Mechanism Of CO Oxidation Research Articles

A W/Cu Synthetic Model for the Mo/Cu Cofactor of Aerobic CODH Indicates That Biochemical CO Oxidation Requires a Frustrated Lewis Acid/Base Pair.

Constructing synthetic models of the Mo/Cu active site of aerobic carbon monoxide dehydrogenase (CODH) has been a long-standing synthetic challenge thought to be crucial for understanding how atmospheric concentrations of CO and CO2 are regulated in the global carbon cycle by chemolithoautotrophic bacteria and archaea. Here we report a W/Cu complex that is among the closest synthetic mimics constructed to date, enabled by a silyl protection/deprotection strategy that provided access to a kinetically stabilized complex with mixed O2-/S2- ligation between (bdt)(O)WVI and CuI(NHC) (bdt = benzene dithiolate, NHC = N-heterocyclic carbene) sites. Differences between the inorganic core's structural and electronic features outside the protein environment relative to the native CODH cofactor point to a biochemical CO oxidation mechanism that requires a strained active site geometry, with Lewis acid/base frustration enforced by the protein secondary structure. This new mechanistic insight has the potential to inform synthetic design strategies for multimetallic energy storage catalysts.

Two-dimensional porphyrin sheet-supported single-atom manganese catalyst for CO oxidation: A DFT-D study

The mechanism of CO oxidation on two-dimensional porphyrin sheet (TDPS)-supported single atom manganese was studied using density functional theory with dispersion (DFT-D). The molecular dynamics (MD) simulation based on the first principles has been used for confirming Mn–TDPS stability. The energetic calculations indicated that CO and O2 were favorable to co-adsorb at the active center. Eley–Rideal, Langmuir–Hinshelwood (LH), and tri-molecular Eley–Rideal (TER) mechanisms were used to investigate the reaction pathways of CO oxidation on the Mn–TDPS. Results indicated that the LH and TER reaction mechanisms were feasible due to the small energy barrier of the rate-determining step. Moreover, the oxidation states of the active center of Mn–TDPS affected the reaction barrier and reaction heat, which was unfavorable for CO oxidation in Mn–TDPS. These results findings suggested that Mn–TDPS could be a high activity catalyst for CO oxidation.

The mechanism of photodriven oxidation of CO over anatase and rutile TiO2 investigated from the hydrogen adsorption behavior

In this study, the reaction mechanisms of photodriven CO oxidation over both anatase and rutile TiO2 have been investigated from the hydrogen adsorption behavior. It can be beneficial when H2 is introduced into CO oxidation system to increase the catalytic activity of rutile, while to decrease the catalytic activity of anatase. Further analysis revealed that H2 molecule is dissociated on anatase surface, and then produce oxygen vacancies by dehydroxylation under UV irradiation. In contrast, the adsorbed H2 (H2 (ads)) with a non-dissociative mode does not activate the lattice oxygen (Olat) over rutile. For anatase, CO oxidation reaction mainly undergoes the Olat activation process, while H2 (ads) can inhibit the Olat activation, resulting in a suppressed CO oxidation. For rutile, the surface hydroxyls are the active species for CO oxidation, while H2 (ads) can promote to form more surface hydroxyl species, resulting in a promoted CO oxidation.

A Comparative Study on the Structure and Catalytic Performance of UiO-66 Supported Pt Nanocatalysts Prepared by NaBH4 and H2 Reduction: Light-Off, Durability and Mechanism for CO Oxidation

Metal–organic framework (MOF) supported nanocatalysts are attracting great attentions in heterogeneous catalysis. However, they are still less explored and poorly understood in catalyzing solid–gas reactions. In current work, two MOF supported Pt nanocatalysts were prepared by immersion method coupled with the most commonly used NaBH4 (Pt/UiO-66-NaBH4) and H2 (Pt/UiO-66-H2) reduction, and their catalytic CO oxidation performances were investigated. NaBH4 and H2 reduction both provide highly dispersed Pt/UiO-66 nanocatalysts, and the two prepared catalysts display excellent CO oxidation activities. Moreover, CO oxidation durability tests reveal that Pt/UiO-66-NaBH4 shows superior durability and structure stability, which are much better than those of Pt/UiO-66-H2. The high structure stability of Pt/UiO-66-NaBH4 also endows it with good H2O and CO2 tolerance. This work attempts to elucidate the influence of reduction methods on the structures and catalytic CO oxidation performances of Pt/UiO-66 catalysts by employing XRD, TEM, N2-sorption, XPS and in situ DRIFTS techniques. Furthermore, the mechanism of CO oxidation was studied by in situ DRIFTS, and the result reveals that the CO oxidation on the two catalysts follows different mechanism.

CO oxidation on the heterodinuclear tantalum–nickel monoxide carbonyl complex anions

The series of heterodinuclear metal oxide carbonyls in the form of TaNiO(CO)n− (n = 5–8) are generated in the pulsed-laser vaporization source and characterized by mass-selected photoelectron velocity-map spectroscopy. During the consecutive CO adsorption, the μ2-O-bent structure initially is the most favorable for TaNiO(CO)5−, and subsequently both μ2-O-bent and μ2-O-linear structures are degenerate for TaNiO(CO)6−, then the μ2-O-linear structure is most preferential for TaNiO(CO)7−, and finally the η2-CO2-tagged structure is the most energetically competitive one for TaNiO(CO)8−, i.e., the CO oxidation occurs at n = 8. In contrast to the literature reported CO oxidation on heteronuclear metal oxide complexes generally proceeding via Langmuir–Hinshelwood-like mechanism, complementary theoretical calculations suggest that both Langmuir–Hinshelwood-like and Eley–Rideal-like mechanisms prevail for the CO oxidation reaction on TaNiO(CO)8− complex. Our findings provide new insight into the composition-selective mechanism of CO oxidation on heteronuclear metal complexes, of which the composition be tailored to fulfill the desired chemical behaviors.

Mechanism investigations on CO oxidation catalyzed by Fe-doped graphene: A theoretical study

The reaction mechanism in heterogeneous catalysis which is involving O2 or atomic oxygen has been dicussed intensely because the spin change of O2 molecule and atomic O during the reaction. To understand the mechanism of CO oxidation is key for design of advanced materials. Here we demonstrate spin conversion occurs in the path of CO oxiation on Fe-doped graphene (Fe-Gr) with or without electric field according to our DFT investigations. Moreover, the reations proceed via the similar favourable spin state along the pathways both without and with the considered electric field. Additionally, the negative eletric field applied is revealed to promote CO oxidation due to its improving activation of O-O bond in O2, but also induce a competition between the atomic O removal via CO + O* → CO2 for the catalyst regeneration and the carbonate-like species formation via CO2 + O* →CO3* when the negative electric field stronger than −0.25 V/Å. Therefore, a more appropriate for CO oxidation on Fe-Gr is proposed as evidenced under −0.10 V/Å with a lower determined reaction barrier of 0.62 eV at 298.15 K and expelling the carbonate-like species formation. These findings offer a new view of CO oxidation on the doped graphene.

Stabilization by Configurational Entropy of the Cu(II) Active Site during CO Oxidation on Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O.

The mechanisms of CO oxidation on the Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O high-entropy oxide were studied by means of operando soft X-ray absorption spectroscopy. We found that Cu is the active metal and that Cu(II) can be rapidly reduced to Cu(I) by CO when the temperature is higher than 130 °C. Co and Ni do not have any role in this respect. The Cu(II) oxidation state can be easily but slowly recovered by treatment of the sample with O2 at ca. 250 °C. However, it should be noted that CuO is readily and irreversibly reduced to Cu(I) when it is treated with CO at T > 100 °C. Thus, the main conclusion of this work is that the high configurational entropy of Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O stabilizes the rock-salt structure and permits the oxidation/reduction of Cu to be reversible, thus permitting the catalytic cycle to take place.

Mechanism of CO oxidation by oxygen in the presence of palladium(ii) bromide complexes: a quantum chemical modeling

Mechanisms of CO oxidation by oxygen in the PdBr2-LiBr-MeCN-H2O system in the absence and in the presence of iron(ii) phthalocyaninate (PcFe) as co-catalyst were studied within the framework of the density functional theory with the PBE functional. According to calculations, palladium(ii) bromide complexes most likely occur as Pd(AN)(CO)Br2 (AN is acetonitrile), Pd(CO)2Br2, and [Pd(CO)Br3]−. From the results of calculations it follows that neutral complexes Pd(AN)(CO)Br2 and Pd(CO)2Br2 can be highly catalytically active in the CO oxidation. Insertion of O2 into the Pd-H bond in the hydride complexes is the rate-limiting step of the CO oxidation reaction proceeding in the absence of PcFe as co-catalyst. Iron(ii) phthalocyaninate present in the reaction system causes a noticeable decrease in the activation energy of regeneration of the PdII complexes due to more efficient activation of O2. The oxidation of palladium hydride complexes by iron superoxophthalocyaninate can follow two routes that differ in the fashion of hydride transfer.

Catalytic oxidation of CO on Pd38 cluster and Pd slab, a computational study

The catalytic oxidation of CO was comparatively investigated on the Pd slab and Pd38 cluster models by density functional theory (DFT) calculation, in order to reveal the mechanism of CO oxidation over Pd catalysts. The results show that the rate-determining step of CO oxidation on the Pd38 cluster is the dissociation of O2, with the energy barrier of 0.65 eV, whereas the oxidation of CO turns to be the rate-determining step on Pd slab, with the energy barrier of 0.87 eV. Obviously, the oxidation of CO on the Pd38 cluster is much easier than that on the Pd slab, suggesting that the activity of Pd catalysts is related to the dispersion of active Pd species; the Pd catalyst with higher Pd dispersion also exhibits higher activity in CO oxidation.

Selective immobilization of single-atom Au on cerium dioxide for low-temperature removal of C1 gaseous contaminants

Selective immobilization of single-atom Au on the specific facets of CeO2 has been successfully performed by redox etching precipitation (REP), which makes it possible to clearly refine the interfacial effect of multiple-facet support on single atom. With systemic characterizations, it is found that single-atom Au is apt to lie on nonpolar facets of CeO2 (111) and (110) rather than polar facet of CeO2 (100). The modification of morphology-dependent properties is attributed to the different interaction between Au atom and each CeO2 interface. Because of synergy between Au and CeO2, more oxygen vacancies and more active oxygen species are generated; meanwhile, the interfacial effect stabilizes the charged Au species which serves as active site. Therefore, the performance in catalytic oxidation of HCHO and CO on CeO2 is facilitated by loading Au. Among them, CeO2 rod-supported Au as an optimal catalyst exhibits a remarkable activity and stability. With in-situ characterization, the reaction mechanisms for HCHO and CO oxidation over Au/r-CeO2 are studied. Meanwhile, it is proved that REP strategy is also valid to obviously promote catalytic performance whenever commercial CeO2 is used or Au is replaced with Ag, so the improvement of recently applied catalyst with REP process is promising.

Oxygen activation on the interface between Pt nanoparticles and mesoporous defective TiO2 during CO oxidation.

Platinum-based heterogeneous catalysts are mostly used in various commercial chemical processes because of their high catalytic activity, influenced by the metal/oxide interaction. To design rational catalysts with high performance, it is crucial to understand the relationship between the metal-oxide interface and the reaction pathway. Here, we investigate the role of oxygen defect sites in the reaction mechanism for CO oxidation using Pt nanoparticles supported on mesoporous TiO2 catalysts with oxygen defects. We show an intrinsic correlation between the catalytic reactivity and the local properties of titania with oxygen defects (i.e., Ti3+ sites). In situ infrared spectroscopy observations of the Pt/mesoporous TiO2-x catalyst indicate that an oxygen molecule bond can be activated at the perimeter between the Pt and an oxygen vacancy in TiO2 by neighboring CO molecules on the Pt surface before CO oxidation begins. The proposed reaction pathways for O2 activation at the Pt/TiO2-x interface based on density functional theory confirm our experimental findings. We suggest that this provides valuable insight into the intrinsic origin of the metal/support interaction influenced by the presence of oxygen vacancies, which clarifies the pivotal role played by the support.

Enhanced Catalytic Oxidation of CO on Sulfur‐Doped Boron Nitride

AbstractBridging the gap between homogeneous and heterogeneous catalysis, the single‐atom catalyst supported on a substrate shows extremely high atom efficiency, low cost, excellent stability, and high activity for CO oxidation. Here, we report the catalytic mechanism of CO oxidation on sulfur‐doped h‐BN. The chemisorption activates O2 via electron transfer from sulfur‐doped h‐BN, inducing a reduction in the bond order of O2 and lengthening the O−O distance. This variation is helpful for the subsequent oxidation of CO. The oxidation process includes O2 adsorption, electron reorganization, and two instances each of CO adsorption, oxygen transfer, and CO2 desorption. The first oxygen transfer from O2 to CO occurs exothermically by ∼−63.8 kcal/mol with a barrier of ∼12.1 kcal/mol, which is thus the rate‐determining step. The significantly enhanced catalytic performance implies that sulfur doping on h‐BN should have potential applications in the areas of metal‐free catalysts with low cost and high activity.

Insights into the reaction mechanism and particle size effects of CO oxidation over supported Pt nanoparticle catalysts

CO oxidation is an extensively studied reaction in heterogeneous catalysis due to its seeming simplicity and its great importance for emission control. However, the role of particle size and more specifically structure sensitivity in this reaction is still controversial. In the present study, colloidal “surfactant-free” Pt nanoparticles (NPs) in a size regime of 1–4 nm with narrow size distribution and control over particle size were synthesized and subsequently supported on Al2O3 to prepare model catalysts. CO oxidation was performed using Pt NPs catalysts with particles sizes of 1, 2, 3, and 4 nm at different reaction temperatures. It is shown that the reaction exhibits a particle size effect that depends strongly on the reaction conditions. At 170 °C, the reaction seems to proceed within the same kinetic regime for all particle sizes, but the surface normalized activity depends strongly on the particle size, with maximum activity for nanoparticles 2 nm in diameter. A temperature increase to 200 °C leads to a change of the kinetic regime that depends on the particle size. For Pt NPs 1 nm in diameter a reaction order of 1 for O2 was observed, indicating that O2 adsorbs molecularly and dissociates in a following step, which represents the generally accepted mechanism on Pt surfaces. The reaction order of −1 for CO demonstrates that the surface is saturated with CO under reaction conditions. With increasing particle size, the reaction orders of O2 and CO change. For particles 2 nm in size, an increase in temperature also results in reaction orders of 1 for O2 and −1 for CO; NPs of 3 and 4 nm, even at higher temperatures, show no clear kinetic behavior that can be explained by a single reaction mechanism. Instead, the Boudouard reaction between two adjacent adsorbed CO molecules was identified as an important additional reaction pathway that occurs preferentially on large particles and causes more complex kinetics.

Theoretical study on the catalytic properties of single-atom catalyst stabilised on silicon-doped graphene sheets

ABSTRACTThe stable configurations, electronic structures and catalytic activities of single-atom metal catalyst anchored silicon-doped graphene sheets (3Si-graphene-M, M = Ni and Pd) are investigated by using density functional theory calculations. Firstly, the adsorption stability and electronic property of different gas reactants (O2, CO, 2CO, CO/O2) on 3Si-graphene-M substrates are comparably analysed. It is found that the coadsorption of O2/CO or 2CO molecules is more stable than that of the isolated O2 or CO molecule. Meanwhile, the adsorbed species on 3Si-graphene-Ni sheet are more stable than those on the 3Si-graphene-Pd sheet. Secondly, the possible CO oxidation reactions on the 3Si-graphene-M are investigated through Eley–Rideal (ER), Langmuir–Hinshelwood (LH) and new termolecular Eley–Rideal (TER) mechanisms. Compared with the LH and TER mechanisms, the interaction between 2CO and O2 molecules (O2 + CO → CO3, CO3 + CO → 2CO2) through ER reactions (< 0.2 eV) are an energetically more favourable. These results provide important reference for understanding the catalytic mechanism for CO oxidation on graphene-based catalyst.

Integrated Experimental-Theoretical Approach To Determine Reliable Molecular Reaction Mechanisms on Transition-Metal Oxide Surfaces.

By combining experimental and theoretical approaches, we investigate the quantitative relationship between molecular desorption temperature and binding energy on d and f metal oxide surfaces. We demonstrate how temperature-programmed desorption can be used to quantitatively correlate the theoretical surface chemistry of metal oxides (via on-site Hubbard U correction) to gas surface interactions for catalytic reactions. For this purpose, both CO and NO oxidation mechanisms are studied in a step-by-step reaction process for perovskite and mullite-type oxides, respectively. Additionally, we show solutions for over-binding issues found in COx, NOx, SOx, and other covalently bonded molecules that must be considered during surface reaction modeling. This work shows the high reliability of using TPD and density functional theory in conjunction to create accurate surface chemistry information for a variety of correlated metal oxide materials.

Catalytic Mechanism of CO Oxidation on Three‐Silicon‐Doped Hexagonal Boron Nitride

AbstractDoped nanomaterials show extremely high atom efficiency, low cost, high stability and activity for CO oxidation, bridging the gap between homogeneous and heterogeneous catalysis. The catalytic mechanism of CO oxidation on three‐silicon‐doped h‐BN is reported. The adsorption of O2 is more advantageous than CO. The electron transfer from the dopant activates the adsorbed O2 and initiates the following CO oxidation. The oxidation process includes O2 adsorption, electron transfer, CO adsorption, oxygen transfer, and CO2 desorption. The energy barriers for the generation of two CO2 molecules in N vacancy dopant are 0.50 and 0.33 eV, respectively. The enhanced catalytic performance indicates that three‐silicon‐doped h‐BN has potential applications to fabricate low cost and high activity BN‐based metal‐free catalysts.

Mechanistic insight into the selective catalytic oxidation for NO and CO on co-doping graphene sheet: A theoretical study

NO and CO are recognized as the major source of air pollution and are expected toward selective catalytic oxidation under different environments. Based on density functional theory (DFT) calculations, the formation configuration and surface activity of metal Co and nonmetal N atoms co-doping graphene (Co-N3-graphene) as substrate are investigated. Firstly, the adsorption behaviors of gas reactants on Co-N3-graphene sheet are comparably analyzed. It is found that the coadsorption of OH/NO and OH/CO molecules have the larger adsorption energies than that of the single NO or CO, thus the Langmuir-Hinshelwood (LH) mechanism for NO or CO oxidation reactions will be considered. Secondly, the coadsorption of 2NO molecules is more stable than that of the 2CO and the interaction between 2NO and 2CO molecules through Eley-Rideal (ER) mechanism are also investigated. So, the possible reaction mechanisms for NO or CO oxidation on Co-N3-graphene are explored. In the O environments, the formation of HONO or HOCO complex and NO or CO are easily generated NO2 or CO2 with smaller energy barriers (<0.1 eV), which is an energetically more favorable than the LH and ER mechanisms. These results will provide an important reference for designing high performance graphene-based catalyst for CO or NO oxidation.

Activity of Atomically Precise Titania Nanoparticles in CO Oxidation.

Understanding the property evolution of atomically precise nanoparticles atom by atom along the size continuum is critical for selecting potential candidates to assemble nanomaterials with desired functionality, but it is very challenging experimentally especially for systems having mixtures of elements such as metal oxides. In this work, the capability to oxidize carbon monoxide has been measured experimentally for titania nanocluster anions of (TiO2 )n Om - (-3≤m≤3) across a broad size range in the gas phase. Stoichiometric (TiO2 )n O- exhibits superior oxidative activity over other clusters of (TiO2 )n Om - (m≠1) even when the cluster dimensions are scaled up to n=60, indicating that each atom still influences the chemical behavior of titania nanoparticles composed of ≈180 atoms. The fascinating result not only identifies a promising building block of Tin O2n+1 for devising new nanoscale titania materials with desirable oxidative activity, but also provides compelling molecular-level evidence for the Mars-van Krevelen mechanism of CO oxidation over titania supports.

Activity of Atomically Precise Titania Nanoparticles in CO Oxidation

AbstractUnderstanding the property evolution of atomically precise nanoparticles atom by atom along the size continuum is critical for selecting potential candidates to assemble nanomaterials with desired functionality, but it is very challenging experimentally especially for systems having mixtures of elements such as metal oxides. In this work, the capability to oxidize carbon monoxide has been measured experimentally for titania nanocluster anions of (TiO2)nOm− (−3≤m≤3) across a broad size range in the gas phase. Stoichiometric (TiO2)nO− exhibits superior oxidative activity over other clusters of (TiO2)nOm− (m≠1) even when the cluster dimensions are scaled up to n=60, indicating that each atom still influences the chemical behavior of titania nanoparticles composed of ≈180 atoms. The fascinating result not only identifies a promising building block of TinO2n+1 for devising new nanoscale titania materials with desirable oxidative activity, but also provides compelling molecular‐level evidence for the Mars–van Krevelen mechanism of CO oxidation over titania supports.

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.