Robert Mark Friedman

Characterization of CuAl2O3 catalysts

Abstract The interaction of cupric ions with γ-alumina supports was studied by physical characterization of a wide range of catalyst preparations. The results from the different techniques, extended X-ray absorption fine structure (EXAFS), X-ray absorption edge shifts, X-ray diffraction, ESR, ESCA, and optical spectroscopy, were combined with prior work to develop a coherent description of the phases present and cation site distribution in the virgin and aged catalysts. Apparent inconsistencies between earlier workers are rationalized in terms of the present model and the details of the experimental probe used. A number of transition γ-aluminas appear to have a similar threshold loading for the appearance of crystalline cupric oxide, ~4 wt% Cu/100 m 2 /g support surface area. In the dispersed phase, the cupric ions are predominantly, but not exclusively, in a tetragonally distorted octahedral environment; there are some tetrahedrally coordinated ions present. The relative proportion of tetrahedral to octahedral cupric ions increases with time at calcination temperatures above 600 °C until the cation distribution of bulk copper aluminate is reached; i.e., 60% tetrahedral and 40% octahedral sites.

Evaluation of matrix-assisted laser desorption ionization mass spectrometry for polymer characterization

A protocol for the preparation of polymeric samples for time-of-flight matrix-assisted laser desorption ionization mass spectrometry (TOF-MALDI-MS) analysis was developed. Dithranol was identified as a good matrix for polystyrene (PS), and the addition of silver for cationization of molecules was determined to be necessary. Based on this preparative method, low molecular weight samples of other polymers [polyisoprene, polybutadiene, poly(ethylene oxide), poly(methyl methacrylate), and polydimethylsiloxane] were analyzed with molecular weights up to 49 ku. The effects of laser intensity were determined to influence the molecular weight distribution of intact oligomers, most significantly for low molecular weight polymers. Linear and reflectron modes of analysis were evaluated; better signal intensity and resolution were obtained in the reflectron mode. The TOF-MALDI-MS measurements are compared with time-of-flight secondary ion mass spectrometry (TOF-SIMS) and gel permeation chromatography (GPC) for the same polymers. The Mn values calculated by TOF-MALDI-MS consistently are higher than values calculated by TOF-SIMS for all classes of polymers with molecular weights up to 8 ku. The molecular weights of the PS calculated from TOF-MALDI-MS are in good agreement with GPC (±10%). The composition of the terminal group on a polymer chain may affect the ion yields. The ion yields of intact oligomers were evaluated as a function of end group composition for both TOF-MALDI-MS and TOF-SIMS. The slight disparity of results between TOF-SIMS and TOF-MALDI-MS for the perfluoroalkyl-terminated PS suggests that the oligomers are desorbed preferentially from the surface in the TOF-SIMS analysis, rather than having an increased ionization probability.

The preparation and use of high surface area silicon carbide catalyst supports

Abstract High surface area SiC has been prepared in a tubular reactor by the high-temperature, vapor phase decomposition of Si-containing compounds such as tetramethylsilane. Surface areas near 50 m2 g−1 for β-phase SiC were obtained in these initial experiments. These powders did not sinter during heating to 1173 K, even in the presence of oxygen, and surface areas actually increased somewhat due to burnoff of residual elemental carbon. Using these SiC powders as a support resulted in well-dispersed Pt/SiC catalysts which possessed normal kinetic behavior for methanation. However, Ni/SiC catalysts exhibited unusual chemisorption behavior, a range of activity for CO hydrogenation, and selectivity only to methane.

A Direct Deposition Method for Coupling Matrix‐assisted Laser Desorption/Ionization Mass Spectrometry with Gel Permeation Chromatography for Polymer Characterization

Matrix-assisted laser desorption/ionization (MALDI) measurements of polymeric materials of narrow polydispersity provide molecular weight measurements that agree closely with those from conventional tools such as gel-permeation chromatography (GPC). However, materials that have a broader molecular weight distribution (polydispersity) > 1.1) have been shown to diminish the accuracy of the MALDI results. In a recent effort to overcome this limitation, some researchers have utilized GPC prior to MALDI analysis to collect fractions with narrower molecular weight distributions. In our research, the GPC eluent was spray deposited onto a rotating matrix-coated substrate and the resulting polymer ‘trail’ was characterized directly by MALDI. Specifically, molecular weight information was determined using various points along poly(methyl methacrylate) sample ‘trails’ deposited on a matrix of trans-3-indoleacrylic acid. The overall number-average molecular weight (M n) and molecular weight distribution (MWD) for each of the samples were calculated. The Mn values are in fairly good agreement with manufacturers values, though the MWD values determined by MALDI are narrower.

An Investigation into the Importance of Polymer–Matrix Miscibility Using Surfactant Modified Matrix‐assisted Laser Desorption/Ionization Mass Spectrometry

Sample preparation, namely finding a suitable matrix–analyte combination, has been one of the most important concerns in using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for synthetic polymers. Herein we report the novel use of surfactants in the polymer sample preparation procedure. Specifically, three nonionic surfactants were investigated for MALDI analysis of poly(methyl methacrylate) (PMMA). This approach resulted in major enhancements in the PMMA molecular ion signals with dithranol as the matrix. The results illustrate the importance of polymer–matrix miscibility in optimizing MALDI investigations.

The structure of copper aluminate: cation distribution at different temperatures and its implications for Cu/A1203 catalysts

Abstract Samples of spinel CuA1 2 0 4 prepared at 639°C and 952°C were studied by structural analysis of their X-ray diffraction powder patterns and their X-ray absorption fine structure (EXAFS). The diffraction analysis showed that 40% of the cupric ions were in octahedral sites. The results were identical within lσ using three datasets, two for the high temperature preparation and one for the low temperature preparation. The cation distribution between the two coordination environments, octahedral and tetrahedral, is consistent with the extended X-ray absorption fine structure, the EXAFS showing 45% of the cupric ions in octahedral environments. The samples were deficient in cupric ions, more closely modeling the case for supported transition metal ion catalysts; the EXAFS refinement explicitly treats the presence of cation vacancies in the spinel lattice. The implications of the constant cation distribution for the two preparations at different temperatures is discussed with regard to the forces determining cation site locations in the bulk and on the surface of the transition aluminas relevant to supported transition metal ion catalysts.

Expanded lattice ruthenium pyrochlore oxide catalysts I. Liquid-phase oxidations of vicinal diols, primary alcohols, and related substrates with molecular oxygen

Abstract Ternary ruthenium oxide oxidation catalysts are reported that function directly with molecular oxygen for the conversion of oxygenated hydrocarbon substrates to carboxyl-containing products. The oxide catalysts have an expanded lattice pyrochlore structure with the general composition A2+xRu2−y(A = Pb, Bi; 0 1,2-diol > primary alcohol) over these oxide catalysts closely follows that reported previously when these oxides are used as anodic electrocatalysts suggesting common surface intermediates. Trickle bed reactor operation provides the highest product selectivity: trans-1,2-cyclohexanediol is oxidatively cleaved to adipate in NaOH solutions with 74 to 95% selectivity at contact times of 0.042 to 0.784 hr and temperatures of 26 to 95°C. The bismuth-containing oxides, Bi2+xRu2−xO7−y, show stable catalyst performance under trickle bed reactor operation for over 180 hr. Complete details of these catalytic oxidations are provided along with a discussion of substrate reaction pathways and oxygen activation by these novel expanded lattice ruthenium pyrochlore oxide catalysts.

Structure and electrochemical behavior of a flavin sulfide monolayer adsorbed on gold

The electrochemical behavior and structure of monolayers of the synthetic flavin 10-(3′-methylthiopropyl)-isoalloxazinyl-7-carboxylic acid adsorbed on gold is reported. The redox behavior of the compound is quasi-reversible. The surface concentration is estimated to be 1.5×10−10 mol cm−2. An electron transfer rate constant of 340 s−1 is estimated for the cathodic process and a value of 540 s−1 is estimated for the anodic process. The reduction potential of the monolayer is found to shift with pH as expected for a 2e−, 2H+ process. The monolayers have also been characterized by X-ray photoelectron spectroscopy (XPS) and low voltage field emission secondary electron microscopy (LVFESEM). The most likely orientation would have the long axis parallel to the surface with the carboxyl group exposed to the solution. A comparison of the C1s XPS spectra at glancing and normal emission indicates that the carboxyl group is at the film surface. The LVFESEM images indicate that the molecules pack in domains that do not follow the topology of the gold grains. Semi-quantitative examination of the micrographs shows that 10–15% of the gold surface is uncovered. The bare gold substrate catalyzes the oxidation of NADH; the presence of the flavin film reduces the observed catalysis by the gold substrate.

Expanded lattice ruthenium pyrochlore oxide catalysts II. Catalyst surface investigations by electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction and oxidation

Abstract Five expanded lattice ruthenium pyrochlore oxide powders with the general formula A 2+ x Ru 2− x O 7− y ( A = Pb: x = 0.06, 0.15, 0.62; y = 0.5 and A = Bi: x = 0.39, 0.86; 0 y ≤ 0.5) were investigated using high-resolution electron microscopy (HREM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction/oxidation (TPR/TPO) to ascertain factors that contribute to their low-temperature catalytic oxidation activity toward 1,2-diols and alcohols in aqueous alkaline solution. HREM data find small crystallites that vary from about 25 to 200 A in diameter. Image analysis techniques applied to one 100-A crystallite of Pb 2.62 Ru 1.38 O 6.5 reveal a 5% range of lattice spacings about the exterior portions of the particle. XPS data were collected in both the valence-band and core-level regions. XPS valence-band spectra for samples with higher levels ( x > 0.15) of Ru-site substitution by the A -site atoms display a less intense band near the Fermi energy level indicating a reduced Ru 4 d character compared to the more stoichiometric analogues. Core level XPS bands contain contributions from two different valence states for each of the Pb, Bi, and Ru atoms in the A 2+ x Ru 2− x O 7− y series. XPS-derived compositions show a higher A /Ru ratio for Pb 2.62 Ru 1.38 O 6.5 than the bulk whereas the other four oxides have A /Ru ratios that are similar in the surface (XPS) and bulk (XRD) compositions. TPR using H 2 of the A 2 + x Ru 2 − x O 7 − y oxides reveals remarkable reactivity below 200°C that is in line with loosely bound oxygen in these oxides. A sample of Pb 2.62 Ru 1.38 O 6.5 with a lead-rich surface shows markedly less reactivity toward H 2 below 100°C than the other oxides. No distinct differences are seen in TPR data for all five expanded lattice ruthenium pyrochlore oxides between the surface and bulk oxygen. Reduced oxides reoxidize reversibly with oxygen after up to 20 mol% reduction (based on oxygen content) but at rates that are slow compared to H 2 reduction. The lack of a correlation between the TPR/TPO data and the liquid-phase catalytic activities toward 1,2-diols and alcohols is explained through differences in the active site preferences by the various substrates.

Thiol protected platinum black and palladium black catalysts in oxidation catalysis

Platinum and palladium metals are common catalysts for formic acid wet oxidation, exhibiting high activity and good stability. However, in the presence of some reactive organic substances, even in low concentrations, they suffer severe deactivation. This paper demonstrates the feasibility of protecting metal catalysts from aggressive media by creating a tertiary structure over the active sites. The approach is based on the known principle of metal surface modification by self-assembled organic monolayer films. We have found that self-assembled monolayers (SAMs) fabricated from heterocyclic thiols containing nitrogen, e.g., 6-mercaptopurine (6MP), can protect platinum and palladium black catalysts for formic acid oxidation in the presence of iminobis[methylenephosphonic acid] (IDMPA). IDMPA was chosen as a model inhibitor because it displays strong chelating properties. Catalysts protected with 6MP demonstrate higher activity and stability and have significantly lower metal leaching (up to 15 times in case of palladium black) than their unprotected counterparts. The surface structure of the catalysts and their surface composition were characterized using X-ray photoelectron spectroscopy (XPS) and Scanning Tunneling Microscopy (STM).