D. Ciuparu - Academia.edu (original) (raw)
Papers by D. Ciuparu
Des catalyseurs a base de palladium supporte sur des oxydes a caractere basique (oxyde de magnesi... more Des catalyseurs a base de palladium supporte sur des oxydes a caractere basique (oxyde de magnesium) ou faiblement acido-basique (dioxyde de zirconium) sont prepares, caracterises et testes en reformage catalytique (pour la premiere fois dans le cas de supports contenant de la zircone). La zircone est elaboree suivant une methode sol-gel originale : l'hydrolyse de l'ion zr 4 + par l'hydrazine suivie de la digestion du precurseur oxyhydroxyde. Ce procede conduit a un oxyde de grande surface specifique (plus de 200 m 2. G - 1), manifestant une excellente resistance au frittage et gardant la structure tetragonale au-dessus de 1100\c ; l'introduction de mgo en solution solide entraine l'apparition de zircone cubique. La spectroscopie de reflexion diffuse (uv-visible et pir) et la resonance paramagnetique electronique (rpe) permettent d'identifier les defauts (zr 3 + et lacunes d'oxygene v o) dans la zircone traitee en atmosphere reductrice et de preciser l...
Journal of Catalysis, 2011
The Journal of Physical Chemistry B, 2004
The mechanisms controlling the reduction of cobalt and the growth of metallic cobalt clusters in ... more The mechanisms controlling the reduction of cobalt and the growth of metallic cobalt clusters in Co-MCM-41 catalysts at different stages in the process of single-wall carbon nanotubes (SWNT) synthesis were investigated both by in-situ and ex-situ X-ray absorption spectroscopy. We have found that prereduction of the catalyst in hydrogen at temperatures below 700°C does not reduce the cobalt ions to metallic cobalt, but removes hydroxyl groups and oxygen ions creating oxygen vacancies and/or a partially reduced cobalt species. The prereduction treatment, however, does increase the density of electrons at the Fermi level weakening the interaction of Co 2+ with the silica framework. Subsequent exposure of the catalyst to CO at 750°C causes CO to strongly interact with the cobalt clusters most likely by the transfer of electrons into the d orbitals of Co. This strong interaction makes the cobalt more mobile at the surface and allows it to nucleate into clusters capable of dissociating CO and initiating the growth of SWNT. Prior to exposure to CO, the reduced cobalt species strongly interacting with the silica framework do not nucleate into larger clusters in the presence of He or H 2 , preserving near atomic cobalt dispersion, as determined by EXAFS.
Applied Catalysis B: Environmental, 2012
The effect of electrochemical promotion of catalysis (NEMCA effect or EPOC) has been studied for ... more The effect of electrochemical promotion of catalysis (NEMCA effect or EPOC) has been studied for the methane oxidation reaction over Rh catalytic films interfaced with YSZ, an oxygen ion conductor, at temperatures from 350 to 550 • C, under reducing, stoichiometric and oxidizing conditions. CO 2 is the main reaction product; however, CO is produced in small amounts at high temperatures. The effect of electrochemical promotion of catalysis on the reaction catalytic rate has been found to decrease by increasing partial pressure of oxygen and temperature. Under reducing conditions, at 430 • C, positive current application can cause a 3-fold increase of the catalytic rate, while the apparent Faradaic efficiency is 170. After positive current interruption the catalytic rate reversibly returns to the initial open-circuit state value. Negative current application results in a 57% decrease of the catalytic rate with an apparent Faradaic efficiency equal to 40. After negative current interruption the catalytic rate slowly increases but remains lower than the initial value. This permanent poisoning effect has been interpreted by formation of a surface oxide layer by the strongly adsorbed oxygen from the gas phase upon negative polarization. A poisoning index, , has been defined to quantify the magnitude of the effect. Moreover, under stoichiometric conditions, a periodical oscillation of the rate is observed both under open-circuit and polarization conditions in a narrow temperature window between 480 and 520 • C, which is attributed to catalyst phase transition phenomena.
Recent Contributions to Physics
In this paper, calculated the solubility of fullerite in order to clarify the features of the tem... more In this paper, calculated the solubility of fullerite in order to clarify the features of the temperature dependence in the process. This takes into account the implementation in different temperature ranges of the crystalline structures of fullerite formed by fullerene molecules of various modifications. Experimental studies of solutions of fullerene molecules in various organic media made it possible to establish the features of the temperature dependence of the solubility of fullerite C 60. Instead of the expected increase in solubility with increasing temperature, the extremes of its temperature dependence with a maximum at room temperature were revealed, the endothermicity of the dissolution process was established. The anomalies of the temperature dependence of the solubility of fullerenes C 60 in organic matrices are due to a change in the structure of bonds in the framework of fullerene molecules, which, in turn, affect the structure of solid-phase fullerite.More accurate results using the formulas obtained can be calculated if the true values of the energy parameters are known from independent experiments. However, it can be assumed that the nature of the revealed patterns will remain.
MRS Proceedings, 2004
ABSTRACTWe report flexible synthesis of III-Nitride nanowires and heterostructures by metal-organ... more ABSTRACTWe report flexible synthesis of III-Nitride nanowires and heterostructures by metal-organic chemical vapor deposition (MOCVD) via a catalytic vapor-liquid-solid (VLS) growth mechanism. Indium is used as an in-situ catalyst to facilitate and sustain the stability of liquid phase droplet for VLS growth based on thermodynamic consideration. The employment of mesoporous molecular sieves (MCM-41) helps to prevent the coalescence of catalyst droplets and to promote nucleation statistics. Cathodoluminescence (CL) of GaN nanowires shows near band-edge emission at 370nm, and strong E2 phonon peak is observed at room temperature in Raman scattering spectra. Both binary GaN and AlN nanowires have been synthesized by MOCVD. Three-dimensional AlN/GaN trunk-branch nanostructures are reported to illustrate the versatility of incorporating the VLS mechanism into MOCVD process.
The Journal of Physical Chemistry B, 2004
Cobalt silicate was studied as a model catalyst to seek an understanding of why Co-MCM-41 catalys... more Cobalt silicate was studied as a model catalyst to seek an understanding of why Co-MCM-41 catalyst is not active for single wall carbon nanotubes (SWNT) upon treatment at high temperature. Cobalt orthosilicate was formed during high-temperature pretreatment of Co-...
The Journal of Physical Chemistry B, 2006
The effect of initial synthesis solution pH and tetramethylammonium silicate concentration in the... more The effect of initial synthesis solution pH and tetramethylammonium silicate concentration in the synthesis solution on the physical and chemical properties of MCM-41 was systematically investigated using N(2) physisorption, X-ray diffraction, temperature-programmed reduction, in situ Fourier transform IR, UV-vis, and X-ray absorption spectroscopies. pH and tetramethylammonium (TMA) fraction affect the porosity of MCM-41 and the reducibility of incorporated Ni cations; higher pH and TMA concentration produced more porosity with higher stability against reduction, which is attributed to more metal ions locating in the interior of the silica walls. The control of the pore diameter of mesoporous MCM-41 at the sub-nanometer scale may be accomplished by adjusting the pH and TMA fraction. pH may be used to control the surface free silanol group density and nickel reduction degree as well, and this is useful in the design of a specific catalyst for particular reactions, such as CO methanation, which requires highly dispersed, stable metallic clusters with controllable size.
Des catalyseurs a base de palladium supporte sur des oxydes a caractere basique (oxyde de magnesi... more Des catalyseurs a base de palladium supporte sur des oxydes a caractere basique (oxyde de magnesium) ou faiblement acido-basique (dioxyde de zirconium) sont prepares, caracterises et testes en reformage catalytique (pour la premiere fois dans le cas de supports contenant de la zircone). La zircone est elaboree suivant une methode sol-gel originale : l'hydrolyse de l'ion zr 4 + par l'hydrazine suivie de la digestion du precurseur oxyhydroxyde. Ce procede conduit a un oxyde de grande surface specifique (plus de 200 m 2. G - 1), manifestant une excellente resistance au frittage et gardant la structure tetragonale au-dessus de 1100\c ; l'introduction de mgo en solution solide entraine l'apparition de zircone cubique. La spectroscopie de reflexion diffuse (uv-visible et pir) et la resonance paramagnetique electronique (rpe) permettent d'identifier les defauts (zr 3 + et lacunes d'oxygene v o) dans la zircone traitee en atmosphere reductrice et de preciser l...
Journal of Catalysis, 2011
The Journal of Physical Chemistry B, 2004
The mechanisms controlling the reduction of cobalt and the growth of metallic cobalt clusters in ... more The mechanisms controlling the reduction of cobalt and the growth of metallic cobalt clusters in Co-MCM-41 catalysts at different stages in the process of single-wall carbon nanotubes (SWNT) synthesis were investigated both by in-situ and ex-situ X-ray absorption spectroscopy. We have found that prereduction of the catalyst in hydrogen at temperatures below 700°C does not reduce the cobalt ions to metallic cobalt, but removes hydroxyl groups and oxygen ions creating oxygen vacancies and/or a partially reduced cobalt species. The prereduction treatment, however, does increase the density of electrons at the Fermi level weakening the interaction of Co 2+ with the silica framework. Subsequent exposure of the catalyst to CO at 750°C causes CO to strongly interact with the cobalt clusters most likely by the transfer of electrons into the d orbitals of Co. This strong interaction makes the cobalt more mobile at the surface and allows it to nucleate into clusters capable of dissociating CO and initiating the growth of SWNT. Prior to exposure to CO, the reduced cobalt species strongly interacting with the silica framework do not nucleate into larger clusters in the presence of He or H 2 , preserving near atomic cobalt dispersion, as determined by EXAFS.
Applied Catalysis B: Environmental, 2012
The effect of electrochemical promotion of catalysis (NEMCA effect or EPOC) has been studied for ... more The effect of electrochemical promotion of catalysis (NEMCA effect or EPOC) has been studied for the methane oxidation reaction over Rh catalytic films interfaced with YSZ, an oxygen ion conductor, at temperatures from 350 to 550 • C, under reducing, stoichiometric and oxidizing conditions. CO 2 is the main reaction product; however, CO is produced in small amounts at high temperatures. The effect of electrochemical promotion of catalysis on the reaction catalytic rate has been found to decrease by increasing partial pressure of oxygen and temperature. Under reducing conditions, at 430 • C, positive current application can cause a 3-fold increase of the catalytic rate, while the apparent Faradaic efficiency is 170. After positive current interruption the catalytic rate reversibly returns to the initial open-circuit state value. Negative current application results in a 57% decrease of the catalytic rate with an apparent Faradaic efficiency equal to 40. After negative current interruption the catalytic rate slowly increases but remains lower than the initial value. This permanent poisoning effect has been interpreted by formation of a surface oxide layer by the strongly adsorbed oxygen from the gas phase upon negative polarization. A poisoning index, , has been defined to quantify the magnitude of the effect. Moreover, under stoichiometric conditions, a periodical oscillation of the rate is observed both under open-circuit and polarization conditions in a narrow temperature window between 480 and 520 • C, which is attributed to catalyst phase transition phenomena.
Recent Contributions to Physics
In this paper, calculated the solubility of fullerite in order to clarify the features of the tem... more In this paper, calculated the solubility of fullerite in order to clarify the features of the temperature dependence in the process. This takes into account the implementation in different temperature ranges of the crystalline structures of fullerite formed by fullerene molecules of various modifications. Experimental studies of solutions of fullerene molecules in various organic media made it possible to establish the features of the temperature dependence of the solubility of fullerite C 60. Instead of the expected increase in solubility with increasing temperature, the extremes of its temperature dependence with a maximum at room temperature were revealed, the endothermicity of the dissolution process was established. The anomalies of the temperature dependence of the solubility of fullerenes C 60 in organic matrices are due to a change in the structure of bonds in the framework of fullerene molecules, which, in turn, affect the structure of solid-phase fullerite.More accurate results using the formulas obtained can be calculated if the true values of the energy parameters are known from independent experiments. However, it can be assumed that the nature of the revealed patterns will remain.
MRS Proceedings, 2004
ABSTRACTWe report flexible synthesis of III-Nitride nanowires and heterostructures by metal-organ... more ABSTRACTWe report flexible synthesis of III-Nitride nanowires and heterostructures by metal-organic chemical vapor deposition (MOCVD) via a catalytic vapor-liquid-solid (VLS) growth mechanism. Indium is used as an in-situ catalyst to facilitate and sustain the stability of liquid phase droplet for VLS growth based on thermodynamic consideration. The employment of mesoporous molecular sieves (MCM-41) helps to prevent the coalescence of catalyst droplets and to promote nucleation statistics. Cathodoluminescence (CL) of GaN nanowires shows near band-edge emission at 370nm, and strong E2 phonon peak is observed at room temperature in Raman scattering spectra. Both binary GaN and AlN nanowires have been synthesized by MOCVD. Three-dimensional AlN/GaN trunk-branch nanostructures are reported to illustrate the versatility of incorporating the VLS mechanism into MOCVD process.
The Journal of Physical Chemistry B, 2004
Cobalt silicate was studied as a model catalyst to seek an understanding of why Co-MCM-41 catalys... more Cobalt silicate was studied as a model catalyst to seek an understanding of why Co-MCM-41 catalyst is not active for single wall carbon nanotubes (SWNT) upon treatment at high temperature. Cobalt orthosilicate was formed during high-temperature pretreatment of Co-...
The Journal of Physical Chemistry B, 2006
The effect of initial synthesis solution pH and tetramethylammonium silicate concentration in the... more The effect of initial synthesis solution pH and tetramethylammonium silicate concentration in the synthesis solution on the physical and chemical properties of MCM-41 was systematically investigated using N(2) physisorption, X-ray diffraction, temperature-programmed reduction, in situ Fourier transform IR, UV-vis, and X-ray absorption spectroscopies. pH and tetramethylammonium (TMA) fraction affect the porosity of MCM-41 and the reducibility of incorporated Ni cations; higher pH and TMA concentration produced more porosity with higher stability against reduction, which is attributed to more metal ions locating in the interior of the silica walls. The control of the pore diameter of mesoporous MCM-41 at the sub-nanometer scale may be accomplished by adjusting the pH and TMA fraction. pH may be used to control the surface free silanol group density and nickel reduction degree as well, and this is useful in the design of a specific catalyst for particular reactions, such as CO methanation, which requires highly dispersed, stable metallic clusters with controllable size.