Anitha Patlolla

Catalysis on singly dispersed bimetallic sites

A catalytic site typically consists of one or more atoms of a catalyst surface that arrange into a configuration offering a specific electronic structure for adsorbing or dissociating reactant molecules. The catalytic activity of adjacent bimetallic sites of metallic nanoparticles has been studied previously. An isolated bimetallic site supported on a non-metallic surface could exhibit a distinctly different catalytic performance owing to the cationic state of the singly dispersed bimetallic site and the minimized choices of binding configurations of a reactant molecule compared with continuously packed bimetallic sites. Here we report that isolated Rh1Co3 bimetallic sites exhibit a distinctly different catalytic performance in reduction of nitric oxide with carbon monoxide at low temperature, resulting from strong adsorption of two nitric oxide molecules and a nitrous oxide intermediate on Rh1Co3 sites and following a low-barrier pathway dissociation to dinitrogen and an oxygen atom. This observation suggests a method to develop catalysts with high selectivity.

How Strain Affects the Reactivity of Surface Metal Oxide Catalysts

Highly dispersed molybdenum oxide supported on mesoporous silica SBA-15 has been prepared by anion exchange resulting in a series of catalysts with changing Mo densities (0.2-2.5 Mo atoms nm(-2) ). X-ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO4 units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temperature-programmed reduction and a steep increase in the catalytic activity observed in metathesis of propene and oxidative dehydrogenation of propane at 8 % of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calculations, NEXAFS spectra at the O-K-edge at high Mo loadings are explained by distorted MoO4 complexes. Limited availability of anchor silanol groups at high loadings forces the MoO4 groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.

Thermochromism in polydiacetylene-metal oxide nanocomposites

Irreversible and reversible chromatic transitions during heating and cooling cycles were investigated in polydiacetylene poly-PCDA (poly-10,12-pentacosadiynoic acid) composites with nanocrystalline zinc oxide (ZnO), titanium oxide (TiO2), zirconium oxide (ZrO2) and ZnO and ZrO2 alloys. In contrast to pure poly-PCDA, poly-PCDA composites with nanocrystalline ZnO displayed rapid reversibility on thermal cycling, whereas the corresponding composites with nanocrystalline TiO2 and ZrO2 were irreversible, and poly-PCDA composites with thermally prepared ZnO and ZrO2 alloys displayed slower reversibility. The mechanism of reversible and irreversible thermochromism in these materials was explored using Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and X-ray absorption fine structure (XAFS) spectroscopy. In pure poly-PCDA, heating leads to an irreversible strain on the polymer backbone to form a red phase, which is not released on cooling. In the presence of ZnO evidence is provided for chelation involving the side chain head groups which can release strain on cooling to rapidly form the blue phase. Chemical interaction coupled with reversible behavior was however observed only when the composites were prepared with ZnO having an average crystallite size of 300 nm and below with a fraction of an amorphous grain boundary phase. Poly-PCDA composites with ZnO/ZrO2 alloys showing slower thermochromic reversibility can be used both as temperature and elapsed time-temperature sensors.

Characterization of Metal-Oxide Catalysts in Operando Conditions by Combining X-ray Absorption and Raman Spectroscopies in the Same Experiment

We have developed a new instrumental setup that combines simultaneous X-ray absorption spectroscopy, Raman spectroscopy and online mass spectrometry for operando studies of catalytic reactions. The importance of combining these techniques in the same experiment is demonstrated with the example of CO oxidation over nanoscale copper oxide catalysts supported on high surface area titanium oxide. X-ray absorption near edge structure (XANES) spectroscopy provides information on the charge state and local geometry of the catalytically active atoms. Extended X-ray absorption fine-structure (EXAFS) technique adds information about their local coordination environment. Raman spectroscopy adds sensitivity to crystallographic phase and long range order that both XANES and EXAFS are lacking. Together, these measurements enable simultaneous studies of the structural and electronic properties of all components present in metal-oxide catalysts. Coupled with online reactant and product analysis, this new setup allows one to elucidate the synergy between different components of a catalytic system and shed light on its catalytic activity and selectivity.

Scalable nano-bioprobes with sub-cellular resolution for cell detection.

Here we present a carbon nanotube based device to noninvasively and quickly detect mobile single cells with the potential to maintain a high degree of spatial resolution. The device utilizes standard complementary metal oxide semiconductor (CMOS) technologies for fabrication, allowing it to be easily scalable (down to a few nanometers). Nanotubes are deposited using electrophoresis after fabrication in order to maintain CMOS compatibility. The devices are spaced by 6 μm which is the same size or smaller than a single cell. To demonstrate its capability to detect cells, we performed impedance spectroscopy on mobile human embryonic kidney (HEK) cells, neurons cells from mice, and yeast cells (S. pombe). Measurements were performed with and without cells and with and without nanotubes. Nanotubes were found to be crucial to successfully detect the presence of cells. The devices are also able to distinguish between cells with different characteristics.

Studies of the blue to red phase transition in polydiacetylene nanocomposites and blends

The conjugated polymeric backbone of polydiacetylenes (PDAs), comprising of alternating ene-yne groups, undergo intriguing stress-, chemical- or temperature-induced chromatic phase transitions associated with the disruption of the backbone structure and shortening of the conjugation length. PDAs, such as polymerized 10, 12 pentacosadiynoic acids (PCDA), when incorporated with inorganic oxides form nanocomposites and uniform blends with polymers. Blends of poly-PCDA with polymers, such as polyvinyl alcohol, polyvinylidene fluoride and cellulose increase the blue to red transition temperature without affecting the irreversibility of the red phase. However, the addition of zinc oxide to pure poly-PCDA makes the red phase highly reversible and substantially increases the blue to red transition temperature. The addition of TiO 2 to poly-PCDA on the other hand does not affect the irreversibility of the red phase and the chromatic transition temperature. In order to understand the atomic scale interactions associated with these changes in the chromatic transitions, we have investigated both the nanocomposites and polymer blends using Raman and Fourier-transform infrared spectroscopy, and extended X-ray absorption fine structure (EXAFS) measurements

Carbon Nanotube-based, Membrane-less and Mediator-free Enzymatic Biofuel Cells

A direct electron transfer single wall carbon nanotube (SWNT)- based biofuel cell on a gold-coated porous silicon (pSi) substrate shows a peak power density of 12 microwatts/cm2 in 4 mM glucose (corresponding to normal blood sugar concentration) with an average power of 1 microwatt over 48 hours of operation. An interesting feature of this biofuel cell is that it can be re-activated to operate with an average power output of 1 microwatt/cm2 after storage for 3 months at 4 C. Gold particles were electrolessly self-assembled on each pSi substrate from HAuCl4 solution in alcohol after similarly depositing an adhesion-promoting Cr layer. The hierarchy of pores on pSi enables nanoscale transport during fuel cell operation via capillary and wicking effects. Carboxylated SWNTs were deposited by an electrophoretic process, followed by electrochemically-induced covalent immobilization of anodic glucose oxidase and cathodic laccase enzymes on the nanotubes to form the biofuel cell structure.