Activity For Low-temperature CO Oxidation Research Articles

Comparative Study of Exsolved and Impregnated Ni Nanoparticles Supported on Nanoporous Perovskites for Low-Temperature CO Oxidation.

This study investigated the redox exsolution of Ni nanoparticles from a nanoporous La0.52Sr0.28Ti0.94Ni0.06O3 perovskite. The characteristics of exsolved Ni nanoparticles including their size, population, and surface concentration were deeply analyzed by environmental scanning electron microscopy (ESEM), transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX) mapping, and hydrogen temperature-programmed reduction (H2-TPR). Ni exsolution was triggered in hydrogen as early as 400 °C, with the highest catalytic activity for low-temperature CO oxidation achieved after a reduction step at 500 °C, despite only a 10% fraction of Ni exsolved. The activity and stability of exsolved nanoparticles were compared with their impregnated counterparts on a perovskite material with a similar chemical composition (La0.65Sr0.35TiO3) and a comparable specific surface area and Ni loading. After an aging step at 800 °C, the catalytic activity of exsolved Ni nanoparticles at 300 °C was found to be 10 times higher than that of impregnated ones, emphasizing the thermal stability of Ni nanoparticles prepared by redox exsolution.

Ultrasmall Pd Clusters in FER Zeolite Alleviate CO Poisoning for Effective Low-Temperature Carbon Monoxide Oxidation.

Ultrasmall Pd4 clusters form in the micropores of FER zeolite during low-temperature treatment (100 °C) in the presence of humid CO gas. They effectively catalyze CO oxidation below 100 °C, whereas Pd nanoparticles are not active as they are poisoned by CO. Using catalytic measurements, infrared (IR) spectroscopy, X-ray absorption spectroscopy (EXAFS), microscopy, and density functional theory calculations, we provide the molecular-level insight into this previously unreported phenomenon. Pd nanoparticles get covered with CO at low temperatures, which effectively blocks O2 activation until CO desorption occurs. Small Pd clusters in zeolites, in contrast, demonstrate fluxional behavior in the presence of CO, which significantly increases the affinity for binding O2. Our study provides a pathway to achieve low-temperature CO oxidation activity on the basis of a well-defined Pd/zeolite system.

Promoted low-temperature CO oxidation activity of CeO2 by Cu doping: The important role of oxygen vacancies

Oxygen vacancies play key roles in improving the activities of catalysts. In this paper, the concentration of oxygen vacancies in a CeO2 catalyst was regulated by a series of Cu doping amounts to promote low-temperature CO catalytic oxidation. Characterizations and DFT calculations show that introduction of an appropriate amount of Cu generates oxygen vacancies and benefits their chemical stabilization, which activates the lattice oxygen and eventually improves the performance of the catalyst. The catalyst exhibits the best CO oxidation activity (T95 = 90.5 °C) when Cu/Ce = 0.2/0.8. This study establishes a theoretical foundation for enhancing CO oxidation by CeO2 at low temperatures by manipulating the oxygen vacancies.

PdCu Alloy Catalyst for Inhibition‐free, Low‐temperature CO Oxidation

AbstractDesigning robust catalysts for low‐temperature oxidation is pertinent to the development of advanced combustion engines to meet increasingly stringent emissions limitations. Oxidation of CO, hydrocarbon, and NO pollutants over platinum‐group catalysts suffer from strong inhibition due to their competitive adsorption, while coinage metals are generally slow at activating O2. Through computational screening, we discovered a PdCu alloy catalyst that completely oxidizes CO below 150 °C without inhibition by NO, propylene or water. This is attributed primarily to geometric effects and the presence of CO bound to Pd sites within the Cu‐rich surface of the PdCu alloy. We demonstrate that the novel PdCu catalyst can be used in tandem with a PtPd catalyst to achieve sequential, inhibition‐free, complete oxidation of CO in a two‐bed system, while also achieving 50 % NO conversion below 120 °C. Moreover, neither water nor propylene adversely affect the low temperature CO oxidation activity.

Sintering resistance of Pd single atoms on steam-modified ceria: deciphering the role of hydroxyl groups

OH-modified CeO2 not only anchors the loading sites of Pd, forming a stable OH-assisted doping structure to enhance sintering resistance, but also increases the active sites of Pd, enabling sustainable activity for low-temperature CO oxidation.

Unraveling the low-temperature activity of Rh-CeO2 catalysts in CO oxidation: probing the local structure and Red-Ox transformation of Rh3+ species.

The local structure of the active sites is one of the key aspects of establishing the nature of the catalytic activity of the systems. In this work, a detailed structural investigation of the Rh-CeO2 catalysts prepared by the co-precipitation method was carried out. The application of a variety of physicochemical methods such as XRD, Raman spectroscopy, XPS, TEM, TPR-H2, and XAS revealed the presence of highly dispersed Rh3+ species in the catalysts: Rh3+ single ions and RhOx clusters. The substitution of Ce4+ ions by Rh3+ species, which provided a strong distortion of the CeO2 lattice, is shown. XAS data ensured the refinement of the Rh local structure. It was shown that single Rh3+ sites located next to each other can merge the formation of RhOx clusters with Rh local environment close to the one in Rh2O3 and CeRh2O5 oxides. The distortion of the CeO2 lattice around single and cluster rhodium species had a beneficial effect on the catalytic activity of the samples in low-temperature CO oxidation (LTO-CO). TEM, XAS, and in situ XRD data allowed establishing the structural transformations of the catalysts under Red-Ox treatments. The reduction treatment led to Rhn metallic cluster formation localized on defects of the reduced CeO2-δ. The reduced sample demonstrated efficient CO conversion at 0 °C. However, this system was not stable: its contact with air led to ceria reoxidation and partial reoxidation of Rh to highly dispersed Rh3+ species at room temperature, while heating in an oxidizing atmosphere resulted in the complete reoxidation of metallic rhodium species. The results of the work shed light on the structural aspects of the reversibility of the Rh-CeO2 catalysts based on the highly dispersed Rh3+ species under treatment in the reaction conditions.

Controlling the metal–support interaction with steam-modified ceria to boost Pd activity towards low-temperature CO oxidation

OH-modified ceria can promote Pd dispersion and weaken CO adsorption strength to greatly improve the low-temperature CO oxidation activity.

Gold single-atoms confined at the CeO x -TiO2 interfaces with enhanced low-temperature activity toward CO oxidation

We use CeO x -TiO2 hetero-interfaces generated on the surface of CeO x -TiO2 hybrid oxide supporting powders to stabilize Au single-atoms (SAs) with excellent low-temperature activity toward CO oxidation. Based on intriguing density functional theory calculation results on the preferential formation of Au-SAs at the CeO x -TiO2 interfaces and the high activity of Au-SAs toward the Mars–van Krevelen type CO oxidation, we synthesized a Au/CeO x -TiO2 (ACT) catalyst with 0.05 wt.% of Au content. The Au-SAs stabilized at the CeO x -TiO2 interfaces by electronic coupling between Au and Ce showed improved low-temperature CO oxidation activity than the conventional Au/TiO2 control group catalyst. However, the light-off profile of ACT showed that the early activated Au-SAs are not vigorously participating in CO oxidation. The large portion of the positive effect on the overall catalytic activity from the low activation energy barrier of ACT was retarded by the negative impact from the decreasing active site density at high temperatures. We anticipate that the low-temperature activity and high-temperature stability of Au-SAs that stand against each other can be optimized by controlling the electronic coupling strength between Au-SAs and oxide clusters at the Au-oxide-TiO2 interfaces. Our results show that atomic-precision interface modulation could fine-tune the catalytic activity and stability of Au-SAs.

Pd-Ce-Ox/MWCNTs and Pt-Ce-Ox/MWCNTs Composite Materials: Morphology, Microstructure, and Catalytic Properties.

The composite nanomaterials based on noble metals, reducible oxides, and nanostructured carbon are considered to be perspective catalysts for many useful reactions. In the present work, multi-walled carbon nanotubes (MWCNTs) were used for the preparation of Pd-Ce-Ox/MWCNTs and Pt-Ce-Ox/MWCNTs catalysts comprising the active components (6 wt%Pd, 6 wt%Pt, 20 wt%CeO2) as highly dispersed nanoparticles, clusters, and single atoms. The application of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) provided analysis of the samples' morphology and structure at the atomic level. For Pd-Ce-Ox/MWCNTs samples, the formation of PdO nanoparticles with an average crystallite size of ~8 nm was shown. Pt-Ce-Ox/MWCNTs catalysts comprised single Pt2+ ions and PtOx clusters less than 1 nm. A comparison of the catalytic properties of the samples showed higher activity of Pd-based catalysts in CO and CH4 oxidation reactions in a low-temperature range (T50 = 100 °C and T50 = 295 °C, respectively). However, oxidative pretreatment of the samples resulted in a remarkable enhancement of CO oxidation activity of Pt-Ce-Ox/MWCNTs catalyst at T < 20 °C (33% of CO conversion at T = 0 °C), while no changes were detected for the Pd-Ce-Ox/MWCNTs sample. The revealed catalytic effect was discussed in terms of the capability of the Pt-Ce-Ox/MWCNTs system to form unique PtOx clusters providing high catalytic activity in low-temperature CO oxidation.

One-pot phyto-mediated combustion technicality for synthesizing Pd adorned Cu2O@CuO heterojunction with great efficiency in CO oxidation and epoxidation applications

In this work, Pd nanoparticles immobilized Cu2O@CuO heterojunction catalysts were synthesized through a facile one-pot green solution combustion technicality using the pomegranate seed extract as reducing mediator/capping fuel during the combustion process. The findings showed that pomegranate seeds extract within a feasible content of biogenic capping components demonstrate a microspheres Pd/Cu2O@CuO with Hydrangea flower-like fibrous structure, higher porosity, the adaptable ratio of Cu+/Cu2+, and Pd°/Pd2+. Even further, the bio-inspired Pd/Cu2O@CuO heterojunction catalysts were exploited in CO oxidation and olefins epoxidation. The bio-inspired 0.5 wt% Pd/Cu2O@CuO provides 100 % conversion of CO at 99 °C and 99 % for cyclohexene epoxidation compared to 220 °C and 81 % for Cu2O@CuO. CuO and Cu2O NPs offer distinctive active centers for respectively, the dissociation of oxygen and CO oxidation. The nanoscale spacing between CuO and Cu2O zone and Pd NPs dispersion are beneficial to the quick movement of the adatom oxygen over Cu2O to cover the vacancies of oxygen on CuO and thereby enhance the catalytic activity for low-temperature CO oxidation and epoxidation processes through a synergetic mechanism.

Non-noble metal single-atom catalyst with MXene support: Fe1/Ti2CO2 for CO oxidation

MXenes have attracted considerable attention owing to their versatile and excellent physicochemical properties. Especially, they have potential applications as robust support for single atom catalysts. Here, quantum chemical studies with density functional theory are carried out to systematically investigate the geometries, stability, electronic properties of oxygen functionalized Ti 2 C (Ti 2 CO 2 ) supported single-atom catalysts M 1 /Ti 2 CO 2 (M = Fe, Co, Ni, Cu Ru, Rh, Pd, Ag Os, Ir, Pt, Au). A new non-noble metal SAC Fe 1 /Ti 2 CO 2 has been found to show excellent catalytic performance for low-temperature CO oxidation after screening the group 8-11 transition metals. We find that O 2 and CO adsorption on Fe 1 atom of Fe 1 /Ti 2 CO 2 is favorable. Accordingly, five possible mechanisms for CO oxidation on this catalyst are evaluated, including Eley-Rideal, Langmuir-Hinshelwood, Mars–van Krevelen, Termolecular Eley-Rideal, and Termolecular Langmuir-Hinshelwood (TLH) mechanisms. Based on the calculated reaction energies for different pathways, Fe 1 /Ti 2 CO 2 shows excellent kinetics for CO oxidation via TLH mechanism, with distinct low-energy barrier (0.20 eV) for the rate-determining step. These results demonstrate that Fe 1 /Ti 2 CO 2 MXene is highly promising 2D materials for building robust non-noble metal catalysts. MXene supported non-noble metal single-atom catalyst Fe 1 /Ti 2 CO 2 is found to exhibit excellent activity for low-temperature CO oxidation through first-principles computational modelling.

Activation of Pt Nanoclusters on TiO2 via Tuning the Metallic Sites to Promote Low-Temperature CO Oxidation

Metallic Pt sites are imperative in the CO oxidation reaction. Herein, we demonstrate the tuning of Pt sites by treating a Pt catalyst in various reductive atmospheres, influencing the catalyst activities in low-temperature CO oxidation. The H2 pretreatment of Pt clusters at 200 °C decreases the T50 from 208 °C to 183 °C in the 0.1 wt % Pt/TiO2 catalyst. The T50 shows a remarkable improvement using a CO pretreatment, which decreases the T50 further to 135 °C. A comprehensive characterization study reveals the integrated reasons behind this phenomenon: (i) the extent of PtO transition to metallic Pt sites, (ii) the ample surface active oxygen triggered by metallic Pt, (iii) the CO selectively adsorbs on metallic Pt sites which participate in low-temperature CO oxidation, and (iv) the formation of the unstable intermediate such as bicarbonate, contributes together to the enhanced activity of CO pretreated Pt/TiO2.

Low‐Temperature CO Oxidation over the Pt−TiN Interfacial Dual Sites

AbstractThe metal‐support interfacial synergistic catalysis allows the competitive reaction steps to occur in close proximity at different sites, thus decreasing the activation barrier and improving the catalytic efficiency. Combining the structural characterization, reactivity assessments and density functional theory (DFT) calculations, it is found that highly dispersed Pt clusters on TiN substrates favor the formation of Pt−TiN interfacial dual sites, which are highly reactive for the low‐temperature CO oxidation with an apparent activation energy as low as 38.7 kJ mol−1, much less than that on the traditional Pt/TiO2 or Pt/SiO2 catalysts. At low temperature, the surface sites of Pt are saturated with CO adsorption due to its high adsorption energy and the molecular O2 dissociatively adsorbs on the Ti sites to form the titanium oxynitride (TiNxOy) skin with a low barrier of 0.1 eV. The O atoms binding on Ti sites readily react with the adjacent CO molecules adsorbed on the perimeter of Pt clusters to attain a catalytic cycle with a reaction barrier of 0.5 eV. The interfacial synergistic effect between Pt and TiN is the origin of the low temperature CO oxidation activity on the metal‐nitride catalysts.

A Robust Cu Catalyst for Low-Temperature CO Oxidation in Flue Gas: Mitigating Deactivation via Co-Doping

CuCeTiOx (CCT) catalyst is considered as a promising prospect attributable to their high activity for low-temperature CO oxidation. However, rapid deactivation when treating humid flue gas hindered their industrial exploitation. The hydroxide ion (OH−) dissociated from H2O, and carbonate intermediates derived from CO/CO2 deposited on the catalyst surface of CCT catalyst, inhibits the CO oxidation by surface oxygen on active sites. In this study, the detrimental effect caused by H2O and CO2 were evaluated, and the performance of CCT catalysts were investigated and compared using in situ DRIFTs study. Further, intentional doping on the CCT using transition metal (e.g., Co and Mn) was performed to mitigate the catalyst deactivation caused by H2O and CO2. The incorporation of cobalt in Co-CCT altered the reaction pathway and mitigated the deactivation via enhancing the consumption of surface adsorbed OH- by CO, reducing the occupancy of active sites. Also, preferential adsorption of CO further suppressed the competition of OH- and CO2 towards active sites on catalyst attributable to the abundant oxygen vacancies and low coordinated metal (i.e., Cu+, Ce3+) in Co-CCT, which significantly enhanced the resistance to H2O and CO2 in the flue gas. This work thoroughly analyzed the mechanism of H2O and CO2 impacting the catalyst activity during low-temperature CO oxidation, is able to provide innovative insights for the design of highly-active and long-shelf life catalysts. The incorporation of cobalt in CuCeTiOx catalyst facilitates the formation of oxygen vacancies, the adsorption of CO, and the consumption of OH-, speeding up the CO oxidation to CO2 and promoting the resistance to deactivation caused by H2O and CO2 in the flue gas.

Tunable Electronic Metal–Support Interactions on Ceria-Supported Noble-Metal Nanocatalysts in Controlling the Low-Temperature CO Oxidation Activity

A fundamental study on the metal-support interactions of supported metal catalysts is of great importance for developing heterogeneous catalysts with high performance, is still attracting and challenging in many heterogeneous catalytic reactions. In this work, we report the catalytic performances of CeO2-supported noble-metal catalysts among single atoms, subnanoclusters (∼1 nm), and nanoparticles (2.2-2.7 nm) upon low-temperature CO oxidation reaction between 50 and 250 °C. The subnanoclusters and nanoparticles of Ru, Rh, and Ir showed much higher activities than those of the single atoms, while a Pd single-atom catalyst was more active than Pd subnanoclusters and nanoparticles. According to the results of multiple ex situ and in situ characterizations, the much different activities of Ru, Rh, Ir, and Pd were derived from the alterable electronic metal-support interactions (EMSI), which determine the concurrent reaction pathway including the famous Mars van Krevelen mechanism and carbonate-intermediate route on the most active metal sites of Mδ+ (0 < δ < 1) for Ru, Rh, and Ir and Pd2+ for Pd. Also, the moderate EMSI of CeO2-supported Rh subnanoclusters furthest benefited activation of the adsorbed CO molecule and ensured it the highest activity among CeO2-supported Ru, Rh, and Ir catalysts with similar metal deposit sizes.

Physical and Chemical Synthesis of Au/CeO2 Nanoparticle Catalysts for Room Temperature CO Oxidation: A Comparative Study

In many heterogeneous catalytic reactions, such as low-temperature CO oxidation, the preparation conditions, and the role of the CeO2 support (oxygen vacancies and redox properties) in the dispersion and the chemical state of Au, are considered critical factors for obtaining gold nanoparticle catalysts with high catalytic performance. In this work, the physical and chemical preparation methods were compared, aiming at understanding how the preparation method influences the catalytic activity. The Au/CeO2 nanoparticle catalysts with 5% Au loading were prepared via the Physical Laser Vaporization Controlled Condensation method (LVCC), and the chemical Deposition-Precipitation method (DP) was used to investigate the effect of synthesis methods on the structure and the catalytic activity toward the CO oxidation. In this manuscript, we compare the activity of nanostructured Au/CeO2 catalysts. The structure and the redox properties of the catalysts were investigated by the XRD, SEM, TEM, TPR, and XPS. The catalytic activity for low-temperature CO oxidation was studied using a custom-built quartz tube flow reactor coupled with an infrared detector system at atmospheric pressure. The study reveals that the prepared CeO2-supported Au nanoparticles’ catalytic activity was highly dependent on the preparation methods. It showed that the sample prepared by the DP method exhibits higher catalytic efficiency toward CO oxidation when compared with the sample prepared by the LVCC method. The high catalytic activity could be attributed to the small particle size and shape, slightly higher Au concentration at the surface, surface-active Au species such as Au1+, along with the large interface between Au and CeO2. This study suggests that the stability, dispersion of Au nanoparticles on CeO2, and strong interaction between Au and CeO2 lead to strong oxidation ability even below room temperature. Considering the universal character of different physical and chemical methods for Au/CeO2 preparation, this study may also provide a base for supported Au-based catalysts for many oxidation reactions in energy and environmental applications.

Improvement in Low Temperature CO Oxidation Activity of CuOx/CeO2−δ by Cs2O Doping: Mechanistic Aspects

The various wt% of Cs promoted CuOx/CeO2−δ catalysts were prepared by impregnation method and examined for CO oxidation. The 0.1 wt% Cs doped CuOx/CeO2−δ showed a maximum CO oxidation (22%) compared to the CuOx/CeO2−δ (9%) at light off temperature (40 oC). The plausible CO oxidation mechanism has explained using characterization techniques like ATR-FTIR, XPS, XRD, SEM, H2-TPR, and HRTEM. The formation of Cs2O was responsible for the stabilization of Cu1+ species. The Cs doping increases the electron density on the catalyst surface due to the charge diffusion. The Cs addition in CuCe leads to the formation of smaller Cu1+ species, CuO nanorod, Ce3+ and adsorbed oxygen. The role of these species for CO oxidation at a lower temperature is explained in detail with plausible mechanism. The synergistic interaction of Cs with CuCe leads to the increase in CO conversion rate with decrease in the activation energy.

Entropy-stabilized single-atom Pd catalysts via high-entropy fluorite oxide supports

Single-atom catalysts (SACs) have attracted considerable attention in the catalysis community. However, fabricating intrinsically stable SACs on traditional supports (N-doped carbon, metal oxides, etc.) remains a formidable challenge, especially under high-temperature conditions. Here, we report a novel entropy-driven strategy to stabilize Pd single-atom on the high-entropy fluorite oxides (CeZrHfTiLa)Ox (HEFO) as the support by a combination of mechanical milling with calcination at 900 °C. Characterization results reveal that single Pd atoms are incorporated into HEFO (Pd1@HEFO) sublattice by forming stable Pd–O–M bonds (M = Ce/Zr/La). Compared to the traditional support stabilized catalysts such as Pd@CeO2, Pd1@HEFO affords the improved reducibility of lattice oxygen and the existence of stable Pd–O–M species, thus exhibiting not only higher low-temperature CO oxidation activity but also outstanding resistance to thermal and hydrothermal degradation. This work therefore exemplifies the superiority of high-entropy materials for the preparation of SACs.

Interplay of Electronic and Steric Effects to Yield Low-Temperature CO Oxidation at Metal Single Sites in Defect-Engineered HKUST-1.

In contrast to catalytically active metal single atoms deposited on oxide nanoparticles, the crystalline nature of metal‐organic frameworks (MOFs) allows for a thorough characterization of reaction mechanisms. Using defect‐free HKUST‐1 MOF thin films, we demonstrate that Cu+/Cu2+ dimer defects, created in a controlled fashion by reducing the pristine Cu2+/Cu2+ pairs of the intact framework, account for the high catalytic activity in low‐temperature CO oxidation. Combining advanced IR spectroscopy and density functional theory we propose a new reaction mechanism where the key intermediate is an uncharged O2 species, weakly bound to Cu+/Cu2+. Our results reveal a complex interplay between electronic and steric effects at defect sites in MOFs and provide important guidelines for tailoring and exploiting the catalytic activity of single metal atom sites.

Shape-controlled CuxCe1-xO2 nanorods catalyst and the active components functioned in selective oxidation of CO in hydrogen-rich stream

A novel shape-controlled CuxCe1-xO2 nanorods catalyst is synthesized with a simple coprecipitation method at room temperature and Cu(I) chloride as copper sources for the first time. Various analysis methods (XRD Rietveld Refinement, N2 adsorption-desorption, dynamic OSC, XPS, HRTEM, H2-TPR and In situ DRIFTs) and catalytic activity test techniques are used to investigate the structural properties and the catalytic performance of the catalysts in CO-PROX. Introducing copper dopant into ceria nanorods has positive effect on enriching the lattice distortion, oxygen vacancy concentration and the lattice oxygen mobility of ceria support. The incorporation of copper ions generates two main types of active copper species that play different roles in CO-PROX. Specifically, the highly dispersed CuOx species determine the low-temperature CO oxidation activity, while strongly bound Cu-[Ox]-Ce species at the copper-ceria interface regulates the high-temperature CO oxidation activity of the catalyst. Appropriate copper doping (7–11 wt %) is in favor of forming sufficient CuOx species highly dispersed on the surface of ceria nanorods and accelerating the oxygen mobility, as well as strengthening the copper-ceria interaction in the catalyst, which significantly improves its catalytic activity and broadens the temperature window in CO-PROX.