Ag–Sn Bimetallic Catalyst with a Core–Shell Structure for CO₂ Reduction

Abstract

Converting greenhouse gas carbon dioxide (CO) to value-added chemicals is an appealing approach to tackle CO emission challenges. The chemical transformation of CO requires suitable catalysts that can lower the activation energy barrier, thus minimizing the energy penalty associated with the CO reduction reaction. First-row transition metals are potential candidates as catalysts for electrochemical CO reduction; however, their high oxygen affinity makes them easy to be oxidized, which could, in turn, strongly affect the catalytic properties of metal-based catalysts. In this work, we propose a strategy to synthesize Ag-Sn electrocatalysts with a core-shell nanostructure that contains a bimetallic core responsible for high electronic conductivity and an ultrathin partially oxidized shell for catalytic CO conversion. This concept was demonstrated by a series of Ag-Sn bimetallic electrocatalysts. At an optimal SnO shell thickness of ∼1.7 nm, the catalyst exhibited a high formate Faradaic efficiency of ∼80% and a formate partial current density of ∼16 mA cm at -0.8 V vs RHE, a remarkable performance in comparison to state-of-the-art formate-selective CO reduction catalysts. Density-functional theory calculations showed that oxygen vacancies on the SnO (101) surface are stable at highly negative potentials and crucial for CO activation. In addition, the adsorption energy of CO at these oxygen-vacant sites can be used as the descriptor for catalytic performance because of its linear correlation to OCHO* and COOH*, two critical intermediates for the HCOOH and CO formation pathways, respectively. The volcano-like relationship between catalytic activity toward formate as a function of the bulk Sn concentration arises from the competing effects of favorable stabilization of OCHO* by lattice expansion and the electron conductivity loss due to the increased thickness of the SnO layer.

Exclusive Formation of Formic Acid from CO2 Electroreduction by a Tunable Pd‐Sn Alloy

“…With respect to the alloy catalysts,itwas found that their HCOOH and CO FEs were highly dependent on the surface configurations at the respective cathodic peak potentials,w here CO 2 electroreduction reaction rates were maximum (Supporting Information, Figure S5). [27] In comparison, the catalyst PdSn/C delivered the HCOOH FE over 99 %a tt he lower potential of À0.43 V(vs.RHE). [25,26] With increasing Sn content, the HCOOH FE increases to 54 %over Pd 3 Sn/C and further up to 63 %o ver Pd 2 Sn/C.S urprisingly,t he highest FE of > 99 %for producing HCOOH was obtained over the PdSn/C catalyst at the lowest overpotential of À0.26 V. We note that negligible H 2 (FE less than 0.3 %) was produced from the HER, and CO formation was completely suppressed over PdSn/C.S ubsequently,H COOH FE decreased gradually to 73 %o ver Pd 0.5 Sn/C and 42 %o ver Pd 0.25 Sn/C when further increasing the Sn content.…”

Reaction Mechanisms of the Electrochemical Conversion of Carbon Dioxide to Formic Acid on Tin Oxide Electrodes

“…Jiao et al . investigated Ag−Sn bimetallic catalysts with a thin SnO x shell for selective HCOOH production . In their computational analysis, surface defects, such as oxygen vacancies (V O ) and embedded hydroxyl groups from proton reduction, were examined on a partially reduced Sn II O(101) surface as shown in Figure a.…”

“…For example, for a specific multi‐step reaction (denoted by EĈ) where a rate‐limiting chemical step occurs after a one‐electron pre‐equilibrium step, n p =1 and n q =0, and the Tafel slope is 60 mV dec −1 . Electrokinetic studies for HCOO − production over SnO 2 catalysts have shown Tafel slopes close to 60 and 120 mV dec −1 . Based on the elementary steps for HCOO − production [Equations (2)–(6)], a Tafel slope of 60 mV dec −1 is interpreted as a rate‐limiting chemical proton transfer from bicarbonate after a one‐electron transfer to CO 2 .…”

“…Based on the elementary steps for HCOO − production [Equations (2)–(6)], a Tafel slope of 60 mV dec −1 is interpreted as a rate‐limiting chemical proton transfer from bicarbonate after a one‐electron transfer to CO 2 . On the other hand, a Tafel slope close to 120 mV dec −1 indicates that the first electron‐transfer step, the formation of a CO 2 ‐ anion, is the RDS . Although Tafel slopes less than 60 mV dec −1 have been rarely observed, a Tafel slope corresponding to 40 mV dec −1 can be assigned to n p =1 and n q =1.…”

“…The key intermediates, selectivity trends and limiting steps for HCOO − formation on pure metals have been investigated through computational analysis . Electrokinetic studies based on Tafel and reaction‐order analyses have proposed possible rate‐determining steps (RDS) for HCOO − formation . Furthermore, in situ IR and Raman analyses have presented direct evidence for intermediates and active sites .…”