Active and Selective Ensembles in Oxide-Derived Copper Catalysts for CO2 Reduction

Copper catalysts are unique in CO2 reduction as they allow the formation of C2+ products. Depending on the catalysts' synthesis, product distribution varies significantly: while Cu nanoparticles produce mainly methane and hydrogen, oxide-derived copper leads to ethylene and ethanol. Here, by means of ab initio molecular dynamics on oxygen-depleted models, we identified the ensembles controlling catalytic performance. Upon reconstruction and irrespective of the starting structure, recurrent patterns defined by their coordination and charges appear: metallic Cu0, polarized Cuδ+, and oxidic Cu+. These species combine to form 14 ensembles. Among them, 4-(6-)coordinated Cu adatoms and Cu3δ+O3 are responsible for tethering CO2, while metastable near-surface oxygens in fcc-(111) or (100)-like Cu domains promote C-C bond formation via glyoxylate species, thus triggering selective C2+ production at low onset potentials. Our work provides guidelines for modeling complex structural rearrangements under CO2 reduction conditions and devising new synthetic protocols toward an enhanced catalytic performance

Influence of Cations on HCOOH and CO Formation during CO2 Reduction on a PdMLPt(111) Electrode

Understanding the role of cations in the electrochemical CO2 reduction (CO2RR) process is of fundamental importance for practical application. In this work, we investigate how cations influence HCOOH and CO formation on PdMLPt(111) in pH 3 electrolytes. While only (a small amount of adsorbed) CO forms on PdMLPt(111) in the absence of metal cations, the onset potential of HCOOH and CO decreases with increasing cation concentrations. The cation effect is stronger on HCOOH formation than that on CO formation on PdMLPt(111). Density functional theory simulations indicate that cations facilitate both hydride formation and CO2 activation by polarizing the electronic density at the surface and stabilizing *CO2-. Although the upshift of the metal work function caused by high coverage of adsorbates limits hydride formation, the cation-induced electric field counterbalances this effect in the case of *H species, sustaining HCOOH production at mild negative potentials. Instead, at the high *CO coverages observed at very negative potentials, surface hydrides do not form, preventing the HCOOH route both in the absence and presence of cations. Our results open the way for a consistent evaluation of cationic electrolyte effects on both activity and selectivity in CO2RR on Pd-Pt catalysts

The Role of Cation Acidity on the Competition between Hydrogen Evolution and CO2 Reduction on Gold Electrodes

CO2 electroreduction (CO2RR) is a sustainable alternative for producing fuels and chemicals. Metal cations in the electrolyte have a strong impact on the reaction, but mainly alkali species have been studied in detail. In this work, we elucidate how multivalent cations (Li+, Cs+, Be2+, Mg2+, Ca2+, Ba2+, Al3+, Nd3+, and Ce3+) affect CO2RR and the competing hydrogen evolution by studying these reactions on polycrystalline gold at pH = 3. We observe that cations have no effect on proton reduction at low overpotentials, but at alkaline surface pH acidic cations undergo hydrolysis, generating a second proton reduction regime. The activity and onset for the water reduction reaction correlate with cation acidity, with weakly hydrated trivalent species leading to the highest activity. Acidic cations only favor CO2RR at low overpotentials and in acidic media. At high overpotentials, the activity for CO increases in the order Ca2+ < Li+ < Ba2+ < Cs+. To favor this reaction there must be an interplay between cation stabilization of the*CO2- intermediate, cation accumulation at the outer Helmholtz plane (OHP), and activity for water reduction. Ab initio molecular dynamics simulations with explicit electric field show that nonacidic cations show lower repulsion at the interface, accumulating more at the OHP, thus triggering local promoting effects. Water dissociation kinetics is increasingly promoted by strongly acidic cations (Nd3+, Al3+), in agreement with experimental evidence. Cs+, Ba2+, and Nd3+ coordinate to adsorbed CO2 steadily; thus they enable*CO2- stabilization and barrierless protonation to COOH and further reduction products

Identifying Promising Ionic Liquids for Electrochemical CO2 Reduction

Electrochemical CO2 reduction (CO2R) is a promising technology to reduce CO2 atmospheric concentrations by simultaneously storing renewable energy and generating high added-value products.1 Among the many possible reaction products, the generation of syngas, i.e. a mixture of carbon monoxide and hydrogen, is particularly considered as it requires low energetic consumption, yet this product ensures a broad market share.2 Such process usually occurs on weak CO binding catalysts, such as Au and Ag,3 and it can be particularly enhanced using ionic liquids (IL) as co-catalysts in the electrolyte.4,5 Earlier computational studies indicate that ionic liquids can either stabilize the CO2 adsorbate via electric interaction 6 or poison the electrocatalytic surface,7 thus blocking CO2 reduction and enhancing water reduction. In our group, we recently carried out a systematic assessment of the role of different EMIM+/BMIM+-based ionic liquids for the electrochemical reduction of CO2 on silver electrodes. Such study resulted in a joint experimental/modeling work,8 where some of us demonstrated that IL anions tune the ratio between the concentration of cations (EMIM+ or BMIM+) and the carbene species (EMIM:/BMIM:) in the electrolyte. Such effect can be rationalized by using few thermodynamic descriptors, such as the formation energy of EMIM:/BMIM: species, their adsorption energy, and their ability to trap CO2 in solution. Consequently, the ratio of cations and carbenes rules the CO2 capture and electrochemical conversion properties of imidazolium based ILs. Herein, we carried out a follow up of the previous study,8 generalizing the previously suggested descriptors to provide predictive guidelines for experimental synthesis. Screening among different IL, we confirmed that the formation energy of EMIM:/BMIM: species is the primary driving force for enhancing water reduction. In fact, such carbenes either increase the local availability of protons to sustain hydrogen evolution (HER) or block the surface, hindering adsorption of CO2 at the surface and thus allowing only HER to occur. Such surface blocking effect was further confirmed by in-house measurements of electrochemically active surface areas (ECSA) in presence of different IL. In addition to surface poisoning, EMIM:/BMIM: can also trap CO2 in solution, further hindering CO2 reduction. Among the considered anions, acetate anion determines the lowest energy for EMIM+/BMIM+ deprotonation, consequently leading to high H2 partial current densities and low ECSA values. Instead, triflate anion prevents the formation of carbenes and thus hinders any surface poisoning effect, enabling Faradaic efficiency toward CO behind 90%. Overall, by generalizing the insights from the previous work,8 we here provide guidelines to identify the best ionic liquids out of simple thermodynamic properties. By extrapolating our results to IL not yet tested, it is possible to predict HER and CO2R activities on silver, thus enabling a direct pathway for the experimental design of IL for CO2R

Determining Structure-Activity Relationships in Oxide Derived CuSn Catalysts During CO2 Electroreduction Using X-Ray Spectroscopy

The development of earth-abundant catalysts for selective electrochemical CO2 conversion is a central challenge. Cu-Sn bimetallic catalysts can yield selective CO2 reduction toward either CO or formate. This study presents oxide-derived Cu-Sn catalysts tunable for either product and seeks to understand the synergetic effects between Cu and Sn causing these selectivity trends. The materials undergo significant transformations under CO2 reduction conditions, and their dynamic bulk and surface structures are revealed by correlating observations from multiple methods—X-ray absorption spectroscopy for in situ study, and quasi in situ X-ray photoelectron spectroscopy for surface sensitivity. For both types of catalysts, Cu transforms to metallic Cu0 under reaction conditions. However, the Sn speciation and content differ significantly between the catalyst types: the CO-selective catalysts exhibit a surface Sn content of 13 at. % predominantly present as oxidized Sn, while the formate-selective catalysts display an Sn content of ≈70 at. % consisting of both metallic Sn0 and Sn oxide species. Density functional theory simulations suggest that Snδ+ sites weaken CO adsorption, thereby enhancing CO selectivity, while Sn0 sites hinder H adsorption and promote formate production. This study reveals the complex dependence of catalyst structure, composition, and speciation with electrochemical bias in bimetallic Cu catalysts

Theoretical models for the electrochemical reduction of CO2 on copper catalysts under working conditions

CO2 reduction is the only process which can generate green fuels with a net negative impact in CO2 emissions. Therefore, the future development of our society needs an industrial scale up of this technology, involving the production of heavily used chemicals such as ethylene. Copper is a unique material for catalyzing these C2+ products, however significant advances need a deep theoretical understanding of the complexity of this material under CO2 reduction conditions. In this thesis I aimed at developing theoretical methods to address the main factors involved in this process: (i) surface reconstruction at negative potential; (ii) chemical effects on copper selectivity; and (iii) the effect of the electrolyte. Chapters I and II were dedicated to the motivations and methods. After having benchmarked in Chapter 3 well-established experimental results, such as the morphology dependence of CO2 product distribution on copper local morphology, I investigated the reconstruction of polycrystalline copper at negative potentials. This process is driven by local surface polarization, which destabilizes close-packed domains and promotes (100) facets and defects. Following theoretical guidelines, I synthesized an effective copper-based catalyst with produced ethylene at high yield and high current density. In Chapter V I studied a complex oxide-derived copper material to provide insights about copper oxidation state, its coordination and surface ensembles active toward C2+ chemicals. Among the outcomes, I demonstrated that polarization drives CO2 reduction activity, whilst a newly reported intermediate, a deprotonated glyoxylate, triggers C2+ selectivity. In chapter VI I dedicated to chemical effects on copper reactivity. Sulfur adatoms, acting as strong tethering centers enable the generation of formate, a chemical employed as preservative for animal food stock. Finally, in Appendix A I introduced cation effect on CO2 reduction, not yet fully understood but having a clear relevance on product distribution

Modelling and mapping pathways of electrochemical CO2 reduction on modified catalytic surfaces

La reducció de CO2 és l'únic procés per generar combustibles verds amb un impacte negatiu net en les emissions de CO2. Per tant, el desenvolupament futur de la nostra societat necessita una aplicació industrial d'aquesta tecnologia per produir productes químics d'ús intensiu com l'etilè. El coure és un material únic per catalitzar aquests productes, però, avenços significatius en aquest procés requereixen una comprensió teòrica profunda de la seva complexitat. En aquesta tesi em vaig proposar desenvolupar mètodes teòrics per abordar els principals factors involucrats en la reducció de CO2 amb coure: (i) reconstrucció superficial a causa de potencial negatiu; (ii) efectes químics sobre la selectivitat; i (iii) l'efecte de l'electròlit. Els capítols I i II es van dedicar a les motivacions i mètodes i el Capítol 3 a comprovar resultats experimentals ben establerts. En el capítol 4 vaig investigar la reconstrucció del coure policristal·lí a potencials negatius. Aquest procés està impulsat per la polarització de la superfície, que promou dominis (100) i defectes. Seguint les previsions teòriques, vaig sintetitzar un catalitzador a base de coure eficaç per produir etilè amb alt rendiment. En el capítol V, vaig estudiar l'òxid de coure per investigar l'estat d'oxidació del coure, la seva coordinació i els llocs superficials actius cap a la producció de químics C2+. Entre els resultats, vaig demostrar que la polarització impulsa la reducció de CO2, mentre un nou intermedi, el glioxilato desprotonado, millora la selectivitat fins als C2+. En el capítol VI em vaig dedicar a efectes químics que influencien la reactivitat el coure. Adatomos de sofre, que actuen com a centres d'ancoratge, permeten la generació de formiat. Finalment, a l'apèndix A vaig introduir l'efecte dels cations sobre la reducció de CO2, que encara no es comprèn completament, però té una clara rellevància en la distribució del producte.La reducción de CO2 es el único proceso para generar combustibles verdes con un impacto negativo neto en las emisiones de CO2. Por lo tanto, el desarrollo futuro de nuestra sociedad necesita una aplicación industrial de esta tecnología para producir productos químicos de uso intensivo como el etileno. El cobre es un material único para catalizar estos productos, sin embargo, avances significativos en este proceso requieren una comprensión teórica profunda de su complejidad. En esta tesis me propuse desarrollar métodos teóricos para abordar los principales factores involucrados en la reducción de CO2 con cobre: (i) reconstrucción superficial debido a potencial negativo; (ii) efectos químicos sobre la selectividad; y (iii) el efecto del electrolito. Los capítulos I y II se dedicaron a las motivaciones y métodos y el Capítulo 3 a comprobar resultados experimentales bien establecidos. En el capítulo 4 investigué la reconstrucción del cobre policristalino a potenciales negativos. Este proceso está impulsado por la polarización de la superficie, que promueve dominios (100) y defectos. Siguiendo las previsiones teóricas, sinteticé un catalizador a base de cobre eficaz para producir etileno con alto rendimiento. En el Capítulo V, estudié el óxido de cobre para investigar el estado de oxidación del cobre, su coordinación y los sitios superficiales activos hacia la producción de químicos C2+. Entre los resultados, demostré que la polarización impulsa la reducción de CO2, mientras un nuevo intermedio, el glioxilato desprotonado, mejora la selectividad hasta los C2+. En el capítulo VI me dediqué a efectos químicos que influencian la reactividad del cobre. Adatomos de azufre, que actúan como centros de anclaje, permiten la generación de formiato. Finalmente, en el Apéndice A introduje el efecto de los cationes sobre la reducción de CO2, que aún no se comprende completamente, pero tiene una clara relevancia en la distribución del producto.CO2 reduction is the only process which can generate green fuels with a net negative impact in CO2 emissions. Therefore, the future development of our society needs an industrial scale up of this technology, involving the production of heavily used chemicals such as ethylene. Copper is a unique material for catalyzing these C2+ products, however significant advances need a deep theoretical understanding of the complexity of this material under CO2 reduction conditions. In this thesis I aimed at developing theoretical methods to address the main factors involved in this process: (i) surface reconstruction at negative potential; (ii) chemical effects on copper selectivity; and (iii) the effect of the electrolyte. Chapters I and II were dedicated to the motivations and methods. After having benchmarked in Chapter 3 well-established experimental results, such as the morphology dependence of CO2 product distribution on copper local morphology, I investigated the reconstruction of polycrystalline copper at negative potentials. This process is driven by local surface polarization, which destabilizes close-packed domains and promotes (100) facets and defects. Following theoretical guidelines, I synthesized an effective copper-based catalyst with produced ethylene at high yield and high current density. In Chapter V I studied a complex oxide-derived copper material to provide insights about copper oxidation state, its coordination and surface ensembles active toward C2+ chemicals. Among the outcomes, I demonstrated that polarization drives CO2 reduction activity, whilst a newly reported intermediate, a deprotonated glyoxylate, triggers C2+ selectivity. In chapter VI I dedicated to chemical effects on copper reactivity. Sulfur adatoms, acting as strong tethering centers enable the generation of formate, a chemical employed as preservative for animal food stock. Finally, in Appendix A I introduced cation effect on CO2 reduction, not yet fully understood but having a clear relevance on product distribution

Diffusion trapped oxygen in oxide derived Copper electrocatalyst in CO2 reduction

Oxide-derived Cu (OD-Cu) catalysts have shown an excellent ability to ensure C-C coupling in the electrochemical carbon dioxide reduction reaction (eCO2RR). However, these materials extensively rearrange under reaction conditions, thus the nature of the active site remains controversial. Here, we studied the reduction process of OD-Cu via large-scale molecular dynamics at first-principles accuracy introducing experimental conditions. The oxygen concentration in the most stable OD-Cu materials increases with the increase of the pH/potential/specific surface area. In long electrochemical experiments, the catalyst would be fully reduced to Cu, but it takes a considerable amount of time to remove all the trapped oxygen, and the highly reconstructed Cu surface provides various sites to adsorb oxygen under relatively stronger reduction potentials (U = –0.58 VSHE at pH=14, 0.25 VRHE). This work provides insight into the evolution of OD-Cu catalysts and residual oxygen during the reaction conditions and a deep understanding of the nature of active sites

Origin of the selective electroreduction of carbon dioxide to formate by chalcogen modified copper

The electrochemical reduction of atmospheric CO2 by renewable electricity opens new routes to synthesize fuels and chemicals, but more selective and efficient catalysts are needed. Herein, by combining experimental and first-principles studies, we explain why chalcogen modified copper catalysts are selective toward formate as the only carbon product. On the unmodified copper, adsorbed CO2 is the key intermediate, yielding carbon monoxide and formate as carbon products. On sulfur, selenium, or tellurium modified copper, chalcogen adatoms are present on the surface and actively participate in the reaction, either by transferring a hydride or by tethering CO2 thus suppressing the formation of CO. These results highlight the active role of chalcogen centers via chemical steps and point toward basicity as the key descriptor for the stability and selectivity of these catalysts.ISSN:1948-718

Origin of the selective electroreduction of carbon dioxide to formate by chalcogen modified copper

The electrochemical reduction of atmospheric CO2 by renewable electricity opens new routes to synthesize fuels and chemicals, but more selective and efficient catalysts are needed. Herein, by combining experimental and first-principles studies, we explain why chalcogen modified copper catalysts are selective toward formate as the only carbon product. On the unmodified copper, adsorbed CO2 is the key intermediate, yielding carbon monoxide and formate as carbon products. On sulfur, selenium, or tellurium modified copper, chalcogen adatoms are present on the surface and actively participate in the reaction, either by transferring a hydride or by tethering CO2 thus suppressing the formation of CO. These results highlight the active role of chalcogen centers via chemical steps and point toward basicity as the key descriptor for the stability and selectivity of these catalysts.ISSN:1948-718