Restructuring of Cu-based Catalysts during CO Electroreduction: Evidence for the Dominant Role of Surface Defects on the C2+ Product Selectivity.

CO is the key reaction intermediate in the Cu-catalyzed electroreduction of CO2 to products containing C-C bonds. Herein, we investigate the impact of the particle size of CuO precursors on the direct electroreduction of CO (CORR) to C2+ products. Flame spray pyrolysis was used to prepare CuO particles with sizes between 4 and 30 nm. In situ synchrotron wide-angle X-ray scattering (WAXS), quasi-in situ X-ray photoelectron spectroscopy, and transmission electron microscopy demonstrated that, during CORR, the CuO precursors transformed into ∼30 nm metallic Cu particles with a crystalline domain size of ∼17 nm, independently of the initial size of the CuO precursors. Despite their similar morphology, the samples presented different Faradaic efficiencies (FEs) to C2+ products. The Cu particles derived from medium-sized (10-20 nm) CuO precursors were the most selective to C2+ products (FE 60%), while those derived from CuO precursors smaller than 10 nm displayed a high FE to H2. As the oxidation state, the particle and the crystallite sizes of these samples were similar after CORR, the differences in product distribution are attributed to the type and density of surface defects on the metallic Cu particles, as supported by studying electrochemical oxidation of the reduced Cu particles during CV cycling in combination with synchrotron WAXS. Cu particles derived from <10 nm CuO contained a higher density of more under-coordinated defects, resulting in a higher FE to H2 than Cu particles derived from 10 to 30 nm CuO. Bulk oxidation was most prominent and stable for Cu particles derived from medium-sized CuO, which indicated the more disordered nature of their surface compared to Cu particles derived from 30 nm CuO precursors and their lower reactivity compared to Cu particles derived from small CuO. Cu particles derived from <10 nm CuO initially displayed intense redox behavior, quickly fading during subsequent CVs. Our results evidence the significant restructuring during the electrochemical reduction of CuO precursors into Cu particles of similar size. The differences in CORR performance of these Cu particles of similar size can be correlated to different surface structures, qualitatively resolved by studying surface and bulk oxidation, which affect the competition between CO dimerization to yield C2+ products and undesired H2 evolution.

Carbon dioxide and nitrate co-electroreduction to urea on CuOxZnOy.

Urea is a commonly used nitrogen fertiliser synthesised from ammonia and carbon dioxide using thermal catalysis. This process results in high carbon dioxide emissions associated with the required amounts of ammonia. Electrocatalysis provides an alternative method to urea production with reduced carbon emissions while utilising waste products like nitrate. This manuscript reports on urea synthesis from the electroreduction of nitrate and carbon dioxide using CuOxZnOy electrodes under mild conditions. Catalysts with different ratios of CuO and ZnO, synthesised via flame spray pyrolysis, were explored for the reaction. The results revealed that all the CuOxZnOy electrocatalyst compositions produce urea, but the efficiency strongly depends on the metal ratio composition of the catalysts. The CuO50ZnO50 composition had the best performance in terms of selectivity (41% at -0.8 V vs RHE) and activity (0.27 mA/cm2 at -0.8 V vs RHE) towards urea production. Thus, this material is one of the most efficient electrocatalysts for urea production reported so far. This study systematically evaluates bimetallic catalysts with varying compositions for urea synthesis from carbon dioxide and nitrate.

Size of cerium dioxide support nanocrystals dictates reactivity of highly dispersed palladium catalysts.

The catalytic performance of heterogeneous catalysts can be tuned by modulation of the size and structure of supported transition metals, which are typically regarded as the active sites. In single-atom metal catalysts, the support itself can strongly affect the catalytic properties. Here, we demonstrate that the size of cerium dioxide (CeO2) support governs the reactivity of atomically dispersed palladium (Pd) in carbon monoxide (CO) oxidation. Catalysts with small CeO2 nanocrystals (~4 nanometers) exhibit unusually high activity in a CO-rich reaction feed, whereas catalysts with medium-size CeO2 (~8 nanometers) are preferred for lean conditions. Detailed spectroscopic investigations reveal support size-dependent redox properties of the Pd-CeO2 interface.

Dispersity under Scrutiny: Phase Behavior Differences between Disperse and Discrete Low Molecular Weight Block Co-Oligomers.

An experimental study is presented in which we compare the bulk phase behavior of discrete and (partially) disperse diblock co-oligomers (BCOs) with high χ-low N. To this end, oligomers of dimethylsiloxane (oDMS) and lactic acid (oLA) were synthesized, each having either a discrete number of repeat units or a variable block length. Ligation of the blocks resulted in oDMS-oLA BCOs with dispersities ranging from <1.00001 to 1.09, as revealed by mass spectroscopy and size exclusion chromatography. The phase behavior of all BCOs was investigated by differential scanning calorimetry and small-angle X-ray scattering. Compared to the well-organized lamellae formed by discrete oDMS-oLA, we observe that an increase in the dispersity of these BCOs results in (1) an increase of the stability of the microphase-segregated state, (2) a decrease of the overall degree of ordering, and (3) an increase of the domain spacing.