synthesis-performance-characterizations-transition-metal-single-atom

Synthesis and Performance Characterizations of Transition Metal Single Atom Catalyst for Electrochemical CO<sub>2</sub> Reduction

This protocol presents both the synthesis method of the Ni single atom catalyst, and the electrochemical testing of its catalytic activity and selectivity ...

Harvard University

Engineering active grain boundaries (GBs) in oxide-derived (OD) electrocatalysts is critical to improve the selectivity in COreduction reaction (CORR), which is becoming an increasingly important pathway for renewable energy storage and usage. Different from traditional in situ electrochemical reduction under CORR conditions, where some metal oxides are converted into active metallic phases but with decreased GB densities, here we introduce the Li electrochemical tuning (LiET) method to controllably reduce the oxide precursors into interconnected ultrasmall metal nanoparticles with enriched GBs. By using ZnO as a case study, we demonstrate that the LiET-Zn with freshly exposed GBs exhibits a CO-to-CO partial current of ∼23 mA cmat an overpotential of -948 mV, representing a 5-fold improvement from the OD-Zn with GBs eliminated during the in situ electro-reduction process. A maximal CO Faradaic efficiency of ∼91.1% is obtained by LiET-Zn on glassy carbon substrate. The CO-to-CO mechanism and interfacial chemistry are further probed at the molecular level by advanced in situ spectroelectrochemical technique, where the reaction intermediate of carboxyl species adsorbed on LiET-Zn surface is revealed.

Li Electrochemical Tuning of Metal Oxide for Highly Selective COReduction.

Steerable hydrogen generation from the hydrogen storage chemical formic acid via heterogeneous catalysis has attracted considerable interest given the safety and efficiency concerns in handling H. Herein, a series of carbon-supported capping-agent-free Pd nanoparticles (NPs) with mean sizes tunable from 2.0 to 5.2 nm are developed due to the demand for more efficient dehydrogenation from a formic acid-formate solution of pH 3.5 at room temperature. The trick for the facile size-controlled synthesis of Pd/C catalysts is the selective addition of NaCO, NH·HO, or NaOH to a Pd(II) solution to attain initial pH values of 7-9.5. For comparison, cuboctahedron modeling and electrochemical COstripping methods are applied to evaluate active surface Pd sites for turnover frequency (TOF) calculation. Both mass activity and specific activity (TOF) of hydrogen production are not only time-dependent but also Pd-size-dependent. An initial Hproduction rate of 246 L·h·gis achieved on 2.0 nm Pd/C at 303 K, together with a TOF of 1815 hon the basis of cuboctahedron modeling of surface-active Pd sites. The initial TOF exhibits a significant rise from 3.5 down to 2.8 nm and then levels off below 2.8 nm and even shows a maxima at ca. 2.2 nm using the electrochemical surface area for calculation. The volcano-shaped dependence of TOF on Pd NP size may be better attributed to the changing ratios of terrace sites to defect sites on Pd NPs.

Surfactant-Free Synthesis of Carbon-Supported Palladium Nanoparticles and Size-Dependent Hydrogen Production from Formic Acid-Formate Solution.

Facile interconversion between COand formate/formic acid (FA) is of broad interest in energy storage and conversion and neutral carbon emission. Historically, electrochemical COreduction reaction to formate on Pd surfaces was limited to a narrow potential range positive of -0.25 V (vs RHE). Herein, a boron-doped Pd catalyst (Pd-B/C), with a high CO tolerance to facilitate dehydrogenation of FA/formate to CO, is initially explored for electrochemical COreduction over the potential range of -0.2 V to -1.0 V (vs RHE), with reference to Pd/C. The experimental results demonstrate that the faradaic efficiency for formate (η) reaches ca. 70% over 2 h of electrolysis in CO-saturated 0.1 M KHCOat -0.5 V (vs RHE) on Pd-B/C, that is ca. 12 times as high as that on homemade or commercial Pd/C, leading to a formate concentration of ca. 234 mM mgPd, or ca. 18 times as high as that on Pd/C, without optimization of the catalyst layer and the electrolyte. Furthermore, the competitive selectivity ηηon Pd-B/C is always significantly higher than that on Pd/C despite a decreases of ηand an increases of the CO faradaic efficiency (η) at potentials negative of -0.5 V. The density functional theory (DFT) calculations on energetic aspects of COreduction reaction on modeled Pd(111) surfaces with and without H-adsorbate reveal that the B-doping in the Pd subsurface favors the formation of the adsorbed HCOO*, an intermediate for the FA pathway, more than that of *COOH, an intermediate for the CO pathway. The present study confers Pd-B/C a unique dual functional catalyst for the HCOOH ↔ COinterconversion.

Article	Year
Li Electrochemical Tuning of Metal Oxide for Highly Selective COReduction. ACS nano\| PubMed ID: 28558186	2017
Surfactant-Free Synthesis of Carbon-Supported Palladium Nanoparticles and Size-Dependent Hydrogen Production from Formic Acid-Formate Solution. ACS applied materials & interfaces\| PubMed ID: 28658569	2017
Boosting Formate Production in Electrocatalytic COReduction over Wide Potential Window on Pd Surfaces. Journal of the American Chemical Society\| PubMed ID: 29409320	2018

Kun Jiang

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