Degree Of Catalyst Reduction Research Articles

Fischer-Tropsch synthesis: Platinum promoted Co@HCS catalysts

Hollow carbon sphere (HCS) nanoreactors were used as a catalyst support for Co, with Pt as a promoter. The hollow morphology of the HCSs nanoreactor was exploited to study the influence of the Pt promoter nanoparticle location relative to that of the Co3O4 (Co 10 wt%) nanoparticles (dCo = 3.5 – 12.5). Furthermore, the effect of Pt on the reduction behaviour and activity of the Co FTS catalyst was evaluated. The synthesis and use of Pt nanoparticles supported on/in Co@HCS FT catalysts to give CoPt@HCS and Pt/(Co@HCS) catalysts are reported. Electron microscopy, ex situ PXRD, BET, and TPR studies revealed that the prepared catalysts were successfully synthesized with well dispersed Co nanoparticles. It was observed that the proximity between the Co and Pt influenced the Co hcp/fcc ratios. A secondary hydrogen spillover effect was invoked to account for the reduction of Co3O4 on 1Pt/(10Co@HCS) at 335 °C. The complete reduction of Co3O4 to Co metal on 10Co1Pt@HCS at 350 °C was attributed to a primary hydrogen spillover effect. Following catalyst activation at 350 °C, the primary spillover process resulted in a catalyst exhibiting higher Fischer–Tropsch activity (approximately 2×) compared to both the unpromoted catalyst and catalysts where Pt and Co were separated by the HCS. This enhanced activity was partially attributed to the Co phases formed on the carbon support during reduction and the degree of catalyst reduction, which depended on the type of hydrogen spillover process. A comparison of the reduction ability of Os, Ru and Pt on the reduction of Co supported on/in HCSs and the impact of the promoter on Co hcp/fcc ratios is reported.

Isolating the Roles of Hydrogen Exposure and Trace Carbon Contamination on the Formation of Active Catalyst Populations for Carbon Nanotube Growth.

Limited understanding of the factors influencing the yield of carbon nanotubes (CNTs) relative to the number of catalyst particles remains an important barrier to their large-scale production with high quality, and to tailoring CNT properties for applications. This lack of understanding is evident in the frequent use of Edisonian approaches to give high-yield CNT growth, and in the sometimes-confusing influence of trace residues on the reactor walls. In order to create conditions wherein CNT yield is reproducible and to enable large-scale and reliable CNT synthesis, it is imperative to understand-fundamentally-how these common practices impact catalytic activity and thus CNT number density. Herein, we use ambient pressure-X-ray photoelectron spectroscopy (AP-XPS) to reveal the influence of carbon and hydrogen on the coupling between catalyst reduction and CNT nucleation, from an iron catalyst film. We observe a positive correlation between the degree of catalyst reduction and the density of vertically aligned CNTs (forests), verifying that effective catalyst reduction is critical to CNT nucleation and to the resulting CNT growth yield. We demonstrate that the extent of catalyst reduction is the reason for low CNT number density and for lack of self-organization, lift-off, and growth of CNT forests. We also show that hydrocarbon byproducts from consecutive growths can facilitate catalyst reduction and increase CNT number density significantly. These findings suggest that common practices used in the field-such as reactor preconditioning-aid in the reduction of the catalyst population, thus improving CNT number density and enabling the growth of dense forests. Our results also motivate future work using AP-XPS and complementary metrology tools to optimize CNT growth conditions according to the catalyst chemical state.

Effect of support for Pt[sbnd]Cu bimetallic catalysts synthesized by electron beam irradiation method on preferential CO oxidation

Supported bimetallic catalysts have been shown to improve properties and catalytic performance of monometallic catalysts in various fuel conversion reactions. To investigate the effect of support on bimetallic PtCu system, PtCu supported on γ-Fe2O3 and CeO2 were synthesized using electron beam irradiation method and preferential CO oxidation (PROX) performance was investigated in relation to structural and chemical properties of the catalysts. The difference in performance among the catalysts existed in both oxygen transport ability and selectivity. While oxygen transport ability was influenced by support property such as crystallite size (within the comparison among catalysts supported on CeO2 and γ-Fe2O3), selectivity was rather influenced by support material which led to clear differences in reducibility, reduction degree of catalyst, and alloying extent of PtCu. CeO2-supported catalysts are unique in the view point that they were able to convert CO selectively in high CO conversion region compared to Fe2O3-supported catalyst. PtCu with small CeO2 crystallite maintained high O2 conversion (high capacity for oxygen transport) in O2-rich atmosphere in addition to high selectivity. Compared to monometallic Cu which lost its activity in the O2-rich wet condition, or compared to monometallic Pt which exhibited poor selectivity, PtCu realized both high activity and selectivity on CeO2 of small crystallite size, leading to more than 99% of CO conversion at 100°C. By modifying the structure, it was demonstrated that PtCu/CeO2 system can be improved to have low-temperature activity comparable to monometallic Pt while keeping high selectivity of the original PtCu catalyst in the practical PROX condition.

Oxidative dehydrogenation of propane over differently structured vanadia-based catalysts in the presence of O 2 and N 2O

The effect of the nature and distribution of VO x species over amorphous and well-ordered (MCM-41) SiO 2 as well as over γ-Al 2O 3 on their performance in the oxidative dehydrogenation of propane with O 2 and N 2O was studied using in situ UV–vis, ex situ XRD and H 2-TPR analysis in combination with steady-state catalytic tests. As compared to the alumina support, differently structured SiO 2 supports stabilise highly dispersed surface VO x species at higher vanadium loading. These species are more selective over the latter materials than over V/γ-Al 2O 3 catalysts. This finding was explained by the difference in acidic properties of silica- and alumina-based supports. C 3H 6 selectivity over V/γ-Al 2O 3 materials is improved by covering the support fully with well-dispersed VO x species. Additionally, C 3H 6 selectivity over all materials studied can be tuned by using an alternative oxidising agent (N 2O). The improving effect of N 2O on C 3H 6 selectivity is related to the lower ability of N 2O for catalyst reoxidation resulting in an increase in the degree of catalyst reduction, i.e. spatial separation of active lattice oxygen in surface VO x species. Such separation favours selective oxidation over CO x formation.

Carbon deposition and catalyst stability over La 2NiO 4/γ-Al 2O 3 during CO 2 reforming of methane to syngas

The use of 9% La 2NiO 4/γ-Al 2O 3 (9NLA) as a catalyst for CO 2 reforming of methane to syngas has been investigated in a fixed-bed reactor. The results revealed that the yields of CO and H 2 over a 800 °C calcined 9NLA catalyst were remarkably higher than those over a 500 °C calcined one. The BET data confirmed that the properties, such as specific surface area, pore volume and average pore diameter, of a used and a regenerated catalyst were similar to those of a fresh catalyst. Temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) results indicated that the activity of the catalyst depended largely on the degree of catalyst reduction, as well as on the dispersion of metallic nickel. The results of X-ray diffraction (XRD) and temperature-programmed oxidation (TPO)-MS studies demonstrated that the formation of carbon species on the catalyst could be related to the structure and surface composition of the catalyst. With the rise in temperature for catalyst calcination, there was a decline in coke formation due to the formation of a stable NiAl 2O 4 spinel structure.

Oxidative Dehydrogenation of Butane over VMgO Catalysts

A VMgO catalyst has been characterized using a battery of both in situ and ex situ techniques in order to gain insight into the relationship between the state of the catalyst surface and the catalytic performance observed during the oxidative dehydrogenation of n-butane. It has been found that treating the catalyst under oxygen-rich or oxygen-lean atmospheres markedly changes the activity, selectivity, and rate of catalyst deactivation. The performance of the catalyst under both types of atmospheres is also notably different. A certain degree of catalyst reduction is desirable for this reaction, because it increases the selectivity to dehydrogenation products by limiting the amount of oxygen available at the catalyst surface. However, the performance of the catalyst under oxygen-depleted reaction atmospheres is not stable, the main cause for catalyst deactivation being the formation of coke. An overall analysis of the observed effects is proposed on the basis of a redox mechanism.

Reduction of nitrogen oxide by ammonia. Oxidation state of V2O5/Al2O3 catalysts and reaction mechanism

When the (NO + NH3 + O2) reaction is carried out over V2O5/Al2O3, the catalysts are partially reduced, and prereduced catalysts are oxidized by the reaction mixture at a slower rate than by an O2 + N2 mixture. The degree of catalyst reduction in the (NO + NH3 + O2) reaction depends on the gas phase composition and particularly on the vanadium loading of the catalyst. The initial oxidation state of catalyst has an effect on the catalyst activity for the reaction. Rate-determining reduction and oxidation steps of the so-called reduction-oxidation mechanism are proposed on the basis of a comparison of the reduction and oxidation of the catalyst by the reaction components.

Individual and combined oxidation of CO and SO2 on vanadium catalysts with and without Pd additives

The function of SO2 in CO oxidation on Pd-V catalysts is to transform a rapid heterogeneous-homogeneous process to a slower heterogeneous one. High concentrations of CO also exert the same effect. In the heterogeneous process an increase in the SO2 concentration increasing the degree of catalyst reduction with and without Pd additives, promotes CO oxidation.