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[8] viXra:2002.0275 [pdf] submitted on 2020-02-14 08:21:30
Authors: Juan-Jesús Velasco-Vélez, Travis Jones, Dunfeng Gao, Emilia Carbonio, Rosa Arrigo, Cheng-Jhih Hsu, Yu-Cheng Huang, Chung-Li Dong, Jin-Ming Chen, Jyh-Fu Lee, Peter Strasser, Beatriz Roldan Cuenya, Robert Schlögl, Axel Knop-Gericke, Cheng-Hao Chuang
Comments: 16 pages manuscript plus 5 pages supplemental material
Redox-active copper catalysts with accurately prepared oxidation states (Cu0, Cu+ and Cu2+) and high selectivity to C2 hydrocarbon formation, from electrocatalytic cathodic reduction of CO2, were fabricated and characterized. The electrochemically prepared copper-redox electro-cathodes yield higher activity for the production of hydrocarbons at lower oxidation state. By combining advanced X-ray spectroscopy and in situ micro-reactors it was possible to unambiguously reveal the variation in the complex electronic structure that the catalysts undergo at different stages (i.e. during fabrication and electrocatalytic reactions). It was found that the surface, sub-surface and bulk properties of the electrochemically prepared catalysts are dominated by the formation of copper carbonates on the surface of cupric-like oxides, which prompts catalyst deactivation by restraining effective charge transport. Furthermore, the formation of reduced or partially-reduced copper catalysts yields the key dissociative proton-consuming reactive adsorption of CO2 to produce CO; allowing the subsequent hydrogenation into C2 and C1 products by dimerization and protonation. These results yield valuable information on the variations in the electronic structure that redox-active copper catalysts undergo in the course of the electrochemical reaction, which, under extreme conditions are mediated by thermodynamics but, critically, kinetics dominate near the oxide/metal phase transitions.
Category: Chemistry
[7] viXra:2002.0252 [pdf] submitted on 2020-02-13 06:58:36
Authors: Katharina Klingan, Tintula Kottakkat, Zarko P. Jovanov, Shan Jiang, Chiara Pasquini, Fabian Scholten, Paul Kubella, Arno Bergmann, Beatriz Roldan Cuenya, Christina Roth, Holger Dau
Comments: 10 Pages.
Electrochemical CO2 reduction is of high interest for production of non-fossil fuels. The reactivity of eight Cu foams with substantial morphology differences was comprehensively investigated by analysis of product spectrum and electrochemical in-situ spectroscopies (XANES, EXAFS, XPS, Raman). This approach provided new insight in reactivity determinants: (1) Morphological details, (2) stable Cu oxide phases, and (3) *CO poisoning of H2-formation are not decisive. (4) The electrochemically active surface area (ECSA) determines reactivity trends. (5) Macroscopic diffusion limits the proton supply, resulting in pronounced alkalization at CuCat surfaces (operando Raman spectroscopy). We propose: (6) H2 and CH4 formation are suppressed by macroscopic buffer alkalization, whereas CO and C2H4 formation still proceed via a largely pH-independent mechanism. (7) C2H4 is formed from two CO precursor species, namely adsorbed *CO and dissolved CO present in the foam cavities.
Category: Chemistry
[6] viXra:2002.0247 [pdf] submitted on 2020-02-12 09:15:32
Authors: Dunfeng Gao, Rosa M. Arán-Ais, Hyo Sang Jeon, Beatriz Roldan Cuenya
Comments: 29 Pages.
CO2 electroreduction reaction (CO2RR) to fuels and feedstocks is an attractive route to close the anthropogenic carbon cycle and store renewable energy. The generation of more reduced chemicals, especially multicarbon oxygenate and hydrocarbon products (C2+) with higher energy density is highly desirable for industrial applications. However, selective conversion of CO2 to C2+ suffers from high overpotential, low reaction rate and low selectivity, and the process is extremely sensitive to the catalyst structure and electrolyte. Here we discuss strategies to achieve high C2+ selectivity through rational design of the catalyst and electrolyte. Current state-of-the-art catalysts, including Cu and Cu-bimetallic catalysts as well as alternative materials are considered. The importance of taking into consideration the dynamic evolution of the catalyst structure and composition are highlighted, focusing on findings extracted from in situ and operando characterizations. Additional theoretical insight into the reaction mechanisms underlying the improved C2+ selectivity of specific catalyst geometries/compositions in synergy with a well-chosen electrolyte are also provided.
Category: Chemistry
[5] viXra:2002.0237 [pdf] submitted on 2020-02-12 02:07:07
Authors: Lukas Pielsticker, Ioannis Zegkinoglou, Nuria J. Divins, Hemma Mistry, Yen-Ting Chen, Aleksander Kostka, Jorge Anibal Boscoboinik, Beatriz Roldán Cuenya
Comments: 26 Pages.
Surface segregation, restructuring and sintering phenomena in size-selected copper-nickel nanoparticles (NPs) supported on silicon dioxide substrates were systematically investigated as a function of temperature, chemical state and reactive gas environment. Using near-ambient pressure (NAP-XPS) and ultra-high vacuum X-ray photoelectron spectroscopy (XPS), we showed that nickel tends to segregate to the surface of the NPs at elevated temperatures in oxygen- or hydrogen-containing atmospheres. It was found that the NP pre-treatment, gaseous environment and oxide formation free energy are the main driving forces of the restructuring and segregation trends observed, overshadowing the role of the surface free energy. The depth profile of the elemental composition of the particles was determined under operando CO2 hydrogenation conditions by varying the energy of the X-ray beam. The temperature dependence of the chemical state of the two metals was systematically studied, revealing the high stability of nickel oxides on the NPs and the important role of high valence oxidation states in the segregation behavior. Atomic force microscopy (AFM) studies revealed a remarkable stability of the NPs against sintering at temperatures as high as 700 °C. The results provide new insights into the complex interplay of the various factors which affect alloy formation and segregation phenomena in bimetallic NP systems, often in ways different from those previously known for their bulk counterparts. This leads to new routes for tuning the surface composition of nanocatalysts, for example through plasma and annealing pre-treatments.
Category: Chemistry
[4] viXra:2002.0236 [pdf] submitted on 2020-02-12 02:30:20
Authors: Dunfeng Gao, Ian T. McCrum, Shyam Deo, Yong-Wook Choi, Fabian Scholten, Weiming Wan, Jingguang G. Chen, Michael J. Janik, Beatriz Roldan Cuenya
Comments: 34 Pages.
CO2 electroreduction reaction (CO2RR) to chemicals and fuels is of both fundamental and practical significance since it would lead to a more efficient storage of renewable energy while closing the carbon cycle. Here we report enhanced activity and selectivity for CO2RR to multicarbon hydrocarbons and alcohols (~69 % Faradaic efficiency and −45.5 mA cm−2 partial current density for C2+ at −1.0 V vs RHE) over O2-plasma-activated Cu catalysts via electrolyte design. Increasing the size of the alkali metal cations in the electrolyte, in combination with the presence of subsurface oxygen species which favor their adsorption, significantly improved C-C coupling on CuOx electrodes. The co-existence of Cs+ and I− induced drastic restructuring of the CuOx surface, the formation of shaped particles containing stable CuI species, and a more favorable stabilization of the reaction intermediates and concomitant high C2+ selectivity. This work combining both experiment and density functional theory, provides insights into the active sites and reaction mechanism of oxide-derived Cu catalysts for CO2RR.
Category: Chemistry
[3] viXra:2002.0235 [pdf] submitted on 2020-02-12 03:10:18
Authors: M. Bernal, A. Bagger, F. Scholten, I. Sinev, A. Bergmann, M. Ahmadi, J. Rossmeisl, B. Roldan Cuenya
Comments: 32 Pages.
Understanding the changes that a catalyst may experience on its surface during a reaction is crucial in order to stablish structure/composition-reactivity correlations. Here, we report on bimetallic size-selected Cu100-xCox nanoparticle (NP) catalysts for CO2 electroreduction reaction (CO2RR) and we identify the optimum Cu/Co ratio and NP size leading to improved activity and selectivity. Operando X-ray absorption spectroscopy (XAS) and quasi in situ X-ray photoelectron spectroscopy (XPS) provided insight into the morphological, structural, and chemical transformations underwent by the CuCo NPs during CO2RR. We illustrate that the as-prepared state of the bimetallic NPs is drastically different from the structure and surface composition of the working catalyst. Under electrochemical conditions, a reduction of both initially oxidized metallic species was observed, accompanied by Cu surface segregation. Density functional theory (DFT) results from a Cu3X model were used to describe the surface segregation. In order to extract mechanistic understanding, the activity of the experimental Cu and CuCo NPs towards CO2RR was described via DFT in terms of the interaction of Cu facets under expansion and compression with key reaction intermediates, in particular CO* and COOH*.
Category: Chemistry
[2] viXra:2002.0234 [pdf] submitted on 2020-02-12 03:27:28
Authors: Hyo Sang Jeon, Ilya Sinev, Fabian Scholten, Nuria J. Divins, Ioannis Zegkinoglou, Lukas Pielsticker, Beatriz Roldan Cuenya
Comments: 4 Pages.
We explored the size-dependent activity and selectivity of Zn nanoparticles (NPs) for the electrochemical CO2 reduction reaction (CO2RR). Zn NPs ranging from 3 to 5 nm showed high activity and selectivity (~70 %) for CO production, while those above 5 nm exhibited bulk-like catalytic properties. In addition, a drastic increase in hydrogen production was observed for the Zn NPs below 3 nm, which is associated with the enhanced content of low-coordinated sites on small NPs. The presence of residual cationic Zn species in the catalysts was also revealed during CO2RR via operando X-ray absorption fine-structure spectroscopy (XAFS) measurements. Such species are expected to play a role in the selectivity trends obtained. Our findings can serve as guidance for the development of highly active and CO-selective Zn-based catalysts for CO2RR.
Category: Chemistry
[1] viXra:2002.0227 [pdf] submitted on 2020-02-12 07:35:10
Authors: Rosa M. Arán-Ais, Dunfeng Gao, Beatriz Roldan Cuenya
Comments: 30 Pages.
The utilization of fossil fuels (i.e., coal, petroleum, and natural gas) as the main energy source gives rise to serious environmental issues, including global warming caused by the continuously increasing level of atmospheric CO2. To deal with this challenge, fossil fuels are being partially replaced by renewable energy such as solar and wind. However, such energy sources are usually intermittent and currently constitute a very low portion of the overall energy consumption. Recently, the electrochemical conversion of CO2 to chemicals and fuels with improved energy density driven by electricity derived from renewable energy has been recognized as a promising strategy towards sustainable energy.
The activation and reduction of CO2, which is a thermodynamically stable and kinetically inert molecule, is extremely challenging. Although the participation of protons in the CO2 electroreduction reaction (CO2RR) helps lower the energy barrier, high overpotentials are still needed to efficiently drive the process. On the other hand, the concurrent hydrogen evolution reaction (HER) under CO2RR conditions leads to lower selectivity toward CO2RR products. Electrocatalysts that are highly active and selective for multicarbon products are urgently needed to improve the energy efficiency of CO2RR. The reduction of CO2 involves multiple proton-electron transfers and has many complex intermediates. Recent reports have shown that the relative stability of the intermediates on the surface of catalysts determines final reaction pathways as well as the product selectivity. Furthermore, this reaction displays a strong structure-sensitivity. The atomic arrangement, electronic structure, chemical composition, and oxidation state of the catalysts significantly influence catalyst performance. Fundamental understanding of the dependence of the reaction mechanisms on the catalyst structure would guide the rational design of new nanostructured CO2RR catalysts. As a reaction proceeding in a complex environment containing gas/liquid/solid interfaces, CO2RR is also intensively affected by the electrolyte. The electrolyte composition in the near surface region of the electrode where the reaction takes place plays a vital role in the reactivity. However, the former might also be indirectly determined by the bulk electrolyte composition via diffusion. Adding to the complexity, the structure, chemical state and surface composition of the catalysts under reaction conditions usually undergo dynamic changes, especially when adsorbed ions are considered. Therefore, in addition to tuning the structure of the electrocatalysts, being able to also modify the electrolyte provides an alternative method to tune the activity and selectivity of CO2RR. In situ and operando characterization methods must be employed in order to gain in depth understanding on the structure- and electrolyte-sensitivity of real CO2RR catalysts under working conditions.
This Account provides examples of recent advances in the development of nanostructured catalysts and mechanistic understanding of CO2RR. It discusses how the structure of a catalyst (crystal orientation, oxidation state, atomic arrangement, defects, size, surface composition, segregation, etc.) influences the activity and selectivity, and how the electrolyte also plays a determining role in the reaction activity and selectivity. Finally, the importance of in situ and operando characterization methods to understand the structure- and electrolyte-sensitivity of the CO2RR is discussed.
Category: Chemistry
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