Oxidation Of CO In Excess Research Articles

Au nanoparticles supported on nanorod-like TiO2 as catalysts in the CO-PROX reaction under dark and light irradiation: Effect of acidic and alkaline synthesis conditions

Gold nanoparticles precipitated-deposited on titania nanostructures (1.0 wt% nominal loading) were studied in the preferential CO oxidation in excess of H2 at room temperature and atmospheric pressure, both in dark and under simulated solar light irradiation. Titania supports were synthesized by means of two hydrothermal methods markedly acid and basic, giving rise to rutile nanorods and anatase deformed nanorods structures, respectively. Characterization techniques such as N2 physisorption, XRD, XPS, DRUV-vis, HRTEM and XRF were performed in order to study the chemical, structural and optical properties of the catalysts. Well defined rutile nanorods structures were obtained from the acidic treatment allowing a regular distribution of gold nanoparticles and resulting quite active in the CO-PROX reaction. In particular the sample from the acidic synthetic approach calcined at 700 °C displayed the best results as it was highly selective to CO2 under both dark and simulated solar light irradiation.

TixCe(1-x)O2 as Pt support for the PROX reaction: Effect of the solvothermal synthesis

TiCe mixed oxides with different Ti/Ce atomic ratios have been synthesized via a solvothermal method and used as supports for platinum nanoparticles. These catalysts were tested in the preferential oxidation of CO in excess of H2 (PROX reaction) using different feed compositions in order to simulate real conditions. Both, supports and catalysts, were characterized by a wide variety of techniques including N2 adsorption, XRF, XRD, Raman spectroscopy, XPS, H2-TPR, UV–Vis and DRIFTS. The best performance was obtained with the Pt/Ti0.9Ce0.1O2 catalyst, what was attributed to the incorporation of Ce atoms in the TiO2 lattice. 10% at. Ce is the maximum concentration we could incorporate in the TiO2 lattice by using the solvothermal method.

A highly active Pt-Fe/γ-Al2O3 catalyst for preferential oxidation of CO in excess of H2 with a wide operation temperature window.

Highly efficient Pt-Fe/γ-Al2O3 catalysts for preferential oxidation of CO in excess of H2 (CO-PROX) were prepared by utilizing single-atom Fe species as active sites for O2 activation, which exhibited high catalytic activity and selectivity from 25 °C to 200 °C, with the highest Pt specific rate of Pt-based catalysts for CO-PROX.

Preferential oxidation of CO in excess of hydrogen over Au/CeO2-ZrO2 catalysts

The preferential oxidation of CO to CO2 in a simulated reformed gas using Au/-CeO2-ZrO2 catalysts was examined. The studied catalyst showed high catalytic activity and stability during long-term tests even in presence of CO2 and water vapour, which inhibited the conversion of CO to a small extent. The catalyst preparation at controlled pH (6) favours the high dispersion of deposited gold particles, while the catalytic activity and selectivity was correlated with the size and oxidation state of gold particles. Furthermore, it was observed that oxidizing treatment resulted in the formation of gold particles having different shapes and/or side length, which was evidenced by a broadening of the plasmon resonance band (From DRS UV–vis).

High performance of Cu/CeO2-Nb2O5 catalysts for preferential CO oxidation and total combustion of toluene

Copper-based catalysts supported on niobium-doped ceria have been prepared and tested in the preferential oxidation of CO in excess of H2 (PROX) and in total oxidation of toluene. Supports and catalysts have been characterized by several techniques: N2 adsorption, ICP-OES, XRF, XRD, Raman Spectroscopy, SEM, TEM, H2-TPR and XPS, and their catalytic performance has been measured in PROX, with an ideal gas mixture (CO, O2 and H2) with or without CO2 and H2O, and in total oxidation of toluene. The effects of the copper loading and the amount of niobium in the supports have been evaluated. Remarkably, the addition of niobia to the catalysts may improve the catalytic performance in total oxidation of toluene. It allows us to prepare cheaper catalysts (niobia it is far cheaper than ceria) with improved catalytic performance.

CuO-CeO2 supported on montmorillonite-derived porous clay heterostructures (PCH) for preferential CO oxidation in H2-rich stream

This study reports on the preparation and characterization of porous clay heterostructures (PCH) as a high-surface-area support for CuO–CeO2 based catalysts for the preferential oxidation of CO in excess of H2 (CO-PROX). After pillaring the montmorillonite clay with silica (Si-PCH) and silica-zirconia (SiZr-PCH), the Cu-Ce active phase was loaded by incipient wet impregnation setting the cerium amount constant (20wt%) and investigating three different copper loadings (3, 6 and 12wt%). The use of pillars of silica or silica-zirconia inserted in the interlayer space of a natural clay provides a high surface area support that can favor the dispersion of both CuO and CeO2 active phases, leading to the formation of a high amount of copper-ceria interfacial sites, responsible for a very high catalytic activity in the CO-PROX reaction.The results obtained from characterization of the materials by XRD, N2 physisorption, H2-TPR, XPS and CO2-TPD suggest that this synthesis method gives rise to catalysts with copper species highly active and selective for the CO-PROX reaction.The catalysts exhibit high CO conversion values and the sample with 6wt% of copper on Si-PCH displays very good performances, comparable to those based on precious metal catalysts, even at low temperatures.The system reducibility was found modified by the incorporation of zirconium in the support, with a slight decrement of the CO conversion value, compared to the same material without Zr. The influence of the presence of CO2 and H2O in the gas feed was also studied in order to simulate the real operating conditions of a PEMFC feed stream. Correlations between catalytic performances and physicochemical properties of the materials have been made.

3-D flower like Ce–Zr–Cu mixed oxide systems in the CO preferential oxidation (CO-PROX): Effect of catalyst composition

Ce–Zr–Cu mixed oxide systems, with different contents of ZrO2 and CuO, were prepared by slow co-precipitation method, leading to the formation of materials with a flower-like morphology. It was evaluated both the role of CuO loading (3–7wt%), by maintaining constant the amount of ZrO2 (7wt%), and the role of ZrO2 (2–10wt%) with a fixed amount of CuO (7wt%). The prepared catalysts were characterized by means of ICP-OES, SEM, N2 physisorption, quantitative XRD, H2-TPR, and XPS. Their catalytic performances in the preferential oxidation of CO in excess of H2 (CO-PROX) were evaluated, in the 40–190°C temperature range. Characterization and catalytic results showed that an optimum Zr/Ce molar ratio and CuO loading are required to attain the best catalytic performance over the studied nanostructured Ce/Zr/Cu oxide system. Characterization results revealed that, regardless of the sample composition, all of them presented similar flower-like morphology and textural properties. Instead, the catalyst composition determined the crystalline phases formed, their reducibility and surface distribution. The catalyst containing an intermediate ZrO2 loading (7wt%) and the highest studied CuO amount (7wt%), FCZCu77, showed the higher fraction of undetectable CuO from XRD, that is the highest proportion of highly dispersed Cu species also observed from H2-TPR. These characteristics justify how FCZCu77 catalyst exhibited a quite higher catalytic activity than the others samples.

Preferential oxidation of CO in excess of H2 on Pt/CeO2–Nb2O5 catalysts

A series of CeO2–Nb2O5 mixed oxides with different Nb content, as well as the pure oxides, have been synthesized by co-precipitation with excess urea. These materials have been used as supports for platinum catalysts, with [Pt(NH3)4](NO3)2 as precursor. Both supports and catalysts have been characterized by several techniques: N2 physisorption at 77K, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, UV–vis spectroscopy, scanning electron microscopy, transmission electron microscopy, temperature-programmed reduction and temperature-programmed desorption (CO and H2), and their catalytic behaviour has been determined in the PROX reaction, both with an ideal gas mixture (CO, O2 and H2) and in simulated reformate gas containing CO2 and H2O. Raman spectroscopy analysis has shown the likely substitution of some Ce4+ cations by Nb5+ to some extent in supports with low niobium contents. Moreover, the presence of Nb in the supports hinders their ability to adsorb CO and to oxidize it to CO2. However, an improvement of the catalytic activity for CO oxidation is obtained by adding Nb to the support, although the Pt/Nb2O5 catalyst shows very low activity. The best results are found with the Pt/0.7CeO2–0.3Nb2O5 catalyst, which shows a high CO conversion (85%) and a high yield (around 0.6) after a reduction treatment at 523K. The effect of the presence of CO2 and H2O in the feed has also been determined.

Effect of Crystalline Phase and Composition on the Catalytic Properties of PdSn Bimetallic Nanoparticles in the PROX Reaction

We present a synthetic strategy for the preparation of palladium–tin alloy and intermetallic nanoparticles. Complexes of palladium(II) and tin(IV) precursors in oleylamine were thermally decomposed in an organic solution in the presence of reducing moieties, leading to the formation of monodispersed nanoparticles with varying crystallographic structures. We found that the nanoparticles crystalline structure closely follows the bulk material phase diagram. The nanoparticles were supported on Al2O3 and their reactivity tested as catalysts for the preferential oxidation of CO in excess of hydrogen (PROX). Different Pd/Sn and O2/CO ratios have been investigated, and the structure–reactivity correlation highlighted. With increasing tin content, the CO ignition temperature remarkably lowers and the CO conversion rate increases, up to the formation of intermetallic phases that concurrently determine a reduction in the catalyst activity; the selectivity of the pure palladium references is preserved.

Influence of synthesis parameters on the performance of CeO2–CuO and CeO2–ZrO2–CuO systems in the catalytic oxidation of CO in excess of hydrogen

Ce–Cu and Ce–Zr–Cu oxide systems with a flower-like morphology were prepared by slow co-precipitation in the absence of any structure directing agent. For the sake of comparison, Ce–Cu and Ce–Zr–Cu oxide samples were also prepared by a classic co-precipitation method. All the samples were calcined in air flow at 650°C. The materials were characterized by SEM and TEM microscopy, quantitative X-ray diffraction, N2 physisorption, H2-TPR, O2-TPD and XPS. The catalytic activity of the prepared samples was evaluated in the preferential oxidation of CO in excess of H2 (CO-PROX), in the 40–190°C temperature range.

Preferential CO oxidation in excess of hydrogen over Au/HMS catalysts modified by Ce, Fe and Ti oxides

The effect of the modification of HMS (hexagonal mesoporous silica) support with Fe3+, Ce4+ or Ti4+ ions on the physico-chemical and catalytic properties of mesoporous Au/HMS catalyst was evaluated in the preferential oxidation (PROX) of CO, in hydrogen-rich stream (50vol.% H2) and in the total CO oxidation (TOX) reaction. The bare supports were prepared via neutral S0I0 templating route, employing one pot synthesis, whereas the supported gold catalysts were prepared by the deposition–precipitation method. The reduced and spent Au catalysts have been studied by different techniques such as N2 physisorption, oxygen storage capacity (OSC), XRD, HRTEM, hydrogen chemisorption (by TPD-H2), DRS UV–vis, Mössbauer and XPS spectroscopic techniques. Among the catalysts studied, Au/HMS–Fe recorded the highest activity and stability for total CO oxidation. The XPS and DRS UV–vis data suggest that the active-sites configuration of this catalyst during this reaction could be small gold particles and ionic gold species stabilized by the support. The Au/HMS–Fe catalyst was also able to oxidize CO selectively in a hydrogen-rich gas mixture (PROX reaction) at a relevant temperature to hydrogen fuel cell applications (T50=83°C). Additionally, the Au/HMS–Fe sample recorded the lowest hydrogen oxidation (undesired reaction) during the CO-PROX reaction in excess hydrogen. From the catalyst activity–structure correlation, the main factors influencing the superior catalytic behaviour of this sample in the CO-PROX reaction are (i) its largest Au species surface exposure, as determined by XPS for the spent catalysts; (ii) the largest population of small Au particles, as determined by HRTEM for the freshly reduced samples; (iii) the large oxygen and hydrogen storage capacities, and (iv) the lowest deactivation by the formation of surface coke species, as determined by coke burning followed by TGA technique.

Gold stabilized aqueous sols immobilized on mesoporous CeO 2–Al 2O 3 as catalysts for the preferential oxidation of carbon monoxide

Nanostructured Au/Al 2O 3–CeO 2 catalysts with a low content of precious metal (0.9% wt.) were prepared immobilizing two different stabilized Au sols on a high surface area Al 2O 3–CeO 2 mixed oxide with a uniform pore size distribution, synthesized by a one-pot methodology. The samples were characterized by elemental analysis, N 2 physisorption, XRPD, TEM and 27Al-MAS NMR techniques. The catalytic activity of the two samples in the preferential oxidation of CO in excess of H 2 (CO-PROX) was comparatively evaluated in the 35–110 °C temperature range. The Au-THPS/AlCe20 sample, prepared immobilizing a sol obtained reducing an aqueous solution of gold tetrachloroaurate salt with bis[tetrakis(hydroxymethyl)phosphonium sulfate], resulted very active and selective at low temperatures and its catalytic activity was correlated with the structural characteristics of the metal particles and of the ordered mesoporous support.

Selective CO oxidation in excess of H 2 over high-surface area CuO/CeO 2 catalysts

The influence of the surface area, crystal size of the support and the Cu loading (3–9 wt.%) on the catalytic behavior of CuO/CeO 2 catalysts was investigated in the preferential CO oxidation (PROX) reaction. Two ceria samples were used: a high-surface area one (SCe) prepared by the templating technique and a low surface commercial one (ACe). Copper was incorporated to the calcined support by classical impregnation. Techniques were used to characterize the textural, structural and chemical properties of the catalysts (BET, XRD, HRTEM, and TPR). For catalysts supported on SCe these techniques evidenced highly dispersed copper species in strong interaction with nanosized CeO 2 crystals. The enhanced redox properties of the CuO–support interface sites at low temperature evidenced by TPR in CuSCe catalysts play a fundamental role in the catalytic behavior for the CO oxidation in presence of excess H 2 (PROX). CuSCe catalysts showed excellent catalytic activity compared to catalysts supported on the commercial CeO 2. Total conversion of CO is obtained at 125 °C with 100% selectivity to CO 2. In the presence of CO 2 and H 2O the maximum CO conversion for temperatures higher than 125 °C was 95%. Selectivity also decreased being more pronounced when H 2O was present. This negative effect was reversible since the original activity and selectivity were practically restored upon elimination of these components from the feed. Partially reduced Cu + species seem to be present in the catalysts according to CO adsorption followed by DRIFT.

Inverse gas chromatographic investigation of the active sites related to CO adsorption over Rh/SiO 2 catalysts in excess of hydrogen

In the present work, the novel methodology of reversed-flow gas chromatography (RF-GC) is extended in a topic of contemporary scientific and technological interest, such as the effect of hydrogen in the “ topography” of the active sites related to CO adsorption. This study concerns CO adsorption on a silica-supported Rh catalyst, at 90 °C. The topography of the catalyst in the absence of hydrogen consists of both randomly and islands of CO bound over chemisorbed CO molecules. In contrast, under H 2-rich conditions, the observed topography is almost entirely patchwise ascribed to long-range lateral attractions between adsorbate molecules. In excess of hydrogen, CO adsorption is shifted at higher lateral attractions values which correspond to weaker adsorbate–adsorbent interactions and lower surface coverage. This provides an indication of a H 2-induced desorption, which can be attributed to the formation of an H–CO complex desorbing from the catalyst surface below the temperature required for CO desorption, in the absence of H 2, and it may explain the well-known enhancement of the rate of selective CO oxidation in excess of hydrogen by H 2.

Kinetics of selective CO oxidation in excess of H 2 over the nanostructured Cu 0.1Ce 0.9O 2− y catalyst

The kinetics of CO oxidation in excess hydrogen over a nanostructured Cu 0.1Ce 0.9O 2− y catalyst prepared by a sol–gel method was studied under simulated preferential oxidation (PROX) reactor conditions. Reaction temperature was varied between 45 and 155 °C. The partial pressures of CO and O 2 in 0.5 bar excess of H 2 and He as a balance gas were varied between 0.001 and 0.025 and between 0.001 and 0.05 bar, respectively. The catalyst was found to be 100% selective in the temperature range from 45 to 90 °C. In this temperature range, the kinetics of the reaction was found to follow the redox mechanism represented by the Mars and van Krevelen type of rate equation. Kinetic parameters of the reaction calculated on the basis of this rate equation were found to be as follows: apparent activation energy for CO oxidation step, 57.2 kJ/mol, and for the catalyst reoxidation step, 60.2 kJ/mol. The observed reaction rate at the 0.01-bar CO partial pressure and stoichiometric O 2 partial pressure at 90 °C was 2.7×10 −6 mol/g cat s. The steady-state experimental data could be regressed almost equally well with the modified Langmuir–Hinshelwood model introduced by Liu et al. However, the transient experiments performed in our study reveal that lattice oxygen could be involved even at low reaction temperatures, thus favoring the use of a steady state Mars and van Krevelen kinetic model.