ADSORPTION OF TRANSITION METAL COMPLEXES ON METAL OXIDE SUPPORTS: PROBING THE SOLID-LIQUID INTERFACE IN HETEROGENEOUS CATALYST SYNTHESIS
Open Access
- Author:
- Mukhopadhyay, Ahana
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 01, 2018
- Committee Members:
- Robert Martin Rioux Jr., Dissertation Advisor/Co-Advisor
Robert Martin Rioux Jr., Committee Chair/Co-Chair
Michael John Janik, Committee Member
Xueyi Zhang, Committee Member
Thomas E Mallouk, Outside Member - Keywords:
- solid-liquid interface
electrostatic adsorption
single atom catalysts
isothermal titration calorimetry
noble metals
reducible oxides
methane combustion - Abstract:
- Widespread industrial applications and large impact of supported late transition precious metal catalysts on the global economy serves as the prime motivation for the dedication of academic researchers towards focusing on the scalable and affordable design of efficient catalysts. Catalyst design requires a fundamental understanding of how the different synthetic steps (adsorption, drying, pretreatment, etc) influence the properties of the final catalyst. Moreover, in current times, single-atom catalysts represent an exciting new class of materials that have demonstrated high activity for chemical reactions relevant to energy production. Among the various stages involved in catalyst synthesis, the initial adsorption step between the support and the precursor is believed to be of most importance as this interaction influences the unit operations that follow and affects the final size distribution of the catalyst nanoparticles. The ability of metal oxide supports to enhance the dispersion of the active metal on their surface and control their morphology and sintering kinetics is fundamentally related to the nature and strength of the metal–support interaction which is determined at the time of adsorption at the solid-liquid interface. Documented studies on the importance of the adsorption step on the overall characteristics of the catalyst nanoparticle are limited in recent literature due to challenges associated with probing a buried solid-liquid interface. In this work, we have examined the molecular level details of catalyst synthesis with substantial emphasis on the adsorption thermodynamics occurring at the solid-liquid interface during the initial adsorption of transition metal complexes (TMCs) on metal oxide supports and its influence on nanoparticle size, growth and stability. Using a number of surface analytical tools, we have probed at the interface during the adsorption process to quantify metal uptake and measure the kinetics and enthalpy of binding in order to identify the effect of different precursors and their ligand chemistry on the electrostatic driving force. Isothermal Titration Calorimetry (ITC) is used to contact reducible and refractory supports like SiO2, γ-Al2O3 and CeO2 with pH adjusted TMC solutions of Pt, Pd, Rh, Ir and Ag at adjusted pH values, providing a strong electrostatic driving force for adsorption and measure equilibrium binding constants, stoichiometry and enthalpies of adsorption. This study is unique in context that it truly probes the interface during adsorption (in situ) of metal precursors on supports rather than as-synthesized nanoparticles. The trends in the estimated thermodynamic parameters as a function of pH for both the cationic and anionic Pt complexes on silica and alumina respectively captures the effect of ligand speciation and complex solvation at acidic and basic solution conditions. Equilibrium adsorption isotherms from bench top bulk uptake studies aid in quantifying the amount of metal adsorbed on the support surface and by varying choice and weight loading of the precursors, we are able to identify that chloride ligand speciation chemistry around main metal center and solvation strongly influenced metal uptake. Next, we compared bulk and interfacial adsorption mechanisms through ex-situ synthesis to determine how the particle size distribution and metal dispersion of the catalysts were influenced by the mode of adsorption. Thereafter, we looked at cerium oxide which is an important support for transition metal catalysts due to its high “oxygen storage capacity”; thus allowing it to successfully stabilize noble metals, inhibit sintering and maintain small sized nanoparticles on its surface compared to other oxide supports. The thermodynamic adsorption parameters of a comprehensive list of late transition metal complexes in Group 9-11 on shape controlled faceted cerium oxide nano-crystals demonstrated by ITC and DFT calculations showed a trend in the enthalpies of binding between support and metal precursors that correlates with the oxide formation tendency of the transition metal and the reducibility of the support. The ability of metals to form atomically dispersed metal nanoparticles on cerium oxide through formation of an M-O-Ce bond under strong oxidative conditions was examined using XPS and TEM. Several combinations of catalysts were synthesized using precursors having various ligand chemistries deposited on different facets of cerium oxide nano-crystals and surface analytical tools were used to evaluate the optimal conditions for stable, highly dispersed catalysts. From these design rules, a series of ceria supported low weight loading single atom Pd catalysts were synthesized and examined for low temperature methane combustion that is highly in demand to reduce methane slip from lean-burn natural gas vehicles. Here, we probed into the effect of the transition from nano-clusters to single atoms on the activity of the reaction. A possible mechanistic change in the Pd catalytic redox cycle is believed to enhance the catalytic turnover at low temperatures while maintaining reduced precious metal usage.