Papers by Ruxandra Birjega
We report a new synthesis technique for the preparation of Fe-Sn0 2 , namely the C0 2 laser pyrol... more We report a new synthesis technique for the preparation of Fe-Sn0 2 , namely the C0 2 laser pyrolysis of gas phase reactants. Tetramethyl tin, air and iron pentacarbonyl were used as precursors. At increased oxygen in mixtures, the Sn0 2 phase (mean crystallite sizes about 12 nm) largely prevails over the other two crystallographic phases: SnO and Sn. Different Fe/Sn ratios (from about 0.01 to 0.08 in at %) were obtained by varying the process parameters. No iron compounds were identified by diffraction techniques. With increased Fe doping, an overall decrease of the diffraction peaks and an inhibited particle growth may be observed.
Sulfated zirconia and modified sulfated zirconia Zr0.85Me IV 0.15O2/SO4 2-(M = Ce IV , Sn IV) hav... more Sulfated zirconia and modified sulfated zirconia Zr0.85Me IV 0.15O2/SO4 2-(M = Ce IV , Sn IV) have been studied as catalysts for skeletal isomerisation of n-butane. The correlation between the structure, porosity, the solid-acid character of these samples and their catalytic performances were reported.
10 concentration have been prepared by the laser pyrolysis method. Titanium tetrachloride, air an... more 10 concentration have been prepared by the laser pyrolysis method. Titanium tetrachloride, air and iron pentacarbonyl (vapors) were used as precursors. In the synthesized powders, a mixture of anatase and rutile was found, wit dominant rutile phase. Mean particle diameters of around 14 nm and 12 nm were estimated for undoped and doped anatase titania, respectively. Major effects of iron doping seem to be: i) the higher proportion of an amorphous phase; ii) the higher rutile proportion in the iron-doped samples. For analyzing the photochemical properties the samples were heat-treated at 450 oC. Doping the photocatalyst with Fe shifted the absorption onset of the semiconductor to 15 visible region, as compared to bare TiO2. This shift depends on the Fe concentration. In contrast, even if the photoefficiency to the degradation of 4-chlorophonel of the undoped sample is with 45 % higher than that of the P25 Degussa sample, the calcined iron doped samples with higher Fe concentrations sh...
Journal of Luminescence, 2014
Ultraviolet (365 nm, 1 D 2-3 H 6), blue (480 nm, 1 G 4-3 H 6), red (650-660 nm, 1 G 4-3 F 4 þ 3 F... more Ultraviolet (365 nm, 1 D 2-3 H 6), blue (480 nm, 1 G 4-3 H 6), red (650-660 nm, 1 G 4-3 F 4 þ 3 F 2,3-3 H 6), and infrared (800 nm, 3 H 4-3 H 6) upconversion luminescence, were obtained for the first time in La 3 Ga 5.5-Ta 0.5 O 14 :Tm:Yb ceramic sample, synthesized by solid-state reaction, pumped at 973 nm. Yb 3 þ-Tm 3 þ energy transfer processes accounting for population of the emitting levels of Tm 3 þ are discussed. Also, the back-transfer process Tm 3 þ-Yb 3 þ was observed. The decay curves of the metastable levels 1 D 2 , 1 G 4 , 3 H 4 , and 3 F 4 of Tm 3 þ and 2 F 5/2 of Yb 3 þ were analyzed. The cross-relaxation microparameters of Tm 3 þ were found. The efficiency of the Yb 3 þ-Tm 3 þ transfer was estimated.
International Journal of Hydrogen Energy, 1997
ABSTRACT
RSC Advances, 2015
Zinc embedded self-templated saccharide derived carbonaceous spheres proved to be multipurpose pr... more Zinc embedded self-templated saccharide derived carbonaceous spheres proved to be multipurpose precursors for hollow, solid and double oxide spheres.
ABSTRACT The catalytic activities for n-butene and n-butane conversion to aromatics and the physi... more ABSTRACT The catalytic activities for n-butene and n-butane conversion to aromatics and the physico-chemical properties of Ga-ZSM-5 and Sb-ZSM-5 prepared by different techniques such as ionic exchange, wet impregnation, or physical mixture of the parent zeolite with the corresponding oxides were compared. It was found that the preparation technique influences both the catalytic behaviour and the physico-chemical properties of the catalysts. Sb-modified ZSM-5 zeolites enhance the selective conversion of light paraffins and olefins to aromatics even if they are not as performing as Ga-modified ZSM-5. In their case, the back spillover mechanism for hydrogen transfer is not promoted as in the case of Ga-modified ZSM-5. The increase of the selectivity to aromatics rich in BTX is due only to the decrease of the protonic acidity, which promotes cracking side-reactions.
Applied Clay Science, 2015
This work investigates the possibility to obtain the meixnerite structure Mg 6 Al 2 (OH) 18 •4H 2... more This work investigates the possibility to obtain the meixnerite structure Mg 6 Al 2 (OH) 18 •4H 2 O in carbonate-free environment by direct co-precipitation of Mg and Al nitrates using only NaOH solution as precipitation agent as well as the physico-chemical and catalytic performances of the mixed oxide (MO) obtained by thermal decomposition of this layered material at 460°C under nitrogen flow. The influence played by different gaseous environments, air or nitrogen, during the hydration of the above mentioned mixed oxide on the structural characteristics and catalytic performances of the derived reconstructed layered double hydroxide (LDH) materials (RLDH-A and RLDH-N, respectively) is also scrutinized. The characterization of the solids was performed by chemical and elemental analyses, XRD, DRIFTS, N 2 adsorption-desorption isotherms and adsorption of molecules with different pK a in order to determine their basicity. The catalytic activity for cyanoethylation was discussed in relation with the structural properties of the catalytic materials. The results showed that the co-precipitation method employed leads to the formation of an LDH containing also NO 3 − and CO 3 2− anions besides HO −. Also, low amounts of C and N (b 1%) were still present in the derived mixed oxide as well as in the reconstructed layered materials. The use of different gaseous environments, air or nitrogen, during the reconstruction leads to different structural properties which affected the catalytic activity of the solids. It has been noticed that the presence of carbonate anion in the structure of parent LDH-materials as well as that of carbonate low amounts in the LDH-derived mixed oxide improved the ability of the latter to recreate the lamellar structure through memory effect. However this fact is not accompanied by an increase of catalytic activity since the presence of carbonate anions decreases the basicity of the solid. Among the layered materials, the sample reconstructed under nitrogen (RLDH-N) which has a low crystallinity, and hence a larger number of defects, has also the highest number of HO − base sites and is the most active for catalytic cyanoethylation.
Surface and Coatings Technology, 2004
Hydrogenated and nitrogenated carbon films were obtained by PECVD with carbon species supplied by... more Hydrogenated and nitrogenated carbon films were obtained by PECVD with carbon species supplied by acetylene gas injected into an argon or argonynitrogen remote plasma generated by an expanding radiofrequency discharge. The properties of carbon material like the nature of bonds, morphology, surface topography, crystallinity are presented. The reaction pathways linking the precursors with the depositing species, as revealed by emission spectroscopy, are discussed.
Solid State Sciences, 2013
Layered basic zinc salt (LBZS) that contains both carbonate and acetate as intercalated anions wa... more Layered basic zinc salt (LBZS) that contains both carbonate and acetate as intercalated anions was obtained by a sonochemical procedure, using a non-basic zinc acetate aqueous solution. The compound was characterized by IR spectroscopy, thermal analysis, SEM and XRD investigations. The LBZS consists in a lamellar structure with two interlayer distances (13.70 and 19.34 A), the XRD pattern and SEM images being characteristic for turbostratic materials. The acetate anions are intercalated as free anions and monodentate ligands, while carbonate anions behave as bidentate ligands. A growth mechanism for the LBZS is advanced. Its calcination at 600 C led to ZnO porous sheets with uniform pores and pore density and a BET surface of 10.9 g/m 2. ZnO exhibits interesting photoluminescence features, with two multipeaks bands covering the visible region. The UV photocatalytical activity tested for phenol degradation shows that ZnO acts as a promising photocatalyst, w90% of the phenol being mineralized.
Particulate Science and Technology, 2013
Particulate Science and Technology, 2013
ABSTRACT Mobile or fixed dental prostheses are a common and important part of our life. Over deca... more ABSTRACT Mobile or fixed dental prostheses are a common and important part of our life. Over decades, a large variety of noble metal based alloys were used in prosthodontics, and they need a correct evaluation of their technical, biological and clinical properties. The present study proposes a new approach for the manufacturing process. We emphasize the key role of each step for combined process of heating, casting and spin for completing works of Ni–Cr alloys with improved physical-chemical properties, as a protocol for quality of the obtained physiognomic dental restorations. The differences in composition distribution between cast and raw materials were investigated by Energy dispersive X-ray spectroscopy (EDX) while the surface morphology was inspected by scanning electron microscopy (SEM). Important aspects related to crystal lattice were revealed through the X Ray Diffraction (XRD) method and they are closely related to the centrifugal casting process. Key words: dental prostheses, Ni-Cr metal alloy, grain – stopper, SEM, EDX.
Catalysis Letters, 2009
The paper presents a comparison between the results obtained for cyclohexene epoxidation in the p... more The paper presents a comparison between the results obtained for cyclohexene epoxidation in the presence of base catalysts: Mg 0/75 Al 0.25-hydrotalcite (HT), the corresponding mixed oxide (CHT), the reconstructed hydrotalcite (RHT), a hydrotalcite containing cobalt in the brucite-type layer ((Co-Mg 0.75)Al 0.25)) and one containing cobalt complex-species supported on HT carrier (Co-complex/HT). The selective conversion of cyclohexene to the corresponding epoxide is closely related to the base strength of the catalysts.
Catalysis Communications, 2008
The aim of this work is to study how the changes of structure, texture and surface morphology of ... more The aim of this work is to study how the changes of structure, texture and surface morphology of as-synthesized LDHs (Mg/Al=3), the reconstructed via “memory effect” LDHs and their corresponding mixed oxides are reflected in the catalytic activity in cyanoethylation of ethanol with acrylonitrile. The as-synthesized LDHs was obtained by co-precipitation at low suprasaturation conditions, the reconstructed LDHs by the
Catalysis Communications, 2004
The basic solid catalysts obtained from Mg–Al hydrotalcite (HT) precursors have been investigated... more The basic solid catalysts obtained from Mg–Al hydrotalcite (HT) precursors have been investigated in the cyanoethylation of ethanol. Low activity was observed on conventional HT (carbonated); the activity reached high values after calcinations and rehydration at room temperature of the calcined samples.The basicity of the catalysts has been increased by the replacement – during preparation – of some small amount
Applied Surface Science, 2014
Heavy metals are toxic and hazardous pollutants in the environment due to their nonbiodegradabili... more Heavy metals are toxic and hazardous pollutants in the environment due to their nonbiodegradability and persistence, which can pose serious threats to living organisms. The ability of Mg-Al based layered double hydroxides (LDHs) thin films to retain heavy metals from aqueous solutions at different concentrations is a novel topic with prospects of attractive applications, such as detection of heavy metals. We report on the ability of a series of Mg-Al based layered double hydroxides thin films to detect Ni and Co cations in aqueous solutions. Uptake of heavy metals ions such as Ni 2+ , Co 2+ from aqueous solutions was studied as function of contact time at a standard metal ion concentration. The LDHs thin films were deposited using pulsed laser deposition (PLD). The different adsorption mechanisms were studied in connection with different heavy metals used as probe cations. X-ray diffraction, atomic force microscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, Fourier transform infra-red spectroscopy were the techniques used for the investigation of as deposited and after heavy metals retention thin films.
Applied Physics A, 2012
Powdered layered double hydroxides (LDHs)also known as hydrotalcite-like (HT)-compounds have been... more Powdered layered double hydroxides (LDHs)also known as hydrotalcite-like (HT)-compounds have been widely studied due to their applications as catalysts, anionic exchangers or host materials for inorganic or organic molecules. Assembling thin films of nano-sized LDHs onto flat solid substrates is an expanding area of research, with promising applications as sensors, corrosion-resistant coatings, components in optical and magnetic devices. The exploitation of LDHs as vehicles to carry dispersed metal nanoparticles onto a substrate is a new approach to obtain composite thin films with prospects for biomedical and optical applications. We report the deposition of thin films of Ag nanoparticles embedded in a Mg-Al layered double hydroxide matrix by pulsed laser deposition (PLD). The Ag-LDH powder was prepared by co-precipitation at supersaturation and pH = 10 using aqueous solutions of Mg and Al nitrates, Na hydroxide and carbonate, and AgNO 3 , having atomic ratios of Mg/Al = 3 and Ag/Al = 0.55. The target to be used in laser ablation experiments was a dry pressed pellet obtained from the prepared Ag-LDH powder. Three different wavelengths of a Nd:YAG laser (266, 532 and 1064 nm) working at a repetition rate of A.M. and R.B. have contributed equally to this study.
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Papers by Ruxandra Birjega