Onset Of CO Oxidation Research Articles

Synthesis of PtRe electrocatalysts supported on mesoporous carbon for the ethanol oxidation reaction

Mesoporous carbon was synthesized to be employed as support in PtRe anodic electrocatalysts. The catalysts were prepared by the polyol method with the following Re loadings: 1, 3, 5, and 10 wt.%. The N2 adsorption-desorption isotherms, thermogravimetric measurements, temperature programmed desorption and temperature programmed reduction of the support were performed and the catalysts were structurally characterized by temperature programmed reduction, H2 chemisorption, X-ray diffraction, X-ray photoelectron spectroscopy, benzene hydrogenation reaction, and transmission electron microscopy. Furthermore, the electrochemical characterization of the catalysts was carried out by cyclic voltammetry, chronoamperometry, and CO stripping techniques. The addition of Re leads to a decrease in the CO oxidation onset potential value. The PtRe(3)/MC catalyst exhibited the best electrocatalytic activity, showing an electrochemical active surface area value of 82.5 m2 gPt−1. Moreover, the potential and anodic current values for ethanol oxidation reaction were 889 mV vs Ag/AgCl and 648 mA mgPt−1, respectively. This result is consistent with electronic effects and the smallest particle size found for this catalyst.

Delafossite-based nitrate treatment for fabricating copper-cerium hybridized interface to achieve enhanced methanol oxidation reaction

A major concern for direct methanol fuel cells is the deactivation of irreplaceable Pt-based anode catalysts. Herein, a simple cerium nitrate hydrothermal treatment was employed to fabricate a copper-cerium hybridized interface that efficiently utilized the characteristic of Cu release from layered Cu-based delafossite. Remarkably, the construction of hybridized interface considerably utilized ligand effect, strain effect, and bifunctional effect of loaded Pt, yielding the mass activity of 1268.6 mA/mgPt and specific activity of 1.57 mA/cm2, which were 3.69, 1.54 times higher than commercial Pt/C, respectively. Meanwhile, nitrate-treated CeT-rGO-Pt achieved a satisfactory anti-CO poisoning ability as evidenced by a high If/Ib ratio (1.86) and a low CO oxidation onset potential (less than Pt/C 55 mV). Density functional theory calculations further confirmed that the formation of hybridized interface optimized the geometry and electronic structure of Pt, which was favorable for inhibiting carbon monoxide adsorption. This straightforward and effective hybridized interface fabricated strategy provided insight into the design of delafossite-based composite catalysts for the high performance of the methanol oxidation reaction.

Tuning surface composition of multiwalled carbon nanotubes supported Pt-Co bimetallic nanoparticles for boosting methanol oxidation catalysis

Developing catalysts with high performance and low cost for methanol oxidation reaction (MOR) is the key to promoting the industrialization of direct methanol fuel cells (DMFCs). In this work, multiwalled carbon nanotubes (MWCNTs) supported PtCo alloys catalysts with improved MOR properties and anti-CO poisoning ability are successfully prepared by integrating low temperature adsorption and high temperature reduction method. The Pt1Co3@NC/MWCNTs sample with moderate Co2+ feeding content (0.81 mA/ugPt) achieves a factor of 1.93 enhancement in MOR mass activity compared to the commercial Pt/C catalyst (0.42 mA/ugPt). In addition, the Pt1Co3@NC/MWCNTs sample displays a lower CO oxidation onset potential respect to pristine Pt/C catalyst (0.74 V vs. 0.82 V). Scuh improvement of MOR activity, durability and anti-CO poisoning ability of the Pt1Co3@NC/MWCNTs catalyst is ascribed to the moderate surface compositions, optimal electronic interaction between PtCo alloys and MWCNTs, and the protection of N-doped carbon (NC) shells. This study provides a new direction to decrease the utilization of platinum and improve the MOR activity, stability and anti-CO poisoning ability of electrocatalysts which will be potential in design and fabrication of the highly efficient electrocatalysts for DMFCs applications.

Investigations of the Effect of H2 in CO Oxidation over Ceria Catalysts

The preferential CO oxidation (so-called CO-PROX) is the selective CO oxidation amid H2-rich atmospheres, a process where ceria-based materials are consolidated catalysts. This article aims to disentangle the potential CO–H2 synergism under CO-PROX conditions on the low-index ceria surfaces (111), (110) and (100). Polycrystalline ceria, nanorods and ceria nanocubes were prepared to assess the physicochemical features of the targeted surfaces. Diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS) shows that ceria surfaces are strongly carbonated even at room temperature by the effect of CO, with their depletion related to the CO oxidation onset. Conversely, formate species formed upon OH + CO interaction appear at temperatures around 60 °C and remain adsorbed regardless the reaction degree, indicating that these species do not take part in the CO oxidation. Density functional theory calculations (DFT) reveal that ceria facets exhibit high OH coverages all along the CO-PROX reaction, whilst CO is only chemisorbed on the (110) termination. A CO oxidation mechanism that explains the early formation of carbonates on ceria and the effect of the OH coverage in the overall catalytic cycle is proposed. In short, hydroxyl groups induce surface defects on ceria that increase the COx–catalyst interaction, revealed by the CO adsorption energies and the stabilization of intermediates and readsorbed products. In addition, high OH coverages are shown to facilitate the hydrogen transfer to form less stable HCOx products, which, in the case of the (110) and (100), is key to prevent surface poisoning. Altogether, this work sheds light on the yet unclear CO–H2 interactions on ceria surfaces during CO-PROX reaction, providing valuable insights to guide the design of more efficient reactors and catalysts for this process.

Thermal CO Oxidation and Photocatalytic CO2 Reduction over Bare and M-Al2O3 (M = Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au) Cotton-Like Nanosheets.

Aluminum oxide (Al2O3) has abundantly been used as a catalyst, and its catalytic activity has been tailored by loading transition metals. Herein, γ-Al2O3 nanosheets were prepared by the solvothermal method, and transition metals (M = Co, Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au) were loaded onto the nanosheets. Big data sets of thermal CO oxidation and photocatalytic CO2 reduction activities were fully examined for the transition metal-loaded Al2O3 nanosheets. Their physicochemical properties were examined by scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction crystallography, and X-ray photoelectron spectroscopy. It was found that Rh, Pd, Ir, and Pt-loading showed a great enhancement in CO oxidation activity while other metals negated the activity of bare Al2O3 nanosheets. Rh-Al2O3 showed the lowest CO oxidation onset temperature of 172 °C, 201 °C lower than that of bare γ-Al2O3. CO2 reduction experiments were also performed to show that CO, CH3OH, and CH4 were common products. Ag-Al2O3 nanosheets showed the highest performances with yields of 237.3 ppm for CO, 36.3 ppm for CH3OH, and 30.9 ppm for CH4, 2.2×, 1.2×, and 1.6× enhancements, respectively, compared with those for bare Al2O3. Hydrogen production was found to be maximized to 20.7 ppm during CO2 reduction for Rh-loaded Al2O3. The present unique pre-screening test results provided very useful information for the selection of transition metals on Al2O3-based energy and environmental catalysts.

Tracking the formation, fate and consequence for catalytic activity of Pt single sites on CeO2

Platinum single sites are highly attractive due to their high atom economy and can be generated on CeO2 by an oxidative high-temperature treatment. However, their location and activity are strongly debated. Furthermore, reaction-driven structural dynamics have not been addressed so far. In this study, we were able to evidence platinum-induced CeO2 surface restructuring, locate platinum single sites on CeO2 and track the variation of the active state under reaction conditions using a complementary approach of density functional theory calculations, in situ infrared spectroscopy, operando high-energy-resolution fluorescence detected X-ray absorption spectroscopy and catalytic CO (as well as C3H6 and CH4) oxidation. We found that the onset of CO oxidation is linked to the migration of platinum single sites from four-fold hollow sites to form small clusters containing a few platinum atoms. This demonstrates that operando studies on single sites are essential to assess their fate and the resulting catalytic properties. Single-atom catalysts hold great promise for process optimization by reducing metal utilization. However, their structure–activity properties remain elusive. Here, a combination of operando techniques and density functional theory analysis is used to capture the evolution of single platinum atoms on CeO2 during CO, C3H6 and CH4 oxidation.

Ionic liquid polyoxometalate-enhanced Pd/N,P-codoped coal-based carbon fiber catalysts for formic acid electrooxidation

Direct formic acid fuel cells have great application potential in the energy field, but their commercialization is hindered by the CO poisoning and low performance of Pd-based anode catalysts. In this study, novel composite catalysts composed of Pd, organic-inorganic hybrid imidazole ionic liquid polyoxometalates, and N,P-codoped coal-based carbon fibers with good dispersion and smaller Pd nanoparticles were synthesized successfully. Physical and electrochemical characterizations revealed that the as-prepared Pd-based composite catalysts have excellent CO poisoning resistance and enhanced electrocatalytic activity in formic acid electrooxidation. Notably, the CO oxidation onset potential of a catalyst modified by imidazole ionic liquid polyoxometalate with long alkyl chain was 0.1 V lower than that of the unmodified catalyst. The mass activity of the as-prepared Pd catalyst in formic acid oxidation was 754.1 mA mg−1Pd. It is expected that organic-inorganic hybrid polyoxometalates can be further applied to fuel cell catalysts.

Tuning Pt-CeO2 interactions by high-temperature vapor-phase synthesis for improved reducibility of lattice oxygen

In this work, we compare the CO oxidation performance of Pt single atom catalysts (SACs) prepared via two methods: (1) conventional wet chemical synthesis (strong electrostatic adsorption–SEA) with calcination at 350 °C in air; and (2) high temperature vapor phase synthesis (atom trapping–AT) with calcination in air at 800 °C leading to ionic Pt being trapped on the CeO2 in a thermally stable form. As-synthesized, both SACs are inactive for low temperature (<150 °C) CO oxidation. After treatment in CO at 275 °C, both catalysts show enhanced reactivity. Despite similar Pt metal particle size, the AT catalyst is significantly more active, with onset of CO oxidation near room temperature. A combination of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and CO temperature-programmed reduction (CO-TPR) shows that the high reactivity at low temperatures can be related to the improved reducibility of lattice oxygen on the CeO2 support.

Electrochemical Behavior of Pt–Ru Catalysts Supported on Graphitized Ordered Mesoporous Carbons toward CO and Methanol Oxidation

In this work, graphitized ordered mesoporous carbons (gCMK-3) were employed as support for Pt and Pt–Ru nanoparticles synthesized by different reduction methods. The catalysts displayed metal contents and Pt:Ru atomic ratios close to 20 wt % and 1:1, respectively. A comparison of the physical parameters of Pt and Pt–Ru catalysts demonstrated that Ru enters into the Pt crystal structure, with well-dispersed nanoparticles on the carbon support. The Pt catalysts exhibited similar surface oxide composition, whereas a variable content of surface Pt and Ru oxides was found for the Pt–Ru catalysts. As expected, the Pt–Ru catalysts showed low CO oxidation onset and peak potentials, which were attributed to the high relative abundances of both metallic Pt and Ru oxides. All the studied catalysts exhibited higher maximum current densities than those observed for the commercial Pt and Pt–Ru catalysts, although the current–time curves at 0.6 V vs. reversible hydrogen electrode (RHE) demonstrated a slightly higher stationary current density in the case of the Pt/C commercial catalyst compared with Pt nanoparticles supported on gCMK-3s. However, the stationary currents obtained from the Pt–Ru/gCMK-3 catalysts surpassed those of the commercial Pt–Ru material, suggesting the suitability of the prepared catalysts as anodes for these devices.

Anodic Electrocatalysts for Fuel Cells Based on Pt/Ti1–xRu x O2

The electrocatalytic activity of materials in the 10% Pt/Ti1–xRu x O2–δ system, where x = 0–0.3 (0 ≤ Ru ≤ 30 mol %), in the reactions of hydrogen electrooxidation in the presence of CO is studied in the liquid three-electrode cell and a model of fuel cell. It is shown that the tolerance of the electrocatalysts towards CO is determined by the crystal structure of the support: the support with the rutile structure provides a higher rate of CO desorption than the support with the anatase structure. The potential of the onset of CO oxidation decreases with increasing concentration of dopant in the support from 650 mV for 10% Pt/TiO2 to 480 mV (NHE) for 10% Pt/Ti0.91Ru0.09O2–δ (rutile). The use of these materials as the anodic catalysts of fuel cell operating with hydrogen containing 30 ppm CO enabled us to obtain a current density by 7 times higher as compared with the 20% PtRu/C E-Tec catalysts.

Ambient pressure phase transitions over Ir(1 1 1): at the onset of CO oxidation

In this study we report on the adsorbate structures on an Ir(1 1 1) surface during the phase transition from the inactive to the active state during CO oxidation. The CO oxidation over Pt(1 1 1) is used as a reference case. Where Pt(1 1 1) either is inactive and CO covered or active and O covered, Ir(1 1 1) exhibits a transition state with co-existing chemisorbed O and CO. The observed structural differences are explained in terms of DFT-calculated adsorption energies. For Pt(1 1 1) the repulsive CO–O interaction makes co-existing chemisorbed CO and O unfavourable, while for Ir(1 1 1) the stronger O and CO adsorption allows for overcoming the repulsive interaction. At the onset of CO oxidation over Ir(1 1 1), a CO structure containing defects forms, which enables O2 to dissociatively adsorb on the Ir(1 1 1) surface, thus enabling the CO oxidation reaction. At the mass transfer limit, the Ir(1 1 1) surface is covered by a chemisorbed O structure with defects; hence, the active surface is predominately chemisorbed O covered at a total pressure of 0.5 mbar and no oxide formation is observed.

Voltage Oscillations in a Polymer Electrolyte Membrane Fuel Cell with Pd-Pt/C and Pd/C Anodes.

Polymer electrolyte membrane fuel cells (PEMFC) fed with H2 contaminated with CO may exhibit oscillatory behavior when operated galvanostatically. The self‐organization of the anodic overpotential is interesting because it can be accompanied by an increase in the average performance. Herein we report experimental studies of voltage oscillations that emerge in a PEMFC equipped with a Pd/C or PdPt/C anode and fed with H2 contaminated with CO (100 ppm). We used on‐line mass spectrometry to investigate how the mass fragments associated with CO2 and CO (m/z 44 and 28, respectively) varied with the voltage oscillations. Overall, we observed that oscillations in the anodic overpotential are in phase with that of the CO and CO2 signals. This fact is consistent with an autonomous adsorption–oxidation cyclic process. For both anodes, it has been observed that, in general, an increase in current density implies an increase in oscillatory frequency. By using CO stripping, we also discuss how the onset of CO oxidation is related to the maximum overpotential reached during a cycle, whereas the minimum overpotential can be associated with the catalytic activity of the electrode for H2 oxidation.

Catalytic activities of Ni-decorated boron particles

Ni-decorated boron (Ni/B) particles were prepared by a ball-milling method, and their physicochemical properties were fully characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX) elemental mapping, and X-ray photoelectron spectroscopy (XPS). Upon decoration by Ni, the previously inactive boron particles showed enhanced activity in CO oxidation and photoelectrochemical hydrogen/oxygen evolution reactions (HERs/OERs). The CO oxidation onset and T10% were observed around 320 and 350°C, respectively. In preliminary tests, the Ni/B (1:3 w/w) sample showed UV and visible light current responses, and current densities of 600 and 240μA/cm2 were observed at a potential of 500mV for HER and OER, respectively. The present study emphasizes the potential application of boron as a catalyst support material for air quality control and photoelectrochemical hydrogen/oxygen evolution.

Fabrication of ZnO, ZnS, Ag-ZnS, and Au-ZnS microspheres for photocatalytic activities, CO oxidation and 2-hydroxyterephthalic acid synthesis

This paper reports the synthesis of ZnS microspheres with cubic and hexagonal crystal phases using a solvothermal method. These materials were characterized by a range of techniques. The effects of Au and Ag-doping on ZnS were also examined. Hexagonal phase ZnO microspheres could be obtained by a CO oxidation process over the ZnS microspheres. The onset of CO oxidation occurred above 350 °C, which was increased by Au and Ag doping. The synthesized materials were used for the photocatalytic degradation of mixed dyes {methylene blue (MB) + methyl orange (MO) + rhodamine B (RhB)}. The observed photocatalytic degradation rate over the photocatalysts was in the order, RhB < MB << MO. The photocatalytic degradation activity of the synthesized materials was observed in the order, Au-ZnS ≈ Ag-ZnS << ZnS < ZnO. ZnO was ∼6 times more efficient in the photocatalytic synthesis of 2-hydroxyterephthalic acid than the ZnS and metal-doped ZnS. These results further support the importance and wide applications of the synthesized ZnS, Ag-ZnS, Au-ZnS and ZnO microspheres.

Methanol oxidation on hybrid catalysts: PtRu/C nanostructures promoted with cerium and titanium oxides

Hybrid catalysts comprising of ceramic, metal, and carbon phase were synthesized by incorporating titanium and cerium oxides into PtRu/C commercial catalyst using an in-situ combustion followed by heat treatment at 600 °C. The structure dependent electrochemical behavior of as-synthesized and heat-treated materials towards methanol oxidation, carbon dioxide (CO) tolerance and chemical stability was studied by XRD, HRTEM, BET, EDS, cyclic voltammetry, chronoamperometry, and CO-stripping method. As a result of heat treatment, amorphous phase of metal oxides was transformed into a crystalline phase with particle size of about 3–7 nm. Improved methanol oxidation activity of the hybrid catalysts was compared to PtRu/C catalyst as a baseline and explained by the changes in Pt electronic behavior and excess adsorption of OH-ions. When heat-treated at 600 °C, CeO2-PtRu/C demonstrated the highest mass activity of 580 mA/mg (∼3× that of PtRu/C) compared to TiO2-PtRu/C (394 mA/mg). Heat-treated hybrid catalysts exhibited higher methanol oxidation activity at higher peak potentials than the corresponding as-synthesized materials. However, as-synthesized hybrid catalysts display higher CO-tolerance, lower CO-oxidation onset potentials, and better chemical stability in comparison to corresponding heat-treated catalysts. To explain the difference, a mechanism for ceramic oxide structure dependent electrochemical behavior of the hybrid catalysts is proposed and discussed.

Fundamental nature and CO oxidation activities of indium oxide nanostructures: 1D-wires, 2D-plates, and 3D-cubes and donuts

Various indium oxide nanostructures of 1D-wires, 2D-hexagonal plates, 3D-cubes and donuts were synthesized, and their fundamental characteristics and CO oxidation activities were studied in detail. X-ray diffraction and Raman analysis revealed that the as-synthesized wires and cubes are orthorhombic InOOH and cubic In(OH)3, respectively. Upon annealing at 700 °C in air, all the as-prepared samples were recrystallized to cubic In2O3. The direct band gap of various as-synthesized nanostructures was estimated to be ∼2.7 eV from the UV-Vis absorption. Two broad photoluminescence peaks were found at 360 and 450 nm, which are attributed to the oxygen vacancies. The CO oxidation activities were in the order of hexagonal plates ≤ donuts < cubes < wires, tested by temperature-programmed reaction mass spectrometry. The difference in activity is explained on the basis of the surface area and oxygen vacancies of different nanostructures. In particular, the wires showed the CO oxidation onset at around 320 °C, which is 280 °C lower than that of hexagonal plates. The detailed morphology dependent properties and CO oxidation activities of various In2O3 nanostructures presented in this study provide new insights into sensor, energy, and environmental applications.

MnO2/CNT Supported Pt and PtRu Nanocatalysts for Direct Methanol Fuel Cells

Pt/MnO2/carbon nanotube (CNT) and PtRu/MnO2/CNT nanocomposites were synthesized by successively loading hydrous MnO2 and Pt (or PtRu alloy) nanoparticles on CNTs and were used as anodic catalysts for direct methanol fuel cells (DMFCs). The existence of MnO2 on the surface of CNTs effectively increases the proton conductivity of the catalyst, which then could remarkably improve the performance of the catalyst in methanol electro-oxidation. As a result, Pt/MnO2/CNTs show higher electrochemical active surface area and better methanol electro-oxidation activity, compared with Pt/CNTs. As PtRu alloy nanoparticles were deposited on the surface of MnO2/CNTs instead of Pt, the PtRu/MnO2/CNT catalyst shows not only excellent electro-oxidation activity to methanol with forward anodic peak current density of 901 A/gPt but also good CO oxidation ability with lower preadsorbed CO oxidation onset potential (0.33 V vs Ag/AgCl) and peak potential (0.49 V vs Ag/AgCl) at room temperature.

A method for rough estimation of the catalyst surface area in a fuel cell

A method for a rough estimation of the catalyst surface area in a fuel cell is developed. It is based on the deconvolution of experimental CO oxidation data by use of a mathematical model. The kinetic parameters of the model are determined by fitting the experimental curves. The experimental data are collected at different sweep rates (2–100 mV s−1) and at different temperatures (room −60.0 °C). The model can predict the sweep rate dependence of the CO oxidation onset potential, the peak current, the peak potential and the peak broadness. The model is further used for the prediction of the baseline in the presence of CO and for calculation of the CO charge consumed up to half peak potential. It is obtained that the latter value is constant at different sweep rates and that the baseline deviates from linearity already at low sweep rates (2 mV s−1), but not very significantly (2.0% in comparison to 8.8% at 100 mV s−1, based on calculated CO charge). It is suggested that lower sweep rates should be used for experimental surface area determination.

Surface electrochemistry of CO as a probe molecule on carbon-supported Se-surface modified Ru nanoparticles via infrared reflection absorption spectroscopy

The carbon monoxide molecule is used to probe the electronic properties of Ru/C and Se surface-modified Ru/C nanocatalysts. Coordination of a Se to a Ru surface strongly affects the vibrational properties and reactivity of the CO adsorbed. Marked alteration of the CO stretching frequency; increase of the Stark tuning rate for C-O vibration; and a positive shift of the onset of CO oxidation are observed for Se-modified Ru particles and attributed to the electronic effect of Se.

FTIR Studies of CO Adsorption on Al2O3- and SiO2-Supported Ru Catalysts

The adsorption of CO on Al2O3- and SiO2-supported Ru catalysts has been investigated through FTIR spectroscopy. Deconvolution of the spectra obtained reveals the presence of 11 distinct bands in the case of Ru/Al2O3 and 10 bands in the case of Ru/SiO2, which were assigned to different carbonyl species adsorbed on reduced as well as partially oxidized Ru sites. Although most of these bands on both supports are similar, they exhibit substantial differences in terms of stability. In general, the analogous CO species on Ru/Al2O3 are adsorbed stronger than those on Ru/SiO2, with the most stable species observed being a dicarbonyl adsorbed on metallic Ru (i.e., Ru0(CO)2). Following sintering of the Ru, the ratio of multicarbonyl to monocarbonyl adsorption is reduced substantially because of the lack of isolated sites or small Ru clusters that enable the formation of multicarbonyl species via oxidative disruption. Finally, in the presence of O2, the main features observed correspond to monocarbonyl, dicarbonyl, and tricarbonyl species adsorbed on partially oxidized Run+. The intensities of all bands decrease drastically at temperatures above 210 degrees C because of the onset of CO oxidation, which results in substantially reduced surface coverage.