CO Oxidation Reaction Research Articles

Synergistic effect of metal-nonmetal substitution on oxygen activation in Pd/C- and Pd/N-substituted [formula omitted

In this study, we report DFT + U analysis of Pd/C- and Pd/N-codoped TiO2 to assess their thermal catalytic activities. Various sites were explored for the codoping of Pd/C and Pd/N in TiO2. The effect of codopants on the net oxygen activation were investigated. Two unique crystal planes (001) and (100) of TiO2 were chosen for codoping. The energetics of substitution in Ti1-xPdxO2-δ-yCy was energetically favoured with an energy change of −4.2 eV while it was energy intensive in Ti1-xPdxO2-δ-2yN2y with an energy requirement of 16.7 eV. Net oxygen activation as high as 23.8% was achieved in Ti1-xPdxO2-δ-yCy while 25.6% activation was observed in Ti1-xPdxO2-δ-2yN2y. Surface reconstructions were examined via transfer of electrons from the codoped sites to the nearest neighbouring sites leading to the superior potential of Ti1-xPdxO2-δ-yCy and Ti1-xPdxO2-δ-2yN2y towards surface oxidation reactions over (001) and (100) surface planes when compared to their monosubstituted counterparts. Redox nature of reducible oxides like TiO2 has been very well established to govern their catalytic activity for reactions like CO oxidation, NOx reduction and water-gas shift reaction. Improvement in lattice oxygen exchange capacity has been reported to improve the catalytic activity. Hence, we have used quantitative net oxygen activation as a probe into the thermal catalytic activity of Pd/C and Pd/N-codopedTiO2.

Atomic scale insights on the electronic and geometric effects in the electro-oxidation of CO on PtxRu1-x/Ru(0001) surface alloys

The enhanced activity of various bimetallic catalysts/electrodes in electrocatalytic reactions is usually ascribed to a bifunctional mechanism. This seems to neglect other effects arising from electronic modifications and further geometric effects, which are known to strongly affect the interaction of these surfaces with adsorbed species. In this work we elucidate the role of electronic and geometric effects on the activity of structurally well-defined PtRu electrodes in the bulk CO oxidation reaction, using model electrodes prepared and structurally characterized on an atomic scale by scanning tunneling microscopy under ultrahigh vacuum (UHV) conditions. Samples include PtxRu1-x/Ru(0001) surface alloys with varying Pt content consisting of a Ru(0001) substrate and a PtxRu1-x alloy surface layer as well as a Ru(0001) electrode covered by a pseudomorphic Pt monolayer (x = 1), and Pt(111) for comparison. Comparing the structural/chemical properties and the relative abundance of specific PtnRum ensembles on the electrode surfaces with the respective bulk CO electro-oxidation activity enabled us to identify Pt1Ru3 ensembles as the most active ensembles under the present conditions. The importance of electronic and geometric ensemble effects are discussed. The results demonstrate that the reactivity of bimetallic surfaces is much more complex than described by the classical bifunctional mechanism, which is proposed as a general feature.

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.

Ligand-Mediated Reactivity in CO Oxidation of Niobium-Nickel Monoxide Carbonyl Complexes: The Crucial Roles of the Multiple Adsorption of CO Molecules.

The heteronuclear metal oxide complexes are of great significance in heterogeneous catalytic oxidation of CO. However, previous studies are mainly focused on the composition of metal oxide, charge state, the support and the active oxygen species, with little attention paid to adsorbed CO ligands. Herein, the ligand-mediated reactivity in CO oxidation of niobium-nickel monoxide carbonyl complexes has been successfully identified. The NbNiO(CO) n- ( n = 5-6) anions are determined to be O-bridged complexes. In contrast, the NbNiO(CO) n- ( n = 7-8) anions are characterized to be η2-CO2-tagged complexes. The crucial roles of the multiply adsorbed CO molecules that can facilitate not only the competitive binding with bridging oxygen atom to the transition metal centers but also the electron accumulation of transition metal atoms have been discovered. The fascinating results are of substantial importance to understand the mechanisms of CO oxidation over heteronuclear metal oxide under CO-rich feed condition.

Pd(II), Cu(II), and pillared clay based nanocatalysts for low-temperature CO oxidation

Pillared clays (PILCs) are little-studied as supports in compositions of anchored metal complex catalysts. In the work, pillared clay, Al-PILC, was prepared by using Ukrainian natural bentonite, N-Bent, and the aluminum polycation, [Al13O4(OH)24(H2O)12]7+ (Al13), as an intercalating agent. The final product was used for preparing К2PdCl4–Cu(NO3)2–KBr/Al-PILC nanocatalysts by an impregnation method. Initial N-Bent and Al-PILC as well as the nanocompositions based on them were characterized by XRD, FT-IR spectroscopy, DTG–DTA, water vapor adsorption, and pH metry. К2PdCl4–Cu(NO3)2–KBr/ $${\bar{\text{S}}}$$ compositions ( $${\bar{\text{S}}}$$ were N-Bent or Al-PILC) were tested in the reaction of CO oxidation with air oxygen at temperature of 20 °C and a relative humidity of 67%. It has been found that the preliminary modification of natural bentonite by Al13 intercalation results in formation of the nanostructured catalyst with a crystallite size of 19 nm showing the maximum activity in the reaction under study at CPd(II) = 2.72 × 10−5 mol/g and copper(II) concentrations of 2.9 × 10−5 and 5.9 × 10−5 mol/g.

Exploring reaction mechanisms for CO oxidation on boron-doped carbon nanotubes: A computational approach

Spin-polarized first-principles computations are carried out to illustrate the catalytic mechanisms of CO oxidation on the (5, 5) boron-doped carbon nanotube (BCNT). BCNTs with two kind of boron content of 1 and 2 atom % are studied. The calculations show that O2 species prefers to partially reduce and chemically bind at CB site via the side-on configuration with the adsorption energies of −0.85 eV for the BCNT with boron content of 1 atom % and −1.27 ~ −1.29 eV for the BCNT with boron content of 2 atom %. The partially reduced O2 species can further react with CO via the Eley-Rideal (ER) mechanism by overcoming a small energy barrier of 0.34 eV for the BCNT with boron content of 1 atom % and 0.42 eV for the BCNT with boron content of 2 atom %. Second reaction of CO oxidation occurs by the reaction of CO + O → CO2 with a slight energy barriers of 0.12 and 0.14 eV for the BCNT with boron content of 1 and 2 atom %, respectively. Our results manifest that the boron-doped CNT can be an advanced metal-free catalyst for CO oxidation.

Co-doped triel-pnicogen graphene as metal-free catalyst for CO oxidation: Role of multi-center covalency.

Third and fifth group atoms, named triel and pnicogen, respectively, were used for graphene doping. AlP and AlN structures were selected as co-doped graphene-based catalysts. The electronic structure and catalytic properties of binary AlN, AlP co-doped graphene were investigated through density functional theory (DFT). Results show that the AlP co-doped graphene strictly enhances the oxygen reactivity compared to AlN one. The CO oxidation on AlP and AlN co-doped graphene sheets is mainly done through Eley-Rideal mechanism as follows: CO + O2➔CO2 + Oads and CO + Oads➔ CO2. The CO oxidation reaction paths over the AlN and AlP-co-doped graphene have revealed that they can be regarded as the competent catalyst for CO oxidation in the presence of O2. Mechanistically, both AlP and AlN co-doped graphene catalysts are appropriately active in the first step while the second step is too hard to do regarding the multi-center covalency character between O2 and AlP co-doped graphene and hence its catalytic efficiency is significantly lower compared to AlN co-doped graphene sheet. Thus, the substitution of a C-C bond with Al-N is an effective way to design the graphene-based catalysts for CO oxidation.

Unravelling origins of Pd ensembles’ activity in CO oxidation via state-to-state microkinetic analysis

The size and configuration of small Pdn ensembles (monomers, dimers, trimers, and tetramers) is of great importance not only for the fundamental understanding of single-site catalysis design, but also for practical applications such as automotive emission control and fuel cells. In this study, by coupling the competitive gaseous adsorptions and multiple reaction pathways via a state-to-state microkinetic formalism, we reveal CO oxidation mechanisms on small Pd ensembles under realistic experimental conditions. It is found that reaction of O2 with preadsorbed CO on adjacent Pd sites is the dominant pathway at low temperatures (denoted as the preadsorbed CO oxidation pathway, or PCOP). As the temperature increases, different PCOPs dominate CO oxidation. At even higher temperatures, the CO oxidation reaction proceeds primarily via the recombination of dissociated O atoms and CO (denoted as the dissociated O2 pathway, or DO2P). Among different ensembles investigated, the Pd dimer possesses the best low-temperature CO oxidation activity, while the tetramer outperforms other forms at higher temperatures. It is concluded that oxygen adsorption ability is the main factor influencing the reactivity. This work unravels the intrinsic activity of the catalyst on the microscopic scale and shed light on ensemble engineering for maximizing its reactivity.

Structural, electronic and catalytic performance of single-atom Fe anchored 3Si-doped graphene

By density functional theory (DFT) calculations, it is found that the single-atom Fe anchored three Si modified defective graphene (3Si-graphene-Fe) exhibits the high stability, and this system is semiconducting property and has non-magnetic moment. Besides the most stable configurations, electronic structures and magnetic properties of adsorbed species (O2, CO, 2CO and CO/O2) on 3Si-graphene-Fe systems are comparably discussed. The adsorption of O2 is more stable than that of CO molecule and the coadsorption of 2CO and CO/O2 has the larger adsorption energy than that of the isolated one. The adsorbed O2, CO and CO/O2 can induce the change in magnetic properties of 3Si-graphene-Fe system, and the coadsorbed CO/O2 on system exhibits the metallic property. Among the reaction mechanisms, the CO oxidation reactions through Eley–Rideal (ER) reactions have lower energy barriers (<0.5 eV) than those of the Langmuir–Hinshelwood (LH) and new termolecular Eley–Rideal (TER) mechanisms, indicating that the ER reaction as starting step is an energetically favourable process. These results provide an important guidance on validating the catalytic activity of single atom on graphene-based materials.

A Single Layer of MoS2 Activates Gold for Room Temperature CO Oxidation on an Inert Silica Substrate

We demonstrate the facile deposition of catalytically active stable Au nanoparticles on single-layer MoS2, on an inert oxide substrate (SiO2). Unlike a gold surface, adsorption of CO on these gold particles proceeds even at room temperature. Subsequent exposure to oxygen leads to CO2 formation and desorption along a reaction pathway described, in detail, by density functional theory. The barrier to CO oxidation, along this pathway, is less than 300 meV, corroborating the facile nature of the CO oxidation reactions in this system. X-ray photoelectron spectroscopy directly reveals that the carbon monoxide, adsorbed on Au nanoparticles on single-layer MoS2, can be repeatedly titrated off the substrate by exposure to oxygen. Comparison of computational results assuming a defect-laden and pristine MoS2 substrate suggest that the pristine single-layer material may be superior in supporting this reaction.

Interaction of atomically dispersed gold with hydrothermally prepared copper-cerium oxide for preferential CO oxidation reaction

Gold nanoclusters were highly dispersed on hydrothermally prepared copper-ceria nanocatalysts and resulted in enhanced catalytic performance for preferential CO oxidation reaction, highlighting the direct interaction between Cu and Au through a Cu–Au interface. The presence of gold nanoclusters promoted the reducibility and oxygen mobility of ceria and copper-ceria catalysts, while a reduction pretreatment before gold deposition resulted in re-dispersion of copper species and further enrichment of ceria surface with these active species, thus enhanced interactions with atomically dispersed gold nanoclusters. The catalytic performance in preferential CO oxidation reaction depicted the promoting role of gold nanoclusters and reductive pretreatment in terms of activity, selectivity and CO2/H2O tolerance.

Kinetics of CO oxidation and redox cycling of Sr2Fe1.5Mo0.5O6-δ electrode for symmetrical solid state electrochemical devices

Resultsof the redox cycling of the Sr2Fe1.5Mo0.5O6-δ electrode in contact with the LaGaO3-based electrolyte at 800°С in air - CO/CO2 - Ar/H2 are presented for the first time. The electrochemical impedance method was used to demonstrate that the electrode polarization resistance at fast gas cycling remained almost unchanged. In the long-term cycling from oxidizing to reducing atmospheres (and vice versa) the polarization resistance equilibrated rapidly (slowly). The kinetics of the CO oxidation in CO/CO2 gas mixtures at 800°С (pO2 ≈ 10−16 – 10−19 atm) near the equilibrium electrode potential was studied. By means of a complex approach to the impedance spectra analysis using the distribution of relaxation times and non-linear least squares methods, it was determined that the CO oxidation is limited by three steps: CO adsorption (CO2 desorption) at the electrode surface, oxygen hetero-exchange and the oxygen ion transport in the electrode bulk. We assume that the charge transfer and the gas diffusion do not limit the CO oxidation reaction on the Sr2Fe1.5Mo0.5O6-δ electrode.

Kinetic control of CeO2 nanoparticles for catalytic CO oxidation

Abstract

CO catalytic oxidation over C59X heterofullerenes (X = B, Si, P, S): A DFT study

The aim of this study is to explore the mechanisms of CO oxidation to CO2 over some C59X heterofullerenes (X = B, P, Si and S) through systematic dispersion-corrected density functional theory calculations. The adsorption energy of O2 over these fullerenes becomes more negative in the order of C59S < C59B < C59P < C59Si. According to our results, the CO oxidation reaction cannot take place over C59B and C59S due to the poisoning of the active site of these surfaces by CO. The oxidation of first CO molecule over C59Si and C59P proceeds via the Langmuir–Hinshelwood mechanism.

Tuning the Catalytic Properties of Copper-Promoted Nanoceria via a Hydrothermal Method

Copper-cerium mixed oxide catalysts have gained ground over the years in the field of heterogeneous catalysis and especially in CO oxidation reaction due to their remarkable performance. In this study, a series of highly active, atomically dispersed copper-ceria nanocatalysts were synthesized via appropriate tuning of a novel hydrothermal method. Various physicochemical techniques including electron paramagnetic resonance (EPR) spectroscopy, X-ray diffraction (XRD), N2 adsorption, scanning electron microscopy (SEM), Raman spectroscopy, and ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) were employed in the characterization of the synthesized materials, while all the catalysts were evaluated in the CO oxidation reaction. Moreover, discussion of the employed mechanism during hydrothermal route was provided. The observed catalytic activity in CO oxidation reaction was strongly dependent on the nanostructured morphology, oxygen vacancy concentration, and nature of atomically dispersed Cu2+ clusters.

CO catalytic oxidation over graphene with double vacancy-embedded molybdenum: a DFT investigation

Based on the M06-2X density functional, the catalytic oxidation of CO by O2 over Mo-embedded graphene was investigated in detail. The model with molybdenum atom embedded in double vacancy (DV) in a graphene sheet was considered. It is found that the complete CO oxidation reactions over Mo-DV-graphene include a two-step process, in which the first step prefers to Langmuir–Hinshelwood mechanism and followed the progress of CO oxidation with a remaining atomic Otop. Compared with the structure of Mo atom decorated at the single carbon vacancy on graphene (Mo-SV-graphene), the catalytic activity of Mo-DV-graphene is weaker. The present results imply that the catalytic activity of Mo-embedded graphene for CO oxidation can be improved by increasing the ratio of single vacancy (SV).

Role of CO2 methanation into the kinetics of preferential CO oxidation on Cu/Co3O4

In the present study, cobalt oxide (Co3O4) and Cu substituted Co3O4 were synthesized using sonochemical method. The role of Cu substitution was studied for CO oxidation in absence of H2 as well as in H2 rich conditions (PrOx). The synthesized catalysts were characterized by XRD, XPS, TEM and BET. The role of side reactions such as H2 oxidation and CO2 methanation in inhibiting the CO oxidation reaction were discussed. H2 – TPR and in situ FTIR were conducted to understand the reducible nature of the support and reaction mechanism. The intermediate species such as carbonates, bicarbonates, formates play an important role for CO oxidation under H2 rich conditions. Further, the utilization of atomic hydrogen by surface CO2 species, enhanced the methane formation. A detailed reaction mechanism was proposed for both CO oxidation and PrOx conditions with the insights obtained from in situ FTIR. 3% Cu/Co3O4 showed high stability for CO oxidation under hydrogen rich conditions for 50 h at 170 °C as well as in methanation prone region at 200 °C.

NRu⋅mPt⋅(Hx−3n−2mMoO3) composite prepared by surface redox reaction as a highly active electrocatalyst for carbon monoxide and methanol oxidation

A new method for the synthesis of nRu⋅mPt⋅(Hx−3n−2mMoO3) composite based on the redox reaction between the reduced forms of molybdenum bronzes and the solution containing platinum and ruthenium species is proposed. The uniform distribution of Pt and Ru over the electrode surface was observed in the case of their codeposition from the solution containing both platinum and ruthenium compounds. The obtained composite contained platinum mainly in the metallic state, while ruthenium was found in both metallic and oxidized states. The manufactured electrodes were characterized by high catalytic activity in the reactions of carbon monoxide and methanol oxidation. The rate of the methanol oxidation reaction (MOR) on nRu⋅mPt⋅(Hx−3n−2mMoO3) was 4–5 times higher in comparison with that on nPt⋅(Hx−2nMoO3) and ∼30 times higher as compared with that on Pted/GC. On the basis of the obtained data, one can state the additive effect of ruthenium and molybdenum species on methanol electrooxidation.

Catalytic performance of Cu/hydroxyapatite catalysts in CO preferential oxidation in H2-rich stream

Abstract A series of Cu/HAP catalysts was prepared by impregnation and characterised by TEM, BET, DRS, XPS, H2-TPR, XRD, TEM and FTIR techniques. H2-TPR and DRS data revealed that at low concentration ( The activity of the reduced samples in COTOX and COPROX processes revealed remarkable differences when the Cu content was increased from 0.8 to 14.8%. For instance, analysing the TOF values calculated for the two processes and the characterisation data, it was concluded that the catalysts with low Cu loadings (0.8% and 3.6%) were the most active ones and their sites involved in the CO oxidation reaction were almost similar. The addition of CO2 and water seemed to affect the performances of the two catalysts via different pathways. Indeed, in contrast to the Cu(4)/HAP, the Cu(1)/HAP catalyst became more selective in the presence of CO2 and water. Moreover, under these realistic COPROX conditions, the latter was 6.6 times more active than the former. A correlation with the characterisation data showed that the most efficient species consisted of those presenting strong interaction with the HAP support. By contrast, the sites exhibiting high concentration of Cu lattice imperfections showed a relatively low activity.

Dynamics of Two-Step CO Oxidation Light-Off on Pt/ $$\gamma$$ γ -Al $$_2$$ 2 O $$_3$$ 3 and Pd/ $$\gamma$$ γ -Al $$_2$$ 2 O $$_3$$ 3 in the Presence of C $$_3$$ 3 H $$_6$$ 6

Although the kinetics of CO and hydrocarbon (HC) oxidation reactions on three-way catalyst and diesel oxidation catalysts for automotive exhaust gas aftertreatment has been studied for a couple of decades, there are still some phenomena affecting the catalyst performance and pollutant conversion that need to be better understood and controlled. Two-step CO light-off is an undesired effect that significantly delays the process of reaching full CO conversion in the exhaust gas mixture including also hydrocarbons. In such a case CO light-off curve exhibits a plateau or shoulder where the CO conversion does not increase or even temporarily decreases with temperature, though there is enough O\(_2\) available for both CO and HC oxidation. The onset of hydrocarbon oxidation may inhibit the CO conversion due to blocking of active catalytic sites by adsorbed reaction intermediates. Furthermore, CO can be released as a by-product of the hydrocarbon oxidation. In this contribution we present the results of a systematic experimental study exploring these phenomena on Pt/\(\gamma\)-Al\(_2\)O\(_3\) and Pd/\(\gamma\)-Al\(_2\)O\(_3\) catalysts. Pt/\(\gamma\)-Al\(_2\)O\(_3\) showed larger decrease of CO conversion caused by accumulation of the surface intermediates. On the contrary, the production of CO by incomplete C\(_3\)H\(_6\) oxidation was higher on Pd/\(\gamma\)-Al\(_2\)O\(_3\). Overall extent of the two-step CO light-off effect was higher on Pt/\(\gamma\)-Al\(_2\)O\(_3\).