Development of Molecular Catalysts for Fuel Cell Applications PDF Download

Author: Jennifer Wood Eddy
Publisher:
ISBN: 9780355255737
Category :
Languages : en
Pages : 230

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Book Description
Formic acid has been proposed as a hydrogen storage medium; however, this necessitates efficient and selective catalysts for the dehydrogenation of formic acid to produce H2 and CO2. Consequently, we have developed palladium based complexes supported by chelating bis-N-heterocyclic carbene (NHC) ligands and probed the activity of such complexes for the dehydrogenation of formic acid. The formic acid dehydrogenation properties of [(MDCMes)Pd(MeCN)2](PF6)2 in MeCN with triethylamine additive were monitored using water displacement and gas chromatography to show a 1:1 ratio of CO2:H2 production with no detection of CO, and a modest turnover frequency (TOF, 325 h-1) and turnover number (TON, 185). The [(MDCMes)Pd(MeCN)2](PF6)2 catalyst was used under relatively mild conditions and is the first example of a homogenous palladium catalyst with any reasonable activity for formic acid dehydrogenation. The original catalyst motif was modified by changing either the NHC wingtip substituents or the coordinating ligands. This family of complexes was characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography, and studied for formic acid dehydrogenation. The modified complexes were found to be less active than the parent catalyst. ☐ From these initial studies, a mechanism was proposed and probed using several kinetic studies, including Eyring and Arrhenius analyses. These studies supported the proposed mechanism and suggested that the opening of a coordination site on palladium for subsequent b-hydride elimination was the rate determining step of H2 liberation. Based on the proposed mechanism, the reaction system with [(MDCMes)Pd(MeCN)2](PF6)2 as catalyst was further optimized by changing the base from triethylamine to Hünig’s base. The initial TOF for the reaction with Hünig’s base was determined to be 414 h-1 and the total TON was increased to 353. Additionally, formic acid could be added up to 18 times with catalytic activity. ☐ The 4e–/4H+ reduction of oxygen to water is an important reaction that takes place at the cathode of fuel cells; therefore, catalysts that are selective for this reaction are highly desired. The calix[4]phyrin is a tetrapyrrole macrocycle that exhibits unique properties due to the incorporation of two sp3 hybridized meso carbons. We wished to explore these unique macrocycles and corresponding metal complexes with the goal of applications to catalysis, in particular the oxygen reduction reaction (ORR). The freebase calix[4]phyrin was synthesized by modifying a streamlined procedure for tetrapyrrole macrocycle synthesis previously utilized in our laboratory for the related phlorin macrocycle. The freebase calix[4]phyrin macrocycle was then metalated to give the corresponding zinc, copper, nickel and cobalt complexes. These metal complexes were characterized using a variety of methods, including X-ray crystallography, UV-vis spectroscopy, differential pulse voltammetry and cyclic voltammetry. ☐ The cobalt calix[4]phyrin was studied as a catalyst for the ORR, both heterogeneously and homogeneously. The homogeneous ORR was monitored using UV-vis spectroscopy, and cobalt calix[4]phyrin was found to catalyze the reduction of O2 to give approximately 50% water production (n = 3). A series of kinetic studies were also performed by varying the concentration of each species in solution, and from these studies a mechanism was proposed. The ORR with cobalt calix[4]phyrin was studied heterogeneously using rotating ring-disk electrode electrochemistry. By using Koutecky-Levich analysis, cobalt calix[4]phyrin was found to reduce O2 with 2.9 electron equivalents transferred under electrochemical conditions, which corresponds to ~50% water production. This selectivity for water production is promising for a monomeric cobalt complex. Initial attempts were made to further optimize the cobalt calix[4]phyrin using a hangman scaffold, however these modifications did not increase the selectivity as compared to the parent compound.