Catalytic oxidation of solid carbon and carbon monoxide over cerium‐zirconium mixed oxides

A series of cerium‐zirconium mixed oxides was prepared and evaluated in the catalytic oxidation of solid coke with 10 vol % O₂ in He at 673 K using a thermogravimetric analyzer. The measured first order rate constant for coke oxidation was proportional to the catalyst loading when the mass ratio of catalyst to coke was low, which enabled the calculation of a surface area specific reaction rate. The validity of the normalization method was confirmed by performing CO oxidation over the cerium‐zirconium mixed oxides in a fixed bed reactor at 573 K. Although there was no correlation between the coke oxidation rate and the oxygen storage capacity or the reducibility of the catalysts, there was an excellent correlation to the CO oxidation rate. Kinetic studies of both coke and CO oxidation suggested an important role of surface lattice oxygen from the catalyst in the two reactions. © 2016 American Institute of Chemical Engineers AIChE J, 63: 725–738, 2017     

Potential rare-earth modified CeO2 catalysts for soot oxidation

The physico-chemical properties of ceria (CeO₂) and rare earth modified ceria (with La, Pr, Sm, Y) catalysts are studied and correlated with the soot oxidation activity with using O₂ and O₂ + NO. CeO₂ modified with La and Pr shows superior soot oxidation activity with O₂ compared with the unmodified catalyst. The improved soot oxidation activity of rare earth doped CeO₂ catalysts can be correlated to the increased meso/micro pore volume and the stabilisation of the external surface area. On the other hand, unreducible ions decrease the intrinsic soot oxidation activity of rare earth modified ceria with both O₂ and NO + O₂ due to the decreased amount of redox surface sites. The catalyst bulk oxygen storage capacity is not a critical parameter in determining the soot oxidation activity. The modification with Pr shows the best soot oxidation with both O₂ and O₂ + NO compared with all other catalysts.     

Thin‐Film Catalytic Coating of a Microreactor for Preferential CO Oxidation over Pt Catalysts

Different Pt‐based catalyst layers have been prepared and tested in a stacked foil microreactor for CO oxidation and preferential oxidation of CO in presence of hydrogen. The reactions were performed on Pt without support by impregnation of a pre‐oxidized microstructured metal plate, Pt/Al₂O₃ and Pt/CeO₂ based on sol methods as well as Pt/nano‐Al₂O₃, a combined method of sol‐gel and nanoparticle slurry coating. The ceria based sol‐gel catalyst was much more active for CO oxidation than alumina based sol‐gel catalysts at low temperature. However, total oxidation was only obtained at higher temperature on the alumina based catalysts. The combined method seems to have advantages in terms of less internal mass transfer limitation when trying to increase the catalyst coating thickness based on sol‐gel approaches due to no reduction of CO selectivity up to 300 °C reaction temperature. Experiments on CO oxidation with the Pt/CeO₂ catalyst have been conducted in an oxygen supply microreactor to evaluate the catalyst performance under sequential oxygen supply to reaction zone (CO excess).     

Understanding the preferential oxidation of carbon monoxide (PrOx) using size‐controlled Au nanocrystal catalyst

Removing CO from hydrogen streams is an important industrial process. The catalytic preferential oxidation of CO (PrOx) is a promising method for CO removal, leaving the hydrogen concentration unchanged. Here, the effect of size and support on the gold‐catalyzed PrOx reaction using size‐controlled Au nanocrystals (NCs) is investigated. For all supports, Au NC sizes of 2–5 nm show the highest rates, whereas for larger sizes rates drop. Ceria‐supported Au shows by far the best performance. By analyzing the dependency of the reaction rate on the NC diameter, the most active centers for CO oxidation on Au/CeO₂ are Au⁺ corner atoms at the interface with the support, resulting in 2.1 nm Au NCs supported on ceria reaching full O₂ conversion and CO selectivity of about 50%. Therefore, it is suggested that increasing the fraction of Au‐ceria interface sites would lead to the best performing materials for this reaction. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3159–3167, 2018     

Simulation of NOx and soot abatement with Cu‐Cha and Fe‐ZSM5 catalysts

The embodiment of the NO_x selective catalytic reduction (SCR) functionality in a diesel particulate filter (DPF), so‐called SCR‐on‐Filter (SCRoF), is investigated through numerical modeling with SCR kinetics corresponding to Cu‐Chabazite and Fe‐ZSM5 catalysts. The results of the simulations of the SCR activity, performed in the absence and presence of soot, indicate that the presence of soot negligibly affects the NO_x conversion efficiency, given the slow dynamics of passive regeneration. Conversely, the reduction in cake thickness by soot passive oxidation is significantly different in the absence of SCR activity (uncatalyzed DPF) compared to that in its presence (SCRoF). In fact, in the SCRoF only 60–80% of the original soot consumption obtained in the absence of SCR reaction over 1 h can be achieved. Individual Cu‐Chabazite and Fe‐ZSM5 catalysts, as well as in‐series layers of the two catalysts, are investigated to devise the widest temperature window for SCRoF. © 2016 American Institute of Chemical Engineers AIChE J, 63: 238–248, 2017     

Catalytic Oxidation of CO over Nanocrystalline La1–xCexNiO3 Perovskite‐Type Oxides

Nanocrystalline La_1–xCe_xNiO₃ (x = 0.1, 0.3, 0.5, 0.7, 0.9) perovskite‐type oxide catalysts prepared by the Pechini method were employed in catalytic CO oxidation and the effect of substitution of La by Ce on CO conversion was evaluated. The results indicated the remarkable effect of La substitution with Ce on the catalytic performance at low temperatures. The reaction temperature had a significant influence on the stability of the catalysts. The La_0.1Ce_0.9NiO₃ sample exhibited the highest activity among the prepared catalysts in CO oxidation reaction. In addition, the influence of different parameters including pretreatment condition, feed ratio, and gas hourly space velocity (GHSV) on the catalytic performance was examined. The optimum catalyst proved high stability under severe reaction conditions in the presence of water vapor and CO₂ in the feed stream.     

Determination of Mechanism for Soot Oxidation with NO on Potassium Supported Mg‐Al Hydrotalcite Mixed Oxides

Soot oxidation with NO (in the absence of gas phase O₂) on potassium‐supported Mg‐Al hydrotalcite mixed oxides (K/MgAlO) was studied using a temperature‐programmed reaction and in situ FTIR techniques. Nitrite and the ketene group were identified as the reaction intermediates and thus a nitrite‐ketene mechanism was proposed in which surface active oxygen on K sites of K/MgAlO is transferred to soot by NO through nitrites. In the absence of gas phase O₂, soot oxidation with NO at lower temperatures (below 450 °C) is limited by the amount of active oxygen on the K sites. This kind of active oxygen is not reusable but can be replenished in the presence of gas phase O₂.     

Nanosized Composite Pt‐Ru Catalysts for Production of Modern Modified Fuels

A Pt‐Ru/2 % Ce/(θ+α)‐Al₂O₃ nanosized catalyst was developed for selective catalytic oxidation of CH₄ to synthesis gas. The process was carried out entirely with the formation of synthesis gas at high selectivity by H₂ and CO with H₂:CO = 2.0 ratio only at Pt:Ru = 2:1 or 1:1 atomic ratio and short contact time on Pt‐, Ru‐, and Pt‐Ru low‐percentage catalysts. Samples, which were reduced by H₂ at high temperature, presented a mixture of Pt‐, Ru‐, and Pt‐Ru nanosized particles, its alloy in the mixed catalysts. The correlation between experimental results and data of physicochemical research was established. The activity together with physicochemical properties and quantum chemical calculations for the developed low‐percentage Pt‐Ru catalysts was investigated.     

Impact of Fe Content in Laboratory-Produced Soot Aerosol on its Composition,Structure, and Thermo-Chemical Properties

Soot aerosol, which is a major pollutant in the atmosphere of urban areas, often contains not only carbonaceous matter but also inorganic material. These species, for example, iron compounds, originated from impurities in fuel or lubricating oil, additives or engine wear may change the physico-chemical characteristics of soot and hence its environmental impact. We studied the change of composition, structure, and oxidation reactivity of laboratory-produced soot aerosol with varying iron content. Soot types of various iron contents were generated in a propane/air diffusion flame by adjusting the doping amount of iron pentacarbonyl Fe(CO)₅ to the flame. Scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX) was combined with cluster analysis (CA) to separate individual particles into definable groups of similar chemical composition representing the particle types in dependence of the iron content in soot. Raman microspectroscopy (RM) and infrared spectroscopy were applied for the characterization of the graphitic soot structure, hydrocarbons, and iron species. For the analysis of soot reactivity, temperature-programmed oxidation (TPO) was used. It is demonstrated that iron is most dominantly present in the form of amorphous Fe (III) oxide crystallizing to hematite α-Fe₂O₃ upon thermal treatment. Iron contaminations do not change the soot microstructure crucially, but Fe(CO)₅ doping of the flame impacts hydrocarbon composition. Soot oxidation reactivity strongly depends on the iron content, as the temperature of maximum carbon (di)oxide emission T _max follows an exponential decay with increasing iron content in soot. Based on the results of the thermo-chemical characterization of laboratory-produced internally mixed iron-containing soot, we can conclude that iron-containing combustion aerosol samples cannot be characterized unambiguously by current thermo-optical analysis protocols.

Copyright 2012 American Association for Aerosol Research     

Preferential CO oxidation in H2-rich gas mixtures over Au/doped ceria catalysts

Nanosized gold catalysts supported on doped ceria were prepared by deposition–precipitation method. A deep characterization study by HRTEM/EDS, XRD, FT-Raman, TPR and FTIR was undergone in order to investigate the effect of ceria modification by various cations (Sm³⁺, La³⁺ and Zn²⁺) on structural and redox properties of gold catalysts. Doping of ceria affected in different way catalytic activity towards purification of H₂ via preferential CO oxidation. The following activity order was observed: Au/Zn–CeO₂ > Au/Sm–CeO₂ > Au/CeO₂ > Au/La–CeO₂. The differences in CO oxidation rates were ascribed to different concentration of metallic gold particles on the surface of Au catalysts (as confirmed by the intensity of the band at 2103 cm⁻¹ in the FTIR spectra collected during CO–O₂ interaction). Gold catalysts on modified ceria showed improved tolerance towards the presence of CO₂ and H₂O in the PROX feed. The spectroscopic experiments evidence enhanced reactivity when PROX is performed in the presence of H₂O already at 90 K.     

More active Ir subnanometer clusters than single‐atoms for catalytic oxidation of CO at low temperature

This work reported the adsorption and reaction performance of FeO_x supported subnanometer cluster and single‐atom Ir catalysts for the oxidation of CO at low temperature. By varying the pretreatment temperature and Ir loading, the single‐atom and subnanometer cluster Ir catalysts were obtained. The Ir subnanometer clusters exhibited higher activity for the oxidation of CO with or without the presence of H₂ than the single‐atom counterpart. By using adsorption microcalorimetry and in situ infrared spectroscopy measurements, it was found that the Ir subnanometer clusters not only promoted the adsorption and reaction of CO and O₂ but also facilitated the formation of OH species from reaction between H₂ and O₂, thus opening a new reaction pathway between CO and OH species to produce CO₂ compared with that between CO and O species on the single‐atom counterpart. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4003–4012, 2017     

Synthesis and physicochemical characterization of nanostructured Pd/ceria‐clinoptilolite catalyst used for p‐xylene abatement from waste gas streams at low temperature

BACKGROUND: The effect of Pd loading, xylene concentration and GHSV on xylene oxidation was tested over Pd/CeO₂(30%)‐clinoptilolite nanocatalysts at low temperatures. The catalysts were prepared by acid treatment of clinoptilolite, followed by the incipient wetness method of synthesized ceria and modified clinoptilolite in PdCl₂ solution. The synthesized nanocatalysts were characterized by XRD, FESEM, EDAX, TEM, BET, FTIR and TG‐DTG analysis. RESULTS: The XRD patterns confirmed the formation of crystalline ceria with an average crystallite size of 11.8 nm. FESEM images showed nanostructures in cavities of natural zeolite, brought about by ceria incorporation and acid activation. TEM analysis showed high dispersion of Pd with a size distribution between 6.6 and 36.7 nm. The quantitative analysis showed that the specific surface area of Pd(1%)/CeO₂(30%)‐clinoptilolite was 77 m² g^?1. The results showed that Pd(1%)/CeO₂(30%)‐clinoptilolite is the most appropriate catalyst, with the conversion more than 90% at 275 °C. CONCLUSIONS: Experimental results established effective performance and durability for the catalysts. As a result, clinoptilolite modification and ceria incorporation significantly altered the samples' morphology at nanoscale, improving the structure of composites and distribution of noble metals. A reaction path was suggested based on the adsorption‐migration of species to reveal the mechanism of p‐xylene oxidation over nanocatalysts. © 2012 Society of Chemical Industry     

Aerobic Oxidation of Cyclohexane using N‐Hydroxyphthalimide Bearing Fluoroalkyl Chains

The N‐hydroxyphthalimide derivatives, F₁₅‐ and F₁₇‐NHPI, bearing a long fluorinated alkyl chain, were prepared and their catalytic performances were compared with that of the parent compound, N‐hydroxyphthalimide (NHPI). The oxidation of cyclohexane under 10 atm of air in the presence of fluorinated F₁₅‐ or F₁₇‐NHPI, cobalt diacetate [Co(OAc)₂], and manganese diacetate [Mn(OAc)₂] without any solvent at 100 °C afforded a mixture of cyclohexanol and cyclohexanone (K/A oil) as major products along with a small amount of adipic acid. It was found that F₁₅‐ and F₁₇‐NHPI exhibit higher catalytic activity than NHPI for the oxidation of cyclohexane without a solvent. However, for the oxidation in acetic acid all of these catalysts afforded adipic acid as a major product in good yield and the catalytic activity of NHPI in acetic acid was almost the same as those of F₁₅‐ and F₁₇‐NHPI. The oxidation by F₁₅‐ and F₁₇‐NHPI catalysts in trifluorotoluene afforded K/A oil in high selectivity with little formation of adipic acid, while NHPI was a poor catalyst under these conditions, forming K/A oil as well as adipic acid in very low yields. The oxidation in trifluorotoluene by F₁₅‐ and F₁₇‐NHPI catalysts was considerably accelerated by the addition of a small amount of zirconium(IV) acetylacetonate [Zr(acac)₄] to the present catalytic system to afford selectively K/A oil, but no such effect was observed in the NHPI‐catalyzed oxidation in trifluorotoluene.     

Insights into the dynamics of oxygen storage/release phenomena in model ceria–zirconia catalysts as inferred from transient studies using H2, CO and soot as reductants

Samples of ceria–zirconia pre-treated under various conditions have been used as catalysts in CO and soot oxidation under stationary and transient conditions, in the presence and in the absence of oxygen. Their behaviour has been compared with that observed under redox conditions in the presence of hydrogen (oxygen storage activity). All the catalysts are active in CO and soot oxidation. Under stationary conditions, the activity in CO oxidation depends on the amount of Ce present, with little contribution from the redox capacity of the support and is strongly influenced by surface area. When the reaction is carried out under transient conditions, especially with low-surface area samples, the performances of ceria–zirconia are higher than those of ceria, with a maximum in the middle composition range. Interestingly, a similar behaviour is observed in soot combustion, where the activity for low-surface area sample is dependent on composition. This suggests that oxygen from the support plays a key role also in the oxidation of large carbon particles under a fully oxidizing mixture.     

Redox-catalytic correlations in oxidised copper-ceria CO-PROX catalysts

Four nanostructured oxidised copper-cerium catalysts prepared by two different methods (impregnation of ceria and coprecipitation of the two components within reverse microemulsions) with varying copper loadings have been examined with the aim of establishing correlations between redox and catalytic properties for preferential oxidation of CO in H₂-rich streams. The analysis is based on ex situ TPR examination both with H₂ or CO as well as operando spectroscopic exploration by DRIFTS and XANES, additionally complemented by conventional catalytic tests. The results reveal redox promoting effects on copper oxide reduction and allow establishing a model of the catalytic behaviour of this type of catalysts which can provide keys to control their CO-PROX catalytic properties.     

Preferential oxidation of carbon monoxide over Pt, Au monometallic catalyst, and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate

The objective of this paper was to study a preferential oxidation (PROX) of carbon monoxide over monometallic catalysts including Pt, Au and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate. Single step sol–gel method (SSG) and impregnation on sol–gel method (ISG) were chosen for the preparation of the catalysts. The characteristics of these catalysts were investigated by X-ray diffractometer (XRD), Brunauer–Emmet–Teller (BET) method, transmission electron microscope (TEM), scanning electron microscope (SEM) and temperature-programmed reduction (TPR). The XRD patterns of the catalysts showed only the peaks of ceria crystallite and no metal peak appeared. From TEM images, the active components were seen to be dispersed throughout the ceria support. The TPR patterns of PtAu/CeO₂ catalyst prepared by SSG showed the reduction peaks were within a low temperature range and therefore, the catalysts prepared by SSG exhibited excellent catalytic activity for preferential oxidation of CO. Bimetallic Pt–Au catalyst improved the activity (90% conversion and 50% selectivity at 90 °C) because of the formation of a new phase. When the metal content of (1:1) PtAu/CeO₂ catalyst prepared by SSG was increased, the CO conversion did not change much while the selectivity decreased in the low temperature range (50–90 °C). The CO conversion increased with increasing W/F ratio. The presence of CO₂ and H₂O had a negative effect on CO conversion and selectivity due to blocking of carbonate and water on active sites.     

Ceria‐coated diesel particulate filters for continuous regeneration

The potential of diesel particulate filters wash‐coated with highly dispersed nano‐metric ceria particles for continuous regeneration has been investigated. To this end, catalytic filters were prepared, soot‐loaded (avoiding the formation of the cake layer), and regenerated—under isothermal conditions—at temperature ranging from 200–600°C. Results have shown that catalytic oxidation of soot starts from 300°C and, at all temperatures, the selectivity to CO₂ is higher than 99%. 475°C is the minimum temperature at which the filter is regenerated via catalytic path. At this temperature, the catalytic filter maintains substantially the same performance over repeated cycles of soot loading and regeneration, indicating that the thermal stability of ceria is preserved. This has been further confirmed by comparison between the outcomes obtained from characterization (X‐ray powder diffraction, N₂ adsorption at 77 K, Hg intrusion porosimetry, and scanning electron microscope/energy dispersive X‐ray analysis) of fresh filter and filter subjected to repeated regeneration tests. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3442–3449, 2017     

Comparative Study on Catalytic Properties for Low-Temperature CO Oxidation of Cu/CeO2 and CuO/CeO2 Prepared via Solvated Metal Atom Impregnation and Conventional Impregnation

Cu/CeO₂ and CuO/CeO₂ catalysts were prepared by solvated metal atom impregnation (SMAI) and conventional impregnation (CI) and used for carbon monoxide oxidation in CO and air. The catalysts were characterized by means of XRD, XPS, AES and H₂-TPR techniques. The Cu/CeO₂ catalyst prepared via SMAI exhibits higher catalytic activity in CO oxidation than that prepared via CI with the same Cu content due to the smaller Cu particles. The CuO/CeO₂ catalyst prepared via SMAI also shows higher catalytic activity than that prepared via CI because the CuO particles of the former are smaller than the latter and can be reduced by CO more easily. The Cu/CeO₂ catalysts display higher catalytic activities than CuO/CeO₂ catalysts with the same Cu content and prepared by the same method. The TPR profile for CuO/CeO₂ catalyst prepared via SMAI has a single peak, indicating a one-step reduction, whereas the TPR profile for CuO/CeO₂ catalyst prepared via CI has two peaks, indicating a two-step reduction due to the existence of two kinds of CuO species.     

Copper oxide catalysts supported on ceria for low-temperature CO oxidation

Copper oxide catalysts supported on ceria were prepared by wet impregnation method using finely CeO₂ nanocrystals, which was derived from alcohothermal synthesis, and copper nitrate dissolved in the distilled water. The catalytic activity of the prepared CeO₂ and CuO/CeO₂ catalysts for low-temperature CO oxidation was investigated by means of a microreactor-GC system. The samples were characterized using BET, XRD, SEM, HRTEM and TPR.     

Characterization and catalytic activity of unpromoted and alkali (earth)-promoted Au/Al2O3 catalysts for low-temperature CO oxidation

Various unpromoted and alkali (earth) promoted gold catalysts were characterized by means of XRD, HRTEM, DR/UV–Vis and TPR. Based on the results we conclude that metallic Au is the active species in CO oxidation and that the reduction of Au³⁺ to Au⁰ proceeds below 200 °C. Pretreatment at mild temperatures, viz. 200 °C, results in the highest catalytic performance of Au/Al₂O₃ in low-temperature CO oxidation. Alkali (earth) metal oxide additives are most probably structural promoters. The best promoting effect is found for BaO.