Exposed surfaces on shape-controlled ceria nanoparticles revealed through AC-TEM and water-gas shift reactivity.

Aberration-corrected transmission electron microscopy and high-angle annular dark field imaging was used to investigate the surface structures and internal defects of CeO2 nanoparticles (octahedra, rods, and cubes). Further, their catalytic reactivity in the water-gas shift (WGS) reaction and the exposed surface sites by using FTIR spectroscopy were tested. Rods and octahedra expose stable (111) surfaces whereas cubes have primarily (100) facets. Rods also had internal voids and surface steps. The exposed planes are consistent with observed reactivity patterns, and the normalized WGS reactivity of octahedra and rods were similar, but the cubes were more reactive. In situ FTIR spectroscopy showed that rods and octahedra exhibit similar spectra for -OH groups and that carbonates and formates formed upon exposure to CO whereas for cubes clear differences were observed. These results provide definitive information on the nature of the exposed surfaces in these CeO2 nanostructures and their influence on the WGS reactivity.

Pure component spectral analysis of surface adsorbed species measured under real conditions. BTEM-DRIFTS study of CO and NO reaction over a Pd/gamma-Al2O3 catalyst.

The adsorption of NO and CO was studied on an alumina-supported palladium catalyst by in situ diffuse reflectance infrared spectroscopy (DRIFTS). The temperature range was 50-160 degrees C and a wide variety of partial pressures was used. The band-target entropy minimization (BTEM) algorithm was applied to the DRIFTS data sets resulting in the pure component spectra of numerous species adsorbed on both the Pd (primarily a variety of Pd-CO and Pd-NO species, in various oxidation states and coordinations) and alumina surface species (i.e. nitrates, nitrites, carbonates, bicarbonates, formates, and isocyanates) as well as gas phase species. Twenty seven previously known species were identified as well as three new and previously unreported or previously unassigned spectra. The present study indicates that BTEM can be meaningfully applied to Pd/Al2O3 DRIFTS in order to provide enhanced spectroscopic analysis. Moreover, the present results are compared in detail with the recent BTEM analysis of CO and NO adsorption on Pt/Al2O3 using DRIFTS (Phys. Chem. Chem. Phys. 2008, 10, 3535).

Spectral reconstruction of surface adsorbed species using band-target entropy minimization. Application to CO and NO reaction over a Pt/gamma-Al2O3 catalyst using in situ DRIFT spectroscopy.

The adsorption of NO and CO was studied on an alumina-supported platinum catalyst by in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). The temperature range was 50-160 degrees C and a wide variety of partial pressures was used. The band-target entropy minimization (BTEM) algorithm was applied to the DRIFTS data sets resulting in the pure component spectra of numerous species adsorbed on both the Pt (primarily a variety of Pt0-CO and Pt2+ -CO species) and alumina surface (i.e. nitrates, nitrites, bicarbonates, isocyanates) as well as gas-phase species. Thirty-one previously known species were identified as well as 6 new and previously unreported or previously unassigned spectra. The present results indicate that BTEM analysis of DRIFTS data is a very promising tool for the study of heterogeneous catalytic in situ spectroscopic data.

Fourier transform Raman spectral measurements of powdered quaternary mixtures of organic compounds Exceptional pure component spectral reconstruction using band-target entropy minimization (BTEM).

Fourier transform Raman spectra of eight mixtures of four organic solids, namely dicyandiamide, melamine, acetamide and urea were measured. Matrices formed from these spectra were first subjected to singular value decomposition to obtain the right singular vectors. The right singular vectors were then subjected to blind source separation using band-target entropy minimization (BTEM), thus no a priori information (i.e. involving the nature of the components present, their spectra, nor their concentrations) was included in the analysis. The recovered pure component spectra are of exceptionally high quality and are consistent with pure reference spectra. Various empirical and statistical tests, such as the Euclidean norm and target transform factor analysis, were employed to assess the quality of the recovered spectra. The present results indicate the applicability of combined Raman and BTEM analysis for solid mixtures.

Tricarbonyls of low-coordinated Au(0) atoms in zeolite-supported gold nanoparticles: Evidence from infrared and X-ray absorption spectroscopies.

Mononuclear gold complexes in zeolite NaY were synthesized from initially physisorbed Au(CH3)2(C5H7O2) and characterized by X-ray absorption and infrared spectra recorded as the samples were exposed to flowing CO. X-ray absorption spectra demonstrate the formation of zero-valent gold nanoparticles during the CO treatment. Three new nu(CO) bands grew in during this treatment, at 2070, 2033, and 2000 cm(-1), characteristic of carbonyls of Au0. Because the relative intensities of these bands decreased monotonically when the flow of CO was replaced by flowing He, it is inferred that they correspond to a single Au0(CO)3 species, on low-coordinated Au atoms. This is the first example of an Au0(CO)3 species.

Synthesis and reactivity of dimethyl gold complexes supported on MgO: characterization by infrared and X-ray absorption spectroscopies.

Dimethyl gold complexes bonded to partially dehydroxylated MgO powder calcined at 673 K were synthesized by adsorption of Au(CH3)2(acac) (acac is C5H7O2) from n-pentane solution. The synthesis and subsequent decomposition of the complexes by treatment in He or H2 were characterized with diffuse reflectance Fourier transform infrared (DRIFT), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The XANES results identify Au(III) in the supported complexes, and the EXAFS and DRIFTS data indicate mononuclear dimethyl gold complexes as the predominant surface gold species, consistent with the lack of Au-Au contributions in the EXAFS spectrum and the presence of nu(as)(CH3) and nu(s)(CH3) bands in the IR spectrum. EXAFS data show that each complex is bonded to two oxygen atoms of the MgO surface at an Au-O distance of 2.16 angstroms. The DRIFT spectra show that reaction of Au(CH3)2(acac) with MgO at room temperature also formed Mg(acac)2 and H(acac) species on the support. Treatment of the dimethyl gold complexes in He or H2 at increasing temperatures varying from 373 to 573 K removed CH3 ligands and caused aggregation forming zerovalent gold nanoclusters of increasing size, ultimately with an average diameter of about 30 angstroms. Analysis of the gas-phase products during the genesis of the gold clusters indicated formation of CH4 (consistent with removal of CH3 groups) and CO2 at 473-573 K, associated with decomposition of the organic ligands derived from acac species. O2 and CO2 were also formed in the decomposition of ubiquitous carbonates present on the surface of the MgO support.