Reaction Pathways For CO Oxidation Research Articles

CO oxidation mediated by Al‐doped ZnO nanoclusters: A first‐principles investigation

AbstractUsing density functional theory the reaction pathways of CO oxidation mediated by Al‐doped Zn12O12 cluster and its assembled wire‐like (Zn12O12)n=2−4 structures were studied. It is revealed that O2 molecule is chemisorbed over the doped clusters while physisorbed over pristine (Zn12O12)n. Moreover, increasing the size of the nanocluster from AlZn11O12 to (AlZn11O12)4 enhances the O2 adsorption energy, although the amount of increase reduces as the cluster size grows. The adsorption energies of O2 over Al‐doped (Zn12O12)n clusters range from −1.83 to −2.14 eV, which are more negative than those of CO molecule (≈−0.80 eV). The Eley–Rideal (ER) and Langmuir–Hinshelwood (LH) pathways are used to investigate the oxidation mechanisms of the CO molecule. The energy barriers for the rate limiting step in the LH mechanism (i.e., OCOO → CO2 + Oads) are around 0.30 eV, which are substantially lower than the energy barriers in the ER process.

Reaction mechanism of Ca2Fe2O5 oxygen carrier with CO in chemical looping hydrogen production

The CO adsorption behavior, CO oxidation and oxygen anion diffusion of Ca2Fe2O5(010) surface were systematically investigated to deeply understand the reaction mechanism of Ca2Fe2O5 oxygen carrier with CO based on density functional theory (DFT) calculations. The results indicate that the activity of CO adsorption on four terminations of Ca2Fe2O5(010) surface follows the order: FeO2- terminated > FeO-terminated > CaO-terminated > OCa-terminated. Four reaction pathways for CO oxidation were proposed. It was found that CO oxidation reaction is very easy to proceed, and that both types of surface oxygen (O1 and O2) could exhibit high activity. The oxygen diffusion in Ca2Fe2O5 is inherently a high-temperature process. In addition, the energy barrier of oxygen diffusion is much higher than that of CO oxidation, implying that oxygen diffusion is the rate-limiting step for the entire reaction between Ca2Fe2O5 oxygen carrier and CO. The superior reactivity of Ca2Fe2O5 oxygen carrier is attributed to its excellent surface oxygen activity and good oxygen diffusion ability.

CO oxidation on Pt based binary and ternary alloy nanocatalysts: Reaction pathways and electronic descriptor

The design of stable, highly active heterogeneous nanocatalyst with no CO poisoning is a challenge for the catalytic converter industry. This can be achieved by defining electronic descriptor and by understanding the bonding mechanism. We present results using density functional theory to design optimal metal nanoalloy catalysts for CO oxidation. The adsorption configuration of O2 and O on different active sites is studied in detail to find the efficient reaction pathway for CO oxidation. The varying extent of back-donation of charge is found to be the factor deciding the CO tolerance. The trade-off between the activity and CO tolerance signifies that there is an upper limit for alloying Pt with Ni and Co. An electronic descriptor based on the occupancy of the d orbital and d band center of the host atom is defined to explain the site dependent activity of nanocatalysts. The role of underneath carbon surface on the CO oxidation activity of metal sites and CO poisoning is described.

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.

Theoretical mechanistic study of CO catalytic oxidation by O2 on an ultra-small 13-atom bimetallic Ag7Au6 cluster

We report an advanced configurational sampling method that uses density functional theory (DFT) to design a highly active catalyst for conversion of CO into less-harmful products, under ambient conditions. The reaction pathway for CO oxidation by O2 on ultra-small 13-Atom bimetallic Ag7Au6 cluster has two possible mechanisms, namely, stepwise adsorption and co-adsorption. The rate-determining step involving with COO association via a co-adsorption process shows a significantly small barrier of 0.21 eV. Furthermore, microkinetic simulation results suggest that CO oxidation rates and the optimal temperature for CO oxidation exhibit both greater performances for the co-adsorption pathway, compared to that for a stepwise-adsorption mechanism. Our new proposed mechanism suggests that the bimetallic Ag7Au6 catalyst is active for CO oxidation at room temperatures. Thus, it has potential application as a highly-active catalyst for conversion of carbon monoxide into less toxic CO2.

Temperature-dependent reaction pathways of CO oxidation and the application as monolithic catalysts for Co3O4 nanorods

Via investigating the evolution of carbonate intermediates by in-situ DRIFTS (25 °C–350 °C) analysis, temperature-dependent reaction pathways of CO oxidation over Co3O4 nanorods are successfully proposed. The types of reactive oxygen species participated in CO oxidation may affect the way CO react with neighboring oxygen atoms. Additionally, the negative effect of water and CO2 on the catalytic activity was also studied. Considering the remarkable reactivity towards CO, the Co3O4 catalysts were integrated onto metallic and cordierite monolithic substrates and used for CO conversion as application. SEM examinations show that the catalyst layer was coated on the substrates uniformly with few microcracks, and desirable mechanical shock and thermal shock resistance was observed. The influence of substrate materials, gas hourly space velocity, the existence of water vapor and CO2, and reaction time on the CO oxidation performance of monolithic catalysts was investigated. Metallic monolithic catalysts performed higher CO oxidation activity mainly owing to superior heat conductivity and larger surface area compared with cordierite monolithic catalysts. It is noticeable that superior activity was also achieved under high gas flow with the existence of H2O (5.6 vol.%) and CO2 (7 vol.%) at 220 °C, suggesting great promise for practical application.

TiC and TiN supported platinum monolayer as high-performance catalysts for CO oxidation: A DFT study.

The reactivity toward CO oxidation of Pt monolayer supported on TiC(001) and TiN(001) is studied by using empirical dispersion-corrected density functional theory calculations. A number of possible reaction pathways for CO oxidation, including the Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanisms, between adsorbed O2 and CO molecules considering the cases that the adsorbed O2 dissociates first or directly reacts with CO. It is found that the dissociation adsorption of O2 molecules as the initial step is more favorable with lower activation barriers compared with the direct reaction mode. Hence the dissociation of adsorbed O2 molecules plays a very key role in the CO oxidation reaction. For both Pt monolayer systems, our analyses also reveal that the reaction is most likely predominant via the initial ER mechanism and the subsequent LH mechanism. Furthermore, by comparing the activation barriers of the rate-limiting steps, CO oxidation on PtML/TiN(001) shows a higher catalytic activity than that on PtML/TiC(001), showing the important role that the support would play in the catalytic reactions. The present results suggest that the TiN supported monolayer Pt catalyst appears to be a good candidate for CO oxidation at low-temperature.

Observation of temperature-dependent kinetics for catalytic CO oxidation over TiO2-supported Pt catalysts

TiO2-supported Pt catalysts were prepared for the study of CO oxidation kinetics at reaction conditions of pressures (1–100Torr) and temperatures (300–500K). We find two distinct temperature ranges with different reaction kinetics distinguished by an abrupt slope change at around 380K in the Arrhenius plot only with the excess O2, not with the stoichiometric O2/CO ratio. We propose that Pt oxides are formed during the catalytic reactions with increasing temperatures under the oxidizing condition and the origin of the slope change is due to the changes in the reaction pathways of CO oxidation due to the Pt oxide formation.

Understanding the catalytic activity of nanoporous gold: Role of twinning in fcc lattice.

Nanoporous gold (NPG) prepared by de-alloying Al2Au exhibits correlation between the high catalytic reactivity towards CO oxidation and the density of twinning defects in the fcc lattice of NPG. It was also discovered that on the internal surface of NPG, quite common twinning defects can create close-packed rows of six-coordinated catalytically active Au atoms denoted as W-chains. In this work, using density functional theory methods, we investigate energy conditions for formation, thermal stability, and chemical reactivity of these active sites. The possibility of dioxygen chemisorption on various surface sites is studied in detail. A contribution from the dispersion interactions is also considered. The calculated surface density of the active six-coordinated atoms in NPG comparable with that of supported gold nanoparticle catalysts, exothermic chemisorption of dioxygen, and the energy profiles of reaction pathways for CO oxidation indicate that the six-coordinated sites created by twinning can significantly contribute to the catalytic activity of NPG.

Structure dependence and reaction mechanism of CO oxidation: A model study on macroporous CeO2 and CeO2-ZrO2 catalysts

Three-dimensionally ordered macroporous (3DOM) CeO2 and CeO2-ZrO2 oxides with different particle sizes, oxygen vacancy concentrations, oxygen mobility and preferentially exposed surface planes were synthesized, which, as model catalysts, give a new approach for understanding the structural dependence and reaction mechanism of CO oxidation over CeO2-based catalysts. The prepared 3DOM CeO2 and CeO2-ZrO2 catalysts exhibited much higher catalytic activity for CO oxidation than the nonporous samples. Although the textural and reducible features of catalysts could affect the catalytic performance, the preferentially exposed surface plane is crucial for determining the catalytic activity. The {110} plane of CeO2 could create more active sites for CO adsorption, resulting in relatively high activity for CO oxidation. Higher concentration of oxygen vacancy would also enhance the reactivity for CO oxidation. Langmuir-Hinshelwood mechanism should be the crucial reaction pathway for CO oxidation over the 3DOM CeO2-based catalysts, although the Mars-van Krevelen mechanism cannot be ignored.

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.

The behaviors of ultra-low-gold-loaded catalysts (Au/CeO2) for CO oxidation in the presence of water on the catalysts

The catalytic behavior of ultra-low-gold-loaded Au/CeO2 during CO oxidation was studied using HRTEM, TGA, in-situ FTIR, CO–TPD, O2–TPD, and O2–TPO. Herein, water, which facilitates CO oxidation, is present on the catalyst rather than obtained from the reactant gas. The experimental results show that the presence of water on the catalyst enhances the adsorption and activation of O2 to produce Oact on the catalyst surface, resulting in excellent catalytic activity. Different reaction pathways for CO oxidation over Au/CeO2 may be present; however, the dominant pathway is influenced by the species on the catalyst surface.

On the structure dependence of CO oxidation over CeO2 nanocrystals with well-defined surface planes

CO oxidation is a model reaction for probing the redox property of ceria-based catalysts. In this study, CO oxidation was investigated over ceria nanocrystals with defined surface planes (nanoshapes) including rods ({110}+{100}), cubes ({100}), and octahedra ({111}). To understand the strong dependence of CO oxidation observed on these different ceria nanoshapes, in situ techniques including infrared and Raman spectroscopy coupled with online mass spectrometer, and temperature-programmed reduction (TPR) were employed to reveal how CO interacts with the different ceria surfaces, while the mobility of ceria lattice oxygen was investigated via oxygen isotopic exchange experiment. CO adsorption at room temperature leads to strongly bonded carbonate species on the more reactive surfaces of rods and cubes but weakly bonded ones on the rather inert octahedra surface. CO-TPR, proceeding via several channels including CO removal of lattice oxygen, surface water–gas shift reaction, and CO disproportionation reaction, reveals that the reducibility of these ceria nanoshapes is in line with their CO oxidation activity, i.e., rods>cubes>octahedra. The mobility of lattice oxygen also shows similar dependence. It is suggested that surface oxygen vacancy formation energy, defect sites, and coordinatively unsaturated sites on ceria play a direct role in facilitating both CO interaction with ceria surface and the reactivity and mobility of lattice oxygen. The oxygen vacancy formation energy, nature and amount of the defect and low coordination sites are intrinsically affected by the surface planes of the ceria nanoshapes. Several reaction pathways for CO oxidation over the ceria nanoshapes are proposed, and certain types of carbonates, especially those associated with reduced ceria surface, are considered among the reaction intermediates to form CO2, while the majority of carbonate species observed under CO oxidation condition are believed to be spectators.

First-Principles Study of Carbon Monoxide Oxidation on Ag(111) in Presence of Subsurface Oxygen and Stepped Ag(221)

The role of subsurface oxygen in Ag(111) and the step edge of vicinal Ag(221) on CO oxidation were studied by using density functional theory calculations. For high oxygen coverage, the formation of subsurface oxygen is not only energetically favorable but also kinetically likely. CO oxidation with on-surface atomic oxygen is facile with a barrier of about 0.16 eV, whereas the reaction with subsurface oxygen is hindered because of the significant barrier for diffusion of the subsurface oxygen to the surface from the subsurface region. It is found that the adsorption of molecules (CO, atomic O and O2) is stabilized in the presence of subsurface oxygen, and surface reactivity is enhanced. An energetically favorable reaction pathway for CO oxidation with O2 is identified, with a reaction barrier of 0.23 eV via a unique four-center Ο2···CΟ intermediate. A catalytic cycle for CO oxidation on Ag(111) in the presence of subsurface oxygen is proposed. On Ag(221) surfaces, our calculations show that both the adsor...

Reaction pathway analysis for CO oxidation over anionic gold hexamers using DFT

The reaction pathways for CO oxidation over anionic gold hexamer (Au6−) as a model catalyst were studied using unrestricted density functional theory (UDFT). The reaction mechanisms for two possible initial structures, in which either CO or O2 was adsorbed on the apex site while the other was on the nearest lateral site, were analyzed. The reaction proceeds through a typical Langmuir–Hinshelwood mechanism taking place in two steps for both initial structures. First, CO reacts with the molecular O2 leading to the first CO2 molecule and Au–O complex through the formation of a four-centered intermediate structure (CO–OO). This is followed by the elimination of the remaining O on the cluster by another CO. It was found that the energy barriers for the formation of the intermediate and its decomposition into CO2 and Au–O complex were much lower when O2 was bonded to the apex site. The second step, on the other hand, was found to be identical for both initial structures.

The Effect of Water on the CO Oxidation on Ag(111) and Au(111) Surfaces: A First-Principle Study

Density functional theory has been used to study the effect of water on the molecular (02, 0, and CO) adsorption, O-2 dissociation, and CO oxidation on the Ag(111) and Au(111) surfaces. It is found that, though CO oxidation with atomic O is facile on both surfaces with barriers less than 0.29 eV, considerable barriers for O-2 dissociation (1.03 eV for Ag(111) and 1.97 eV for Au(111)) and weak adsorption of reactants limit their overall reactivity. Our calculations show that the reactants can be stabilized by coadsorption of water via the formation of an H bond and/or the interaction mediated through the substrates. The stabilization induced by coadsorbed water affects not only adsorption of reactants but also the transition states and intermediates, which enhances overall reactivity for CO oxidation, correspondingly. H bonds facilitate the dissociation of O-2 with reduction of barrier by 0.36 eV on Ag(111). Moreover, a highly active reaction pathway for CO oxidation via molecular assistant by water with overall barrier of 0.15 eV is identified. Atomic oxygen formed may either react with adsorbed CO or react with adsorbed water to form hydroxyls, which oxidizes CO subsequently, to complete the catalytic cycle.

Similarities and differences on the molecular mechanism of CO oxidation on Rh(111) and bimetallic RhCu(111) surfaces

The reaction between adsorbed CO and atomic O on various sites of Rh(111) and on the bimetallic RhCu(111) surface has been investigated by first principles density functional theory using slab models. The most likely reaction pathway for CO oxidation on Rh(111) involves probably migration of atomic oxygen from fcc to hcp sites. On the bimetallic surface the mechanism is similar, although depending on the type of bimetallic site a reduction of the energy barrier is predicted. Consequences for the NO reduction by CO reaction are analyzed.

Formation of subsurface oxygen species and its high activity toward CO oxidation over silver catalysts

Silver is well known to show peculiar catalytic activities in several oxidation reactions. In the present paper, we investigate the catalytic activity of silver catalysts toward CO-selective oxidation in H 2. XRD, TEM, TPD, and in situ FTIR techniques were used to characterize the catalysts. The pretreatment of the catalysts was found to have great influence on their performance. The pretreatment in O 2 improves the activity of the silver catalyst, whereas He pretreatment at 700 °C or direct hydrogen pretreatment shows an inverse effect. Silver catalysts undergo massive structural change during oxygen pretreatment at high temperatures ( > 500 °C ), and there is solid evidence for the formation of subsurface oxygen species. The existence of this silver-subsurface oxygen structure facilitates the formation of active sites on silver catalysts for CO oxidation, which are related to the size, morphology, and exposed crystal planes of the silver particles. Its formation requires a certain temperature, and a higher pretreatment temperature with oxygen is required for the silver catalyst with a smaller particle size. It is observed, for the first time, that adsorbed CO on the surface of silver particles can directly react with subsurface oxygen species at low temperatures (e.g., RT), and the surface oxygen can migrate into and refill these subsurface sites after the consumption of subsurface oxygen by the reaction with CO. This finding provides a new reaction pathway for CO oxidation on silver catalyst.

Density-functional theory study of the catalytic oxidation of CO over transition metal surfaces

In recent years, due to improvements in calculation methods and increased computer power, it has become possible to perform first-principles investigations for ‘simple’ chemical reactions at surfaces. We have carried out such studies for the catalytic oxidation of CO at transition metal surfaces, in particular, at the ruthenium surface for which unusual behavior compared to other transition metal catalysts has been reported. High gas pressure catalytic reactor experiments have revealed that the reaction rate over Ru for oxidizing conditions is the highest of the transition metals considered — in contrast, under ultra-high vacuum conditions, the rate is by far the lowest. We find it important for understanding the pressure dependence of the reaction that Ru(0001) can support high concentrations of oxygen at the surface. Under these conditions, the O–metal bond is atypically weak compared to that at lower coverages. We have investigated a number of possible reaction pathways for CO oxidation for the conditions of high oxygen coverage, including scattering reactions of gas-phase CO at the oxygen covered surface (Eley–Rideal mechanism) as well as the Langmuir–Hinshelwood mechanism involving reaction between adsorbed CO molecules and O atoms.

Yttria-Stabilized Zirconia Supported Copper Oxide Catalyst: II. Effect of Oxygen Vacancy of Support on Catalytic Activity for CO Oxidation

Copper oxide was supported on yttria-stabilized zirconia (YSZ) and γ-alumina, respectively, using impregnation methods. The supported copper oxide catalysts were characterized by temperature-programmed reduction. The catalytic activity of copper oxide supported on YSZ for CO oxidation was measured and compared with those supported on γ-alumina and with commercial γ-alumina supported precious metals (PMs) (i.e., Pd and Pt). Results of the activity tests indicated that CuO/YSZ catalysts can exhibit PM-like behaviors such as the light-off characteristic and the hysteresis phenomenon but CuO/γ-alumina catalysts cannot do so. The significant and conspicuous activity enhancement of the YSZ supported copper oxide has been ascribed to the modification of reaction mechanism due to the surface oxygen vacancy of YSZ. An active center composed of an interfacial Cu+ion and a surface oxygen vacancy of YSZ and formed by an interfacial metal oxide–support interaction is proposed to explain the activity enhancement. These interfacial active centers could provide a second reaction pathway for CO oxidation via the so-called “metal–support interfacial reaction” step, for which the turnover frequency is much higher than that of redox cycle mechanism operated on the CuO/γ-alumina catalyst. These interfacial active centers can be destroyed by chlorine addition due to blocking of the surface oxygen vacancies of YSZ by the strongly adsorbed chlorine. The reaction mechanism and catalytic behavior of PM/γ-alumina, CuO/γ-alumina, and CuO/YSZ catalysts are compared and discussed.