Restructuring-induced catalytic activity is an intriguing phenomenon of fundamental importance to rational design of high-performance catalyst materials. We study three copper-complex materials for electrocatalytic carbon dioxide reduction. Among them, the copper(II) phthalocyanine exhibits by far the highest activity for yielding methane with a Faradaic efficiency of 66% and a partial current density of 13 mA cm−2 at the potential of – 1.06 V versus the reversible hydrogen electrode. Utilizing in-situ and operando X-ray absorption spectroscopy, we find that under the working conditions copper(II) phthalocyanine undergoes reversible structural and oxidation state changes to form ~ 2 nm metallic copper clusters, which catalyzes the carbon dioxide-to-methane conversion. Density functional calculations rationalize the restructuring behavior and attribute the reversibility to the strong divalent metal ion–ligand coordination in the copper(II) phthalocyanine molecular structure and the small size of the generated copper clusters under the reaction conditions.
A versatile two-step wet process to fabricate Pt, Pd, Rh, and Ru nanoparticle films (simplified as nanofilms hereafter) for in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) study of electrochemical interfaces is presented, which incorporates an initial chemical deposition of a gold nanofilm on the basal plane of a silicon prism with the subsequent electrodepostion of desired platinum group metal overlayers. Galvanostatic electrodeposition of Pt, Rh, and Pd from phosphate or perchloric acid electrolytes, or potentiostatic electrodeposition of Ru from a sulfuric acid electrolyte, yields sufficiently "pinhole-free" overlayers as evidenced by electrochemical and spectroscopic characterizations. The Pt group metal nanofilms thus obtained exhibit strongly enhanced IR absorption. In contrast to the corresponding metal films electrochemically deposited directly on glassy carbon and bulk metal electrodes, the observed enhanced absorption for the probe molecule CO exhibits normal unipolar band shapes. Scanning tunneling microscopic (STM) images reveal that fine nanoparticles of Pt group metals are deposited around wavy and stepped bunches of Au nanoparticles of relatively large sizes. This ubiquitous strategy is expected to open a wide avenue for extending ATR surface-enhanced IR absorption spectroscopy to explore molecular adsorption and reactions on technologically important transition metals, as exemplified by successful real-time spectroscopic and electrochemical monitoring of the oxidation of CO at Pd and that of methanol at Pt nanofilm electrodes. The spectral features of free water molecules coadsorbed with CO on Pt, Pd, Rh, and Ru are also discussed.
Bimetallic electrocatalysts can improve the activity and selectivity over their monometallic counterparts by tuning the structure, morphology, and composition. However, there scarcely was a systematic model to understand the structural effect relationship on CO 2 electrochemical reduction reaction, especially for a product tuning process by introduction of a second metal to grow into outer layers. Herein, we report a structure-controlled model of the growth process of Ag@Cu bimetallic nanoparticles that are fabricated by a polyol method, that is, reducing mixtures of Ag + and Cu 2+ (excess amount) in ethylene glycol (reducing agent) in the presence of polyvinylpyrrolidone. Structural characterizations reveal that a series of Ag@Cu NPs are tuned from the Ag core, Cu modified Ag, to the Cu outer shell by controlling the heating time (0−25 min). Moreover, highly selective catalysts with the tuning reduction products from carbon monoxide to hydrocarbons can be realized. Different from the "dilution" effects between Ag and Cu, the volcanic curve for carbon monoxide production is detected for the introduction of Cu and the peak point is the Ag@Cu-7 electrocatalyst (heating time is 7 min). Similarly, interestingly, when the Cu cladding layer continuously grows, the hydrocarbons are not a simple proportional addition and optimized at Ag@Cu-20 (heating time is 20 min). The geometric effects dominantly account for the synergistic effect of CO product and control the surface activity to hydrocarbons. This study serves as a good starting point to tune the energetics of the intermediate binding to achieve even higher selectivity and activity for core−shell structured catalysts.
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