TPR Experiments Research Articles

Characterization of Chromium Cations in CrAPO-5 Metal Aluminophosphate

Chromium-substituted aluminophosphate (CrAPO-5) with the AFI crystal structure was prepared for the first time by using microwave conditions at the stage of gel formation and crystallization. This approach allowed the reduction of the time required for the synthesis of CrAPO-5 from 60–70 h to 6 h. CrAPO-5 metal aluminophosphate prepared by microwave-assisted synthesis is studied by UV-visible and IR spectroscopy and X-ray photoelectron spectroscopy. The material is characterized by the presence of Cr3+ and Cr2+ ions in the framework with predominating Cr3+ ions introduced via isomorphous substitution, as well as some minor amounts of extra-framework Cr3+ species, which are present presumably in the state of α-Cr2O3. The latter species can be partially reduced to Cr2+ species in the presence of CO or H2. XPS study of CrAPO-5 revealed the presence of Cr3+ ions in the framework. A TPR experiment showed that the reduction of chromium starts at about 450–500 °C.

Ordered Mesoporous Ceria and Cerium Gadolinium Oxide Prepared by Vacuum-Assisted Nanocasting.

Four ceria-based mesoporous oxide materials were prepared using a new vacuum impregnation (VI) templating method developed by the authors, namely, vacuum-assisted nanocasting (VAN). Two hard templates (SBA-15 and KIT-6) were employed, and products with compositions CeO2 and Ce0.9Gd0.1O1.9 (CGO) were made with each. The desired fluorite phase and composition were confirmed by powder XRD and EDS. The product structures were characterised by XRD, TEM, gas physisorption and SAXS. All products contained ordered mesoporous material in high yields. The specific surface areas (SSAs) and pore volumes of the products were determined to be high and the pore size and pore spacings related well to the templates from which the materials were synthesised. The TEM studies confirmed that the samples had a 3D pore structure, with this being the negative of the original template. The target materials were not only produced in high yields, but also displayed a porous single-crystal morphology with non-linear lattice planes. The highest SSA values and pore volumes were reported for materials impregnated using the KIT-6 template and with the CGO composition. The results suggest that VAN is an excellent and reproducible method for producing ordered mesoporous cerias and has considerable potential for wider application. All the mesoporous products showed dramatically increased reducibility in TPR experiments compared with a high-SSA nanoparticulate ceria reference. This is very promising for their potential applications in oxidation catalysts and in fuel cell components.

BiOCl/BiOBr composites for efficient photocatalytic carbon dioxide reduction

BiOBr (BOB) photocatalysts have low photocatalytic CO2 reduction performance due to their low electron-hole pair separation efficiency. To improve this problem, BiOCl (BOCl) is used as a co-catalyst. BOCl is loaded into BOB to form BiOCl/BiOBr (BOCl/BOB) type Ⅰ heterojunction, which enhances the photocatalytic CO2 reduction performance of BOB by promoting the separation of photogenerated electrons and holes. The results of TPR and PL experiments show that the 20% BOCl/BOB composites have the highest photogenerated carrier separation rate, and the results of CO2 reduction experiments show that 20% BOCl/BOB composites have the best photocatalytic CO2 reduction performance of 7.353 μmolg−1h−1, which is 3.22 and 1.99 times higher than pure BOB and BOCl, respectively. This work provides an effective reference for the development of modified photocatalysts.

Interpretation of H2-TPR from Cu-CHA Using First-Principles Calculations.

Temperature-programmed reduction and oxidation are used to obtain information on the presence and abundance of different species in complex catalytic materials. The interpretation of the temperature-programmed reaction profiles is, however, often challenging. One example is H2 temperature-programmed reduction (H2-TPR) of Cu-chabazite (Cu-CHA), which is a material used for ammonia assisted selective catalytic reduction of NOx (NH3-SCR). The TPR profiles of Cu-CHA consist generally of three main peaks. A peak at 220 °C is commonly assigned to ZCuOH, whereas peaks at 360 and 500 °C generally are assigned to Z2Cu, where Z represents an Al site. Here, we analyze H2-TPR over Cu-CHA by density functional theory calculations, microkinetic modeling, and TPR measurements of samples pretreated to have a dominant Cu species. We find that H2 can react with Cu ions in oxidation state +2, whereas adsorption on Cu ions in +1 is endothermic. Kinetic modeling of the TPR profiles suggests that the 220 °C peak can be assigned to Z2CuOCu and ZCuOH, whereas the peaks at higher temperatures can be assigned to paired Z2Cu and Z2CuHOOHCu species (360 °C) or paired Z2Cu and Z2CuOOCu (500 °C). The results are in good agreement with the experiments and facilitate the interpretation of future TPR experiments.

Influence of temperature and volume ratio of Cu:Ni exchange solutions on the chemical properties of the guest metals in prepared CuNi–mordenite materials: Evidence of direct competition between copper and nickel ions

Mordenite-type zeolites with a SiO2/Al2O3 ratio of 13 in sodium form were subjected to ion exchange using 0.1 N solutions of Ni(NO3)2 and CuSO4, or mixtures thereof with variable ratio of components at two different temperatures, 20 and 90 °C, varying the volume of Cu:Ni ratio in solution. The diffraction patterns revealed the preservation of the zeolite structure after the ion exchange process. Further ICP–AES measurements showed that the ions exchanged percentage depended on the volume ratio of the components added to the exchange solution. The electronic and chemical states of the species in zeolite were studied by diffuse UV–Vis spectroscopy, TPD–NO, and TPR–H2 methods. According to the UV–Vis DRS spectra, absorption increased in the charge transference region (200–350 nm) as the ion exchange temperature increased from 20 to 90 °C. On the other hand, evidence for changes in the chemical state was obtained from TPR and TPD–NO profiles. In the TPR experiments, we observed differences in reductivity with increasing ion exchange temperature. Meanwhile by TPD–NO experiments, all materials showed a capacity to desorb NO molecule at low temperature (<350 °C) and changes in the types of present species as was related with the variation in the ion-exchange temperature. The composition and properties of the Ni:Cu/mordenite bimetallic system were affected by the ion–exchange temperature and the volume ratio of Cu:Ni metal solutions.

Impact of Mg on Pd-based methane oxidation catalysts for lean-burn natural gas emissions control

More efficient lean-burn, natural gas engines are limited by greenhouse gas emissions due to methane oxidation catalysts (MOC) that suffer from water inhibition and high temperature activation. Herein, we report that the addition of Mg to supported 1 wt% Pd MOCs improved hydrothermal stability even after severe hydrothermal aging. The superior methane oxidation activity compared to the corresponding Mg-free catalyst was attributed to (1) influence of Mg during surface roughening and restructuring at 700 °C on metal-support interaction, (2) reducibility of PdOx sites and (3) preferential stabilization of active Pd (1 0 0) facets in the sample as was evidenced by H2 TPR and CO TPD characterization experiments. Methane conversion under synthetic exhaust conditions relevant to natural gas, lean-burn engines were investigated. BET, TPR, CO pulse chemisorption followed by TPD provided valuable insights into the surface area, pore volume, reducibility, Pd dispersion and Pd particle size of the selected catalyst samples.

One-step non-hydrolytic sol-gel synthesis of mesoporous SiO2-Al2O3-NiO catalysts for ethylene oligomerization

Mesoporous SiO2-Al2O3-NiO mixed oxide catalysts with different Si/Al ratio and a Ni content of ≈5.5 wt% have been prepared by a simple, one-step non-hydrolytic sol-gel route and evaluated in the oligomerization of ethylene. The catalysts were characterized by TEM-EDX, N2-physisorption, NH3-TPD, powder XRD, 29Si and 27Al MAS-NMR, XPS, and H2-TPR. The Si/Al ratio was found to influence both the texture and the acidity of the materials. According to XRD, part of the Ni formed NiO crystallites, but according to XPS, no bulky NiO was detected at the surface of the catalysts. XPS spectra and TPR experiments are consistent with the formation of dispersed Ni2+ species strongly interacting with the surface. The samples with high Al content displayed excellent performances with very high specific activity up to 270 mmol ethylene converted per g of catalyst per hour at 150 °C, and a percentage of butenes in the products of ≈70%. These catalysts compare well with model catalysts prepared by time-consuming multistep procedures from expensive ordered mesoporous silica supports.

Copper-nickel mixed oxide catalysts from layered double hydroxides for the hydrogen-transfer valorisation of lignin in organosolv pulping

Copper and nickel mixed catalysts obtained by calcination of iron and aluminium hydrotalcites (layered double hydroxides, LDH) have been tested in the conversion of a lignin model dimer in subcritical methanol. Phase distribution and textural properties of the catalysts were characterized by X-ray diffraction Rietveld analysis and N2 physisorption. The presence of copper was critical for effective hydrogenation, both by direct hydrogen transfer from methanol to aldehyde groups and by reactivity of products from methanol reforming. TPR experiments showed that the hydrogenation activity was promoted by an enhanced reducibility of the Cu-catalysts, related to the presence of other oxide components.Characterisation of the catalysts after reaction indicated that metallic copper was formed by the reduction of CuO by methanol and that modifications of the oxide catalysts in the reaction medium played a major role in the formation of active sites.

Enhancing the formation of nickel catalysts (111) crystal plane and CO2 methanation reactivity by external magnetic field

Nickle catalysts with acicular nanostructures are prepared by chemical reduction under different external magnetic fields. The characteristics reveal by XRD and HR-TEM illustrated that the growth of the Ni (111) plane is enhanced with the increasing in external magnetic field intensity. The shape of the Ni catalyst changes from particle to wire when the external magnetic field was greater than 1000 G. The TOF of catalysts increases linearly as the ratio of the (111)/(200) crystal planes increases, according to TPR and CO chemisorption experiments. The Ni catalyst prepared under 500 G external magnetic field shows good catalytic performance for CO2 methanation below 200 °C. Up to 99% conversion and CH4 selectivity of CO2 is obtained at 200 °C with H2/CO2 = 4.5 and GHSV ~ 1,000 h−1. Furthermore, the catalysts remine unchanged after 120 h at 180 °C. The activation energy of CO2 methanation on Ni catalysts is 84.7 kJ/mol.

Experimental and theoretical investigations into the activity and mechanism of the water–gas shift reaction catalyzed by Au nanoparticles supported on Zn–Al/Cr/Fe layered double hydroxides

Water–gas shift reaction (WGSR) is an industrialized reaction with numerous applications concerning CO removal and H2 generation. Since this process is widely used and occurs at elevated temperatures, the development of high efficiency and stable catalysts for WGSR and investigating their catalytic mechanism is a hot topic. In this paper, we demonstrate Au nanoparticles supported on different layered double hydroxides (LDHs) as highly efficient and stable catalysts for WGSR. The incorporation of Au nanoparticles significantly decreases the activation energy and enhances the catalytic activity of LDHs for WGSR, with Au/ZnCr–LDHs exhibiting the best catalytic performance including: 79.4% CO conversion, 102.1 μmol gcat−1 s−1 of reaction rate, 1.01 s−1 TOF values and 41.7 kJ mol−1 of activation energy. TPR experiments suggest that the addition of Au alters the redox cycle on the surface of the catalyst, a key intermediary step involved in the catalytic process. In situ DRIFTS shows that the production of CO2 during WGSR involves the reaction between CO and adsorbed O, which comes from the dissociation of OH species and not the decomposition of formates. DFT calculations indicate that Au–based catalysts can effectively lower the energy barrier of the kinetically relevant step of H2O dissociation, which is the most probable reason for the enhancement of activity. The calculated activation barriers coincide with the experimentally measured values with the order of Au/ZnCr–LDHs<Au/ZnFe–LDHs<Au/ZnAl–LDHs<LDHs. Particularly, redox mechanism B has the lowest activation barriers which is the most potential reaction pathway and perfectly supports the in–situ DRIFTS results.

Novel ceramic paper structures for diesel exhaust purification.

The catalytic combustion of diesel soot is addressed with flexible and structured "paper catalysts". Two different series of catalysts were prepared either by drip impregnation or by a spray method to deposit a mixture of Co, Ba, and K or a mixture of Co and Ce onto SiO2-Al2O3 ceramic paper matrixes. In every case, CeO2 nanoparticles were added to bind the ceramic fibers. SEM images showed that the impregnation method generated catalytic particles concentrated as large chunks (> 10μm), mainly at ceramic fiber crossings, whereas the spray method produced smaller catalytic particles (< 1μm) well distributed throughout the ceramic paper. Besides, Co-Ba-K particles appeared better dispersed on the surface of ceramic fibers than Co-Ce due to the presence of K. Additionally, FTIR spectra showed the formation of O22- and O2- species associated with CeO2 (binder) on the samples containing potassium which gave the Co-Ba-K-ceramic paper good catalytic properties, thus making the Co-Ba-K drop impregnated the best catalyst both considering activity and stability. Successive temperature programmed oxidation (TPO) runs up to 700°C caused the formation of cobalt silicates in the catalytic ceramic paper prepared by the spray method, as indicated by TPR. The formation of these species was probably favored by the smaller size of cobalt particulates and their higher dispersion in the catalysts prepared by the spray method. This provoked the partial loss of the redox properties of Co3O4. TPR experiments also indicated the formation of BaCoO3 in Ba-containing ceramic paper, which could help in maintaining the catalyst activity after several TPO runs through the capacity of this mixed perovskite-type oxide to trap and release NOx.

The Reducibility and Cluster Size of Vanadium Oxide as Potential Descriptors of Its Catalytic Activity in the Partial Oxidation of Ethanol

Abstract A series of vanadium oxide (VOx) catalysts prepared using three different supports were tested for the partial oxidation of ethanol to acetaldehyde. Optical absorption spectroscopy and Temperature Programmed reductions experiments indicate that the reducibility and average domain size of the vanadia clusters anchored on the supports are very sensitive to vanadia loading. The catalytic activity results were modeled using a pseudo steady state approximation using the ethanol hydrogen abstraction step as rate limiting. The results obtained strongly suggest that catalytic activity can be correlated to both average vanadia cluster size and the ability of vanadia to uptake hydrogen during TPR experiments.

Fischer-Tropsch synthesis: Foregoing calcination and utilizing reduction promoters leads to improved conversion and selectivity with Co/silica

Foregoing calcination and utilizing direct reduction of cobalt nitrate led to the formation of smaller cobalt oxide nanoclusters in stronger interaction with silica support as intermediates of the activation process to Co° nanoparticles; this was demonstrated by TPR, TPR-MS, TPR-XANES, and TPR-EXAFS experiments using hydrogen. These intermediate cobalt oxides included a spinel (e.g., Co3O4) formed from oxidation of Co2+ species by NO2, which in turn converted to CoO prior to formation of the metal. To improve the reducibility, metal promoters such as Pt, Re, Ru, and Ag were added. Hydrogen chemisorption and EXAFS experiments revealed smaller nanoparticles; Co-Co metal coordination numbers were significantly lower for the H2-activated Co metal nanoparticles when direct reduction of the nitrate was used relative to H2-activated air calcined catalysts. Comparing at the same space velocity, the best catalysts were Re and Pt promoted 12%Co/SiO2 catalysts utilizing direct reduction of the nitrate, where initial conversions in a CSTR were up to 3.8 times higher and 71% higher than unpromoted and Pt promoted air calcined catalysts, respectively. At these conditions, methane production was lower (6.8 and 8.0% for Re and Pt promoted catalysts, respectively, by direct reduction versus 12.5 and 10.1% for unpromoted and Pt promoted air calcined catalysts) and C5+ selectivity was higher (81.2% and 81.5% for Re and Pt promoted catalysts, respectively, by direct reduction versus 73.4 and 78.8% for unpromoted and Pt promoted air calcined catalysts). The uncalcined catalysts were slightly less stable than the calcined samples, with the only exception being the rhenium promoted sample, where no visible deactivation was observed; this catalyst also had the highest catalytic activity on a per gram catalyst basis.

The Effects of Chelating Agents on CoMo/TiO2–Al2O3 Hydrodesulfurization Catalysts

A series of CoMo/TiO2–Al2O3 catalysts with different chelating agents were prepared by co-impregnation method. The influence of chelating agents on dispersions of active phase, textural properties, and hydrodesulfurization (HDS) activity for CoMo/TiO2–Al2O3 catalysts was studied. XRD characterization demonstrates that the addition of chelating agents can promote the dispersion degree of active components. TPR experiments exhibit an important effect of chelating agents on the catalysts. CoMo/TiO2–Al2O3 catalysts prepared with different chelating agents shows different HDS activity, and the addition of EDTA results in a remarkable improvement in both thiophene and DBT HDS conversions compared with others.

Over– and low–exchanged Co/BEA catalysts: General characterization and catalytic behaviour in ethane ammoxidation

Ethane ammoxidation into acetonitrile was successfully catalyzed between 380 and 450°C over Co/BEA solids issued from solid–state ion exchange with different metal loads. During the preparation, in the presence of NH4+–Beta zeolite (Si/Al=12.5), CoCl2 precursor decomposes under helium stream without evaporation, leading to the stabilization of bare Co2+ at the exchange cationic sites as revealed by spectroscopic tools. However, at 4.13 and 5.63wt.% of Co (Co/Al molar ratio equal to 0.75 and 1, respectively), the corresponding solids stabilized, besides bare β–type Co2+ ions, Co oxo [Co(III)O]+ species, revealed by H2–TPR. These species exhibit highest catalytic activity on the basis of TOF calculation and play a key role in ethane ammoxidation into acetonitrile at low temperature (380°C). Nevertheless, the excess of cobalt is transformed into Co3O4 oxide (as revealed by XRD and TPR experiments) which catalyzes the hydrocarbons combustion into CO2.

Use of pillared clays in the preparation of washcoated clay honeycomb monoliths as support of manganese catalysts for the total oxidation of VOCs

Manganese catalysts supported by impregnation onto honeycomb monoliths extruded from commercial clay, previously coated with aluminium-pillared clay, were prepared and tested in the total combustion of propane and acetone. Samples characterization included chemical analysis, nitrogen physisorption, electron microscopies (SEM-EDS, HAADF and EELS), XRD, TPR and O2-TPD experiments. The active phase (around 5wt%), consisting of MnO2 particles as majority phase, with homogeneous size and shape, that tends to concentrate in some regions of the surface of the clay support, exhibited high efficiency to oxidize the two model VOCs investigated and stability in severe reaction conditions. Light-off temperatures as low as 225 and 330°C were found for the oxidation of acetone and propane respectively. The better performance observed in comparison to the monoliths without pillared clay was attributed to the higher active phase loading. Differences found as function of the VOC's nature and concentration were related to the different oxidation mechanisms proposed in literature, either just Mars-van Krevelen or this with also Eley-Rideal contribution, for acetone and propane respectively. These results combined with the intrinsic advantages of the honeycomb monolithic design open up new possibilities for using pillared clays as catalytic support in VOCs oxidation under more affordable conditions.

Pd nanoparticles supported on iron oxide nanorods for CO oxidation: Effect of preparation method

In this work, Pd nanoparticles was precipitated over iron oxide that prepared through two basic approaches to look for better dispersion of Pd nanoparticles and robust catalyst for carbon monoxide oxidation. The first involves direct precipitation of iron oxide using sodium hydroxide, and the second involves hydrothermal treatment after precipitation. The catalysts were characterized by many techniques such as, FTIR, XRD, N2 adsorption-desorption, H2-TPR and TEM images. The catalytic activity of the prepared catalysts was tested in CO oxidation reaction. The XRD showed that, α-FeOOH crystalline structure is formed. While, TEM images showed that, α-FeOOH has nanorods structure with different dimensions. The TPR experiment indicated that the reducibility of Pd/FeOOH catalyst was much more enhanced. The results also demonstrate that, as compared with the direct precipitation approach, the hydrothermal treatment after precipitation approach of iron oxide enable better dispersion of Pd nanoparticles as well as hinder the growth of large Pd crystallites which are important to produce outstanding catalysts for carbon monoxide oxidation.

Study on chemical looping reforming of ethanol (CLRE) for hydrogen production using NiMn2O4 spinel as oxygen carrier

This paper deals with an investigation of the reactivity of spinel type mixed oxide, i.e., NiMn2O4, in the chemical loop reforming of ethanol (CLRE) to CO and H2. The Ni ferrite showed a higher activity in CLRE, which was also confirmed by TPR experiments. During the first minutes of reaction, when the materials still acted as reactants while being reduced at the same time, the two oxides showed differences in regard to the nature of the products formed. During reduction, the structural evolution was monitored by means of several physical techniques, thus showing the formation of MnO and metallic species (Ni and Mn) as well.

Investigation of the surface reaction network of the oxidative coupling of methane over Na2WO4/Mn/SiO2 catalyst by temperature programmed and dynamic experiments

In this work a series of temperature programmed experiments were carried out on a Na2WO4/Mn/SiO2 catalyst. In TPR experiments we tested the reducibility of this catalyst and O2 desorption behavior was investigated by TPD. TPSR experiments in a flow of methane, ethane or ethene gave information about the reaction network of OCM on the catalyst surface, without the presence of gas phase reactions, induced by gas phase oxygen. We found indications of involvement of two different active oxygen species on the catalyst surface. Furthermore an activation energy of 275kJ/mole for selective methane activation was determined. Dynamic experiments were performed to determine the amount of available oxygen species for the OCM reaction. Variation of methane partial pressure and flow rate showed a linear correlation between methane partial pressure and surface oxygen conversion in dynamic experiments.

Simultaneous removal of soot and NOx over K- and Ba-doped ruthenium supported catalysts

Ruthenium-based materials are investigated for the simultaneous removal of soot and NOx and their behavior is compared with that of model Pt-Ba/Al2O3 and Pt-K/Al2O3 catalysts.The materials, 1%wt Ru–10%wt AM/MO (AM=Ba or K; MO=Ce0.8Zr0.2O2, ZrO2, Al2O3), have been prepared by incipient wetness impregnation and characterized by BET, XRD, HRTEM and TPR experiments. The catalytic activity for diesel soot combustion is studied under loose contact conditions by means of TPO (Temperature Programmed Oxidation) while the reactivity in the NOx removal is investigated by Isothermal Concentration Step Change experiments.All the ruthenium-based formulations are active in the combustion of diesel soot, particularly potassium-doped materials that show low onset temperatures, near 230°C. The catalytic activity in the simultaneous removal of soot and NOx shows that Ru-based catalysts are very efficient in the oxidation of soot also under isothermal conditions and that their NOx removal capacity (i.e. storage capacity) is almost comparable to that of traditional Pt-based LNT systems. However, the NOx reduction efficiency of the Ru-containing catalysts is lower than that of traditional LNT Pt-based catalysts.