4,709 research outputs found
The promotional role of Mn in CO2 hydrogenation over Rh-based catalysts from a surface organometallic chemistry approach
No full textRh-based catalysts modified by transition metals have been intensely studied for CO2 hydrogenation due to their high activity. However, understanding the role of promoters at the molecular-level remains challenging due to the ill-defined structure of heterogeneous catalysts. Here, we constructed well-defined RhMn@SiO2 and Rh@SiO2 model catalysts via surface organometallic chemistry combined with thermolytic molecular precursors (SOMC/TMP) approach to rationalize the promotional effect of Mn in CO2 hydrogenation. We found that the addition of Mn shifts the products from almost pure CH4 to a mixture of methane and oxygenates (CO, CH3OH, and CH3CH2OH) upon going from Rh@SiO2 to RhMn@SiO2. In situ X-ray absorption spectroscopy (XAS) confirms that the MnII is atomically dispersed in the vicinity of metallic Rh nanoparticles, and enables to induce the oxidation of Rh to form Mn-O-Rh interface under reaction conditions. The formed interface is key to maintain Rh+ site to promote the formation of CO, CH3OH and CH3CH2OH
Unveiling the Structure Sensitivity for Direct Conversion of Syngas to C2-Oxygenates with a Multicomponent-Promoted Rh Catalyst
No full textAbstract: Mn and Li promoted Rh catalysts supported on SiO2 with a thin TiO2 layer were synthesized by stepwise incipient wetness impregnation approach. The thin TiO2 layer on the surface of SiO2 was proved to stabilize those small Rh nanoparticles and hinder their agglomeration. The reducibility of Rh on these catalysts depends on Rh particle size as well as the position of manganese oxide, and large Rh nanoparticles with MnO on Rh nanoparticles can be only reduced at an elevated temperature. Catalyst with large Rh particles exhibits a higher CO conversion and higher products selectivity towards long chain hydrocarbons and C2-oxygenates at the expense of decreasing methane formation than a similar catalyst with smaller Rh particles. This was attributed to the synergistic effect of Mn and Li promotion and molar ratio between Rh0 and Rhδ+ sites on the surface of Rh nanoparticles. Moreover, Rh nanoparticles on MnO are proved to be more efficient in promoting hydrogenation of acetaldehyde to ethanol than its counterpart with MnO on Rh nanoparticles. Finally, in order to target high C2-oxygenates selectivity, low reaction temperature together with a low H2/CO ratio in the feed is recommended. Graphic Abstract: [Figure not available: see fulltext.].ChemE/Catalysis EngineeringChemE/O&O groe
Computational Exploration of Rh-III/Rh-V and Rh-III/Rh-I Catalysis in Rhodium(III)-Catalyzed C-H Activation Reactions of N-Phenoxyacetamides with Alkynes
No full textThe selective rhodium-catalyzed functionalization of arenes is greatly facilitated by oxidizing directing groups that, act both as directing groups and internal oxidants. We report density functional theory (B3LYP and M06) investigations on the mechanism of rhodium(III)-catalyzed redox coupling reaction of N-phenoxyacetamides with alkynes. The results elucidated the role of the internal oxidizing directing group, and the role of Rh-III/Rh-I and Rh-III/Rh-V catalysis of C-H functionalizations. A novel Rh-III/Rh-V-Rh-III cycle successfully rationalizes recent experimental observations by Liu and Lu et al. (Liu, G. Angew. Chem. Int. Ed. 2013, 52, 6033) on the reactions of N-phenoxyacetamides with alkynes in different solvents. Natural Bond Orbital (NBO) analysis confirms the identity of Rhy intermediate in the catalytic cycle.National Natural Science Foundation of China [21133002, 21203004]; Shenzhen Peacock Program [KQTD201103]; National Science Foundation of the USA [CHE-1361104]; National Science Foundation under the CCI Center for Selective C-H Functionalization [CHE-1205646]; National Science Foundation [OCI-1053575]SCI(E)[email protected]; [email protected]
Trends in sexual activity and demand for and use of modern contraceptive methods in 74 countries. A retrospective analysis of nationally representative surveys
No full textBackground A quarter of a century ago, two global events—the International Conference on Population and Development
in Cairo, and the Fourth World Conference on Women in Beijing—placed gender equality and reproductive health and
rights at the centre of the development agenda. Progress towards these goals has been slower than hoped. We used
survey data and national-level indicators of social determinants from 74 countries to examine change in satisfaction of
contraceptive need from a contextual perspective.
Methods We searched for individual-level data from repeated nationally representative surveys that included information
on sexual and reproductive health, and created a single dataset by harmonising data from each survey to a standard data
specification. We described the relative timings of sexual initiation, first union (cohabitation or marriage), and first birth
and used logistic regression to show the change in prevalence of sexual activity, demand for contraception, and modern
contraceptive use. We used linear regression to examine country-level associations between the gender development
index and the expected length of time in education for women and the three outcomes: sexual activity, demand for
contraception, and modern contraceptive use. We used principal component analysis to describe countries using a
combination of social-structural and behavioural indicators and assessed how well the components explained countrylevel variation in the proportion of women using contraception with fractional logistic regression.
Findings In 34 of the 74 countries examined, proportions of all women who were sexually active, not wanting to conceive,
and not using a modern contraceptive method decreased over time. Proportions of women who had been sexually active
in the past year changed over time in 43 countries, with increases in 30 countries; demand for contraception increased in
42 countries, and use of a modern method of contraception increased in 37 countries. Increases over time in met need
for contraception were correlated with increases in gender equality and with women’s time in education. Regression
analysis on the principal components showed that country-level variation in met contraceptive need was largely explained
by a single component that combined behavioural and social-contextual variables.
Interpretation Progress towards satisfying demand for contraception should take account of the changing context in
which it is practised. To remove the remaining barriers, policy responses—and therefore research priorities—could
require a stronger focus on social-structural determinants and broader aspects of sexual health.
Funding UNDP/UNFPA/UNICEF/WHO/World Bank Special Programme of Research, Development and Research
Training in Human Reproduction
Catalytic activation of Rh,Pd nanoclusters supported on Ceria
No full textRh,Pd/CeO2 nanocatalysts exhibit excellent activity and stability for ethanol steam reforming (ESR) at 400°C. This thesis encompasses experimental and computational investigations of this nanocatalyst. The activation mechanism of Rh,Pd/CeO2 nanocatalysts under ethanol steam reforming conditions is unclear, though it has been previously suggested that oxidic forms of Rh and Pd initially present on the surface of CeO2-x are reduced to metallic form under the reducing environment of the ESR reaction. Here we report X-Ray Photoelectron Spectroscopy (XPS), X-Ray Absorption Spectroscopy (XAS) and High Resolution Transmission Electron Microscopy (HRTEM) studies on the catalytic activation of Rh,Pd/CeO2 nanocatalysts during ethanol steam reforming (ex-situ), as well as EXAFS and HRTEM studies of catalyst activation under reducing H2 (ex-situ) and in-situ EXAFS under CO atmospheres. The studies reveal the catalyst auto-activates under ESR conditions, with metallic Rh and Pd being the dominant noble metal species present after catalyst activation. By comparison, attempts to activate the catalyst using H2 proved ineffective. XAS and HRTEM showed the formation of the Rh2O3 clusters from RhCl3 after H2 reduction at 400°C. CO reduction showed different behaviour depending on the temperature. Bimetallic Rh-Pd cluster formation occurs within 80 minutes at 400°C during CO reduction. At lower temperatures, no transformation occurred. The metal/support interaction of Rh,Pd/CeO2 is investigated using DFT calculations for tetrahedral M10 clusters on the reduced and oxidised CeO2 support. A differing interaction with the supports is traced to the occupancy of the 4f states of ceria. The metal displays differing behaviour on the support, Pd10 does not show the donation to the support of Rh10. The bonding mechanism was seen to be different for both metal clusters where Pd10 shows a partial ionic mechanism while Rh10 shows only inter-metallic bonding. Rh4Pd6 is found to exhibit a bonding mechanism similar to Rh10. The interfacial Ce 4f display occupancy in the Ce-Rh interaction but remain empty for Ce-Pd interface. Both rhodium containing clusters show significant distortion as compared to their structure in the gas phase where Pd10 does not. The electronic and geometric properties of the interface are presented in delving the stability and activity of the bimetallic clusters
Solvent effects in heterogeneous selective hydrogenation of acetophenone: differences between Rh/C and Rh/Al2O3 catalysts and the superiority of water as a functional solvent
No full textSelective hydrogenation of acetophenone (AP) to 1-phenylethanol (PhE) was investigated over Rh/Al2O3 and Rh/C catalysts in 13 solvents including water and conventional organic solvents. Strong solvent effects on the overall rate of AP conversion were observed in different manners depending on the catalysts used. The conversion obtained is correlated with hydrogen-bond-donation (HBD) capability for Rh/C but with hydrogen-bond-acceptance (HBA) capacity for Rh/Al2O3. The solvent effects should result from interactions between the carbonyl group of AP and the solvent molecules through hydrogen bonding for Rh/C and from those between the solvent molecules and the catalyst surface for Rh/Al2O3 having HBD hydroxyl groups on its surface. Water is the most effective functional solvent in the selective hydrogenation of AP for C and Al2O3-supported Rh catalysts due to its high HBD capability (a) and low HBA capability (beta), respectively. For the hydrogenation with Rh/Al2O3 in water, its large polarity/polarizability index (pi*) may contribute to the high selectivity to PhE
Di-μ-carbonyl-bis-[bis-(triphenyl-phos-phane)rhodium(0)](Rh-Rh) acetone disolvate
No full textThe dirhodium complex, [Rh(2)(C(18)H(15)P)(4)(CO)(2)]·2(CH(3))(2)CO, has crystallographic twofold symmetry and the Rh-Rh distance is 2.6266 (8) Å. The four atoms proximate to each Rh atom [Rh-P = 2.3222 (7) and 2.3283 (8) Å, and Rh-C = 1.961 (3) and 2.045 (3) Å] form a distorted tetra-hedron with large deviations from the putative tetra-hedral angles [r.m.s. deviation = 23 (1)°]. The six angles more closely approximate those of a trigonal bipyramid [r.m.s. deviation = 14 (1)°] with one missing equatorial ligand. The two bridging carbonyl ligands are much more linearly coordinated to one Rh [Rh-C O = 151.0 (2)°] than to the other [127.0 (2)°], and the two Rh(2)CO planes form a dihedral angle of 45.43 (5)°. The two acetone solvent mol-ecules are disordered, and their estimated scattering contribution was subtracted from the observed diffraction data using the SQUEEZE routine in PLATON [Spek (2009 ▶). Acta Cryst.D65, 148-155]
Adverse effects of potassium on NO<sub>x</sub> reduction over Di-Air catalyst (Rh/La-Ce-Zr)
No full textThe influence of potassium in Rh on a lanthium promoted zirconia stablised ceria (CZ) catalysts was studied toward NOxreduction reactivity and selectivity. The results are compared with a Rh/CZ catalyst. The samples were characterised by N2 adsorption, XRD, SEM, ICP, and H2-TPR. The study highlighted the importance of stored NOx regeneration over potassium in determining the overall performance of the Rh/K/CZ catalyst. The NOx stored over Rh/K/CZ in the previous NO gas stream cannot be regenerated sufficiently during the C3H6 gas stream, and stored NOxgradually decreased from one cycle to the next, resulting in deteriorating performance of Rh/K/CZ. Besides, problem of NOx slip, the formation of both NH3 and N2O (selectivities up to 30% for each side product) were observed by the addition of potassium into the Rh/CZ catalyst system, depending on the reaction conditions applied and the severity of the catalyst deactivation.</p
Modified Kinematic Technique for Measuring Pathological Hyperextension and Hypermobility of the Interphalangeal Joints
No full textDynamic finger joint motion is difficult to measure using optical motion analysis techniques due to the limited surface area allowed for adequate marker placement. This paper describes an extension of a previously validated kinematic measurement technique using a reduced surface marker set and outlines the required calculations based on a specific surface marker placement to calculate flexion/extension and hyperextension of the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints. The modified technique has been assessed for accuracy using a series of static reference frames (absolute residual error = ±3.7°, cross correlation between new method and reference frames; r = 0.99). The method was then applied to a small group of participants with rheumatoid arthritis (seven females, one male; mean age = 62.8 years ± 12.04) and illustrated congruent strategies of movement for a participant and a large range of finger joint movement over the sample (5.8–71.1°, smallest to largest active range of motion). This method used alongside the previous paper [1] provides a comprehensive, validated method for calculating 3-D wrist, hand, fingers, and thumb kinematics to date and provides a valuable measurement tool for clinical research
Rh promoted In2O3 as a highly active catalyst for CO2 hydrogenation to methanol
No full textSynthesis of methanol with high selectivity and productivity through hydrogenation of CO2 is highly attractive. This work uses a Rh doped In2O3 catalyst to achieve a high methanol productivity of 1.0 g(MeOH) h(-1) g(cat)(-1) while maintaining the intrinsic high selectivity of pure In2O3. Rh facilitated the dissociation of H-2 leading to creation of oxygen vacancies over the In2O3 surface. In addition, Rh atoms also participated in the activation of CO2 to produce formate species with a low activation barrier as evidenced by DFT calculation. Rh species were atomically dispersed in the In2O3 matrix and were stable during a long term reaction. Under reaction conditions, the surface Rh atoms were reduced and were stabilized by charge transfer from neighbouring In atoms. Our results show that incorporation of atomic Rh species in In2O3 can lead to high methanol productivity by creation of oxygen vacancies as well as Rh centred active sites for CO2 activation
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