CeO2 Support Research Articles

Enhancing the electrocatalytic oxygen evolution performance of iridium oxide in acidic media by inducing a lower valence state using CeO2 support

In this study, a composite catalyst consisting of IrOx nanoparticles dispersed in CeO2 support (IrOx/CeO2) was prepared for oxygen evolution reaction (OER) using a simple polyol method. The oxygen transfer from IrOx to oxygen defects in CeO2 induced a high ratio of lower-valence Ir3+ species, which are known to be an active phase for OER. The prepared IrOx/CeO2 catalyst exhibited excellent OER performance with a small overpotential of 263.4 mV at 10 mA/cm2, and good stability for 10 h under acidic condition, outperforming IrOx/C and commercial IrO2 catalysts. The lower-valence Ir3+ species near the catalyst surface functioned as possible active site ensembles, promoting the chemical and electrochemical steps during OER process and improving the OER activity. Additionally, the metal-support interaction between IrOx and CeO2 enhanced the durability of the catalyst.

Ammonia Cracking Catalyzed by Ni Nanoparticles Confined in the Framework of CeO2 Support.

For the extraction of hydrogen from ammonia at low temperatures, we investigated Ni-based catalysts fabricated by the thermal decomposition of RNi5 intermetallics (R = Ce or Y). The interconnected microstructure formed via phase separation between the Ni catalyst and the resulting oxide support was observed to evolve via low-temperature thermal decomposition of RNi5. The resulting Ni/CeO2 nanocomposite exhibited superior catalytic activity of ∼25% at 400 °C for NH3 cracking. The high catalytic activity was attributed to the interlocking of Ni nanoparticles with the CeO2 framework. The growth of Ni nanoparticles was prevented by this interconnected microstructure, in which the Ni nanoparticles incorporated nitrogen owing to the size effect, whereas Ni does not commonly form nitrides. To the best of our knowledge, this is a unique example of a microstructure that enhances catalytic NH3 cracking.

Synthesis of Propargylamine and Indolazine Derivatives via Three‐Component Reaction Catalyzed by CuOx/CeO2 Materials

AbstractIn this paper, nanorod CeO2 supports were prepared by hydrothermal method using Ce(NO3)3 ⋅ 6H2O as raw material. CuOx/CeO2‐r materials were prepared by calcination using Cu salt‐loaded CeO2‐r supports as raw materials. The three‐component coupling reaction of phenylacetylene, amine and aldehyde was successfully catalyzed by CuOx/CeO2‐r material as a non‐homogeneous catalyst, which showed good catalytic activity. Furthermore, a series of indolizine derivatives can be obtained in higher isolated yields by replacing benzaldehyde with pyridinecarbaldehyde and catalyzing the three‐component synthesis with the same CuOx/CeO2‐r material. The synthesis mechanisms of propargylamine and indolizine were discussed. It is worth noting that the CuOx/CeO2‐r material system is provided for two three‐component reactions, and has the advantages of less catalyst dosage, short reaction time, simple post‐treatment, and a wide range of substrates.

Ultralow Ru Single Atoms Confined in Cerium Oxide Nanoglues for Highly-Sensitive and Robust H2 O2 -Related Biocatalytic Diagnosis.

Exploring highly efficient, portable, and robust biocatalysts is a great challenge in colorimetric biosensors. To overcome the challenging states in creating single-atom biocatalysts, such as insufficient activity and stability, here, this work has engineered a unique CeO2 support as nanoglue to tightly anchor the Ru single-atom sites (CeO2 -Ru) with strong electronic coupling for achieving highly sensitive and robust H2 O2 -related biocatalytic diagnosis. The morphology and chemical/electronic structure analysis demonstrates that the Ru atoms are well-dispersed on CeO2 surface to form high-density active sites. Benefiting from the unique structure, the prepared CeO2 -Ru exhibits outstanding peroxidase (POD) like catalytic activity and selectivity to H2 O2 . Steady-state kinetic study results show that the CeO2 -Ru presents the highest Vmax and turnover number than the state-of-the-art POD-like biocatalysts. Consequently, the CeO2 -Ru discloses a high efficiency, good selectivity, and robust stability in the colorimetric detection of L-cysteine, glucose, and uric acid. Notably, the limit of detection (LOD) can reach 0.176 × 10-3 m for the L-cysteine, 0.095 × 10-3 m for the glucose, and 0.088 × 10-3 m for the uric acid via cascade reaction. This work suggests that the proposed unique CeO2 nanoglue will offer a new path to create single-atom noble metal biocatalysts and take a step closer to future biotherapeutic and biocatalytic applications.

A study on the activity recovery behavior of noble metal catalysts against sulfur poisoning

Various noble metals (Pt, Rh, Ru, and Pd) were supported on CeO2 to understand their sulfur-tolerant catalytic activity. The synthesized catalysts were applied to the water-gas shift reaction in the waste-to-hydrogen process with the injection of 500 ppm H2S. Among the samples, the PtCe catalyst showed the highest sulfur tolerance and recovery of catalytic activity. Considering the catalytic reaction results and physicochemical properties, the sulfur tolerance of the catalyst is affected by oxygen vacancies and redox properties, which are closely related to the interaction between the metal and the CeO2 support. In addition, the oxygen storage capacity concentration was used as an indicator for classifying the recovered catalyst and the non-recovered catalyst which had been poisoned by sulfur.

(Invited) Understanding the Effects of Charge Distribution on the Oxidation of Water

Water oxidation is an important first step of photosynthesis. Presently, it remains a challenge to carry out this reaction efficiently using low-cost materials. Among the questions that remain to be answered are the influences by charge distribution at or on the catalyst surfaces where water oxidation takes place. Recent literature has shown that the reaction can be sensitive to the charge density or the charge potential or both. Generally, high charge density or high charge potential favors faster kinetic constants. However, the existing studies face challenges as most of them were carried out on different materials that feature different chemical catalytic properties, as well. As such, it is not yet possible to definitely conclude the observed differences in water oxidation kinetics is necessarily a function of charge properties but not the inherent chemical catalytic differences. The study we plan to present addresses this issue. We used a catalyst system that features a well-defined catalytic center but employed supports that feature varying charge redistribution properties. This system allowed us to systematically compare how charge redistribution affects the water oxidation reactions. It was observed that when charge redistribution indeed plays an important role in defining the reaction kinetics, but it is so only when the subsequent chemical steps are fast. For the first time, we found a different dependence of the reaction kinetics on temperatures. Under otherwise identical conditions, comparable reaction kinetics were measured on ITO support and CeO2 support at room temperature. When the temperature increased to 50ºC, the reaction rate on ITO support increased to twice that on CeO2 support. The results shed new light on the role played by charge redistribution in governing reactions such as water oxidation.

Observation of Support-Dependent Water Oxidation Kinetics on Molecularly-Derived Heterogeneous Ir-Oxide Catalysts

Heterogeneous water oxidation catalysis is central to the development of renewable energy technologies. Understanding the material properties that determine water oxidation activity is critically important for the design of efficient and durable water oxidation catalysts. The electronic structure and surface chemistry of the catalyst influence the accumulation of oxidative charge (holes) on catalytically active sites. Hole distribution dynamics and molecular structure of the active sites are generally accepted to jointly determine activity. However, these properties are typically inherent to materials; it has been difficult to independently tune them on most heterogeneous catalysts, hindering the development of an understanding of their relative influence. Herein, heterogenized dinuclear Ir catalysts (Ir-DHC) supported on different oxides were employed to overcome this challenge. It was found that at low temperatures (288-298 K), the photocatalytic activities of Ir-DHC on indium tin oxide (ITO) and CeO2 supports are comparable within the studied range of fluences (62-151 mW/cm2). By contrast, at higher temperatures (310-323 K), Ir-DHC on ITO exhibits a ∼100% higher water oxidation activity than on CeO2. The divergent activities under these conditions were attributed to the distinct abilities of the supporting substrates to redistribute surface holes. Specifically, ITO is relatively efficient in distributing photogenerated holes between Ir-DHC active sites, whereas CeO2 exhibits sluggish hole redistribution because of its reducibility, thereby limiting hole accumulation at Ir-DHC active sites. The temperature dependence was explained by a temperature induced switch in the rate-determining step of the reaction. These findings highlight the critical role of the supporting substrate in determining the turnover at active sites of heterogeneous catalysts.

Effect of precipitation variables on the performance of CeO2-based catalysts for waste-to-hydrogen

Waste-to-hydrogen technology, a future net-zero energy mix as a clean hydrogen source, is currently a research hotspot. To upcycle waste to high-value chemicals such as hydrogen, it is necessary to study customized catalysts for water-gas shift (WGS) reactions considering waste-derived synthesis gas. Herein, we aimed to enhance the physicochemical properties of CeO2 supported Pt catalysts by controlling the precipitation variables to develop a highly sulfur-tolerant WGS catalyst using waste-derived synthesis gas. The CeO2 support was prepared by the classic precipitation method. During precipitation, the precipitating agent was injected at different methods and rates. In the case of the normal titration method, the KOH solution was injected into the Ce precursor dissolved solution at rates of 5, 20, and 50 mL/min. In the case of the reverse titration method, the Ce precursor dissolved solution was injected into the KOH solution at rates of 5, 20, and 50 mL/min. The as-prepared Pt/CeO2 catalysts were applied to the WGS reaction with 500 ppm H2S. This work demonstrated that the titration method and titration rate affected the oxygen vacancy concentration and the active metal dispersion of the catalysts, respectively. And these properties worked in combination to determine the sulfur resistance of the catalysts. Subsequently, the high sulfur tolerance and catalytic performance of Pt/CeO2 catalyst prepared by normal titration method with the titration rate of 50 mL/min were due to the high Pt dispersion and concentration of oxygen vacancies.

Electro-oxidation Reaction of Methanol over Reducible Ce1-x-yNixSryO2-δ: A Mechanistic Probe of Participation of Lattice Oxygen.

Methanol oxidation reaction crucially depends on the formation of -OOH species over the catalyst's surface. Ni-based catalysts are by far the choice of materials, where the redox couple of Ni2+/Ni3+ facilitates the formation of -OOH species by surface reconstructions. However, it is challenging to oxidize Ni2+ as it generates charge-transfer orbitals near the Fermi energy level. One possible solution is to substitute Ni2+ with a reducible oxide support, which will not only facilitate the Ni2+ → Ni3+ oxidation but also adsorb oxygenated species like -OOH at a lower potential owing to its oxophilicity. This work shows with the help of structural and surface studies that the reducible CeO2 support in Ni and Sr co-doped Ce1-x-yNixSryO2-δ solid solution can easily facilitate Ni2+ → Ni3+ oxidation as well as evolution of lattice oxygen during the methanol oxidation reaction. While the Ni3+ species helped in formation of -OOH surface intermediates, the evolved lattice oxygen eased the CO oxidation process in order to bring out the better CO-tolerant methanol oxidation activity over Ce1-x-yNixSryO2-δ. The study shows the unique importance of the electronic interactions between the active site and support and involvement of lattice oxygen in the methanol oxidation reaction.

Atomic‐level insights into single Pt atom catalysts on CeO2 support

AbstractPrecious Pt metals deposited on reducible CeO2 oxide have been widely used in exhaust gas treatment. However, the high cost of Pt metals is the main barrier that prevents from applying the Pt metals. Therefore, a lot of efforts have been dedicated to reducing the Pt size as small as possible, aiming at maximizing Pt metal utilization efficiency. The smallest ultimate size that Pt metal can be deposited on CeO2 as single atom structures. Furthermore, the supported single Pt atom catalysts enhance catalytic performance and selectivity compared to Pt cluster or Pt particle counterparts. However, the electronic structures of single Pt atom catalysts at the atomic level are a challenge for the experimentalist, leading to a lack of important information in using these catalysts. Here, we aim to figure out the nature of the high thermal stability of single Pt atom catalysts on CeO2 support using density theory functional and compared to experimental data from previous studies. This study will provide useful information on single Pt atom catalysts on CeO2 support aiming at boosting synthesis and applications applying CeO2‐supported Pt.

Acidic TiNb2O7 nanospheres supported with CeO2 to enhance NH3-SCR denitrification activity and resistance to H2O and SO2: Synergistic effect of CeO2 and TiNb2O7

TiNb2O7 material can be selected as the good support for NH3-SCR catalysts because of its shear structure and surface acidity. Increasing the acidity of catalyst is the key to achieving excellent NH3-SCR performance. In this work, m%CeO2/TiNb2O7 (m = 1, 2, 5, 8 and 10) catalysts were prepared by adjusting the Ce amount supported on TiNb2O7 with good acidity. Among them, 8%CeO2/TiNb2O7 exhibited good activity, N2 selectivity, and excellent SO2/H2O resistance. A series of analytical techniques were employed to examine the relationship among catalyst structure, surface properties and performance. The results show that the short-range ordered Cen+–O2--Nbn+ and Cen+–O2--Tin+ species from interactions between CeO2 and acidic support TiNb2O7 result in improved redox ability and acidity (especially Brønsted acids). The influence of SO2/H2O on SCR reaction and the reaction mechanism were researched using in-situ DRIFTS. The results indicate that SO2/H2O can promote NH3 adsorption and slightly inhibit NOx adsorption. The SCR reaction over 8%CeO2/TiNb2O7 mainly follows the E-R mechanism within the activation window, thus, the influence of SO2 and H2O on the reaction pathway is low, which probably contributes to the better tolerance against SO2 and H2O of the catalyst.

Surface reactivity of VOx/CeO2 (1 1 1) and the impact of transition metal doping of CeO2 support on oxidative dehydrogenation of propane

Propane oxidative dehydrogenation (ODH) was studied over VOx/CeO2 nanoparticle catalysts for various vanadia nuclearities using Density Functional Theory simulations. On monomers and dimers, propene formation involved V=O, bridged (V–O–Ce, V–OV) and ceria surface oxygens but with low selectivity, while on trimers and possibly higher oligomers it was kinetically favourable over ceria surface oxygens, with higher selectivity than on monomers and dimers. On trimer, overoxidation via acetone formation was inhibited, but at the expense of overall catalyst reactivity. Doping of ceria support with Ni induced strong Ni–VOx interactions, stabilizing the vanadia monomer. The V–O–Ni bridged oxygen was less accessible for overoxidation, resulting in an increase in activation energy barrier for acetone formation by 0.69 eV on VO2/Ce0.89Ni0.11O2(111), while there was a minimal impact on V3O6/Ce0.89Ni0.11O2 (111). Hence, transition metal doping of ceria support is a potential strategy to improve propene selectivity over VOx/CeO2 catalysts for a wide range of V loadings.

Effects of CeO2, TiO2, and TiO2-CeO2 Supports on Catalytic Performance of Ni2P in Hydrodeoxygenation of Anisole

This work examined the effects of supports for nickel phosphide (Ni2P) catalysts on anisole hydrodeoxygenation (HDO) using synthesized CeO2, TiO2, and TiO2-CeO2 supports and commercial supports (denoted as CeO2-L and TiO2-L) along with bulk Ni2P. Characterization revealed that Ni2P nanoparticles were well dispersed on the synthesized CeO2. Ni2P/CeO2 exhibited the highest activity on a catalyst weight basis among those Ni2P on synthesized supports as well as the common materials such as SiO2, Al2O3, and activated carbon. Ni2P/TiO2-CeO2 (at a Ti/Ce molar ratio of 0.98) afforded superior reactivity to Ni2P/CeO2 based on turnover frequency (TOF, h–1), which points to more active sites on Ni2P/TiO2-CeO2 than on Ni2P/CeO2. The TOF of anisole on catalysts decreased in the following order: Ni2P/Ti0.98Ce0.02O2 > Ni2P/TiO2 > Ni2P/CeO2 > bulk Ni2P. The type of supports employed for Ni2P also influenced the product distributions and thus HDO pathways. Ni2P/CeO2 favored benzene selectivity, and this may be related to the relative electron enrichment on the Niδ+ site of Ni2P/CeO2 that facilitated benzene desorption from the catalyst surface after demethoxylation of anisole. Ni2P/TiO2-CeO2 exhibited significantly higher cyclohexane selectivity, which may be attributed to the lower electron density on Niδ+ of Ni2P/TiO2-CeO2 and the acidity of binary support, promoting the deep hydrogenation of benzene to cyclohexane. The presence of phenol when Ni2P/TiO2 and Ni2P/TiO2-CeO2 were employed in anisole HDO could be correlated with the acidity of TiO2, which led to methylation of anisole. Adding a small amount of CeO2 in TiO2 and using it as support synergistically improved the TOF and suppressed phenol formation while increasing cyclohexane selectivity. Both Ni2P/CeO2 and Ni2P/TiO2-CeO2 are promising catalysts in anisole HDO for the tunable target products of benzene and cyclohexane, respectively.

Effect of alkali addition on a Cu/SmCeO2 @TiO2 catalyst for NO reduction with CO under oxidizing conditions

The accumulation of alkali metals generated during biomass combustion onto the catalysts employed for off-gas treatment can have a detrimental effect on their performance. This work investigates the effect of potassium as a model alkali on an efficient copper-based catalyst supported on SmCeO2 @TiO2. The addition of potassium modifies the redox properties of the Cu species or, more specifically, the metal-ceria interface, providing better catalytic activity for NO reduction in oxidizing conditions. SmCeO2 @TiO2 stabilizes highly dispersed copper species, and the presence of potassium introduces surface distortions that modify the lability of oxygen atoms. The results demonstrate the key role of surface defects and labile oxygen species associated with the CeO2 support. It was found that both Cu/SmCeO2 @TiO2 and K/Cu/SmCeO2 @TiO2 catalysts fully oxidized CO below 350 °C and actively reduced NO with CO in the presence of excess oxygen, reaching a maximum NO conversion of 65% at 316 °C and 83% at 330 °C, respectively, while undesired NO2 release was minimized in the alkaline-loaded sample.

Single‐Atom Rh on High‐Index CeO2 Facet for Highly Enhanced Catalytic CO Oxidation

AbstractReducible oxide‐supported noble metal nanoparticles exhibit high activity in catalyzing many important oxidation reactions. However, atom migration under harsh reaction conditions leads to deactivation of the catalyst. Meanwhile, single‐atom catalysts demonstrate enhanced stability, but often suffer from poor catalytic activity owing to the ionized surface states. In this work, we simultaneously address the poor activity and stability issues by synthesizing highly active and durable rhodium (Rh) single‐atom catalysts through a “wrap‐bake‐peel” process. The pre‐coated SiO2 layer during synthesis of catalyst plays a crucial role in not only protecting CeO2 support against sintering, but also donating electron to weaken the Ce−O bond, producing highly loaded Rh single atoms on the CeO2 support exposed with high‐index {210} facets. Benefiting from the unique electronic structure of CeO2 {210} facets, more oxygen vacancies are generated along with the deposition of more electropositive Rh single atoms, leading to remarkably improved catalytic performance in CO oxidation.

Single-Atom Rh on High-Index CeO2 Facet for Highly Enhanced Catalytic CO Oxidation.

Reducible oxide-supported noble metal nanoparticles exhibit high activity in catalyzing many important oxidation reactions. However, atom migration under harsh reaction conditions leads to deactivation of the catalyst. Meanwhile, single-atom catalysts demonstrate enhanced stability, but often suffer from poor catalytic activity owing to the ionized surface state. In this work, we address the poor activity and stability issues simultaneously by synthesizing highly active and durable rhodium (Rh) single-atom catalysts through a "wrap-bake-peel" process. The pre-coated SiO2 layer during synthesis of catalyst plays a crucial role in not only protecting CeO2 support against sintering, but also donating electron to weaken the Ce-O bond, producing highly loaded Rh single atoms on the CeO2 support exposed with high-index{420}. Benefiting from the unique electronic structure of CeO2facets, more oxygen vacancies are generated along with the deposition of more electropositive Rh single atoms, leading to remarkably improved catalytic performance in CO oxidation.

New insights into the relationship between Cu–Ce interaction and reactive Cu species in CuOx-CeO2 catalysts for preferential oxidation of carbon monoxide in excess hydrogen

A group of CuOx-CeO2 catalysts for preferential oxidation of carbon monoxide (CO-PROX) is designed by constructing different distribution status of Cu species on similar CeO2 support to adjust the Cu–Ce interaction strength. Cycled temperature programmed reduction by hydrogen (H2-TPR) indicates the strength of Cu–Ce interaction can impact the amount and redox properties of surface highly dispersed CuOx and strongly bonded Cu-[Ox]-Ce species, which respectively determines the catalytic performance at low and high temperature. Temperature programed desorption of H2 (H2-TPD) reveals the presence of large amounts of Cu-[Ox]-Ce species can inhibit H2 adsorption and dissociation, which increases the O2 selectivity toward CO oxidation. Moreover, temperature programmed desorption of CO2 (CO2-TPD) and in situ diffuse reflectance infrared Fourier transform spectra (DRIFTs) illustrates that H2O and CO2 influences the low-temperature catalytic performance mainly by competitive adsorption with CO on surface highly dispersed reactive Cu+ sites. This work aims to shed light on the underlying Cu–Ce interaction regime, and guide the design of high-performance CuO–CeO2 catalysts in realistic reaction conditions.

Engineering Oxygen Vacancies in IrOx Clusters Supported on Metal–Organic Framework Derived Porous CeO2 for Enhanced Oxygen Evolution in Acidic Media

Developing highly active and stable electrocatalysts for the oxygen evolution reaction (OER) in acidic media for proton exchange membrane water electrolysis is urgent but still challenging. Herein, porous CeO2-supported IrOx clusters with different oxygen vacancy (Ov) concentrations are synthesized by the aid of N-rich and N-free isostructural Ce-based metal–organic frameworks (Ce-MOFs) as templates. Ov concentration in the derived IrOx clusters is effectively regulated by the presence of heteroatoms in MOFs. The optimized IrOx/CeO2 catalyst with high Ov (hov-IrOx/CeO2) exhibits a low overpotential of 251 mV at 10 mA cm–2, a higher turnover frequency (TOF) of 5400 h–1 and outstanding durability for at least 98 h of catalysis in 0.1 M HClO4, outperforming other analogues. Density functional theory calculations also show that the rich Ov in IrOx can weaken the binding energy between Ir and *O intermediates, which decreases the energy barriers for forming *OOH from *O in the rate-determining step. Additionally, the suppressed over-oxidation of Ir species as well as the corrosion resistance of the CeO2 support also contribute to the high stability.

Efficient electrochemical CO2 reduction via CuAg doped CeO2

CO2 electrocatalysis reduction (CO2RR) is considered to be one of the most efficient methods to achieve carbon neutralization and realize renewable energy conversion. Unfortunately, at present, CO2 electrocatalysts confront the challenges of high overpotential, low selectivity, and poor stability. Ag based electrocatalysts are considered to be the most promising catalysts for CO production via CO2RR on a large scale, due to their relatively higher catalytic performance and more abundant reserves compared with other noble metals. Whereas, they still suffer from high cost. In this paper, we design bimetallic CuAg/CeO2-6 catalysts by the simple impregnation method for CO2RR, exhibiting the highest FECO of 84% at –1.1 V vs. RHE compared with other bimetallic catalysts with different Cu/Ag ratio and monometal catalysts. XPS results demonstrate the electron delocalization effect not only between Cu and Ag bimetals but also among CeO2 support. This effect results in the electronic structure change and the formation of the highest concentration of oxygen vacancies, promoting the CO2 adsorption and activation as well as facilitating the formation of COOH* intermediates as evidenced by the in situ ATR-SEIRAS measurements. Additionally, the highest active surface for CO2RR and the smallest charge resistance of CuAg/CeO2-6 also contributes to its good CO2RR performance. This work sheds light on the design of other efficient CO2RR catalysts through electronic structure tuning by bimetallic strategy.

Improved metal-support interaction in Ru/CeO2 catalyst via plasma-treated strategy for dichloroethane oxidation

Plasma-treatment strategy has become an important technique to modify heterogenous catalyst. Here, the plasma-treated Ru/CeO2 catalysts were synthesized and applied to dichloroethane (DCE) oxidation. Compared with the untreated catalysts (CeO2-T and Ru/CeO2-T), the plasma-treated catalysts (CeO2-P, Ru/CeO2-P and Ru/CeO2-TP) show higher catalytic activity. Amongst, the Ru/CeO2-P catalyst shows the highest activity (T90 =243 °C, TOFSSA=6.98 μmol·m−2·h−1 and Ea=26 kJ·mol−1). Based on various characterizations, it reveals that the plasma-treatment can enrich the concentration of surface oxygen vacancies, promote the formation of acid sites, and strengthen the interaction between Ru and CeO2 support. The interaction benefits the dispersion of Ru species, forming more active sites to enhance the DCE conversion. The more acid sites can improve the HCl selectivity. Moreover, the Ru/CeO2-P catalyst exhibits excellent thermal stability and water-resistance performance. This signification result promotes the development of catalysts by plasma-treated strategy, and its wide application in heterogenous catalysis.