Gary L. Haller

Metal–Support Interaction: Group VIII Metals and Reducible Oxides

Publisher Summary This chapter discusses the interaction of small metal particles with reducible oxide supports, and focuses on Group VIII (or Group 8-10) metals supported on TiO 2 . The strong suppression in catalytic activity and chemisorption capacity observed on TiO 2 -supported catalysts after reduction at high temperatures can be, at least partially, explained by a geometric blocking of sites by TiO 2 species. The dominant mechanism of formation of TiO 2 species on metal particles is migration during the high-temperature reduction, but some deposition of metal precursors during preparation may also occur. This picture finds an analog in the structure of the bimetallic clusters observed in Group VIII-Group Ib catalysts, in the sense that the structure sensitive reactions, e.g., alkane hydrogenolysis, are most affected whereas only modest effects are observed for structureinsensitive reactions, e.g., hydrogenation-dehydrogenation. The migration of reduced species from the support is accompanied by the formation of metal-Ti bonds, which provide the thermodynamic driving force for the migration. The metal-support bonding is not identical to that in the intermetallic compounds because of the associated oxygen that imparts cationic character to the Ti and Group VIII metal partner; in other words, it may be analogous to the Group VIII-Ti bonding that persists on intermetallic compounds surfaces after oxygen chemisorption.

A model of metal-oxide support interaction for Rh on TiO2

Abstract Using the Rh/TiO 2 catalyst system, a simple model for the different kinds of interaction that occur following a low- and a high-temperature reduction of Group VIII noble metals supported on TiO 2 , was developed. The model proposes a delocalized transfer of charge from Rh to TiO 2 after low-temperature reduction and a localized (chemical bonding) transfer of charge from the support to Rh after a high-temperature reduction. In addition, it is suggested that migration of a reduced species of the support is responsible, in part, for the depression of hydrogenolysis activity following a high-temperature reduction. The experimental evidences for the latter comes from comparing H 2 and H atom reduction, the kinetics of the onset of strong-metal-support interaction, and a strong analogy between group VIII-group Ib and group VIII-TiO 2 interaction effects on structure-sensitive and insensitive reactions. The kind of charge transfer is based on observed effects on kinetic parameters and previously reported spectroscopic experiments.

Density functional theory model for calculating pore size distributions: pore structure of nanoporous catalysts

Using the example of nanoporous catalysts, we discuss the non-local density functional . theory NLDFT model applied to physical adsorption of nitrogen and argon. The model has been used for predicting adsorptionrdesorption isotherms in nanopores of different geome- . tries over a wide range of pore sizes 0.5)100 nm , and for calculating pore size distributions from adsorption isotherms based on given intermolecular fluid)fluid and fluid)solid poten- tials. The development of new nanoporous catalysts requires reliable characterization methods. We critically analyze different methods which are currently used for pore structure characterization in the range of nanometers. Calculations of the pore size distributions from nitrogen and argon adsorption isotherms are presented. Our primary method is based on the NLDFT model of adsorption on MCM-41, developed earlier. The results obtained with the NLDFT model are compared with other methods. It is shown, that the pore structure of nanoporous catalysts can be quite complex, and that Ar and N isotherms contain compli- 2 mentary information. The NLDFT model is recommended for evaluation of pore size distributions in nanoporous catalysts and other MCM-41 based materials. Q 1998 Elsevier Science B.V. All rights reserved.

Metal-support effects in Pt/L-zeolite catalysts

A series of Pt/L-zeolites with different cations (varying acidity) were prepared and characterized by H2 and CO chemisorption and competitive toluene/benzene hydrogenation. The ratio of adsorption equilibrium constants for toluene/benzene was extracted from rates of hydrogenation. The variation of this ratio across a series of alkaline-earth exchanged Pt/L-zeolites indicates that there is a varying support interaction with Pt clusters in the zeolite and suggests that the clusters interact electronically with the support.

Chemisorptive and Catalytic Behavior of Chromia

Publisher Summary The catalytic properties of chromia have received considerable attention, probably more than those of any other catalysts except Group VIII metals, silica, and alumina. This chapter describes the unsupported chromia as a catalyst for relatively low temperature reactions under reducing conditions, and provides an outline of the wide variety of reactions catalyzed by chromia or supported chromia. A chromia gel activated only at 150° has no detectable catalytic activity for the hydrogenation of olefins at room temperatures nor does it chemisorb oxygen or carbon monoxide. Catalytic and adsorptive capacity develops upon pretreating the gel at higher temperatures. After heating to 400°, chromia rapidly chemisorbs oxygen and carbon monoxide at −78° and it leads to rapid hydrogenation of ethylene at −78°. The surface of chromia appears to be an ideal case for study at the present. By activation at increasing temperatures, one can vary the number of active sites from none to some maximum number. At a low density of sites, one can hope that the sites are well separated and non-interacting. One can compare chemisorption of various gases with specific catalytic activities at various levels of site densities and hope to gain information about site heterogeneity.

The dynamics of CO oxidation on Pd, Rh, and Pt studied by high-resolution infrared chemiluminescence spectroscopy

The dynamics of carbon monoxide oxidation on Pd, Rh, and Pt foils were probed under nearly collision‐free conditions using high resolution infrared chemiluminescence. Auger electron spectroscopy was used to verify the absence of impurities on the surfaces. The reactants were supplied to the surface through a free jet nozzle source, while a Fourier transform infrared spectrometer operating at 0.012 cm−1 resolution was used to fully resolve the rotational structure of several vibrational transitions in the product CO2 (22 in the case of Pd). In all cases, the product CO2 is vibrationally excited and the apparent vibrational temperatures are in the same order as the peak reaction rates, i.e., Pd>Pt>Rh. The surface coverage of oxygen on Pd was varied by changing the CO: O2 ratio and the surface temperature and, in both cases, increasing oxygen coverage causes an increase in vibrational excitation of product CO2. On Pt and Rh, the apparent temperatures of different vibrational modes are similar, while on Pd, t...

The exciting oxidation of CO on Pt

Abstract An uncollimated molecular beam (free jet) of CO and O 2 molecules incident on a polycrystalline Pt surface at 775 K in vacuo produces vibrationally hot CO 2 molecules at a density sufficient for infrared emission spectrometry. Analysis of spectra at a resolution of 0.1 cm −1 clearly shows that nascent product molecules have much more internal excitation than would be the case for equilibrium at the surface temperature.

Enhanced cellular activation with single walled carbon nanotube bundles presenting antibody stimuli.

Efficient immunotherapy can be accomplished by expanding T cells outside the body using single walled carbon nanotube (SWNT) bundles presenting antibody stimuli. Owing to the large surface area of these bundles, which can reach 1560 m (2)/g, T cell stimulating antibodies such as anti-CD3, can be presented at high local concentrations inducing potent activation of T cells. We show that anti-CD3 adsorbed onto SWNT bundles stimulate cells more effectively than equivalent concentrations of soluble anti-CD3. Stimulation by antibody adsorbed onto SWNT is significantly higher than other high surface area materials (activated carbon, polystyrene, and C60 nanoparticles), suggesting unique properties of SWNT bundles for stimuli presentation. We demonstrate the surface area tunability of these bundles by chemical treatment and its effect on antibody adsorption and subsequent T cell activation. In addition, the T cell response varied with the concentration of SWNT in a concentration dependent manner. Antibody stimuli adsorbed onto SWNT bundles represent a novel paradigm for efficient activation of lymphocytes, useful for basic science applications and clinical immunotherapy.

A carbon nanotube–polymer composite for T-cell therapy

Clinical translation of cell therapies requires strategies that can manufacture cells efficiently and economically. One promising way to reproducibly expand T cells for cancer therapy is by attaching the stimuli for T cells onto artificial substrates with high surface area. Here, we show that a carbon nanotube-polymer composite can act as an artificial antigen-presenting cell to efficiently expand the number of T cells isolated from mice. We attach antigens onto bundled carbon nanotubes and combined this complex with polymer nanoparticles containing magnetite and the T-cell growth factor interleukin-2 (IL-2). The number of T cells obtained was comparable to clinical standards using a thousand-fold less soluble IL-2. T cells obtained from this expansion were able to delay tumour growth in a murine model for melanoma. Our results show that this composite is a useful platform for generating large numbers of cytotoxic T cells for cancer immunotherapy.

Chiral-selective CoSO4/SiO2 catalyst for (9,8) single-walled carbon nanotube growth

Electronic and optical properties of single-walled carbon nanotubes (SWCNTs) correlate with their chiral structures. Many applications need chirally pure SWCNTs that current synthesis methods cannot produce. Here, we show a sulfate-promoted CoSO(4)/SiO(2) catalyst, which selectively grows large-diameter (9,8) nanotubes at 1.17 nm with 51.7% abundance among semiconducting tubes and 33.5% over all tube species. After reduction in H(2) at 540 °C, the catalyst containing 1 wt % Co has a carbon yield of 3.8 wt %, in which more than 90% is SWCNT. As compared to other Co catalysts used for SWCNT growth, the CoSO(4)/SiO(2) catalyst is unique with a narrow Co reduction window under H(2) centered at 470 °C, which can be attributed to the reduction of highly dispersed CoSO(4). X-ray absorption spectroscopy (XAS) results suggested the formation of Co particles with an average size of 1.23 nm, which matches the diameter of (9,8) tubes. Density functional theory study indicated that the diameter of structurally stable pure Co particles is scattered, matching the most abundant chiral tubes, such as (6,5) and (9,8). Moreover, the formation of such large Co particles on the CoSO(4)/SiO(2) catalyst depends on sulfur in the catalyst. XAS results showed that sulfur content in the catalyst changes after catalyst reduction at different conditions, which correlates with the change in (n,m) selectivity observed. We proposed that the potential roles of sulfur could be limiting the aggregation of Co atoms and/or forming Co-S compounds, which enables the chiral selectivity toward (9,8) tubes. This work demonstrates that catalysts promoted with sulfur compounds have potentials to be further developed for chiral-selective growth of SWCNTs.