O2 TPD Research Articles

Structure–activity relation of iron oxide catalysts in soot oxidation

In this paper, a systematic series of bare iron oxide catalysts was evaluated towards their activity for the oxidation of soot under oxygen-rich conditions. The catalytic studies were performed by temperature programmed oxidation (TPO) using tight contact mixtures of catalyst and soot. A home-made carbon black originated from propene combustion was taken as model soot. The catalysts were thoroughly characterised by employing powder X-ray diffraction (PXRD), N2 physisorption, high resolution transmission electron microscopy (HRTEM), temperature programmed reduction by H2 (HTPR), temperature programmed desorption of NH3 (NH3-TPD), temperature programmed desorption of O2 (O2-TPD) and thermogravimetry coupled with difference thermal analysis (TG/DTA). Special notice was put on the NH3-TPD data, which were used for the modelling of the NH3 adsorption and desorption. This model included elementary reactions and implied the specific number of Bronsted and Lewis acid sites. The obtained kinetic parameters were found to be close to that of related oxides such as ZrO2 and H-BEA zeolite. The results of the physical–chemical characterisation as well as NH3 adsorption/desorption kinetics were coupled with the catalytic performance to identify determining properties of the iron oxide catalysts. From these correlations it was derived that both Lewis acid surface sites as well as crystallinity drive the catalytic activity for soot oxidation. The effect of these characteristics is in line with the postulated role of the catalyst implying the transport of oxygen from iron oxide to soot by surface and bulk diffusion.

Promotional role of ceria on cobaltosic oxide catalyst for low-temperature CO oxidation

Ceria modified Co3O4 catalysts for low temperature CO oxidation were prepared by a precipitation–oxidation method, and characterized by low-temperature N2 adsorption/desorption, TPR, O2–TPD, CO–TPD and transient–response reaction. The roles of ceria in CeO2–Co3O4 catalyst and the effect of pretreatment on the performance of CeO2–Co3O4 for CO oxidation were investigated in detail. The results show that the presence of CeO2 can increase its surface area, reduce the crystal size of Co3O4, and improve obviously the catalytic activity and stability of Co3O4 for CO oxidation, such as its T100 is only −60 °C. It was also found that the addition of CeO2 can not only promote the adsorption of O2 and the reaction of adsorbed CO with surface oxygen species to form CO2, but also increase the CO2 desorption speed. The pretreatment method can affect the catalytic activity of CeO2–Co3O4, the catalyst treated in N2 exhibits higher catalytic activity for low-temperature CO oxidation due to formation of oxygen vacancy. The catalyst reduced in H2 shows lower activity for CO oxidation although it has more surface oxygen vacancies, because of the difficult desorption of CO2 on the reduced CeO2–Co3O4 catalyst.

Removal of Carbon Monoxide by Oxidation from Excess Hydrogen Gas on Co3O4-NiO/AC Catalyst

The present work investigates the catalytic properties of the Co-Ni-supported activated carbon (AC) catalyst for the oxidation removal of CO in excess hydrogen gas to obtain High purity hydrogen. X-ray diffraction (XRD), temperature-programmed desorption of O2 (O2-TPD), and temperature-programmed reduction (TPR) were used to study the properties of the prepared catalyst. XRD, and TPR results indicate that the highly dispersed NiO and Co3O4 are the main phase in the prepared catalyst. The O2-TPD results show that the Co3O4-NiO/AC catalyst has strong adsorption and activation capacity to O2 molecules. Active oxygen species can be formed on the surface of Co3O4-NiO/AC catalyst, which is the active species to oxidize CO molecules. CO conversion can exceed 99.5% in the temperature range of 443~463 K, and CO oxidation selectivity exceeds 62.2% in less than 463 K.

Investigation of the effect of Cu addition on the SO2-resistance of a Ce[sbnd]Ti oxide catalyst for selective catalytic reduction of NO with NH3

A series of characterizations including TEM–EDS, XRD, FTIR, SO2+O2 TPD, XPS, H2-TPR and NH3-TPD has been carried out to explore the mechanism of the promoting effect of Cu addition on the SO2-resistance of the CeTi oxide catalyst. SO2 had been found to strongly interact with surface cerium oxygen and result in the reduction of Ce4+ to Ce3+ on the CeTi oxide catalyst. The Cu atoms in the CeCuTi oxide catalyst will preferentially interact with SO2 and preserve the adjacent Ce4+. The redox ability of the CeTi catalyst was found to significantly decrease due to the sulfation by SO2. Due to the formation of CuSO4 and the preserved Ce4+, the reducibility of the CeCuTi catalyst has not significantly decreased like the CeTi catalyst but slightly decreased to an adequate level for SCR reaction after sulfation. The surface acidities of CeTi and CeCuTi were both enhanced by sulfation. The preservation of the redox ability and the increase of surface acidity after sulfation had caused CeCuTi to be a high SO2-resistance catalyst for SCR reaction. A graphical description of the effect of the Cu addition on the SO2-resistance of a CeTi oxide catalyst has also been proposed.

CONVERSION OF METHANOL TO FORMALDEHYDE ON TiO2 SUPPORTED Ag NANOPARTICLES

TiO2 supported Ag nanoparticles with metal loading ranging from 2.0 to 12.0 wt.% have been prepared by reduction of a silver precursor with formic acid under mild conditions. XPS showed that silver remains in a reduced state and TEM has demonstrated that the procedure was effective to produce silver nanoparticles. Only small changes in the metal particle size from 2.0-3.4 nm occurs when the Ag content increases from 2.0 to 12.0 wt.%. O2 TPD has revealed the presence of different types of adsorbed oxygen species being the one assigned to the active specie called Ob which one displayed the highest proportion in the catalysts with higher metal loading. Catalytic activity in the conversion of methanol to formaldehyde showed that those samples with metal loading higher than 7.0 wt% displays higher yields compared to the unsupported silver catalyst.

Low-cost catalysts for the control of indoor CO and PM emissions from solid fuel combustion

Cu–Mn based mixed oxide type low-cost catalysts have been prepared in supported form using mesoporous Al2O3, TiO2 and ZrO2 supports. These supports have been prepared by templating method using a natural biopolymer, chitosan. The synthesized catalysts have been characterized by XRD, BET-SA, SEM, O2–TPD and TG investigations. The catalytic activity for CO as well as PM oxidation was studied, in a view of their possible applications in the control of emissions from solid fuel combustion of rural cook-stoves. The trend observed for the catalytic activity of the synthesized catalysts for CO oxidation was ZrO2>TiO2>Al2O3 while for PM oxidation it was observed to be TiO2>ZrO2>Al2O3. The effect of CO2, SO2 and H2O on CO oxidation activity was also investigated, and despite partial deactivation, the catalysts show good CO oxidation activity. An effective regeneration treatment was attempted by heating the partially deactivated catalysts in presence of oxygen. Redox properties of TiO2 and ZrO2 and their structure appeared to be responsible for their promotional activity for CO and PM oxidation reactions. These unordered mesoporous materials could be useful for such reactions where mass transfer is more important than shape and size selectivity.

La0.8Sr0.2Ni0.4Fe0.6O3–Ce0.8Gd0.2O2–δ Nanocomposite as Mixed Ionic–Electronic Conducting Material for SOFC Cathode and Oxygen Permeable Membranes: Synthesis and Properties

Mixed ionic–electronic conducting nanocomposite La0.8Sr0.2Ni0.4Fe0.6O3 (LSNF)–Ce0.8Gd0.2O2– δ (GDC) was prepared via ultrasonic dispersion of nanocrystalline powders of perovskite and fluorite oxides in water with addition of surfactant, followed by drying and sintering up to 1300°C. Analysis of the real structure of nanocomposite (studied by XRD and TEM with EDX) and its surface composition (studied by XPS) revealed moderate redistribution of elements between phases favoring their epitaxy. Results of impedance spectroscopy, oxygen isotope exchange, O2 TPD and H2 TPR experiments revealed a positive effect of composite interfaces on the oxygen mobility and reactivity agreeing with the ambipolar transport behavior of MIEC composite. Preliminary testing of button-size cell with functionally graded LSNF–GDC cathode layer supported on thin YSZ layer covering Ni/YSZ cermet demonstrated high and stable performance, which is promising for its practical application.

Reduction of NOx by H2 on Pt/WO3/ZrO2 catalysts in oxygen-rich exhaust

This work addresses the low-temperature NOx abatement under oxygen-rich conditions using H2 as reductant. For this purpose Pt/ZrO2 and Pt/WO3/ZrO2 catalysts are developed and characterised by temperature-programmed desorption of H2 (H2-TPD), N2 physisorption (BET) and powder X-ray diffraction (PXRD). The most active catalyst is a Pt/WO3/ZrO2 pattern with a Pt load of 0.3wt.% and a W content of 11wt.%. This material reveals high deNOx activity below 200°C and high overall N2 selectivity of about 90%. Additionally, the catalyst exhibits outstanding hydrothermal stability as well as resistance against SOx. Furthermore, the transfer from the powder level to real honeycomb systems leads to promising performance as well.Diffuse reflectance Fourier transform infrared spectroscopic studies, kinetic modelling of temperature-programmed desorption of O2 (O2-TPD) and NOx-TPD examinations indicate that the pronounced H2-deNOx performance of the Pt/WO3/ZrO2 catalyst is related to the electronic interaction of WO3 with the precious metal. The tungsten promoter increases the electron density on the Pt thus activating the sample for H2-deNOx and N2 formation, respectively. Contrary, NOx surface species formed on the WO3/ZrO2 support are not supposed to be involved in the H2-deNOx reaction.

Design and Characterization of Nanocomposites Based on Complex Perovskites and Doped Ceria as Advanced Materials for Solid Oxide Fuel Cell Cathodes and Membranes

AbstractLa0.8Sr0.2Fe0.6Ni0.4O3-x - Ce0.9Gd0.1O2-x and La0.8Sr0.2Fe0.8Co0.2O3-x - Ce0.9Gd0.1O2-x nanocomposites were synthesized via ultrasonic dispersion of nanocrystalline powders of perovskite and fluorite oxides in acetone with addition of a surfactant, followed by drying and sintering at temperatures up to 1200°C. The evolution of the structure of samples with sintering temperature was studied by XRD and TEM with EDX and compared with the data on conductivity, oxygen isotope exchange, O2 TPD, H2 and CH4 TPR. Preliminary testing of button-size cells with cathodes supported on thin YSZ layer covering Ni/YSZ cermet demonstrated a high and stable performance of LSNF–GDC composite promising for the practical application.

EFFECT OF LANTHANUM MANGANITE MODIFICATION BY CALCIUM AND/OR FLUORINE ON THE BONDING STRENGTH, MOBILITY AND REACTIVITY OF THE LATTICE AND SURFACE OXYGEN

ABSTRACTCa and/or F-modified samples of LaMnO3have been prepared by the Pechini method. The bulk structure of samples was characterized by TEM, EXAFS and XRD, while the surface composition was studied by SIMS. Thermal analysis, O2TPD, H2TPR and isothermal pulse/flow samples reduction by CO were applied to characterize the accessible surface/bulk oxygen mobility and reactivity. A reasonable description of the experimental energetic spectrum of the surface oxygen for various types of regular and defect surface sites on the perovskite faces was achieved by using semiempirical Interacting Bonds Method in the slab approximation with a due regard for the surface face termination and relaxation. Fluorine was found to decrease the surface coverage by reactive weakly bound oxygen forms while increasing the bulk oxygen excess and mobility. Calcium generated reactive weakly bound oxygen forms while decreasing the oxygen excess in the lattice and converting the regular M-O oxygen forms into the bridging ones through migration to the surface.

Perovskite-Type Oxides: II. Redox Properties of LaMn1−xCuxO3 and LaCo1−xCuxO3 and Methane Catalytic Combustion

Redox properties of high-surface-area LaM1−xCuxO3 (M=Mn or Co) perovskites prepared by the citrate method were studied by H2 TPR and O2 TPD techniques. We have found that the reduction of Mn4+ occurs in all La–Mn–Cu perovskites at temperatures lower than that of Co3+ in La–Co–Cu samples. The presence of copper affects the reduction of both Mn4+ and Co3+, increasing the temperature of reduction. Furthermore, Mn-based perovskites with a low Cu substitution (x< 0.4) release a great amount of O2 at high temperature. Catalytic activity toward methane combustion was investigated under diluted conditions (0.4% CH4 and 10% O2, N2 as balance) in the temperature range 673–1073 K using a fixed bed reactor with a space velocity of 40,000 N cm3 h−1 g−1. All samples catalyze methane oxidation in the temperature range 673–773 K, giving complete methane conversion below 1073 K and producing 100% CO2. No deactivation phenomena were observed. Substitution with copper decreases catalytic activity. The higher activity of Mn-based perovskites compared with that of the corresponding Co samples with the same composition was attributed to the stronger oxidative properties of LaM1−xCux O3. An apparent activation energy of 23 kcal mol−1 was evaluated on the basis of a first-order kinetic equation for all samples.

Active Oxygen Species and Mechanism for Low-Temperature CO Oxidation Reaction on a TiO2-Supported Au Catalyst Prepared from Au(PPh3)(NO3) and As-Precipitated Titanium Hydroxide

The active oxygen species and mechanism for catalytic CO oxidation with O2 on a highly active TiO2-supported Au catalyst (denoted as Au/Ti(OH)*4), which was prepared by supporting a Au–phophine complex on as-precipitated wet titanium hydroxide followed by calcination at 673 K, have been studied by means of oxygen isotope exchange, O2 temperature-programmed desorption (O2 TPD), electron spin resonance (ESR), and Fourier-transformed infrared spectroscopy (FT-IR). Surface lattice oxygen atoms on the Au/Ti(OH)*4 catalyst were inactive for oxygen exchange with O2 and CO and also for CO oxidation at room temperature. The surface lattice oxygen atoms were exchanged only with the oxygen atoms of CO2, probably via carbonates. O2 did not dissociate to atomic oxygen on the catalyst. The catalyst showed a paramagnetic signal at g=2.002 due to unpaired electrons trapped at oxygen vacancies mainly at the surface. O2 adsorbed on the oxygen vacancies to form superoxide O−2 with g1=2.020, g2=2.010, and g3=2.005, which are characteristic of O−2 with an angular arrangement. Upon CO exposure, all the adsorbed oxygen species disappeared. The adsorbed oxygen on Au/Ti(OH)*4 desorbed below 550 K. O−2 species were also observed on TiO*2 prepared by calcination of as-precipitated wet titanium hydroxide at 673 K, but were unreactive with CO. FT-IR spectra revealed that CO reversibly adsorbed on both Au particles and Ti4+ sites on the Au/Ti(OH)*4 surface. No band for adsorbed CO was observed on the TiO*2, which indicates that the presence of Au particles has a profound effect on the surface state of Ti oxide. No shifts of νCO peaks on Au/Ti(OH)*4 occurred upon O2 adsorption, suggesting that O2 was not directly bound to the Au particles on which CO adsorbed. Annealing of Au/Ti(OH)*4 under O2 atmosphere significantly suppressed the O2 adsorption and the CO oxidation due to a decrease in the amount of oxygen vacancies, while CO adsorption was not affected by annealing. From the systematic oxygen isotope exchange experiments along with O2-TPD, ESR, and FT-IR, it is most likely that CO adsorbed on Au metallic particles and O−2 adsorbed on oxygen vacancies at the oxide surface adjacent to the Au particles contribute to the low-temperature catalytic CO oxidation. The mechanism for the catalytic CO oxidation on the active Au/Ti(OH)*4 catalyst is discussed in detail and compared with mechanisms reported previously.

置換チタン酸ストロンチウムを触媒とするメタンの酸化カップリング反応 構造と触媒活性

Perovskite type strontium titanate, in which Ti4+ion was partly substituted by Mg2+ and the oxide ion defects were formed, was prepared by doping Mg0 to SrCO3-Ti02 mixture at 1473K. We found that this oxide was active catalyst for the oxidative coupling of methane (0CM) and its activity increased with increasing the amountof oxide ion defects and reached the maximum level at the substitution ratio of 40-60%. The reaction of methane with SrTi0.4Mg0.6O3-δ and SrTiO3 in the absence of oxygen gave C2 hydrocarbons and COx. The selectivity at the initial stage agreed with that of the steady-state catalytic reaction of the oxidative coupling of methane.From these results and O2 TPD result which has been reported, it is suggested that the addition of Mg2+modify the reactivity of the lattice oxide ion and the lattice oxide ion, which desorbed in the temperature range 873 K-1273 K on TPD measurement, is re-sponsible for the selective oxidative coupling of methane.

Study of high coverages of atomic oxygen on the Pt(111) surface

Atomic oxygen coverages of up to 0.75 ML may be adsorbed cleanly on Pt(111) surfaces under UHV conditions by exposure to NO2 at 400 K. We have studied this adsorbed oxygen layer by using AES, LEED, UPS, HREELS, TPD, and work function (ΔΦ) measurements. The (2 × 2)-O structure formed at θO = 0.25 ML is still apparent at θO = 0.60 ML and a faint (2 × 2) pattern persists even up to θO = 0.75 ML. AES and ΔΦ measurements show no evidence for chemically distinct species in the adlayer as a function of oxygen coverage. HREELS spectra clearly rule out the presence of molecular oxygen and oxide species over the range of oxygen coverage studied. UPS also shows no shift in binding energy of the oxygen-derived peak as the coverage is increased. These spectroscopic probes indicate that all oxygen is present as atomic oxygen with no indication of oxide formation or molecular oxygen at any coverage. Multiple O2 desorption peaks observed in TPD are interpreted as arising largely from kinetic effects rather than a result of multiple, distinctly different chemical species, even though large changes in the Pt-O bond energy are determined from the TPD data. The activation energy for O2 desorption reflects the sum of the heat of dissociative adsorption of O2 and the activation energy for O2 dissociation. The structure in the O2 TPD spectrum is due to large changes in the activation energy for O2 desorption resulting from increases in the barrier to dissociative O2 chemisorption and decreases in the Pt-O bond energy. These barriers arise from strong repulsive interactions between adsorbed oxygen adatoms that cause sharp reductions in the Pt-O bond strength at these coverages. Finally, we note that our spectroscopic probes are quite insensitive to the changes in the Pt-O bond strength over the entire range of oxygen coverage studied.