Temperature Programmed Reductions Research Articles

Insights on copper, manganese, and Nickel/ZSM-5 catalytic mechanisms for nitric oxides selective reduction with ammonia

The elucidation of the selective catalytic reduction mechanisms over state-of-the-art metal-promoted zeolites is essential for nitric oxides removal in automobile and stationary source applications. In this work, H/ZSM-5 catalysts modified with transition metals, including copper, manganese, and nickel, were prepared by using an incipient wetness impregnation method and were evaluated for the selective reduction of nitric oxides with ammonia. Results indicate that copper/ZSM-5 exhibits the highest catalytic activity, with > 90% nitric oxide conversion at a broad operation temperature window (221–445 °C). The nitric oxide conversion profiles of nickel/ZSM-5 shows two peaks that correspond to weak activity among the catalysts; the low-temperature peak (290 °C) was induced by nickel clusters dispersed on the ZSM-5 surface, while the high-temperature peak (460 °C) was assigned to the bulk nickel oxides. The size of granular nickel monoxide crystallites with an exposed (2 0 2) plane is 2–30 nm, as confirmed by Scanning electron microscopy, X-ray diffraction, and Transmission electron microscope measurements. Temperature-programmed reductions with hydrogen results testified that the copper and nickel cations, as the main species contributing to selective catalytic reduction, were reduced via Cu2+/Cu+→Cu0 and Ni2+→Ni0 for copper/ZSM-5 and nickel/ZSM-5, respectively, while for the manganese/ZSM-5, the Mn3+ species in manganese clusters were reduced to Mn2+ by hydrogen. Particularly, temperature-programmed desorption coupled with mass spectrometer (TPD-MS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were comprehensively used to reveal the relationship between zeolite structure and catalysts’ properties for improving selective catalytic reduction. These results confirm that the ammonia is adsorbed and activated on both Brønsted and Lewis acid sites. The nitrous oxide desorbs in two stages during nitric oxide-TPD-MS measurements, corresponding to the desorption of nitric oxide bounded to amorphous clusters and the nitric oxide strongly bounded to bulk metal oxides, respectively. The selective catalytic reduction process follows the L-H mechanism at low temperatures, in which nitric oxide and ammonia molecules were adsorbed and activated on the catalyst surface. The selective catalytic reduction rates reached the maximum value of 1.8 × 108 (218 °C), 6.4 × 107 (227 °C), and 3.9 × 107 s−1 (235 °C) for copper, manganese, and nickel /ZSM-5, respectively.

Desorption-dominated synthesis of CuO/SBA-15 with tunable particle size and loading in supercritical CO2

Here, we present a novel method to control the size of CuO nanoparticles (NPs) and Cu loading in SBA-15 via fast desorption of supercritical CO2 (scCO2). After calcination, the average size of the CuO NPs (6.47 ± 2.89∼2.18 ± 0.97 nm) decreased with the increase of the depressurization rate (20–14 MPa, 50 °C) from transmission electron microscopy, and the x-ray diffraction results also indicated the decrement of the average particle size (8.6∼4.3 nm by a Scherrer equation). Two reduction peaks situated at 195 °C and 220 °C were found from the temperature-programmed reductions with H2 profiles, and the intensity of the low-temperature peak increased with increasing the rate for a profile. The hydrogenation of dimethyl oxalate (DMO) to ethylene glycol (EG) was selected to evaluate the catalytic activity of the as-prepared sample. The reaction was conducted at p = 3.0 MPa, T = 200 °C, H2/DMO = 120, the weight hourly space velocity = 1.2 h−1, and the EG selectivity remained at about 90% for over 100 h. The fast desorption of scCO2 caused mechanical perturbations and crystallization of the adsorbed salt ions on the supports, decreasing the particle size and increasing Cu loading (8∼12 wt%).

Environmentally-friendly tourmaline modified CeMnFeOx catalysts for low-temperature selective catalytic reduction of NOx with NH3

Abstract The tourmaline/manganese-iron-cerium-oxide composites prepared by the hydrothermal method was investigated for NOx removal in simulated flue gas. The microstructure of the catalyst has been characterized by X-ray diffractometer (XRD), temperature-programmed reductions (TPR) and X-ray photoelectron spectra (XPS). The incorporation of tourmaline could remarkably enhance the activity of tourmaline/Mn-Fe-Ce catalyst at low temperature. H2-TPR results indicated that H2 adsorption amount decreased after the incorporation of tourmaline. The XPS results revealed that the incorporation of tourmaline could enhanced the contents of high valence Oα (Oα/O) in the composite catalysts, which is favorable for oxidation process of elemental manganese. Overall, the catalytic performance of tourmaline/Mn-Fe-Ce was closely related to the addition of tourmaline. When the addition of tourmaline was 2%, the composite catalyst showed the best catalytic performance: The NOx conversion of the composite catalyst is 100% at 170–230 °C. Moreover, the denitrification mechanism of composites was studied. The addition of tourmaline improved the catalytic process, and stable bidentate nitrate species was activated at low temperature during the denitrification reaction. As a mineral material, tourmaline has excellent far-infrared properties and spontaneous polarization properties, which can significantly improve the denitration performance of the catalyst. The use of mineral materials as raw materials to prepare denitration catalysts provides new ideas for future research.

Active Component Migration and Catalytic Properties of Nitrogen Modified Composite Catalytic Materials

During the catalytic combustion reaction of methane, the migration of the active species on surface facilitates the catalytic reaction, and the element doping can improve the redox performance of the catalyst. Nitrogen-modified perovskite type composite catalysts were prepared by hydrothermal method and then characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), temperature-programmed reductions (TPR), and X-ray photoelectron spectra (XPS). The results revealed that nitrogen sources (urea, biuret, melamine, carbohydrazide, and semicarbazide hydrochloride) and nitrogen source addition changed the catalytic performance in physical and chemical properties, the migration of reactive species and the catalytic performance. When the addition amount of semicarbazide hydrochloride was three times that of LaCoO3, the composite catalysts had high Co3+/Co2+ (1.39) and Oads/Olat (15.18) and showed the best catalytic performance: the temperatures that are required for achieving methane conversion of 50% and 90% were 277 and 360 °C, which are more effective than noble metal oxides. Moreover, the in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) were applied to elucidate the efficient for CH4 removal and also can further explain the surface reaction mechanism of the composite catalyst during the methane catalytic combustion.

Unique phase identification of trimetallic copper iron manganese oxygen carrier using simultaneous differential scanning calorimetry/thermogravimetric analysis during chemical looping combustion reactions with methane

Chemical looping combustion (CLC) is a promising combustion technology that generates heat and sequestration-ready carbon dioxide that is undiluted by nitrogen from the combustion of carbonaceous fuels with an oxygen carrier, or metal oxide. This process is highly dependent on the reactivity and stability of the oxygen carrier. Thus, the development of oxygen carriers remains one of the major barriers for commercialization of CLC. Synthetic oxygen carriers, consisting of multiple metal components, have demonstrated enhanced performance and improved CLC operation compared to single metal oxides. However, identification of the complex mixed metal oxide phases that form after calcination or during CLC reactions has been challenging. Without an understanding of the dominant metal oxide phase, it is difficult to determine reaction parameters and the oxygen carrier reduction pathway, which are necessary for CLC reactor design. This is particularly challenging for complex multi-component oxygen carriers such as copper iron manganese oxide (CuFeMnO4).This study aims to differentiate the unique phase formation of a highly reactive, complex trimetallic oxygen carrier, CuFeMnO4, from its single and bimetallic counterparts using thermochemical and reaction data obtained from simultaneous differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) during temperature programmed reductions (TPR) with methane. DSC/TGA experiments during TPR with methane provides heat flow data and corresponding reaction rate data that can be used to determine reaction routes and mechanisms during methane reduction. Furthermore, non-isothermal TPR data provides the advantage of distinguishing reactions that may not be observable in isothermal analysis.The detailed thermochemical and reaction data, obtained during TPR with methane, distinguished a unique reduction pathway for CuFeMnO4 that differed from its single and bimetallic counterparts. This is remarkable since X-ray diffraction (XRD) data alone could not be used to distinguish the reactive trimetallic oxide phase due to overlapping peaks from various single and mixed metal oxides. The unique reduction pathway of CuFeMnO4 was further characterized in this study using in-situ XRD TPR with methane to determine changes in the dominant trimetallic phase that influenced the thermochemical and reaction rate data.

Partial oxidation of ethanol on vanadia catalysts on supporting oxides with different redox properties compared to propane

The influence of the support material of vanadia catalysts on the reaction rate, activation energies, and defect formation enthalpies was investigated for the oxidative dehydrogenation of ethanol and propane. Characterization by infrared absorption–reflection spectroscopy (IRAS), Raman and UV–vis spectroscopy verifies a high dispersion of vanadia for powder and thin-film model catalysts. The support effect of ceria, alumina, titania, and zirconia is reflected in activation energy, oxidative dehydrogenation (ODH) rate, and temperature-programmed reductions (TPR) for both catalyst systems, ethanol and propane. Impendence spectroscopy and density functional theory (DFT) calculations were used to determine the defect formation enthalpy of the vanadyl oxygen double bond, providing the scaling parameter for a Bell–Evans–Polanyi relationship. On the basis of a Mars–van-Krevelen mechanism, an energy profile for the oxidative dehydrogenation is proposed.

Kinetics of mixed copper–iron based oxygen carriers for hydrogen production by chemical looping water splitting

Water splitting for hydrogen production through chemical looping was investigated in a micro fluidized bed reactor. Iron, copper and mixed iron–copper based oxygen carriers were prepared by coprecipitation and incipient wetness impregnation and compared in terms of H2 production via decomposition of water. The powders were completely reduced in hydrogen (10%), and exposed to steam at temperature ranging from 500 to 800 °C producing about 0.2 mmol of H2 per gram of metal oxide in the case of a mixed Cu–Fe powder. Powders prepared by coprecipitation showed poor fluidization properties as well as poor reactivity compared to those synthesized through impregnation. Temperature Programmed Reductions (TPR) experiments carried out on a TGA showed that particles have different oxygen carrying capacities, ranging from 16% in the case of a copper oxide prepared by coprecipitation, to around 1% for other particles prepared impregnation. Kinetics were evaluated following gas–solid mechanisms for two mixed Fe–Cu carriers. Oxidation of the mixed Fe–Cu carrier prepared by impregnation is well represented by a shrinking core model (phase-boundary-controlled). The activation energy for oxidation of this carrier was 46 kJ/mol. Oxidation of the mixed Fe–Cu carrier prepared by coprecipitation is well represented by a nuclei growth model (Avrami) with random nucleation and an activation energy of 51 kJ/mol.

Effects of excess manganese in lanthanum manganite perovskite on lowering oxidation light-off temperature for automotive exhaust gas pollutants

The effects of excess manganese on enhancement of oxidation activity of LaMn 1+ x O 3+ δ perovskite for CO and propane removal from a synthetic automotive exhaust gas are reported. Various LaMn 1+ x O 3+ δ ( x = 0.05–0.4) perovskite-type nanocatalysts were prepared by a microwave-assisted “gel combustion” method and calcined at 700 °C in air. Scanning electron microscopy (SEM) of the samples demonstrates highly porous and frothy particles. More than twice enhancement in the BET surface area of the catalyst samples is observed for the LaMn 1.2O 3+ δ catalyst, as compared to LaMnO 3. X-ray powder diffraction (XRD) analyses show only the perovskite structure for the catalysts. H 2-, CO-, and C 3H 8- temperature programmed reductions (TPR) indicate that the excess manganese oxides in LaMn 1+ x O 3+ δ significantly enhance the reducibility of the catalysts. The manganese oxides may be in the form of a separate phase of amorphous and/or small crystallites. Redox titration test results show that, as the amount of Mn in the structure increases, the average oxidation state of manganese reduces resulting in lowering cation vacancies. The light off temperature for CO and propane oxidation on LaMn 1.2O 3+ δ is decreased by 57 and 38 °C, respectively as compared to those of LaMnO 3 perovskite. LaMn 1.2O 3+ δ is stable under harsh conditions of the pollutants oxidation.

Preparation of ZnNiMo/γ-alumina catalysts from recycled Ni for hydrotreating reactions

Ni, recovered from Ni–Cd cellular phone batteries, was used in the preparation of ZnNiMo/Al 2O 3 catalysts. The catalysts were characterized by temperature programmed reductions (TPR), surface area determinations (BET) and chemical analysis. Vanadyl octaethyl porphyrin (VOOEP) hydrodeporphyrinization (HDP) and thiophene hydrodesulfurization (HDS) were used as catalytic tests. It was found that the addition of Zn increases the ratio between octahedral and tetrahedral Mo in ZnMo and ZnNiMo catalysts, and that Ni addition lowers the reduction temperature of Mo species. Both results induce a positive synergetic effect for HDP and HDS reactions. An activity maximum was found for the catalyst with a Zn/(Zn + Ni) atomic ratio equal to 0.29, for both reactions. Finally, the use of a possible pollutant (Ni–Cd batteries) to produce a catalyst to eliminate contaminants in fuels was shown to be feasible.

Design of dual functional adsorbent/catalyst system for the control of VOC’s by using metal-loaded hydrophobic Y-zeolites

To design a good combined adsorbent/catalyst dual functional system for the control of the low concentration VOC, both adsorption and catalytic activity test over metal-loaded zeolite HY were carried out. Hydrophobic HY zeolite was selected as a good adsorbent candidate among the tested zeolite adsorbents and extended to the catalyst support material by adding various transition metals. The temperature programmed surface reaction (TPSR) of toluene and methylethylketone (MEK) suggested the silver as the best candidate among the tested transition metals. Temperature programmed reductions (TPR) and O 2-temperature programmed desorption (O 2-TPD) on Ag/HY catalysts were carried out to explain the nature of active centre of Ag catalyst for the toluene oxidation. Silver oxide species or partially oxidized metallic silver on to the surface of metallic silver phase was proposed as an active redox site during the oxidation reaction.

Redox properties of a TiO2 supported Cu-V-K-Cl catalyst in low temperature soot oxidation

Abstract Strong activity of a Cu-V-K-Cl/TiO 2 catalyst for soot oxidation was found by comparing results of constant temperature carbon oxidation tests in the presence and in the absence of catalyst. In addition, temperature programmed reductions (TPR) with different reductants (H 2 or soot) and re-oxidations (TPO) with air were carried out. Mass-spectrometric (MS) analyses of the gaseous products of the mentioned TPR and TPO processes were also performed on line. The aim was to investigate the behaviour of the catalyst under reducing and oxidising environments in order to draw indications on the role of the redox properties in the catalytic reaction mechanism. Results showed that the catalyst can be reduced and re-oxidised under proper environments. In particular, the catalyst delivers oxygen in the whole range of temperatures from 480 to 1000 K when carbon or H 2 are employed as reducing substances. The relevant differences between the two cases are the main products of the reductions (H 2 O with H 2 and CO 2 with soot) and the high HCl desorption in the case of reduction by H 2 . In addition, the temperature range within which the catalyst re-oxidises after such a reduction is practically coincident with those where soot catalytic oxidation occurs.

Catalytic wet oxidation of H 2S to sulfur on Fe/MgO catalyst

The room temperature wet catalytic oxidation was conducted in a batch reactor with Fe/MgO catalyst. Fe/MgO catalyst was prepared by the dissolution–precipitation method. XRD and temperature-programmed reductions (TPR) indicate that Fe oxide in the Fe/MgO is finely dispersed in the MgO support. The high H 2S removal capacities of Fe/MgO can be explained by the finely dispersed iron oxide MgO. The H 2S removal capacities of Fe/MgO are dependent on oxygen partial pressure (1.0 g H 2S/g cat in air and 2.6 g H 2S/g cat in oxygen). The valence state analysis of Fe/MgO catalyst suggests that the H 2S oxidation on Fe/MgO can occur by a redox couple reaction, reducing Fe 3+ into Fe 2+ by H 2S and oxidizing Fe 2+ to Fe 3+ by O 2.

Temperature-programmed reduction of V2O5 and coprecipitated V2O5−TiO2 by hydrogen

Temperature-programmed reductions (TPR) with H2 of both pure V2O5 and coprecipitated V2O5−TiO2 systems with different titanium concentrations was performed. The original and the reduced samples following each TPR step were characterized by X-ray diffraction, Fourier transform infrared analysis and scanning electron microscopy. Within the temperature range in which TPR analysis was carried out (100–600°C) the V2O5 phase was reduced in two or three steps, while no variation in the TiO2 phase (anatase or rutile) was observed. In the first reduction step only superficial reduction of the oxides was detected. In the following steps, the H2 reacted with oxygen atoms of the V=O and V−O−V bonds. This led to important changes in the structure and morphology of the system. The experimental evidence allowed the conclusion that titanium stabilizes certain phases of vanadium oxides in which vanadium appears as V(+4) or as a mixture of V(+4) and V(+5). Moreover, when moderate and high titanium concentrations were used, the reduction temperature of the bulk V2O5 decreased markedly.