CO2 reduction to higher value products is a promising way to produce fuels and key chemical building blocks while reducing CO2 emissions. The reaction at atmospheric pressure mainly yields CH4 via methanation and CO via the reverse water-gas shift (RWGS) reaction. Describing catalyst features that control the selectivity of these two pathways is important to determine the formation of specific products. At the same time, identification of morphological changes occurring to catalysts under reaction conditions can be crucial to tune their catalytic performance. In this contribution we investigate the dependency of selectivity for CO2 reduction on the size of Ru nanoparticles (NPs) and on support. We find that even at rather low temperatures (210 °C), oxidative pretreatment induces redispersion of Ru NPs supported on CeO2 and leads to a complete switch in the performance of this material from a well-known selective methanation catalyst to an active and selective RWGS catalyst. By utilizing in situ X-ray absorption spectroscopy, we demonstrate that the low-temperature redispersion process occurs via decomposition of the metal oxide phase with size-dependent kinetics, producing stable single-site RuO x /CeO2 species strongly bound to the CeO2 support that are remarkably selective for CO production. These results show that reaction selectivity can be heavily dependent on catalyst structure and that structural changes of the catalyst can occur even at low temperatures and can go unseen in materials with less defined structures.

Templated encapsulation of platinum-based catalysts promotes high-temperature stability to 1,100 °C.

Catalytic CO2 reduction to fuels and chemicals is a major pursuit in reducing greenhouse gas emissions. One approach utilizes the reverse water‐gas shift reaction, followed by Fischer–Tropsch synthesis, and iron is a well‐known candidate for this process. Some attempts have been made to modify and improve its reactivity, but resulted in limited success. Now, using ruthenium–iron oxide colloidal heterodimers, close contact between the two phases promotes the reduction of iron oxide via a proximal hydrogen spillover effect, leading to the formation of ruthenium–iron core–shell structures active for the reaction at significantly lower temperatures than in bare iron catalysts. Furthermore, by engineering the iron oxide shell thickness, a fourfold increase in hydrocarbon yield is achieved compared to the heterodimers. This work shows how rational design of colloidal heterostructures can result in materials with significantly improved catalytic performance in CO2 conversion processes.

Low-temperature removal of noxious environmental emissions plays a critical role in minimizing the harmful effects of hydrocarbon fuels. Emission-control catalysts typically consist of large quantities of rare, noble metals (e.g., platinum and palladium), which are expensive and environmentally damaging metals to extract. Alloying with cheaper base metals offers the potential to boost catalytic activity while optimizing the use of noble metals. In this work, we show that Pt x Cu 100−x catalysts prepared from colloidal nanocrystals are more active than the corresponding Pt catalysts for complete propene oxidation. By carefully controlling their composition while maintaining nanocrystal size, alloys with dilute Cu concentrations (15−30% atomic fraction) demonstrate promoted activity compared to pure Pt. Complete propene oxidation was observed at temperatures as low as 150 °C in the presence of steam, and five to ten times higher turnover frequencies were found compared to monometallic Pt catalysts. Through DFT studies and structural and catalytic characterization, the remarkable activity of dilute Pt x Cu 100−x alloys was related to the tuning of the electronic structure of Pt to reach optimal binding energies of C* and O* intermediates. This work provides a general approach toward investigation of structure−property relationships of alloyed catalysts with efficient and optimized use of noble metals.

Pd-and Pt-based catalysts are highly studied materials due to their widespread use in emissions control catalysis. However, claims continue to vary regarding the active phase and oxidation state of the metals. Different conclusions have likely been reached due to the heterogeneous nature of such materials containing various metal nanoparticle sizes and compositions, which may each possess unique redox features. In this work, using uniform nanocrystal catalysts, we study the effect of particle size and alloying on redox properties of Pd-based catalysts and show their contribution to methane combustion activity using operando quick extended x-ray absorption fine structure measurements. Results demonstrate that for all studied Pd sizes (3 nm-16 nm), Pd oxidation directly precedes CH 4 combustion to CO 2 , suggesting Pd oxidation as a prerequisite step to methane combustion, and an oxidation pretreatment shows equal or better catalysis than a reduction pretreatment. Results are then extended to uniform alloyed PtxPd 1−x nanoparticles, where oxidative pretreatments are shown to enhance low-temperature combustion. In these uniform alloys, we observe a composition-dependent effect with Pt-rich alloys showing the maximum difference between oxidative and reductive pretreatments. In Pt-rich alloys, we initially observe that the presence of Pt maintains Pd in a lower-activity reduced state. However, with time on stream, PdO eventually segregates under oxidizing combustion conditions, leading to a slowly increasing activity. Overall, across particle sizes and alloy compositions, we relate increased catalytic activity to Pd oxidation, thus shedding light on previous contrasting results related to the methane combustion activity of these catalysts.

Colloidal nanocrystals allow investigating sintering phenomena in supported catalysts.

Supported metal nanoparticles are essential components of high-performing catalysts, and their structures are intensely researched. In comparison, nanoparticle spatial distribution in powder catalysts is conventionally not quantified, and the influence of this collective property on catalyst performance remains poorly investigated. Here, we demonstrate a general colloidal self-assembly method to control uniformity of nanoparticle spatial distribution on common industrial powder supports. We quantify distributions on the nanoscale using image statistics and show that the type of nanospatial distribution determines not only the stability, but also the activity of heterogeneous catalysts. Widely investigated systems (Au−TiO 2 for CO oxidation thermocatalysis and Pd−TiO 2 for H 2 evolution photocatalysis) were used to showcase the universal importance of nanoparticle spatial organization. Spatially and temporally resolved microkinetic modeling revealed that nonuniformly distributed Au nanoparticles suffer from local depletion of surface oxygen, and therefore lower CO oxidation activity, as compared to uniformly distributed nanoparticles. Nanoparticle spatial distribution also determines the stability of Pd−TiO 2 photocatalysts, because nonuniformly distributed nanoparticles sinter while uniformly distributed nanoparticles do not. This work introduces new tools to evaluate and understand catalyst collective (ensemble) properties in powder catalysts, which thereby pave the way to more active and stable heterogeneous catalysts.

Metal nanoparticles have superior properties for a variety of applications. In many cases, the improved performance of metal nanoparticles is tightly correlated with their size and atomic composition. To date, colloidal synthesis is the most commonly used technique to produce metal nanoparticles. However, colloidal synthesis is currently a laboratory scale technique that has not been applied at larger scales. One of the greatest challenges facing large-scale colloidal synthesis of metal nanoparticles is the large volume of long-chain hydrocarbon solvents and surfactants needed for the synthesis, which can dominate the cost of nanoparticle production. In this work, we demonstrate a protocol, based on solvent distillation, which enables the reuse of colloidal nanoparticle synthesis surfactants and solvents for over 10 rounds of successive syntheses and demonstrates that pure solvents and surfactants are not necessarily needed to produce uniform nanocrystals. We show that this protocol can be applied to the production of a wide variety of mono- and bimetallic nanoparticles with reproducible sizes and compositions, which leads to reproducible performance as heterogeneous catalysts. A techno-economic assessment demonstrates the potential of this technique to greatly reduce the solvent-related costs of colloidal metal nanoparticle synthesis, which could contribute to its wider application at commercial scale.