Shape-dependent CO2 hydrogenation to methanol over Cu2O nanocubes supported on ZnO

The hydrogenation of CO2 to methanol over Cu/ZnO-based catalysts is highly sensitive to the surface composition and catalyst structure. Thus, its optimization requires a deep understanding of the influence of the pre-catalyst structure on its evolution under realistic reaction conditions, including the formation and stabilization of the most active sites. Here, the role of the pre-catalyst shape (cubic vs spherical) in the activity and selectivity of ZnO-supported Cu nanoparticles was investigated during methanol synthesis. A combination of ex situ, in situ, and operando microscopy, spectroscopy, and diffraction methods revealed drastic changes in the morphology and composition of the shaped pre-catalysts under reaction conditions. In particular, the rounding of the cubes and partial loss of the (100) facets were observed, although such motifs remained in smaller domains. Nonetheless, the initial pre-catalyst structure was found to strongly affect its subsequent transformation in the course of the CO2 hydrogenation reaction and activity/selectivity trends. In particular, the cubic Cu particles displayed an increased activity for methanol production, although at the cost of a slightly reduced selectivity when compared to similarly sized spherical particles. These findings were rationalized with the help of density functional theory calculations.Peer ReviewedPostprint (published version

Operando Raman spectroscopy uncovers hydroxide and CO species enhance ethanol selectivity during pulsed CO2 electroreduction.

Pulsed CO2 electroreduction (CO2RR) has recently emerged as a facile way to in situ tune the product selectivity, in particular toward ethanol, without re-designing the catalytic system. However, in-depth mechanistic understanding requires comprehensive operando time-resolved studies to identify the kinetics and dynamics of the electrocatalytic interface. Here, we track the adsorbates and the catalyst state of pre-reduced Cu2O nanocubes ( ~ 30 nm) during pulsed CO2RR using sub-second time-resolved operando Raman spectroscopy. By screening a variety of product-steering pulse length conditions, we unravel the critical role of co-adsorbed OH and CO on the Cu surface next to the oxidative formation of Cu-Oad or CuOx/(OH)y species, impacting the kinetics of CO adsorption and boosting the ethanol selectivity. However, a too low OHad coverage following the formation of bulk-like Cu2O induces a significant increase in the C1 selectivity, while a too high OHad coverage poisons the surface for C-C coupling. Thus, we unveil the importance of co-adsorbed OH on the alcohol formation under CO2RR conditions and thereby, pave the way for improved catalyst design and operating conditions

Selectivity Control of Cu Nanocrystals in a Gas-Fed Flow Cell through CO2_2 Pulsed Electroreduction

In this study, we have taken advantage of a pulsed CO$_2$ electroreduction reaction (CO$_2$RR) approach to tune the product distribution at industrially relevant current densities in a gas-fed flow cell. We compared the CO$_2$RR selectivity of Cu catalysts subjected to either potentiostatic conditions (fixed applied potential of −0.7 V$_{RHE}$) or pulsed electrolysis conditions (1 s pulses at oxidative potentials ranging from $E_{an}$ = 0.6 to 1.5 V$_{RHE}$, followed by 1 s pulses at −0.7 VRHE) and identified the main parameters responsible for the enhanced product selectivity observed in the latter case. Herein, two distinct regimes were observed: (i) for $E_{an}$ = 0.9 V$_{RHE}$ we obtained 10% enhanced C$_2$ product selectivity (FE$_{C_2H_4}$ = 43.6% and FE$_{C_2H_5OH}$ = 19.8%) in comparison to the potentiostatic CO$_2$RR at −0.7 V$_{RHE}$ (FE$_{C_2H_4}$ = 40.9% and FE$_{C_2H_5OH}$ = 11%), (ii) while for $E_{an}$ = 1.2 V$_{RHE}$, high CH$_4$ selectivity (FE$_{CH_4}$ = 48.3% vs 0.1% at constant −0.7 V$_{RHE}$) was observed. Operando spectroscopy (XAS, SERS) and ex situ microscopy (SEM and TEM) measurements revealed that these differences in catalyst selectivity can be ascribed to structural modifications and local pH effects. The morphological reconstruction of the catalyst observed after pulsed electrolysis with $E_{an}$ = 0.9 V$_{RHE}$, including the presence of highly defective interfaces and grain boundaries, was found to play a key role in the enhancement of the C$_2$ product formation. In turn, pulsed electrolysis with $E_{an}$ = 1.2 V$_{RHE}$ caused the consumption of OH$^–$ species near the catalyst surface, leading to an OH-poor environment favorable for CH$_4$ production

Influence of the cobalt content in cobalt iron oxides on the electrocatalytic OER activity

Sub 10 nm cobalt ferrite Co$_x$Fe$_{3−x}$O$_4$ (x ≤ 1.75) nanoparticles and cobalt-rich wüstite (Co$_{x/3}$Fe$_{(1−x)/3}$)O nanoparticles (x ≥ 2) were synthesized in a solvothermal approach and characterized by powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) as well as energy dispersive X-ray spectroscopy (EDX), IR, Raman, and $^{57}$Fe-Mössbauer spectroscopy. Their electrocatalytic activity in the oxygen evolution reaction (OER) was evaluated and the active state formation was tracked by operando X-ray absorption spectroscopy (XAS). Our studies demonstrate that the cobalt-rich wüstite (Co$_{x/3}$Fe$_{(1−x)/3}$)O nanoparticles underwent a phase-transformation into the spinels Co$_x$Fe$_{3−x}$O$_4$ (x ≥ 2) under the applied OER conditions. The overpotential $η$10 at 10 mA cm$^{−2}$, serving as a benchmark for the OER activity of the cobalt ferrite nanoparticles in alkaline media, was lower than that of magnetite Fe$_3$O$_4$ even with low cobalt concentrations, reaching a minimum of 350 mV for Co$_{2.25}$Fe$_{0.75}$O$_4$ with a Tafel slope of 50 mV dec$^{−1}$. Finally, we identified that the catalytic activity is linked to the nanoparticle size as well as to the degree of Co redox activity and change in coordination during OER

Covalent Organic Framework (COF) Derived Ni‐N‐C Catalysts for Electrochemical CO2_2 Reduction: Unraveling Fundamental Kinetic and Structural Parameters of the Active Sites

Electrochemical CO$_2$ reduction is a potential approach to convert CO$_2$ into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel-nitrogen-doped carbon (Ni-N-C) is an efficient catalyst for CO$_2$ reduction to CO, and the single-site Ni−Nx motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)-derived Ni-N-C catalysts, for which the Ni−Nx content could be adjusted by the pyrolysis temperature. The combination of high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure evidenced the presence of Ni single-sites, and quantitative X-ray photoemission addressed the relation between active site density and turnover frequency

Covalent Organic Framework (COF) Derived Ni‐N‐C Catalysts for Electrochemical CO2_2 Reduction: Unraveling Fundamental Kinetic and Structural Parameters of the Active Sites

Electrochemical CO$_2$ reduction is a potential approach to convert CO$_2$ into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel-nitrogen-doped carbon (Ni-N-C) is an efficient catalyst for CO$_2$ reduction to CO, and the single-site Ni−N$_x$ motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)-derived Ni-N-C catalysts, for which the Ni−N$_x$ content could be adjusted by the pyrolysis temperature. The combination of high-angle annular dark-field scanning transmission electron microscopy and extended X-ray absorption fine structure evidenced the presence of Ni single-sites, and quantitative X-ray photoemission addressed the relation between active site density and turnover frequency

Shape-Dependent CO₂ Hydrogenation to Methanol over Cu₂O Nanocubes Supported on ZnO

The hydrogenation of CO2 to methanol over Cu/ZnO-based catalysts is highly sensitive to the surface composition and catalyst structure. Thus, its optimization requires a deep understanding of the influence of the pre-catalyst structure on its evolution under realistic reaction conditions, including the formation and stabilization of the most active sites. Here, the role of the pre-catalyst shape (cubic vs spherical) in the activity and selectivity of ZnO-supported Cu nanoparticles was investigated during methanol synthesis. A combination of ex situ, in situ, and operando microscopy, spectroscopy, and diffraction methods revealed drastic changes in the morphology and composition of the shaped pre-catalysts under reaction conditions. In particular, the rounding of the cubes and partial loss of the (100) facets were observed, although such motifs remained in smaller domains. Nonetheless, the initial pre-catalyst structure was found to strongly affect its subsequent transformation in the course of the CO2 hydrogenation reaction and activity/selectivity trends. In particular, the cubic Cu particles displayed an increased activity for methanol production, although at the cost of a slightly reduced selectivity when compared to similarly sized spherical particles. These findings were rationalized with the help of density functional theory calculations