Synthesis, characterization, and kinetic evaluation of planar and supported heterogeneous catalysts
Abstract
An integrated approach for biomass upgrading to fuels requires catalyst synthesis, characterization, and kinetic evaluation. The work was divided in three areas, 1-preparation and characterization of model catalysts containing metal oxide overcoats and metal nanoparticles synthesized by atomic layer deposition (ALD), a gas-phase material deposition technique, 2-the study of structure sensitivity for formic acid decomposition on planar Pt catalysts, and 3-characterization of bimetallic Pt-M (M = Re, Mo) catalysts using surface sensitive techniques and theory. In the first project, metal oxide overcoats, which have been found to prevent metal nanoparticle deactivation via sintering and coking in biomass upgrading reactions, were prepared by ALD. In this work, model alumina-overcoated catalysts were synthesized by exposing Pd(111), Pt(111), and Cu(111) surfaces to trimethylaluminum at ca. 10-6 mbar and a co-reactant (H2O or O2) at ca. 10-6 or 0.1 mbar. Nominal Al coverages differed on each surface after saturation trimethylaluminum exposure (≥ 500 L) at 200°C in the order Pd(111) (1.4 ML) > Pt(111) (1.0 ML) > Cu(111) (no Al), and Al alloyed only with Pd(111). Trimethylaluminum adsorbed on Cu 2O-covered Cu(111), consuming adsorbed oxygen. Overlayer morphologies and adsorbed carbon species were found to be different on each surface, indicating that the interaction of trimethylaluminum with transition metal surfaces is substrate-dependent. Details of each reaction mechanism are discussed. Atomic layer deposition was also used for the synthesis of metallic nanoparticles. Dissociative adsorption sites for M(II) (M = Pd or Cu) hexafluoroacetylacetonates (hfac) on rutile TiO2(110) to form adsorbed (hfac) and M(hfac) were determined, and annealing this surface resulted in the formation of M nanoparticles. In the second project, formic acid decomposition kinetics were evaluated on planar Pt catalysts. Hydrogen is necessary for oxygen removal from biomass, and formic acid, a byproduct of biomass upgrading, catalytically decomposes to produce H2 and CO2 or H2O and CO. Batch reactor kinetics were found to be structure-insensitive on Pt(111), (100), and a polycrystalline foil under standard reaction conditions (1% HCOOH, 1.875% H2, 1.875% CO, PTotal = 800 Torr, 220°C), within measurement precision. Approximate CO2 formation turnover rates were 2.6 ± 0.6, 3.7 ± 1.0, and 3 ± 2 s-1 at 220°C on Pt(111), (100), and Pt foil, respectively, while CO selectivity remained < 1% for conversions < 10%. Finally, bimetallic Pt-M (M = Re, Mo) catalysts were characterized in the third project. Exposing Re/Pt(111) surfaces synthesized by ultra-high vacuum chemical vapor deposition to oxygen formed ReOx (0.5 < x < 1). Oxygen desorbed above 973 K, and a Pt skin formed over Re, which bound O and CO more weakly than either monometallic surface. Adsorbate frequencies were calculated by density functional theory and compared to experiment, which confirmed that rhenium oxide clusters were present on the O-exposed, Pt skin surface. Theoretical calculations showed that the binding energy trends observed by XPS and HREELS experiments can be explained on the basis of d-band centers of Pt-Re systems. Pt-Mo catalysts supported on multiwall carbon nanotubes were also characterized by x-ray photoelectron spectroscopy which revealed the presence of PtxMo (2 < x < 3) alloy, Mo carbide-like, Mo4+, and Mo6+ phases, confirming x-ray absorption results. Catalysts with more Mo relative to Pt contained fewer Pt monometallic and more PtxMo bimetallic nanoparticles, and at the highest Mo loadings Mo oxide and carbide-like phases dominated, indicating that Mo phase distribution is a function of Mo loading.
Degree
Ph.D.
Advisors
Delgass, Purdue University.
Subject Area
Chemical engineering
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