Gold Stabilized by Nanostructured Ceria Supports: Nature of the Active Sites and Catalytic Performance

The interaction of gold atoms with CeO₂ nanocrystals having rod and cube shapes has been examined by cyanide leaching, TEM, TPR, CO IR and X-ray absorption spectroscopy. After deposition?Cprecipitation and calcination of gold, these surfaces contain gold nanoparticles in the range 2?C6 nm. For the ceria nanorods, a substantial amount of gold is present as cations that replace Ce ions in the surface as follows from their first and second coordination shells of oxygen and cerium by EXAFS analysis. These cations are stable against cyanide leaching in contrast to gold nanoparticles. Upon reduction the isolated Au atoms form finely dispersed metal clusters with a high activity in CO oxidation, the WGS reaction and 1,3-butadiene hydrogenation. By analogy with the very low activity of reduced gold nanoparticles on ceria nanocubes exposing the {100} surface plane, it is inferred that the gold nanoparticles on the ceria nanorod surface also have a low activity in such reactions. Although the finely dispersed Au clusters are thermally stable up to quite high temperature in line with earlier findings (Y. Guan and E. J. M. Hensen, Phys Chem Chem Phys 11:9578, 2009), the presence of gold nanoparticles results in their more facile agglomeration, especially in the presence of water (e.g., WGS conditions). For liquid phase alcohol oxidation, metallic gold nanoparticles are the active sites. In the absence of a base, the O?CH bond cleavage appears to be rate limiting, while this shifts to C?CH bond activation after addition of NaOH. In the latter case, the gold nanoparticles on the surface of ceria nanocubes are much more active than those on the surface of nanorod ceria.     

H/D Exchange on Silica-Grafted Tantalum(V) Imido Amido [(≡SiO)<Subscript>2</Subscript>Ta<Superscript>(V)</Superscript>(NH)(NH<Subscript>2</Subscript>)] Synthesized from Either Ammonia or Dinitrogen: IR and DFT Evidence for Heterolytic Splitting of D<Subscript>2</Subscript>

The silica-grafted Ta^(V) imido amido complex [(≡SiO)₂Ta(NH)(NH₂)], 2, obtained from the reaction of either ammonia or dinitrogen plus hydrogen with the silica-grafted hydrides [(≡SiO)₂Ta^(III)H], 1a, and [(≡SiO)₂Ta^(V)H₃], 1b, undergoes H/D exchange with D₂. In situ IR spectroscopy shows that the fully labelled compound [(≡SiO)₂Ta(ND)(ND₂)], 2-d, can be obtained by moderate heating (60 °C, 3 h) under D₂ atmosphere (550 torr, 300 eq. with respect to Ta), and that the exchange is reversible. The observed stretching and bending frequencies of 2-d are in agreement with the expected isotopic shift upon H/D replacement with respect to literature values reported for 2 and have been corroborated by the independent synthesis of 2-d by reaction of deuterated 1a and 1b with N₂ and D₂. Density functional theory (DFT) calculations, performed using a periodic or a cluster model, explored the structures and energetics of all minima involved in the reaction with H₂ and showed that among the explored pathways the energetically preferred mechanisms for H₂ reaction with [{(μ-O)[(HO)₂SiO]₂}Ta^(V)(NH)(NH₂)], 2q, is the heterolytic cleavage of either the imido Ta=N or the amido Ta-N bonds, to yield respectively [{(μ-O)[(HO)₂SiO]₂}TaH(NH₂)₂], 3q (ΔE = −9.5 kcal mol⁻¹ and ΔG_298K = +2.6 kcal mol⁻¹ with respect to 2q) and [{(μ-O)[(HO)₂SiO]₂}Ta(NH)(NH₃)], 4q (ΔE = −6.0 kcal mol⁻¹ and ΔG_298K = +7.9 kcal mol⁻¹ with respect to 2q). All activation barriers are moderate (between 17.7 and 30.2 kcal mol⁻¹) in agreement with the observed mild heating conditions necessary for the reaction to occur.     

The role of collagen in bone apatite formation in the presence of hydroxyapatite nucleation inhibitors

Bone is a composite material in which collagen fibrils form a scaffold for a highly organized arrangement of uniaxially oriented apatite crystals. In the periodic 67 nm cross-striated pattern of the collagen fibril, the less dense 40-nm-long gap zone has been implicated as the place where apatite crystals nucleate from an amorphous phase, and subsequently grow. This process is believed to be directed by highly acidic non-collagenous proteins; however, the role of the collagen matrix during bone apatite mineralization remains unknown. Here, combining nanometre-scale resolution cryogenic transmission electron microscopy and cryogenic electron tomography with molecular modelling, we show that collagen functions in synergy with inhibitors of hydroxyapatite nucleation to actively control mineralization. The positive net charge close to the C-terminal end of the collagen molecules promotes the infiltration of the fibrils with amorphous calcium phosphate (ACP). Furthermore, the clusters of charged amino acids, both in gap and overlap regions, form nucleation sites controlling the conversion of ACP into a parallel array of oriented apatite crystals. We developed a model describing the mechanisms through which the structure, supramolecular assembly and charge distribution of collagen can control mineralization in the presence of inhibitors of hydroxyapatite nucleation.     

An examination of the relationship between general life stress, racism-related stress, and psychological health among black men.

This study explored the relationship among general life stress, racism-related stress, and psychological health in a sample of 220 Black men. Participants completed a personal data form, the Perceived Stress Scale (S. Cohen, T. Kamarck, & R. Mermelstein, 1983), a modified version of the Schedule of Racist Events (H. Landrine & E. A. Klonoff, 1996), and the Mental Health Inventory (C. T. Veit & J. E. Ware, 1983). Results of hierarchical regressions indicated that when general stress was controlled, racism-related stress predicted an additional 4% of variance in psychological distress for working class men and an additional 7% for middle-upper class men. Racism-related stress also predicted an additional 5% of variance in psychological well-being for middle-upper class men; however, it was not predictive of psychological well-being for working class men. Implications for counseling practice and future research are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Differential induction of systemic resistance in Arabidopsis by biocontrol bacteria

Selected nonpathogenic, root-colonizing bacteria are able to elicit induced systemic resistance (ISR) in plants. To elucidate the molecular mechanisms underlying this type of systemic resistance, an Arabidopsis-based model system was developed in which Pseudomonas syringae pv. tomato and Fusarium oxysporum f. sp. raphani were used as challenging pathogens. In Arabidopsis thaliana ecotypes Columbia and Landsberg erecta, colonization of the rhizosphere by P. fluorescens strain WCS417r induced systemic resistance against both pathogens. In contrast, ecotype RLD did not respond to WCS417r treatment, whereas all three ecotypes expressed systemic acquired resistance upon treatment with salicylic acid (SA). P. fluorescens strain WCS374r, previously shown to induce ISR in radish, did not elicit ISR in Arabidopsis. The opposite was found for P. putida strain WCS358r, which induced ISR in Arabidopsis but not in radish. These results demonstrate that rhizosphere pseudomonads are differentially active in eliciting ISR in related plant species. The outer membrane lipopolysaccharide (LPS) of WCS417r is the main ISR-inducing determinant in radish and carnation, and LPS-containing cell walls also elicit ISR in Arabidopsis. However, mutant WCS417rOA-, lacking the O-antigenic side chain of the LPS, induced levels of protection similar to those induced by wild-type WCS417r. This indicates that ISR-inducing bacteria produce more than a single factor that trigger ISR in Arabidopsis. Furthermore, WCS417r and WCS358r induced protection in both wild-type Arabidopsis and SA-nonaccumulating NahG plants without activating pathogenesis-related gene expression. This suggests that elicitation of an SA-independent signaling pathway is a characteristic feature of ISR-inducing biocontrol bacteria.     

From divided to integrated city?

Good urban governance results in economic efficiency, social equity, gender-aware policies, overall sustainability and ultimately, improved living conditions of not only the urban poor but all city residents. […] Good urban governance is not merely a matter of efficient management; it also has political dimensions related to democracy, human rights and civic participation in decision-making processes. (UNCHS, 2001: 211, emphasis in original) The Commission proposes the urgent establishment of a City Development Partnership to enable the development and implementation of a shared development agenda. This partnership must be about concerted action and must be co-owned by the Council, other government agencies, parastatals, the private sector, labour and civil society organisations. The emphasis of the city development partnership will be on a highly focussed campaign or campaigns which can mobilise the commitment, energy and resources of a wide range of stakeholders but which substantively address our core issues of a divided city, dysfunctional communities, and a dual economy. (Unicity Commission, 2000: 9) Previous versions of this paper were prepared for a conference on urban governance in Boston during September 2000, under the aegis of the National Academy of Sciences, and the “Urbanizing World and UN Human Habitat II” Conference, 4–6 June 2001, Columbia University, New York.     

Oligonucleotide optimization for DNA synthesis

Large DNA constructs can be synthesized from smaller oligonucleotides using the polymerase chain reaction. The set of oligonucleotides should be designed so that the melting temperature amongst oligonucleotide hybridization pairs do not vary greatly and the length of each oligonucleotide should not exceed 50 nucleotides. A near optimal oligonucleotide set is calculated using reliable gradient optimization methods. This was accomplished by defining a set of discrete arrays that is used to determine the melting temperature of a subset of the larger DNA sequence, depending on the subset start and end positions. These arrays were then incorporated into an objective function, which was optimized using the Broyden‐Fletcher‐Goldfarb‐Shanno method. This method is adjusted slightly to incorporate explicit length and temperature constraints. Experimental results confirmed that the method performs better than similar software programs for the cases investigated and produces suitable oligonucleotide sets for DNA assembly. © 2010 American Institute of Chemical Engineers AIChE J, 2011