Catalytic Oxidation Of CO Research Articles

Tuning Single‐atom Pt1−CeO2 Catalyst for Efficient CO and C3H6 Oxidation: Size Effect of Ceria on Pt Structural Evolution

AbstractTo reveal the effect of ceria particle size on dispersion and structure of supported Pt catalysts during preparation, activation, and reaction testing, a unique CeO2/Al2O3 support (CA−c) with smaller CeO2 particle size and more surface defects was prepared using a colloidal CeO2 precursor, comparing with a conventional CeO2/Al2O3 support (CA−n) using cerium nitrate as precursor. More atomically dispersed Pt and abundant Pt−O−Ce structures were observed in the Pt/CA−c catalyst than in the Pt/CA−n catalyst. Both parent catalysts received significant enhancement on their catalytic CO oxidation activities if activated by 10% hydrogen at 400 °C before reaction. Between the two representative catalysts, the extent of activity enhancement upon activation was more pronounced for Pt/CA−c. We found that smaller Pt clusters with more active ionic Pt sites were generated on the activated Pt/CA−c catalyst, while agglomerated larger Pt particles with more metallic sites were formed on the activated Pt/CA−n. The facile formation of Ce3+ was also indicative of more active metal‐support interfaces in the activated Pt/CA−c catalyst. These results highlight the importance of regulating ceria support particles to enable a controlled anchoring and subsequent activation of Pt single atoms for low‐temperature CO oxidation reaction.

Modeling and optimization of synthesis condition of CuCr2O4 Spinel for CO oxidation

In this paper, the CuCr2O4 spinel catalyst was synthesized by the Pechini method, and its activity was evaluated in catalytic oxidation of CO. CuCr2O4 spinel catalyst was characterized by XRD, BET, H2-TPR, and SEM. This catalyst has a good ability in CO oxidation. The effects of three synthesis variables (EG/citrate, citrate/nitrate ratio, and calcination temperature) and reaction temperature as an operational variable on CO conversion were investigated. Based on the results, the optimum neural network architecture succeeded to predict CO conversion data with an acceptable level of accuracy. The model predicted that the relative importance of variables is as follows: calcination temperature > citrate/nitrate ratio > EG/citrate ratio. The optimum neural network architecture was used as a fitness function for the genetic algorithm to find the optimum catalyst. Under the optimum condition (EG/citrate: 3.24, citrate/nitrate ratio: 0.62 and calcination temperature: 620 °C), the predicted optimum value of CO conversion was found to be in a good agreement with the corresponding actual value.

Tracking the formation, fate and consequence for catalytic activity of Pt single sites on CeO2

Platinum single sites are highly attractive due to their high atom economy and can be generated on CeO2 by an oxidative high-temperature treatment. However, their location and activity are strongly debated. Furthermore, reaction-driven structural dynamics have not been addressed so far. In this study, we were able to evidence platinum-induced CeO2 surface restructuring, locate platinum single sites on CeO2 and track the variation of the active state under reaction conditions using a complementary approach of density functional theory calculations, in situ infrared spectroscopy, operando high-energy-resolution fluorescence detected X-ray absorption spectroscopy and catalytic CO (as well as C3H6 and CH4) oxidation. We found that the onset of CO oxidation is linked to the migration of platinum single sites from four-fold hollow sites to form small clusters containing a few platinum atoms. This demonstrates that operando studies on single sites are essential to assess their fate and the resulting catalytic properties. Single-atom catalysts hold great promise for process optimization by reducing metal utilization. However, their structure–activity properties remain elusive. Here, a combination of operando techniques and density functional theory analysis is used to capture the evolution of single platinum atoms on CeO2 during CO, C3H6 and CH4 oxidation.

Cu/CeO2 as efficient low-temperature CO oxidation catalysts: effects of morphological structure and Cu content

Cu loaded on different morphologies of CeO2 were synthesized and tested by SEM and CO catalytic oxidation experiment, the results indicated that with the same Cu load, nanorod-like Cu/CeO2 performed the best catalytic activity than 3D flower-like Cu/CeO2 and gear-like Cu/CeO2. Then nanorod-like Cu/CeO2 were chose to explore the best Cu load on CeO2 nanorods for CO oxidation. Cu/CeO2 nanorods were characterized by TEM, XRD, XPS as well as physical and chemical adsorption. The results indicate that 0.15Cu/CeO2 nanorods have the best catalytic activity because of more reducing copper species (Cu+), adsorbed oxygen (Oads) and Ce3+ species on the catalysts surface, which can achieve 99% CO conversion at 100 °C. The effect of CO2 and water vapor on catalytic activity was also examined.

Three-dimensional strain dynamics govern the hysteresis in heterogeneous catalysis

Understanding catalysts strain dynamic behaviours is crucial for the development of cost-effective, efficient, stable and long-lasting catalysts. Here, we reveal in situ three-dimensional strain evolution of single gold nanocrystals during a catalytic CO oxidation reaction under operando conditions with coherent X-ray diffractive imaging. We report direct observation of anisotropic strain dynamics at the nanoscale, where identically crystallographically-oriented facets are qualitatively differently affected by strain leading to preferential active sites formation. Interestingly, the single nanoparticle elastic energy landscape, which we map with attojoule precision, depends on heating versus cooling cycles. The hysteresis observed at the single particle level is following the normal/inverse hysteresis loops of the catalytic performances. This approach opens a powerful avenue for studying, at the single particle level, catalytic nanomaterials and deactivation processes under operando conditions that will enable profound insights into nanoscale catalytic mechanisms.

Mechanistic study of the CO oxidation reaction on the CuO (111) surface during chemical looping combustion

Cu-based oxides oxygen carriers and catalysts are found to exhibit attractive activity for CO oxidation, but the dispute with respect to the reaction mechanism of CO and O2 on the CuO surface still remains. This work reports the kinetic study of CO oxidation on the CuO (111) surface by considering the adsorption, reaction and desorption processes based on density functional theory calculations with dispersion correction (DFT-D). The Eley–Rideal (ER) CO oxidation mechanism was found to be more feasible than the Mars-van-Krevelen (MvK) and Langmuir–Hinshelwood (LH) mechanisms, which is quite different from previous knowledge. The energy barrier of ER, LH, and MvK mechanisms are 0.557, 0.965, and 0.999 eV respectively at 0 K. The energy barrier of CO reaction with the adsorbed O species on the surface is as low as 0.106 eV, which is much more active in reacting with CO molecules than the lattice O of CuO (111) surface (0.999 eV). A comparison with the catalytic activity of the perfect Cu2O (111) surface shows that the ER mechanism dictates both the perfect Cu2O (111) and the CuO (111) surface activity for CO oxidation. The activity of the perfect Cu2O (111) surface is higher than that of the perfect CuO (111) surface at elevated temperatures. A micro-kinetic model of CO oxidation on the perfect CuO (111) surface is established by providing the rate constants of elementary reaction steps in the Arrhenius form, which could be helpful for the modeling work of CO catalytic oxidation.

Role of Spin in the Catalytic Oxidation of CO by N2O Enabled by Co+: New Insights from Temperature-Dependent Kinetics and Statistical Modeling.

The catalytic oxidation of CO by N2O promoted by Co+ was studied as a function of temperature in a variable-ion source temperature-adjustable selected-ion flow tube (VISTA-SIFT). Each step of the cycle, Co+ + N2O and CoO+ + CO was studied individually for unambiguous interpretation of the results. The rate constant of CoO+ + CO is (1.5 ± 0.4) × 10-10 × (T/300 K)-0.7±0.2 cm3 s-1 is in disagreement with a previously reported upper limit of 10-13 cm3 s-1, with the discrepancy likely due to the earlier report having studied the reactions in tandem. The reaction of Co+ + N2O produces CoO+ with a much smaller rate constant of 1.4 ± 0.4 × 10-12 cm3 s-1 at 300 K. The association product, Co(N2O)+, was also produced with a rate constant of 1.6 × 10-28 cm6 s-1. While the rate constant for termolecular association decreased with temperature in accordance with a decreasing time scale for stabilization, the production of CoO+ increased with temperature in a manner that is not well described by simple functional forms. Statistical modeling of calculated reaction coordinates was employed and the experimental data reproduced only by assuming an intersystem crossing to yield ground state CoO+ occurring competitively with the spin-allowed formation of excited state CoO+.

The Sintering of Au Nanoparticles on Flat {100}, {111} and Zigzagged {111}-Nanofacetted Structures of Ceria and Its Influence on Catalytic Activity in CO Oxidation and CO PROX

The thermal stability of Au nanoparticles on ceria support of various morphology (nanocubes, nanooctahedra, and {111}-nanofacetted nanocubes) in oxidizing and reducing atmospheres was investigated by electron microscopy. A beneficial effect of the reconstruction of edges of ceria nanocubes into zigzagged {111}-nanofacetted structures on the inhibition of sintering of Au nanoparticles was shown. The influence of different morphology of Au particles on various ceria supports on the reducibility and catalytic activity in CO oxidation, and CO PROX of Au/ceria catalysts was also investigated and discussed. It was shown, that ceria nanocubes with flat {110} terminated edges are more suitable as a support for Au nanoparticles, used to catalyze CO oxidation, than zigzagged {111}- nanofacetted structures.Graphic

Kinetic and mechanistic study of CO oxidation over nanocomposite Cu−Fe−Al oxide catalysts

AbstractThe oxidation of CO has been studied over Fe−Al and Cu−Fe−Al oxide nanocomposite catalysts prepared by melting of copper, iron, and aluminum nitrates. It was shown that the addition of copper significantly increases the catalytic activity of the Fe−Al nanocomposites. The catalysts were characterized by low‐temperature nitrogen adsorption, X‐ray diffraction (XRD), and X‐ray photoelectron spectroscopy (XPS). It was found that the catalysts contain Fe2O3 with the hematite structure modified by aluminum. Copper in the three‐component catalyst is in the Cu2+ state, forming CuO and CuFeOx clusters on the catalyst surface. An increase in the copper content leads to the formation of a CuxAlyFe3‐x‐yO4 spinel phase. In situ XPS study showed that a treatment of the catalysts in a CO flow leads to the reduction of both copper and iron cations into the metallic state. In contrast, a treatment in a CO/O2 flow leads only to partial reduction of Cu2+ to Cu1+, while Fe3+ are not reduced. The tests of catalytic activity performed in a flow fixed bed reactor using a CO pulse technique showed that the light‐off temperature in the oxidation of CO over the Cu−Fe−Al nanocomposite catalysts depends on the copper content. The minimal light‐off temperature was achieved over the catalyst containing 5 wt% CuO. In addition, we performed kinetic measurements in a differential reactor and obtained the activation energy and the reaction orders with respect to the reactants. The reaction mechanism of the catalytic oxidation of CO and the origin of active species are discussed.

Catalytic Oxidation of CO on a Curved Pt(111) Surface: Simultaneous Ignition at All Facets through a Transient CO‐O Complex**

AbstractThe catalytic oxidation of CO on transition metals, such as Pt, is commonly viewed as a sharp transition from the CO‐inhibited surface to the active metal, covered with O. However, we find that minor amounts of O are present in the CO‐poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near‐ambient pressure X‐ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. Analysis of C and O core levels across the curved crystal reveals that, right before light‐off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces: a CO‐Pt‐O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.

Catalytic Oxidation of CO on a Curved Pt(111) Surface: Simultaneous Ignition at All Facets through a Transient CO-O Complex*.

The catalytic oxidation of CO on transition metals, such as Pt, is commonly viewed as a sharp transition from the CO-inhibited surface to the active metal, covered with O. However, we find that minor amounts of O are present in the CO-poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near-ambient pressure X-ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. Analysis of C and O core levels across the curved crystal reveals that, right before light-off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces: a CO-Pt-O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.

Role of surface coverage of alumina with Pt nanoparticles deposited by laser electrodispersion in catalytic CO oxidation

This work elucidates the role of surface coverage of alumina with Pt nanoparticles on the catalyst efficiency in CO oxidation. Size-selected Pt nanoparticles were deposited on the outer surface of alumina pellets by the laser electrodispersion technique. The alumina surface coverage with Pt varied from 0.04 to 3.5 nanoparticle layers and affected the Pt electronic state and catalyst efficiency. Even in the multilayer coatings nanoplatinum particles remained isolated. The catalysts were tested in CO oxidation at CO/O2 ratios of 0.2, 1 and 2 in the temperature-programmed and pulse reaction modes. At CO/O2 = 0.2 the temperature of 50% CO conversion increased with decreasing the surface coverage with Pt particles. At CO/O2 = 2 the decrease in the surface coverage enhanced the catalyst activity. Pt0 dominates only in multilayer catalysts, however, approximately half of platinum remained non-oxidized even at such a low metal loading as 0.01 wt%. The change in the electronic state of platinum under the thermal treatment in the reaction mixture affected the catalyst efficiency. The oxidation state of Pt depended on the metal loading, reaction temperature, CO/O2 ratio in the reaction mixture, mode of its feeding and surface coverage with Pt nanoparticles that influenced the interparticle and particle – support interactions.

Simultaneous removal of gaseous CO and elemental mercury over Cu-Co modified activated coke at low temperature

Cu-Co multiple-oxides modified on HNO3-pretreated activated coke (ACN) were optimized for the simultaneous removal of gaseous CO and elemental mercury (Hg0) at low temperature (< 200 °C). It was found that 2%CuOx-10%CoOx/ACN catalyst calcined at 400°C resulted in the coexistence of complex oxides including CuO, Cu2O, Co3O4, Co2O3 and CoO phases, which might be good for the simultaneous catalytic oxidation of CO by Co-species and removal of Hg0 by Cu-species, benefiting from the synergistic catalysis during the electro-interaction between Co and Cu cations (CoO ⇌ Co3O4 and Cu2O ⇌ CuO). The catalysis removal of CO oxidation was obviously depended on the reaction temperature obtaining 94.7% at 200 °C, while no obvious promoting effect on the Hg0 removal (68.3%-78.7%). These materials were very substitute for the removal of CO and Hg° from the flue gas with the conditions of 8–20 vol.% O2 and flue-gas temperature below 200 °C. The removal of Hg° followed the combination processes of adsorption and catalytic oxidation reaction via Langmuir-Hinshelwood mechanism, while the catalysis of CO abided by the Mars-van Krevelen mechanism with lattice oxygen species.

High-performance and long-term stability of mesoporous Cu-doped TiO2 microsphere for catalytic CO oxidation

Although the low-temperature reaction mechanism of catalytic CO oxidation reaction remains unclear, the active sites of copper play a crucial role in this mechanism. One-step aerosol-assisted self-assembly (AASA) process has been developed for the synthesis of mesoporous Cu-doped TiO2 microspheres (CuTMS) to incorporate copper into the TiO2 lattice. This strategy highly enhanced the dispersion of copper from 41.10 to 83.65%. Long-term stability of the as-synthesized CuTMS materials for catalytic CO oxidation reaction was monitored using real-time mass spectrum. Isolated CuO and Cu-O-Ti were formed as determined by X-ray photoelectron spectroscopy (XPS). The formation of the Cu-O-Ti bonds in the crystal lattice changes the electron densities of Ti(IV) and O, causing a subsequent change in Ti(III)/Ti(IV) and Onon/OTotal ratio. 20CuTMS contained the highest lattice distortion (0.44) in which the Onon/OTotal ratio is lowest (0.18). This finding may be attributed to the absolute formation of the Cu-O-Ti bonds in the crystal lattice. However, the decrease of Ti(III)/Ti(IV) ratio to about 0.35 of 25CuTMS was caused by the CuO cluster formation on the surface. N2O titration-assisted H2 temperature-programmed reduction and in-situ Fourier transform infrared spectroscopy revealed the properties of copper and effects of active sites.

Catalytic oxidation of CO using a silicon-coordinated carbon nitride fullerene

We report, for the first time, the possibility of using a silicon atom coordinated carbon nitride fullerene (Si@C24N24) as an efficient catalyst for oxidation of CO by O2 molecule. According to our results, the CO oxidation reaction over Si-C24N24 proceeds via a two-step Langmuir–Hinshelwood mechanism: i.e. (a) CO + O2 → OCOO → CO2 + O and (b) CO + O → CO2. The activation energy for the rate-determining step of this process is only 0.42 eV, which seems to be easily overcome at room temperature. Moreover, spin-crossing between singlet and triplet spin states is found to play an important role in CO oxidation via the Eley–Rideal mechanism. Besides, it is found that the moisture effect can promote the catalytic activity of Si@C24N24 and makes a large decrease in the activation energy of O2 dissociation reaction.

Highly Efficient CuO/α-MnO2 Catalyst for Low-Temperature CO Oxidation.

Copper manganese composite (hopcalite) catalyst has been widely explored for low-temperature CO oxidation reactions. However, the previous reports on the stabilization of such composite catalysts have shown that they deactivated severely under moist conditions. Herein, we developed an α-MnO2 nanorod-supported copper oxide catalyst that is very active and stable for the conditions with or without moisture by the deposition precipitation (DP) method. Incredibly, the CuO/MnO2 DP catalyst (with 5 wt % copper loading) achieves superior activity with a reaction rate of 9.472 μmol-1·gcat-1·s-1 even at ambient temperatures, which is at least double times of that for the reported copper-based catalyst. Additionally, the CuO/MnO2 DP catalyst is significantly more stable than the copper manganese composite catalysts reported in the literature under the presence of 3% water vapor as well as without moisture. A correlation between the catalytic CO oxidation activity and textural characteristics was derived via multitechnique analyses. The results imply that the superior activity of the CuO/MnO2 DP catalyst is associated with the proper adsorption of CO on partially reduced copper oxide as Cu(I)-CO and more surface oxygen species at the interfacial site of the catalyst.

The coordination of Al on pyridinic-N doped graphene as electrons reservoir for efficiently catalyzing CO oxidization

Single atom catalysts (SACs) have gained paramount attention due to their superior catalytic reactivity. Herein, as the most abundant metal on earth, the aluminum (Al) was used to construct a novel SAC with configuration of Al-GN4 for catalytic oxidation of CO, where a single Al atom was coordinated with four pyridinic nitrogen atoms doped in graphene grids. Four CO catalytic oxidization mechanisms including Eley-Rideal (ER), Langmuir-Hinshelwool (LH), New Eley-Rideal (NER) and termolecular Eley-Rideal (TER) were investigated to explore the catalytic performance of Al-GN4. It was found that the energy barriers of the rate-determining step for ER, LH and NER mechanisms were in the range from 0.41 to 0.59 eV, denoting the high catalytic activity of Al-GN4. It is worthy to note that the CO oxidization via TER mechanism is nearly barrierless although the probability may be low for its high adsorption energy at the initial state. Through detailed electronic structure analysis, an Al-N-C local structure functioning as electrons reservoir for electrons collection and release during adsorption and activation of O2 was proposed. Our results may provide clues for understanding the catalytic performance of SACs, moreover, we expect it will be helpful for designing other SACs with superior activity.

Highly dispersed boron-nitride/CuOx-supported Au nanoparticles for catalytic CO oxidation at low temperatures

Supported-Au catalysts show excellent activity in CO oxidation, where the nature of the support has a significant impact on catalytic activity. In this work, a hexagonal boron nitride (BN) support with a high surface area and adequately exposed edges was obtained by the ball-milling technique. Thereafter, impregnation of the BN support with Cu(NO3)2 followed by calcination under air at 400 °C yielded a CuO-modified support. After Au loading, the obtained Au-CuOx/BN catalyst exhibited high CO oxidation activity at low temperatures with a 50% CO conversion temperature (T50%) of 25 °C and a complete CO conversion temperature (T100%) of 80 °C, well within the operational temperature range of proton exchange membrane fuel cells. However, the CO oxidation activity of Au/BN, prepared without CuOx for comparison, was found to be relatively low. Our study reveals that BN alone disperses both Cu and Au nanoparticles well. However, Au nanoparticles on the surface of BN in the absence of CuO species tend to aggregate upon CO oxidation reactions. Conversely, Au nanoparticles supported on the surface of CuO-modified BN remain small with an average size of ∼2.0 nm before and after CO oxidation. Moreover, electron transfer between Au and Cu species possibly favors the stabilization of highly dispersed Au nanoparticles on the BN surface and also enhances CO adsorption. Thus, our results demonstrate that thermally stable and conductive CuO-modified BN is an excellent support for the preparation of highly dispersed and stable Au catalysts.

CeO2 nanoparticles modified by CuO nanoparticles for low-temperature CO oxidation with high catalytic activity

CeO2 nanocomposites modified by CuO are successfully synthesized by a facile hydrothermal method. The influence of different Cu contents on the crystal structure, morphology, surface element valence, specific surface area, and material reduction temperature has been characterized by many different technical characterization means of XRD, SEM, TEM, Raman, XPS, specific surface area measurement and temperature programmed control under hydrogen, respectively. With the increasing of Cu metal content, the grain size tends to decrease and specific surface area of the sample tends to increase. The CO catalytic oxidation performance of CuO–CeO2 nanocomposites is studied by gas chromatography. Compared with pure CeO2, the CuO–CeO2 nanocomposite catalysts showed more active catalytic performance. The temperature with catalyst rate of 50% in all CuO–CeO2 nanocomposite catalysts for CO conversion is relatively low (77 < T50 < 172 °C). In all as-prepared nanocatalysts, the CuO–CeO2 nanocomposite catalyst with 30 wt% Cu plays the best catalytic activity and the temperature of 50% CO conversion could be down to 77 °C. The introduction of transition metal element Cu could facilitate the interaction of the material surface, promoting the generation of oxygen vacancy, so as to improve the conversion efficiency of CuO modified CeO2 nanocomposite for CO catalytic oxidation at low temperature.

Enhanced moisture resistance of Cu/Ce catalysts for CO oxidation via Plasma−Catalyst interactions

Copper/cerium bimetallic catalyst is an efficient material for the removal of carbon monoxide, while the rapid deactivation under moisture-rich conditions in the conventional thermal-catalysis limited its wide application. Here, we investigated the plasma-assisted catalytic oxidation of CO over Cu/Ce oxides supported on γ-Alumina in comparison with the conventional thermal catalytic oxidation. The TOF values of the Cu/Ce catalysts showed that the plasma catalysis was the better catalytic system for CO oxidation (2.96 s−1 for thermal catalysis, 5.13 s−1 for plasma catalysis). Importantly, the energy barriers for plasma catalysis were much lower than that for thermal catalysis, especially under moisture-rich conditions (e.g. 130.3 kJ/mol versus 246.1 kJ/mol under 9.8 vol% water vapor). The loss of activity caused by water was reversible for the plasma process, but not for the thermal process. The Cu/Ce catalyst remained good stability within 60 h in the presence of 6.1% water for plasma oxidation, while the thermal catalytic activity declines gradually. Also, water could inhibit the formation of gas byproducts (O3 and NOx). The promoting role of plasma could be mainly ascribed to the enhanced strength of oxygen mobility and plasma-assisted decomposition of surface carbonate in the presence of water, as revealed by the in-situ NTP-TPR, XPS, and the ex-situ DRIFTS analyses.