It is the dream of every catalyst designer to design catalysts that continue to operate without loss of performance. However, heterogeneous catalysts are susceptible to loss of performance due to sintering, the loss of physical surface area and the growth of nanoparticles, especially catalysts that operate at elevated temperatures. To restore catalyst activity, a regeneration step is needed. This is possible in petroleum refining or alkane dehydrogenation where the reactor or the catalyst must be taken offline for regeneration. But this is not possible for emission-control catalysts, which must operate in the honeycomb monolith for the full useful life of the vehicle. Here, we elucidate the design features that allow emission-control catalysts to mitigate the loss of activity under normal operating conditions. We seek to determine why certain metal/support combinations perform better than others. In this study, we subjected catalysts to accelerated aging protocols, which are used in industry to screen catalyst formulations. For diesel oxidation catalysts, this involves heating under oxidizing conditions with steam at 800 °C. For three-way catalysts, the aging is performed in the presence of steam at 980 °C or higher, under cyclic reducing and oxidizing conditions. At these aging temperatures, the primary mechanism of sintering involves Ostwald ripening, where mobile species, typically single atoms or oxides or hydroxides, diffuse over the surface or are transported through the vapor phase and condense to form larger particles, leading to loss of activity. To allow these catalysts to continue to operate indefinitely, without requiring regeneration, the catalyst must provide opportunities to trap the atoms and then to reconstitute the active sites. Three-way catalysts operate under oscillatory conditions, providing opportunities for self-healing that are not available in diesel oxidation catalysts, which operate under lean conditions (excess oxygen). Unravelling the principles that lead to self-healing behaviour in both these application areas yields insights that can guide design of robust heterogeneous catalysts.

Nanosized cerium oxide (CeO2) has been extensively used as the oxygen storage component in automotive emission control systems. However, the possible influence of atomically dispersed Ce in these catalysts has not been recognized. Here, we demonstrate the controllable transformation of ceria nanoparticles into isolated cerium cations on γ-Al2O3 via reductive atom trapping in 10% H2 at 800 °C, achieving over half-monolayer coverage. Dispersed Ce1 ions anchored by surface penta- and octa-coordinated Al sites exhibit outstanding thermal stability in air up to 500 °C, enabling further loading of active metals with well-defined catalyst structures. With this strategy, supported single-atom Rh1 surrounded by dispersed Ce1 is confirmed to exhibit much superior performance to Rh1 on bare Al2O3 or nanocrystalline CeO2 in catalyzing NO reduction by CO, exhibiting a striking one-order-of-magnitude increase in activity. Dispersed Ce1 exhibits greatly enhanced oxygen transfer capability compared to ceria nanoparticles and introduces a modified reaction mechanism that involves an adjacent Rh1-Ce1 motif, resulting in a greatly decreased activation barrier (from 192 to 96 kJ/mol). The reactivity enhancements are also seen with Ce1-promoted Pt nanoparticles for oxidation of CO and hydrocarbons.

When three-way catalysts (TWC) are exposed to exhaust at high temperatures, metal sintering is one of the primary causes of catalyst deactivation. Other deactivation mechanisms include poisoning and fouling of the catalyst, and as shown in this paper, the conversion of metallic nanoparticles into single atoms can also lead to catalyst deactivation. The various components of an exhaust treatment catalyst (for example, Pt, Pd and Rh) coexist along with the alumina support and ceria used for oxygen storage. This makes it difficult to precisely identify the mechanisms for deactivation in a fully formulated TWC. To overcome this limitation, we prepared monometallic catalysts containing similar metal loading (3 wt%) of Pt, Pd, and Rh on alumina and subjected them to accelerated aging under industry standard protocols involving lean/rich aging at 980 degrees C for 5 h. After aging, catalytic reactivity tests using fully formulated exhaust showed that Pd performed the best, with Rh showing comparable performance. The Pt catalyst shows significant deactivation. Both Pt and Rh show significant volatility in air due to mobile oxide species, but the mechanisms of deactivation are very different. Pt forms anomalously large particles due to the inability of PtO2 to bind to the alumina support while volatile Rh oxides react with the alumina to form atomically dispersed single atom species. Pd shows no vapor phase transport. We describe various strategies to transform the volatile species created during accelerated aging into active metallic particles yielding self-healing performance in emission control catalysts.

Ceria nanoparticles supported on alumina are widely used in various catalytic reactions, particularly in conjunction with platinum group metals (PGMs)1-9. Here we found that treating these catalysts at temperatures between 750 and about 1,000 °C in the presence of CO and NO in steam (reactive treatment under reducing atmosphere) leads to the dispersion of ceria nanoparticles into high-density 2D (roughly one atomic layer thin) CexOy domains, as confirmed by microscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), infrared spectroscopy and density functional theory (DFT) calculations. These domains, which densely cover the alumina, exhibit substantially enhanced oxygen mobility and storage capacity, facilitating easier extraction of oxygen and the formation of Ce3+ sites and oxygen vacancies. As a result, these catalysts-whether with or without PGMs, such as Rh and Pt-show improved activity for several industrially important catalytic reactions, including NO and N2O reduction, as well as CO and NO oxidation, even after exposure to harsh ageing conditions. This study shows a catalyst architecture with superior redox properties under conditions that typically cause sintering, offering a pathway to more efficient metal-ceria catalysts for enhanced general catalysis.

Ceria nanoparticles supported on alumina have found wide applications for various catalytic reactions, especially in contact with precious metals. We discovered that treatment of these catalysts at temperatures between 750 and ~1,000 ºC under the flow of CO and NO in the presence of steam (reactive aging in reducing atmosphere) leads to dispersion of ceria nanoparticles and creates a novel catalytic architecture with high density (up to 10 wt%) of atomically dispersed, ultra small CexOy clusters densely covering alumina, as confirmed by XPS, FTIR and AC-STEM characterization. These clusters possess markedly higher oxygen mobility (and therefore oxygen storage capacity), leading to easier extraction of oxygen with the formation of abundant Ce+3 sites and oxygen vacancies. Because of this, these catalysts (in the absence or presence noble metals, such as Rh and Pt) possess much improved activity for multiple industrially important catalytic reactions such as NO and N2O reduction, as well as CO and NO oxidation even after exposure to harsh aging conditions, with activity superior to fresh catalyst even for aged samples, providing a general pathway to creating more efficient PGM/ceria catalysts. Our study therefore reveals novel catalyst architecture with atomically dispersed ceria clusters with superior redox properties under conditions where typical catalyst sintering is generally assumed to occur and allows to utilize these materials as supports for more effective general catalysis.

Nanosized cerium oxide (CeO2) has been extensively used as an oxygen storage component in automotive emission control systems. However, the possible involvement of atomically dispersed cerium (Ce) has not been explored. Here, we demonstrate the controllable transformation of CeO2 nanoparticles into isolated Ce1 cations on the surface of gamma-type alumina (γ-Al2O3) via reductive atom trapping, achieving over half-monolayer coverage. Supported single-atom rhodium (Rh1) surrounded by dispersed Ce1 shows superior performance to Rh1 on bare Al2O3 or crystalline CeO2 in catalyzing NO reduction, exhibiting a striking one-order-of-magnitude increase in turnover frequency. Dispersed Ce1 also exhibits greatly enhanced oxygen transfer capability and introduces a modified reaction mechanism that involves adjacent Rh1-Ce1 dual-sites, resulting in a greatly decreased activation barrier (96 vs. 192 kJ/mol). The understanding of reductive atom trapping of Ce1 as well as its structure-property relationships obtained in this work could be implemented in the rational design of Ce1-promoted catalysts for many other applications. Benefiting from the greatly enhanced OSC, activity enhancements are also seen with Ce1-promoted platinum nanoparticles for the oxidation of CO and hydrocarbons. Additionally, dispersing Ce1 on Al2O3 results in modified surface properties, which could be further utilized to explore the field of acid-base catalysis

Cerium oxide (ceria) has been shown to be very effective at trapping platinum atoms due to formation of stable surface complexes at step edges, where coordinatively unsaturated cerium cations are present. But ceria loses its effectiveness when heated to high temperatures due to loss of surface area and growth in particle size associated with sintering of the oxide. Being a rare earth, and with limited supplies worldwide, it is important to develop methods to improve the effectiveness of ceria as a catalyst support. Here, we explore the performance for trapping Pt atoms when the ceria is supported on a high surface area alumina carrier. This helps create a more sustainable catalyst formulation, especially if we can retain the high dispersion of Pt seen on ceria supports. For this work, we studied the atom trapping efficacy of ceria/alumina samples with increasing ceria content (8-50 wt %) and contrasted the behavior with pure ceria. Electron microscopy reveals that when dispersed on alumina, ceria is present in the form of crystalline nanoparticles and isolated cerium ions. These two forms of ceria differ markedly in their ability to trap Pt atoms. Atomically dispersed cerium is present in the form of Ce3+ cations on alumina; however, this form of ceria is not effective for trapping Pt atoms. Our results show that the atom-trapped Pt resides primarily on crystalline ceria nanoparticles. CO oxidation was used as a probe reaction to evaluate the performance of these Pt-AT/ceria-alumina catalysts. We conclude that over the range of ceria loadings we investigated, 50% ceria/alumina represents the optimal catalyst support for achieving high surface area and atom trapping efficiency while helping reduce the total ceria content in this catalyst system.

Ceria nanoparticles supported on alumina have low activity for industrially relevant NO reduction by CO. We discovered that treatment of these catalysts at temperatures between 750 and ~1,000 ºC under the flow of CO and NO in the presence of steam, which typically leads to catalyst deterioration and sintering, in fact, leads to dispersion of ceria nanoparticles into high density (up to 10 wt%) of fully exposed Ce atoms mostly in +3 oxidation state. We characterize them with XPS, FTIR and HAADF-STEM imaging. Their presence changes the alumina surface, as evidenced by XPS and FTIR with probe molecules. Ce+3 ions show dramatically enhanced NO reduction ability in the presence of CO and steam. Infra-red studies reveal close interaction of NO molecules on Ce+3/Alumina surfaces with the formation of N2O species, indicating redox activity of Ce+3 structures. Heating these samples in oxygen (in wet or dry streams) at 800 ºC and above leads to coalescence of Ce into CeO2 nanoparticles, resulting in reversible loss of activity. Further, reactive treatment of CeO2/Al2O3 under high temperature reaction conditions restores Ce+3 cations as well as catalytic activity. Our study shows reversible redispersion of ceria into isolated Ce+3 cations under conditions where typical catalyst sintering is generally assumed to occur and suggests a pathway to utilize these materials as supports for more effective catalysis. Indeed, supporting only 0.1-0.5 wt% Rh on these supports containing atomically dispersed Ce+3 cations on the surface of alumina, shows synergies between a noble metal and atomically dispersed Ce ions with greatly improved activity and stability for NO reduction with CO compared with Rh/CeO2nanoparticles/Al2O3. Furthermore, exposed Ce cations dispersed on alumina in contact with metals (Rh) show much higher catalytic CO oxidation activity than Rh on CeO2 nanoparticles/Al2O3 system. XO2 oxides (where X is Ce, Ti and Zr) show improvements of NO reduction activity via high temperature treatments suggesting redispersion of XO2 nanoparticles into X exposed cations (with abundance of X+3) is a general phenomenon that occurs, leads to improved catalysts and allows these catalysts to maintain high activity after exposure to extreme aging conditions.

Ceria nanoparticles supported on gamma-alumina prepared via wet impregnation and sourced commercially have low activity for industrially relevant NO reduction by CO in the presence of steam. These supports contain ceria nanoparticles as well as small (~1%) amount of Ce atomically dispersed and anchored by penta-Al sites. We discovered that treatment of these catalysts at temperatures ~750-950 ºC under the flow of CO and NO in the presence of steam, which typically leads to catalyst deterioration and sintering, in fact, leads to dispersion of ceria nanoparticles into isolated Ce+3 atoms. We extensively characterize them with XPS, FTIR and HAADF-STEM imaging. Their presence changes the alumina surface, as evidenced by XPS and FTIR with probe molecules. Ce+3 ions show dramatically enhanced NO reduction ability in the presence of CO and steam. Infra-red studies reveal close interaction of NO molecules on Ce+3/Alumina surfaces with the formation of N2O species. Heating these samples in oxygen (in wet or dry streams) at 800 ºC and above leads to coalescence of Ce+3 into CeO2 nanoparticles, resulting in reversible loss of activity. Further, reactive treatment of CeO2/Al2O3 under high temperature reaction conditions restores Ce+3 cations as well as catalytic activity. Our study shows reversible redispersion of ceria into isolated Ce+3 cations under conditions where typical catalyst sintering is generally assumed to occur and suggests a pathway to utilize these materials as supports for more effective catalysis. Indeed, supporting only 0.1-0.5 wt% Rh on these CeAl supports, shows synergies between Rh and atomically dispersed Ce ions with excellent activity and stability for NO reduction with CO.