Showing papers on "Temperature-programmed reduction published in 2003"

Study of reverse water gas shift reaction by TPD, TPR and CO2 hydrogenation over potassium-promoted Cu/SiO2 catalyst

Abstract: The reverse water gas shift (RWGS) reaction over Cu/SiO2 with and without potassium promoter was studied by means of CO2 hydrogenation, temperature programmed reduction (TPR) and temperature programmed desorption (TPD). After addition of even a little amount of potassium, Cu/K2O/SiO2 obviously offers better catalytic activity than Cu/SiO2. The coverage of formate species increases on Cu/K catalyst. TPD of CO2 adsorbed on Cu/K2O/SiO2 and TPR profile of Cu/K2O/SiO2 showed that new active sites could be created at interface between Cu and K. The main role of K2O was to provide catalytic activity for decomposition of formates, besides acting as a promoter for CO2 adsorption. A new reaction mechanism was suggested. Hydrogen was dissociatively adsorbed on Cu and could spill over to K2O to associate with CO2. This resulted in the formation of formate species for the production of CO.

Cu-Ni-K/γ-Al2O3 supported catalysts for ethanol steam reforming: Formation of hydrotalcite-type compounds as a result of metal–support interaction

Abstract: The interaction of Cu2+, Ni2+ and Al3+ ions during the impregnation step of K/γ-Al2O3 support was studied. Cu-Ni catalyst precursors (“just impregnated solids”), just reduced precursors (H2, 300 °C), and calcined precursors in the range of 400–800 °C were characterised by X-ray diffraction (XRD), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) measurements. In addition, the catalytic behaviour of reduced precursors and calcined catalysts was analysed in the ethanol steam reforming reaction at 300 °C and atmospheric pressure. XRD results of different precursors indicated that, after the impregnation step, copper is present in two different phases: a copper basic nitrate and a CuAl and/or CuNiAl hydrotalcite-type compound (HT). In the sample containing only nickel, this metal is present as a NiAl-HT compound. At constant copper content of 6 wt.%, the ratio between the copper phases depends on the nickel content. Adding nickel favours the formation of HT compounds. While calcination of Cu-Ni precursors in the range of 400–800 °C produces a CuO segregated phase and/or a phase of copper called “surface spinel”, nickel is always found as a nickel aluminate after the calcination treatment. The catalytic behaviour of the samples strongly depends on the conditions of the thermal treatments. Thus, the increase in the calcination temperature of the precursors produces a strong interaction between nickel and aluminium, decreasing nickel reducibility and selectivity to C1 compounds. On the other hand, the just reduced precursors showed to have the best catalytic performance for the ethanol steam reforming reaction at 300 °C.

Ni based mixed oxide materials for CH4 oxidation under redox cycle conditions

Abstract: The preparation, characterization and redox properties of Ni–Al–O and Ni–Mg–Al–O mixed oxides for CH 4 chemical looping combustion (CLC) is addressed in this study. Ni–Al–O samples having different Ni/Al ratios (0.5–2.25), prepared by coprecipitation, consist after calcination at 1000 °C of cubic NiO and NiAl 2 O 4 spinel. A similar phase composition is obtained for Ni–Mg–Al–O, with Mg partitioned in the two phases. The presence of NiAl 2 O 4 prevents the crystal size growth of NiO with respect to pure NiO; further limit of the sintering of the cubic oxide was observed in presence of Mg. Reduction of the samples by H 2 occurs in two steps, associated with reduction of Ni 2+ in NiO and NiAl 2 O 4 . Mg stabilizes Ni 2+ in both the cubic oxide and the spinel phase and improves regenerability upon repeated redox cycles. Temperature programmed reduction with CH 4 (CH 4 -TPR) experiments showed poor selectivity to CO 2 and H 2 O, being CO and H 2 the most abundant products. Also, formation of coke is observed over the samples. The same behavior is observed in CH 4 /O 2 pulse experiments; however, in the case of the Mg-containing system, coke formation can be avoided by co-feeding H 2 O along with CH 4 .

Ni and Mo interaction with Al-containing MCM-41 support and its effect on the catalytic behavior in DBT hydrodesulfurization

Abstract: A series of Mo and NiMo catalysts supported on Al-containing MCM-41 was prepared and characterized by N2 physisorption, XRD, ammonia TPD, temperature programmed reduction (TPR), UV-Vis diffuse reflectance spectroscopy (DRS) and 27 Al MAS-NMR. It was shown that the incorporation of Al atoms into the siliceous MCM-41 framework causes a deterioration of the textural characteristics and some loss in the periodicity of the MCM-41 pore structure. However, the acidity of the Al-containing MCM-41 is substantially higher. The dispersion of Mo and Ni oxidic species increases with the incorporation of aluminum in the MCM-41 support due to the strong interaction of Mo and Ni oxidic species with aluminum atoms of the support. However, the strong interaction of metal species with the Al-containing MCM-41 supports, up to the formation of Al2(MoO4)3 in the case of unpromoted Mo catalysts, produces an increase in the proportion of Ni and Mo species difficult to reduce. When Ni and Mo are impregnated simultaneously the formation of Al2(MoO4)3 is prevented because of the competitive interaction of both, Ni and Mo species, with Al atoms of the support. For both, Mo and NiMo catalysts, maximum catalytic activity in dibenzothiophene (DBT) hydrodesulfurization is observed for the catalysts supported on Al-MCM-41 with SiO2/Al2O3 molar ratio of 30. When Al-containing MCM-41 is used as a support for NiMo catalyst, some cracking of the main reaction products (biphenyl (BiP), cyclohexylbenzene (CHB) and dicyclohexyl (DCH)) is observed.

Effect of promoters including WO3 and BaO on the activity and durability of V2O5/sulfated TiO2 catalyst for NO reduction by NH3

Abstract: The effect of tungsten and barium oxides on the activity and durability of V2O5/TiO2 catalyst for NO reduction by NH3 was examined. Tungsten enhanced the NO removal activity of V2O5 catalyst supported on sulfur-free TiO2, while no effect of tungsten was observed for the V2O5 catalyst supported on sulfated TiO2. The tungsten oxide promotes the formation of polymeric vanadate that is a strong active reaction site for NO reduction by NH3. When both tungsten and sulfur simultaneously exist on the surface of V2O5/TiO2, the sulfur species seems to play a more important role for NO removal activity than tungsten. The tungsten oxide on the V2O5/TiO2 catalyst also enhances the activity for SO2 oxidation by promoting the adsorption of SO2, regardless of the presence of sulfur species on the catalyst surface. The NO removal activity of V2O5 catalyst supported on sulfur-free TiO2 has been significantly reduced by barium oxide, mainly due to the formation of inactive VOBa compound through the strong interaction of vanadia with barium oxide. No change of NO removal activity over V2O5-BaO/sulfated TiO2, however, was examined by the addition of barium oxide, since the structure of vanadium oxide was not altered on the surface of the sulfated TiO2. The SO2 oxidation reaction over V2O5-BaO/TiO2 catalysts was significantly suppressed by the addition of barium oxide to the catalyst. The barium oxide seems to reduce the redox ability of vanadium oxide on the catalyst surface as well as the adsorption capacity of SO2. Based on the temperature programmed reduction (TPR), Raman and XPS observations, the surface structure of vanadium and its interaction with tungsten and barium oxides has been illustrated when sulfur exists on the surface of TiO2.

Catalytic decomposition of N2O and catalytic reduction of N2O and N2O + NO by NH3 in the presence of O2 over Fe-zeolite

Abstract: The decomposition of N2O, and the catalytic reduction by NH3 of N2O and N2O + NO, have been studied on Fe-BEA, -ZSM-5 and -FER catalysts. These catalysts were prepared by classical ion exchange and characterized by TPR after various activation treatments. Fe-FER is the most active material in the catalytic decomposition because “oxo-species” reducible at low temperature, appearing upon interaction of FeII-zeolite with N2O (α-oxygen), are formed in largest amounts with this material. The decomposition of N2O is promoted by addition of NH3, and even more with NH3 + NO in the case of Fe-FER and -BEA. It is proposed that the NO-promoted reduction of N2O originated from the fast surface reaction between α-oxygen O∗ and NO∗ to yield NO2∗, which in turn reacts immediately with NH3.

CoOx catalysts supported on alumina and alumina-baria: influence of the support on the cobalt species and their activity in NO reduction by C3H6 in lean conditions

Abstract: CoO x catalysts (Co 1 and 3 wt.%) were prepared by incipient-wetness impregnation of the supports, Al 2 O 3 and Al 2 O 3 (80 wt.%)-BaO (20 wt.%), and calcined at 500 and 800 °C. The samples were characterized by X-ray diffraction (XRD) and BET techniques. H 2 -temperature programmed reduction (TPR) and UV-Vis diffuse reflectance spectroscopy (DRS) spectra were recorded with the aim to identify the different Co species formed. The presence of Co 2+ species tetrahedrally and octahedrally coordinated appears strongly influenced by the nature of the support and the cobalt content. Co 3 O 4 particles were detected after calcination at 500 °C, a successive treatment at 800 °C promotes the dispersion of Co 3 O 4 clusters in the network of alumina with formation of CoAl 2 O 4 spinels. The catalysts were tested in the selective catalytic reduction (SCR) of NO with C 3 H 6 in the presence of excess O 2 , using a reactant mixture containing NO 1000 ppm, C 3 H 6 1000 ppm, O 2 5%. The effectiveness of the cobalt catalysts in the SCR process depends strongly on the type of support, metal loading and calcination temperature. The presence of barium oxide in the alumina network is effective in the stabilization on the surface of dispersed Co 2+ ions which are active and selective for NO reduction in excess of O 2 . Clusters of cobalt oxide, present as Co 3 O 4 in the alumina samples calcined at 500 °C, are active mainly for C 3 H 6 combustion.

State of Ru on CeO2 and its catalytic activity in the wet oxidation of acetic acid

Abstract: The relationship between the state of Ru on CeO 2 and catalytic activity in the wet oxidation of acetic acid was investigated for Ru/CeO 2 catalysts prepared by different methods. The temperature programmed reduction (TPR) experiments of Ru/CeO 2 showed that the oxygen species of RuO 2 was reduced at different temperatures depending upon the methods of preparation. Ru species reduced at low temperatures could not be observed by TEM and XRD. It was concluded that RuOCe bonds in the well-dispersed Ru species are highly fragile and its mobile oxygen is the active species in the wet oxidation.

Pulse study of CO2 reforming of methane over LaNiO3

Abstract: The perovskite type oxide LaNiO 3 was used as starting material for the CO 2 reforming of methane. The reaction was studied by a pulse technique using a CH 4 /CO 2 ratio close to one, in order to understand the catalyst behavior. LaNiO 3 was reduced prior to the reaction by temperature programmed reduction (TPR) under hydrogen. The catalyst was thus composed of La 2 O 3 and Ni 0 . We have shown that good catalytic performances were obtained at 700 °C and 800 °C. At 800 °C all the CH 4 was transformed whereas the CO 2 conversion reached 92% with a H 2 /CO ratio equal to 0.91. The crystallographic phases present after catalyst stabilization by the reaction depends on the reaction temperature. At 800 °C the only phases present are La 2 O 3 and metallic nickel whereas at 700 °C the spinel phase La 2 NiO 4 was identified leading to the conclusion that the CO 2 reforming of CH 4 involve consecutive reactions occurring simultaneously in our experimental conditions: Ni 0 + La 2 O 3 + CO 2 ↔ CO + La 2 NiO 4 and CH 4 + La 2 NiO 4 → CO +2 H 2 + La 2 O 3 + Ni 0 .

Investigation of the interaction between NiO and yttria-stabilized zirconia (YSZ) in the NiO/YSZ composite by temperature-programmed reduction technique

Abstract: Temperature-programmed reduction (TPR) was performed to investigate the reducibility of the NiO/YSZ composite that is related to the anode of solid oxide fuel cells (SOFCs). The TPR profiles were found to be affected by preparation method, composition and pretreatment conditions. The calcination temperature strongly affected the reduction behavior; the TPR profile of the sample calcined at 1723 K shows a high temperature reduction peak that was not found in the TPR profile of the sample calcined at 1473 K. Such a difference in the TPR profiles can be attributed to the presence of distinct states of NiO species, which are due to the difference of interactions between NiO and YSZ. Five distinct states of NiO species are shown to be present in the NiO/YSZ composite.

Methane combustion and CO oxidation on LaAl1−xMnxO3 perovskite-type oxide solid solutions

Abstract: Structural, redox and catalytic deep oxidation properties of LaAl 1− x Mn x O 3 ( x =0.0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) solid solutions prepared by the citrate method and calcined at 1073 K were investigated. XRD analysis showed that all the LaAl 1− x Mn x O 3 samples are single phase perovskite-type solid solutions. Particle sizes and surface areas (SA) are in the 280–1180 A and 4–33 m 2 g −1 ranges, respectively. Redox properties and the content of Mn 4+ were derived from temperature programmed reduction (TPR) with H 2 . Two reduction steps are observed by TPR for pure LaMnO 3 , the first attributed to the reduction of Mn 4+ to Mn 3+ and the second due to complete reduction of Mn 3+ to Mn 2+ . The presence of Al in the LaAl 1− x Mn x O 3 solid solutions produces a strong promoting effect on the Mn 4+ →Mn 3+ reducibility and inhibits the further reduction to Mn 2+ . Both for methane combustion and CO oxidation all Mn-containing perovskites are much more active than LaAlO 3 , so pointing to the essential role of the transition metal ion in developing highly active catalysts. Partial dilution with Al appears to enhance the specific activity of Mn sites for methane combustion.

Characterisation of copper-manganese oxide catalysts: Effect of precipitate ageing upon the structure and morphology of precursors and catalysts

Abstract: The effect of ageing time on the composition an morphology of copper manganes oxide catalysts (Cu:Mn = 2:1) prepared using a coprecipitation method is described and discussed. The ageing time was varied from 0min (for the unaged precursor) to 1440min. The effect of ageing time was investigated using powder X-ray diffraction (XRD), temperature programmed reduction (TPR), transmission electron microscopy (TEM) and thermal gravimetric analysis (TGA). The effects of ageing on the catalyst structure are discussed in terms of the catalyst performance. © 2003 Elsevier B.V. All rights reserved.

Highly stable Pd/CeO2 catalyst for hydrodechlorination of chlorobenzene

Abstract: Catalysts containing 1 wt.% Pd supported on CeO2 were prepared by the deposition–precipitation (DP) and conventional impregnation (IMP) methods. The effect of the nature of precursor was also investigated using the chloride and nitrate salts of Pd. The catalysts were characterized by nitrogen adsorption (for BET surface area), CO chemisorption, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). Their activity and stability was evaluated in the vapor phase hydrodechlorination (HDC) of chlorobenzene, using molecular hydrogen. The catalyst prepared by the DP method using the chloride precursor exhibited the highest stability during the reaction. Contrary to the general observation that the low dispersed catalysts are preferred for the HDC, by adopting the DP method of catalyst preparation, the present investigation demonstrates that high dispersed catalysts also offer better stability. This is a significant observation in terms of considerable reduction in Pd content of the catalyst. The higher stability is ascribed to the formation of cationic Pd species which resist deactivation. On the contrary, the IMP catalysts are susceptible for transformation into inactive palladium chloride, thus loosing stability.

Effects of potassium addition on the acidity and reducibility of chromia/alumina dehydrogenation catalysts

Abstract: Dehydrogenation catalysts based on chromia supported on γ-alumina, containing about 6 wt.% of chromium and increasing amounts of potassium (up to 1.8 wt.%), were prepared and characterised by means of several techniques, such as N2 adsorption at 77 K, X-ray powder diffraction (XRPD), UV-Vis diffuse reflectance spectroscopy (DRS), electron paramagnetic resonance (EPR), temperature programmed reduction (TPR), microcalorimetry and chemical analyses. The catalytic behaviour of the samples was investigated in the propane dehydrogenation reaction at 813 K and the effect of potassium loading was examined. The addition of increasing amounts of the alkali metal, as expected, determined a general decrease in the surface acidity. The sites strength distribution was greatly influenced, due to the fact that, at loadings above 0.5 wt.%, potassium preferentially caused the disappearance of weak and medium acid sites. EPR showed a decrease of CrIII species (especially α-Cr2O3) on potassium increasing. TPR profiles of the K-containing samples showed the presence of two reduction peaks, ascribable to two different kinds of CrVI surface species with a different reduction behaviour. All catalysts showed a maximum of activity as a function of time-on-stream, due to reduction of CrVI to CrIII species in the first reaction period, and then a decrease, due to deactivation by coking. Propene selectivity was high in all cases and was positively affected by the presence of potassium in concentrations up to 1 wt.%.

Acetylene hydrogenation over Ni–Si–Al mixed oxides prepared by sol–gel technique

Abstract: The effect of the support (here silica and silica-alumina) and of the composition of Ni-based hydrogenation catalysts, elaborated by sol–gel method, on their physico-chemical characteristics and on their activity and selectivity (towards ethylene) for acetylene hydrogenation has been studied with the help of temperature programmed reduction (TPR) and oxidation (TPO), X-ray photoelectron spectroscopy (XPS) and reaction tests (acetylene to ethylene hydrogenation). These techniques have shown the existence of two different types of sites. Firstly, the hydrogenolytic (naked) metallic centres, corresponding to nickel without (or little) interaction with the support, are responsible for a large part of the coke and are active for side-reactions. Secondly, the hydrogenating sites correspond to nickel in interaction with the support and are active for the main reaction. Three types of coke have been detected: filaments, amorphous coke, and partially hydrogenated carbons strongly adsorbed on the surface. The latter are facilitated by the acidity of the silica-alumina. Because the metal–support interactions related to a better dispersion of reduced nickel, silica-alumina-based catalysts produce a much smaller amount of coke than silica-based catalysts.

The role of the titania and silica supports in Ru-Fe catalysts to partial hydrogenation of benzene

Abstract: Titania (TiO 2 ) or silica (SiO 2 ) supported ruthenium-iron catalysts were prepared by the dry co-impregnation, using aqueous solutions of their chlorides precursors. This study analyzes the structural characteristics influence of these supports, as well as that of the iron additive, over the performance of these catalysts on the partial hydrogenation of benzene. The results of the X-ray diffraction (XRD) analyses, oxygen chemisorption and temperature programmed reduction (TPR) have showed that the reducibility and dispersion of metallic phase strongly depend on the support structural characteristics. These analyses have also revealed the influence of the iron over supported ruthenium morphology and its catalytic performance. That influence was more significant on the ruthenium catalysts supported by silica. Higher selectivity in cyclohexene was observed on the titania supported ruthenium catalysts, opposite to those supported on silica.

The gold–ruthenium–iron oxide catalytic system for the low temperature water–gas-shift reaction: The examination of gold–ruthenium interactions

Abstract: The water–gas-shift reaction has been carried out over Au–Ru supported on α-Fe2O3 catalyst at various reaction temperatures ranging from 373 to 513 K at atmospheric pressure. Activity of the bimetallic Au–Ru/iron oxide compared with Au/iron oxide and Ru/iron oxide catalysts has been found to be higher over all the reaction temperatures studied. All the fresh and spent samples were characterised by transmission electron microscopy (TEM), X-ray powder diffraction (XRD), BET-surface area (SBET), temperature programmed reduction (TPR) and carbon estimations were carried out for the spent catalysts. XRD analysis revealed the presence of Au metallic particles on supported gold and α-Fe2O3 compound in all the fresh catalysts and Fe3O4 phases along with metallic Au in spent catalysts. However, no reflections due to Ru or RuO2 species were found either fresh or in spent catalysts. TPR analysis indicated the shift in reduction temperature due to Au and/or Ru over Au–Ru/iron oxide compared to individual Au/iron oxide and Ru/iron oxide catalysts. Increase in particles sizes are observed by TEM analysis of the spent forms compared to fresh, which is accompanied by a significant reduction in BET-surface areas of fresh catalysts.

Highly dispersed activated carbon supported platinum catalysts prepared by OMCVD: a comparison with wet impregnated catalysts

Abstract: This study shows that the air oxidation treatment of an activated carbon originates drastic changes in its chemical surface composition leading to the formation of a less acidic surface with thermally stable oxygenated groups, mainly hydroxyl and carbonyl groups. The oxidation treatment also induces changes in the textural properties, although less pronounced than the extent of the chemical changes. The degree of oxidation was characterized by temperature programmed desorption (TPD). Pt/AC catalysts were prepared on the activated carbon supports by organometallic chemical vapor deposition (OMCVD). Another set of catalysts was prepared on the same supports by the classical incipient wetness impregnation method. The platinum loading of each sample was kept fixed at 1 wt.%. These Pt/AC catalysts were characterized by H 2 adsorption, temperature programmed reduction (TPR), electron microscopy (TEM), infrared spectroscopy (DRIFTS) and tested for their activity in benzene hydrogenation. The changes in the physical and chemical structures of the activated carbons have pronounced effects on the characteristics of both impregnated and OMCVD samples. The results revealed that the preparation by OMCVD on the air oxidized support leads to a drastic increase in both Pt dispersion and catalytic activity. No pronounced differences were observed between the dispersion and activity of impregnated and OMCVD samples that were prepared on the non-oxidized activated carbon.

The drastic effect of platinum on carbon-supported ruthenium-tin catalysts used for hydrogenation reactions of carboxylic acids

Abstract: The incorporation of platinum on bimetallic ruthenium-tin catalysts supported on active carbon has profound effects on the catalytic behavior. The reaction rate was accelerated nearly three times compared with that of the bimetallic system for the hydrogenation of 1,4-cyclohexanedicarboxylic acid (CHDA). Moreover, it turned out that Pt can completely prevent Sn, one component of the catalyst, from dissolving into the reaction mixture, whereas a considerable amount of dissolved Sn was detected in a binary Ru-Sn catalyst during hydrogenation. We verified the durability of our catalyst by carrying out a catalyst recycling test over seven cycles. The characterization of the catalyst has been evaluated by the temperature programmed reduction, X-ray photoelectron spectroscopy and X-ray diffraction as well as electron microscopy. A significant difference in the catalytic properties of the resulting Ru-Pt-Sn catalyst from a binary one exists in the valence state of the tin species, which is present mostly as Sn(0). It is postulated that the high performance of this novel catalyst in terms of activity and durability are related to this point.

Influence of Ni content on physico-chemical characteristics of Ni, Mg, Al-Hydrotalcite like compounds

Abstract: The physico-chemical properties of a series of Ni,Mg,Al-HTLC with Al/(Al+Mg+Ni) = 0.25 and low Ni/Mg ratios were studied by means of X-ray diffraction (XRD), thermogravimetric (TGA) and thermodifferential (DTA) analysis, N2 physissorption and temperature programmed reduction (TPR). The as-synthesized materials were well-crystallized, with XRD patterns typical of the HTLCs in carbonate form. Upon calcination and dehydration the dehydroxilation of the layers with concurrent decomposition of carbonate anions produced mixed oxides with high surface area. XRD analysis indicated that the different nickel and aluminum oxides species are well-dispersed in a poor-crystallized MgO periclase-type phase. As observed by TPR, the different Ni species showed distinct interactions with Mg(Al)O phase, which were influenced by both nickel content and calcination temperature. Regardless of the the nickel content, the reduction of nickel species was not complete as indicated by the presence of metallic dispersions.