Changing the Selectivity of O₂ Reduction Catalysis with One Ligand Heteroatom

Abstract

The development of catalytic systems that selectively reduce O2 to water is needed to continue the advancement of fuel cell technologies. As an alternative to platinum catalysts, derivatives of iron (Fe) and cobalt (Co) porphyrin molecular catalysts provide one benchmark for catalyst design, but incorporation of these catalysts into heterogeneous platforms remains a challenge. Co-porphyrins can be heterogeneous O2 reduction catalysts when immobilized on to edge plane graphite (EPG) electrodes, but their selectivity for the desired four-electron reduction of O2 to H2O is often poor. Herein, we demonstrate substantial improvements in the O2 reduction selectivity using a Co-porphyrin that incorporates a 2-pyridyl group at one of the meso-positions of a Co-tetraarylporphyrin (cobalt­(II) 5-(2-pyridyl)-10,15,20-triphenylporphyrin, CoTPPy). The properties of CoTPPy immobilized on EPG were investigated using cyclic voltammetry, rotating disk, and rotating ring-disk electrochemistry. The presence of a single 2-pyridyl group in the CoTPPy gives rise to the four-electron reduction of O2, as opposed to the two-electron reduction commonly associated with cobalt porphyrins. Detailed electrochemical studies of CoTPPy and related Co and Fe porphyrins are described. Use of Co instead of Fe improves overpotentials by over 200 mV with a factor of 2 increase in maximum turnover frequency (TOFmax). This work demonstrates that a simple change in catalyst structure can dramatically change the selectivity for O2 reduction.

Secondary-Sphere Effects in Molecular Electrocatalytic CO2 Reduction

“…While this review focuses on molecular catalysts which were specifically examined for electrochemical CO 2 reduction, secondary-sphere effects have been successfully harnessed in related catalytic processes, including thermal CO 2 hydrogenation (Himeda et al, 2004, 2005, 2007; Hull et al, 2012; Wang et al, 2012, 2013, 2014; Manaka et al, 2014; Suna et al, 2014; Cammarota et al, 2017), hydrogen evolution (Curtis et al, 2003; Henry et al, 2005, 2006; Wilson et al, 2006; Fraze et al, 2007; Jacobsen et al, 2007a,b; DuBois and DuBois, 2009; Gloaguen and Rauchfuss, 2009; Helm et al, 2011; Reback et al, 2013; Ginovska-Pangovska et al, 2014), hydrogen oxidation (Curtis et al, 2003; Henry et al, 2005, 2006; Wilson et al, 2006; Fraze et al, 2007; Jacobsen et al, 2007a,b; Dutta et al, 2013, 2014; Ginovska-Pangovska et al, 2014), formate oxidation (Galan et al, 2011, 2013; Seu et al, 2012), and oxygen reduction (Collman, 1977; Collman et al, 2004; Lewis and Tolman, 2004; Mirica et al, 2004; Fukuzumi, 2013; Ray et al, 2014; Fukuzumi et al, 2015; Nam, 2015; Sahu and Goldberg, 2016; Elwell et al, 2017; Hong et al, 2017; Sinha et al, 2019) reactions. In this review, we focus on how the mechanism of CO 2 reduction relates to the type of secondary-sphere effects employed in molecular systems.…”

“…This overview is followed by a careful examination of secondary-sphere effects in several abiotic molecular electrocatalyst examples, beginning with the [Fe(tetraphenylporphyrin)] + [Fe(TPP)] + systems pioneered by Savéant, Robert, and Constentin, including a discussion of the effects of pendent proton source placement and the distance dependence of through-space effects induced by charged residues (Costentin et al, 2014a,b; Manbeck et al, 2015; Mohamed et al, 2015; Azcarate et al, 2016; Zahran et al, 2016; Margarit et al, 2018; Nichols et al, 2018b; Sinha and Warren, 2018). Next, M(bpy)(CO) 3 X catalysts (M = Mn or Re; X = solvent molecule or halide) (Wong et al, 1998; Bourrez et al, 2011; Smieja et al, 2013; Chabolla et al, 2014, 2017; Franco et al, 2014; Machan et al, 2014a, 2015, 2016; Riplinger et al, 2014; Agarwal et al, 2015; Manbeck et al, 2015; Riplinger and Carter, 2015; Machan and Kubiak, 2016; Ngo et al, 2017; Sahu et al, 2017; Sinha et al, 2019) in which steric parameters, pendent Lewis acid effects, and charged residues have been shown to be effective will be discussed. Finally, [Ni(cyclam)] 2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane), which contains pendent proton donors on the coordinating N atoms of the macrocycle, is discussed (Beley et al, 1984; Barefield et al, 1986; Balazs and Anson, 1993; Kelly et al, 1999; Froehlich and Kubiak, 2012; Song et al, 2014; Nichols and Chang, 2018).…”

Supramolecular Tuning Enables Selective Oxygen Reduction Catalyzed by Cobalt Porphyrins for Direct Electrosynthesis of Hydrogen Peroxide

“…Figure A shows the RRDE responses for each catalyst in which significant ring currents are observed, indicating H 2 O 2 generation. The product selectivity for ORR for each catalyst can then be calculated and is plotted as a function of applied potential in Figure B. Co‐TPP displays minimal product selectivity with around 50 % Faradaic efficiency for H 2 O 2 , which is common with mononuclear cobalt porphyrins and may be attributed to catalyst aggregation that creates intermolecular active sites that can catalyze the off‐pathway 4 e − reduction of O 2 into H 2 O. The selectivity of porphyrin ORR catalysts is also known to be highly dependent on the proton source and catalyst medium .…”

Electrocatalytic O2 Reduction by an Organometallic Pd(III) Complex via a Binuclear Pd(III) Intermediate

“…(2): 19,23 100 %H2O2 = 100 2 100 ; Eq. 2where I is the disk current, I is the ring current, and N is the collection efficiency (= 0.18).…”