Activity For CO Oxidation Research Articles

Optimizing the Pt‐FeOx Interaction over Atomic Pt/FeOx/CeO2 Catalysts for Improved CO Oxidation Activity

AbstractThe metal‐supported CeO2 catalysts (such as Pt/CeO2) have been considered as the most promising exhaust‐treatment catalyst to meet the future emission standard. However, the Pt‐O‐Ce interface on Pt/CeO2 system seems to over‐stabilize the Pt sites to cause low activity for CO oxidation. In this work, by adding Fe oxides, the enhanced Pt‐FeOx interaction was formed with the disappearance of the Pt‐O‐Ce interface, facilitating the electron transfer from the support to the atomically dispersed Pt on the Pt‐FeOx interface for the dramatically CO (preferential) oxidation performance. The single‐atom Pt/1.5FeOx/CeO2 performed the best catalytic activity with a reaction rate of 0.58 s−1 at 144 °C (T100). Such strategy in resorting the special metal‐support interaction to tune the active sites can be extended to other metal‐oxide systems for further optimized catalysis.

Pt-Based Nanostructures for Electrochemical Oxidation of CO: Unveiling the Effect of Shapes and Electrolytes.

Direct alcohol fuel cells are deemed as green and sustainable energy resources; however, CO-poisoning of Pt-based catalysts is a critical barrier to their commercialization. Thus, investigation of the electrochemical CO oxidation activity (COOxid) of Pt-based catalyst over pH ranges as a function of Pt-shape is necessary and is not yet reported. Herein, porous Pt nanodendrites (Pt NDs) were synthesized via the ultrasonic irradiation method, and its CO oxidation performance was benchmarked in different electrolytes relative to 1-D Pt chains nanostructure (Pt NCs) and commercial Pt/C catalyst under the same condition. This is a trial to confirm the effect of the size and shape of Pt as well as the pH of electrolytes on the COOxid. The COOxid activity and durability of Pt NDs are substantially superior to Pt NCs and Pt/C in HClO4, KOH, and NaHCO3 electrolytes, respectively, owing to the porous branched structure with a high surface area, which maximizes Pt utilization. Notably, the COOxid performance of Pt NPs in HClO4 is higher than that in NaHCO3, and KOH under the same reaction conditions. This study may open the way for understanding the COOxid activities of Pt-based catalysts and avoiding CO-poisoning in fuel cells.

In situ Synthesis of Ultrasmall Au Clusters on Thiol-modified CeO2 with Enhanced Stability and CO Oxidation Activity

In situ Synthesis of Ultrasmall Au Clusters on Thiol-modified CeO2 with Enhanced Stability and CO Oxidation Activity

Crystal plane induced metal-support interaction in Pd/Pr-CeO2 catalyst boosts H2O-assisted CO oxidation

Catalytic oxidation is an effective method to remove CO in exhaust gas, yet it is challenging to enhance the H2O tolerance of catalysts at low temperature due to the strong competitive adsorption between H2O and reactants (CO and O2). Here, we report that the precise control of the metal-support interaction by regulating the support crystal plane of Pd/Pr-CeO2 catalysts leads to highly efficient CO oxidation catalysts under wet conditions. The addition of H2O inhibits the CO oxidation activity of Pr-CeO2 nano-cube and octahedron supported highly dispersed Pd catalysts, but enhances the CO oxidation activity over Pr-CeO2 nanorod supported highly dispersed Pd catalysts. After introduction of H2O in feed gas, the T99 (Temperature to achieve 99 % conversion of CO) decreases from 157 °C to 115 °C for Pd/Pr-CeO2-NRs (Nanorods) catalyst at a gas hourly space velocity of 70, 000 h−1 and remains stable for more than 2000 min in the presence of H2O. Systematic characterization reveals that the (110) crystal plane of Pr-CeO2 nanorod enhance the interaction between Pd species and support, resulting in the formation of abundant high dispersed Pd-oxygen vacancy (Ov) active sites. The kinetics study and in-situ DRIFTS characterization confirmed that CO oxidation Pd/Pr-CeO2-NRs mainly follows the H2O-mediated Langmuir-Hinshelwood mechanism in the presence of H2O. H2O promote the formation of active OH groups and accelerate CO oxidation through formate pathway at low temperature. In contrast, the CO oxidation follow traditional MvK mechanism in the absence of H2O.

Nature of support plays vital roles in H2O promoted CO oxidation over Pt catalysts

Pt nanoparticle catalysts supported on a series of TiO2-SiO2 composites with different molar ratios were prepared, characterized, and their CO oxidation activities were evaluated under dry and humid conditions. Among the catalysts, Pt/1Ti-3Si showed the best performance under both conditions and potentials for future industrial applications. H218O experiments were designed and the CO2 composition was calculated to quantify the promotion effect of H2O, which was highly correlated with the concentration of H2O and Ti-Si ratio. The XRD, XPS and BET results revealed that the defects on the supports inhibited phase transformation and lattice growth for anatase TiO2. These defects also led to an increase in the number of acid sites on Pt/TiO2-SiO2. The TEM, EDS mapping, and CO chemosorption results indicated that metallic Pt0 particles were formed, which was beneficial for CO oxidation during reaction. It was found that the generation of OH from H2O dissociation and the desorption of OH on TiO2 were much easier than those on SiO2, illustrating that the H2O promotion effect could be controlled by regulating the nature of support. The mechanism of H2O promotion was proposed by experimental and theoretical methods, which confirmed the carboxyl intermediate pathway rather than the formate pathway.

Identifying the surface active sites of FeOx-modified Pt/Nb2O5 catalysts in CO and propane oxidation

A comparative study on FeOx-modified Pt/Nb2O5 catalyzing CO and propane oxidation has been conducted to differentiate the surface active sites demanded for these two important reactions. Pt-Fe/Nb2O5 displays improved activity for CO oxidation but decreased activity for propane oxidation compared to Pt/Nb2O5. Detailed characterizations reveal the alteration of the geometrical and electronic states of Pt species and surface acidity induced upon the addition of FeOx, thus clarifying the divergent catalytic responses in CO and propane oxidation. Pt-FeOx interface sites promotes oxygen activation that is a crucial step in CO oxidation, thereby accelerating the CO oxidation. However, the rate-determining step in propane oxidation is propane activation that requires the synergistic catalysis of acid sites and Pt0, the critical active sites for propane oxidation. The inferior activity of Pt-Fe/Nb2O5 in propane oxidation is explained by the coverage of acid sites and Pt species.

CO oxidation over the Cu2O/CuO hollow sphere heterojunction catalysts with enhanced low-temperature activities

In this work, a series of the Cu2O/CuO hollow sphere heterojunction catalysts with various Cu+/Cu2+ ratios were designed and synthesized to improve the low-temperature catalytic activity of CO oxidation. The physicochemical properties of all the catalysts were characterized by the SEM, TGA, N2-physisorption, XRD, FT-IR, XPS, and H2-TPR. The results revealed that the Cu2O hollow sphere catalyst had the large specific surface area due to the formation of the hollow structure, which could provid more accessible active sites for the gaseous reactants. Meanwhile, the Cu2O/CuO hollow sphere heterojunction catalysts possessed high concentration of the surface oxygen vacancies and further promoted the adsorption and activation of O2. As a result, the apparent activation energy of the CO oxidation was decreased and thus low-temperature activity of the CO oxidation was finally promoted. Therefore, the Cu2O/CuO hollow sphere heterojunction catalysts were considered as a series of high-efficient catalyst candidates for CO oxidation with promising low-temperature performance.

Investigation into the roles of interfacial H2O structure in catalytic oxidation of HCHO and CO over CuMnO2 catalysts

The rapid deactivation of cost-effective MnO2-based catalysts in humid air limits their application in practice, and the identification of the role of water in an oxidation process is significant for developing water-resistant MnO2-based catalysts. Here, CuMnO2 showed a 20.3% HCHO conversion in 10 hr at room temperature in humid air with relative humidity of 40%, but deactivated in 3 hr in dry air. The excellent activity and stability of HCHO oxidation in humid air were attributed to the positive effect of H2O on HCHO oxidation to the H2O-HOCH2OH supermolecule assemblies via hydrogen bonds formed on CuMnO2. H2O-HOCH2OH supermolecule assemblies tend to be oxidized to carbonate, which is further oxidized to CO2. Furthermore, CuMnO2 exhibited a much poorer activity of CO oxidation in humid air, but the CO conversion was still 100% in 10 hr in dry air. H2O showed a competitive adsorption effect to CO on CuMnO2. CuMnO2 could be applied in HCHO elimination in humid air and CO elimination in dry air.

Preferential CO Oxidation on a Highly Active Cu Single Atom Catalyst Supported by Ce−TiOx

AbstractA heterogeneous single‐atom catalyst (SAC) based on a tiny Cu loading (0.05 wt.%) supported on CeO2−TiO2 (Ce/Ti=0.18) shows very high Cu mass‐normalized CO oxidation activity, 100 % selectivity for CO2 between 60 and 100 °C and essentially no deactivation during 17 h on stream in the preferential oxidation of CO (PROX) at 120 °C. Based on state of the art, this catalyst is among the most active and PROX‐selective catalysts. Employing a combination of ex‐situ and operando methods, we infer that the isolated Cu single sites are incorporated with redox‐active Cu2+−O−Ce4+⇆Cu+−□−Ce3+ moieties connected by labile oxygen and are exposed on the surface of highly dispersed ceria. This high dispersion is promoted by TiO2 which itself does not participate in redox steps during PROX, as Ti ions remain essentially tetravalent while Cu and Ce undergo reversible redox shuttles.

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.

Effect of Pd precursors on the catalytic properties of Pd/CeO2 catalysts for CH4 and CO oxidation

Herein, Pd/CeO2 catalysts were prepared using different Pd precursors and applied to CH4 and CO oxidation. Significantly, the Pd/CeO2 catalyst prepared using palladium nitrate (Pd(2)/CeO2 (N)) was highly active toward CH4 oxidation, whereas the catalyst prepared using palladium acetate (Pd(2)/CeO2 (A)) exhibited high CO oxidation activity. The effects of the precursor on the catalytic activity and other properties were studied using various characterization techniques. The Pd(2)/CeO2 (N) catalyst contained surface PdO nanoparticles, which were active sites for CH4 oxidation. On the contrary, the Pd(2)/CeO2 (A) catalyst possessed Pd ions stabilized at CeO2 defect sites, which were not very reactive. However, the high Pd dispersion was beneficial for the CO oxidation reaction. Therefore, the Pd precursor clearly altered the Pd dispersion and state in the Pd/CeO2, which are critical for catalytic activity.

Pd/CeO2-SnO2 catalysts with varying tin content: Promotion of catalytic properties and structure modification

1%Pd/CeO2-SnO2 catalysts with varying Ce/Sn ratio were synthesized by counter-precipitation followed by calcination in a wide temperature range. The catalysts with Ce/Sn < 3/1 possess high thermal stability after calcination up to 1000 °C while maintaining low-temperature activity in CO oxidation. The PdOx clusters serving as active centers in CO oxidation are modified by Sn upon calcination. High tin content (Ce/Sn = 1/3) provides the activity of the catalysts in CH4 oxidation due to stabilization of PdO nanoparticles in the form of core@shell PdO@(CeO2 + SnO2) structures. Formation of the nanoheterophase structure upon calcination plays a key role in the stabilization of Pd-active centers of different types.

The Study of Thermal Stability of Mn-Zr-Ce, Mn-Ce and Mn-Zr Oxide Catalysts for CO Oxidation.

MnOx-CeO2, MnOx-ZrO2, MnOx-ZrO2-CeO2 oxides with the Mn/(Zr + Ce + Mn) molar ratio of 0.3 were synthesized by coprecipitation method followed by calcination in the temperature range of 400-800 °C and characterized by XRD, N2 adsorption, TPR, TEM, and EPR. The catalytic activity was tested in the CO oxidation reaction. It was found that MnOx-CeO2, MnOx-ZrO2-CeO2, MnOx-ZrO2 catalysts, calcined at 400-500 °C, 650-700 °C and 500-650 °C, respectively, show the highest catalytic activity in the reaction of CO oxidation. According to XRD and TEM results, thermal stability of catalysts is determined by the temperature of decomposition of the solid solution Mnx(Ce,Zr)1-xO2. The TPR-H2 and EPR methods showed that the high activity in CO oxidation correlates with the content of easily reduced fine MnOx particles in the samples and the presence of paramagnetic defects in the form of oxygen vacancies. The maximum activity for each series of catalysts is associated with the start of solid solution decomposition. Formation of active phase shifts to the high-temperature region with the addition of zirconium to the MnOx-CeO2 catalyst.

Functional CeOx nanoglues for robust atomically dispersed catalysts.

Single-atom catalysts1 make exceptionally efficient use of expensive noble metals and can bring out unique properties1-3. However, applications are usually compromised by limited catalyst stability, which is due to sintering3,4. Although sintering can be suppressed by anchoring the metal atoms to oxide supports1,5,6, strong metal-oxygen interactions often leave too few metal sites available for reactant binding and catalysis6,7, and when exposed to reducing conditions at sufficiently high temperatures, even oxide-anchored single-atom catalysts eventually sinter4,8,9. Here we show that the beneficial effects of anchoring can be enhanced by confining the atomically dispersed metal atoms on oxide nanoclusters or 'nanoglues', which themselves are dispersed and immobilized on a robust, high-surface-area support. We demonstrate the strategy by grafting isolated and defective CeOx nanoglue islands onto high-surface-area SiO2; the nanoglue islands then each host on average one Pt atom. We find that the Pt atoms remain dispersed under both oxidizing and reducing environments at high temperatures, and that the activated catalyst exhibits markedly increased activity for CO oxidation. We attribute the improved stability under reducing conditions to the support structure and the much stronger affinity of Pt atoms for CeOx than for SiO2, which ensures the Pt atoms can move but remain confined to their respective nanoglue islands. The strategy of using functional nanoglues to confine atomically dispersed metals and simultaneously enhance their reactivity is general, and we anticipate that it will take single-atom catalysts a step closer to practical applications.

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 &lt; 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.

A Tale of Two Sites: Neighboring Atomically Dispersed Pt Sites Cooperatively Remove Trace H2 in CO-Rich Stream.

Single-atom catalysts (SACs) exhibit distinct catalytic behavior compared with nano-catalysts because of their unique atomic coordination environment without the direct bonding between identical metal centers. How these single atom sites interact with each other and influence the catalytic performance remains unveiled as designing densely populated but stable SACs is still an enormous challenge to date. Here, a fabrication strategy for embedding high areal density single-atom Pt sites via a defect engineering approach is demonstrated. Similar to the synergistic mechanism in binuclear homogeneous catalysts, from both experimental and theoretical results, it is proved that electrons would redistribute between the two oxo-bridged paired Pt sites after hydrogen adsorption on one site, which enables the other Pt site to have high CO oxidation activity at mild-temperature. The dynamic electronic interaction between neighboring Pt sites is found to be distance dependent. These new SACs with abundant Pt-O-Pt paired structures can improve the efficiency of CO chemical purification.

PtPd nanoparticles on LaCoO3 with ruthenium in the interfaces for CO oxidation

In this work, trimetallic Pt-Pd-Ru nanoparticles (NPs) loaded on LaCoO3 perovskite were prepared by using LaCo0.9PtxPdyRu0.1-x-yO3 as precursor through citrate complexing sol-gol method and characterized by N2 adsorption/desorption, XRD, H2 Temperature-programmed reduction, XPS, CO-TPD, ICP-OES and TEM techniques. The special structure of the trimetallic catalyst and its catalytic performance for CO oxidation were investigated. The results demonstrated that Pt, Ru and Pd ions were successfully incorporated into the lattice of LaCoO3 to form with perovskite structure. After the reduction treatment, the catalyst Pt-Pd-Ru/LaCoO3 was obtained, which exhibited special structure of Pt surface, Pd sub-surface and Ru interface. The trimetallic Pt-Pd-Ru/LaCoO3 displayed the highest activity and best stability for CO oxidation as compared with corresponding bimetallic and monometallic catalysts. The significant improvement on the catalytic activity was ascribed to the combination of the oxygen vacancies on the surface of LaCoO3 and the highly dispersed NPs. The excellent stability was owing to the special metal-support interaction, that is the Pt-Pd in the NPs are connected and adhered to the support of LaCoO3 by the Ru species in the interface.

Solid Fe Resources Separated from Rolling Oil Sludge for CO Oxidation.

The efficient recycling of valuable resources from rolling oil sludge (ROS) to gain new uses remains a formidable challenge. In this study, we reported the recycling of solid Fe resources from ROS by a catalytic hydrogenation technique and its catalytic performance for CO oxidation. The solid Fe resources, after calcination in air (Fe2O3-H), exhibited comparable activity to those prepared by the calcinations of ferric nitrate (Fe2O3-C), suggesting that the solid resources have excellent recycling value when used as raw materials for CO oxidation catalyst preparation. Further studies to improve the catalytic performance by supporting the materials on high surface area 13X zeolite and by pretreating the materials with CO atmosphere, showed that the CO pretreatment greatly improved the CO oxidation activity and the best activity was achieved on the 20 wt.%Fe2O3-H/13X sample with complete CO conversion at 250 °C. CO pretreatment could produce more oxygen vacancies, facilitating O2 activation, and thus accelerate the CO oxidation reaction rate. The excellent reducibility and sufficient O2 adsorption amount were also favorable for its performance. The recycling of solid Fe resources from ROS is quite promising for CO oxidation applications.

Versatile Layered Hydroxide Precursors for Generic Synthesis of Cu‐Based Materials

The ability to mix multiple elements in a structure is crucial for obtaining Cu‐based nanostructures and microstructures with desirable physicochemical properties. Precursors containing multiple metal cations, such as layered double hydroxides, have been used for the synthesis of multielement materials. However, these precursors experience difficulty containing large cations, which limits the functionalities of the derived materials. Herein, the development of Cu‐based hydroxy double salts (HDSs), versatile precursors that accommodate a broad range of metal cations with homogeneous distributions, is reported. Up to 25 different metal cations are mixed with Cu in an HDS single phase, individually and simultaneously. The HDSs further exhibit useful properties with respect to anion exchange, exfoliation, and phase transformation to metal oxides. During the metal oxide transformation process, the formed crystal structures are mainly dependent on the ionic radius of the secondary metal cations. To prove their utility as precursors, the metal oxides derived from the HDSs are tested and found that they exhibited high catalytic activities for CO oxidation. This study significantly expands the compositional and structural freedom of Cu‐based multi‐elemental materials.

Determination of Intrinsic Active Sites on CuO–CeO2–Al2O3 Catalysts for CO Oxidation and NO Reduction by CO: Differences and Connections

Based on a CeO2–Al2O3 support (CA-T) prepared by two-step incipient wetness impregnation (T-IWI) method, an efficient CuO catalyst supported on CA-T (Cu/CA-T) was successfully developed, which showed better catalytic performance in CO oxidation and NO reduction by CO (NO + CO reaction) together with higher thermal stability than the CuO catalyst loaded on conventional CeO2–Al2O3 support (Cu/CA). Based on the results of HAADF-STEM, Raman spectra, electron paramagnetic resonance, X-ray absorption spectroscopy, ex situ and quasi in situ X-ray photoelectron spectroscopy, in situ diffuse reflectance infrared Fourier transform spectroscopy, and dynamic oxygen storage capacity (OSC) testing, etc., the microstructure of CuO–CeO2–Al2O3 catalysts, especially the state of Cu species, was systemically investigated. The mechanisms of CO oxidation and NO + CO reactions on Cu/CA and Cu/CA-T catalysts were collaboratively revealed. It was found that the CA-T support with smaller CeO2 particles could facilitate the formation of more highly dispersed CuOx clusters with better redox property. In CO oxidation, more active Cu+ species could be formed on Cu/CA-T under reaction conditions due to the higher reducibility of CuOx clusters on CeO2, which contributed to its superior CO removal efficiency. After aging, a significant decrease in CO oxidation activity was observed on Cu/CA catalyst, which was mainly resulted from the disappearance of highly dispersed CuOx clusters. In clear contrast, for aged Cu/CA-T catalyst, the highly dispersed CuOx clusters could be well preserved, well explaining its much higher catalytic activity and thermal stability than the aged Cu/CA counterpart as probed by the CO oxidation reaction. In NO + CO reaction, besides the formation of more Cu+ sites for CO adsorption and activation, much higher OSC function was also achieved on the Cu/CA-T catalyst. This was due to the higher concentration of surface oxygen vacancies and moderate strength of CuOx–CeO2 interaction on Cu/CA-T, which could effectively promote the NO dissociation into [N] for N2 formation and the transfer of dissociated [O] to CO@Cu+ sites to complete the CO depletion.