45 research outputs found
Perkembangan Teori Sewa Tanah dalam Perspektif Pemikiran Ekonomi
A history of Land Rent Theorities have several opinions, as mazhab of Physiocratic, classical tradition, and new. The different opponions can be understanding for knowing two factors that land value increasingly location to central bussines and fertile soil
Tracking Rh Atoms in Zeolite HY: First Steps of Metal Cluster Formation and Influence of Metal Nuclearity on Catalysis of Ethylene Hydrogenation and Ethylene Dimerization
The
initial steps of rhodium cluster formation from zeolite-supported
mononuclear RhÂ(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> complexes in
H<sub>2</sub> at 373 K and 1 bar were investigated by infrared and
extended X-ray absorption fine structure spectroscopies and scanning
transmission electron microscopy (STEM). The data show that ethylene
ligands on the rhodium react with H<sub>2</sub> to give supported
rhodium hydrides and trigger the formation of rhodium clusters. STEM
provided the first images of the smallest rhodium clusters (Rh<sub>2</sub>) and their further conversion into larger clusters. The samples
were investigated in a plug-flow reactor as catalysts for the conversion
of ethylene + H<sub>2</sub> in a molar ratio of 4:1 at 1 bar and 298
K, with the results showing how the changes in catalyst structure
affect the activity and selectivity; the rhodium clusters are more
active for hydrogenation of ethylene than the single-site complexes,
which are more selective for dimerization of ethylene to give butenes
Hydrogen Activation and Metal Hydride Formation Trigger Cluster Formation from Supported Iridium Complexes
The formation of iridium clusters from supported mononuclear
iridium
complexes in H<sub>2</sub> at 300 K and 1 bar was investigated by
spectroscopy and atomic-resolution scanning transmission electron
microscopy. The first steps of cluster formation from zeolite-supported
IrÂ(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> complexes are triggered
by the activation of H<sub>2</sub> and the formation of iridium hydride,
accompanied by the breaking of iridiumâsupport bonds. This
reactivity can be controlled by the choice of ligands on the iridium,
which include the support
Oxide- and Zeolite-Supported Isostructural Ir(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> Complexes: Molecular-Level Observations of Electronic Effects of Supports as Ligands
Zeolite Hβ- and γ-Al<sub>2</sub>O<sub>3</sub>-supported
mononuclear iridium complexes were synthesized by the reaction of
IrÂ(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>(acac) (acac is acetylacetonate)
with each of the supports. The characterization of the surface species
by extended X-ray absorption fine structure (EXAFS) and infrared (IR)
spectroscopies demonstrated the removal of acac ligands during chemisorption,
leading to the formation of essentially isostructural IrÂ(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> complexes anchored to each support by
two IrâO<sub>support</sub> bonds. Atomic-resolution aberration-corrected
scanning transmission electron microscopy (STEM) images confirm the
spectra, showing only isolated Ir atoms on the supports with no evidence
of iridium clusters. These samples, together with previously reported
IrÂ(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> complexes on zeolite HY,
zeolite HSSZ-53, and MgO supports, constitute a family of isostructural
supported iridium complexes. Treatment with CO led to the replacement
of the ethylene ligands on iridium with CO ligands, and the ν<sub>CO</sub> frequencies of these complexes and white line intensities
in the X-ray absorption spectra at the Ir L<sub>III</sub> edge show
that the electron density on iridium increases in the following order
on these supports: zeolite HY < zeolite Hβ < zeolite HSSZ-53
⪠γ-Al<sub>2</sub>O<sub>3</sub> < MgO. The IR spectra
of the iridium carbonyl complexes treated in flowing C<sub>2</sub>H<sub>4</sub> show that the CO ligands were replaced by C<sub>2</sub>H<sub>4</sub>, with the average number of C<sub>2</sub>H<sub>4</sub> groups per Ir atom increasing as the amount of iridium was increasingly
electron-deficient. In contrast to the typical supported catalysts
incorporating metal clusters or particles that are highly nonuniform,
the samples reported here, incorporating uniform isostructural iridium
complexes, provide unprecedented opportunities for a molecular-level
understanding of how supports affect the electronic properties, reactivities,
and catalytic properties of supported metal species
Photocatalytic Water Splitting with Suspended Calcium Niobium Oxides: Why Nanoscale is Better than Bulk â A Kinetic Analysis
The layered DionâJacobson phase KCa<sub>2</sub>Nb<sub>3</sub>O<sub>10</sub> is known to catalyze photochemical water
reduction
and oxidation under UV light in the presence of sacrificial agents.
The same reactions are catalyzed by tetrabutylammonium hydroxide-supported
HCa<sub>2</sub>Nb<sub>3</sub>O<sub>10</sub> nanosheets obtained by
chemical exfoliation of the parent phase. Here we describe a factorial
study into the effects of nanoscaling, sacrificial charge donors,
cocatalysts,
and cocatalyst deposition conditions on the activity of these catalysts.
In water,
nanoscaling leads to a 16-fold increase in H<sub>2</sub> evolution
and an 8-fold increase in O<sub>2</sub> evolution over the bulk phase
under the same conditions.
The sacrificial electron donor methanol improves H<sub>2</sub> production
by 2â3 orders of magnitude to 20â30 mmol of H<sub>2</sub>/h/g, while the electron acceptor AgNO<sub>3</sub> increases O<sub>2</sub> production to 400 Îźmol
of O<sub>2</sub>/h/g. Rates for H<sub>2</sub> and O<sub>2</sub> evolution
further depend on the presence of cocatalysts (Pt or IrO<sub><i>x</i></sub>) and, in the case of H<sub>2</sub>, inversely
on their particle size. To rationalize these findings and the increased
activity of the nanoscale particles, we propose a kinetic model for
photocatalysis with semiconductor particles. The model calculates
the electronic rate of the catalysts as a product of terms for charge
generation, charge and mass transport, chemical conversion, and charge
recombination. The analysis shows that the activity of the catalysts
is limited mainly by the kinetics of the redox reactions and by the
rate of charge transport to the waterâcatalyst interface. Mass
transport in the solution phase does not play a major role, and neither
does surface charge recombination
Ir<sub>6</sub> Clusters Compartmentalized in the Supercages of Zeolite NaY: Direct Imaging of a Catalyst with Aberration-Corrected Scanning Transmission Electron Microscopy
By use of the precursor Ir(CO)<sub>2</sub>(acac) (acac is acetylacetonate), a ship-in-a-bottle synthesis was used to prepare Ir<sub>6</sub>(CO)<sub>16</sub> and, by decarbonylation, clusters well approximated as Ir<sub>6</sub> in the supercages of zeolite NaY. The samples were characterized by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies and imaged by aberration-corrected scanning transmission electron microscopy with a high-dose electron beam (âź10<sup>8</sup> e<sup>â</sup>/Ă
Ě<sup>2</sup>, 200 kV), and the catalyst performance was characterized by turnover frequencies for ethene hydrogenation at 298 K and atmospheric pressure. The images characterizing a sample with about 17% of the supercages occupied by decarbonylated nanoclusters indicated clusters well approximated as Ir<sub>6</sub>, with diameters consistent with such clusters, and some of the images show the clusters with atomic resolution and indicating each of the 6 Ir atoms. The cluster size was confirmed by EXAFS spectra. Two bonding positions of the Ir<sub>6</sub> clusters in the supercages were distinguished; 25% of the clusters were present at T5 sites and 75% at T6 sites. The results represent the first example of the application of high-dose electron beam conditions to image metal nanoclusters in a nanoporous material; the data are characterized by a high signal-to-noise ratio, and their interpretation does not require any image processing or simulations. These statements are based on images determined in the first 5 s of exposure of the catalyst to the electron beam; thereafter, the electron beam caused measurable deterioration of the zeolite framework and thereupon aggregation of the iridium clusters
Atomically Resolved Site-Isolated Catalyst on MgO: Mononuclear Osmium Dicarbonyls formed from Os<sub>3</sub>(CO)<sub>12</sub>
Supported triosmium clusters, formed from Os<sub>3</sub>(CO)<sub>12</sub> on MgO, were treated in helium at 548 K for 2 h, causing
fragmentation of the cluster frame and the formation of mononuclear
osmium dicarbonyls. The cluster breakup and the resultant fragmented
species were characterized by infrared and X-ray absorption spectroscopies,
and the fragmented species were imaged by scanning transmission electron
microscopy. The spectra identify the surface osmium complexes as OsÂ(CO)<sub>2</sub>{O<sub>support</sub>}<sub><i>n</i></sub> (<i>n</i> = 3 or 4) (where the braces denote support surface atoms).
The images show site-isolated Os atoms in mononuclear osmium species
on MgO. The intensity analysis on the images of the MgO(110) face
showed that the Os atoms were located atop Mg columns. This information
led to a model of the OsÂ(CO)<sub>2</sub> on MgO(110), with the distances
approximated as those determined by EXAFS spectroscopy, which are
an average over the whole MgO surface; the results imply that these
complexes were located at Mg vacancies
Direct <i>in Situ</i> Determination of the Mechanisms Controlling Nanoparticle Nucleation and Growth
Although nanocrystal morphology is controllable using conventional colloidal synthesis, multiple characterization techniques are typically needed to determine key properties like the nucleation rate, induction time, growth rate, and the resulting morphology. Recently, researchers have demonstrated growth of nanocrystals by <i>in situ</i> electron beam reduction, offering direct observations of single nanocrystals and eliminating the need for multiple characterization techniques; however, they found nanocrystal morphologies consistent with two different growth mechanisms for the same electron beam parameters. Here we show that the electron beam current plays a role analogous to the concentration of reducing agent in conventional synthesis, by controlling the growth mechanism and final morphology of silver nanocrystals grown via <i>in situ</i> electron beam reduction. We demonstrate that low beam currents encourage reaction limited growth that yield nanocrystals with faceted structures, while higher beam currents encourage diffusion limited growth that yield spherical nanocrystals. By isolating these two growth regimes, we demonstrate a new level of control over nanocrystal morphology, regulated by the fundamental growth mechanism. We find that the induction threshold dose for nucleation is independent of the beam current, pixel dwell time, and magnification being used. Our results indicate that <i>in situ</i> electron microscopy data can be interpreted by classical models and that systematic dose experiments should be performed for all future <i>in situ</i> liquid studies to confirm the exact mechanisms underlying observations of nucleation and growth
Quality of Graphite Target for Biological/Biomedical/Environmental Applications of <sup>14</sup>C-Accelerator Mass Spectrometry
Catalytic graphitization for <sup>14</sup>C-accelerator mass spectrometry (<sup>14</sup>C-AMS) produced various forms of elemental carbon. Our high-throughput Zn reduction method (C/Fe = 1:5, 500 °C, 3 h) produced the AMS target of graphite-coated iron powder (GCIP), a mix of nongraphitic carbon and Fe<sub>3</sub>C. Crystallinity of the AMS targets of GCIP (nongraphitic carbon) was increased to turbostratic carbon by raising the C/Fe ratio from 1:5 to 1:1 and the graphitization temperature from 500 to 585 °C. The AMS target of GCIP containing turbostratic carbon had a large isotopic fractionation and a low AMS ion current. The AMS target of GCIP containing turbostratic carbon also yielded less accurate/precise <sup>14</sup>C-AMS measurements because of the lower graphitization yield and lower thermal conductivity that were caused by the higher C/Fe ratio of 1:1. On the other hand, the AMS target of GCIP containing nongraphitic carbon had higher graphitization yield and better thermal conductivity over the AMS target of GCIP containing turbostratic carbon due to optimal surface area provided by the iron powder. Finally, graphitization yield and thermal conductivity were stronger determinants (over graphite crystallinity) for accurate/precise/high-throughput biological, biomedical, and environmental<sup>14</sup>C-AMS applications such as absorption, distribution, metabolism, elimination (ADME), and physiologically based pharmacokinetics (PBPK) of nutrients, drugs, phytochemicals, and environmental chemicals
Direct Observation of Aggregative Nanoparticle Growth: Kinetic Modeling of the Size Distribution and Growth Rate
Direct observations of solution-phase
nanoparticle growth using
in situ liquid transmission electron microscopy (TEM) have demonstrated
the importance of ânon-classicalâ growth mechanisms,
such as aggregation and coalescence, on the growth and final morphology
of nanocrystals at the atomic and single nanoparticle scales. To date,
groups have quantitatively interpreted the mean growth rate of nanoparticles
in terms of the LifshitzâSlyozovâWagner (LSW) model
for Ostwald ripening, but less attention has been paid to modeling
the corresponding particle size distribution. Here we use in situ
fluid stage scanning TEM to demonstrate that silver nanoparticles
grow by a length-scale dependent mechanism, where individual nanoparticles
grow by monomer attachment but ensemble-scale growth is dominated
by aggregation. Although our observed mean nanoparticle growth rate
is consistent with the LSW model, we show that the corresponding particle
size distribution is broader and more symmetric than predicted by
LSW. Following direct observations of aggregation, we interpret the
ensemble-scale growth using Smoluchowski kinetics and demonstrate
that the Smoluchowski model quantitatively captures the mean growth
rate and particle size distribution