Activity For CO Oxidation Research Articles

Cover Feature: CeO2 Supported Gold Nanocluster Catalysts for CO Oxidation: Surface Evolution Influenced by the Ligand Shell (ChemCatChem 14/2022)

The Cover Feature shows the “four seasons” of ligand-protected Au nanoclusters on CeO2, used as catalysts in heterogeneous CO oxidation. In their Research Article, N. Barrabés and co-workers followed the evolution of the protecting ligand sphere by in situ IR spectroscopy, which allowed to investigate the dynamics of surface species in detail. After preparation of the gold clusters in solution, they were immobilized on ceria and subsequently subjected to oxidative pretreatment to facilitate ligand removal. For phosphine-protected Au nanoclusters – shown in the image – residues of the ligand sphere were found to remain in the system at the interfacial sites. This affected the CO oxidation activity of the cluster catalysts, leading to conversions lower than that of their thiolate-protected counterparts. This study shows that the protecting ligands play an important role in heterogeneous catalysis and need to be considered when designing a catalytic system.More information can be found in the Research Article by N. Barrabés and co-workers.

Influence of oxidizing and reducing pretreatment on the catalytic performance of CeO2 for CO oxidation

Cerium oxide (CeO2) and ceria-based materials have been extensively investigated as catalyst and support materials for various catalytic reactions, due to higher oxygen storage capacity and excellent redox properties. In the current work, we investigated the impact of pretreatment conditions (e.g., oxidation and reduction) on the physical properties of bulk CeO2 and catalytic activity for CO oxidation as a model reaction. To understand the physical properties of pretreated CeO2 catalysts, a suite of complementary characterization techniques, including X-ray diffraction (XRD), surface area analysis (BET), X-ray photoemission spectroscopy (XPS), and Raman spectroscopy, were applied. The results showed that a higher pretreatment temperature led to a decreased specific surface area (SSA), a decrease in oxygen vacancy/defect sites, and increased crystallite size, while surface Ce3+/Ce4+ ratio did not show a specific relationship to the treatment conditions. The 700 °C treated CeO2 samples under oxidizing and reducing conditions showed higher specific oxidation rate (μmolCO/s/m2) compared to other samples at 280 and 300 °C (or < 15% CO conversion). The CO conversion per total mass of catalysts, however, decreased with increasing temperatures, especially at 700 °C under reducing condition, indicating that the catalytic performance was affected by the physical properties (SSA, oxygen vacancy/defect sites, and crystallite size)..

Surface Oxygen Vacancies Confined by Ferroelectric Polarization for Tunable CO Oxidation Kinetics.

Surface oxygen vacancies have been widely discussed to be crucial for tailoring the activity of various chemical reactions from CO, NO, to water oxidation by using oxide-supported catalysts. However, the real role and potential function of surface oxygen vacancies in the reaction remains unclear because of their very short lifetime. Here, it is reported that surface oxygen vacancies can be well confined electrostatically for a polarization screening near the perimeter interface between Pt {111} nanocrystals and the negative polar surface (001) of ferroelectric PbTiO3. Strikingly, such a catalyst demonstrates a tunable catalytic CO oxidation kinetics from 200°C to near room temperature by increasing the O2 gas pressure, accompanied by the conversion curve from a hysteresis-free loop to one with hysteresis. The combination of reaction kinetics, electronic energy loss spectroscopy (EELS) analysis, and density functional theory (DFT) calculations, indicates that the oxygen vacancies stabilized by the negative polar surface are the active sites for O2 adsorption as a rate-determining step, and then dissociated O moves to the surface of the Pt nanocrystals for oxidizing adsorbed CO. The results open a new pathway for tunable catalytic activity of CO oxidation.

Quality or Quantity? How Structural Parameters Affect Catalytic Activity of Iron Oxides for CO Oxidation

The replacement of noble metal catalysts by abundant iron as an active compound in CO oxidation is of ecologic and economic interest. However, improvement of their catalytic performance to the same level as state-of-the-art noble metal catalysts requires an in depth understanding of their working principle on an atomic level. As a contribution to this aim, a series of iron oxide catalysts with varying Fe loadings from 1 to 20 wt% immobilized on a γ-Al2O3 support is presented here, and a multidimensional structure–activity correlation is established. The CO oxidation activity is correlated to structural details obtained by various spectroscopic, diffraction, and microscopic methods, such as PXRD, PDF analysis, DRUVS, Mössbauer spectroscopy, STEM-EDX, and XAS. Low Fe loadings lead to less agglomerated but high percentual amounts of isolated, tetrahedrally coordinated iron oxide species, while the absolute amount of isolated species reaches its maximum at high Fe loadings. Consequently, the highest CO oxidation activity in terms of turnover frequencies can be correlated to small, finely dispersed iron oxide species with a large amount of tetrahedrally oxygen coordinated iron sites, while the overall amount of isolated iron oxide species correlates with a lower light-off temperature.

Effects of Subsurface Oxide on Cu1/CeO2 Single-Atom Catalysts for CO Oxidation: A Theoretical Investigation.

Supported atomic dispersion metals are of great interest, and the interfacial effect between isolated metal atoms and supports is crucial in heterogeneous catalysis. Herein, the behavior of single-atom Cu catalysts dispersed on CeO2 (100), (110), and (111) surfaces has been studied by DFT + U calculations. The interactions between ceria crystal planes and isolated Cu atoms together with their corresponding catalytic activities for CO oxidation are investigated. The CeO2 (100) and (111) surfaces can stabilize active Cu+ species, while Cu exists as Cu2+ on the (110) surface. Cu+ is certified as the most active site for CO adsorption, which can promote the formation of the reaction intermediates and reduce reaction energy barriers. For the CeO2 (100) surface, the interaction between CO and Cu is weak and the CO adsorbate is more likely to activate the subsurface oxygen. The catalytic performance is closely related to the binding strength of CO to the active Cu single atoms on the different subsurfaces. These results bring a significant insight into the rational design of single metal atoms on ceria and other reducible oxides.

Effect of TiO2 Calcination Pretreatment on the Performance of Pt/TiO2 Catalyst for CO Oxidation.

In order to improve the CO catalytic oxidation performance of a Pt/TiO2 catalyst, a series of Pt/TiO2 catalysts were prepared via an impregnation method in this study, and various characterization methods were used to explore the effect of TiO2 calcination pretreatment on the CO catalytic oxidation performance of the catalysts. The results revealed that Pt/TiO2 (700 °C) prepared by TiO2 after calcination pretreatment at 700 °C exhibits a superior CO oxidation activity at low temperatures. After calcination pretreatment, the catalyst exhibited a suitable specific surface area and pore structure, which is beneficial to the diffusion of reactants and reaction products. At the same time, the proportion of adsorbed oxygen on the catalyst surface was increased, which promoted the oxidation of CO. After calcination pretreatment, the adsorption capacity of the catalyst for CO and CO2 decreased, which was beneficial for the simultaneous inhibition of the CO self-poisoning of Pt sites. In addition, the Pt species exhibited a higher degree of dispersion and a smaller particle size, thereby increasing the CO oxidation activity of the Pt/TiO2 (700 °C) catalyst.

Catalytic Oxidation of Carbon Monoxide over CeO2 and La2O3 Oxides Supported Nickel Catalysts: The Effect of the Support and NiO Loading

AbstractCeO2 and La2O3 oxides supported NiO loading (5 and 20 wt%) were synthesized, characterized by ICP‐AES, XRD, IR‐ATR, SEM, B.E.T, H2‐TPR and applied to CO oxidation with and without pretreatment under H2 atmosphere. It was shown that the support has a significant effect on the bulk, surface and reactivity properties. The reduction of nickel to metallic species occurred at higher temperatures in ceria case. 20‐Ni/La sample showed the best catalytic activity without reduction but this behavior mitigates when the sample pretreated under H2 indicating that the presence of different kinds sites responsible of CO oxidation. An opposite reactivity property was obtained in 5‐Ni/La and 20‐Ni/La cases and the interfacial γ‐type NiO interactions decreases with NiO loading which was coincident with CO oxidation activity. The active species in 20‐Ni/La sample is of type metal/support interface≡γ‐NiO/La2O3. In contrast, for 5‐Ni/La and 20‐Ni/Ce catalysts, γ‐NiO active sites incorporated into support is available.

Catalytic behavior of gold nanoparticles supported on a TiO2-Al2O3 mixed oxide for CO oxidation at low temperature.

The present work highlights the versatility of a TiO2-Al2O3 mixed oxide bearing highly dispersed gold nanoparticles that was applied in the CO oxidation reaction at room temperature. The TiO2, Al2O3, and TiO2-Al2O3 supports were synthesized by the sol-gel method, while gold nanoparticles were added by the deposition-precipitation with urea method using a theoretical Au loading of 2 wt.%. A promotional effect of the TiO2-Al2O3 support on the activity of gold catalysts with respect to TiO2 and Al2O3 was observed; Au/TiO2-Al2O3 showed outstanding CO oxidation, being active from 0°C and stable throughout a 24-h test. As for the alumina content (5, 10, and 15 wt.%) in TiO2, it improved the textural properties by retarding the crystal growth and anatase-rutile phase transformation of TiO2, suppressing the deposition of carbon on the catalyst surface and stabilizing the Au nanoparticles even at high temperatures. Gold was highly dispersed with nanoparticle sizes ranging from 1 to 2nm when H2 was used to treat thermally the Au/TiO2-Al2O3, Au/TiO2, and Au/Al2O3 materials. In addition, the XPS technique helped elicit that Au0 and Au1+ boosted their interaction with the TiO2, Al2O3, and TiO2-Al2O3 supports by means of charge transfer, which resulted in outstanding CO oxidation activity from 0°C. Likewise, the key factors that control the peculiar catalytic performance in the CO oxidation reaction are discussed, which represents a step forward in the versatility behavior of gold catalysts supported on mixed oxide catalysts.

LDH-derived preparation of CuMgFe layered double oxides for NH3-SCR and CO oxidation reactions: Performance study and synergistic mechanism

The NH3-SCR and CO oxidation reactions provides a promising route for the synergistic removal of NOx and CO. Herein, a novel two-dimensional CuMgFeO layered double oxides by the co-precipitation method and employed it for the NH3-SCR and CO oxidation reactions. The catalytic activity results showed that the CuMgFeO sample reached 96.8 % NO conversion at 250 °C, and 100 % CO conversion in temperature of 200–300 °C. Impressively, the CuMgFeO catalyst showed outstanding N2 selectivity, water resistance, and maintained splendid long-term catalytic stability. The superior NH3-SCR and CO oxidation activity of CuMgFeO catalyst can be related to the well dispersion of CuO species, higher reducibility, and abundance of acid sites. Meanwhile, XPS analysis indicated that the presence of electron transfer between the Cu2+ and Fe3+ redox cycles, as well as adsorbed oxygen on the sample surface were the main reactive oxygen species. The in situ DRIFT species indicate that the NH3-SCR reaction over CuMgFeO catalysts mainly complied with the E-R mechanism, while the CO oxidation followed the L-H mechanism. Finally, the deactivation mechanism of SO2 was investigated by in situ DRIFT species, indicating that SO2 hinders the NH3-SCR reaction through competition with NH3 and NOx for adsorption sites and the formation of ammonium sulfate salts. This work provides insight into the simultaneous implementation of the NH3-SCR and CO oxidation reactions on a single catalyst, which is of great value for the future synergistic control of flue gas pollutants.

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

Identifying Key Descriptors for the Single-Atom Catalyzed CO Oxidation

ZSM-5 and BEA zeolites modified with Pd nanoparticles by laser electrodispersion. The structure and catalytic activity in CO and CH4 oxidation

ZSM-5 and BEA zeolites modified with Pd nanoparticles by laser electrodispersion. The structure and catalytic activity in CO and CH4 oxidation

Robust Joule-heating ceramic reactors for catalytic CO oxidation

Joule-heating reactors have the higher energy efficiency and product selectivity compared with the reactors based on radiative heating. Current Joule-heating reactors are constructed with electrically-conductive metals or carbon materials, and therefore suffer from stability issue due to the presence of corrosive or oxidizing gases during high-temperature reactions. In this study, chemically-stable and electrically-conductive (La0.80Sr0.20)0.95FeO3 (LSF)/Gd0.1Ce0.9O2 (GDC) ceramics have been used to construct Joule-heating reactors for the first time. Taking the advantage of the resistance decrease of the ceramic reactors with temperature increase, the ceramic reactors heated under current control mode achieved the automatic adjustment of heating to stabilize reactor temperatures. In addition, the electrical resistance of LSF/GDC reactors can be tuned by the content of the high-conductive LSF in composite ceramics and ceramic density via sintering temperature, which offers flexibility to control reactor temperatures. The ceramic reactors with dendritic channels (less than 100 urn in diameter) showed the catalytic activity for CO oxidation, which was further improved by coating efficient MnO2 nanocatalyst on reactor channel wall. The Joule-heating ceramic reactors achieved complete CO oxidation at a low temperature of 165 °C. Therefore, robust ceramic reactors have successfully demonstrated effective Joule heating for CO oxidation, which are potentially applied in other high-temperature catalytic reactions.

Boosting Thermal Stability of Volatile Os Catalysts by Downsizing to Atomically Dispersed Species.

Os-based catalysts present remarkable catalytic activity; however, their use has been limited by the undesirable side reactions that generate highly toxic and volatile OsO4 even at room temperature. Herein, we demonstrate that the thermal stability of Os-based catalysts can be dramatically improved by downsizing Os nanoparticles (NPs) into atomically dispersed species. We observed that Os NPs were converted into OsO4 after calcination at 250 °C followed by sublimation, whereas single Os sites retained their structure after calcination. Temperature-programmed oxidation analysis confirmed that Os NPs started to undergo oxidation at 130 °C, whereas atomically dispersed Os preserved its state up to 300 °C. The CO oxidation activity of the atomically dispersed Os catalyst at 400 °C (100% conversion) was stably preserved over 30 h. By contrast, the activity of Os NP catalyst declined drastically. This study highlights the unique catalytic behavior of atomically dispersed catalysts, which is distinct from that of NP-based catalysts.

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.

Electronic Metal-Support Interactions Between CuxO and ZnO for CuxO/ZnO Catalysts With Enhanced CO Oxidation Activity.

Metal-support interaction has been one of the main topics of research on supported catalysts all the time. However, many other factors including the particle size, shape and chemical composition can have significant influences on the catalytic performance when considering the role of metal-support interaction. Herein, we have designed a series of CuxO/ZnO catalysts as examples to quantitatively investigate how the metal-support interaction influences the catalytic performance. The electronic metal-support interactions between CuxO and ZnO were regulated successfully without altering the structure of CuxO/ZnO catalyst. Due to the lower work function of ZnO, electrons would transfer from ZnO to CuO, which is favorable for the formation of higher active Cu species. Combined experimental and theoretical calculations revealed that electron-rich interface result from interaction was favorable for the adsorption of oxygen and CO oxidation reaction. Such strategy represents a new direction to boost the catalytic activity of supported catalysts in various applications.

How H2O may influence ambient CO oxidation over Au/BN

Water is considered as a promoter for the CO oxidation over Au catalysts but experimental evidence of how adsorbed H2O may influence the CO oxidation is scarce. In this study, Au/BN catalysts prepared by deposition method are subjected to calcination at different temperature. Increasing calcination temperature results in more Au particles distributed on BN basal plane, increased water affinity, increased defects on BN, and altered CO oxidation activity. All catalysts exhibit two CO adsorption bands at high relative humidity which are attributable to COad on Au0 and CO(H2O)n species, although only a blue-shifted COad is found at nominal dry condition. The turnover frequency is found to correlate with H2O physisorption, with a volcano-shape dependency on surface density and first-layer physisorption enthalpy of H2Oad. How the physisorbed H2O may influence CO oxidation over Au/BN is discussed and it implies that (H2O)n,ad cluster is involved in CO activation.

Engineering surface segregation of perovskite oxide through wet exsolution for CO catalytic oxidation

Cation segregation occurring near the surface or interfaces of solid catalysts plays an important role in catalytic reactions. Unfortunately, the native surface of perovskite oxides is dominated by passivated A-site segregation, which severely hampers the catalytic activity and durability of the system. To address this issue, herein, we present a wet exsolution method to reconstruct surface segregation in perovskite cobalt oxide. Under reduction etching treatment of glycol solution, inert surface Sr segregation was transformed into active Co3O4 segregation. By varying the reaction time, we achieved differing coverage of the active Co3O4 segregation on the La0.5Sr0.5CoO3-δ (LSCO) perovskite oxide surface. This study reveals that CO oxidation activity exhibits a volcano-shaped dependence on the coverage of Co3O4 segregation at the surface of a perovskite cobalt oxide. Furthermore, we find that a suitable coverage of Co3O4 segregation can dramatically improve the catalytic activity of the perovskite catalyst by enhancing interface interactions. Co K-edge, Co L-edge, and O K-edge X-ray absorption spectra confirm that the synergistic effect optimizes the covalence of the metal-oxygen bond at the surface and interface. This work not only contributes to the design and development of perovskite-type catalysts, but also provides important insight into the relationship between surface segregation and catalytic activity.

Reducibility Studies of Ceria, Ce0.85Zr0.15O2 (CZ) and Au/CZ Catalysts after Alkali Ion Doping: Impact on Activity in Oxidation of NO and CO

The aim of these studies was to perform thorough research on the influence of alkali metal ions (Li, Na, K and Cs) on the properties of nanogold catalysts supported on ceria–zirconia. The addition of alkali metal ions onto CeO2 further affected the reducibility, which was not noted for the Zr-doped support (Ce0.85Zr0.15O2). Despite the substantial impact of alkali metal ions on the reducibility of ceria, the activity in CO oxidation did not change much. In contrast, they do not have a large effect on the reducibility of Au/CZ but suppressed the activity of this system in CO oxidation. The results show that for CO oxidation, the negative effect of potassium ions is greater than that of sodium, which corresponds to the shift in the Tmax of the reduction peak towards higher temperatures. The negative effect of Li+ and Cs+ spans 50% CO conversion. The negative effect was visible for CO oxidation in both the model stream and the complex stream, which also contained hydrocarbons and NO. In the case of NO oxidation to NO2, two temperature regimes were observed for Au + 0.3 at% K/CZ, namely in the temperature range below 350 °C; the effect of potassium ions was beneficial for NO oxidation, whereas at higher temperatures, the undoped gold catalyst produced more NO2.

Reticular Synthesis of Hydrogen-Bonded Organic Frameworks and Their Derivatives via Mechanochemistry.

Rational synthesis of hydrogen-bonded organic frameworks (HOFs) with predicted structure has been a long-term challenge. Herein, by using the efficient, simple, low-cost, and scalable mechanosynthesis, we demonstrate that reticular chemistry is applicable to HOF assemblies based on building blocks with different geometry, connectivity, and functionality. The obtained crystalline HOFs show uniform nano-sized morphology, which is challenging or unachievable for conventional solution-based methods. Furthermore, the one-pot mechanosynthesis generated a series of Pd@HOF composites with noticeably different CO oxidation activities. In situ DRIFTS studies indicate that the most efficient composite, counterintuitively, shows the weakest CO affinity to Pd sites while the strongest CO affinity to HOF matrix, revealing the vital role of porous matrix to the catalytic performance. This work paves a new avenue for rational synthesis of HOF and HOF-based composites for broad application potential.

Reticular Synthesis of Hydrogen‐Bonded Organic Frameworks and Their Derivatives via Mechanochemistry

AbstractRational synthesis of hydrogen‐bonded organic frameworks (HOFs) with predicted structure has been a long‐term challenge. Herein, by using the efficient, simple, low‐cost, and scalable mechanosynthesis, we demonstrate that reticular chemistry is applicable to HOF assemblies based on building blocks with different geometry, connectivity, and functionality. The obtained crystalline HOFs show uniform nano‐sized morphology, which is challenging or unachievable for conventional solution‐based methods. Furthermore, the one‐pot mechanosynthesis generated a series of Pd@HOF composites with noticeably different CO oxidation activities. In situ DRIFTS studies indicate that the most efficient composite, counterintuitively, shows the weakest CO affinity to Pd sites while the strongest CO affinity to HOF matrix, revealing the vital role of porous matrix to the catalytic performance. This work paves a new avenue for rational synthesis of HOF and HOF‐based composites for broad application potential.