8 research outputs found
Low-temperature water-gas shift on Pt/Ce0.8La0.2O2âδâCNT: The effect of Ce0.8La0.2O2âδ/CNT ratio
Hybrid materials of (100 â x) wt% Ce0.8La0.2O2âδâx wt% CNT composition (x = 0, 7.5, 20.5, 32.5, 44.1 and 100) were synthesized using the urea-assisted co-precipitation method and used as supports of 0.5 wt% Pt toward the low-temperature WGS (LT-WGS) reaction. The main focus of this work was to provide fundamental understanding of the effect of Ce0.8La0.2O2âδ/CNT ratio on the LT-WGS catalytic activity of such materials. It was found that the material containing 44.1 wt% CNT presented the best catalytic activity (kinetic rate and CO conversion), result that is correlated with the following parameters: (i) Pt-CO bond strength (TPD-CO), (ii) extent of dispersion of the Ce0.8La0.2O2âδ phase in the hybrid support system and, thus of the Pt phase; the larger dispersion of the Ce0.8La0.2O2âδ phase had a direct impact on its reducibility ability (labile oxygen species), (iii) concentration of surface Ce3+ species (XPS), indication for an increased concentration of oxygen vacant sites, (iv) PtH bond strength (H2-TPD studies), and (v) concentration of active carbon-containing intermediates, âC-poolâ formed around each Pt nanoparticle (SSITKA studies). WGS kinetic studies at 300 °C revealed that the reaction order with respect to CO was 0.17 and 0.13 for the catalysts containing 20.5 and 44.1 wt% CNT, respectively, while the reaction order with respect to H2O was 1.40 for the latter CNT loading. Oxidation of CNTs over the catalyst containing 44.1 wt% of CNTs occurred at temperatures larger than 400 °C, result of practical importance for the LT-WGS reaction. The proposed WGS reaction mechanism over the present catalytic materials is that of âredoxâ in parallel with the âassociative with âOH group regenerationâ mechanism.The European Regional Development Fund, the Republic of Cyprus and the Research Promotion Foundation of Cyprus are gratefully acknowledged for their financial support through the project TECHNOLOGY/0308(BE)/05.Peer Reviewe
Stability of nanoparticle production by atmospheric-pressure spark ablation
The stability of nanoparticle (NP) production by atmospheric-pressure spark ablation was studied and found to depend on the composition of the electrodes and the carrier gas (here N2 or Ar). For materials that do not react with N2, such as Pd and Ni, NP production was rather stable regardless of the carrier gas employed. In contrast, for materials that can easily produce nitride species (e.g., Al and Mg), both the concentration and size of the resulting NPs exhibited noticeable fluctuations, when ablating them in N2, which are more pronounced when the electrical energy input to the system is low. The variation in concentration and particle size is attributed to the formation of a metal-nitride region on the face of the electrodes where the sparks hit, as a result of its reaction with the carrier gas, altering the electrical and thermal conductivity, and consequently the ablatability of the electrode at that region. This explanation was corroborated by offline analysis of the face surface of the electrodes, showing two chemically distinct regions: one with high content of N and one without. In addition, the concentration of the Al and Mg NPs produced in N2 decreased gradually over time until it reached a plateau after several hours. When using Ar, the fluctuation and decreasing trend in NP production, and consequently the formation of nitride compounds on the face surface of the electrodes, were negligible, providing an effective solution for stable ablation of materials that can easily react with N2.ChemE/Materials for Energy Conversion and StorageAtmospheric Remote Sensin
Water-gas shift reaction on Pt/Ce1- xTixO 2-δ: The effect of Ce/Ti ratio
Pt nanoparticles (1.2â2.0 nm size) supported on Ce1âxTixO2âδ (x = 0, 0.2, 0.5, 0.8, and 1.0) carriers synthesized by the citrate solâgel method were tested toward the waterâgas shift (WGS) reaction in the 200â350 °C range. A deep insight into the effect of two structural parameters, the chemical composition of support (Ce/Ti atom ratio), and the Pt particle size on the catalytic performance of Pt-loaded catalysts was realized after employing in situ X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM) and HAADF/STEM, scanning electron microscopy (SEM), in situ Raman and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopies under different gas atmospheres, H2 temperature-programmed reduction (H2-TPR), and temperature-programmed desorption (NH3-TPD and CO2-TPD) techniques. The 0.5 wt % Pt/Ce0.8Ti0.2O2âδ solid (dPt = 1.7 nm) was found to be by far the best catalyst among all the other solids investigated. In particular, at 250 °C the CO conversion over Pt/Ce0.8Ti0.2O2âδ was increased by a factor of 2.5 and 1.9 compared to Pt/TiO2 and Pt/CeO2, respectively. The catalytic superiority of the Pt/Ce0.8Ti0.2O2âδ solid is the result of the supportâs (i) robust morphology preserved during the WGS reaction, (ii) moderate acidity and basicity, and (iii) better reducibility at lower temperatures and the significant reduction of âcokingâ on the Pt surface and of carbonate accumulation on the Ce0.8Ti0.2O2âδ support. Several of these properties largely influenced the reactivity of sites (k, sâ1) at the Ptâsupport interface. In particular, the specific WGS reaction rate at 200 °C expressed per length of the Ptâsupport interface (Îźmol CO cmâ1 sâ1) was found to be 2.2 and 4.6 times larger on Pt supported on Ce0.8Ti0.2O2âδ (Ti4+-doped CeO2) compared to TiO2 and CeO2 alone, respectively.The European Regional Development Fund, the Republic of Cyprus, the Research Promotion Foundation of Cyprus, and the Research Committee of the University of Cyprus are gratefully acknowledged for their financial support through the project TEXNO/0308(BE)/05. S.B. acknowledges financial support from the COST Action CM1104.Peer Reviewe
Tuning atomic-scale mixing of nanoparticles produced by atmospheric-pressure spark ablation
Nanoparticles (NPs) mixed at the atomic scale have been synthesized by atmospheric-pressure spark ablation using pairs of Pd and Hf electrodes. Gravimetric analysis of the electrodes showed that the fraction of each material in the resulting mixed NPs can be varied from ca. 15-85 at% to 85-15 at% by employing different combinations of electrode polarities and thicknesses. These results were also qualitatively corroborated by microscopy and elemental analysis of the produced NPs. When using pairs of electrodes having the same diameter, the material from the one at negative polarity was represented at a substantially higher fraction in the mixed NPs regardless of whether a pair of thin or thick electrodes were employed. This can be attributed to the higher ablation rate of the electrodes at the negative polarity, as already known from earlier experiments. When using electrodes of different diameters, the fraction of the element from the thinner electrode was always higher. This is because thinner electrodes are ablated more effectively due to, at least in part, the increased importance of the associated heat losses compared to its thicker counterpart. In those cases, the polarity of the electrodes had a significantly smaller effect. Overall, our results demonstrate, for the first time, that spark ablation can be used to control atomic scale mixing and thus produce alloyed NPs with compositions that can be tuned to a good extent by simply using different combinations of electrode diameters and polarities. This expands the capabilities of the technique for producing mixed nanoparticle building blocks of well-defined composition that are highly desired for a wide range of applications.ChemE/Materials for Energy Conversion and StorageAtmospheric Remote Sensin
WaterâGas Shift Reaction on Pt/Ce<sub>1â<i>x</i></sub>Ti<sub><i>x</i></sub>O<sub>2âδ</sub>: The Effect of Ce/Ti Ratio
Pt nanoparticles (1.2â2.0
nm size) supported on Ce<sub>1â<i>x</i></sub>Ti<sub><i>x</i></sub>O<sub>2âδ</sub> (<i>x</i> = 0, 0.2, 0.5, 0.8, and 1.0) carriers synthesized by the citrate
solâgel method were tested toward the waterâgas shift
(WGS) reaction in the 200â350 °C range. A deep insight
into the effect of two structural parameters, the chemical composition
of support (Ce/Ti atom ratio), and the Pt particle size on the catalytic
performance of Pt-loaded catalysts was realized after employing in
situ X-ray diffraction (XRD), high-resolution transmission electron
microscopy (HR-TEM) and HAADF/STEM, scanning electron microscopy (SEM),
in situ Raman and diffuse reflectance infrared Fourier transform (DRIFT)
spectroscopies under different gas atmospheres, H<sub>2</sub> temperature-programmed
reduction (H<sub>2</sub>-TPR), and temperature-programmed desorption
(NH<sub>3</sub>-TPD and CO<sub>2</sub>-TPD) techniques. The 0.5 wt
% Pt/Ce<sub>0.8</sub>Ti<sub>0.2</sub>O<sub>2âδ</sub> solid
(<i>d</i><sub>Pt</sub> = 1.7 nm) was found to be by far
the best catalyst among all the other solids investigated. In particular,
at 250 °C the CO conversion over Pt/Ce<sub>0.8</sub>Ti<sub>0.2</sub>O<sub>2âδ</sub> was increased by a factor of 2.5 and
1.9 compared to Pt/TiO<sub>2</sub> and Pt/CeO<sub>2</sub>, respectively.
The catalytic superiority of the Pt/Ce<sub>0.8</sub>Ti<sub>0.2</sub>O<sub>2âδ</sub> solid is the result of the supportâs
(i) robust morphology preserved during the WGS reaction, (ii) moderate
acidity and basicity, and (iii) better reducibility at lower temperatures
and the significant reduction of âcokingâ on the Pt
surface and of carbonate accumulation on the Ce<sub>0.8</sub>Ti<sub>0.2</sub>O<sub>2âδ</sub> support. Several of these properties
largely influenced the reactivity of sites (<i>k</i>, s<sup>â1</sup>) at the Ptâsupport interface. In particular,
the specific WGS reaction rate at 200 °C expressed per length
of the Ptâsupport interface (Îźmol CO cm<sup>â1</sup> s<sup>â1</sup>) was found to be 2.2 and 4.6 times larger
on Pt supported on Ce<sub>0.8</sub>Ti<sub>0.2</sub>O<sub>2âδ</sub> (Ti<sup>4+</sup>-doped CeO<sub>2</sub>) compared to TiO<sub>2</sub> and CeO<sub>2</sub> alone, respectively
Structural and Redox Properties of Ce<sub>1â<i>x</i></sub>Zr<sub><i>x</i></sub>O<sub>2âδ</sub> and Ce<sub>0.8</sub>Zr<sub>0.15</sub>RE<sub>0.05</sub>O<sub>2âδ</sub> (RE: La, Nd, Pr, Y) Solids Studied by High Temperature <i>in Situ</i> Raman Spectroscopy
<i>In situ</i> Raman spectroscopy at temperatures up
to 450 °C is used to probe the structural and redox properties
of Ce<sub>1â<i>x</i></sub>Zr<sub><i>x</i></sub>O<sub>2âδ</sub> solids (<i>x</i> = 0â0.8)
prepared by the citrate solâgel and coprecipitation with urea
methods. The anionic sublattice structure of the solids is dependent
on the preparation route. The composition effects exhibited by the
Raman spectra are adequate for characterizing the phases present and/or
eventual phase segregations. For <i>x</i> = 0.5 the pseudocubic <i>t</i>âł phase occurs for the solid prepared by the citrate
solâgel method, while phase segregation (cubic, tetragonal)
is evidenced for the corresponding material prepared by the coprecipitation
with urea method. A larger extent of defects and interstitial O atoms
is evidenced for the materials prepared by the citrate solâgel
method. The well-known âdefectâ (âDâ)
band around 600 cm<sup>â1</sup> for CeO<sub>2</sub> as well
as for Ce<sub>1â<i>x</i></sub>Zr<sub><i>x</i></sub>O<sub>2âδ</sub> consists of at least two components:
âD1â above 600 cm<sup>â1</sup> and âD2â
below 600 cm<sup>â1</sup>. Doping of Ce<sub>0.8</sub>Zr<sub>0.2</sub>O<sub>2âδ</sub> with rare earth cations (La<sup>3+</sup>, Nd<sup>3+</sup>, Y<sup>3+</sup>, Pr<sup>3+</sup>) results
in strengthening of the âD2â band that, however, is
found to be insensitive under reducing conditions of flowing 5% H<sub>2</sub>/He at 450 °C. A novel approach based on sequential <i>in situ</i> Raman spectra under alternating oxidizing (20% O<sub>2</sub>/He) and reducing (5% H<sub>2</sub>/He) gas atmospheres showed
that the âD1â band is selectively attenuated under reducing
conditions at 450 °C and is therefore assigned to a metalâoxygen
vibrational mode involving interstitial oxygen atoms that can be delivered
under suitable conditions. A reversible temperature-dependent evolution
of the anionic sublattice structures of Ce<sub>1â<i>x</i></sub>Zr<sub><i>x</i></sub>O<sub>2âδ</sub> solids is evidenced by <i>in situ</i> Raman spectroscopy.
The results are corroborated by powder XRD and oxygen storage capacity
measurements, and observed structure/function relationships are discussed.
It is shown that at low temperatures (e.g., 450 °C) the function
of oxygen release and refill is based on a mechanism involving oxygen
atoms in interstitial sites rather than on defects induced by hetrovalent
M<sup>4+</sup>â RE<sup>3+</sup> doping, the latter improving
the pertinent function at high (e.g., >600 °C) temperatures