Chaitanya K. Narula

CO Oxidation on Supported Single Pt Atoms: Experimental and ab Initio Density Functional Studies of CO Interaction with Pt Atom on θ-Al2O3(010) Surface

Although there are only a few known examples of supported single-atom catalysts, they are unique because they bridge the gap between homogeneous and heterogeneous catalysis. Here, we report the CO oxidation activity of monodisperse single Pt atoms supported on an inert substrate, θ-alumina (Al2O3), in the presence of stoichiometric oxygen. Since CO oxidation on single Pt atoms cannot occur via a conventional Langmuir-Hinshelwood scheme (L-H scheme) which requires at least one Pt-Pt bond, we carried out a first-principles density functional theoretical study of a proposed pathway which is a variation on the conventional L-H scheme and inspired by the organometallic chemistry of platinum. We find that a single supported Pt atom prefers to bond to O2 over CO. CO then bonds with the oxygenated Pt atom and forms a carbonate which dissociates to liberate CO2, leaving an oxygen atom on Pt. Subsequent reaction with another CO molecule regenerates the single-atom catalyst. The energetics of the proposed mechanism suggests that the single Pt atoms will get covered with CO3 unless the temperature is raised to eliminate CO2. We find evidence for CO3 coverage at room temperature supporting the proposed mechanism in an in situ diffuse reflectance infrared study of CO adsorption on the catalysts supported single atoms. Thus, our results clearly show that supported Pt single atoms are catalytically active and that this catalytic activity can occur without involving the substrate. Characterization by electron microscopy and X-ray absorption studies of the monodisperse Pt/θ-Al2O3 are also presented.

Remarkable NO oxidation on single supported platinum atoms

Our first-principles density functional theoretical modeling suggests that NO oxidation is feasible on fully oxidized single θ-Al2O3 supported platinum atoms via a modified Langmuir-Hinshelwood pathway. This is in contrast to the known decrease in NO oxidation activity of supported platinum with decreasing Pt particle size believed to be due to increased platinum oxidation. In order to validate our theoretical study, we evaluated single θ-Al2O3 supported platinum atoms and found them to exhibit remarkable NO oxidation activity. A comparison of turnover frequencies (TOF) of single supported Pt atoms with those of platinum particles for NO oxidation shows that single supported Pt atoms are as active as fully formed platinum particles. Thus, the overall picture of NO oxidation on supported Pt is that NO oxidation activity decreases with decreasing Pt particle size but accelerates when Pt is present only as single atoms.

Precursors to Boron-Nitrogen Macromolecules and Ceramics

Boron nitride has been prepared in the past from classical high temperature reactions and more recently by CVD methods. Few attempts have been made to prepare this important material from pyrolyses of preceramic oligomers or polymers. In the present study oligomerization reactions of substituted borazenes with silyamine crosslinking groups have been found to provide useful gel materials which upon pyrolysis form boron nitrogen materials. Selected aspects of this chemistry and some characterization of the materials is presented.

Incorporation of lanthanides in alumina matrices by a sol–gelprocess employing heterometallic alkoxides,M[Al(OPri)4]3 , as precursors

Spectroscopic studies of the hydrolysis of heterometallic alkoxides, M[Al(OPr i ) 4 ] 3 , M=La, Ce, suggest that they do not dissociate during the early stages of hydrolysis. Gels from these heterometallic alkoxides can be prepared by two methods (i) direct hydrolysis in Pr i OH, and (ii) peptization with acetic acid. Changes that occur in the structure and surface properties of gels during heat treatment are monitored by BET surface area measurements and X-ray powder diffraction (XRD) studies. No crystalline phase is observed below 500 °C in xerogels derived from La[Al(OPr i ) 4 ] 3 and Ce[Al(OPr i ) 4 ] 3 or their mixtures. The XRD of the gel derived from La[Al(OPr i ) 4 ] 3 shows diffraction peaks at 900 °C due to LaAlO 3 . Cerium oxide starts to separate out at 600 °C in xerogels prepared from Ce[Al(OPr i ) 4 ] 3 , and at 700 °C in xerogels prepared from mixtures of La[Al(OPr i ) 4 ] 3 and Ce[Al(OPr i ) 4 ] 3 . Raman spectra of the gels prepared from the mixtures of La[Al(OPr i ) 4 ] 3 and Ce[Al(OPr i ) 4 ] 3 show that the CeO 2 crystallite sizes are >100 A, and CeO 2 , in fact, contains lanthanum in a mixed fluorite structure. High-resolution electron microscopy confirms that the lanthanum is part of the CeO 2 fluorite structure even in lanthanum rich areas. X-Ray photoelectron spectroscopic (XPS) analysis of the lanthanum containing gels, heated to 500 and 1000 °C, shows a peak at 836.5 eV for La 3d 5/2 , which could be due to the presence of M–O–Al type bridges in the samples. The contribution of µ‴ in the Ce 3d peaks suggests that both oxides, CeO 2 and Ce 2 O 3 , are present in cerium containing gels. The presence of lanthanum does not affect the distribution of Ce III and Ce IV in our samples.

Synthesis of Ain and AlN/SiC Ceramics from Polymeric Molecular Precursors

Reactions of (Me 3 Si) 3 AI·Et 2 O with NH 3 in different stoichiometric ratios have been studied in search of useful precursors to AIN. Two molecular condensation-elimination compounds and an insoluble oligomer have been obtained from these reactions, and pyrolysis of the compounds leads to the convenient formation of AlN/SiC solid solutions.

Novel zirconium nitride precursor: synthesis, decomposition pathway, and pyrolysis of [(CH3)3Si]2NH·ZrCl4

The reaction of hexamethyldisilazane with zirconium tetrachloride furnishes an adduct, [(CH3)3Si]2NHZrCl4, which is soluble in dichloromethane. The decomposition of [(CH3)3Si]2NHZrCl4 proceeds with the elimination of trimethylchlorosilane and hydrogen chloride and is complete at 600C. The resulting powder is amorphous zirconium nitride which is highly susceptible to hydrolysis and oxidation. The pyrolysis of the precursor in a dynamic vacuum or ammonia atmosphere at 600C and subsequent sintering of the powder at 900C furnishes crystalline zirconium nitride. Transmission electron microscopy of the powder shows that the particles are coated with a thin zirconium oxide layer. Crystalline zirconium nitride powder, free from zirconium oxide impurities, is obtained if the sintering is carried out at 1075C.

Synthesis and molecular structure of a P2B3 cage compound: P2(BNPri2)3

The reaction of (Pri2N)B(Cl)P(SiMe3)2 with (Pri2N)BCl2 in a 2:1 ratio resulted in the formation of a trigonal bipyramidal cage compound P2(BNPri2)3 whose structure was determined by X-ray diffraction techniques.

A novel trico-ordinated boronium salt: 1,3-dimethyl-2-phenanthridin-5-yl-1,3,2-diazaborolidinium trifluoromethansesulphonate

N.m.r. evidence suggests that pyridine but not phenanthridine reacts with 2-chloro-1,3-dimethyl-1,3,2-diazaborolidine (1a) in a 1 : 1 ratio with formation of an ionic compound; however, 1,3-dimethyl-2-trifluoromethanesulphonato-1,3,2-diazaborolidine (1b) produces a 1 : 1 addition product with phenanthridine which was shown to be the title compound by an X-ray structure determination.

Indium derivatives of monothio-β-diketones and the X-ray structure of tris[benzoyl(thiobenzoyl)methanato-o,s]indium(III)

Six-co-ordinated mono-, bis-, and tris-(monothio-β-diketonates) of iridium(III) have been synthesized and character ized. The X-ray crystal structure of the title compound reveals a distorted octahedral geometry with facia I arrangement of the sulphur and oxygen ligand atoms.

Novel Approach for High Resolution Tem Studies of Ceramic-Ceramic Interfaces

Ceramic coatings on oxides can be studied by high resolution transmission electron microscopy (HRTEM), with minimal sample preparation, if the substrate consists of nonporous particles of simple geometric shape. Interfaces suitable for ‘end-on’ examination by HRTEM can be readily obtained without any necessity for ion-beam thinning. All the interface orientations that are thermodynamically stable are available for examination from a single sample. This technique is of general applicability and can be used for studies of metal-ceramic and ceramic-ceramic interfaces. We have examined the nature of boron nitride interfaces with oxides such as MgO, TiO 2 and Al 2 O 3 and find that BN appears to wet the oxide surface and form tough, adherent coatings. The hexagonal crystalline BN grows with the (0001) planes always being locally parallel to the oxide surface in every instance.