Activation Energy For CO Oxidation Research Articles

Mechanistic study of low-temperature CO oxidation over CuO/Cu2O interfaces with oxygen vacancy modification

Copper oxides have attracted great attention in the oxidation of CO due to their high activity at low temperatures. In this work, novel CuO/Cu2O heterojunction catalysts for low-temperature CO oxidation was prepared via hydrothermal in-situ oxidizing in H2O2. Based on studies of surface composition and the corresponding catalytic activities, the CuO/Cu2O catalysts contain rich oxygen vacancies (OVs), which improve the activation and migration ability of O2. The coupling effect between OVs and heterojunction reduces the activation energy of CO oxidation, leading the temperature of CO completed oxidation temperature from 250 °C to 140 °C. In addition, the in-situ Fourier transform infrared spectroscopy (in-situ FTIR) and Density functional theory (DFT) studies confirm two reaction paths under Langmuir-Hinshelwood (LH) mechanism: a low barrier path takes place at low temperature, and another new reaction path with a barrier of 1.8 eV can take place at a high temperature. The new reaction site formed at the CuO/Cu2O interface resulted in different activation modes of CO and O2. The modification of OVs significantly reduces the reaction temperature required. This work provides a method to improve the performance of co-catalytic oxidation via the coupling effect of OVs and heterojunctions.

The π back-donation influence in CO oxidation on small and oxidized Au–Ag clusters

CO oxidation catalyzed by bimetallic Au–Ag clusters. • The π back-donation as an indicator of reactivity in CO oxidation reactions. • The π back-donation depletion in the M−CO interaction promotes CO oxidation. • The bimetallic Au 2 Ag 2 presents the lowest energy barrier for CO 2 formation. • AuAg represents an improvement over pure Au and Ag clusters in CO oxidation. Carbon monoxide is a pollutant that can be removed by oxidation in catalytic converters. This work presents CO oxidation on small and oxidized Au n , Ag m , and bimetallic Au n Ag m ( n + m ≤ 6) clusters using theoretical calculations by the ZORA-DFT approach. During CO adsorption, the Mayer bond order follows the trend: Au–CO > AuAg–CO > Ag–CO. The CO adsorption on bimetallic AuAg clusters decreases π back-donation, which reduces the M−CO bond covalent nature. The bimetallic II Au 2 Ag 2 presents the lowest energy barrier for the CO 2 formation, 0.08 eV, where the Langmuir-Hinshelwood mechanism constitutes a viable route for CO oxidation. Bimetallic Au–Ag represents an improvement compared to pure Au and Ag clusters because they reduce the M−CO bond’s covalent nature, causing weaker M−CO bonds. These factors reduce the activation energy of CO oxidation. We propose the π back-donation as an indicator of cluster reactivity.

Cobalt oxide confined in mesoporous SiO2 as effective catalyst for CO oxidation

Ordered mesoporous SiO 2 samples (SBA-15) with different pore sizes were prepared as carriers, and a series of catalysts (CoO x @SBA-15( X )) were synthesized to confine CoO x in SBA-15 through the solid-state grinding method for CO oxidation. The characterization results showed that the aggregation of CoO x in the pores of the carrier SBA-15 was effectively inhibited by the confinement effect, which further facilitated the formation of the main catalytic site Co(III) species. The results of the density-functional theory calculations further confirmed that Co(III) was the important catalytic site for CO oxidation. Compared with the catalyst prepared through the impregnation method, catalysts CoO x @SBA-15( X ) exhibited a lower CO conversion temperature and activation energy for CO oxidation. In addition, the pore size of the carrier SBA-15 had a significant impact on the catalytic activity of CoO x , and the catalyst prepared with a larger pore size SBA-15 as carrier exhibited a higher catalytic activity. This result was mainly attributed to the fact that the confinement effect could effectively enhance the defect formation in metal oxides. Furthermore, the catalyst with a larger pore size SBA-15 as a carrier presented a higher content of Co(III) species, which significantly enhanced the catalytic activity of CoO x for CO oxidation. The results demonstrated that the pore structure of SBA-15 could affect the formation of the metal oxide (CoO x ) species, which further significantly affected the catalytic activity of CoO x for CO oxidation. The results are expected to provide a strategy to synthesize efficient catalysts for CO oxidation by using ordered mesoporous materials. • Solid state grinding method is an effective way to confine CoO x in SBA-15 pores. • Confinement effect could enhance the catalytic activity of catalysts for CO oxidation. • SBA-15 with larger pore size was more favorable for the formation of Co(III) species.

Insight into the synergy effect on the improved reactivity of spinel NiMn2O4 oxygen carrier in chemical looping combustion

Spinel NiMn2O4 is a promising oxygen carrier due to the synergistically enhanced performance, but it remains elusive how the synergy effect acts on the reactivity. Herein, the synergistically improved reactivity of spinel NiMn2O4 was systematically studied by thermogravimetric analysis (TGA) tests and theoretical calculations. Experimental results suggested that the reduction of NiMn2O4 by CO is divided into two stages, which corresponds to the successive reduction steps of NiO and Mn2O3. The reduction products of NiMn2O4 are Ni and MnO, and the former will synergistically promote the reduction of Mn2O3 into MnO. Owing to the presence of Ni atoms, the reactivity of NiMn2O4 is superior to Mn2O3. Moreover, the reaction mechanism of NiMn2O4 with CO was unraveled by the density functional theory (DFT) approach. The results indicate that CO prefers the chemisorption on the Ni–O bridge site. Two-step reactions are involved in the CO oxidation reaction: CO adsorption and CO2 desorption, and the latter is the rate-determining step. Three types of O atoms exhibit different reactivity because they bond with different metal atoms. The activation energy of CO oxidation via the three types of O atoms are 18.20, 36.53 and 67.53 kJ·mol−1. The O atom bonding with two Ni and one Mn atoms has the highest reactivity, implying that the Ni atom can improve the reactivity of Mn-based oxygen carrier. This work reveals a new insight on the synergistically improved reactivity of spinel NiMn2O4.

Fabricating Cu2O-CuO submicron-cubes for efficient catalytic CO oxidation: The significant effect of heterojunction interface

In this work, the uniform Cu2O submicron-cubes were facilely synthesized by liquid phase reduction method. Then, the Cu2O submicron-cubes were further oxidized into Cu2O-CuO heterojunction with tunable Cu2+/Cu+ ratios and CuO submicron-cubes by controlling the calcination temperature. The phase transition period during calcination was real-time monitored by the in-situ XRD and in-situ DRIFTS. The obtained materials were investigated as the catalysts of CO oxidation. The results revealed that the Cu2O-CuO heterojunction catalysts performed much higher catalytic activities than the Cu2O and CuO counterparts. Because the synergistic effect of the heterojunction (Cu2+/Cu+) could increase the surface oxygen vacancy concentration. Furthermore, it was also found that only the Cu2O-CuO heterojunction structure with the appropriate Cu2+/Cu+ ratio behaved the optimum catalytic activity. The kinetic studies indicated that the apparent activation energy of CO oxidation was greatly affected by the Cu2+/Cu+ ratio. Therefore, these Cu2O-CuO submicron-cubes with heterostructure were considered as the promising CO oxidation catalysts.

Effect of electron tunneling through the oxide film grown on metal substrate upon the efficiency of molecular reaction over the oxide supported metal nanoparticles: A case of CO oxidation on Au/Al2O3/Mo(110)

Formation of gold nanosized particles supported by aluminum oxide film grown on Mo(110) substrate and oxidation of carbon monoxide molecules on their surface have been in-situ studied in ultra-high vacuum by means of Auger electron spectroscopy (AES), reflection-absorption infrared spectroscopy (RAIRS), low energy electron diffraction (LEED), atomic force microscopy (AFM), temperature programmed desorption (TPD) and work function measurements. The main focus was to follow how the thickness of the alumina film influences the efficiency of CO oxidation in an attempt to find out an evidence of possible effect of electron tunneling between the metal substrate and the Au particle through the oxide interlayer. Providing the largest degree of surface identity of the studied metal/oxide system at different thicknesses of the alumina film (2, 4, 6 and 8 monolayers), it was found that the CO oxidation efficiency, defined as CO2 to CO TPD peaks intensity ratio, as well as the activation energy of CO oxidation, exponentially depend upon the oxide film thickness. Taking into account the known fact that the CO oxidation efficiency depends on the amount of excess charge acquired by Au particle, the latter suggests that electron tunneling adds efficiency to the oxidation process, although not significantly.

Surface reduction properties of ceria-zirconia solid solutions: a first-principles study.

Based on the density functional theory (DFT), the reduction properties of Ce1−xZrxO2 (110) surfaces were systematically calculated using CO as a probe for thermodynamic study, and a large supercell was applied to build the whole composition range (x = 0.125, 0.250, 0.375, 0.500, 0.625, 0.750, 0.875). From the calculated energy barriers of CO oxidation by lattice oxygen, we found that composition Ce0.875Zr0.125O2 exhibited the most promising catalytic effectiveness with the lowest activation energy of 0.899 eV. Moreover, the active surface O3c ions coordinated by two Zr ions and one Ce ion were facilely released from their bulk positions than the O3c ions surrounded by two Ce ions and one Zr ion on Ce0.625Zr0.375O2, Ce0.500Zr0.500O2, and Ce0.375Zr0.625O2 (110) surfaces. This difference could be explained by the binding strength of O3c with different neighboring cations.

Catalytic Enhancement of CO Oxidation on LaFeO3 Regulated by Ruddlesden-Popper Stacking Faults.

The influence of planar defects, in the form of stacking faults, within perovskite oxides on catalytic activity has received little attention because controlling stacking-fault densities presents a major synthetic challenge. Furthermore, stacking faults in ceramics are not thought to appreciably impact surface chemistry, which partly explains why their direct effect on catalysis is generally ignored. Here, we show that Ruddlesden-Popper (RP) stacking faults in otherwise stoichiometric LaFeO3 can be broadly controlled by modulating the ceramic synthesis route. Electronic structure calculations along with electron microscopy and spectroscopy show that energetically favorable RP faults occur both near the surface and in bunches and enhance CO oxidation kinetics. Density functional theory (DFT) + U shows that subsurface RP faults strengthen the adsorption and co-adsorption of CO, O, and O2, which could lower the apparent activation energy of CO oxidation on faulted catalysts compared to that on their pristine counterparts. Our work suggests that planar defects should be considered a new and useful feature in hierarchal nanoscale design of future catalysts.

Catalytic performance of Ce1-xLnxOy of nanocrystalline Ln (III-IV)-substituted ceria

The activity of several lanthanide promoted Ceria in CO oxidation was studied. Samples were obtained by a high yield and sustainable method by urea thermal decomposition. Precursors basic carbonates, Ce1-xLnx(OH)CO3, being Ln = La (III), Sm(III), Gd(III), Pr(III) were prepared in the range of compositions 0 to 30 at.%. These precursors were annealed to moderate temperature obtain Ln-Ce mixed oxides, at 450ºC. These exhibit large surface areas up to 120 m2/g. Their catalytic performance revealed good activity towards CO oxidation (COOX) for all samples. However, a different behavior may be observed for Ln content higher than 20 at.%. By the reactor operation in differential conditions, the activation energy for COOX were obtained for all samples. The rise of activation energy as a function of Ln(III-IV) content is analyzed in the frame of phase stability and surface segregation.

The Vibronic Theory of Size Effects in CO Oxidation by Gold Nanocatalysts

A theoretical model, based on the vibronic theory of catalysis, has been developed for the CO oxidation reaction catalyzed by gold nanoparticles. The charge fraction transferred between the catalyst and the adsorbed CO molecule depends upon the relative positions of the size-dependent Fermi level of the cluster and the energy levels of the molecular orbitals of CO adsorbed onto the cluster. The charge transferred from the nanocluster to the adsorbed CO molecule increases the absolute values of forces deforming the molecule chemical bond and force constants defining the curvature of potential for nuclear oscillation leading to the decrease in activation energy for CO oxidation. The application of vibronic and jellium theories to CO oxidation provides an explanation of the observed cluster size effects. The relative locations of electronic energy levels of a gold cluster and a CO molecule before and after the adsorption onto the cluster surface

Copper oxide catalyst supported on niobium oxide for CO oxidation at low temperatures

CuOx/Nb2O5 shows higher CO oxidation activity than CuOx/Al2O3. Analysis of X-ray photoelectron spectra shows the presence of Cu1+ species on Nb2O5, which is known to be the active species for CO oxidation. In contrast, only Cu2+ is found to be present on Al2O3. Enhanced activity of CuOx/Nb2O5 is consistent with its lower activation energy for CO oxidation compared with CuOx/Al2O3. It is argued that the formation of stable Cu1+ species on Nb2O5 is a result of presence of deep copper acceptor states within the band gap of Nb2O5. 6%CuOx/Nb2O5 shows comparable activity to 1%Pt/Al2O3 but loses its advantage above 500°C.

The effect of H2 treatment at 423–573 K on the structure and synergistic activity of Pd–Cu alloy catalysts for low-temperature CO oxidation

The effect of H2 treatment at 423–573K on the structure of Cu/Pd/Al2O3 catalysts for low-temperature CO oxidation was studied by means of XRD, TEM, EDS, FFT, DRIFTS, and XPS methods. The formation of 5nm alloy particles in the Cu/Pd catalyst reduced at 423K was detected. The reaction on Cu/Pd, Pd, and Cu catalyst reduced at 423K starts at 275, 400, and 475K, respectively. The activation energy for CO oxidation on Cu/Pd, Pd, and Cu catalyst is 30, 90, and 110kJ/mol, respectively. The enhanced activity of Cu/Pd catalyst is explained by oxidation on Pd0–MOx interfaces (M=Pd, Cu). The key steps of this process are: oxidation of CO molecules strongly bound to Pd0 by lattice oxygen of MOx; fast desorption of CO2 molecules; replenishment of oxygen vacancies in MOx by oxygen molecules from reaction media. As a result, a substantial part of “blocked” Pd0 sites become available for adsorption and dissociation of O2 molecules. This is favorable for low-temperature oxidation via Langmuir–Hinshelwood mechanism. The increase in the reduction temperature results in a separation of alloy particles into metallic particles. As a result, Cu/Pd catalysts reduced at high temperatures are characterized with a decreased content of Pd0–MOx interfaces; an essential amount of “blocked” Pd0 sites and a decreased light-off activity.

Benchmarking of manganese oxide materials with CO oxidation as catalysts for low temperature selective oxidation

CO oxidation behavior of different manganese oxide materials was studied as a model probe reaction to benchmark catalytic activities that could be useful for other aerobic oxidations. K-OMS-2, which showed intermediate activity [Ea=3.0 and 6.3kcal/mol for K-OMS-2 (made in solvent free procedure or SF) and K-OMS-2 (made via hydrothermal route or REF) respectively], were chosen for metal dopant studies using microwave assisted hydrothermal synthesis. These were further characterized using X-ray diffraction (XRD), N2 sorption, Raman spectroscopy, scanning electron microscopy (SEM), and carbon monoxide temperature programmed reduction (CO-TPR). Additional characterization was carried out for Ni-K-OMS-2 materials using atomic absorption spectroscopy (AAS) and thermogravimetric analysis-mass spectroscopy (TGA-MS). Doping K-OMS-2 with Ni resulted in a large increase in oxygen mobility and re-adsorption, and the moderate activation energy for CO oxidation (13.kcal/mol) by Ni-K-OMS-2(SF) suggests that Ni-doped materials are good candidates for low temperature selective oxidation.

Synthesis and characterization of pure and (Ce, Zr, Ag) doped mesoporous CuO-Fe2O3 as highly efficient and stable nanocatalysts for CO oxidation at low temperature

A series of single and mixed oxide nanocatalysts of mesoporous CuO-Fe2O3 with different CuO contents (1–50wt.%) were prepared by a co-precipitation method and further promoted by trace amounts of CeO2, ZrO2 and Ag2O (0.1–0.5wt.%) dopants. The original and calcined catalysts were characterized by TG, DTA, XRD, TEM, VSM, N2 sorption analysis, surface chemisorbed oxygen and DC electrical conductivity measurements. The catalytic performance of these nanocatalysts toward CO oxidation was studied using a conventional fixed bed flow type reactor. The results revealed that the addition of 1–20wt.% CuO to Fe2O3 monotonically increases the specific surface area, the amount of surface chemisorbed oxygen, electrical conductivity and catalytic activity of the nanocatalysts. In addition, the catalytic activity indicated that Fe-Cu mixed oxide nanocatalyst promoted with the three dopants (CeO2, ZrO2 and Ag2O) exhibited the highest catalytic activity with a total conversion of CO into CO2 at 100°C. Moreover, the activation energy of CO oxidation decreased from 38.4 to 23.1kJmol−1 upon treating the catalyst containing 20wt.% CuO with the three dopants. Finally the effects of various operational parameters were also studied.

Low-temperature CO oxidation by Co3O4 nanocubes on the surface of Co(OH)2 nanosheets

CO oxidation has been performed on Co3O4 nanocubes and Co(OH)2 nanosheets as model catalysts. The reaction rate of CO on Co3O4/Co(OH)2 nanocomposites obtained by one-pot synthesis is about ~two orders of magnitude higher than that on Co3O4 nanomaterials. The catalytic behaviors of different nanomaterials revealed that the assembly with nano building blocks cause the catalytic sites much more active for CO oxidation. The kinetic data showed that the activation energy for CO oxidation over Co3O4/Co(OH)2 nanocomposites was lower than that of other nanomaterials. Since Co(OH)2 nanosheets can prevent Co3O4 nanocubes aggregating, nanocomposites kept good catalytic stability.

Reaction of CO with Preadsorbed Oxygen on Low-Index Copper Surfaces: An Ambient Pressure X-ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study

The reaction of CO with chemisorbed oxygen on three low-index faces of copper was studied using ambient pressure X-ray photoelectron spectroscopy (XPS) and high-pressure scanning tunneling microscopy. At room temperature, the chemisorbed oxide can be removed by reaction with gas-phase CO in the 0.01–0.20 Torr pressure range. The reaction rates were determined by measuring the XPS peak intensities of O and CO as a function of time, pressure, and temperature. On Cu(111) the rate was found to be one order of magnitude faster than that on Cu(100) and two orders of magnitude faster than that on Cu(110). The apparent activation energies for CO oxidation were measured as 0.24 eV for O/Cu(111), 0.29 eV for O/Cu(100), and 0.51 eV for O/Cu(110) in the temperature range between 298 and 473 K. These energies are correlated to the oxygen binding energies on each surface.

Selectively combusting CO in the presence of propylene

Acrylic acid plant capacity can be increased by 30% by substituting propane for nitrogen (in air) thereby increasing the overall gas heat capacity. To minimize propane purge rates, the effluent is recycled but the CO in the recycle stream must be oxidized to avoid poisoning the catalyst. We studied the kinetics of CO combustion in a micro-fluidized bed with a catalyst inventory of 1g and a 4cm ID reactor charged with 5g of catalyst – Pd/zeolite – and 145g of inert. Experiments were run at temperatures between 90 and 240°C and at 1 and 3.2 bara. At temperatures below 140°C propylene conversion was less than 30% while CO conversion approached 90% at gas hourly space velocities near 10,000h−1. A first order kinetic model characterized the data over the whole range of conditions in which conversion was varied between 2% and 99+%. The rate constant for CO conversion was equal to 0.75s−1 whereas it equaled 0.04s−1 for propylene oxidation; the activation energy for CO oxidation was 95kJmol−1K−1 whereas it was 140kJmol−1K−1 for propylene.

Insight into the active phase of CO oxidation on Ni/Pt and NiO1 − x/Pt model catalysts from a first principles investigation

CO oxidation on bimetallic and metal oxide has drawn much attention in the past years due to its importance both technologically and theoretically, but the active phase as well as the detailed reaction mechanism on the bimetallic surface oxide (i.e., a sandwich-like surface structure) are still unclear. In this work, the CO oxidation on the various Pt–Ni model catalysts [including Pt(111), Pt/Ni/Pt(111), Ni/Pt(111), NiO1−x/Pt(111) and NiO1−x/Pt/Ni/Pt(111)] was studied by performing the density functional theory calculations. It was found that the CO oxidation reaction would process with a higher reaction barrier on metals at lower oxygen coverage via the Langmuir-Hinshelwood (L-H) mechanism, whereas CO oxidation reaction would take place with a lower barrier at higher oxygen coverage on metals or in the presence of molecular oxygen/CO (on NiO1−x-like systems) via the Eley-Rideal mechanism. The calculation results show that the activation energy of CO oxidation follows the order: Pt(111) (0.75eV)>Pt/Ni/Pt(111) (0.69eV)>Ni/Pt(111) (0.47eV at 1ML oxygen), which is in general agreement with the experimental observations. On the surface oxide NiO1−x/Pt(111) and NiO1−x/Pt/Ni/Pt(111) systems, it was found that the molecular CO can subtract the surface lattice oxygen to form CO2 spontaneously through the Eley-Rideal mechanism on NiO1−x/Pt/Ni/Pt(111), whereas such kinetic behavior cannot occur on the NiO1−x/Pt(111) system, suggesting the high reactivity of CO oxidation on NiO1−x/Pt/Ni/Pt(111). The possible reason was analyzed by the magnitude of surface oxygen vacancy formation energy, namely NiO1−x/Pt/M/Pt(111) with relatively low vacancy formation energy as compared to that of NiO1−x/Pt(111) (3.46 vs 4.51eV). Moreover, we extend the above study to a more general case in which the subsurface metals in NiO1−x/Pt/M/Pt(111) system including VIII group metals like Fe/Co/Ni and the IB group metals like Cu, and it was found that the molecular CO can subtract the surface lattice oxygen atom to form CO2 spontaneously via the E-R reaction mechanism for all these NiO1−x/Pt/M/Pt(111) systems.

Site Selectivity for CO Adsorption and Stripping on Stepped and Kinked Platinum Surfaces in Alkaline Medium

This work reports experimental evidence that the adsorption of CO on Pt surfaces composed by (111) terraces and steps and/or kinks in alkaline media occurs faster on sites with (100) symmetry, followed by (110) sites. On the other hand, the (111) terrace has the lowest adsorption rate. CO electrooxidation demonstrates the existence of a close relationship between the preferential site CO occupancy and the peak multiplicity observed in CO stripping voltammetry. For the stepped Pt(554) and Pt(544) and kinked Pt(875) single crystal surfaces, the CO stripping process takes place at high potentials (∼0.80 V vs RHE) when only the (110) or (100) step sites are blocked by CO. However, when the terrace sites with (111) symmetry are fully occupied, new CO stripping peaks appear at lower potentials (<0.68 V vs RHE). For all surfaces, it is observed that the first released sites after partial CO oxidation are (111) terrace sites, followed by the step (110) sites and sites with (100) symmetry. The results of partial CO oxidation suggest that the diffusion of adsorbed CO from sites with (110) or (100) symmetry toward unoccupied (111) terrace sites is negligible. However, CO diffusion from terrace sites to step or kink sites cannot be discarded during the growth of the adsorbed CO adlayer, because of the preferential site occupancy for these latter sites. Due to the fact that the CO oxidation process on stepped Pt surfaces in alkaline media occurs at different potentials on different sites, the activation energies for CO oxidation on the different sites have been estimated. The results for full CO coverage and CO decorated stepped surfaces are in good agreement, indicating that the oxidation of CO on the different sites is not coupled. In the absence of CO on terrace sites, in situ FTIR spectroscopy shows that CO molecules on the (110) step sites are essentially linearly bonded, while on the (100) step sites both linearly and bridge bonded CO are observed. The comparison of these spectra with those obtained when a full coverage is attained shows that the band frequencies for CO on step sites are highly coupled with those on the terrace sites.

Inhibition at Perimeter Sites of Au/TiO2 Oxidation Catalyst by Reactant Oxygen

TiO(2)-supported gold nanoparticles exhibit surprising catalytic activity for oxidation reactions compared to noble bulk gold which is inactive. The catalytic activity is localized at the perimeter of the Au nanoparticles where Au atoms are atomically adjacent to the TiO(2) support. At these dual-catalytic sites an oxygen molecule is efficiently activated through chemical bonding to both Au and Ti(4+) sites. A significant inhibition by a factor of 22 in the CO oxidation reaction rate is observed at 120 K when the Au is preoxidized, caused by the oxygen-induced positive charge produced on the perimeter Au atoms. Theoretical calculations indicate that induced positive charge occurs in the Au atoms which are adjacent to chemisorbed oxygen atoms, almost doubling the activation energy for CO oxidation at the dual-catalytic sites in agreement with experiments. This is an example of self-inhibition in catalysis by a reactant species.