25 research outputs found
Effect of Support Particle Size in Steam Reforming of Ethanol over Co/CeO<sub>2</sub> Catalysts
Co catalysts supported on ceria supports with two different
particle
sizes, one in the micro- and the other in the nano-range, were investigated
for their ethanol and ethylene steam reforming performance. Pre- and
post-reaction characterization techniques, including high-resolution
transmission electron microscopy, temperature-programmed oxidation,
dispersion, pore size measurements, in situ X-ray diffraction (XRD)
and X-ray absorption fine structure spectroscopy (XAFS) studies were
performed to examine the reducibility of the catalysts. Steady-state-activity
testing has shown nanoparticles to have a higher reforming activity
for ethanol, but also high ethylene yields. In spite of the high ethylene
yields, catalysts supported on nanoparticles proved to be highly resistant
to coking while the catalysts supported on larger ceria particles
suffered from coke formation. Reforming experiments performed with
ethylene showed significant differences in activity and stability.
Bare supports were also tested for activity and the nanoparticle support
was seen to have high dehydration activity. <i>Operando</i> DRIFTS experiments performed during ESR showed differences in surface
species. Pulse experiments performed to use methanol oxidation as
a probe reaction suggested differences in the relative abundance of
redox sites and basic sites. The bare ceria supports also exhibited
significant activity for ethanol dehydration, but not for C–C
cleavage. The superior performance of the catalysts supported on nanoparticles
is thought to be due to a combination of factors, including increased
reducibility, improved metal dispersion, and a difference in relative
abundance of redox sites on the surface. All of these properties and,
in turn, the catalytic performance, appear to be affected by the particle
size of the support
Effect of Microgravity on Synthesis of Nano Ceria
Cerium oxide (CeO2) was prepared using a controlled-precipitation method under microgravity at the International Space Station (ISS). For comparison, ceria was also synthesized under normal-gravity conditions (referred as control). The Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size analysis results indicated that the ceria particles grown in space had lower surface area and pore volume compared to the control samples. Furthermore, the space samples had a broader pore size distribution ranging from 30–600 Å, whereas the control samples consisted of pore sizes from 30–50 Å range. Structural information of the ceria particles were obtained using TEM and XRD. Based on the TEM images, it was confirmed that the space samples were predominantly nano-rods, on the other hand, only nano-polyhedra particles were seen in the control ceria samples. The average particle size was larger for ceria samples synthesized in space. XRD results showed higher crystallinity as well as larger mean crystal size for the space samples. The effect of sodium hydroxide concentration on synthesis of ceria was also examined using 1 M and 3 M solutions. It was found that the control samples, prepared in 1 M and 3 M sodium hydroxide solutions, did not show a significant difference between the two. However, when the ceria samples were prepared in a more basic medium (3 M) under microgravity, a decrease in the particle size of the nano-rods and appearances of nano-polyhedra and spheres were observed
Quantum Properties of Dichroic Silicon Vacancies in Silicon Carbide
Various defect centers have displayed promise as either quantum applications, single photon emitters or light-matter interfaces. However, the search for an ideal defect with multifunctional ability still remains as open questions. Here, we study the dichroic silicon vacancies in silicon carbide that feature two well-distinguishable zero-phonon lines (ZPLs) and analyze the quantum properties in their optical emission and spin control. It is demonstrated that this center combines 40% optical emission into the ZPLs showing the contrasting difference in optical properties with varying temperature and polarization, and a 100% increase in the fluorescence intensity upon the spin resonance, and long spin coherence time up to 0.6 ms. These results indicate this defect center as a promising system for spin-based quantum applications
Molybdenum Carbides, Active and <i>In Situ</i> Regenerable Catalysts in Hydroprocessing of Fast Pyrolysis Bio-Oil
This paper describes
properties of molybdenum carbides as a potential
catalyst for fast pyrolysis bio-oil hydroprocessing. Currently, high
catalyst cost, short catalyst lifetime, and lack of effective regeneration
methods are hampering the development of this otherwise attractive
renewable hydrocarbon technology. A series of metal-doped bulk Mo
carbides were synthesized, characterized, and evaluated in sequential
low-temperature stabilization and high-temperature deoxygenation of
a pine-derived bio-oil. During a typical 60 h run, Mo carbides were
capable of upgrading raw bio-oil to a level suitable for direct insertion
into the current hydrocarbon infrastructure with residual oxygen content
and total acid number of upgraded oils below 2 wt % and 0.01 mg KOH
g<sup>–1</sup>, respectively. The performance was shown to
be sensitive to the type of metal dopant, Ni-doped Mo carbides outperforming
Co-, Cu-, or Ca-doped counterparts; a higher Ni loading led to a superior
catalytic performance. No bulk oxidation or other significant structural
changes were observed. Besides the structural robustness, another
attractive property of Mo carbides was <i>in situ</i> regenerability.
The effectiveness of regeneration was demonstrated by successfully
carrying out four consecutive 60 h runs with a reductive decoking
between two adjacent runs. These results strongly suggest that Mo
carbides are a good catalyst candidate which could lead to a significant
cost reduction in hydroprocessing bio-oils. We highlight areas for
future research which will be needed to further understand carbide
structure–function relationships and help design practical
bio-oil upgrading catalysts based on Mo carbides
High-Fidelity Spin and Optical Control of Single Silicon-Vacancy Centres in Cilicon Carbide
Scalable quantum networking requires quantum systems with quantum processing capabilities. Solid state spin systems with reliable spin–optical interfaces are a leading hardware in this regard. However, available systems suffer from large electron–phonon interaction or fast spin dephasing. Here, we demonstrate that the negatively charged silicon-vacancy centre in silicon carbide is immune to both drawbacks. Thanks to its 4A2 symmetry in ground andexcited states, optical resonances are stable with near-Fourier-transform-limited linewidths, allowing exploitation of the spin selectivity of the optical transitions. In combination with millisecond-long spin coherence times originating from the high-purity crystal, we demonstrate high-fidelity optical initialization and coherent spin control, which we exploit to show coherent coupling to single nuclear spins with ∼1 kHz resolution. The summary of our findings makes this defect a prime candidate for realising memory-assisted quantum network applicationsusing semiconductor-based spin-to-photon interfaces and coherently coupled nuclear spins