Papers by Keyword: Deactivation

Abstract: It is well known that the presence of phosphorus in hypoeutectic Al-Si alloys causes coarsening of Al-Si eutectic even when the phosphorus concentration is as low as a few ppm. The reason is that phosphorus reacts with aluminum to form AlP, the particles of which serve as potential nuclei for silicon due to a very similar lattice parameter with Si. The purpose of this study is to investigate the possibilities for reducing the nucleation potency of AlP by changing the alloy chemistry. The experiments include melting and solidification of a number of aluminum alloys under different conditions. After that, the samples are examined using SEM and TEM microscopy to elucidate the presence of phosphorus-containing compounds and to explain their formation mechanism. The experimentally obtained data are discussed on a systematic basis of thermodynamic calculations and compared with the corresponding data from the relevant literature. Particularly, the results reveal that such elements as Mg, Ca and Sr are potentially capable of neutralizing the nucleation activity of AlP although their effect is dependent on the presence of other chemical elements in the aluminium alloy.

Abstract: The increasing demand for producing dimethyl ether from synthesis gas explains the renewed interest in studying the activity and stability of catalysts. In the present work, resource catalytic testing of ZSM-5 zeolites was carried out in the one-step synthesis of dimethyl ether from synthesis gas for 120 hours. Formation of condensation products was observed on zeolite surface after catalytic tests which leads to lower catalytic activity of samples. Condensation products were investigated by thermal analysis in an oxidizing atmosphere. Textural characteristics of zeolites before and after reaction were investigated. It was shown that methanol significantly contribute to formation of condensation products on the catalyst surface in the process of dimethyl ether production.

Abstract: Dry reforming reaction of CH₄ by CO₂ was carried out over alumina-supported Ni catalysts. Main products were H₂ and CO with small amount of H₂O by-production because of reverse water gas shift reaction. The activity was gradually decreased with time on stream by carbon deposition. The addition of Ce component was investigated to improve the catalyst stability. The Ce addition brought decrease of carbon deposition during the reaction. The role of Ce has been considered that an inactive coke formation is significantly suppressed.

Abstract: A 1000 hour hydrotreating experiment was performed to investigate the hydrotreating behavior of heteroatom compounds (first stage) and HDM catalyst deactivation (second stage) using Venezuela De-Asphalted Oil. The effect of reaction severity on impurities removal was the expected one, the deeper the hydrotreating degree, the higher the conversion of impurities. The Characterization of spent HDM catalyst shows that the content of coke deposition on spent HDM catalyst is only 4 wt% while that of metal is more than 30 wt%. Vanadium compounds in DAO with less diffusion resistance can deposit inside of the HDM catalyst grain. Lower coke formation also retard the HDM catalyst by keeping the diffusion pores and active cites. The removal of asphaltenes largely improved the stability of the HDM catalyst.

Abstract: The main results of thermodynamic analysis and mathematical simulation of the deactivation of a platinum dehydrogenation catalyst by coke-generating compounds are presented. Developed model allows calculating the optimal flow rate of water fed to the reactor to maintain the conditions of thermodynamic equilibrium of the coke formation reaction and oxidation of intermediate condensation products with water. A comparative evaluation of different raw cycles of platinum-dehydrogenation catalysts is presented and shown to cause reduced production of the desired product was the change the composition of the feedstock.

Abstract: With the wide application of SCR technology, the deactivation of SCR catalyst has caught attention in the coal-fired power plants. The activities of fresh and used SCR catalysts were compared and the deactivation reasons were analyzed by using scanning electron microscope (SEM), BET, X-ray diffraction (XRD), and X-ray fluorescence (XRF). The results showed that the specific surface area of the used catalyst decreased significantly, caused by the pore blockage of fly ash or sulfates on the surface of the used catalyst. The activity of the used catalyst decreased seriously, and the relative activity of the field sample had declined to 0.60. Moreover, according to the results of X-ray fluorescence, alkali poisoning was one of the main reasons for the decline in the activity of the catalyst.

Abstract: The synthesis of α-pinene oxide mediated by Novozym 435 (lipase from Candida antarctica) in a three-phase system was studied in this work. Novozyme 435 catalysed the formation of peroxyoctanoic acid directly from octanoic acid and hydrogen peroxide, which was then applied for in situ oxidation of α-pinene. The highest conversion of a-pinene (approximately 80%) was obtained when the reaction was performed at 30°C and initial hydrogen peroxide concentration in the water phase was set to be 30%. The parameters affecting the lipase activity were also investigated，where the peracid generated in organic phase was obvserved to greatly inactivate the enzyme compared to other components in the organic phase.

Abstract: A series of CuxCe1-xO2-x/SBA-15/cordierite (x = 0-1) catalysts were prepared. The activity of the catalysts for CO combustion was evaluated. The catalysts were characterized by X-ray diffraction (XRD), N2 adsorption-desorption, and X-ray photoelectron spectroscopy (XPS). Deactivation behavior of the catalysts for the catalytic combustion of CO was investigated. The results show that all of the catalysts retained the SBA-15 mesoporous structure. It is proposed that deactivation of the catalysts is associated with the increase of the Cu+ and the decrease of the Cu2+ in the catalysts.

Abstract: A Mo-Ni/Al₂O₃ catalyst for converting mercaptans into thioethers to remove the mercaptans in the FCC gasoline was studied. Under the optimized conditions, the catalyst showed its good catalytic reaction performance and the activity remained stable after the 1500 h life experiment. After the accelerated aging experiment under severe conditions, the catalyst finally lost its activity at 1850 h. The samples of the fresh and used catalyst were characterized by BET, TEM and XPS. The results showed that the binding energy of sulfided Ni species shifted to the low energy level indicated that the sulfided Ni species might have a reducible tendency in the catalytic run. This could be considered as a main reason of the catalysts deactivation. On the other hand, the metal agglomeration and carbon deposition and nitrides adsorption in the catalytic runs also led to the deterioration of the catalyst.

Abstract: Among the new oxidation methods in advanced oxidation processes, heterogeneous photocatalysis based on UV/ titanium dioxide is one of the emerging technologies for degrading and totally mineralizing toxic and highly stable carboxylic acids from waste water has attracted great attention in last decade. The aliphatic and aromatic carboxylic acids are widely used as reactants or produced as main products or byproducts in various chemical process industries. These chemical process industries generates waste water containing significant amount of carboxylic acids, which need to be removed from waste water before disposal to natural water bodies due to the environmental restrictions and their hazardous effects on flora and fauna. This paper reviews photocatalytic degradation (PCD) process for various carboxylic acids in detail, including basic mechanism of titanium dioxide, effect of carboxylic acid structure and various reaction parameters like effect of initial concentration and kinetic study, catalyst loading, pH, adsorption and deactivation of the photocatalyst. A critical analysis of the available literature has been made and some general conclusions have been drawn related to the above mentioned parameters. The photocatalytic degradation pathways for carboxylic acids are also discussed to understand this process thoroughly.