Effect of CeO2 on Supported Pd Catalyst in the SCR of NO: A DRIFT Study

The activity of Pd/Al₂O₃ and Pd/Al₂O₃–CeO₂ samples has been tested in the selective catalytic reduction of NO by propene. It is found that the activity of Pd/Al₂O₃ decreases with calcination temperature, while the activity of Pd/Al₂O₃–CeO₂ increases abnormally with increasing calcination temperature. Surface-area measurement shows both samples suffer a linear decrease in their surface area, so it is reasonable to attribute the activity enhancement to the effect of CeO₂. The adsorption behavior and state of surface-active sites have been characterized by diffuse reflectance FTIR spectroscopy using CO and NO as probes and the effect of CeO₂ has been revealed. The CeO₂ component increases and stabilizes the dispersion of surface Pd species to prevent it from aggregating at high temperature. CeO₂ may also act as a buffer during the redox cycle of Pd, lengthen the period of Pd redox procedure and render Pd a property of inertia in its redox process, thus increasing the activity of the Pd/Al₂O₃–CeO₂ sample. The essential feature of both effects is the strong interaction between Pd and CeO₂. The intensity of interaction increases linearly with calcination temperature and so does the sample activity.     

Selective Reduction of NO with CO Over Titania Supported Transition Metal Oxide Catalysts

A series of transition metal oxides promoted titania catalysts (MO _x /TiO₂; M = Cr, Mn, Fe, Ni, Cu) were prepared by wet impregnation method using dilute solutions of metal nitrate precursors. The catalytic activity of these materials was evaluated for the selective catalytic reduction (SCR) of NO with CO as reductant in the presence of excess oxygen (2 vol.%). Among various promoted oxides, the MnO _x /TiO₂ system showed very promising catalytic activity for NO + CO reaction, giving higher than 90% NO conversion over a wide temperature window and at high space velocity (GHSV) of 50,000 h⁻¹. It is remarkable to note that the catalytic activity increased with oxygen, up to 4 vol.%, under these conditions leading primarily to nitrogen. Our TPR studies revealed the presence of mixed oxidation states of manganese on the catalyst surface. Characterization results indicated that the surface manganese oxide phase and the redox properties of the catalyst play an important role in final catalytic activity.     

Influence of Hydrocarbon Chemisorption on the NOx Adspecies over Supported Noble-Metal Catalysts

In situ and time-resolved DRIFT methods were used to monitor the change in NO _x adspecies on Pt(1%)–TiO₂ and Rh(1%)–TiO₂ catalysts during interaction with propene with the aim to determine whether or not propene chemisorption and interaction with the catalyst induces a change in the nature of the NO _x adspecies prior to their reduction. The nature of NO _x adspecies produced by interaction of the NO + O₂/He feed with the catalyst is different on Pt- and Rh–TiO₂ (in the Pt–TiO₂ catalyst the IR more intense adspecies are nitrate, while in the Rh–TiO₂ catalyst nitrosyl species are the IR more intense), but modification of the nature of the adspecies prior to their conversion is observed in both cases. The interpretation of the data provides indication about the nature of the reactive NO _x species and the presence of multiple pathways in the mechanism of their conversion.     

Co-precipitated Copper Zinc Oxide Catalysts for Ambient Temperature Carbon Monoxide Oxidation: Effect of Precipitate Aging Atmosphere on Catalyst Activity

A study of the effect of the aging atmosphere on the activity of co-precipitated copper zinc oxide catalysts for the ambient temperature oxidation of carbon monoxide is described and discussed. Four aging atmospheres are reported: air, N₂, H₂ and CO₂, and both the precipitation and the aging of the precipitate were carried out by flowing these gases through the precipitation cell at constant pH and temperature. For all atmospheres, the surface area of the final CuO-ZnO catalyst increases with aging time and, consequently, the specific activity (mol CO converted/g catalyst/h) also increases. However, the intrinsic activity (mol CO converted/m²/h) initially decreases with aging time before attaining a steady level. The highest activity catalysts were obtained using air as the aging atmosphere and TPR studies indicate that this catalyst is less readily reduced. Catalysts prepared using CO₂ as the aging atmosphere have lower activity, although the surface areas of these catalysts are not markedly lower. The study demonstrates that selection of the appropriate aging atmosphere, as well as the aging time, is an important parameter for the preparation of co-precipitated catalysts.     

TiO2/ZnO Supported on Sepiolite: Preparation,Structural Characterization,and Photocatalytic Degradation of Flumequine Antibiotic in Aqueous Solution

In this study, TiO₂, ZnO, TiO₂/ZnO (Ti/Zn), and TiO₂/ZnO/Sep (Ti/Zn/Sep) catalysts have been synthesized using sol–gel and chemical precipitation method. Their photocatalytic performances have been compared using Flumequine (FLQ) antibiotic. X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), scanning electron microscopy (SEM), N₂-adsorption, and the determination of a zero point charge has been used to characterize the synthesized catalysts. The degradation studies showed that the catalytic efficiency of Ti/Zn/Sep is higher than that for other catalysts. The operational parameters such as pH, initial FLQ concentration, and catalyst dosage were evaluated. UV–vis and high-resolution mass spectroscopy (HRMS) analyses were used to determine the degradation efficiency and products. ZnO played a major role in the FLQ degradation process, and sepiolite contributed to adsorption of FLQ on the catalyst surface enormously. The catalysts exhibited 11%, 23%, 63%, and 85% degradation efficiency for ZnO, TiO₂, Ti/Zn, and Ti/Zn/Sep in the decomposition of FLQ, respectively.     

CO hydrogenation to iso-C4 hydrocarbons over CeO2–TiO2 catalysts

CeO₂–TiO₂ (Ce:Ti = 0.25–9, molar ratio) catalysts were synthesized by a sol–gel method and the catalytic performances were evaluated in the selective synthesis of isobutene and isobutane from CO hydrogenation under the reaction conditions of 673–748 K, 1–5 MPa and 720–3000 h⁻¹. The physical properties, such as specific surface area, cumulative pore volume, average pore diameter, crystal phase and size, of the catalysts were characterized by N₂ adsorption/desorption and XRD. All the CeO₂–TiO₂ composite oxides showed higher surface areas than pure TiO₂ and CeO₂. No TiO₂ phase was detected on the samples of CeO₂–TiO₂ in which TiO₂ contents were in the range of 10–50 mol%. Crystalline Ce₂O₃ was detected in CeO₂–TiO₂ (8:2). The reaction conditions, temperature, pressure and space velocity, had obvious influences on the CO conversion and distribution of the products over CeO₂–TiO₂ (8:2) catalyst.     

Preferential oxidation of CO in excess hydrogen over a nanostructured CuO–CeO2 catalyst with high surface areas

CuO–CeO₂ is prepared by coprecipitation and ethanol washing and characterized using BET, HR-TEM, XRD and TPR techniques. The results show that CuO–CeO₂ is nanosized (r_TEM = 6.5 nm) and possesses high surface area (S_BET = 138 m² g⁻¹). Furthermore, some lattice defects in the surface of CuO–CeO₂ are found, which are beneficial to enhance catalytic performance of CuO–CeO₂ in preferential oxidation of CO in excess hydrogen (PROX). Consequently, the nanostructured CuO–CeO₂ exhibits perfect catalytic performance in PROX. Namely, CO content can be lowered to less than 100 ppm at 150 °C with 100% selectivity of O₂ in the presence of 8% CO₂ and 20% H₂O at .     

A comparative study of oxygen storage capacity over Ce0.6Zr0.4O2 mixed oxides investigated by temperature-programmed reduction and dynamic OSC measurements

The effects of redox-ageing on the temperature-programmed reduction and dynamic oxygen storage were investigated on two samples of Ce_0.6Zr_0.4O₂ prepared under different synthesis conditions. It was observed that a high-temperature reduction/mild oxidation redox cycle can generate temperature-programmed reduction (TPR) profiles featuring a reduction peak at a temperature as low as 537 K. However, despite such favourable reduction behaviour, a strong deactivation of the oxygen storage is observed under dynamic conditions, indicating the limitations of the TPR method for investigation of oxygen storage.     

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.     

First In Situ Raman Study of Vanadium Oxide Based SO2 Oxidation Supported Molten Salt Catalysts

In situ Raman spectroscopy at temperatures up to 500°C is used for the first time to identify vanadium species on the surface of a vanadium oxide based supported molten salt catalyst during SO₂ oxidation. Vanadia/silica catalysts impregnated with Cs₂SO₄ were exposed to various SO₂/O₂/SO₃ atmospheres and in situ Raman spectra were obtained and compared to Raman spectra of unsupported model V₂O₅–Cs₂SO₄ and V₂O₅–Cs₂S₂O₇ molten salts. The data indicate that (1) the V^V complex V^VO₂(SO₄)₂ ^3– (with characteristic bands at 1034 cm^–1 due to (V=O) and 940 cm^–1 due to sulfate) and Cs₂SO₄ dominate the catalyst surface after calcination; (2) upon admission of SO₃/O₂ the excess sulfate is converted to pyrosulfate and the V^V dimer (V^VO)₂O(SO₄)₄ ^4– (with characteristic bands at 1046 cm^–1 due to (V=O), 830 cm^–1 due to bridging S–O along S–O–V and 770 cm^–1 due to V–O–V) is formed and (3) admission of SO₂ causes reduction of V^V to V^IV (with the (V=O) shifting to 1024 cm^–1) and to V^IV precipitation below 420°C.     

Reforming of Methane with Carbon Dioxide over a Catalyst Consisting of Ruthenium Metal and Cerium Oxide Supported on Mordenite

Reforming of CH₄ with CO₂ proceeds at 400 °C over a catalyst consisting of ruthenium metal and CeO₂ highly dispersed on mordenite. The catalyst, Ru-CeO₂/MZ, is highly active for the reforming of CH₄ under the conditions at which a carbon formation reaction is thermodynamically apt to take place. The reforming selectively forms H₂ and CO. An increase in the weight of the catalyst resulting from carbon deposits was scarcely observed. IR spectra for the catalyst indicate that the reforming proceeds via the formation of the intermediate species such as Ru-CO and Ru-CH_x on the surface of ruthenium. The data of H₂ adsorption support the idea that ruthenium is highly dispersed in Ru-CeO₂/MZ.     

Dependency of the Oxygen Storage Capacity in Zirconia–Ceria Solid Solutions upon Textural Properties

The oxygen storage capacity of Ce_0.2Zr_0.8O₂ solid solutions, prepared by different synthesis routes and aged at 1000°C, is investigated by means of temperature-programmed reduction, total- and dynamic-OSC measurements. The use of CO and H₂ as reducing agent is compared under dynamic conditions. It is observed that the dynamic OSC is critically dependent on sample pre-treatment and the nature of the reducing agent in the texturally unstable sample. In contrast, on a thermally stable Ce_0.2Zr_0.8O₂ only minor effects of the pre-treatment upon dynamic OSC are noticed.     

Catalytic combustion of chlorobenzene over MnOx–CeO2 mixed oxide catalysts

A series of MnOx–CeO₂ mixed oxide catalysts with different compositions prepared by sol–gel method were tested for the catalytic combustion of chlorobenzene (CB), as a model of volatile organic compounds of chlorinated aromatics. MnOx–CeO₂ catalysts with different ratios of Mn/Ce + Mn were found to possess high catalytic activity in the catalytic combustion of CB, and MnOx(0.86)–CeO₂ was identified as the most active catalyst, on which the temperature of complete combustion of CB was 254 °C. Effects of systematic variation of reaction conditions, including space velocity and inlet CB concentration on the catalytic combustion of CB were investigated. Additionally, the stability and deactivation of MnOx–CeO₂ catalysts were studied by various characterization methods and other assistant experiments. MnOx–CeO₂ catalysts with high Mn/Ce + Mn ratios present a stable high activity, which is related to their high ability to remove the adsorbed Cl species and a large amount of active surface oxygen.     

Mechanistic Study of Selective Oxidation of Dimethyl Ether to Formaldehyde over Alumina-supported Molybdenum Oxide Catalyst

XPS and IR spectroscopies were used to investigate the surface intermediates of dimethyl ether (DME) oxidation to formaldehyde over MoOx/Al₂O₃ catalyst. The reaction performances were tested by employing three typical reaction conditions, depending on the O₂/DME ratio and the reaction temperature. When there was sufficient oxygen present in the reaction media, a terminal or bridged CH₃O* species formed by DME dissociation was highly active and rapidly reacted with lattice oxygen to produce formaldehyde, leading to higher selectivity of HCHO. When oxygen was consumed completely or only DME was present in the reaction media, CH₃O species bonded to more than two Mo atoms (μ-OCH₃) and CH_x (x=1–3) species attached to the Mo atoms were observed and the relative ratio of (μ-OCH₃) /Mo–CH_x was significantly dependent on the reduction degree of MoO_x domains. The (μCH₃O) species was related to the formation of CH₃OH or CO_x, and the Mo–CH_x species led to the formation of CH₄.     

Efficient gas phase photodecomposition of acetone by Ru-doped Titania

Nanocrystalline titania particles doped with ruthenium oxide have been prepared by homogenous hydrolysis of TiOSO₄ in aqueous solutions in the presence of urea. The synthesized particles were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM), Selected Area Electron Diffraction (SAED) and surface area (BET) and porosity determination (BJH). The photocatalytic activity of Ru-doped titania samples was determined in the gas phase by decomposition of acetone during irradiation at 365 nm and 400 nm. The Ru-doped titania samples demonstrated enhanced photocatalytic activity under visible light. Ruthenium oxide causes the anatase to rutile transformation to occur at lower temperatures and decreasing of band-gap energy of Ru-doped samples.     

Nanocatalysis on Supported Oxides for CO Oxidation

Active gold and palladium nanoparticles supported on a variety of oxides (CeO₂, ZrO₂, Al₂O₃, SiO₂, MgO and ZnO) were synthesized using laser vaporization and microwave irradiation methods. The catalytic activities for CO oxidation on the nanoparticle catalysts were evaluated and compared among different oxide supports. The effect of shape on the catalytic activity is demonstrated by comparing the activities of the Au and Pd catalysts deposited on MgO nanocubes and ZnO nanobelts. The Au/CeO₂ nanoparticles deposited on MgO nanocubes exhibit high catalytic activity and stability. The enhanced catalytic activity is attributed to the presence of a significant concentration of the corner and edge sites in MgO nanocubes. The Au- and Pd-doped Mn₂O₃ nanoparticles show promising results for the low temperature CO oxidation. Several approaches for incorporating the Au and Pd nanocatalysts within mesoporous oxide supports are presented and discussed.     

pH effects on catalytic ozonation of carboxylic acids with metal on metal oxides catalysts

Summary The pH influence on the catalytic ozonation of small carboxylic acids (succinic (SA), chloroacetic (CAA) and pyruvic (PA) acids) was studied and compared to ozonation alone. In the acidic systems, the conversion of these carboxylic acids and total organic carbon (TOC) during the catalytic ozonation is mainly due to the catalytic effect. Experiments on SA indicate that in the pH range 3.6–5.0, in contrast to the ozone behavior, the efficiency of the catalytic ozonation system is decreased with increasing pH due to the change of charges on the catalyst surface. In the basic pH range (7.2–10.0), the effect of the ozonation alone becomes important and the reaction in the presence of the catalyst is a combined effect of ozonation alone and catalytic oxidation. Although the apparent SA conversion by the catalytic ozonation in the basic systems is higher than in the acidic systems, the Total Organic Carbon conversion is not so much higher than in the latter. During the ozonation with the same catalyst, more than 80 of the CAA converted are mineralized. The oxidation of CAA leads to almost equimolar release of chloride ions. This ion has no detrimental influence on the catalytic effects under our experimental conditions. However, in the very dilute systems with pH value lower than the pKa of CAA, when the CAA concentration is decreased to about 0.6mM, the catalytic oxidation is almost stopped. The low concentration of the dissociated CAA induces only weak adsorption and consequently small reaction rates. The total elimination of CAA with low concentration can be reached only if the pH is increased above the pKa. In addition to the distinct pH effect in the ozonation and catalytic ozonation system observed in this work, a further demonstration that separate mechanisms are involved in the two processes is provided by the analysis of oxidation by-products of pyruvic acid.     

Deactivation of Supported Copper Catalysts for Methanol Synthesis

Binary Cu/ZnO and Cu/Al₂O₃ as well as ternary Cu/ZnO/Al₂O₃ catalysts were investigated with respect to their catalytic activity and stability in methanol synthesis. In a rapid aging test, activity measurements were carried out in combination with the determination of the specific Cu surface area. A close correlation between the loss of catalytic activity and the decrease in specific Cu surface area was found due to sintering of the Cu particles. Differences in the deactivation behavior and the area-activity relationship of each catalyst system imply that the catalysts should be grouped in different classes.     

Luminescence,Paramagnetic, and Electrochemical Properties of Copper Oxides-Decorated TiO2/Graphene Oxide Nanocomposites

The properties of newly synthesized Cu₂O/CuO-decorated TiO₂/graphene oxide (GO) nanocomposites (NC) were analyzed aiming to obtain insight into their photocatalytic behavior and their various applications, including water remediation, self-cleaning surfaces, antibacterial materials, and electrochemical sensors. The physico-chemical methods of research were photoluminescence (PL), electron paramagnetic resonance (EPR) spectroscopy, cyclic voltammetry (CV), and differential pulse voltammetry (DPV). The solid samples evidenced an EPR signal that can be attributed to the oxygen-vacancy defects and copper ions in correlation with PL results. Free radicals generated before and after UV-Vis irradiation of powders and aqueous dispersions of Cu₂O/CuO-decorated TiO₂/GO nanocomposites were studied by EPR spectroscopy using two spin traps, DMPO (5,5-dimethyl-1-pyrroline-N-oxide) and CPH (1-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine), to highlight the formation of hydroxyl and superoxide reactive oxygen species, respectively. The electrochemical characterization of the NC modified carbon-paste electrodes (CPE) was carried out by CV and DPV. As such, modified carbon-paste electrodes were prepared by mixing carbon paste with copper oxides-decorated TiO₂/GO nanocomposites. We have shown that GO reduces the recombination process in TiO₂ by immediate electron transfer from excited TiO₂ to GO sheets. The results suggest that differences in the PL, respectively, EPR data and electrochemical behavior, are due to the different copper oxides and GO content, presenting new perspectives of materials functionalization.     

Study of the Supported K2MoO4 Catalyst for Methanethiol Synthesis by One Step from High H2S-Containing Syngas

ESR and XPS are used to study the Mo-based catalysts MoO₃/K₂CO₃/SiO₂ and K₂MoO₄/SiO₂ prepared with two kinds of precursors, (NH₄)₆Mo₇O₂₄4H₂O and K₂MoO₄. The catalytic properties of the catalysts for methanethiol synthesis from high H₂S-containing syngas are explored. The activity assay shows that the two catalysts have much the same activity for the reaction. By the ESR characterization of both functioning catalysts, the resonant signals of oxo-Mo(V) (g=1.93), thio-Mo(V) (g=1.98) and S (g=2.01 or 2.04) can be detected. In the catalyst MoO₃/SiO₂ modified with K₂CO₃, as increasing amounts of K₂CO₃ are added, the content of oxo-Mo(V) increases, but thio-Mo(V) decreases. The XPS characterization indicates that Mo has mixed valence states of Mo⁴⁺, Mo⁵⁺ and Mo⁶⁺, and that S includes three kinds of species: S^2– (161.5 eV), [S–S]^2– (162.5 eV) and S⁶⁺ (168.5 eV). Adding K₂CO₃ promoter to the catalysts, the Mo species of high valence state is easily sulphided and reduced to Mo₂S and oxo-M(V), and the derivation of [S–S]^2– and S^2– species from S is promoted simultaneously. The methanethiol synthesis is favored if the mole ratio of (Mo⁶⁺ + Mo⁵⁺)/Mo⁴⁺ 0.8 and S^2–/[S–S]^2– is kept at a value of about 1.