Papers by Santhosh K Matam
Journal of Colloid and Interface Science, 2015
We report a facile synthesis of superhydrophobic silica-iron oxide nanocomposites via a co-precur... more We report a facile synthesis of superhydrophobic silica-iron oxide nanocomposites via a co-precursor sol-gel process. The choice of the silica precursor (Methyltrimethoxysilane, MTMS) in combination with iron nitrate altered the pore structure dramatically. The influence of iron oxide doping on the structural properties of pristine MTMS aerogel is discussed.
ACS applied materials & interfaces, Jan 13, 2015
With growing public interest in portable electronics such as micro fuel cells, micro gas total an... more With growing public interest in portable electronics such as micro fuel cells, micro gas total analysis systems, and portable medical devices, the need for miniaturized air pumps with minimal electrical power consumption is on the rise. Thus, the development and downsizing of next-generation thermal transpiration gas pumps has been investigated intensively during the last decades. Such a system relies on a mesoporous membrane that generates a thermomolecular pressure gradient under the action of an applied temperature bias. However, the development of highly miniaturized active membrane materials with tailored porosity and optimized pumping performance remains a major challenge. Here we report a systematic study on the manufacturing of aerogel membranes using an optimized, minimal-shrinkage sol-gel process, leading to low thermal conductivity and high air conductance. This combination of properties results in superior performance for miniaturized thermomolecular air pump application...
CHIMIA International Journal for Chemistry
CHIMIA International Journal for Chemistry
CHIMIA International Journal for Chemistry
Chemistry of Materials
Incorporation of palladium within the perovskitetype lattice is a valuable strategy to prevent si... more Incorporation of palladium within the perovskitetype lattice is a valuable strategy to prevent sintering phenomena that reduce the performance of automotive catalysts. However, occupation of B-site coordination sites by Pd can strongly depend on the nature of the perovskite-type oxide and care must be taken for structural interpretation. Pd-based La(B,Pd)O 3±δ (B = Mn, Fe, Co) perovskite-type oxides and Y(Fe,Pd)O 3±δ have been prepared by the amorphous citrate method by introducing the Pd salt to the precursors solution with the aim to substitute the B-site. The state of Pd was thoroughly characterized using X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction with hydrogen. Pd incorporation was achieved for La(Fe,Pd)O 3±δ and La(Co,Pd)O 3±δ that showed the XANES white line of Pd in octahedral coordination. On the contrary, La(Mn,Pd)O 3±δ showed XANES features and reduction behavior typical of segregated PdO particles. The state of Pd in Y(Fe,Pd)O 3±δ appeared to be dependent on the calcination temperature. Calcination at 700°C produced cationic palladium with strong interaction with the YFeO 3 structure but likely in a different coordination environment than in LaFeO 3 . Calcination at 800°C provided improved crystallinity and forced Pd to form a PdO phase. The markedly different states of Pd are interpreted on the base of ionic radii arguments. The smaller size of Mn cations does not favor occupation of octahedral sites by the larger Pd 3+ . The observed states adopted by Pd strongly influenced methane oxidation activity. The highest catalytic activity was found for the samples where palladium was predominantly dispersed as supported PdO particles. Lowest catalytic activity was measured for samples containing octahedral Pd, thus confirming that Pd incorporation in the perovskite-type lattice may not be favorable for methane oxidation.
The phosphor CaTiO 3 :Pr 3+ was synthesized via a solid-state reaction in combination with a subs... more The phosphor CaTiO 3 :Pr 3+ was synthesized via a solid-state reaction in combination with a subsequent annealing under flowing NH 3 . Comparatively large off-center displacements of Ti in the TiO 6 octahedra were confirmed for as-synthesized CaTiO 3 :Pr 3 by XANES. Raman spectroscopy showed that the local crystal structure becomes highly symmetric when the powders are ammonolyzed at 400 °C. Rietveld refinement of powder X-ray diffraction data revealed that the samples ammonolyzed at 400 °C have the smallest lattice strain and at the same time the largest average Ti-O-Ti angles were obtained. The samples ammonolyzed at 400 °C also showed the smallest mass loss during the thermal re-oxidation in thermogravimetric analysis (TGA). Enhanced photolumincescence brightness and an improved decay curve as well as the highest reflectance were obtained for the samples ammonolyzed at 400 °C. The improved photoluminescence and afterglow by NH 3 treatment are explained as a result of the reduced ...
Journal of Alloys and Compounds, 2014
Fluorine-substituted CaTiO 3 :Pr phosphors were prepared by a solid-state reaction. Rietveld refi... more Fluorine-substituted CaTiO 3 :Pr phosphors were prepared by a solid-state reaction. Rietveld refinements of powder X-ray diffraction patterns revealed that increasing fluorine-substitution leads to the gradual shrinkage of the unit-cell. Enhanced afterglow intensities were observed with fluorine-substitution. Furthermore, the effect of annealing atmosphere was investigated by thermochemical treatment in different atmospheres (Ar, air and NH 3 ). UV-Vis diffuse reflectance spectra and photoluminescence excitation spectra revealed that Pr 4+ in the pristine CaTi(O,F) 3 :Pr phosphor was partially reduced to Pr 3+ under NH 3 flow leading to an intensity improvement of ca. 450% compared to CaTiO 3 :Pr. The substantial improvement of afterglow intensity by fluorine substitution and annealing in NH 3 is considered to be connected with the generation of oxygen vacancies and the partial reduction of Pr 4+ to Pr 3+ .
Optical Materials Express, 2013
The phosphor CaTiO 3 :Pr 3+ was synthesized via a solid-state reaction in combination with a subs... more The phosphor CaTiO 3 :Pr 3+ was synthesized via a solid-state reaction in combination with a subsequent annealing under flowing NH 3 . Comparatively large off-center displacements of Ti in the TiO 6 octahedra were confirmed for as-synthesized CaTiO 3 :Pr 3 by XANES. Raman spectroscopy showed that the local crystal structure becomes highly symmetric when the powders are ammonolyzed at 400 °C. Rietveld refinement of powder X-ray diffraction data revealed that the samples ammonolyzed at 400 °C have the smallest lattice strain and at the same time the largest average Ti-O-Ti angles were obtained. The samples ammonolyzed at 400 °C also showed the smallest mass loss during the thermal re-oxidation in thermogravimetric analysis (TGA). Enhanced photolumincescence brightness and an improved decay curve as well as the highest reflectance were obtained for the samples ammonolyzed at 400 °C. The improved photoluminescence and afterglow by NH 3 treatment are explained as a result of the reduced concentration of oxygen excesses with simultaneous relaxation of the lattice strain.
Catalysis Today, 2015
ABSTRACT Perovskite-type oxides LaCo1−x−yPdxZnyO3±δ are studied for the methanol steam reforming ... more ABSTRACT Perovskite-type oxides LaCo1−x−yPdxZnyO3±δ are studied for the methanol steam reforming (MSR) reaction. LaCo1−x−yPdxZnyO3±δ with and without Co substitution by Pd and/or Zn are synthesized by the amorphous citrate method. The effect of substitution on the reducibility of the materials is investigated by in situ XRD and differential thermal/thermogravimetric analysis (DTA/TG) during temperature programmed reduction (TPR) with hydrogen. From this, the phase composition evolving as a function of temperature is determined. The TPR data indicate that the reducibility of the materials strongly depends on the substituent, i.e., Pd or Zn. The former increases the reducibility, while substitution by Zn decreases the reducibility. Materials containing both Pd and Zn exhibit intermediate reducibility as compared to the un-substituted LaCoO3±δ (LCO) and mono-metal substituted LaCo0.873Pd0.127O3±δ (LCPO) and LaCo0.89Zn0.11O3±δ (LCZO). In general, the metal ions in the materials are completely reduced at 590 °C, except La2O3. After reductive pre-treatment at 590 °C for 2 h, the MSR performance of the materials is evaluated between 150 and 400 °C while keeping comparable CH3OH conversion to assess the product-concentration-dependent CO2 selectivity. These results confirm that the CO2 selectivity is strongly dependent on the material composition. For example, the un-substituted LCO, LCPO and LCZO show a CO2 selectivity of around 13% at 300 °C, while it is improved to 80% on both Pd and Zn containing LaCo0.85Pd0.075Zn0.075O3±δ (LCPZO-15).
International Journal of Photoenergy, 2013
ABSTRACT Phase-pure tantalum oxynitride (β-TaON) powders were synthesized by thermal ammonolysis ... more ABSTRACT Phase-pure tantalum oxynitride (β-TaON) powders were synthesized by thermal ammonolysis of Ta2O5 powders. X-ray diffraction revealed an enlargement of the unit cell and an increase of the crystallite size with increasing ammonolysis temperature. Scanning electron microscopy showed reduced particle sizes for β-TaON synthesized at 800 and compared to the precursor oxide. With increasing nitridation temperature the Brunauer-Emmett-Teller surface area was reduced and the nitrogen content increased. UV-Vis spectroscopy showed a bandgap energy of 2.6–2.4 eV. The highest oxygen evolution rate of 220 μmol·g−1·h−1 was achieved for β-TaON synthesized at . The factors determining the photocatalytic activity of β-TaON powders were found to be the specific surface area and defects in the β-TaON.
Catalysts, 2014
Automotive Three-Way Catalysts (TWC) were introduced more than 40 years ago. Despite that, the de... more Automotive Three-Way Catalysts (TWC) were introduced more than 40 years ago. Despite that, the development of a sustainable TWC still remains a critical research topic owing to the increasingly stringent emission regulations together with the price and scarcity of precious metals. Among other material classes, perovskite-type oxides are known to be valuable alternatives to conventionally used TWC compositions and have demonstrated to be suitable for a wide range of automotive applications, ranging from TWC to Diesel Oxidation Catalysts (DOC), from NO x Storage Reduction catalysts (NSR) to soot combustion catalysts. The interest in these catalysts has been revitalized in the past ten years by the introduction of the concept of catalyst regenerability of perovskite-based TWC, which is in principle well applicable to other catalytic processes as well, and by the possibility to reduce the amounts of critical elements, such as precious metals without seriously lowering the catalytic performance. The aim of this review is to show that perovskite-type oxides have the potential to fulfil the requirements (high activity, stability, and possibility to be included into structured catalysts) for implementation in TWC.
Chemical communications (Cambridge, England), Jan 21, 2001
A titania-supported ammonium salt of 12-molybdophosphoric acid has been synthesized, and the salt... more A titania-supported ammonium salt of 12-molybdophosphoric acid has been synthesized, and the salt-support interaction, which enhanced the reaction rate, has been correlated with the activity of the catalyst in the ammoxidation of 2-methylpyrazine.
The Journal of Physical Chemistry C, 2014
ABSTRACT An empirical study of the gas solid reaction of carbon dioxide (CO2) with alanates is pr... more ABSTRACT An empirical study of the gas solid reaction of carbon dioxide (CO2) with alanates is presented. This investigation was triggered by reports of hazards related to the reaction of lithium aluminum hydride with carbon dioxide, together with the recent emergence of alanates as potential hydrogen storage materials. Furthermore, the reduction of CO2 by hydrides is an alternative to the conventional CO2 reduction employing hydrogen gas in combination with a catalyst. Experimentally this work was carried out by studying the decomposition of alanate samples in a carbon dioxide atmosphere with thermogravimetric and ex situ IR spectroscopic techniques. It is shown that alanates react with CO2 at atmospheric pressure in two distinct temperature regions, yielding methane, hydrogen gas, and metal oxides as the major products. The experimental findings allowed us to postulate a mechanism for the complex metal hydride reduction of CO2, involving alane as a highly reactive intermediate. It is suggested that nucleophilic attack of alane hydride ions onto the carbonyl carbon of CO2 leads to sequential formation of aluminum formate and methoxy species which get converted into methane and metal oxides as the final products.
Topics in Catalysis, 2011
... Kimmerle B, Grunwaldt JD, Baiker A, Glatzel P, Boye P, Ste-phan S, Schroer CG (2009) J Phys C... more ... Kimmerle B, Grunwaldt JD, Baiker A, Glatzel P, Boye P, Ste-phan S, Schroer CG (2009) J Phys Chem C 113:3037 21. Santhosh Kumar M, Aguirre MH, Weidenkaff A, Ferri D (2010) J Phys Chem C 114:9439 22. Ressler T (1998) J Synchrotron Radiat 5:118 23. ...
The Journal of Physical Chemistry C, 2011
The catalytic performance of methane oxidation catalysts, i.e., LaFe 0.95 Pd 0.05 O 3 , 2 wt % Pd... more The catalytic performance of methane oxidation catalysts, i.e., LaFe 0.95 Pd 0.05 O 3 , 2 wt % Pd/LaFeO 3 , and 2 wt % Pd/Al 2 O 3 , has been compared at 500°C under periodic red-ox conditions. The state of palladium has been followed under operando conditions using XANES and QEXAFS at the Pd K-edge combined with mass spectrometry (MS) for product detection. Online MS data reveal that in correspondence to every change of feed composition (O 2 pulses) CO 2 production is enhanced over LaFe 0.95 Pd 0.05 O 3 that is associated with a drop in methane concentration. This is not the case for the samples where PdO nanoparticles are deposited on the support material (Pd/LaFeO 3 and Pd/Al 2 O 3 ). The time-resolved QEXAFS spectra have been treated with a modulation excitation spectroscopy approach. Phase sensitive detection (PSD) enabled to highlight the subtle changes in the whiteline region. Continuous reduction-oxidation of Pd occurs in all samples at every change of feed composition. However, by this process Pd in LaFe 0.95 Pd 0.05 O 3 reversibly emerges on the LaFeO 3 surface under reducing conditions and enters the LaFeO 3 structure under oxidizing conditions. On the contrary, Pd oscillates between the reduced and partially oxidized state in Pd/Al 2 O 3 and Pd/LaFeO 3 in which welldefined Pd nanoparticles are already available. This structural difference is responsible for the activity enhancement in correspondence of each switch and is attributed to the self-regenerative property of perovskitetype oxides. † Part of the "Alfons Baiker Festschrift".
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Papers by Santhosh K Matam