Temperature CO Oxidation Research Articles

Lattice expansion and smaller CuOxCeO2−δ particles formation by magnesium interaction for low temperature CO oxidation

A series of magnesia doped CuOxCeO2−δ catalysts were prepared by co-precipitation followed by impregnation method and investigated for CO oxidation. The manuscript is devoted to explaining the role of MgO for the formation of active species on the CuOxCeO2−δ surface. The improvement in catalytic activity is ascribed to the formation of various active species due to the interaction of magnesia with CuOxCeO2−δ. The catalysts were characterized by PXRD, N2 adsorption, H2-TPR, XPS, SEM, EDS and HRTEM techniques. The Mg doped catalyst shows lattice expansion of ceria due to the formation of smaller Ce3+ species with oxygen vacancies. The Mg–O bond also takes part in CO activation and oxidation, which results in the increase of CO oxidation. The Mg doped CuOxCeO2−δ catalyst shows improvement in low temperature activity compared with the CuOxCeO2−δ.

Low temperature CO oxidation by doped cerium oxide electrospun fibers

We investigated CO oxidation behavior of doped cerium oxide fibers. Electrospinning technique was used to fabricate the inorganic fibers after burning off polymer component at 600 °C in air. Cu, Ni, Co, Mn, Fe, and La were doped at 10 and 30 mol% by dissolving metal salts into the polymeric electrospinning solution. 10 mol% Cu-doped ceria fiber showed excellent catalytic activity for low temperature CO oxidation with 50% CO conversion at just 52 °C. This 10 mol% Cu-doped sample showed unexpected regeneration behavior under simple ambient air annealing at 400 °C. From the CO oxidation behavior of the 12 samples, we conclude that absolute oxygen vacancy concentration estimated by Raman spectroscopy is not a good indicator for low temperature CO oxidation catalysts unless extra care is taken such that the Raman signal reflects oxide surface status. The experimental trend over the six dopants showed limited agreement with theoretically calculated oxygen vacancy formation energy in the literature.

A Route to Develop the Synergy Between CeO2 and CuO for Low Temperature CO Oxidation

Highly active nano-composite oxide of cerium-copper was synthesized via physical grinding of respective metal oxides using mortar and pestle for the oxidation of carbon monoxide. The prepared oxides were characterized using X-ray diffraction (XRD), Thermal-gravimetric studies (TG), Infrared spectroscopy (IR), Scanning electron microscopy (SEM) and BET surface area. In addition, CO adsorption and surface reduction–oxidation property was studied through CO pulse titration, H2-TPR and O2-TPO to understand their surface sensitivity towards it. Among the tested catalysts, synergy interaction produced between cerium and copper oxides after grinding them using mortar and pestle leads to an excellent CO to CO2 conversion. High CO chemisorption and an enhanced redox property are the evidence for the synergy exhibited by CeO2–CuO composite catalyst. The grinding route helped in improving the CO oxidation by decreasing the active temperature region for the reaction showed a good correlation with high CO adsorption and increased oxygen mobility at lower temperature gradient over CeO2–CuO composite. Further, good reaction stability was also seen with the addition of moisture in the reaction mixture.

The Catalytic Stability of Au/FeLaO3/Al2O3 Catalyst for Low Temperature CO Oxidation

Alumina modified by LaFeO3 was used as the support of gold (Au) catalysts for low temperature CO oxidation. In dry reactant feed, the catalyst showed catalytic stability similar to that found for the Al2O3 supported Au catalyst. However, the modification of FeLaO3 perovskite significantly improved the catalytic stability of the Au catalyst in the co-presence of gaseous reactants and water vapor. The modification gives the catalyst a double advantage: it enables the presence of moisture that improves the catalytic activity and promotes the formation of larger pores in the modified catalyst that facilitate a timely removal of water vapor from the catalyst. Furthermore, it also prevents the aggregation of gold nanoparticles as well as the deactivation caused by excessive water vapor.

Identification of single-atom active sites in CO oxidation over oxide-supported Au catalysts

Here we present a combined operando infrared spectroscopic and theoretical analysis of ceria-supported gold catalysts during room temperature CO oxidation that identifies an active site for the reaction as a single gold site on the ceria support forming an Olattice–Au+–CO species. As monitored by operando infrared spectroscopy, the isolated Au+ gold site is either present as a result of the catalyst synthesis or formed under reaction conditions after CO adsorption at the perimeter of the Au–ceria interface. Our results provide new insights into the chemical nature of the active gold site and the reaction mechanism by detecting the formation of active and inhibiting species simultaneously.

Origin of the High CO Oxidation Activity on CeO2 Supported Pt Nanoparticles: Weaker Binding of CO or Facile Oxygen Transfer from the Support?

AbstractPt clusters supported on CeO2 are highly active for low temperature CO oxidation. The enhanced reactivity could be caused by weakening of the CO binding on Pt, allowing adsorbed oxygen to more effectively compete for sites. An alternative explanation is that interfacial sites allow adsorbed CO on Pt to react with lattice oxygen in the ceria. Here we explore the origins of enhanced CO oxidation reactivity on Pt/CeO2 using in‐situ/operando infrared and x‐ray spectroscopies, microcalorimetry, and reaction kinetics. We show that CO adsorbs strongly (∼110–120 kJ/mol) on Pt clusters (∼1.5 nm) and Pt is almost fully covered by CO during reaction, indicating that the high activity can be related to reactive interfacial O* species. Using in‐situ infrared spectroscopy we show that when the reaction mixture of CO and O2 stops flowing over the catalyst, the adsorbed CO on Pt is lost since it reacts with interfacial O*, but if this O* is depleted, the CO band does not disappear. The role of CeO2 is not to alter the binding of CO, but rather to enhance the reactivity of the interfacial metal‐support lattice oxygens. The reaction mechanism involving the interfacial Pt−CeO2 sites (two‐site mechanism) is further confirmed by kinetic measurements which show near zero order dependence on CO and O2 partial pressures.

Cerium(III) Nitrate Derived CeO2 Support Stabilising PtOx Active Species for Room Temperature CO Oxidation

AbstractA new approach for preparing a PtOx/CeO2 catalyst for low‐temperature CO oxidation has been developed. The approach includes; i) preparation of a CeO2 support through the controllable thermal decomposition of Ce(NO3)3 ⋅ 6H2O, and then ii) deposition of polynuclear platinum nitrato complexes ([H3O⊂18‐crown‐6]2[Pt2(μ2‐OH)2(NO3)8][Pt4(μ3‐OH)2(μ2‐OH)4(NO3)10]) on the CeO2 support. The NO3−‐rich ceria support produced at the onset temperature of Ce(NO3)3 ‐> CeO2 transformation (220 °C) yields the “PtOx/CeO2‐220” catalyst that shows intriguingly high activity in CO oxidation, at room temperature and below. To understand the nature of the low‐temperature catalytic activity, prepared catalysts have been studied using X‐ray photoelectron spectroscopy (XPS), Raman and operando X‐ray absorption fine structure spectroscopy (XAFS) and transmission electron microscopy (TEM).

Geometric effect of Au nanoclusters on room temperature CO oxidation.

We report the effect of in situ transforming ZnO supported single Au atoms to 3-dimensional Au clusters (Au3D) on room temperature CO oxidation activity. We discovered that an intermediate, highly distorted 2-dimensional Au layer species is much more active than both the Au3D clusters and the Au single atoms. The geometric arrangement of Au clusters and their interactions with support surfaces play a pivotal role in determining their catalytic performance in room temperature CO oxidation on ZnO supported Au catalysts.

Microwave-assisted synthesis of gold nanoparticles supported on Mn3O4 catalyst for low temperature CO oxidation

ABSTRACT In the present work, Mn3O4 was prepared by various methods and successfully loaded with metallic Au nanoparticles reduced by hydrazine hydrate using microwave irradiation (MWI) method. The surface morphology and composition of the prepared samples were characterized with X-ray diffraction (XRD), N2 adsorption–desorption, temperature programmed reduction (H2-TPR), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The experimental results showed that no significant changes in some textural and structural properties of the samples due to preparation method or Au nanoparticles deposition. While the surface composition and reducibility of the samples were greatly affected by preparation method and Au deposition. The CO oxidation reaction over the samples was selected as a model reaction to study the relation between surface properties of the samples and their catalytic performance. The results showed that a direct proportionality exists between the reducibility and the CO oxidation activity of catalysts. The kinetic study of the reaction showed that the reaction is first order. Moreover, the samples exhibited good stability in CO oxidation at 100% conversion for around 30 h under the reaction conditions.

Environmentally benign synthesis of a PGM-free catalyst for low temperature CO oxidation

Dopants enhance the catalytic properties of ceria. However, conventional techniques for synthesizing doped ceria have limitations in terms of structural homogeneity, surface area, and catalytic activity of the resulting oxide. Use of toxic and corrosive chemicals presents further challenges. The sol-gel method described in this work provides a facile approach for incorporating high concentrations of dopants in a uniform, high surface area structure, yielding excellent catalytic activity. Addition of polyvinylpyrrolidone (PVP) complexing agent prevents the segregation of cerium and dopant atoms during synthesis. Surface areas up to 179 m2/g are achieved, which represents a substantial improvement over doped ceria produced through coprecipitation, solution combustion, or melt-synthesis methods. The resulting powders exhibit dramatically improved CO oxidation activity (T90 = 132 °C for 3.2 wt% Cu-CeO2 compared to 274 °C for a 2 wt% Pt-Al2O3 reference catalyst). First principles calculations suggest a Mars Van Krevelen mechanism, which is facilitated by dopants causing oxygen vacancies.

Unravelling the Different Reaction Pathways for Low Temperature CO Oxidation on Pt/CeO2 and Pt/Al2O3 by Spatially Resolved Structure–Activity Correlations

Spatially resolved operando HERFD-XANES (high energy resolution fluorescence detected X-ray absorption near edge structure) complemented by CO concentration gradient profiles along the catalyst bed (SpaciPro) was used to identify the dominant reaction paths for the low and high temperature CO oxidation on Pt/CeO2 and Pt/Al2O3. At low temperatures, features associated with CO adsorption on Pt were found for both catalysts. During the oxidation reaction light-off, the evolution of the spectral and catalytic profile diverged along the catalyst bed. The CO oxidation rate was high on Pt/CeO2 from the beginning of the catalyst bed with CO being adsorbed on Pt, whereas low CO conversion due to strong CO poisoning was found on Pt/Al2O3. This correlation of the CO concentration gradient with unique insight by HERFD-XANES gave direct proof of the crucial contribution of the Pt-CeO2 perimeter sites overcoming the CO self-inhibition effect at low temperatures.

Engineering the epitaxial interface of Pt-CeO2 by surface redox reaction guided nucleation for low temperature CO oxidation

The interface between metal nanoparticles (NPs) and support plays a vital role in catalysis because both electron and atom exchanges occur across the metal-support interface. However, the rational design of interfacial structure facilitating the charge transfer between the neighboring parts remains a challenge. Herein, a guided nucleation strategy based on redox reaction between noble metal precursor and support-surface is introduced to construct epitaxial interfaces between Pt NPs and CeO2 support. The Pt/CeO2 catalyst exhibits near room temperature catalytic activity for CO oxidation that is benefited from the well-defined interface structure facilitating charge transfer from CeO2 support to Pt NPs. Meanwhile, this general approach based on support-surface-induced-nucleation was successfully extended to synthesize Pd and Cu nanocatalysts on CeO2, demonstrating its universal and feasible characteristics. This work is an important step towards developing highly active supported metal catalysts by engineering their interfaces.

Synthesis of highly active Cobalt catalysts for low temperature CO oxidation

Abstract Cobalt-oxide catalysts has shown a huge potential for CO oxidation in a catalytic converter for their high thermal stability and tailoring flexibility. A novel route of reactive calcination (RC) of Co-salts (oxalate = O, oxalate synthesize by reactive grinding = ORG, carbonate basic = C, acetate = A, nitrate = N) for the preparation of highly active catalysts was studied. The route concerned feeding a low concentration of chemically reactive CO–Air mixture over the cobalt salts at a low temperature. The RC of the precursor produced cobalt species (Co3O4 and CoO) in thermodynamic equilibrium, with the major nano-size Co3O4 phase having a large surface area, while applying in a stagnant air at 400 °C resulted in a CoO phase of better crystallites with the poorer surface area. The amazing performances of novel catalysts over conventional ones (obtained by calcination of the precursors in stagnant air and flowing air) in CO oxidation was associated with the presence of Co3O4 and its unusual morphology as evidenced by XRD, SEM-EDX, XPS and FTIR characterization. The catalysts obtained by RC of various precursors showed activities for CO oxidation in the following order: Cat-ORGr>Cat-Or>Cat-Cr> Cat-Ar>Cat-Nr between (60–130 °C) while the traditional route of catalysts followed the same activity order but at the higher temperatures (75–170 °C). Further, the activity order of the catalysts obtained by various calcination conditions was as follows: RC> flowing-air>stagnant-air. The current research focused highly active Co3O4 catalysts would be applied in wide range of reactions for commercial importance, as well as those associated with environmental cleanliness and production of clean energy sources etc.

Enhanced catalytic activity of Au-CeO2/Al2O3 monolith for low-temperature CO oxidation

In this study, hierarchical porous structured CeO2/Al2O3 monolith was prepared through the combination of sol-gel and heating-drying-calcination methods. Subsequently, the Au-CeO2/Al2O3 monoliths of different Au loadings were prepared by the deposition-precipitation method. When the Au3.2%-CeO2/Al2O3 monolith was used for low- temperature CO oxidation, compared with Au3.2%-CeO2/Al2O3 powder particles, excellent activity (temperature of 34 °C for complete conversion) and stability (up to 100 h) for the CO oxidation were achieved. Such an excellent performance was attributed to the unique hierarchical porous structure of monolith, the good oxygen storage capacity of CeO2, as well as the excellent CO adsorption and catalytic properties of the monodisperse Au nanoparticles.

Bimetallic palladium-supported halloysite nanotubes for low temperature CO oxidation: Experimental and DFT insights

The design and fabrication of novel metal-supported catalysts for energy conversion and heterogeneous catalysis is a pivotal theme. Herein, we present the synthesis of bimetallic palladium nanoalloys supported on halloysite nanotubes, PdM@HNTs, where M = Co, Cu, and Ni and their catalytic performance towards CO oxidation. The synthesis procedure involves simple co-reduction of the metals precursors using NaBH4 on halloysite nanotubes support. The synthesized catalysts retain the tubular morphology of halloysite support with surface area of 90–107 m2 g−1. Transmission electron microscopy (TEM) revealed smaller size for bimetallic nanoparticles (6–8 nm) compared to Pd (14 nm). PdNi displayed the highest catalytic activity towards CO oxidation. Moreover, PdCo and PdNi demonstrated enhanced CO oxidation kinetics compared to PdCu and PdNi as revealed from the calculated activation energies. DFT calculations revealed that the order of catalytic activity is PdNi > PdCo > PdCu > Pd which is in agreement with the experimental results and that the adsorption energy of CO2 on the different catalysts has no apparent role in the whole activity of the catalyst.In conclusion, PdM@HNTs catalysts expressed homogeneous distribution for metallic nanoparticles as well as high dispersion and expressed promising potential to be applied in real flare control processes.

Activation mechanisms of silver toward CO oxidation by tungsten carbide substrate: A density functional theory study

The development of efficient catalysts for low temperature CO oxidation is important to the application of fuel cells. In this work, we report that the Ag monolayer on WC (0001) surface (AgML/WC) could effectively catalyze CO oxidation through the L-H mechanism (CO + O2 → OOCO → CO2 + O). The most sluggish reaction step is suggested to be CO + O → CO2 with a barrier of 0.48 eV, which is 1.21 eV lower than the barrier of O2 dissociation. The electronic structure and d-band center analyses demonstrate that the promoted activity may originate from the synergistic effect between Ag monolayer and WC. The present study is conducive to design new efficient and cost-effective catalysts for CO oxidation without using of the noble platinum.

Preparation and optimization of the MnCo2O4 powders for low temperature CO oxidation using the Taguchi method of experimental design

The MnCo2O4 powders were prepared using a coprecipitation method and employed in low temperature CO oxidation. The preparation parameters in the coprecipitation method were optimized by the Taguchi method of experimental design to synthesize the MnCo2O4 catalyst with high catalytic performance in a low temperature CO oxidation reaction. The results showed that the optimized sample indicated a mesoporous structure with a high specific surface area of 108.42 m2/g and a small pore size of 9.44 nm. The obtained results confirmed that the Taguchi method was effective in optimization of the preparation factors. Moreover, the highest CO conversion was observed on the optimized sample by the Taguchi method and this sample possessed a CO conversion of 100% at 150 °C, which was higher than those observed for other samples at the same temperature.

Catalytic behaviour of Mn2.94M0.06O4-δ (M=Pt, Ru and Pd) catalysts for low temperature water gas shift (WGS) and CO oxidation

Noble metal (Pt, Ru and Pd) substituted Mn3O4 catalysts have been synthesized in this work by a sonochemical route. The catalysts were characterised by XRD, XPS, TEM, H2-TPR and BET surface area analyser and the activity of these catalysts was tested towards low temperature water gas shift reaction (WGS) and CO oxidation reaction. It was observed that these catalysts have the tetragonal crystalline structure of Mn3O4 and the average particle size was found in the range of 12 nm–22 nm. H2-TPR results show that the strong metal support interaction between substituted metal and Mn3O4 leads to high reducibility and makes these catalysts active for WGS and CO oxidation. Pt substituted Mn3O4 showed higher activity towards WGS compared to other synthesized catalysts and 99.9% conversion was observed at 260 °C without methane formation. The activation energy of Mn2.94Pt0.06O4-δ was found to be 59 ± 0.6 kJ/mol. DRIFTS analysis was carried out to propose the reaction mechanism for water gas shift and CO oxidation. Redox mechanism was hypothesized for WGS and used to correlate the experimental data over Pt substituted Mn3O4. Similarly, kinetic parameters were estimated based on Langmuir-Hinshelwood mechanism for CO oxidation over Pd substituted Mn3O4 which showed better activity compare to other synthesized catalysts and 99.9% conversion was observed at 175 °C. The activation energy was calculated from Arrhenius plot which was found to be 30 ± 0.4 kJ/mol.

Defect-induced efficient CO elimination over nonstoichiometric ZnO Nanocrystals supported Pd catalyst

Oxalate was selected as a precursor to generate nonstoichiometric ZnO through a controlled pyrolysis protocol. The decomposition and combustion of the oxalate under air not only produced a large number of oxygen vacancies but also resulted in mesostructured ZnO. After deposition of Pd nanoparticles through in situ reduction, the resulting composites showed defect-dependent catalytic activity for low temperature CO oxidation. Under a space velocity of 15 000 ml·h−1·gcat−1, 1.0 vol% CO could be totally converted at 0 °C for 5.0 wt% Pd loaded catalyst under ambient condition. Even for the lower noble-metal loading catalyst (1.0 wt% Pd/ZnO), T50 was 36 °C, much lower than that containing less defect.

Low temperature CO oxidation on cerium dioxide nanorods

CeO2 is a highly efficient catalyst for low temperature oxidation of CO owing to its non-toxicity and fast Ce4+/Ce3+ redox cycle. In this research work, needle-like cerium dioxide nanorods were prepared by hydrothermal method using cerium nitrate in an alkaline solution. The diameters of the nanorods were about 10 nm and had lengths of up to several micrometers. The unique morphology of ceria prevents the nanoparticles from agglomeration and significantly improves the catalytic activity for CO oxidation.