Novel approaches to catalyst characterization in planar and porous systems

Bradley R Fingland, Purdue University

Abstract

Catalysts are critical to nearly all industrial processes. They transform toxic effluents to more environmentally friendly compounds, they make fiscally unattractive processes economically viable, and reduce the quantities of feedstock needed to make a product. Understanding how a catalyst functions enables one to better utilize a catalyst’s efficacy but can require a host of characterization techniques. This dissertation focuses on characterizing catalysts by transforming them to planar, model systems and by probing high surface area, realistic catalysts with an operando x-ray absorption spectroscopy (XAS) reactor. The operando XAS technique was first validated, then used to probe the WGS reaction over Pt and Au catalysts, then further utilized as a technique for aqueous phase glycerol reforming over a Pt-Mo/C catalyst. Model catalysts were developed such that all the catalytically active material was exposed on the surface of a planar substrate, where surface science studies are well suited. Additionally, an ultra high vacuum (UHV) apparatus with XPS, STM, and a batch reactor was designed, constructed and tested to accommodate these model systems. To create the planar model catalyst, a TiO 2 (110) single crystal was loaded with Pt using two deposition techniques: mass selection deposition (0.03 ML Pt and 0.01 ML Pt) and atomic layer deposition (0.02 ML Pt). Characterization of these surfaces showed them to be heavily contaminated by carbonaceous species and, thus, inactive for ethylene hydrogenation. However, the concept of the model catalyst and the custom built, validated UHV system are expected to be powerful tools for future surface science studies. To create operando catalyst characterization, a fixed-bed, plug-flow reactor was designed and built to allow XAS and kinetic data to be measured simultaneously. Analysis of a well characterized Pd-Zn/Al 2O3 water-gas shift (WGS) catalyst under reaction conditions revealed that the degree of Pd-Zn alloy formation was smaller than what was observed by ex-situ extended x-ray absorption fine structure (EXAFS), demonstrating the importance of characterizing the catalyst under reaction conditions. In addition to traditional XAS, the reactor was utilized in a novel approach to quantify adsorbates on catalysts under reaction conditions with difference x-ray absorption near edge spectroscopy (ΔXANES). This unique type of analysis was critical to understanding why the turnover rate for the WGS reaction of 1.4 nm Au/Al2O3 was approximately 20 times higher than that of 1.6 nm Pt/Al2O3. Adsorption of CO, H 2 or H2O led to changes in the L3 XANES spectra that were used to determine the surface coverage of the individual species. Analysis of the operando ΔXANES led to adsorbate surface coverages during the WGS reaction. The results showed that Pt catalysts adsorb CO, H2, and H2O more strongly than the analogous Au, in agreement with the lower CO reaction order and the observed higher rates over Au. The ability of operando XAS to probe a catalyst while concurrently measuring a reaction rate also has applications in aqueous phase reactions. For the first time, the coverages of H2, CO, and H 2O were measured on the surface of a Pt-Mo/C glycerol reforming catalyst at true reaction conditions, 260 °C and 25 bar, in a 30 wt% glycerol aqueous solution. It was shown that the Pt and Mo form a Pt rich alloy under reaction conditions and that, in a separate gas phase analysis, the Mo enhanced the WGS rate of per total Pt by a factor of 10. Because the WGS reaction and glycerol reforming are so closely linked, as evidenced by the presence of CO on the surface of Pt from XANES analysis and by effluent CO in low CO/CO2 ratios, high throughput WGS experiments are proposed to help select the best glycerol reforming catalysts.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

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