Shape-controlled Copper Nanocrystals as Electrocatalysts for CO2RR

The electrochemical CO2 reduction reaction (CO2RR) has the potential to mitigate the rising CO2 levels while storing renewable energy in chemical bonds. Copper is the only single metal electrocatalyst producing high energy dense hydrocarbons, albeit into 16 products on polycrystalline Cu. Therefore, more selective catalysts need to be developed. Inspired by single crystal studies, well defined metal nanocrystals (NCs) can leverage controllable surface facets and high surface/volume ratio to improve activity and selectivity. This thesis aims to use colloidal Cu NCs with tunable shape and size to firstly demonstrate the importance of these features to direct CO2RR selectivity, and then leverage the facet dependence of Cu NCs to expand on the knowledge regarding mechanistic pathways to high value CO2RR products.Firstly, the known selectivity of Cu{111} to CH4 is investigated at the nano-scale by synthesizing octahedral Cu NCs (Cuoh) with different sizes. The major Cu{111} facet directs CO2RR selectivity to CH4, and the size-dependent performance reveals that the ratio between Cu{111} and {110} edges determines the promotion of CO2RR over the competing hydrogen evolution reaction (HER). The smallest synthesized (75 nm) Cuoh show an overall CO2RR selectivity of 77% with a CH4 faradaic efficiency (FE) of 55%. A contemporary topic of debate is whether the high value product C2H5OH forms via *CHx-*CO coupling or *CO-*CO coupling. As a platform to address this dichotomy, we use CH4 favoring (*CHx populated) Cuoh and C2H4 favoring (*CO-*CO populated) Cucub under elevated *CO coverage induced by Ag NCs. We show that at the expense of CH4, Cuoh selectively promote C2H5OH, while Cucub make both C2H5OH and C2H4. We propose that C2H5OH formation proceeds via *CHx-*CO coupling, and the preferred sites for this coupling are the {111} faces on Cuoh and {110} edges on Cucub. Hence, we posit the counter-intuitive phenomenon of a CH4 selective catalyst becoming C2H5OH selective under CO saturation. Additionally, we suggest that catalysts having larger defect-to-terrace ratios are expected to be C2H5OH selective under CO saturation.We then proceed to tune the Cucub edge/face ratio by size control, as smaller Cucub are expected to possess increased selectivity towards C2H5OH by virtue of their higher proportion of edge sites. Indeed, smaller Cucub show larger C2H5OH/C2H4 ratio under CO saturation conditions further strengthening our hypothesis that Cucub edge sites are C2H5OH selective via the *CHx-*CO coupling pathway.This thesis showcases the strengths of colloidal NCs to reveal structure-sensitivity relations for which the achieved size control (i.e. importance of facet ratio) proved crucial. Furthermore, the possibility to use shape-controlled NCs as a platform to investigate reaction pathways is proposed. The mechanistic insights provided here could aid catalyst design targeting high selectivity for valuable CO2RR products.

Cu/Metal Oxide Hybrid Nanocrystals as Electrocatalysts for the CO2 Reduction Reaction

Seyedehbehnaz Varandili

The electrochemical CO2 reduction reaction (CO2RR) is envisioned to play a significant role in achieving carbon neutrality while contributing to storing renewable energies. Cu-based materials are among the most promising electrocatalysts. However, 16 different products are obtained from a Cu foil, therefore strategies to make more selective catalysts are needed. This thesis explores Cu/metal oxide interfaces as a mean to direct selectivity towards hydrocarbon products in CO2RR. The aim is to deviate from the scaling relationships which exist on single metal surfaces. Cu/metal oxides hybrid nanocrystals (HNCs) with tunable morphologies and spatial configuration are synthesized via colloidal chemistry. The achieved level of control and tunability is demonstrated to be crucial in understanding the impact of these parameters on the reaction outcome. First, we start by designing a synthesis procedure for Cu/CeO2-x heterodimers (HDs) and then we study their catalytic behavior toward CO2RR. We show that the Cu/CeO2-x interface promotes CO2RR, particularly methane with around 54% selectivity, which is ~5 times higher than those obtained with a physical mixture of isolated Cu and CeO2-x nanocrystals, thus highlighting the important role played by the intimate bonding at the interface. A miscellanea of characterization techniques indicates the higher extent of ceria reduction in the HDs during CO2RR. DFT elucidates the presence of unique catalytic motifs that enable bidentate adsorption of critical intermediates at both Cu and the CeO2-xO-vacancy site, creating an opportunity for reaction engineering beyond scaling relationship limitations. Having assessed the importance of the Cu/ceria interface, we investigate the impact of increasing the Cu/CeO2 interfacial area on the CO2RR outcome. We find that increasing the number of ceria domain around copper inhibits CO2RR. This result highlights that a balance between interfacial area and accessibility to the active catalytic sites must be sought after to exploit synergies arising at the interface between different domains. Finally, we explore Cu/ZnO HNCs as CO2RR pre-catalysts to elucidate the structural and compositional parameters which direct the reaction pathway towards ethanol. Using different sizes of ZnO seeds for the synthesis of Cu/ZnO HNCs, we are able to achieve two different configurations including ZnO decorated Cu core (Cu@ZnO) and Cu decorated ZnO core (ZnO@Cu) HNCs. Upon activation via cathodic voltage, we find that Cu@ZnO HNCs favor methane production while ZnO@Cu exhibits a noticeable selectivity toward ethanol. Operando XAS, XPS, and electron microscopy reveal that both HNCs transform into Cu@CuZn core@shell NCs during the activation step. However, the Zn content in the surface alloy (i.e. degree of alloying) and the presence of surface cationic Zn species are found to be crucial in determining the selectivity of these catalysts, which possibly elucidates some of the contrasting results reported on Cu-Zn across the literature. Overall, this thesis showcases the importance of well-defined and tunable HNCs as platforms to study structure/properties relations and to advance the current knowledge in CO2RR. Furthermore, the library of nanomaterials accessible through the developed synthesis routes might also be utilized in the future to investigate other catalytic reactions and applications where the hybrid interfaces might be relevant.

EPFL2021

Rational Design of Pd-Based Catalysts for Selective Alkyne Hydrogenations

Rocio Micaela Crespo Quesada

Traditionally, the search for active and selective catalysts involved a rather tedious "hit-and-miss" approach in which hundreds, if not thousands catalysts were tested until the optimal substance was identified. With the advancing theoretical understanding of catalysis and the development of computational power, a new era of rational catalyst design is dawning. This approach, grounded on first principles, is based on new advances in synthesis, characterization and modeling with the ultimate aim of predicting the expected behavior of a catalyst based on chemical composition, molecular structure and morphology. The rational design of a catalyst is a complex process which spans across several levels of scale. In this thesis, the pursuit of a rational design for Pd-based catalysts effective in alkyne hydrogenations is presented. A multi-level integrated approach was thus applied ranging from the nano-scale design of the active sites for a specific reaction, taking into account its structure sensitivity, to the micro-scale design of the supported Pd nanoparticles including metal-additive and metal-support interactions as well as mass and heat transfer phenomena. In order to rationally design a catalyst at a nano-scale, i.e. a catalyst's active site, several methodologies have been hitherto applied, such as the study on single crystals or model catalysts. Here we present the use of metal nanoparticles with tuned sizes (5-30 nm) and shapes (cubes, octahedra and cube-octahedra) prepared via colloidal techniques. These nanoparticles can be tested per se and represent a new generation of model catalysts, complementing single crystal studies, which inherently lack the complexity of industrial catalysis. However, metal nanoparticles, especially those prepared in a controlled manner in order to tailor their shape and size, require the use of stabilizing and/or capping agents capable of directing their growth. These substances can mask the true catalytic behavior of the nanoparticles. Thus, it is of great importance to study the interactions of the active phase with the substances that are in close contact with it, in the so-called meso-scaled level of rational catalyst design. Therefore, the effect of the nature of the stabilizing agent on the catalytic response was studied, as well as the promoting effect of some of these substances. Finally, a methodology was developed capable of eliminating organic stabilizing agents from the surface of nanoparticles without compromising their morphological stability. The knowledge gathered in the first two levels can be applied further to reach the microscale rational catalyst design. In this thesis, a final catalyst consisting on well-defined stabilizer-free supported Pd nanoparticles was used in the hydrogenation of acetylene. This deep study of the catalytic behavior of well-defined Pd catalysts throughout several levels of scale and complexity have given us the tools needed to perform a rational catalyst design for alkyne hydrogenations. Depending on the specific reaction, the active phase can be optimized in terms of the desired activity and selectivity and can be tuned even further with the use of specific additives. Finally, the appropriate support also exerts a promoting effect on the nanoparticles and ensures their anchoring in addition to avoiding heat and mass transfer artifacts.

EPFL2011

Engineering the ZrO2–Pd Interface for Selective CO2 Hydrogenation by Overcoating an Atomically Dispersed Pd Precatalyst

Alimohammad Bahmanpour, Yuan-Peng Du, Florent Emmanuel Héroguel, Oliver Kröcher, Jeremy Luterbacher, Mounir Driss Mensi, Luka Milosevic

Tailoring the interfacial sites between metals and metal oxides can be an essential tool in designing heterogeneous catalysts. These interfacial sites play a vital role in many renewable applications, for instance, catalytic CO2 reduction. Postsynthesis deposition of metal oxide on supported metal catalysts can not only create such interfacial sites but also prevent particle sintering at high temperature. Here, we report a sol–gel-based strategy to synthesize an atomically dispersed “precatalyst”. In contrast to the deposition on catalysts containing preformed nanoparticles, overcoating this material before reductive treatment can inhibit particle growth during thermal activation steps, yielding highly accessible, sintering-resistant Pd clusters that are less than 2 nm in diameter. This synthetic approach allows us to engineer interfacial sites while maintaining high metal accessibility, which was difficult to achieve in previous overcoated materials. Notably, engineering the Pd–ZrO2 interface into an inverted interface with an amorphous ZrO2 overcoat might facilitate a C–O cleavage route instead of a mechanism containing a bicarbonate intermediate during CO2 hydrogenation. We also observed that carbon deposition occurring on methanation sites could be a key factor for improving CO selectivity. Alteration of the reaction pathway, along with the deactivation of certain sites, led to 100% CO selectivity on the ZrO2@Pd/ZrO2 catalyst. This work demonstrated that overcoated materials could represent a promising class of heterogeneous catalysts for selective CO2 conversion over noble metals, which have higher rates and thermal stability compared to state-of-the-art Cu-based catalysts.