Development of molecular catalysts for fuel cell applications
Date
2017
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
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.
Description
Keywords
Pure sciences, Applied sciences, Catalysts, Formic acid, Fuel