1,136 research outputs found
In Situ Surface Studies of Site-Isolated Hydrogenation Catalysts – The Intermetallic Compound PdGa
Selective acetylene hydrogenation is an important method for removing traces of acetylene in the ethylene feed for the production of polyethylene. Typical catalysts, like Pd dispersed on metal oxides are widely used for this reaction and show a limited selectivity and long-term stability. This can be attributed to the presence of active-sites ensembles on the catalyst surface. This drawback can be overcome by using the intermetallic compound PdGa which possesses palladium atoms in the crystal structure well isolated from each other by a gallium shell. PdGa shows higher selectivity and increased long-term stability compared to the commercial catalysts, including PdAg alloys. In the present work the surface of the intermetallic compound PdGa was probed by in situ XPS as well as CO adsorption using FTIR spectroscopy. The XPS investigation before hydrogenation revealed a significant modification of the Pd electronic state in the intermetallic compound compared to Pd metal: the Pd3d5/2 peak is shifted by 1 eV to higher binding energy. In situ XPS measurements, performed at ~1 mbar pressure, showed a high stability of the Pd surface states without appearance of any additional components or significant shifts of the Pd3d5/2 peak when applying the reactive atmosphere and temperature (1.0 mbar of H2 + 0.1 mbar of C2H2 at 120 ºC). This is in contrast to Pd metal for which the formation of an additional Pd component during alkyne hydrogenation was detected. Investigation of carbon and palladium depth profiles for PdGa indicates the absence of a subsurface carbon-containing phase, distinguishing this material decidedly from metallic palladium catalysts. The adsorption of CO on the PdGa compound at room temperature results in the appearance of only one band with a maximum at 2047 cm-1, which corresponds to linear Pd–CO carbonyls. It should be mentioned that the observed band (2047 cm–1) is shifted to lower wavenumbers compared to the respective CO (on-top) species forming upon adsorption on metallic palladium (2100-2080 cm–1), which is an indication for the modification of the Pd electronic states by covalent bonding in the investigated intermetallic compound. The absence of bands due to bridged carbonyls in the observed spectra and the fact that the observed band is not coverage dependent indicated that the active sites in PdGa are really isolated. Characterization of PdGa by FTIR and in situ XPS revealed high surface stability during the reaction of acetylene hydrogenation and confirms the isolation of the active Pd site on the surface. In combination with modified electronic Pd states due to covalent bonding in the intermetallic compound it leads to superior catalytic properties like high selectivity and long-term stability during the partial hydrogenation of acetylene
In Situ Studies of Site-Isolated Hydrogenation Catalysts – The Intermetallic Compound PdGa
Selective acetylene hydrogenation is an important method for removing traces of acetylene in the ethylene feed for the production of polyethylene. Typical catalysts show a limited selectivity and long-term stability. This can be attributed to the presence of active-site ensembles. This drawback can be overcome by using the intermetallic compound PdGa which possesses palladium atoms in the crystal structure well isolated by a gallium shell. PdGa shows higher selectivity and increased long-term stability compared to commercial catalysts. The XPS investigation before the reaction revealed a significant modification of the Pd electronic state in the intermetallic compound compared to Pd metal: the Pd3d5/2 peak is shifted by 1 eV to higher binding energy. In situ XPS measurements showed a high stability of the Pd surface states without appearance of any additional components or significant shifts of the Pd3d5/2 peak when applying the reactive atmosphere and temperature (1.0 mbar H2 0.1 mbar C2H2 at 120 ºC). This is in contrast to Pd metal for which the formation of an additional Pd component during alkyne hydrogenation was reported recently. The adsorption of CO on PdGa at room temperature results in the appearance of only one band with a maximum at 2047 cm-1, which should correspond to linearly bound CO (Pd–CO). It should be mentioned that the observed band (2047 cm–1) is shifted to lower wavenumbers compared to the respective CO (on-top) species forming upon adsorption on metallic palladium (2100-2080 cm–1), which may be an indication for the modification of the Pd electronic states by covalent bonding in the investigated intermetallic compound. The absence of bands due to bridged carbonyls in the spectra and the fact that the observed band is not coverage dependent indicates that the active sites in PdGa are really isolated. Characterization of PdGa revealed high surface stability during the hydrogenation of acetylene and confirms the isolation of the active Pd sites on the surface. In combination with the modified electronic Pd states – perhaps due to the covalent bonding – it leads to superior catalytic properties high selectivity and long-term stability
Perturbations in the relaxation mechanism for a large cosmological constant
Recently, a mechanism for relaxing a large cosmological constant (CC) has
been proposed [arxiv:0902.2215], which permits solutions with low Hubble rates
at late times without fine-tuning. The setup is implemented in the LXCDM
framework, and we found a reasonable cosmological background evolution similar
to the LCDM model with a fine-tuned CC. In this work we analyse analytically
the perturbations in this relaxation model, and we show that their evolution is
also similar to the LCDM model, especially in the matter era. Some tracking
properties of the vacuum energy are discussed, too.Comment: 18 pages, LaTeX; discussion improved, accepted by CQ
Rodrigues Formula for the Nonsymmetric Multivariable Laguerre Polynomial
Extending a method developed by Takamura and Takano, we present the Rodrigues
formula for the nonsymmetric multivariable Laguerre polynomials which form the
orthogonal basis for the -type Calogero model with distinguishable
particles. Our construction makes it possible for the first time to
algebraically generate all the nonsymmetric multivariable Laguerre polynomials
with different parities for each variable.Comment: 6 pages, LaTe
The decomposition of level-1 irreducible highest weight modules with respect to the level-0 actions of the quantum affine algebra
We decompose the level-1 irreducible highest weight modules of the quantum
affine algebra with respect to the level-0 --action defined in q-alg/9702024. The decomposition is
parameterized by the skew Young diagrams of the border strip type.Comment: 22 pages, AMSLaTe
Spatially Resolved Near-Infrared Spectroscopy of Seyfert 2 Galaxies Mk 1066, NGC 2110, NGC 4388, and Mk 3
We present near-infrared spectra with resolutions of lambda/dlambda~1200 in
the emission lines of Pa-beta, [FeII] (1.2567um), Br-gamma, and H2 v=1-0S(1) of
the nuclei and circumnuclear regions of the four Seyfert 2 galaxies Mk 1066,
NGC 2110, NGC 4388, and Mk 3. All of these galaxies show strong near-infrared
line emission that is detected at radii several times the spatial resolution,
corresponding to projected physical scales of 0.07 to 0.7 kpc. Velocity
gradients are detected in these nuclei, as are spatial variations in line
profiles and flux ratios. We compare the spatial and velocity distribution of
the line emission to previously observed optical line and radio emission. The
evidence indicates that the [FeII] emission is associated with the Seyfert
activity in the galaxies. Our data are consistent with X-ray heating being
responsible for most of the [FeII] emission, although differences in [FeII] and
Pa-beta line profiles associated with radio emission suggests that the [FeII]
emission is enhanced by fast shocks associated with radio outflows. The H2
emission is not as strongly associated with outflows or ionization cones as is
the emission in other lines, but rather appears to be primarily associated with
the disk of the galaxy.Comment: 35 pages, 24 figure
Métodos de análise de dados longitudinais para o melhoramento genético da pinha.
O objetivo deste trabalho foi comparar formas de análise de medidas repetidas para o melhoramento da produção de frutos de pinha (Annona squamosa). Vinte progênies de meias?irmãs foram avaliadas por três anos (2003, 2004 e 2005) em delineamento de blocos ao acaso, com cinco repetições, com cada parcela constituÃda de quatro plantas. A caracterÃstica avaliada foi o número de frutos por indivÃduo. Os modelos de simetria composta, de simetria composta com variâncias heterogêneas, autorregressivo com variâncias heterogêneas, e antedependência estruturada, foram analisados com o programa ASReml. A estimação dos componentes de variância e a predição dos valores genéticos foram feitas com o procedimento REML/BLUP. A comparação dos modelos foi realizada pelo teste de razão de verossimilhança e pelo critério de Akaike. O modelo antedependência estruturada, para os fatores progênie e parcela, e o modelo multivariado, para o fator resÃduo, são as melhores abordagens para a análise dos dados, pois propiciam eficiência e parcimônia em relação ao modelo multivariado completo. Com o modelo antedependência estruturada, é possÃvel a identificação de famÃlias superiores, em cada colheita, e também de famÃlias com maior número total de frutos
Rodrigues Formula for the Nonsymmetric Multivariable Hermite Polynomial
Applying a method developed by Takamura and Takano for the nonsymmetric Jack
polynomial, we present the Rodrigues formula for the nonsymmetric multivariable
Hermite polynomial.Comment: 5 pages, LaTe
In situ Surface Characterization of the Intermetallic Compound PdGa – A Highly Selective Hydrogenation Catalyst
The structurally well-defined intermetallic compound PdGa – a highly selective catalyst for the semi-hydrogenation of acetylene – was characterized by Fourier transform infrared spectroscopy (FTIR) in situ X-ray photoelectron spectroscopy and in situ Prompt gamma activation analysis. A strong modification of the electronic states in PdGa compared to elemental Pd was revealed as well as the complete isolation of the Pd atoms on the surface of PdGa. In situ investigations proved the high stability of the surface, thus excluding segregation phenomena (common for alloys) or sub-surface chemistry involving C and/or H atoms (known for elemental Pd). By suppressing the sub-surface chemistry, the electronic modification as well as the site isolation lead to the high selectivity and long-term stability of PdGa in the semi-hydrogenation of acetylene
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