Nanoparticle Catalysts for Chemical Valorization of Carbon Dioxide
Human activity in the last two centuries that has revolutionized the way of using resources and brought marked improvements in the quality of our lives has now put us in a position to rethink of the many operations that support our society. We are at a stage where the practices of the past and present cannot be continued due to their severe negative impacts, such as the climate change. At the center of those practices lies our heavy reliance on fossil fuels, for harvesting energy and manufacturing chemical products. We need a disruptive technology that can fundamentally change our common practices and poses minimal threats to our society.
Electrochemical (or photochemical) conversion of carbon dioxide to value-added products is one of the technologies with a potential to make the change. It targets the use of atmospheric carbon dioxide as reagents to generate valuable carbon products, such as hydrocarbons or multicarbon oxygenates, which can be utilized for various purposes. Eventually, it allows creating an artificial carbon cycle, just like the one that nature established, which the carbon compounds that we use are constantly recycled through the system by renewable energy inputs such as solar or wind. In order to realize its potentials, a high-performance catalyst material that can efficiently convert carbon dioxide has to be developed. In this regard, the unique catalytic properties of nanoparticles have brought significant advancements in the field.
Here, numerous structural factors of nanoparticles that are linked to their CO2 electrocatalytic performance are discussed. With the notion that these factors span over large length scales, studies have been conducted focusing on not only the properties originating from the atomic scale but structural considerations at the micro- or macroscopic scales. The latter is related to the goal of establishing a complete system capable of CO2 valorization by renewable energy inputs. In addition, a critical factor often neglected, the structural dynamics of nanoparticles have been studied which will shed light into the ways of identifying the true catalytic origin of materials and developing catalysts that will naturally transform into optimized structures during operation.
Bimetallic systems offer a great amount of structural diversity by their change of composition and the physical arrangements of both elements. With regards to CO2 electroreduction, gold-copper bimetallic nanoparticles were explored as catalysts by changing the composition and the atomic orderedness. The composition-dependent activity of Au-Cu nanoparticles showed that the shift in the composition brought modifications to their surface electronic structure that dictated the intermediate binding strength. Furthermore, evidence of geometric effects to intermediate stabilization could be observed as well. However, even with a fixed composition, it was found that the atomic ordering transformation could enable significant shifts in the catalytic behavior (i.e. H2 active to CO active). Investigations involving atomic resolution imaging and X-ray absorption showed that atomic ordering of Au-Cu systems induced a few-atoms thick gold layer on the surface which was responsible for the NPs becoming active for CO2 reduction.
Eventually, catalysts developed for CO2 reduction need to be coupled with other materials or systems to enable CO2 valorization using renewable energy sources. With solar photons being one of the most attractive options as a renewable energy input, two conceptually different ways of utilizing nanoparticles for light-driven CO2 reduction have been studied. The first method involves utilizing nanoparticles as CO2 electrocatalysts and interfacing them with light-absorbing platforms. In this approach, often the issue is how to enable an efficient coupling of the two different materials without compromising their inherent capabilities. Using Si NWs as the light-absorber, we found that the one-dimensional geometry of the NWs guided the colloidal NP assembly process that allowed uniform and controlled assembly of NPs. This integration process led to performance improvements in light-driven CO2 conversion compared to what can be achieved in other platforms and showed the potential of Si NWs to act as a general platform for interfacing a wide range of electrocatalysts.
The other approach involved the use of nanoparticles as catalytic enhancers. Photoactive rhenium complexes were used as visible light-absorbing CO2 catalysts. These complexes were covalently attached within a metal-organic framework (MOF) to understand how the molecular environment within the MOF`s pores can affect their catalytic behavior. With the optimized configuration of the MOF structure, Re-complex containing MOFs were coated on silver nanocubes where MOFs spatially concentrated the photoactive complexes in the vicinity of the plasmon-enhanced near surface electric fields of silver nanocubes. The Ag-MOF nanoparticle structure exhibited maximum 7-fold enhancements in catalytic turnover with stability demonstrated for 48 hours. The success of this approach clearly demonstrates the catalytic potential of combining various inorganic and organic materials with each component having discrete functionalities.
An important aspect of nanomaterials is there tendency to readily transform under various conditions due to their increased energetics. This is particularly important for catalytic applications as the nanoscale catalysts can restructure during catalysis, which can make the structure-activity correlation elusive and also lead to changes in activity over time. This idea has been recently considered for CO2 electrocatalysts. Using spherical copper nanoparticles as the starting material, structural transformation to cuboidal nanoparticles by their fusion and redistribution was observed under catalytic conditions. This transition led to significant improvements in the multicarbon formation activity from CO2. The in-situ formed catalyst exhibited record low overpotentials for achieving over 50% of C2-3 products faradaic efficiency, clearly demonstrating the importance of understanding the materials dynamics under electrochemical conditions. In addition, apart from CO2 electrocatalysis, a rare phenomenon of disappearing copper nanoparticles on amorphous silica supports under ambient conditions was observed. Structural investigations probing the elemental environment of copper atoms showed that they were migrating into the silica support surrounded by oxygen and silicon atoms. This observation opens up many questions regarding the nature of active sites for oxide-supported metal nanoparticles in catalysis.
Overall, the works discussed here illustrate the structural complexity of catalytic systems. Acknowledging their complexity and having a systematic understanding of its factors will provide the fundamental basis for developing efficient catalyst materials and systems for CO2 electroreduction. With all these taken into consideration, we may see an expansion of the potentials of nanoparticles used for CO2 conversion, to the levels where any type of a desired product can be readily and selectively gained.