Oxide-Encapsulated Silver Electrocatalysts for Selective and Stable Syngas Production from Reactive Carbon Capture Solutions.

Electrolysis of bicarbonate-containing CO2 capture solutions is a promising approach towards achieving low-cost carbon-neutral chemicals production. However, the parasitic bicarbonate-mediated hydrogen evolution reaction (HER) and electrode instability in the presence of trace impurities remain major obstacles to overcome. This work demonstrates that the combined use of titanium dioxide (TiO2) overlayers with the chelating agent ethylene diamine tetra-acetic acid (EDTA) significantly enhances the selectivity and stability of Ag-based electrocatalysts for bicarbonate electrolysis. The amorphous TiO2 overlayers suppress the HER by over 50% at potentials more negative than -0.7 V vs. RHE, increasing the CO faradaic efficiency (FE) by 33% (relative). In situ surface-enhanced Raman spectroscopy (SERS) measurements reveal the absence of near-surface bicarbonate species and an abundance of CO2 reduction intermediates at the Ag|TiO2 buried interface, suggesting that the overlayers suppress HER by blocking bicarbonate ions from reaching the buried active sites. In accelerated degradation tests with 5 ppm of Fe(III) impurity, the addition of EDTA allows stable CO production with >47% FE, while the electrodes rapidly deactivate in the absence of EDTA. This work highlights the use of TiO2 overlayers for enhancing the CO:H2 ratio while simultaneously protecting electrocatalysts from impurities likely to be present in "open" carbon capture systems.

Condensed Layer Deposition of Nanoscopic TiO2 Overlayers on High-Surface-Area Electrocatalysts.

Encapsulating an electrocatalytic material with a semipermeable, nanoscopic oxide overlayer offers a promising approach to enhancing its stability, activity, and/or selectivity compared to an unencapsulated electrocatalyst. However, applying nanoscopic oxide encapsulation layers to high-surface-area electrodes such as nanoparticle-supported porous electrodes is a challenging task. This study demonstrates that the recently developed condensed layer deposition (CLD) method can be used for depositing nanoscopic (sub-10 nm thick) titanium dioxide (TiO2) overlayers onto high-surface-area platinized carbon foam electrodes. Characterization of the overlayers by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) showed that the films are amorphous, while X-ray photoelectron spectroscopy confirmed that they exhibit TiO2 stoichiometry. Electrodes were also characterized by hydrogen underpotential deposition (Hupd) and carbon monoxide (CO) stripping, demonstrating that the Pt electrocatalysts remain electrochemically active after encapsulation. Additionally, copper underpotential deposition (Cuupd) measurements revealed that TiO2 overlayers are effective at blocking Cu2+ from reaching the TiO2/Pt buried interface and were used to estimate that between 43 and 98% of Pt surface sites were encapsulated. Overall, this study shows that CLD is a promising approach for depositing nanoscopic protective overlayers on high-surface-area electrodes.

Unraveling Unique Surface Chemistry of Transition Metal Nitrides in Controlling Selective C-O Bond Scission Pathways of Glycerol.

Controlled C-O bond scission is an important step for upgrading glycerol, a major byproduct from the continuously increasing biodiesel production. Transition metal nitride catalysts have been identified as promising hydrodeoxygenation (HDO) catalysts, but fundamental understanding regarding the active sites of the catalysts and reaction mechanism remains unclear. This work demonstrates a fundamental surface science study of Mo2N and Cu/Mo2N for the selective HDO reaction of glycerol, using a combination of model surface experiments and first-principles calculations. Temperature-programmed desorption (TPD) experiments showed that clean Mo2N cleaved two or three C-O bonds of glycerol to produce allyl alcohol, propanal, and propylene. The addition of Cu to Mo2N changed the reaction pathway to one C-O bond scission to produce acetol. High-resolution electron energy loss spectroscopy (HREELS) results identified the surface intermediates, showing a facile C-H bond activation on Mo2N. Density functional theory (DFT) calculations revealed that the surface N on Mo2N interacted with the H atoms in glycerol and blocked some Mo sites to enable selective C-O bond scission. This work shows that Mo2N and Cu/Mo2N are active and selective for the controlled C-O bond scission of glycerol and in turn provides insights into the rational catalyst design for selective oxygen removal of relevant biomass-derived oxygenates.

Controlling reaction pathways of selective C-O bond cleavage of glycerol.

The selective hydrodeoxygenation (HDO) reaction is desirable to convert glycerol into various value-added products by breaking different numbers of C-O bonds while maintaining C-C bonds. Here we combine experimental and density functional theory (DFT) results to reveal that the Cu modifier can significantly reduce the oxophilicity of the molybdenum carbide (Mo2C) surface and change the product distribution. The Mo2C surface is active for breaking all C-O bonds to produce propylene. As the Cu coverage increases to 0.5 monolayer (ML), the Cu/Mo2C surface shows activity towards breaking two C-O bonds and forming ally-alcohol and propanal. As the Cu coverage further increases, the Cu/Mo2C surface cleaves one C-O bond to form acetol. DFT calculations reveal that the Mo2C surface, Cu-Mo interface, and Cu surface are distinct sites for the production of propylene, ally-alcohol, and acetol, respectively. This study explores the feasibility of tuning the glycerol HDO selectivity by modifying the surface oxophilicity.