High CO Oxidation Activity Research Articles

Temperature-dependent reaction pathways of CO oxidation and the application as monolithic catalysts for Co3O4 nanorods

Via investigating the evolution of carbonate intermediates by in-situ DRIFTS (25 °C–350 °C) analysis, temperature-dependent reaction pathways of CO oxidation over Co3O4 nanorods are successfully proposed. The types of reactive oxygen species participated in CO oxidation may affect the way CO react with neighboring oxygen atoms. Additionally, the negative effect of water and CO2 on the catalytic activity was also studied. Considering the remarkable reactivity towards CO, the Co3O4 catalysts were integrated onto metallic and cordierite monolithic substrates and used for CO conversion as application. SEM examinations show that the catalyst layer was coated on the substrates uniformly with few microcracks, and desirable mechanical shock and thermal shock resistance was observed. The influence of substrate materials, gas hourly space velocity, the existence of water vapor and CO2, and reaction time on the CO oxidation performance of monolithic catalysts was investigated. Metallic monolithic catalysts performed higher CO oxidation activity mainly owing to superior heat conductivity and larger surface area compared with cordierite monolithic catalysts. It is noticeable that superior activity was also achieved under high gas flow with the existence of H2O (5.6 vol.%) and CO2 (7 vol.%) at 220 °C, suggesting great promise for practical application.

CO Oxidation over Au/ZnO: Unprecedented Change of the Reaction Mechanism at Low Temperature Caused by a Different O2 Activation Process

Au/ZnO prepared by coprecipitation exhibited extremely high catalytic activity for low-temperature CO oxidation below room temperature. The catalytic activity was influenced by the reduction atmosp...

The effect of reactants adsorption and products desorption for Au/TiO2 in catalyzing CO oxidation

Supported gold catalysts exhibit very high activity for CO oxidation. However, there is lack of systematic study for supports effect on reactants adsorption and products desorption. Herein, we successfully prepared supported gold catalysts on TiO2 matrix exposing either {1 0 1} or {0 0 1} crystal plane (labeled as Au/TiO2-1 0 1 and Au/TiO2-0 0 1 respectively) by deposition-precipitation (DP) method. It is found that the reactivity of Au/TiO2-1 0 1 was much higher than Au/TiO2-0 0 1 in CO oxidation reaction (9.33 × 10−3 molCOgAu−1 s−1vs. 6.23 × 10−4 molCOgAu−1 s−1 at 30 °C). The gold nanoparticles (∼2 nm) were detected as sole gold species in Au/TiO2-1 0 1, while not only gold nanoparticles (∼4 nm) but also clusters (∼1.3 nm) were present in Au/TiO2-0 0 1. The difference in gold status induced different CO adsorption situations that CO was more effectively adsorbed on gold nanoparticles in Au/TiO2-1 0 1 which existed more Au0 site. Meanwhile, we revealed that the bidentate carbonate, which could either block active sites for the reaction or hinder access of reactants, were adsorbed much more strongly on TiO2-0 0 1. While on TiO2-1 0 1, bicarbonate was the main surface species which was easier decomposed to form CO2. Therefore, the differences between Au/TiO2-1 0 1 and Au/TiO2-0 0 1 in the efficiency of CO adsorption and the ability of CO2 desorption resulted in disparate reactivity. We reasonably gave the deep insight into the support effect that was intrinsically related to the adsorption and desorption behavior of surface species on the catalyst.

Promoted oxygen activation of layered micro-mesoporous structured titanium phosphate nanoplates by coupling nano-sized δ-MnO2 with surface pits for efficient photocatalytic oxidation of CO

It is highly desired to fabricate efficient micro-mesoporous structured photocatalysts with strong oxygen activation ability for CO oxidation. Herein, a layered micro-mesoporous structured titanium phosphate (L-MMS-TiP) with high photocatalytic activity for CO oxidation has been successfully synthesized by a two-step solvothermal method. The photocatalytic activity of resulting L-MMS-TiP could be greatly improved by coupling a proper amount of nano-sized δ-MnO2, especially for that with surface pits. The improved activities are mainly attributed to the roles of coupled δ-MnO2 in enhancing photogenerated charge separation and promoting oxygen activation, which is revealed based on measurements of the steady-state surface photovoltage spectra, nitroblue tetrazolium absorption spectra, electron paramagnetic resonance spectra and in situ DRIFTS spectra. Interestingly, the photocatalytic activity of pitted δ-MnO2-coupled L-MMS-TiP is superior to that of P25 TiO2, even much higher than that of Au-coupled L-MMS-TiP. Significantly, δ-MnO2-coupled L-MMS-TiP also has excellent oxidation removal effect for other harmful or even mixed gases. Moreover, it is confirmed that δ-MnO2 with the two-dimensional structure is the best one to couple L-MMS-TiP for photocatalytic CO oxidation among all four kinds of crystal phases and compared with other transitional metal oxides. This work opens up a new way to develop a layered micro-mesoporous structure-based nanocomposite, and further improve the photocatalytic activity by promoting oxygen activation with two-dimensional pitted δ-MnO2.

Au/CeO2 nanorods modified by TiO2 through a combining dealloying and calcining method for low-temperature CO oxidation

TiO2/Au/CeO2 catalysts with different TiO2 contents were successfully synthesized through combining a dealloying Al-Ce-Ti-Au precursor alloy and calcining in O2. XRD, SEM, HRTEM, TEM-EDX mapping, N2 adsorption-desorption, H2-TPR, O2-TPD, Raman spectroscopy, XPS, and in situ DRIFTS were used to characterize the structural phases, microstructures, and physicochemical properties of the samples. The results revealed that the TiO2/Au/CeO2 catalysts prepared from Al89.7Ce9.5Ti0.5Au0.3 had a structure in which Au and TiO2 nanoparticles were loaded onto the surfaces of CeO2 nanorods in a porous framework, resulting in a large specific surface area and a small pore size. The results of an experimental test of the catalytic oxidation of CO indicated that the TiO2/Au/CeO2 catalysts prepared from Al89.7Ce9.5Ti0.5Au0.3 showed the highest activity for CO oxidation. The temperature required for 50% CO conversion was as low as 5 °C and 98% CO conversion was achieved at 40 °C. The catalysts were stable even after 100 h of catalytic oxidation. The introduction of a small amount of TiO2 into the Au/CeO2 is thought to have created an increased number of oxygen vacancies and an increased concentration of Auδ+ species. This leads to stronger interactions between the Au and CeO2 and more active oxygen species at the surface, which enhances the catalytic activity. The present work further develops catalytic materials with high performance and low cost.

Sorption-enhanced CO capture over Cu-Mn-Ce composite oxides with LiOH addition: CO oxidation and in-situ CO2 sorption

CuO-MnOx-CeO2 composite catalysts coupled with different amounts of LiOH sorbent were prepared using a solid-state impregnation method and characterized by XRD, N2 adsorption, FE-SEM/EDS and ICP-OES. Then the samples were tested dynamically and isothermally for CO capture involving CO oxidation and CO2 sorption reactions in the temperature range of 30–180 °C under the dry and wet conditions. The results showed that the samples were able to catalytically oxidize CO and chemically absorb CO2 in situ at low temperature. Particularly, CuMnCe/10LiOH exhibited the higher CO oxidation activity than the traditional CuO-MnOx-CeO2 catalyst, and significantly the resistance to water vapor of the sample was also promoted. Isothermal CO2 capture experiment results further revealed that CuMnCe/LiOH could achieve the larger CO2 sorption efficiency by the in-situ way when compared with trapping CO2 diluted in the feed gas. The promoted CO catalytic performance with the excellent tolerance to water poison, and the higher CO2 sorption capacity and efficiency, could be attributed to the synergistic interaction between the CO catalyst and the CO2/H2O sorbent induced by the sorption and transfer of the intermediate gases CO2 and H2O, which modified the influences of two-sidedness CO2 and H2O gases on CO oxidation and CO2 sorption reactions, respectively. These were summarized as a novel concept called Sorption-Enhanced CO Capture (SECC).

Sulfur resistance of Ca-substituted LaCoO3 catalysts in CO oxidation

La1-xCaxCoO3 perovskite catalysts were prepared by using citrate sol-gel method with the various calcium ratios (denoted as La1-xCaxCoO3 (x = 0, 0.1, 0.2, 0.3, 0.4)). The prepared catalysts were poisoned with simulated gas including SO2 to investigate the sulfur resistance of the catalysts. The fresh and sulfur-aged catalysts were then applied to CO oxidation reaction. X-ray diffraction analysis revealed that the catalysts containing calcium also showed perovskite structure. La0.8Ca0.2CoO3 showed the highest CO oxidation activity among the Ca-substituted LaCoO3 catalysts. H2 temperature-programmed reduction indicated that the addition of Ca to LaCoO3 promotes the facile reduction of Co3+, which can explain the higher activity for CO oxidation. Surprisingly, small amounts of SO2 poisoning were observed to promote CO oxidation in Ca-substituted catalysts, although further exposure to SO2 gave rise to the decline of catalytic activity. In addition, Ca-substituted catalysts had higher catalytic activity than LaCoO3 after poisoning with small amounts of sulfur. XPS analysis indicated that Ca-substituted catalysts possessed the less amount of sulfate on the surface than LaCoO3 upon SO2 poisoning. It was summarized that La1-xCaxCoO3 perovskite catalysts had stronger resistance to sulfur poisoning and higher catalytic activity for CO oxidation.

Bulk single crystal growth and sample surface preparation of catalytic NaAu2

Here in we have grown bulk single crystals of NaAu2 for the first time to enable surface studies on the nature of the (111) bulk surface. This intermetallic compound exhibits surprisingly high catalytic activity for CO oxidation, a benchmark reaction. Theory predicts NaAu2 to be the most thermodynamically stable composition in the Na-Au binary phase diagram and NaAu2 has been seen to preferentially form in experiments containing Na and Au which supports this prediction. The (111) surface was also predicted to be the most stable and is nearly bulk-terminated making single crystal samples prepared with this crystallographic orientation a fitting choice. The crystal quality and surface composition of the metallographically prepared surface was determined by x-ray diffraction methods in addition to optical and electron microscopy.

Mn-doped CuO[sbnd]Co3O4[sbnd]CeO2 catalyst with enhanced activity and durability for hydrocarbon oxidation

A ternary oxide catalyst composed of CuOCo3O4CeO2 (CCC) has great potential for automotive exhaust treatment due to its excellent CO oxidation activity and high resistance against hydrocarbon inhibition. However, current CCC catalyst exhibits significantly lower hydrocarbon oxidation activity comparing to CO oxidation, and deactivation occurs severely after hydrothermal aging. Here, we show that a Mn-doped CCC (CCCM) catalyst exhibits enhanced activity and durability for C3H6 oxidation, while maintaining the high activity for CO oxidation. The Mn-doping prevented sintering and phase separation of the Co3O4 phase even after hydrothermal aging at 750 °C for 25 h. The Mn was selectively doped into Co3O4 by substituting Co3+ octahedral sites in the Co3O4 spinel structure. The presence of Mn with a larger cation radius induced lattice expansion of the Co3O4 phase with more surface active oxygen. This strategy can provide a facile way to develop better non-PGM catalysts for emission control.

CeO2 supported low-loading Au as an enhanced catalyst for low temperature oxidation of carbon monoxide

A series of low loading and high activity Au/CeO2 supported catalysts were synthesized using a coprecipitation method. Au/CeO2 catalysts with a low Au content (0.2 wt%) showed extremely high activity for CO oxidation with 100% conversion of CO around 60 °C.

Identification of the active complex for CO oxidation over single-atom Ir-on-MgAl2O4 catalysts

Supported single atoms provide an opportunity to design new heterogeneous catalysts while optimizing the utilization of noble metals. However, identification of the active single-atom structure is required for understanding the reaction mechanism and guiding catalyst design. Here, we use in situ infrared spectroscopy, operando X-ray absorption spectroscopy and quantum chemical calculations to identify the active single-atom complex as well as the resting state of the Ir/MgAl2O4 catalysts during the low-temperature CO oxidation. In contrast to poisoning of iridium nanoparticles by CO, here we show that the formation of Ir(CO) on single atoms results in a different reaction mechanism and high activity for low-temperature CO oxidation. This is due to the ability of single atoms to coordinate with multiple ligands, where Ir(CO) provides an interfacial site for facile O2 activation between Ir and Al and lowers the reaction barrier between gas-phase CO(g) and *O in Ir(CO)(O) through an Eley–Rideal mechanism. Single-atom catalysts are receiving much attention, but insights into their active sites or the differences in reactivity with conventional nanoparticles are still controversial. Now, operando studies on CO oxidation with Ir/MgAl2O4 accompanied by computational investigations reveal important features of this class of catalyst.

Synthesis and characterization of new isostructural series LnFe0.5Sb1.5O6 (Ln = La-Sm) exhibiting high catalytic activity in CO oxidation

The existence of the LnIIIFeIII0.5SbV1.5O6 (Ln = La-Sm) series has been established for the first time. These compounds were prepared using the co-precipitation method with subsequent annealing. The systematic investigation, which includes phase equilibria study of the Ce2Ox–Fe2O3–Sb2Oy system, XRD, XANES, Rietveld refinement method and magnetic measurements has been carried out. It was shown that these ternary oxides are realized in PbSb2O6 structure type (sp. g. P-31 m) and contain RE and Fe ions in +3 oxidation state. The comparative study of the morphology showed that the composition of the compounds do not affect the shape of the obtained quasi-spherical particles while the size decreases from 400 up to 100 nm in LaSm series. These antimonates exhibit high activity and stability in CO oxidation. The best result was obtained for LaFe0.5Sb1.5O6 catalyst. The temperature of 90% CO conversion increases from 272 up to 426 °C in the series La > Ce > Pr > Nd > Sm.

Fabrication of Ag/La(OH)3 Nanorod Framework Composites Through Dealloying for CO Oxidation

Ag/La(OH)3 nanorod framework composites were fabricated by dealloying melt-spun Al-La-Ag alloys for CO oxidation. After the removal of Al from the precursory alloys, La(OH)3 nanorods bonded with each other to form a novel three-dimension (3D) architecture with nanopores. When the dealloyed samples were calcined at 300°C, Ag nanoparticles (NPs) were in situ supported on the surface of the nanorods and simultaneously contact interfaces were generated between the Ag NPs and the La(OH)3 nanorods. The large specific surface areas, unique 3D framework structure and active OH radicals were believed to contribute to the improvement of the catalytic activity for CO oxidation. The catalytic performance of Ag/La(OH)3 composite was highly dependent on the Ag content and calcination temperature. CO oxidation studies showed that the nanorod composite calcined at 300°C exhibited higher activity for CO oxidation in comparison with the uncalcined sample and pure La(OH)3 nanorods. The Ag/La(OH)3 nanorod framework should be a promising catalyst for CO catalytic oxidation owing to the combination of novel nanostructures and functionalized particles.

Supported gold cluster catalysts prepared by solid grinding using a non-volatile organogold complex for low-temperature CO oxidation and the effect of potassium on gold particle size

We have found that physical mixing (solid grinding, SG) of dimethylgold(III) acetylacetonate (Me2Au(acac)) adsorbed on SiO2 with potassium t-butoxide (KOtBu) significantly improved the catalytic activity for CO oxidation at low temperature, and T1/2 (the temperature at 50% CO conversion) became extremely low such as −43 °C. We have also developed an alternative Au complex for SG, and 2-(p-tolyl)pyridinebis(trifluoroacetate)gold(III) (AuTFA) could be adsorbed on metal oxide (MOx) supports by physical mixing of AuTFA with MOx in air at room temperature for 20 min, and small Au nanoparticles (NPs) were deposited after calcination. The sizes of Au NPs were almost the same as those prepared using Me2Au(acac) and smaller than those prepared by DP for Al2O3 and TiO2 supports. In addition, grinding AuTFA adsorbed on SiO2 with KOtBu was also effective for reducing the size of Au particles and gave Au clusters with a mean diameter of 2.2 nm. Moreover, Au/Al2O3 and Au/K-SiO2 exhibited high catalytic activity for CO oxidation under simulated real exhaust gas conditions (low CO concentration and high humidity up to 100% relative humidity) at ambient temperature.

Surface Engineering Protocol To Obtain an Atomically Dispersed Pt/CeO2 Catalyst with High Activity and Stability for CO Oxidation

Atomically dispersed metal catalysts often exhibit superior performance compared to that of nanoparticle catalysts in many catalysis processes. However, these so-called “single-atom” catalysts have a consistently low loading density on the support surface and easily aggregate at high temperatures, hindering their practical application. Herein, we demonstrate a facile surface engineering protocol using molecule–surface charge transfer adducts to fabricate highly stable noble metal catalysts with atomic dispersion, using a Pt/CeO2 catalyst as an example. The key of this approach is the generation of an adequate amount of Ce3+ defective sites on the porous CeO2 surface through the adsorption of reductive ascorbic acid molecules and a subsequent surface charge transfer process. Subsequently, noble metal Pt atoms can be well-dispersedly anchored onto the generated Ce3+ sites of porous CeO2 nanorods with a loading density of up to 1.0 wt %. The as-prepared highly dispersed Pt/CeO2 catalyst showed outstanding ca...

Structural origin of high catalytic activity for preferential CO oxidation over CuO/CeO2 nanocatalysts with different shapes

Understanding how the shape of a nanocatalyst alters its catalytic performance is essential for the design of active catalysts at the nanoscale. This report describes novel findings of an investigation of CuO/CeO2 nanocatalysts with different shapes for preferential CO oxidation (CO-PROX), aiming at unraveling the structural origin of the high catalytic activity. CuO/CeO2 catalysts with a series of shapes, including octahedron, rod, cube, sphere and spindle morphologies, were synthesized by a combination of hydrothermal and impregnation methods. By probing the structure, elementary valence and reductivity of the catalysts using an array of techniques, an intriguing structure-activity synergy is revealed, pinning the origin of the highest catalytic activity to the intersection of the nanocrystal planes. The CuO/CeO2 catalysts with spheres and spindles featuring exposed {111} and {002} crystal planes of CeO2 were found to exhibit the lowest reaction temperature in terms of 50% CO conversion and a wider temperature window for the complete CO conversion in CO-PROX. The catalytic synergy reflects a combination of high surface area, reduced state of copper sites, high oxygen vacancies, as well as stronger interaction between CuO and CeO2. This synergy was further substantiated by analysis of the results based on DFT calculation, showing that the oxygen vacancies were the highest at the intersection of {111} and {002} facets, but the lowest on CeO2 {111} surface. The finding of {111} and {002} facets with an increased oxygen vacancy and a stronger metal-support interaction for improving the catalytic performance has important implications for designing facet-tunable nanocatalysts for efficient PROX.

Ozone-assisted catalysis of CO: In situ Fourier transform IR evidence of the cooperative effect of a bimetallic Ag-Pd catalyst

The room-temperature oxidation of CO using ozone-assisted catalysis (OAC) over monometallic or bimetallic catalysts supported on γ-Al2O3 is presented. Within the tested particle size range, ozone decomposition was insensitive to the size of the Ag particles (6.6–11.9 nm) at weight hourly space velocities of up to 360 L·g−1·h−1. Among the tested monometallic catalysts (Ag Pd, Fe, Mn, and Cu), Ag showed the highest activity for CO oxidation. Bimetallic catalysts (Ag-M or Pd-M where M = Cu, La, Ru, and Fe) were also considered by adding a second metal or perovskite oxide (LaFeO3) to Ag. A cooperative effect was observed with the bimetallic Ag-Pd catalyst for the OAC of CO oxidation at room temperature, whereas the other bimetallic catalysts showed slightly lower performance compared to the monometallic catalyst. A high ozone utilization efficiency of 0.94 was achieved with the bimetallic Ag-Pd/γ-Al2O3 catalyst. High-angle annular dark-field scanning tunneling electron microscopy (HAADF) and energy dispersive X-ray (EDX) spectroscopy measurements confirmed the proximity of the two components, which is essential for their interaction. The in situ FTIR measurements revealed that the cooperative effect in the bimetallic Ag-Pd catalyst involved modification of CO adsorption and the suppression of product accumulation. Two typical IR absorption bands of linear- (2090 cm−1) and bridge-CO (1918 cm−1) on Pd disappeared in the presence of Ag nanoparticles. Thus, the presence of Ag in contact with Pd inhibited the accumulation of carbonates, which led to enhanced catalytic performance. Adsorption site-dependent CO oxidation on Pd was also confirmed by the in situ Fourier transform IR measurement.

Functional MXene Materials: Progress of Their Applications.

Nowadays, two-dimensional materials have many applications in materials science. As a novel two-dimensional layered material, MXene possesses distinct structural, electronic, and chemical properties; thus, it has potential applications in many fields, including battery electrodes, energy storage materials, sensors, and catalysts. Up to now, more than 70 MAX phases have been reported. However, in contrast to the variety of MAX phases, the existing MXene family merely includes Ti2 C, Ti3 C2 , (Ti1/2 , Nb1/2 )2 C, (V1/2 , Cr1/2 )3 C2 , Nb2 C, Ti3 CN, Ta4 C3 , V2 C, and Nb4 C3 . Among these materials, the Ti3 C2 Tx MXene exhibits prominently high volumetric capacitance, and the rate at which it transports electron is suitable for electrode materials in batteries and supercapacitors. Hence, Ti3 C2 Tx is commonly utilized as an electrode material in ion batteries such as Li+ , Na+ , K+ , Mg2+ , Ca2+ , and Al3+ batteries. What is more, Ti2 C has the biggest specific surface area among all of these potential MXene phases, and therefore, Ti2 C has remarkably high gravimetric hydrogen storage capacities. In addition, Ti2 CO2 materials display extremely high activity for CO oxidation, which makes it possible to design catalysts for CO oxidation at low temperatures. Furthermore, Ti3 C2 Tx with O, OH, and/or F terminations can be used for water purification owing to excellent water permeance, favorable filtration ability, and long-time operation ability. This review supplies a relatively comprehensive summary of various applications of MXenes over the past few years.

Understanding the Intrinsic Surface Reactivity of Single-Layer and Multilayer PdO(101) on Pd(100)

We investigated the intrinsic reactivity of CO on single-layer and multilayer PdO(101) grown on Pd(100) using temperature-programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) experiments, as well as density functional theory (DFT) calculations. We find that CO binds more strongly on multilayer than single-layer PdO(101) (∼119 kJ/mol vs 43 kJ/mol), and that CO oxidizes negligibly on single-layer PdO(101), whereas nearly 90% of a saturated layer of CO oxidizes on multilayer PdO(101) during TPRS experiments. RAIRS further shows that CO molecules adsorb on both bridge-Pdcus and atop-Pdcus sites (coordinatively unsaturated Pd sites) of single-layer PdO(101)/Pd(100), while CO binds exclusively on atop-Pdcus sites of multilayer PdO(101). The DFT calculations reproduce the much stronger binding of CO on multilayer PdO(101), as well as the observed binding site preferences, and reveal that the stronger binding is entirely responsible for the higher CO oxidation activity o...

Synthesis of Co3O4 nanoparticles with controllable size and their catalytic properity

Co3O4 nanoparticles of controllable size were synthesized via a variant microemulsion method combined with hydrothermal treatment. With this improved method, Co3O4 nanoparticles with uniform shape and narrow size distribution were controllably synthesized. By varying the surfactant and water ratio, particles of around 27.97 nm, 42.88 nm and 48.99 nm were synthesized. These nanoparticle catalysts behaved differently at low and high temperature. These samples showed high activity for CO oxidation and 100% CO conversion was achieved at 30 °C. However, the catalysts gradually lost their activity after about 100 min. Our research found that CO2 had a huge influence on the stability of catalysts at low temperature. The relationship between stability at 30 °C and ratio of Co3+/Co2+, as well as particle size and desorption rate of CO2, was also thoroughly investigated in this work.