In situ x-ray absorption investigations of a heterogenized molecular catalyst and its interaction with a carbon nanotube support

Abstract

A highly active heterogenized molecular CO2 reduction catalyst on a conductive carbon support is investigated to identify if its improved catalytic activity can be attributed to strong electronic interactions between catalyst and support. The molecular structure and electronic character of a [Re+1(tBu-bpy)(CO)3Cl] (tBu-bpy = 4,4′-tert-butyl-2,2′-bipyridine) catalyst deposited on multi-walled carbon nanotubes (CNT) are characterized using Re L3-edge X-ray absorption spectroscopy (XAS) under electrochemical conditions and compared to the homogeneous catalyst The Re oxidation state is characterized from the near-edge absorption region, while structural changes of the catalyst are assessed from the extended x-ray absorption fine structure (EXAFS) under reducing conditions. Chloride ligand dissociation and a Re-centered reduction are both observed under applied reducing potential. The results confirm weak coupling of [Re(tBu-bpy)(CO)3Cl] with the support, since the supported catalyst exhibits the same oxidation changes as the homogenous case. However, these results do not preclude strong interactions between a reduced catalyst intermediate and the support, preliminarily investigated here using Quantum Mechanical (QM) calculations. Thus, our results suggest that complicated linkage schemes and strong electronic interactions with the initial catalyst species are not required to improve the activity of heterogenized molecular catalysts.

Steric Effects on CO2 Reduction with Substituted Mn(bpy)(CO)3X-Type Catalysts on Multiwalled Carbon Nanotubes Reveal Critical Mechanistic Details

“…The absence of measurable Faradaic waves in CV has been reported elsewhere for molecular electrocatalysts covalently coupled to and physiosorbed to graphitic surfaces. ,, In certain covalently attached systems, redox silence is thought to arise from strong electronic coupling between the electrode and molecular catalyst. , On the other hand, a recent report using in situ X-ray adsorption to probe the Re-oxidation state in Re(tbu-bpy)(CO) 3 Br adsorbed on MWCNT demonstrated that strong electronic coupling is not a requirement for redox silence to be observed in physiosorbed molecular species …”

“…Electrodes of catalysts 1−5 were prepared according to a method previously reported by our lab. 12 Briefly, an equal part (mg/mg) mixture of Mn(bpy-R)(CO) 3 Br and MWCNT was suspended in isopropyl alcohol (IPA) with sonication to produce an ink that was spray-coated onto carbon fiber paper and dried at 80 °C to produce an electrode. The preparation of samples was done in the dark to minimize the light exposure and degradation of the Mn catalyst.…”

“…27,28 On the other hand, a recent report using in situ X-ray adsorption to probe the Re-oxidation state in Re(tbu-bpy)(CO) 3 Br adsorbed on MWCNT demonstrated that strong electronic coupling is not a requirement for redox silence to be observed in physiosorbed molecular species. 12 The apparent redox silence of catalyst 5 was further investigated by measuring the CV response of catalyst 5 under Li 2 CO 3 (0.25 M, pH adjusted to 9), Cs 2 CO 3 (0.25 M, pH adjusted to 9), and tetraethylammonium (Et 4 N + ) bicarbonate (0.5 M) (Figures 5 and S5). We expected that exchanging the electrolyte cation would elicit a Faradaic response from catalyst 5, if the apparent redox silence under K + arises from the position of the outer Helmholtz plane.…”

“…The orientation and position of the molecular catalyst at the CNT surface do appear to be important factors in general. 12 Herein, we investigate substituent effects on CO 2 reduction with CNT-immobilized molecular catalysts. We do so by studying the behavior of a well-known electrocatalyst Mn(bpy-R)(CO) 3 Br (bpy-R = 4,4′-diR-2,2′-bipyridine) immobilized on MWCNTs with sterically varied substituents installed on the bipyridyl ligand.…”

“…These include positive interactions of interfacial ions with key catalytic intermediates, strong electronic coupling of the electrocatalyst to the CNT, access to an aqueous medium, and molecular strain induced by adsorption on the CNT. − Evidence underpinning generalizable enhancement mechanisms, however, is forthcoming in part due to the relatively recent emergence of these materials. The orientation and position of the molecular catalyst at the CNT surface do appear to be important factors in general …”