Low-temperature Reduction Peak Research Articles

Application of Potassium Ion Deposition in Determining the Impact of Support Reducibility on Catalytic Activity of Au/Ceria-Zirconia Catalysts in CO Oxidation, NO Oxidation, and C3H8 Combustion

The purpose of the study was to show how a controlled, subtle change of the reducibility of the support by deposition of potassium ions impacts the activity of gold catalysts. Since the activity of supported gold catalysts in carbon monoxide oxidation is known to strongly depend on the reducibility of the support, this reaction was chosen as the model reaction. The results of tests conducted in a simple system in which the only reagents were CO and O2 showed good agreement with the CO activity trend in tests performed in a complex stream of reagents, which also contained CH4, C2H6, C3H8, NO, and water vapor. The results of the X-ray Diffraction (XRD) studies revealed that the support has the composition Ce0.85Zr0.15O2, that its lattice constant is the same for all samples, and that gold is mostly present in the metallic phase. The reducibility of the systems was established based on Temperature Programmed Reduction (TPR) and in situ XRD measurements in H2 atmosphere. The results show that the low temperature reduction peak, which is due to the presence of gold, is shifted to a higher value by the presence of 0.3 at% potassium ions on the surface. Moreover, the increase of the potassium loading leads to a more pronounced shift. The T50 of CO oxidation in the simple model stream was found to exhibit an excellent linear correlation with the maximum temperature of the low temperature reduction peak of Au catalysts. This means that stabilizing oxygen with a known amount of potassium ions can be numerically used to estimate the T50 in CO oxidation. The results in the complex stream also showed a similar dependence of CO conversion on reducibility, though there was no substantial difference in the activity of the catalysts in other reactions regardless of the potassium loading. These studies have shown that the influence of potassium varies depending on the reaction, which highlights differences in the impact of reducibility and importance of other factors in these reactions.

The promotional role of Nd on Mn/TiO2 catalyst for the low-temperature NH3‑SCR of NOx

A series of rare-earth metal doped 30% Mn-3% RE/TiO2 (RE = Ce, Sm, Nd, Er, Y) catalysts were prepared by adopting two step incipient wetness impregnation method and investigated for the low-temperature SCR of NOx with NH3. It was found that 3% of rare earth elements (Nd, Er, Y) was added to 30% Mn/TiO2 catalyst even could increase the catalytic performance at the low temperature, in addition to usual used Ce and Sm. And the promoting effect of Nd was among the most significant. Then the 30%Mn-3%Nd/TiO2 catalyst was synthesized by another two methods, Coprecipitation and Sol-gel, for a comparative study. Various techniques were applied to characterize the catalysts. The catalysts prepared by Coprecipitation and Sol-gel method exhibited higher surface area and smaller average particle size in comparison with catalyst prepared by impregnation with TiO2 support. The addition of Nd made surface area and dispersion of MnOx species increase. It also led both the content of amorphous MnO2 and the content of crystalline Mn2O3 to decrease, but the proportion of MnOx species with low oxidation state increased. H2-TPR results showed that the low temperature reduction peak of MnOx was shifted to much lower temperatures with the doping of Nd, which improved the redox property of the catalysts. The all characteristics shown above could well explain the promotional role of Nd doping in the 30%Mn-3%Nd/TiO2 catalyst, that improved the low temperature SCR catalytic activity.

The effects of calcination temperature of support on PtIn/Mg(Al)O catalysts for propane dehydrogenation reaction

The dehydrogenation of propane to propylene on bimetallic PtIn supported on calcined hydrotalcite, PtIn/Mg(Al)O-T, was investigated with the aim of understanding the effects of calcination temperature of hydrotalcite (MgAlO) on the structure/activity relationship of catalysts. The results showed that the highest catalysis reactivity and propylene selectivity were achieved over PtIn/Mg(Al)O-600 catalyst, which was improved obviously compared to our previous studies. The SEM images showed that all PtIn/Mg(Al)O-T catalysts had the flake structure collapsing constantly with the increasing calcination temperature of supports, which resulted in the change of textural properties of catalysts and the formation of more stable structure. Calcination temperature of support had obvious effect on the surface acid sites of PtIn/Mg(Al)O-T catalysts, and PtIn/Mg(Al)O-600 sample exhibited the lowest fraction of strong acid sites that were extremely unfavorable for facilitating propane dehydrogenation reaction. Besides, PtIn/Mg(Al)O-600 catalyst had the smallest average metal particle size as well as the largest specific surface area. XPS revealed that enhancing the interaction between indium and support was conductive to promoting the reduction of platinum. H2-TPR data suggested that the low-temperature reduction peak of PtIn/Mg(Al)O-600 was related to the unique catalytic behavior in the initial stage of propane dehydrogenation reaction. It can be concluded that PtIn/Mg(Al)O-600 catalyst possessed the largest surface area, the lowest fraction of strong acidic sites and In0 species, the best reducibility and distribution of Pt particles with the smallest Pt particle size, and the lowest coke amount corresponding to the best catalytic performance.

Effect of Gallium Loading on Reducibility and Dispersion of Copper-Based Catalyst

The effect of gallium-promoted copper-based catalysts has been investigated in connection with the characteristic of the active copper phase. CuO-ZnO-Ga2O3catalysts with different gallium loadings were prepared using oxalate co-precipitation method. The effects of gallium loading on the properties of catalysts were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and temperature-programmed reduction (TPR). The dispersion and metal area of copper were also determined by dissociative nitrous (N2O) adsorption technique conducted on a metal dispersion analyzer (BELCAT). The TPR profiles showed that the presence of two different reduction regions in the CuO-ZnO catalysts can be attributed to the reduction of highly dispersed copper oxide species (reduced at 246 °C) and bulk-like CuO (reduced at above 390 °C). By contrast, the only low-temperature reduction peak was presented in the TPR profiles after the Ga2O3loading was higher than 4 wt%. With the same molar ratio (Cu/Zn = 2:1), the reducibility of CuO-ZnO-Ga2O3was found to be more facile than CuO-ZnO due to the lower copper oxide crystallite sizes of gallium-promoted catalysts. Higher Ga2O3loadings resulted in an increase in both copper dispersion and metal surface area of all the catalysts studied in good agreement with the reduction behaviors in the TPR profiles, although all the gallium-promoted catalysts were slightly different for the reducibility.

Reduction Kinetics of Perovskite-Based Oxygen Carriers for Chemical Looping Combustion

LaMnO3 and LaMnO3@mSiO2 oxygen carriers were studied in a mini-fluidized bed reactor for chemical looping combustion, and their reduction behavior was first simulated using the following two classical kinetic models: the nucleation and nuclei growth model (NNGM) and the unreacted shrinking-core model. Furthermore, another model designated as the oxygen ions diffusion model was derived by assuming oxygen ion diffusion as the rate-limiting step for the reduction of both oxygen carriers. These materials were characterized using XRD for phase identification, BET for determination of specific surface area, pore volume and pore size distribution, SEM and TEM for morphology, particle size and shell thickness determination, H2-TPR and TPD-O2 for oxido–reduction properties analysis. H2-TPR results of LaMnO3@mSiO2 core–shell structured material indicated that the reduction of Mn3+ into Mn2+ is more difficult than in LaMnO3 as the low-temperature reduction peak was shifted to a higher temperature. Both carriers exhi...

Controllable preparation of CeO2 nanostructure materials and their catalytic activity

Well-crystalline CeO2 nanostructures with the morphology of nanorods and nanocubes were synthesized by a template-free hydrothermal method. X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption measurements were employed to characterize the synthesized materials. The reducibility and catalytic activity of nanostructured CeO2 were examined by hydrogen temperature-programmed reduction (H2-TPR) and CO oxidation. The results showed that CeO2 nanorods could be converted into CeO2 nanocubes with the increasing of the reaction time and the hydrothermal temperature, CeO2 nanorods became longer gradually with the increasing of the concentrations of NaOH. H2-TPR characterization demonstrated that the intense low-temperature reduction peak in the CeO2 nanorods indicated the amount of hydrogen consumed is larger than CeO2 nanocubes. Meantime the CeO2 nanorods enhanced catalytic activity for CO oxidation, the total conversion temperature was 340°C. The reasons were that CeO2 nanorods have much smaller crystalline sizes and higher surface areas than CeO2 nanocubes.

Nickel-doped Mn/TiO2 as an efficient catalyst for the low-temperature SCR of NO with NH3: Catalytic evaluation and characterizations

The Mn/TiO2 and a series of Mn–Ni/TiO2 catalysts were prepared by adopting incipient wetness technique and investigated for the low-temperature SCR of NO with NH3 in the presence of excess oxygen. Our XPS results illustrated that the MnO2 is the dominant phase with respect to the Mn2O3 phase (Mn4+/Mn3+=22.31, 96%), thus leading to a large number of Mn4+ species (Mn4+/Ti) over the titania support for the Mn–Ni(0.4)/TiO2 catalyst. It is remarkable to note that the SCR performance of all the nickel-doped Mn/TiO2 catalysts is accurately associated with the surface Mn4+ concentrations. The co-doping of nickel into the Mn/TiO2 with 0.4 Ni/Mn atomic ratio promotes the formation of surface MnO2 phase and inhibits the formation of surface Mn2O3 sites. Our TPR results revealed that the addition of nickel oxide to titania-supported manganese results in the stabilization of the former in the form of MnO2 rather than Mn2O3. Our TPR data results are in agreement with XPS results that the absence of the high-temperature (736K) peak indicates that the dominant phase in the Mn–Ni/TiO2 catalysts is MnO2. The low-temperature reduction peak is shifted to much lower temperatures in nickel-doped Mn/TiO2 catalysts. This increase in reducibility and the extremely dominant MnO2 phase seem to be the reason for the high SCR activity of the Mn–Ni/TiO2 catalysts.

Supported manganese dioxide catalyst for seawater flue gas desulfurization application

In seawater flue gas desulfurization (FGD) process, ceramic supported MnOx catalysts were developed to improve SO2 removal efficiency of the scrubber by increasing S(IV) oxidation rate at the low pH values. These supported MnOx catalysts were prepared by impregnation–precipitation method, calcined at different temperatures, and exposed to simulated flue gases to investigate the catalytic oxidation of sulfur dioxide in seawater buffering system. Compared with “blank” ceramic, the most active MnOx/ceramic catalyst, 623MnOx/ceramic, that was prepared by doping (NH4)2CO3 as the precipitant and calcined at 623K, exhibited up to 17% higher SO2 conversion at the GHSV 3000h−1 in the industrial feasible temperature range of 318–358K. XPS spectra revealed manganese was in the state of oxidation +4 on the surface of 623MnOx/ceramic. XRD patterns and SEM-EDS spectra indicated that the high activity of 623MnOx/ceramic for S(IV) oxidation at low temperatures could be attributed to oxygen activation and transfer ability of highly dispersed MnO2 on ceramic support. H2-TPR profiles showed the low temperature reduction peak was shifted to higher temperature when the calcination temperature increased from 573K to 723K, and low temperature reducible manganese species were related to catalytic activity for S (IV) oxidation. The good performance of MnO2/ceramic catalyst on the SO2 removal appeared to be promising in seawater FGD application when high sulfur coals were burned.

Au-mixed lanthanum/cerium oxide catalysts for water gas shift

We report on the synthesis of highly homogeneous mixed La/Ce-oxides via a microemulsion-templated approach, and their evaluation as active supports for Au in the water gas shift (WGS) reaction. Both structure and reducibility of the oxides could be tailored by adjusting the La content across the entire range of La:Ce-ratios. The reducibility of the Au-free oxides shows an optimum at ∼25% La content, which can be traced back to improved oxygen mobility due to formation of oxygen vacancies and to the formation of more strongly bound oxygen upon La addition.Deposition of Au onto these oxides gives rise to an additional, low-temperature reduction peak, presumably due to hydrogen spill-over from the noble metal onto the oxide support. The WGS activity of Au/LaxCe1−xO2−0.5x catalysts correlates closely with the reducibility of the oxide supports, and hence with La content, demonstrating that carefully controlled synthesis of nanostructured catalysts with uniform, tailored composition allows for fine control of reactive properties of these materials, and might ultimately open the way towards a more rational design of catalysts.

Characterization of sepiolite as a support of silver catalyst in soot combustion

Turkish sepiolite–zirconium oxide mixtures were applied as a support for the silver catalyst in a soot combustion. Sepiolite–Zr–K–Ag–O catalyst was characterized by XRD, N2 adsorption, SEM, TPR-H2 and EGA-MS. The combustion of soot was studied with a thermobalance (TG-DTA). The modification resulted in a partial degradation of the sepiolite structure, however, the morphology was preserved. The adsorption of N2 of the modified sepiolite is a characteristic for mesoporous materials with a wide distribution of pores. The specific surface area SBET equals 83 m2/g and the pores volume is 0.23 cm3/g. The basic character of the surface centers of sepiolite is indicated by CO2 desorption (TPD-MS) at 170 °C and at about 620 °C due to a surface carbonates decomposition. The thermodesorption of oxygen at 650–850 °C indicates the decomposition of AgOx phases at the surface. The presence of AgOx phases is also confirmed by TPR-H2 spectrum (low temperature reduction peak at 130 and 180 °C). The high-temperature reduction at about 570 °C is probably related to Ag–O–M phases on the support.The soot combustion takes place at T50=575 °C. Without silver (sepiolite–Zr–K–O) T50=560 °C but sepiolite modified with silver (sepiolite–Zr–K–Ag–O) undergoes the same process at T50=490 °C.

Oxidative performance and surface properties of Co-containaing mixed oxides having the K2NiF4 structure

The complexed oxides Nd2?xSrxCoO4 (0.4?x?1.2) and LnSrCoO4 (Ln = Pr, Nd Eu) having the K2NiF4 structure were synthesized by the citric-acid-complexion method. The results of XRD confirmed that the complexed oxides have the K2NiF4 structure. Other phases were found when x = 0.4 and x = 1.2. The influences of Nd, Pr, Eu on the activities of LnSrCoO4 for CO oxidation and C3H8 oxidation were different. The oxidative activities average crystalline size and lattice distortion of Nd2?xSrxCoO4 increased with increasing x value in the oxides. When x > 0.8, the lattice distortion decreased with increasing x. The results of O2?TPD showed that amount of desorption of lattice oxygen over Nd2?xSrxCoO4 increased with x, however, the amount of chemidesorption of oxygen decreased. With increasing x, the high-temperature reduction peak in H2?TPR of Nd2?xSrxCoO4 shifted to higher temperatures, however, the low-temperature reduction peak shifted to lower temperatures, which showed that the activity of the lattice oxygen and the thermal stability of Nd2?xSrxCoO4 increased with increasing x.

Preparation, characterization, and activity of Al 2O 3-supported V 2O 5 catalysts

A series of activated alumina-supported vanadium oxide catalysts with various V 2O 5 loadings ranging from 5 to 25 wt% have been prepared by wet impregnation technique. A combination of various physicochemical techniques such as BET surface area, oxygen chemisorption, X-ray diffraction (XRD), temperature-programmed reduction (TPR), thermal gravimetric analysis (TGA), and Fourier transform infrared (FTIR) were used to characterize the chemical environment of vanadium on the alumina surface. Oxygen uptakes were measured at 370 °C with prereduction at the same temperature, which appears to yield better numerical values of dispersion and oxygen atom site densities. XRD and FTIR results suggest that vanadium oxide exists in a highly dispersed state below 15 wt% V 2O 5 loading and in the microcrystalline phase above this loading level. TPR profiles of V 2O 5/Al 2O 3 catalysts exhibit only a single peak at low temperature up to 15 wt% V 2O 5. It is suggested that the low-temperature reduction peak is due to the reduction of surface vanadia, which has been ascribed to the tetrahedral coordination geometry of the V ions. TPR of V 2O 5/Al 2O 3 at higher vanadia loadings exhibited three peaks at reduction temperatures, indicating that bulk-like vanadia species are present for these catalysts only at higher vanadia loadings, with V ions in an octahedral coordination. The TPR profiles of V 2O 5/Al 2O 3 catalysts indicate that at loadings lower than 15% vanadia forms isolated surface vanadia species, while two-dimensional structure and V 2O 5 crystallites become prevalent in highly loaded (>15% V 2O 5) systems. Liquid-phase oxidation of ethylbenzene to acetophenone has been employed as a chemical probe reaction to examine the catalytic activity. Ethylbenzene oxidation results reveal that 15%V 2O 5/Al 2O 3 is more active than higher vanadia loading catalysts.

Thermal behavior of (Ce,Zr)Ox/Al2O3 complex oxides prepared by a microemulsion method

A study is reported on the thermal behavior of a (Ce,Zr)Ox/Al2O3 specimen prepared by a microemulsion method and subjected to aging treatments. A similarly prepared classical CeOx/Al2O3 support was used as a reference. Attention is paid to the 1273–1373 K region of treatment, where previous experience demonstrates that this type of material suffers deactivation processes. The structural/morphological characterization of the complex support was performed using X-Ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. Surface and redox properties were also examined using electron paramagnetic resonance (EPR) and temperature programmed reduction (TPR). The (Ce,Zr)Ox/Al2O3 sample contains a mixed oxide component that essentially maintains its structural stability after aging treatment under air at 1273 K although an abrupt loss of the structural homogeneity, leading to segregation into two distinct phases, is produced upon aging at 1373 K. Ce–Zr promoter phases detected during the study maintain the t″ tetragonal symmetry and display a moderate surface composition evolution (small with respect to bulk samples) irrespective of the aging treatment. This is attributed to (Ce,Zr)Ox–Al2O3 interactions, which induce beneficial effects on both components, the Ce–Zr promoter and the alumina carrier. TPR experiments show that the aged (Ce,Zr)Ox/Al2O3 materials display a low temperature reduction peak around 650 K, which was previously observed only on activated bulk materials subjected to redox cycling, and may be of importance in the oxygen handling properties of these materials.

Effect of water extraction on the surface properties of Mo/Al 2O 3 and NiMo/Al 2O 3 hydrotreating catalysts

The effect of water extraction on the surface structure and gas oil HDS activity of alumina-supported Mo and NiMo catalysts, prepared at acidic and near neutral pH, and by sequential and simultaneous impregnation procedures, was studied by measuring structural characterization using temperature-programmed reduction (TPR) and catalytic activity before and after metal extraction with water. From oxide catalysts, depending on their preparation, significant amounts of Mo and Ni present as heteropolycompounds, and crystalline MoO 3 were extracted with water. The increase in catalyst activity observed after water extraction indicates that the water-soluble heteropolymolybdate species are not the oxide precursors of the active sulfide phase, and that a portion of Mo does not contribute significantly to the catalytic activity due to its poor dispersion. This effect was more pronounced on the catalysts prepared at acidic pH, where the formation of bulk MoO 3 was observed, therefore, rendering, the catalysts less active. No clear correlation was obtained between HDS activity and TPR data for oxide catalysts. However, on sulfided catalysts, a satisfactory linear correlation was found between the H 2 consumption of the reduction peak observed in the low-temperature region 390–523 K and the HDS activity of water-extracted NiMo catalysts, where Mo species are more homogeneously dispersed on the support. The nonextracted NiMo catalysts were, however, outside of this correlation due to the higher heterogeneity of the Mo species. The low-temperature reduction peak (390–523 K) of sulfided catalysts is associated with the hydrogenation of nonstoichiometric sulfur species and the adsorption of hydrogen on anionic vacancies.