Showing papers on "Temperature-programmed reduction published in 1994"

The quantity of reduced nickel in synthetic takovite: effects of preparation conditions and calcination temperature*

Abstract: Takovi te, Ni6A12(OH)t6CO 3. 5H20 , with molar Ni/A1 ratios of 2.5 and 3 is prepared by pre- cipitation at pH levels of 7 and 10, periods of reaction of 3 and 20 hr, and calcination temperatures varying between 120 ~ and 853~ The quantity of reduced Ni is determined as a function of the calcination temperature, and the solid phases remaining after reduction are determined with XRD. The Ni/A1 ratio, pH of precipitation, and calcination temperature are important preparation conditions. A precursor of a Ni/A1 ratio of 2.5 precipitated at a pH-level of 10 exhibits on reduction metallic Ni particles of about 15 nm irrespective of the drying or calcination temperature. After reduction, a considerable amount of NaA102 is present. Metallic Ni particles of 6 nm are present in a reduced precursor of Ni/A1 ratio of 2.5 precipitated at a pH-level of 7. The size of the Ni particles present in a reduced precursor of Ni/A1 ratio of 3 precipitated at a pH-level of 10 rises from 4-8 nm to 16 nm after calcination at temperatures increasing from 120* to 853"C. The last precursor contains much less sodium, and shows after reduction a disordered NiO phase containing some alumina. Especially, the takovite with the molar Ni/A1 ratio of 2.5, thoroughly washed to remove Na, and calcined at T < 260"C before reduction of Ni provides a promising catalyst for the production of hydrogen-carbon monoxide flows out of methane and steam.

A corrected procedure and consistent interpretation for temperature programmed reduction of supported MoO3

Abstract: Experiments in which bulk MoO3 has been diluted in silica or alumina reveal a “corrected” TPR pattern in which the bulk oxide reduces at the same temperature as the supported crystalline phase. The reduction patterns for diluted bulk MoO3, in conjunction with an XRD study of the reduction process and a recent comprehensive speciation theory have greatly assisted the assignment of peaks for silica and alumina supported samples. A simple interpretation of TPR patterns has been applied with good agreement to the body of MoO3 TPR literature. With consistent interpretation, temperature programmed reduction might be employed with increased confidence to characterize supported MoO3 and perhaps other supported oxides.

X-ray photoelectron spectroscopy, temperature-programmed desorption and temperature-programmed reduction study of LaNiO3 and La2NiO4 +δ catalysts for methanol oxidation

Abstract: An X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD) and reduction (TPR) study of LaNiO3 and La2NiO4+δ perovskites has been carried out. The existence of at least two different forms of oxygen in these compounds is shown by both oxygen reactivity (TPD and TPR) and XPS characterisation. XP spectra have also revealed a surface enrichment in lanthanum and oxygen. TPR profiles have shown a reduction of LaNiO3 through the formation of La2Ni2O5 as an intermediate compound. Above 450 °C, LaNiO3 and intergrowth nickelates (La2NiO4 and La2NiO4 +δ) undergo a final reduction to metallic Ni and La2O3. Between 200 and 400 °C all three compounds exhibit a high catalytic activity in the total catalytic oxidation of methanol.

Activation and Regeneration of a NiMo/Al2O3 Hydrotreatment Catalyst

Abstract: Activation and regeneration procedures applied to a nickel-molybdenum on alumina catalyst, both fresh and spent, were tested by the hydrodesulfurization of thiophene. Characterization techniques used included temperature programmed reduction and oxidation (TPR, TPO), diffuse reflectance spectroscopy (DRS), and X-ray diffraction (XRD). The fresh catalyst was treated by sulfiding, reoxidation, and resulfiding. This sequence was found to be more effective than one sulfiding step, possibly because of the formation of a nickel molybdate phase during reoxidation. The spent catalyst could not be regenerated completely although its original surface properties were attained. The loss of activity of the spent catalysts was alluded by TPO to result from nickel-molybdenum segregation which probably happened because of the excessive heat from burning the coke present on the catalyst

Chromia on Silica and Alumina Catalysts: Temperature-Programmed Reduction and Structure of Surface Chromates

Abstract: • 9H20 and Cr03 followed by calcination at 573 and 873 K. For comparison purposes unsupported materials were also prepared by decomposition of the parent compounds at the same temperatures, namely 573 and 873 K. The materials were structurally characterized by UV-VIS diffuse reflectance spectroscopy (DRS) and XRD, and their stoichiometry was determined by temperature programmed reduction (TPR). Decomposition of the nitrate at 573 K yielded dominantly poorly crystallized a-Cr203 with surface Cr4+ and Cr6+ minority species being also present, while Cr205 was preferentially formed from Cr03. In contrast, high temperature decomposition of both parent compounds gave well crystallized 0t-Cr2O3 containing also surface chromates. These bulk and surface oxides were irreducible in H2 at temperatures < 970 K. Chromia-support interactions turned out to be very weak with Si02, leading to essentially the same structural characteristics as found for the unsupported oxides. The CrO,/Si02 samples calcined at 573 K were more easily reduced than the unsupported analogues due to higher dispersion of the chromia. Due to strong support interactions in the Al203-supported systems, highly dispersed Cr-0 species are anchored to the alumina surface with Cr4+ and Cr6+ being the dominant oxidation states.

Reactions of ceria supported rhodium with hydrogen and nitric oxide studied with tpr/tpo and xps techniques

Abstract: The reactions of ceria supported rhodium with H2 and NO have been investigated by temperature programmed reduction (TPR), temperature programmed oxidation (TPO) and X-ray photoelectron spectroscopy (XPS). In precious metal/ceria systems, both the support and the precious metal may be subject to reduction and reoxidation processes. Since partly reduced ceria is an ion-conducting material, the electrical charging of the whole sample during the XPS measurements depends on the oxidation state of the supporting ceria. Thus, the charging can be used as a sensitive indicator of the oxidation state of the support material which allows distinction between reactions of the support and of the precious metal. Rhodium oxide precursors are reduced to Rh0 at temperatures below 200°C, but a slight reduction of the ceria support in this temperature range was also evident from the XPS spectra. At higher temperatures, the reduction of the ceria becomes more pronounced. Reoxidation of the support by NO is very fast and complete even at room temperature. Rh is oxidised to Rh2O3 at 200–300°C, i.e. below the normal working conditions of car exhaust catalysts.

Formation of the rhodium oxides Rh2O3 and RhO2 in Rh/NaY

Abstract: The oxidation states of Rh in NaY supported catalysts have been studied by temperature programmed reduction (TPR). After calcination of the exchanged catalyst to 380°C, both RhO2 and Rh2O3 are identified, besides small amounts of RhO+ and Rh3+. Quantitative reduction is possible for samples calcined at temperatures not exceeding 500°C. Re-oxidation of the reduced samples leads to formation of RhO2 and Rh2O3, with negligible protonolysis to Rh3+. The dioxide prevails after re-oxidation at 320°C, but the sesquioxide after oxidation at 500°C. In the temperature regime where both oxides coexist the reduction of NO with propane is catalyzed even at an O2/C3H8 ratio of 10. Total oxidation of propane reaches 80% at 350°C.

Passivation layer in a pre-reduced ammonia synthesis catalyst: A temperature-programmed reduction study

Abstract: Industrial ammonia synthesis catalyst, in its fresh, reduced + passivated and used + passivated conditions, was studied by temperature-programmed reduction (TPR). The results indicate that about 10% of the total iron atoms or about 5 atomic layers from the surface are reoxidized during the passivation treatment. This is of the same order as estimated by others from gravimetric, microcalorimetric and Mossbauer studies. Reductions after exposure to oxygen at 20° and 100°C are comparable, but at 200°C and above bulk oxidation sets in. The TPR of a passivated catalyst after five years of operation in the plant is still similar to that of the fresh catalyst after the first reduction and passivation, but the former has apparently only2/3 of the iron area of the latter.

Hydrogen temperature programmed reduction based on water analysis

Abstract: Profiles of temperature programmed reduction (TPR) of V2O5 under H2 have been studied through water analysis. This new approach enhances the sensitivity of the method. The influence of experimental parameters on the temperature corresponding to the maximum reduction rate,Tm, has been studied. The hydrogen concentration and heating rate are the two major factors that influenceTm. In our method, the amount of solid and the flow rate are also important factors in monitoring of profile distortions attributed to water condensation at the outlet of the reactor. An approach of the mechanism of reduction of V2O5 has been carried out. The best kinetic model which describes the reduction rocess is the Avrami-Erofeev tridimensional nucleation model.

Structure and reactivity of undoped and sodium- doped PbO/α-Al2O3 catalysts for oxidative coupling of methane

Abstract: Undoped and Na-doped lead oxides supported on α-Al2O3 were used in the oxidative coupling of methane (OCM) by cofeeding methane and oxygen at atmospheric pressure. In general, the methane conversion increased with increasing Pb-loading but the C2+ selectivity decreased slightly. Another major effect observed on incorporation of PbO into α-Al2O3 was the dramatic increase of carbon dioxide with lead content. The combined use of X-ray diffraction, temperature-programmed reduction and photoelectron spectroscopy revealed that a certain proportion of Pb4+ species is present in the undoped catalysts, and this is greater for the catalysts with higher lead oxide content. These techniques also revealed that lead oxide is partially evaporated under severe conditions imposed by the OCM, although the evaporation can be diminished by doping with sodium. The beneficial effect of sodium on the catalyst stability is also accompanied by a better catalyst performance due to the ability of sodium oxide to activate methane.

Effect of alloying on the sulfur resistance of bimetallic Pt-based catalysts

Abstract: The effect of alloying on the thiotolerance of bimetallic Pt-Ge/Al2O3 and Pt-Sn/Al2O3 catalysts was studied. Unalloyed and alloyed samples were obtained by reducing both catalysts at 573 and 773 K, respectively. Gas chemisorption, catalytic testing, temperature programmed reduction (TPR) and temperature programmed desorption (TPD) experiments showed that after H2 reduction at 773 K a part of tin (germanium) cations reduces and forms a metallic Pt-Sn(Ge) alloy. The relative sensitivity to sulfur poisoning of the catalysts was established using a cyclohexane dehydrogenation as test reaction and thiophene as the poisoning molecule. It was found that alloying does not increase the thiotolerance of Pt-Sn catalysts compared to monometallic Pt catalyst. On the contrary, alloyed Pt-Ge/Al2O3 was the most thiotolerant catalyst because Ge decreases the density of charge of alloyed Pt thereby weakening the bond strength with electrophilic sulfur.

1.14 Selective Hydrogenation of CFC-12 to HFC-32 on Zr-Pd/C Catalyst

Abstract: Reaction of dichlorodifluoromethane(CFC-12) and chlorodifluoromethane(HCFC-22) with H 2 was carried out on modified Pd catalysts. On 5wt% Pd/C, hydrogenation of CFC-12 gave mainly difluoromethane(HCF-32) at 200°C and methane at 350°C, respectively. Zr and V modifiers promoted to increase in activity with no change in selectivity and, thus, to enhance the yield of HFC-32. From temperature programmed reduction (TPR) measurements, formation of Pd-Zr hydride, which desorbed hydrogen at ca. 200°C, was observed. This hydride may play a role to supply active hydrogen in the reaction at the reaction temperature.

Reactivity and Preparation of Perovskite-Type Mixed Oxides LaBO3(B = Mn, Fe, Co) by Citrate Sol-Gel Method

Abstract: Perovskite-type mixed oxides LaBO(B = Mn, Fe, Co) were prepared by citrate sol-gel method in , 24h). The oxygen stoichiometries and structures of these oxides were determined by XRD and TPR results as followings; LaMnO(a = 5.507, c = 13.329 , hexagonal), LaFeO(a = 5.554, b = 5.555, c = 7.863 , orthorhomibic), LaCoO(a = 5.436, c = 13.095 , hexagonal). The temperature programmed reduction(TPR) experiments in static 300 torr H atmosphere shows that the reduction reaction of LaBO(B = Mn, Fe, Co) proceeds into two stages, and thermal stabilities of these oxides decreased in the order of LaMnO > LaFeO > LaCoO. According to the kinetic analysis the lowest activation energy was obtained for LaCoO.

A study of metals supported on active carbon with a temperature programmed reduction (TPR) technique

Abstract: The influence of active carbon as support on the reducibility of supported metals (Ni, Cu, Cd, Zn) has been studied by means of a temperature programmed reduction (TPR) technique. The TPR profiles indicate that active carbon behaves as a dispersion agent and the supported metal is dispersed in a disordered phase rather than as a stoichiometric compound. The hydrogen consumed in the reduction indicates that the metal residues are present in monovalent and divalent forms. High-temperature reduction peaks were also observed and are explained on the basis of metal-surface interactions and the porosity of the active carbon. Nitrogen adsorption reveals that the active carbon porosity decreases due to progressive closure of the pores when metals are supported on the surface of active carbon.

Long-term stability of the V2O5/Al2O3 catalyst for the selective reduction of nitrogen oxides

Abstract: Changes of the V2O5/Al3O3 catalyst aged for up to 10 years under real conditions of the selective catalytic reduction of NO x by ammonia (SCR) at the tail gases of the nitric acid plant were characterized by51V NMR spectroscopy, porosimetry, temperature programmed reduction (TPR) and catalytic activity measurements. The catalytic activity and the redox properties of the catalyst were found intact. Only small variations of the ratio of the octahedral and tetrahedral vanadia species were documented by51V NMR on aged catalyst.

Preparation of La1−xSrxMnO3 Perovskite Catalysts Supported on Ceramic Foam Materials

Abstract: The new method of preparation of La1−xSrxMnO3 catalysts supported on ceramic foam materials was developed. The synthesized supported perovskite catalysts were examined by X-ray diffraction analysis and BET methods. It was shown that in coated samples there is a pure perovskite phase on the support surface up to temperatures of 1000°C. Unsupported La1−xSrxMnO3 catalysts were synthesized applying co-precipitation technique using La, Sr and Mn acetylacetonates and were studied by X-ray diffraction analysis. Temperature programmed reduction method revealed that catalysts contain 2 types of active centers: low temperature (α) and high temperature (β), the quantity of these centers and correlation between them depend on the value of x. It was shown that supported perovskite catalysts, similarly to the massive ones, maintain two types of active centers (α and β). The activity of La1−xSrxMnO3 on mullite foam was characterized in the methane oxidation reaction.

Better sulphide catalysts through optimized active phase-support interaction

Abstract: In this project, improved hydrotreating catalysts have been synthesized by varying the active phase-support interaction. This interaction has been changed by differing the calcining procedures and choice of catalyst precursors and carriers. The active phase consisted basically of molybdenum or tungsten disulphide, and Ni, Co, Cr or Fe as the promoter. Also phosphorus has been added as secondary promoter. As for the preparation of the catalysts, a new type of carrier, filamentary carbon supported by alumina, has been synthesized. To improve the interaction between Co and Mo in the active catalyst, nitrilotriacetic acid (NTA) has been added as a complexing agent to a solution containing both metal salts. Also, a new type of active phase precursor (acetylacetonate) has been used with a number of solvents (water, ethanol, toluene). The final consequences of these variations have been investigated from a physico-chemical point of view in relation to the actual performance of the newly prepared catalysts in the hydrodesulphurization of thiophene. In the physico-chemical characterization of the catalysts, Mossbauer emission and absorption spectroscopy (MES, MAS), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge structure (XANES) have been used. As a result, a detailed model of the so-called ‘Co-Mo-S’ phase could be proposed. Mossbauer experiments on the sulphidation of carbon-supported Co and Co-Mo catalysts indicate the formation of Co species (consisting of Co and Co-Mo, respectively), which have the same spectral features in Mossbauer spectroscopy. Hence, care should be taken into account when referring to the so called ‘Co-Mo-S’ phase. In addition, fingerprinting techniques such as TPR (temperature programmed reduction) and TPS (temperature programmed sulphidation) have been applied extensively. The sulphided catalysts contain different sulphur species. The existence of a very reactive sulphur species called ‘excess sulfur’ is correlated with the hydrodesulphurization activity. A model has been developed describing the coke deposition on hydrotreating catalysts under commercial reaction conditions. As a result, improved hydrotreating catalysts have been developed and a better understanding of the fundamental characteristics of the catalysts has been acquired. Novel equipment has been developed. Two of its major features are a simple design and the possibility for the in situ measurement of physical parameters of the autoclave contents. For future development of the catalysts, a detailed kinetic analysis of their performance, and the application of in situ characterization techniques is of paramount importance.