Date of Award
Campus Access Dissertation
John R Monnier
Christopher T Willaims
Bimetallic catalysts have replaced many monometallic catalysts for a wide range of catalytic applications, due to their enhanced selectivity, stability, and/or activity relative to their corresponding monometallic components. These bimetallic catalysts are typically prepared using precipitation or impregnation methods which often results in a complex mixture of both monometallic and bimetallic particles of varying compositions and makes any correlations between catalyst performance, catalyst characterization, and catalyst composition very difficult. Electroless deposition (ED) is one of the preparative methodologies that provides a way to catalytically deposit in a controlled manner, a second metal only on the surface of a pre-existing metallic surface (i.e., not on support). Thus, bimetallic surface compositions can be more effectively controlled to permit more precise correlation of catalyst composition with performance using much lower amounts of the second metal.
Electroless deposition has been used extensively in industry for the preparation of continuous film coatings in fields ranging from electronics to corrosion, not for the preparation of bimetallic catalyst particles. The application of ED in preparing bimetallic particles was first started few years back at USC for catalytic- chemical (Ag-Pt) and fuel cell (Pt-Pd and Pt-Co) applications. The same principles used for formation of thick films are employed for catalyst(s) synthesis, except that the reaction conditions must be tailored to deposit only small, controlled amounts of the reducible metal ion onto the surface of pre-existing, catalytic metal particles. The successful application of ED requires a bath containing a reducible metal salt and an appropriate reducing agent that (1) is thermodynamically unstable, yet kinetically stable in the absence of a catalyst, (2) does not result in the electrostatic adsorption of the reducible metal salt on the catalyst support, and (3) gives controlled rates of catalytic deposition on the primary catalyst surface. In this dissertation, the systematic development of such stable electroless bath(s) using metal bis-cyano salts of Au+ or Ag+ or Cu+) as metal sources in comparison to less stable chloride precursors with HCHO or N2H4 (for Au and Ag) or DMAB (for Cu) as reducing agents were discussed. The ED bath conditions such as pH was maintained above the point of zero charge (PZC) for silica such that electrostatic adsorption or spontaneous deposition of metal on to the support is avoided in the absence of catalytic substrate.
These electroless baths were optimized to synthesize a series of each group IB-Pd/SiO2 bimetallic catalysts to obtain incremental surface coverages and compositions of each group IB metal on Pd. The catalytic deposition of second metal (Au) deposition on Pd was also kinetically evaluated. Thus synthesized bimetallic surfaces were thoroughly characterized using selective chemisorption, atomic absorption spectroscopy (AAS), Fourier transform infrared spectroscopy (FTIR) of adsorbed CO and X-ray photoelectron (XPS) spectroscopy techniques for possible structural and/or electronic interactions between deposited metals and Pd. The decrease in Pd surface sites upon addition of IB metal confirms that electroless deposition is a surface deposition technique. The results from FTIR of CO adsorbed sites suggest that deposition of Cu and Ag are selective towards Pd(111) sites, while Au deposits non-discriminately on all Pd sites. Finally XPS analysis indicates the possibility of electronic interactions between deposited group IB metal and Pd. Further, these bimetallic were being evaluated for selective olefin hydrogenation and selective oxidation of biomass derived polyols like glycerol.
The effect of Au on Pd surface was kinetically evaluated for structure insensitive propylene hydrogenation probe reaction. As the fractional coverage of Au on Pd (θAu ≥ 0.60) increases, the Pd activity i.e., turnover frequency was increased 10-15 times showing structure sensitivity. The enhanced catalytic activity can be explained by the disruption of continuous Pd ensembles by Au deposition, which prevents formation of the multiply bonded and less reactive propylidyne, while permitting formation of highly reactive and weakly π-bonded propylene. Further Ag-Pd, Cu-Pd catalysts were being examined for selective hydrogenation of acetylene in presence of excess ethylene to provide pure feed for ethylene polymerization process.
The initial study on aqueous-phase catalytic glycerol oxidation reactions with molecular oxygen was described in chapter 5. The reactions were performed in both the absence and presence of a commercial 5 wt% Pd/C catalyst, at T=60 °C, P(O2) = 40 psig, 0.1M glycerol concentration, at varying (pH 10 to 14) initial pH conditions. As the reaction progresses OH‾ ions were consumed, due to their central role in initiating H-abstraction from the OH group. Thus the activity was decreased and the oxidation products were found to change unpredictably in the absence of pH control. The results suggest that the conditions under which glycerol (and other polyol) oxidation reactions are carried out must be chosen with great care if true catalytic phenomena are to be measured. Furthermore, control of pH during the reaction is crucial for measuring accurate kinetic rates and selectivity that can be correlated with catalyst structure.
Rebelli, J.(2011). Preparation, Characterization and Evaluation of Bimetallic Catalysts Prepared by Electroless Deposition Methods. (Doctoral dissertation). Retrieved from http://scholarcommons.sc.edu/etd/590