CO Oxidation Behavior Research Articles

Catalyst Design Based on Morphology- and Environment-Dependent Adsorption on Metal Nanoparticles

Understanding metal–adsorbate interactions is key to controlling and improving the functionality of metal nanoparticles (NPs) in energy and biomedical application areas. However, adsorption is dependent on the morphological characteristics of the NPs, such as their size and shape, and in turn, the NP morphology is dependent on the chemical environment (presence of adsorbates). In this work, we introduce a novel and computationally tractable framework that is able to capture adsorption trends as a function of NP size and shape, including the impact of chemical environment on NP morphology. Our methodology is tested in the area of catalysis and specifically on the CO oxidation behavior on gold (Au), a highly structure-sensitive reaction. Our results reveal a strong correlation between the experimentally observed CO oxidation activity and the average CO adsorption (binding) energy on Au NPs enabling catalytic behavior prediction as a function of NP morphology. We demonstrate that the Au NP size plays a pivotal role on CO adsorption, whereas the Au NP shape appears to be less significant. Most importantly, the developed methodology introduces NP morphology effects on adsorption that are key for the rational design of materials with fine-tuned properties in applications ranging from catalysis to targeted medical imaging to drug delivery.

Vertically Aligned Double-Walled Platinum Nanotubes Decorated with Inner Fibrils for Their Enhanced Electrocatalytic Properties

Vertically aligned double-walled platinum nanotubes (Pt DNTs) with inner fibrils have been synthesized by means of homogeneous coating of Pt onto a Pd thin-wall nanotube array template using electrochemical deposition, followed by chemical etching of the template. The resulting Pt DNTs with inner fibrils have advantageous morphological characteristics including high crystallinity of the coated Pt layers, high porosity of the inner nanotubes, the non-carbon-supported substrate, and a three-dimensional orientation, whereby the catalysts are directly and vertically connected with a conducting electrode. In addition, tube length can be controlled by tuning the total amount of charge injected during the electrochemical deposition. Electrochemical measurements have demonstrated that the resulting Pt DNTs have considerably greater electrochemically active surface area (ECSA) and higher electrocatalytic activity toward methanol oxidation and CO oxidation behavior compared to Pt single nanotube and Pt nanorod structures as well as other reported Pt-based catalysts under similar testing conditions. These results suggest that Pt DNTs are excellent nanocatalytic systems for low-temperature catalytic reactions and surface chemical processes.

Evaluating the Behavior of CO/CO2 in Ni/GDC Solid Oxide Fuel Cell Anodes

Ni/GDC anodes for solid oxide fuel cells have been reported to be more resistant to carbon deposition than conventional Ni/YSZ anodes. To facilitate understanding the nature of carbon chemistry in Ni/GDC anode functional layers, we have studied the behavior of CO oxidation and CO2 dissociation on thin-film Ni/GDC electrodes. Sputtered thin-film GDC electrodes with open Ni sputtered overlayers provide an optically accessible and well-characterized electrode in which to study fundamental reactions of CO and CO2 in Ni/GDC systems for both dry and wet environments. To further separate out the behavior of the GDC surface chemistry from the Ni chemistry, Au overlayers were deposited on the thin-film GDC for an alternative electrode with the Au serving as a low-activity current collector. Results indicate that dry CO oxidation and CO2 dissociation are substantially slower than their H2 and H2O analogs on Ni/GDC with fitted exchange current densities for CO/CO2 approximately one order of magnitude lower than H2/H2O under similar partial pressures and temperatures ranging from 600 to 700 deg. C. By replacing the CO2 with H2O, exchange current densities associated with CO oxidation rise dramatically although still lower than H2 oxidation at similar conditions. By using our studies with ambient pressure XPS, we are able to obtain local overpotentials across the Ni/GDC for some test conditions and thereby extract fundamental information about the CO and CO2 chemistry in the Ni/GDC cell. Although the CO/CO2 reactions are slow, they may play a vital role in the suppression of carbon formation in Ni/GDC cells. Thus, to better understand this, we fit our experimental data for the thin-film electrodes to a microkinetic model that includes CO/CO2 chemistry on both the Ni and GDC. By fitting these rates to match the model results to the performance of Ni/GDC and Au/GDC electrodes (and assuming the Au functions as an inert current collector), we have developed kinetic expressions for surface chemistry of CO and CO2 on the GDC surface. Coupling this surface chemistry to previous Ni surface chemistry and to our own developed mechanism for H2 oxidation on Ni/GDC provides a basis for exploring how the Ni and GDC surface chemistry interact with exposure to carbon-rich syngas. The fitted model for the thin-film Ni/GDC performance suggests that the presence of surface hydrogen and hydroxyls on the Ni can have significant boosts to CO conversion to CO2 on both the GDC and Ni surfaces. These synergies may be critical for providing rapid CO conversion in lower temperature SOFCs which rely on GDC as part of the anode and electrolyte materials.

Modified diatomite-supported CuO–TiO2 composite: Preparation, characterization and catalytic CO oxidation

Modified diatomite (acid treated and ZrO2-coated diatomite) coated by nano copper and titanium mixed oxides was used to investigate the catalytic oxidation of carbon monoxide. The prepared catalysts were characterized by Brunauer–Emmett–Teller (BET) surface area, field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), temperature-programmed reduction (TPR), porosimetry, and X-ray photoelectron spectroscopy (XPS). Catalytic behavior for CO oxidation was studied in different conditions in a tubular reactor packed with catalyst. The variation of catalytic activity of the prepared composite catalysts with different molar fractions of CuO and TiO2 for CO oxidation in different reaction temperatures was investigated. The catalytic activity of catalyst was dependent on the concentration of coated catalyst over diatomite, reaction temperature and retention time. The results illustrated that diatomite is the promising candidate for catalyst support due to its unique characteristics. It was found that CO conversion of 100% can be achieved at 220 °C over catalyst with 70 mol% CuO and 30 mol% TiO2 content.

Effects of alloyed Si on the oxidation behaviour of Co–29Cr–6Mo alloy for solid-oxide fuel cell interconnects

To evaluate the suitability of the Co–29Cr–6Mo alloy as an interconnect in solid-oxide fuel cells (SOFCs), the oxidation behaviour of Co–29Cr–6Mo alloys with various Si concentrations was investigated in air at 700–1000°C. The presence of alloyed Si improved the oxide-scale adhesion by promoting the formation of a continuous Cr2O3 film. The SiO2 particles in the inner layer and the grain boundaries of alloy matrix beneath the inner layer inhibited the inward oxygen diffusion and outward chromium diffusion. The results indicate that the Co–29Cr–6Mo alloy with added Si is appropriate for SOFC applications.

Synergetic effect and oscillatory behavior of CO oxidation over a bimetallic composite catalyst

This article describes the results of simulations of CO oxidation under low pressure conditions over a bimetallic Pt/Rh composite catalyst. The model is represented by a system of four differential–algebraic equations (DAEs) for the coverages of COads and Oads, on the two parts of the composite surface. Surface diffusion of both adsorbates is assumed to be fast. The experimentally observed synergetic effect concerning CO oxidation on a Rh surface partly covered by Pt monolayer islands is reproduced in the simulations. It is shown that this synergetic effect is due to the difference in the CO sticking coefficients on Pt and Rh surfaces. In addition, a new type of kinetic oscillations, namely “synergetic kinetic oscillations” are predicted by the simulations. These oscillations exist under conditions at which no oscillations can exist over Pt and Rh monometallic catalysts. The driving force of this new type of oscillations is connected to the large difference in binding energy of adsorbed oxygen atoms on Rh and Pt.

The effect of alloyed nickel on the short-term high temperature oxidation behaviour of Co–Re–Cr-based alloys

The effect of Ni on the oxidation behaviour of the alloy Co–17Re–23Cr (at.%) is studied. The results show that the alloying of 15at.% Ni improves the alloy oxidation resistance significantly by promoting the formation of a protective Cr2O3 layer. This effect can be attributed at least partially to the enhancement of Cr diffusion by Ni in the metallic Co–Re–Cr–Ni matrix. Moreover, the formation of relatively slow growing Co(Ni)O solid–solution in the transient stage supports the lateral growth of Cr2O3 nuclei and thus accelerates the establishment of the compact chromia layer.

On the nature of the active Au species: CO oxidation on cyanide leached Au/TiO2 catalysts

In order to learn more about the nature of the active Au species for CO oxidation on TiO2 supported Au catalysts, we studied the influence of different gold oxidation states on the CO oxidation activity at 80°C on structurally well defined Au/TiO2 (P25) catalysts. For that purpose, the metallic Au0 nanoparticles were selectively removed from the catalyst by cyanide leaching, while dispersed ionic Aun+ species would remain on the catalyst. The catalysts were prepared by deposition-precipitation. The composition of the active Au phase before leaching was varied by either using a fresh catalyst or calcining it in an O2/N2 mixture, and by applying different conditioning procedures after leaching. The size of the gold particles and their chemical state were characterized by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), the CO adsorption and oxidation behavior was evaluated by kinetic measurements and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements. In the latter measurements, we did not detect any influence of ionic Au3+ on the reaction. Metallic Au0 nanoparticles, obtained mainly by conditioning in O2 atmosphere at elevated temperatures (400°C), but also during the CO oxidation reaction on a N2 pre-treated or fresh catalyst, seem to be a requisite for a high CO oxidation activity of Au/TiO2 catalysts.

A theoretical study of single-atom catalysis of CO oxidation using Au embedded 2D h-BN monolayer: a CO-promoted O₂ activation.

The CO oxidation behaviors on single Au atom embedded in two-dimensional h-BN monolayer are investigated on the basis of first-principles calculations, quantum Born-Oppenheim molecular dynamic simulations (BOMD) and micro-kinetic analysis. We show that CO oxidation on h-BN monolayer support single gold atom prefers an unreported tri-molecular Eley-Rideal (E-R) reaction, where O2 molecule is activated by two pre-adsorbed CO molecules. The formed OCOAuOCO intermediate dissociates into two CO2 molecules synchronously, which is the rate-limiting step with an energy barrier of 0.47 eV. By using the micro-kinetic analysis, the CO oxidation following the tri-molecular E-R reaction pathway entails much higher reaction rate (1.43 × 105 s−1) than that of bimolecular Langmuir-Hinshelwood (L-H) pathway (4.29 s−1). Further, the quantum BOMD simulation at the temperature of 300 K demonstrates the complete reaction process in real time.

Effect of alloying on the oxidation behaviour of Co–Al–W superalloys

The 800°C isothermal and cyclic oxidation of alloys based on Co–7Al–7W (at.%) was examined. Cr, Fe and Si additions were beneficial. Repeated oxide bilayers formed during cycling of non-Cr alloys, which were not protective (>100μm thick). In these layers, the outer was Co3O4 and the inner, a porous Co3O4 and Al2O3 mixture. 10at.% Cr substitution for Co was sufficient to produce a protective, ∼1μm thick scale which contained an outer layer of Al2CoO4, a very thin (100nm) middle layer of Cr2O3 and an inner layer of Al2O3. Therefore protective oxide scales are possible in γ/γ′, Co–Co3(Al,W) superalloys.

The effect of ZnO addition on Co/C catalyst for vapor and aqueous phase reforming of ethanol

The effect of ZnO addition on the oxidation behavior of Co along with catalytic performance in vapor and aqueous phase reforming of ethanol were investigated on Co supported on carbon black (XC-72R). Carbon was selected to minimize the support interactions. Effect of ZnO addition during both vapor and aqueous phase reforming were compared at 250°C. ZnO addition inhibited the reduction of cobalt oxides by H2 and created surface sites for H2O activation. During vapor phase reforming at 450°C the redox of cobalt, driven by steam oxidation and H2 reduction, trended to an equilibrium of Co0/Co2+. ZnO showed no significant effect on cobalt oxidation, inferred from the minor changes of C1 product yield. Surface sites created by ZnO addition enhanced water activation and oxidation of surface carbon species, increasing CO2 selectivity. At 250°C cobalt reduction was minimal, in situ XANES demonstrated that ZnO addition significantly facilitated oxidation of Co0 under vapor phase reforming conditions, demonstrated by lower C1 product yield. Sites introduced by ZnO addition improved the COx selectivity at 250°C. Both Co/C and Co-ZnO/C rapidly oxidized under aqueous phase reaction conditions at 250°C, showing negligible activity in aqueous phase reforming. This work suggests that ZnO affects the activation of H2O for Co catalysts in ethanol reforming.

Reactivity of Ultra-Thin ZnO Films Supported by Ag(111) and Cu(111): A Comparison to ZnO/Pt(111)

We studied structure and reactivity of ZnO(0001) ultrathin films grown on Ag(111) and Cu(111) single crystal surfaces. Structural characterization was carried out by scanning tunneling microscopy, Auger electron spectroscopy, low-energy electron diffraction, and temperature programmed desorption. The CO oxidation behavior of the films was studied at low temperature (450 K) at near atmospheric pressures using gas chromatography. For ZnO/Cu(111), it is shown that under reaction conditions ZnO readily migrates into the Cu crystal bulk, and the reactivity is governed by a CuOx oxide film formed in the reaction ambient. In contrast, the planar structure of ZnO films on Ag(111) is maintained, similarly to the previously studied ZnO films on Pt(111). At sub-monolayer coverages, the “inverse” model catalysts are represented by two-monolayer-thick ZnO(0001) islands on Pt(111) and Ag(111) supports. While the CO oxidation rate is considerably increased on ZnO/Pt(111), which is attributed to active sites at the metal/oxide boundary, sub-monolayer ZnO films on Ag(111) did not show such an effect, and the reactivity was inhibited with increasing film coverage. The results are explained by much stronger adsorption of CO on Pt(111) as compared with Ag(111) in proximity to O species at the oxide/metal boundary. In addition, the water–gas shift and reverse water–gas shift reactions were examined on ZnO/Ag(111), which revealed no promotional effect of ZnO on the reactivity of Ag under the conditions studied. The latter finding suggests that wetting phenomena of ZnO on metals does not play a crucial role in the catalytic performance of ZnO-based real catalysts in those reactions.

On the behavior of CO oxidation on shape-controlled Pt nanoparticles in alkaline medium

In this work, the behavior of the CO electro-oxidation reaction on shape-controlled Pt nanoparticles in alkaline medium was examined in order to understand the effect of the surface structure on this reaction. A series of experiments using Pt nanoparticles of different surface structures/shapes was used and the results obtained were compared with the previous knowledge gained from stepped platinum single crystal electrodes. Independently of the preferential orientation of the nanoparticles, the CO oxidation voltammetry exhibits two main peaks: one at ca. 0.56–0.59V and the second one at 0.66–0.67V, being the intensity of the peaks dependent on the shape of the nanoparticle. These two peaks have been assigned to the oxidation of CO on the (111) terraces and on the rest of the sites, respectively. The appearance of two differentiated peaks reveals that these (111) terraces and the rest of the sites on the nanoparticle surface behave independently of the presence of the other type of sites, that is, they are not connected. The results are discussed considering the effects of the surface mobility of CO and of the OH adsorption properties on the different sites in the oxidation peaks.

Preparation of Pd–MgO model catalysts by deposition of Pd from aqueous precursor solutions onto Ag(0 0 1)-supported MgO(0 0 1) thin films

The preparation of Pd–MgO model catalysts via liquid-phase deposition of Pd from aqueous Pd precursor solutions was studied. Thin, single-crystalline MgO(001) films grown on a Ag(001) substrate were used as support and allowed surface science techniques such as Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning tunneling microscopy, and temperature programmed desorption to be applied for characterization. Thin MgO(001) films were unstable and rapidly dissolved in acidic and neutral environments, but remained stable in thickness in alkaline solutions after an initial dissolution of a few layers of MgO. Pd was deposited by exposure of the thin film MgO substrate to alkaline (pH 12) precursor solutions containing Pd-hydroxide complexes. Scanning tunneling microscopy images taken from ultrathin MgO films revealed the formation of Pd particles 3nm in diameter after thermal decomposition of the precursor at 600K, as well as roughening of the MgO substrate, including the formation of etch pits, which leads to partial exposure of the Ag substrate. For Pd deposited on thick MgO films, the formation of Pd nanoparticles by thermal decomposition of the adsorbed Pd-hydroxide precursor was followed by X-ray photoelectron spectroscopy. Pd–MgO model catalysts with similar Pd coverage prepared either by liquid-phase deposition or physical vapor deposition in UHV exhibited similar properties, as revealed by their comparable behavior in CO adsorption and CO oxidation.

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr2O3 layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr2O4 spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr2O3-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.

Synthesis of bimetallic Pt/Pd nanotubes and their enhanced catalytic activity in methanol electro-oxidation

In this work, we successfully synthesized a vertically-aligned bimetallic Pt/Pd nanotube array by using anodic aluminum oxide (AAO) template-assisted electro-deposition. The resulting nanostructures exhibited higher catalytic activity in methanol oxidation than either individual constituent (Pt or Pd). The facile synthesis of the Pt/Pd tubular nanostructure was inspired by the previously developed thin-walled Pd nanotube formation mechanism, which takes advantage of the surface hydroxyl chemistry of the AAO template. The content of Pt in the bimetallic Pt/Pd nanotubes could reach up to 50%. The inner and outer walls of the perpendicular open Pt/Pd nanotubes increased the ratio of the surface area to the mass relative to the solid nanorod structure, resulting in a higher current density in methanol oxidation. More importantly, the synergistic electronic effect between Pt and Pd gave rise to the enhanced catalytic activity in methanol oxidation, as evidenced from comparisons of the onset potentials and CO oxidation behaviors. Thus, vertically-aligned bimetallic Pt/Pd nanotubes may act as a better catalyst in methanol oxidation than the single-component Pt counterpart.

Self-sustained carbon monoxide oxidation oscillations on size-selected platinum nanoparticles at atmospheric pressure

High-quality mass spectrometry data of the oscillatory behavior of CO oxidation on SiO(2) supported Pt-nanoparticles at atmospheric pressure have been acquired as a function of pressure, coverage, gas composition and nanoparticle size. The oscillations are self-sustained for several days at constant temperature, pressure and CO/O(2) ratio. The frequency of the oscillations is very well defined and increases over time. The oscillation frequency is furthermore strongly temperature dependent with increasing temperature resulting in increasing frequency. A plausible mechanism for the oscillations is proposed based on an oxidation-reduction cycle of the nanoparticles which change the rate of CO oxidation on the particles.

High temperature degradation behavior of sputtered nanostructured Co–Al coatings on superalloy

Microstructure and cyclic high temperature oxidation behavior of nanostructured Co–Al coatings on Superni-718 substrate have been investigated. Cyclic high temperature oxidation tests were conducted on uncoated and coated samples at peak temperatures of 800°C and 900°C for up to 100 thermal cycles between the peak and room temperatures. The results showed that bare substrate has higher oxidation resistance at 800°C as compared to 900°C and coated sample has slightly higher oxidation resistance at 900°C. The external scale of both coated sample exhibited good spallation resistance during cyclic oxidation testing at both temperatures. The improvement in oxide scale spallation resistance is believed to be related to the fine-grained structure of the coating. Nanostructured Co–Al coatings on Superni-718 substrate were deposited by DC/RF magnetron sputtering. FE-SEM/EDS and XRD were used to characterize the morphology and formation of different phases in the coatings, respectively. The Co–Al coating on superalloy substrate showed better performance of cyclic high temperature oxidation resistance due to its possession of β-CoAl phase as Al reservoir and the formation of Al2O3 and spinel phases such as CoCr2O4 and CoAl2O4 in the scale. The oxidation results confirmed an improved oxidation resistance of the Co–Al coating on superalloy as compared to bare substrate up to 100 cycles.

Investigation of Pt-Dy co-doping effects on isothermal oxidation behavior of (Co,Ni)-based alloy

A Co32Ni21Cr8Al0.6Y (wt.%) alloy with and without doping 3 wt.% platinum, or co-doping 3 wt.% platinum and 0.1 wt.% dysprosium was produced by arc melting. The hardness of both base alloy and composition-modified alloy was measured by using a Vickers hardness tester. Isothermal oxidation tests at 1000 °C in static air atmosphere were conducted to assess the isothermal oxidation behavior of the alloys. The microstructure and composition of the tested alloys before and after oxidation were investigated by means of X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS) and back scatter detector. Results showed that platinum had significant influence on microstructure of the tested alloy by the formation of β-(Ni,Pt)Al phase. Addition of 3 wt.% platinum could slightly increase the hardness of the tested alloy. Platinum accelerated phase transformation of alumina from metastable θ-Al2O3 to stable α-Al2O3 and suppressed the consumption of β-phase. Co-doping both 3 wt.% platinum and 0.1 wt.% dysprosium induced the fastest transformation of θ- to α- alumina and the formation of a fine-grained oxide scales. The most effective reduction of oxidation rate was achieved by the Pt-Dy co-doping effects.

Catalytic Oxidation of Carbon Monoxide Using Copper-Zinc Mixed Oxide Nanoparticles Supported on Diatomite

: Background: Diatomite coated by nanoparticles of copper and zinc mixed oxides were used to investigate catalytic oxidation of carbon monoxide. Objectives: Aim of this paper was to investigatediatomite performance as a supporting host to confine the CuO and ZnO nanoparticles and its catalytic activity for carbon monoxide oxidation. Materials and Methods: The prepared catalysts were characterized by Brunauer-Emmett-Teller (BET) surface area, scanning electron microscopy (SEM) and X-ray diffraction (XRD). Catalytic behavior for CO oxidation was studied at different conditions in a tubular reactor packed with catalyst. Results: The results illustrated that diatomite is the promising candidate for catalyst support due to its unique characteristics. The variation of catalytic activity of the prepared composite catalysts withdifferent molar fractions of CuO and ZnO for CO oxidation in different reaction temperatures was investigated. It was found that a CO conversion of 100% can be achieved at 300 oC over catalyst with 80 mol% CuO and 20 mol% ZnO content.