Synthesis and characterization of Nickel Platinum and Cobalt Platinum pseudomorphic overlayer catalysts intended for aqueous phase reforming of lactose

• Main Author: Skoglund, Michael D
• Format: Dissertation
• Language: English
• Subjects: chemistry, inorganic; engineering, chemical
• ISBN: 1303700956; 9781303700958

Alumina supported bimetallic overlayer Re Pd, Ni Pt, and Co Pt (core shell) catalysts have been synthesized using the directed deposition technique. Single crystal and first principles computational studies have indicated that the formation of an overlayer atop a base metal can have an electronic effect on the surface overlaying metal that ultimately can increase or decrease binding energies for adsorbed species such as H2 and CO. In order to detect electronic modifications of the overlaying Pd or Pt metal several characterization techniques were employed. These techniques include: hydrogen and carbon monoxide chemisorption, an ethylene hydrogenation descriptor reaction, and X-ray absorption spectroscopy. Non-structured bimetallic alloy catalysts were also examined for the Ni and Co systems and when compared to overlayer catalysts exhibited notably different behavior. To determine if changes in catalytic behavior previously reported for Re Pd overlayer catalyst was not caused by particle size effects, samples were made with different metal loading, dispersion, and particle size. Particle size effects have long been known to play a role in catalytic behavior as smaller particles tend to have more unsaturated, highly reactive, metal sites (e.g., corners, kinks, and edges). It was found that the 7 wt% Re Pd (~6 nm particle size) demonstrated similar reductions in quality and strength of hydrogen adsorption as were seen for the smaller particle 2 wt% Re Pd (~3 nm particle size) counterpart. Ethylene hydrogenation activity also proved consistent when normalized based on active Pd sites. Results indicate that the formation of a Pd overlayer was the reason for observed changes in adsorption and hydrogenation activity and the effect cannot be merely attributed to particle size effects. Aqueous phase reforming (APR) has proven capable of reforming a feed into H2 under moderate pressures and temperatures. Because the APR reaction produces H2 and CO products, a catalyst that has a decrease in product adsorptions strength should make for an ideal APR catalyst. Ni Pt and Co Pt catalyst have demonstrated a decrease in hydrogen and carbon monoxide adsorption strength and have shown a reduction in ethylene hydrogenation activity when compared to a pure Pt catalyst. XAS studies have shown that overlayer catalysts demonstrate near edge absorption characteristics dissimilar to that of bimetallic catalyst of similar loading indicating that overlayer synthesis is not merely creating a bimetallic alloy. Successive overlayer depositions resulted in an increase in both Pt-M coordination number and interatomic distance indicating that multiple depositions could be depositing multiple overlayers of platinum. However, even with multiple layers deposited, overlayer catalytic activity remains different than that of non-structured bimetallic alloys and is in line with single crystal and computational predictions for overlayer catalyst systems.