WHO criteria for suspected, probable, or confirmed infection; or must have evidence of no prior infection. Recruitment occurs at 86 sites in 33 U.S. states, Washington, DC and Puerto Rico, via facility- and community-based outreach. Participants complete quarterly questionnaires about symptoms, social determinants, vaccination status, and interim SARS-CoV-2 infections. In addition, participants contribute biospecimens and undergo physical and laboratory examinations at approximately 0, 90 and 180 days from infection or negative test date, and yearly thereafter. Some participants undergo additional testing based on specific criteria or random sampling. Patient representatives provide input on all study processes. The primary study outcome is onset of PASC, measured by signs and symptoms. A paradigm for identifying PASC cases will be defined and updated using supervised and unsupervised learning approaches with cross-validation. Logistic regression and proportional hazards regression will be conducted to investigate associations between risk factors, onset, and resolution of PASC symptoms.DiscussionRECOVER-Adult is the first national, prospective, longitudinal cohort of PASC among US adults. Results of this study are intended to inform public health, spur clinical trials, and expand treatment options.RegistrationNCT05172024

Genome-wide association and transcriptome studies identify target genes and risk loci for breast cancer

Abstract Genome-wide association studies (GWAS) have identified more than 170 breast cancer susceptibility loci. Here we hypothesize that some risk-associated variants might act in non-breast tissues, specifically adipose tissue and immune cells from blood and spleen. Using expression quantitative trait loci (eQTL) reported in these tissues, we identify 26 previously unreported, likely target genes of overall breast cancer risk variants, and 17 for estrogen receptor (ER)-negative breast cancer, several with a known immune function. We determine the directional effect of gene expression on disease risk measured based on single and multiple eQTL. In addition, using a gene-based test of association that considers eQTL from multiple tissues, we identify seven (and four) regions with variants associated with overall (and ER-negative) breast cancer risk, which were not reported in previous GWAS. Further investigation of the function of the implicated genes in breast and immune cells may provide insights into the etiology of breast cancer

WTO must ban harmful fisheries subsidies

Sustainably managed wild fisheries support food and nutritional security, livelihoods, and cultures (1). Harmful fisheries subsidies—government payments that incentivize overcapacity and lead to overfishing—undermine these benefits yet are increasing globally (2). World Trade Organization (WTO) members have a unique opportunity at their ministerial meeting in November to reach an agreement that eliminates harmful subsidies (3). We—a group of scientists spanning 46 countries and 6 continents—urge the WTO to make this commitment..

Reproducibility of fluorescent expression from engineered biological constructs in E. coli

We present results of the first large-scale interlaboratory study carried out in synthetic biology, as part of the 2014 and 2015 International Genetically Engineered Machine (iGEM) competitions. Participants at 88 institutions around the world measured fluorescence from three engineered constitutive constructs in E. coli. Few participants were able to measure absolute fluorescence, so data was analyzed in terms of ratios. Precision was strongly related to fluorescent strength, ranging from 1.54-fold standard deviation for the ratio between strong promoters to 5.75-fold for the ratio between the strongest and weakest promoter, and while host strain did not affect expression ratios, choice of instrument did. This result shows that high quantitative precision and reproducibility of results is possible, while at the same time indicating areas needing improved laboratory practices.Peer reviewe