Showing papers on "Temperature-programmed reduction published in 2015"

The role of ceria on the activity and SO2 resistance of catalysts for the selective catalytic reduction of NOx by NH3

Abstract: We investigated the influence of Ce on the catalytic activity of V/Sb/Ce/Ti and its deactivation due to SO2. We studied the properties of the catalyst using physio-chemical techniques, including transmission infrared spectroscopy (IR), NH3 and SO2 temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), H2 temperature programmed reduction (H2-TPR), and thermal gravimetric analysis (TGA). The catalysts V/Sb/Ti and V/Sb/Ce/Ti showed an excellent NOx conversion and N2 selectivity in the temperature range of 200–400 °C. Increasing Bronsted acid sites and the NH3 adsorption positively affected the efficiency of the catalyst. The Ce4+ ratio increased upon the addition of Sb and V to Ce/Ti. The catalyst V/Sb/Ce/Ti was prepared by controlling the Ce4+ ratio and exhibited an excellent activity upon increasing the Ce4+ ratio. The V/Sb/Ti (or V/W/Ti) showed one route in which NH4HSO4 formed by converting SO2 into SO3 upon the injection of SO2 in the selective catalytic reduction (SCR) reaction. In addition to this route, the reaction in the presence of V/Sb/Ce/Ti can proceed via a second route, in which Ce2(SO4)3 is formed in the reaction of Ce with SO2 and O2. Thus, V/Sb/Ce/Ti can inhibit the formation of NH4HSO4 due to the consumption of SO2 in the formation of Ce2(SO4)3. Therefore, V/Sb/Ce/Ti was found to have excellent SO2 resistance compared to V/Sb/Ti (or V/W/Ti).

Synthesis and Evaluation of Cu/SAPO-34 Catalysts for NH3-SCR 2: Solid-state Ion Exchange and One-pot Synthesis

Abstract: Cu/SAPO-34 catalysts are synthesized using two methods: solid-state ion exchange (SSIE) and one-pot synthesis. SSIE is conducted by calcining SAPO-34/CuO mixtures at elevated temperatures. For the one-pot synthesis method, Cu-containing chemicals (CuO and CuSO4) are added during gel preparation. A high-temperature calcination step is also needed for this latter method. Catalysts are characterized with surface area/pore volume measurements, temperature programmed reduction (TPR), electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopies, and scanning electron microscopy (SEM). Catalytic properties are examined using standard ammonia selective catalytic reduction (NH3-SCR) and ammonia oxidation reactions. For Cu/SAPO-34 samples prepared by SSIE, Cu presents both as isolated Cu2+ ions and unreacted CuO. The former are highly active and selective in NH3-SCR, while the latter catalyzes a side reaction; notably, the non-selective oxidation of NH3 above 350 °C. Using the one-pot method followed by a high-temperature aging treatment, it is possible to form Cu/SAPO-34 samples with predominately isolated Cu2+ ions at low Cu loadings. However at much higher Cu loadings, isolated Cu2+ ions that bind weakly with the CHA framework and CuO clusters also form. These Cu moieties are very active in catalyzing non-selective NH3 oxidation above 350 °C. At very low reaction temperature temperatures (

Cu and Co oxides supported on halloysite for the total oxidation of toluene

Abstract: This paper shows the chemical, structural and textural characterization of materials obtained from chemical modification of a clay mineral (halloysite) with species of Cu, Co and Cu Co. The synthesized materials were characterized by elemental, structural and textural analyses using X-ray fluorescence (XRF), X-ray diffraction (XRD), temperature programmed reduction (H2-TPR), N2-adsorption and transmission electron microscopy (TEM). The catalytic performance was evaluated on the complete combustion of toluene, a model molecule of volatile organic compounds (VOCs). The characterization results showed the effective incorporation of chemical species of the metals used and the formation of copper oxides, cobalt oxides or mixed oxides as active phases of the catalysts. In addition, the XRD and TEM analyses indicated the structural and morphological conservation of the catalytic support (halloysite). The N2-adsorption analysis showed that, in general, the type of porosity is maintained and predominantly determined by the support, although a reduction in a specific pore population was detected. The catalytic activity tests showed excellent performance of the materials as catalysts of the studied reaction, and revealed a cooperative effect due to the simultaneous incorporation of copper and cobalt species: the catalysts containing copper and cobalt were substantially more active than the catalysts containing only one metal in the oxide phase.

Selective catalytic reduction of NOx by ammonia over phosphate-containing Ce0.75Zr0.25O2 solids

Abstract: A zirconium phosphate @ Ce 0.75 Zr 0.25 O 2 (ZP/CZ) catalyst was synthesized by impregnating zirconium phosphates on Ce 0.75 Zr 0.25 O 2 microspheres. Catalysts with different structures were also synthesized by various methods to illustrate the structure advantage of ZP/CZ catalyst in selective catalytic reduction of NO x by ammonia (NH 3 -SCR). The catalysts were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), BET surface area, Infrared (IR) spectra, Ultraviolet-visible (UV–vis) spectroscopy, X-ray photoelectron spectroscopy (XPS), H 2 -temperature programmed reduction (TPR) and diffuse reflectance infrared Fourier transformed spectra (DRIFTS) of NH 3 /NO x adsorption. ZP/CZ catalyst presents excellent catalytic activity at 250–450 °C with a good N 2 selectivity under high space velocity of 300,000 h −1 . Firstly, the zirconium phosphate provides adsorption sites for ammonia on the surface of the catalyst and the cerium sites in the core act as the redox sites for NO oxidation. The pre-combination of zirconium and phosphate can reduce the strong interactions between phosphates and cerium ions. Therefore, the mobility of surface lattice oxygen on Ce 0.75 Zr 0.25 O 2 catalyst is retained, which is essential to obtain a catalyst with high NH 3 -SCR activity at low temperatures. Moreover, NH 4 NO 3 has proved to be an important intermediate for the NH 3 -SCR reaction on the surface of ceria based catalysts. The ZP/CZ catalyst preserves plenty of oxygen vacancies, facilitating the Ce 4+ → Ce 3+ redox circles and therefore promotes the nitrites/nitrates formation and desorption on catalyst. Two distinct adsorption sites for NH 3 and NO x result in a close contact between ads-NH 3 /NH 4 + and ads-NO 3 − /NO 2 − species on ZP/CZ catalyst, which can react with NO rapidly with the participation of active surface lattice oxygen.

Effect of metal–support interactions in Ni/Al2O3 catalysts with low metal loading for methane dry reforming

Abstract: Nickel catalysts prepared by a variety of different methods are commonly used for reforming reactions such as methane dry reforming. Two preparation methods, controlled adsorption and dry impregnation, are implemented to explore the effect of preparation method on the formation of active sites on alumina supported nickel catalysts. By varying only the preparation method, comparison of catalysts that differ primarily in metal–support interactions, strong metal–support interaction (controlled adsorption) and weak metal–support interactions (dry impregnation), are obtained. For controlled adsorption, optimal synthesis conditions are identified using point of zero charge measurements, pH-precipitation experiments, and adsorption isotherms. Using these conditions, a catalyst with a higher dispersion and strong metal–support interactions is prepared. Physicochemical characterization by N2 physisorption, H2 chemisorption, temperature programmed reduction (TPR), transmission electron microscopy (TEM), and environmental TEM (ETEM) shows that the types of nickel sites formed strongly depend on the synthesis method. Methane dry reforming reactivity studies show stable catalytic performance for at least 9 h and provide additional insight into the types of active centers present. After reductive pretreatment, the nickel catalyst prepared by dry impregnation is found to primarily have nickel present as a surface NiAl2O4. In contrast, the active centers for the nickel catalyst prepared by controlled adsorption consist of nickel particles that are encapsulated by a nickel aluminate layer with 1–2 nm in thickness. Combustion analysis and XPS of spent catalysts reveal different amounts and nature of carbonaceous deposits as a function of the synthesis method.

Influence of Mo on catalytic activity of Ni-based catalysts in hydrodeoxygenation of esters

Abstract: The effect of molybdenum on activity of Ni-based catalysts in the hydrodeoxygenation of fatty acid esters was studied. Catalysts Ni-Cu/Al 2 O 3 , Ni-Mo/Al 2 O 3 , Cu-Mo/Al 2 O 3 , Mo/Al 2 O 3 , and Ni-Cu-Mo/Al 2 O 3 with different ratios Ni/Mo were prepared and tested in the hydrodeoxygenation of methyl palmitate and ethyl caprate at 300 °С, 1 MPa. It was found that an increase in the Mo content (from 0.0% to 6.9%) in the Ni-Cu-Mo/Al 2 O 3 catalysts leads to an increase in the yield of normal alkanes. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS), temperature programmed reduction and X-ray diffraction techniques. The XPS data showed that the increase in the conversion of fatty acid esters is related to changes in the ratio between different oxidation states of molybdenum (Mo 0 , Mo 4+ , and Mo 6+ ) on the surface of the Ni-Cu-Mo/Al 2 O 3 catalysts.

Synthesis of γ-valerolactone by hydrogenation of levulinic acid over supported nickel catalysts

Abstract: Ni/Al2O3 catalysts were tested for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) as an important bio-based platform molecule for chemical products based on renewable feedstocks. The catalysts were prepared by wet impregnation, incipient wetness impregnation, precipitation, and flame spray pyrolysis; both the influence of different solvents (monovalent alcohols and water) as well as solvent free reaction conditions were screened in batch autoclaves. Whereas alcohols led to a number of side reactions that could only be suppressed by high hydrogen pressures (>20 bar), water as solvent resulted in a GVL selectivity of 100%. The GVL yields reached 57%. Further improvement was achieved without any solvent, whereby the GVL yield increased to 92% at 100% LA conversion. Reuse of the Ni catalysts resulted in a significant drop in activity. The catalysts were thoroughly characterized by temperature programmed reduction (TPR), X-ray diffraction (XRD), linear combination analysis of X-ray absorption near edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS). The results indicated that incorporated Ni2+, as present in flame-derived catalysts, was less active for GVL synthesis compared to supported Ni particles, as present in the wet impregnated catalyst.

Effect of Co/Ni ratios in cobalt nickel mixed oxide catalysts on methane combustion

Abstract: A series of cobalt nickel mixed oxide catalysts with the varying ratios of Co to Ni, prepared by co-precipitation method, were applied to methane combustion. Among the various ratios, cobalt nickel mixed oxides having the ratios of Co to Ni of (50:50) and (67:33) demonstrate the highest activity for methane combustion. Structural analysis obtained from X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) evidently demonstrates that CoNi (50:50) and (67:33) samples consist of NiCo2O4 and NiO phase and, more importantly, NiCo2O4 spinel structure is largely distorted, which is attributed to the insertion of Ni2+ ions into octahedral sites in Co3O4 spinel structure. Such structural disorder results in the enhanced portion of surface oxygen species, thus leading to the improved reducibility of the catalysts in the low temperature region as evidenced by temperature programmed reduction by hydrogen (H2 TPR) and X-ray photoelectron spectroscopy (XPS) O 1s results. They prove that structural disorder in cobalt nickel mixed oxides enhances the catalytic performance for methane combustion. Thus, it is concluded that a strong relationship between structural property and activity in cobalt nickel mixed oxide for methane combustion exists and, more importantly, distorted NiCo2O4 spinel structure is found to be an active site for methane combustion.

Hydrodeoxygenation of 2-methoxyphenol over different Re active phases supported on SiO2 catalysts

Abstract: The conversion of guaiacol, a lignin model compound, over SiO 2 -supported Re metal, oxide and sulfide catalysts in a batch reactor at 300 °C and 5 MPa of hydrogen pressure were studied. The oxide catalyst was prepared by wet impregnation with a loading of 2.5 atom of Re per nm 2 of support. The sulfide and metal catalysts were prepared by sulfidation and reduction, respectively, of the Re oxide catalyst. The catalysts were characterized by N 2 adsorption, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and transmission electron microscopy (TEM). Catalytic activity was correlated with XPS analysis of the post-reaction catalysts. The ReOx/SiO 2 catalyst was the most active and selective towards deoxygenated products, and this was attributed to increase in the concentration of metal oxide defect sites (due to in situ partial reduction of the oxide catalyst during the reaction). This result reveals the high potential of ReOx/SiO 2 catalyst for bio-oil upgrading because it shows, among other things, that it is unnecessary to add undesirable sulfiding agent to maintain catalytic activity.

One‐Pot 2‐Methyltetrahydrofuran Production from Levulinic Acid in Green Solvents Using Ni‐Cu/Al2O3 Catalysts

Abstract: The one-pot hydrogenation of levulinic acid to 2-methyltetrahydrofuran (MTHF) was performed using a series of Ni-Cu/Al2 O3 catalysts in green solvents, such as water and biomass-derived alcohols. Ni/Al2 O3 provided the highest activity, whereas Cu/Al2 O3 was the most selective, reaching a 75 % MTHF yield at 250 °C after 24 h reaction time. Synergetic effects were observed when bimetallic Ni-Cu/Al2 O3 catalysts were used, reaching a 56 % MTHF yield in 5 h at 250 °C for the optimum Ni/Cu ratio. Remarkably, these high yields were obtained using non-noble metal-based catalysts and 2-propanol as the solvent. The catalytic activity and selectivity results are correlated to temperature programmed reduction (TPR), XRD, and STEM characterization data, identifying the role associated with mixed Ni-Cu particles in addition to monometallic Cu and Ni.

Promotional effect of vanadium on the selective catalytic oxidation of NH3 to N2 over Ce/V/TiO2 catalyst

Abstract: The promotional effect of vanadium on Ce/TiO2 catalyst for the selective catalytic oxidation of NH3 to N2 was investigated. The catalytic activity and SO2 tolerance of 10 wt% Ce/TiO2 catalyst were greatly enhanced by the addition of 2 wt% vanadium, in the temperature range of 250–350 °C. The NH3 conversion of 10 wt% Ce/TiO2 had been highly enhanced from 50 to 90% at 300 °C, after vanadium addition. The BET and XPS results indicated that the addition of vanadium could result in better dispersion of Ce4+ species on TiO2, leading to the enhancement of BET surface area. H2 temperature programmed reduction (TPR) results suggest that vanadium enhances the redox properties, and the absence of bulk CeO2 in high temperature (700 °C) peak indicates it could be attributed to the strong interactions among CeOx, VOx, and TiOx. More importantly, 10 wt% Ce/2 wt% V/TiO2 catalyst showed strong resistance to SO2 poisoning. DRIFT results suggest that the 10 wt% Ce/2 wt% V/TiO2 catalyst could reduce the formation of NH4HSO4 species, which was beneficial for easy decomposition, in the presence of SO2. In summary, these results indicated that the 10 wt% Ce/2 wt% V/TiO2 catalyst greatly improved SCO activity and SO2 tolerance.

Promotional effect of Ce on Cu-SAPO-34 monolith catalyst for selective catalytic reduction of NOx with ammonia

Abstract: The activity and hydrothermal stability of Cu-SAPO-34 and CuCe-SAPO-34 for selective catalytic reduction of NO x with ammonia (NH 3 -SCR) were investigated systematically The catalysts were prepared by wet-impregnation method, and characterized by N 2 adsorption, X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), Ultraviolet-visible diffuse reflectance spectrum (UV–vis-DRS), H 2 -temperature programmed reduction (H 2 -TPR) and NH 3 -temperature programmed desorption (NH 3 -TPD) The experimental results of fresh catalysts suggested that Ce mainly existed on the surface of the catalyst and was well dispersed in the form of Ce 3+ , and its interaction with copper could improve the dispersion of copper species and increase the amount of isolated Cu 2+ ions, so that CuCe-SAPO-34 performed better NH 3 -SCR activity than Cu-SAPO-34 After hydrothermal aging at 800 °C for 12 h, the characterization results indicated that the introduction of cerium effectively improved the textural and structural stability of SAPO-34 since more cations at the exchange site of SAPO-34 could decrease the concentration of Si O(H) Al bond which was closely related to the damage of the SAPO-34 framework Moreover, the addition of Ce could prevent the decrease of acid densities, promote the redistribution of CuO during hydrothermal aging and provide higher amount of isolated Cu 2+ ions, leading to superior hydrothermal stability of CuCe-SAPO-34 catalyst

Glycerol steam reforming on bimetallic NiSn/CeO2–MgO–Al2O3 catalysts: Influence of the support, reaction parameters and deactivation/regeneration processes

Abstract: NiSn bimetallic catalysts supported over Al 2 O 3 modified with different promoter (Mg and/or Ce) were prepared and characterized by powder X-ray diffraction (XRD), N 2 sorptometry, and temperature programmed reduction (TPR). Hydrogen production by glycerol steam reforming over these catalysts was investigated. Among the catalysts, NiSn/AlMgCe catalyst shows the highest hydrogen yield as well as the best stability during the reaction. The effect of reaction temperature, water/glycerol molar ratio and space velocity on the glycerol steam reforming over NiSn/AlMgCe were also investigated. Finally, it was verified that the catalyst can be regenerated by oxidation of carbonaceous deposits.

Pd catalysts for total oxidation of methane: Support effects

Abstract: We prepared a series of Pd catalysts for total oxidation of methane on different supports: Al2O3, ZrO2–CeO2 and CeO2. We characterized the catalysts’ structures by XRD and their surface chemistries by temperature programmed reduction (TPR) and oxidation (TPO); and measured their activities for methane oxidation in temperature programmed reaction experiments (TPRx). By fitting a kinetic model to the TPRx results, we extracted apparent activation barriers (Eapp) and pre-exponents (Aapp) for methane oxidation over each of the catalysts. Activity correlates strongly with Aapp, which we interpret as a relative measure of the density of PdO–Pd* (* is an O-vacancy) site pairs. TPR results support this view: PdO in low activity (low Aapp) catalysts, such as Pd/CeO2, is relatively resistant to reduction, meaning that formation of the Pd* partner of the site pair is difficult—and the number of site pairs that can form is low.

Investigation of co-effect of 12-tungstophosphoric heteropolyacid, nickel citrate and carbon-coated alumina in preparation of NiW catalysts for HDS, HYD and HDN reactions

Abstract: Effects of activated carbon in a carbon-coated alumina (CCA) support, active phase morphology and its composition of Ni 6 -PW 12 S/C x /Al 2 O 3 catalysts in hydrotreating of model compounds were studied. The catalysts were synthesized using 12-tungstophosphoric heteropolyacid, nickel citrate and CCA and characterized with multiple methods: N 2 physisorption, X-ray powder diffraction, H 2 temperature programmed reduction, temperature-programmed desorption of ammonia, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The catalytic properties were determined using a fixed-bed microreactor in hydrotreating of dibenzothiophene, naphthalene and quinoline. It was found that with the increase of carbon content in the CCA up to 5 wt.%, reducible reactivity, sulphidation degree, average length and stacking number of WS 2 crystallites in the catalysts increased. Observed changes can be explained by weakening interaction between metal oxide species and carbon-coated support. Full promotion of the NiWS edges by nickel was achieved in the catalysts supported on the CCA with carbon content equal 0.3 wt.% and more. Activities of the catalysts in dibenzothiophene hydrodesulphurization, naphthalene hydrogenation and quinoline hydrodenitrogenation were essentially depended on the carbon content in the CCA-support. Ni 6 -PW 12 S/C 1 /Al 2 O 3 catalyst showed maximal conversions of the substrates in studied reactions. This result was achieved due to an optimal balance between turnover frequency value of the active sites and their content.

Characterization of olivine-supported nickel silicate as potential catalysts for tar removal from biomass gasification

Abstract: In this work olivine-supported nickel silicate, which was prepared by thermal impregnation is considered as a potential tar removal catalyst for cleaning the gas stream during biomass gasification. Previous work on Ni-olivine catalysts has shown that these catalysts have good activity for the tar-reforming reaction as well as good stability and tolerance to coking. In this work, various characterization techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, laser Raman spectroscopy, temperature programmed reduction, temperature programmed reaction, and subsequent temperature programmed oxidation are employed to reveal the properties of the catalyst surface and bulk as well as their relationship to catalytic activity. Higher thermal impregnation temperatures produce stronger interactions between the active component and support, leading to better coke tolerance. Relative amounts of reducible Ni, Fe species and surface Mg have an influence on catalytic behavior. Moreover, compared to olivine, the Ni2SiO4/olivine catalyst exhibits good catalytic activity for dry reforming, water gas shift, reverse water gas shift and methanation reactions, as well as the steam reforming reaction.