Coadsorption Of O2 Research Articles

CO Oxidation Catalyzed by a Single Ti Atom Supported on Divacancy Defective Graphene: A Dispersion‐Corrected DFT Study

AbstractWe report a detailed theoretical investigation on the possible reaction mechanisms of CO oxidation catalyzed by a single Ti atom supported on divacancy defective graphene (TiDVG). The reaction between the adsorbed O2 with CO molecules on TiDVG is compared via three different mechanisms, i. e. Eley‐Rideal (ER), Langmuir‐Hinshelwood (LH) and termolecular Eley‐Rideal (TER). According to our results, the energy barrier for the rate determining step of CO + O2 reaction in the TER mechanism is smaller than those of ER and LH ones. This can be attributed to the significant mixing of electronic states of adsorbed CO and O2 at the reaction site, as well as due to a sizable charge‐transfer from the surface to these molecules. On the other hand, the calculated adsorption energies indicate that the coadsorption of O2 and CO molecules is slightly more energetically favored with respect to the coadsorption of two CO molecules. This result together with the relatively small reaction barriers reveal that the LH mechanism is also possible to occur at low temperature. Our results reported here suggest that TiDVG can be regarded as a green, low‐cost and highly active catalyst for CO oxidation at room temperature.

Species formed during NO adsorption and NO + O2 co-adsorption on ceria: A combined FTIR and DFT study

Adsorption of NO and co-adsorption of NO and O2 on ceria have been re-investigated by FTIR spectroscopy. To provide unambiguous assignments of the IR bands, adsorption of 15NO and co-adsorption of 14NO + 15NO isotopic mixtures have also been studied and DFT calculations performed. At the initial adsorption stages mainly symmetric nitrite species (2472, 1304, ca. 1160 and 822 cm‐1) and trans-hyponitrites ([N2O2]2−, ca. 1100 cm‐1) are simultaneously produced as a result of NO disproportionation. At further stages non-symmetric NO2− species (1393 and 1014 cm‐1), N2O (2240, 1252 cm‐1) and small amount of nitrates are formed. Nitrates are the only surface products of the thermal decomposition of the nitrites and hyponitrites. In the presence of small amounts of oxygen, the hyponitrites are very easily oxidized. In contrast, all nitrites increase in concentration after addition of small amount of O2 to the NO/CeO2 system but are gradually oxidized in the presence of higher amount of oxygen giving rise to different surface nitrates.

First-Principle Investigations of the Interaction between CO and O2 with Group 11 Atoms on a Defect-Free MgO(001) Surface.

In this contribution, we investigate the interaction between CO and O2 with metal atoms of group 11 deposited on a defect-free magnesium oxide surface using density functional theory with periodic point charge embedding. We present the first transversal study of the adsorption and coadsorption of CO and O2 on coinage metal adatoms deposited on metal oxide surfaces from the perspective of single-atom catalysis. Various analysis tools shed light on the binding situation of the metal atoms to the substrate as well as on the situation of the two molecules on the different metal centers. Our analysis demonstrates that cooperative electronic effects enhance the stability of CO upon coadsorption with O2 for all three metal centers. Our results also explain the lack of catalytic activity of group 11 metal atoms with respect to CO oxidation under thermal conditions as a competition between OC-O2 bond activation and surface diffusion, leading to metal atom agglomeration. Additionally, it is shown how coadsorption of CO and O2 on Au/Mg(001) could pave the way to single-atom photocatalysis.

A computational study of CO oxidation reactions on metal impurities in graphene divacancies.

Based on the density functional theory calculations, the formation geometry, electronic properties, and catalytic activity of metal impurities in divacancy graphene (M-DG, M = Mo, Fe, Co, and Ni) were systematically investigated. It has been found that the reactive gases have different stabilities on M-DG substrates, and these quite stable substrates exhibit high catalytic activity for CO oxidation by comparing the traditional Eley-Rideal (ER) and Langmuir-Hinshelwood (LH), as well as the new termolecular ER (TER) mechanisms. For the Co-DG substrate, the coadsorption of O2 and CO as a starting step is an energetically more favorable process, whereas the dissociation reaction of O2 molecules on Mo-DG substrate has a much smaller energy barrier, and the generation of atomic oxygen is active for CO oxidation. These results indicate that the varied adsorption behaviors of reactive gases on M-DG substrates can determine the catalytic pathways and energy barriers, which give us insight into the surface reactivity of graphene-metal composite catalysis in energy-related devices.

Atomic-Scale Insights into the Low-Temperature Oxidation of Methanol Over a Single-Atom Pt1-Co3o4 Catalyst

Heterogeneous catalysts with single-atom active sites offer a means of expanding the industrial application of noble metal catalysts. Herein, we present an atomically dispersed Pt1-Co3O4 catalyst, which exhibits an exceptionally high catalytic activity for the total oxidation of various representative volatile organic compounds. Experimental and theoretical investigations indicated that this catalyst consisted of Pt sites with a large proportion of occupied high electronic states. These sites exhibited a strong affinity for the 3d orbital of Co, which increased electron transfer from Pt to Co3O4 and ultimately, accelerated the rate of oxygen vacancy regeneration. In turn, this was determined to promote the co-adsorption of probe VOC and O2. With methanol, DFT calculations confirmed that the electron transfer over the oxygen vacancy reduces both the methanol adsorption energy and activation barriers for methanol oxidation, which was proposed to significantly enhance the dissociation of the C-H bond in the methanol decomposition reaction. This investigation serves as a solid foundation for characterizing and understanding single-atom catalysts for heterogeneous oxidation reactions.

CO oxidation on Al[sbnd]Au nano-composite systems

Using first principles method we report the CO oxidation behaviour of AlAu nano-composites in three different size ranges: Al6Au8, Al13Au42 and a periodic slab of AlAu(1 1 1) surface. The clusters prefer enclosed structures with alternating arrangement of Al and Au atoms, maximising Auδ−Alδ+ bonds. Charge distribution analysis suggests the charge transfer from Al to Au atoms, corroborated by the red shift in the density of states spectrum. Further, CO oxidation on these nano-composite systems was investigated through both Eley – Rideal and Langmuir Hinshelwood mechanism. While, these clusters interact with O2 non-dissociatively with an elongation of the OO bond, further interaction with CO led to formation of CO2 spontaneously. On contrary, the CO2 evolution by co-adsorption of O2 and CO molecules has a transition state barrier. On the basis of the results it is inferred that nano-composite material of AlAu shows significant promise toward effective oxidative catalysis.

Interfacial-Bonding-Regulated CO Oxidation over Pt Atoms Immobilized on Gas-Exfoliated Hexagonal Boron Nitride

We compared the electronic structure and CO oxidation mechanisms over Pt atoms immobilized by both B-vacancies and N-vacancies on gas-exfoliated hexagonal boron nitride. We showed that chemical bonds are formed between the B atoms associated with dangling bonds around the vacancies and Pt atoms. These bonds not only alter the thermodynamics and kinetics for the aggregation and effectively immobilize Pt atoms, but also significantly change the composition and energetic distribution of the electronic states of the composites to circumvent CO poisoning and to favour coadsorption of CO and O2, which further regulates the reactions to proceed through a Langmuir-Hinshelwood mechanism. The CO oxidation over Pt atoms immobilized at N-vacancies involves formation of an intermediate with –C(O)-O−O- bonded to Pt, the generation of CO2 by peroxo O−O bond scission and the reduction of the remnant oxygen, and the calculated energy barriers are 0.49, 0.23 and 0.18 eV, respectively. Such small energy barriers are comparable to those over Pt atoms trapped at B-vacancies, showing the effectiveness of Pt/hexagonal boron nitride atomic composites as catalysts for CO oxidation. These findings also suggest the feasibility of regulating the reaction pathways over single atom catalysts via interfacial engineering.

DFT study of the interactions of H2O, O2 and H2O + O2 with TiO2 (1 0 1) surface

The interactions of H2O, O2 and H2O+O2 with TiO2 (101) surface was studied using density functional theory (DFT). The adsorption energy of H2O+O2 on the TiO2 surface is the largest, followed by O2, and H2O. H2O molecule and O2 molecule adsorbed respectively on TiO2 surface are not dissociated, while the coadsorption of H2O and O2 results in the dissociation of H2O molecule and O2 molecule. H2O and O2 molecules not only interact respectively with TiO2 surface, but also interact with each other when H2O and O2 molecules adsorb simultaneously on the TiO2 surface. The interaction of H2O+O2 with TiO2 surface is more complex than those of single H2O or O2, including the formations of five new bonds and the cleavages of four bonds, which suggests the strong interaction between H2O+O2 and TiO2 surface. Two hydrogen-oxygen bonds of H2O molecule are all broken, and two hydrogen atoms interact with two oxygen atoms of TiO2 surface, respectively, while oxygen atom of H2O interacts simultaneously with a titanium atom of TiO2 surface and an oxygen atom of O2 molecule. Another oxygen atom of O2 molecule is bonded to another titanium atom of TiO2 surface.

New mechanistic pathways for CO oxidation catalyzed by single-atom catalysts: Supported and doped Au1/ThO2

Single-atom catalysts are of great interest and importance for designing new high-performance low-cost catalysts. We investigated CO oxidation catalyzed by single gold atoms supported on thoria (Au/ThO2) and doped ThO2 using density functional theory with Hubbard-type on-site Coulomb interaction (DFT + U). The calculation results show that the Au-doped ThO2(111) catalyst exhibits remarkable catalytic activity for CO oxidation via the Eley–Rideal mechanism in three steps, where the rate-determining step is decomposition of the OCOO* intermediate with an energy barrier of 0.58 eV. Moreover, our results also reveal a new mechanism of CO oxidation on a gold adatom supported by ThO2(111), where O2 is adsorbed only at the Th site on the surface, and the gas-phase CO then reacts directly with the activated O2* to form CO2, which is the rate-limiting step, with a barrier of 0.46 eV. It is found that CO oxidation can occur without CO and O2 coadsorption on Au, which was previously considered a key intermediate. Therefore, these results provide new insights into CO oxidation on isolated gold atoms supported by the 5f-element compound ThO2(111). This mechanism can help clarify the catalytic cycle of CO oxidation, support the design of high-performance low-cost catalysts, and elucidate the redox properties of actinide oxides.

Ethylene Epoxidation Catalyzed by a Cu38 Nanoparticle: A Computational Study

The reaction mechanisms of ethylene epoxidation on a Cu38 nanoparticle have been investigated by the first-principles calculations. The adsorption of C2H4 and O2 as well as the coadsorption of O2–C2H4 were also examined. Our results show that C2H4 and O2 are likely to adsorb on the top (T) and hollow (H) sites with adsorption energies of −0.51 and −2.19 eV, respectively. The climbing image nudged elastic band method was carried out to illustrate the potential energy profile of the ethylene oxidation reaction. Both epoxide and acetaldehyde are formed by the Langmuir–Hinshelwood mechanism. The overall reaction for the formation of epoxide is predicted to be exothermic by 3.20–3.56 eV whereas it is exothermic by 3.83–4.31 eV for the formation of acetaldehyde. The Cu38 nanoparticle exhibits higher catalytic properties and selectivity for the epoxide formation than the Au38 nanoparticle.

Co‐Adsorption of O2 and CO on Au2−: Infrared Photodissociation Spectroscopy and Theoretical Study of [Au2O2(CO)n]− (n=2–6)

The co-adsorption of O2 and CO on anionic sites of gold species is considered as a crucial step in the catalytic CO oxidation on gold catalysts. In this regard, the [Au2 O2 (CO)n ]- (n=2-6) complexes were prepared by using a laser vaporization supersonic ion source and were studied by using infrared photodissociation spectroscopy in the gas phase. All the [Au2 O2 (CO)n ]- (n=2-6) complexes were characterized to have a core structure involving one CO and one O2 molecule co-adsorbed on Au2- with the other CO molecules physically tagged around. The CO stretching frequency of the [Au2 O2 (CO)]- core ion is observed around ν˜ =2032-2042 cm-1 , which is about 200 cm-1 higher than that in [Au2 (CO)2 ]- . This frequency difference and the analyses based on density functional calculations provide direct evidence for the synergy effect of the chemically adsorbed O2 and CO. The low lying structures with carbonate group were not observed experimentally because of high formation barriers. The structures and the stability (i.e., the inertness in a sense) of the co-adsorbed O2 and CO on Au2- may have relevance to the elementary reaction steps on real gold catalysts.

Surface reaction network of CO oxidation on CeO2/Au(110) inverse model catalysts.

CeO2/Au(110) inverse model catalysts were prepared and their activity toward the adsorption and co-adsorption of O2, CO, CO2 and water was studied by means of X-ray photoelectron spectroscopy, low energy electron diffraction, thermal desorption spectra and temperature-programmed reaction spectra. The Au surface of CeO2/Au(110) inverse model catalysts molecularly adsorbs CO, CO2 and water, and the polycrystalline CeO2 surface of CeO2/Au(110) inverse model catalysts molecularly adsorbs O2, and molecularly and reactively adsorbs CO, CO2 and water. By controllably preparing co-adsorbed surface species on CeO2/Au(110) inverse model catalysts, we successfully identified various surface reaction pathways of CO oxidation to produce CO2 with different barriers both on the CeO2 surface and at the Au-CeO2 interface, including CO oxidation by various oxygen species, and water/hydroxyl group-involved CO oxidation. These results establish a surface reaction network of CO oxidation catalyzed by Au/CeO2 catalysts, greatly advancing the fundamental understandings of catalytic CO oxidation reactions.

In situ IR studies of Co and Ce doped Mn/TiO2 catalyst for low-temperature selective catalytic reduction of NO with NH3

The Mn–Co–Ce/TiO2 catalyst was prepared by wet co-impregnation method for selective catalytic reduction of NO by NH3 in the presence of oxygen. The adsorption and co-adsorption of NH3, NO and O2 on catalysts were investigated by in situ FTIR spectroscopy. The results suggested that addition of cobalt and cerium oxides increased the numbers of acid and redox sites. Especially, the cobalt oxide produced lots of Brønsted acid sites, which favor to the adsorption of coordinated NH3 through NH3 migration. Ce addition improved amide ions formation to reach best NO reduction selectivity. A mechanistic pathway over Mn–Co–Ce/TiO2 was proposed. At low-temperature SCR reaction, coordinated NH3 reacted with NO2−, and amide reacted with NO (ad) or NO (g) to form N2. NO2 was related to the formation of nitrite on Co-contained catalysts and the generation of NH2− on Ce-contained catalysts. At high temperature, the other branch reaction also occurred between the coordinated NH3 and nitrate species, resulting in N2O yield increase.

CO catalytic oxidation on Pt-doped single wall boron nitride nanotube: first-principles investigations

The catalytic oxidation of CO at Pt-doped BNNT (5,5) has been investigated theoretically using density functional theory. The electronic structures and thermochemical properties of CO and O2 that adsorbed on Pt embedded at the B- and N-vacancy sites of BNNTs are analyzed. It is demonstrated that the different BNNT substrates can modify the electronic structure of the Pt catalysts and cause different effects in the catalytic activities. With the N-vacancy (Pt(N)–BNNT), the Pt behaves as a Lewis acid for accepting an electron from the substrate, thus O2 binds stronger than CO molecules, thus alleviating the CO poisoning of the platinum catalysts. Coadsorption of CO and O2 on Pt(N)–BNNT results in additional charge transfer to O2. CO oxidation proceeds via the Eley–Rideal (ER) mechanism entails lower activation barrier and higher reaction rate than that of Langmuir–Hinshelwood (LH) mechanism suggesting the superiority of the ER mechanism for CO oxidation at Pt(N)–BNNT. Therefore, Pt(N)–BNNT might be a good candidate for low-cost, highly active, and stable catalysts for CO oxidation.

Theoretical investigation on coadsorption effect of O2 and H2O on Pt(111) surface

First-principles theory has been applied to study the coadsorption of O2 and H2O on Pt(1 1 1) surface. A series of coadsorption structures were considered in order to determine the most stable configuration. This study shows that the chemisorption state of H2O stabilizes the chemisorption state of O2, while the physisorption state of H2O does not affect the chemisorption state of O2. The adsorption reaction pathways for these structures were identified, and the chemisorption state of H2O was found to promote the O2 chemisorption, while the physisorption state of H2O hinders the O2 chemisorption. These results could shed light on the detailed interpretation of the O2 adsorption processes on Pt(1 1 1) surface.

Reaction mechanisms for CO catalytic oxidation on monodisperse Mo atom-embedded graphene

The geometric stability, electronic structure and catalytic activity of the Mo-embedded graphene (Mo/SV-graphene) are investigated by using the first-principle calculations. Compared with a Mo atom on pristine graphene, the Mo dopant in defective graphene exhibits more positively charged, which helps to weaken the CO adsorption and facilitates the O2 adsorption. Besides, the two mechanisms (Langmuir–Hinshelwood, LH and Eley–Rideal, ER) for the sequential CO oxidation reactions are investigated comparison. Among the reaction processes, the coadsorption of O2 and CO exists at the Mo/SV-graphene surface, the first step (CO + O2 → OOCO) with energy barrier is 0.60 eV and then form a CO2 molecule through the reaction (OOCO → CO2 + Oads) without any energy barrier, and thus the formation of OOCO complex is viewed as rate-controlling step. In the ER reaction, although the CO molecule reacts with the preadsorbed O2 by the low-energy barrier (CO + O2 → CO3, 0.13 eV), the formation of CO3 is more stable than the generating CO2 and Oads (0.84 eV). Hence, the LH mechanism as the starting step is energetically more favorable. The results provide the valuable guidance to fabricate graphene-based catalysis and validate the reactivity of atomic-scale catalyst.

High Catalytic Activity of Au Clusters Supported on ZnO Nanosheets

Catalytic activity of metal clusters supported on a two-dimensional (2d) nanosheet has attracted great interest recently. In this paper, via first-principles calculations, we investigate for the first time Au catalysis on recently proposed 2d one- atom-thick ZnO nanosheets. An Au8 cluster adsorbed on a 2d ZnO sheet (Au8/ZnO) is chosen as a model catalyst, and the CO oxidation is used as the probe reaction for the catalytic activity. When adsorbed on 2d one-atom-thick ZnO, the Au8 cluster is found to adopt a planar structure and exhibit high catalytic activities for the CO oxidation with a low reaction barrier around 0.3 eV. The catalyzed CO oxidation highly prefers the Eley− Rideal (ER) mechanism, and the coadsorption of CO and O2 on the Au8 cluster plays an essential role in the ER type of reaction in which an additional CO approaches O2 in a vacuum. The high catalytic activity (low reaction barrier) is due to the charge transfer from Au8/ZnO to the antibonding 2π* orbital of the O2 molecule that is facilitated by the coadsorption of a CO molecule. These findings provide new opportunities for the future development of ZnO-based catalysts.

Modelling of catalytically oxidative decomposition of carbon tetrachloride on a ZnS nanocluster using density functional theory

Using density functional theory (DFT), adsorption of O2 and co-adsorption of O2 and CCl4 on (ZnS)n clusters (n = 6, 7, 11, 12, 13, 15, 16, 17, 25, 26) and the mechanism of oxidative decomposition of CCl4 on a ZnS nanocluster were investigated. The decomposition of CCl4 without the ZnS cluster was also studied for comparison. The ZnS cluster achieved a significant catalytic effect during the oxidative decomposition of CCl4. It is found that CCl4 in gas phase undergoes a homolytic dissociation process with the activation by ZnS, reacting with O2 to produce a CCl3˙ radical and a ClOO˙ radical first. CO2 and Cl2 are identified as the final neutral reaction products, while COCl2, CO and Cl2O are produced as intermediates. The released energy of the total reaction, 149.83 kcal mol−1, is sufficient to afford the transition energy of the cataluminescence, based on which a novel and sensitive gas sensor for the determination of CCl4 could be constructed.

CO Oxidation on TiO2(110) Supported Subnanometer Gold Clusters: Size and Shape Effects

We performed a comprehensive study of catalytic activities of subnanometer Au clusters supported on TiO2(110) surface (Aun/TiO2, n = 1-4, 7, 16-20) by means of density functional theory (DFT) calculations and microkinetics analysis. The creditability of the chosen DFT/microkienetics methodologies was demonstrated by the very good agreement between predicted catalytic activities with experimental measurement (J. Am. Chem. Soc, 2004, 126, 5682-5483) for the Au1-4/TiO2 and Au7/TiO2 benchmark systems. For the first time, the size- and shape-dependent catalytic activities of the subnanometer Au clusters (Au16-Au20) on TiO2 supports were systematically investigated. We found that catalytic activities of the Aun/TiO2 systems increase with the size n up to Au18, for which the hollow-cage Au18 isomer exhibits highest activity for the CO oxidation, with a reaction rate ∼30 times higher than that of Au7/TiO2 system. In stark contrast, the pyramidal isomer of Au18 exhibits much lower activity comparable to the Au3-4/TiO2 systems. Moreover, we found that the hollow-cage Au18 is robust upon the soft-landing with an impact velocity of 200 m/s to the TiO2 substrate, and also exhibits thermal stability upon CO and O2 co-adsorption. The larger pyramidal Au19 and Au20 clusters (on the TiO2 support) display much lower reaction rates than the pyramidal Au18. Results of rate of reactions for unsupported (gas-phase) and supported Au clusters can be correlated by a contour plot that illustrates the dependence of the reaction rates on the CO and O2 adsorption energies. With the TiO2 support, however, the catalytic activities can be greatly enhanced due to the weaker adsorption of CO on the TiO2 support than on the Au clusters, thereby not only the ratio of O2/CO adsorption energy and the probability for the O2 to occupy the Ti sites are increased but also the requirement for meeting the critical line becomes weaker. The obtained contour plot not only can provide guidance for the theoretical investigation of catalytic activity on other metal cluster/support systems, but also assist experimental design of optimal metal cluster/support systems to achieve higher catalytic efficiency.

Theoretical investigation on CO oxidation catalyzed by a copper nanocluster

The mechanism of CO oxidation catalyzed by a 55-atom copper nanocluster was studied at the BVP86/DNP level with all-electron scalar relativity. Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) mechanisms were studied in detail. Adsorption of O2 on the copper cluster was prior to that of CO. The ER mechanism included three pathways, i.e. the O2 dissociation, the O abstract from the O2-pre-adsorbed Cu55 cluster by the gaseous CO, and the insertion of CO into the O–O bond of the O2-precovered Cu55 cluster to form a carbonate-like intermediate to complete the CO oxidation. The LH mechanism was originated from the co-adsorption of CO and O2 on the Cu55 cluster and had three pathways to the final product. The reaction channels involving the O2 dissociation, the O-abstraction, and the insertion of CO into the O–O bond vied with each other. The commonly accepted LH mechanism via the peroxo-like intermediate was unfeasible because of the unstable co-adsorption of CO and O2 on the Cu55 cluster. After incorporating the entropy effect, the CO-assisted O2 dissociation pathway was the most feasible reaction channel above 250 K. The strong interaction of oxygen atom with the copper surface was in favour of the O2 dissociation step, and was adverse to the reaction of the atomic oxygen toward CO to form CO2.