Adsorption and reaction of thiophene and H2S on Mo2C/Al2O3 catalyst studied by in situ FT-IR spectroscopy

The reactions of both thiophene and H2S onMo(2)C/Al2O3 catalyst have been studied by in situ FT-IR spectroscopy. CO adsorption was used to probe the surface sites of Mo2C/Al2O3 catalyst under the interaction and reaction of thiophene and H2S. When the fresh Mo2C/Al2O3 catalyst is treated with a thiophene/H-2 mixture above 473 K, hydrogenated species exhibiting IR bands in the regions 2800-3000 cm(-1) are produced on the surface, indicating that thiophene reacts with the fresh carbide catalyst at relatively low temperatures. IR spectra of adsorbed CO on fresh Mo2C/Al2O3 pretreated by thiophene/H-2 at different temperatures clearly reveal the gradual sulfidation of the carbide catalyst at temperatures higher than 473 K, while H2S/H-2 can sulfide the Mo2C/Al2O3 catalyst surface readily at room temperature (RT). The sulfidation of the carbide surface by the reaction with thiophene or H2S maybe the major cause of the deactivation of carbide catalysts in hydrotreating reactions. The surface of the sulfided carbide catalyst can be only partially regenerated by a recarburization using CH4/H-2 at 1033 K. When the catalyst is first oxidized and then recarburized, the carbide surface can be completely reproduced.

Unique properties of Ir/ZSM-5 catalyst for NO reduction with CO in the presence of excess oxygen

The reduction of NO with CO in the presence of excess oxygen was investigated over different noble metal catalysts for probing the relationship between catalytic properties and adsorption behaviors. Among the four precious metal catalysts investigated, Ir/ZSM-5 was found to be the only active one for NO reduction with CO under lean conditions. With the decreasing of the Ir content, higher NO conversion and CO selectivity was obtained. Temperature-programmed reaction (TPR) studies of NO/H-2/O-2 and NO/CO/O-2 showed that the Pt/ZSM-5 was active when H-2 was used as the reductant, whereas, the Ir/ZSM-5 was active when CO was the reducing agent. This difference is due to the different mechanisms of the two reactions. Temperature-programmed desorption (TPD) of NO, CO and O-2 showed that NO could dissociate more easily over the Ir/ZSM-5 than on the Pt/ZSM-5, while the oxidation of CO by O-2 proceeded more rapidly on the Pt/ZSM-5 than on the Ir/ZSM-5. The presence of excess O-2 inhibited drastically the dissociation of NO, which is considered as the key step for the NO-CO reaction. The high dissociation rate of NO over the Ir/ZSM-5 is visualized as the key factor for its superior high activity in NO reduction with CO under lean conditions. (C) 2002 Elsevier Science B.V. All rights reserved.

On the surface sites Of MOP/SiO2 catalyst under sulfiding conditions: IR spectroscopy and catalytic reactivity studies

The surface sites of MoP/SiO2 catalysts and their evolution under sulfiding conditions were characterized by IR spectroscopy using CO as the probe molecule. The HDS activities of thiophene were measured on the MoP/SiO2 catalyst that was subjected to different sulfidation and reactivation pretreatments. Cus Modelta+ (0 < delta less than or equal to 2) sites are probed on the surface of fresh MoP/SiO2 by molecularly adsorbed CO, exhibiting a characteristic IR band at 2045 cm(-1). The surface of MoP/SiO2 is gradually sulfided in HDS reactions, as revealed by the shift of the IR band at 2045 to ca. 2100 cm(-1). Although the surface of a MoP/SiO2 catalyst becomes partially sulfided, the HDS activity tests show that MoP/SiO2 is fairly stable in the initial stage of the HDS reaction, providing further evidence that molybdenum phosphide is a promising catalytic material for industrial HDS reactions. Two kinds of surface sulfur species are formed on the sulfided catalyst: reversibly and irreversibly bonded sulfur species. The MoP/SiO2 catalyst remains stable in the HDS of thiophene because most sulfur species formed under HDS conditions are reversibly bonded on the catalyst surface. A detrimental effect of presulfidation on the HDS activity is observed for the MoP/SiO2 catalyst treated by H2S/H-2 at temperatures higher than 623 K, which is ascribed to the formation of a large amount of the irreversibly bonded sulfur species. The irreversibly sulfided catalyst can be completely regenerated by an oxidation and a subsequent reduction under mild conditions. (C) 2003 Elsevier Inc. All rights reserved.

In situ IR spectroscopic studies on molybdenum nitride catalysts: active sites and surface reactions

Recent IR spectroscopic studies on the surface properties of fresh Mo2N/gamma-Al2O3 catalyst are presented in this paper. The surface sites of fresh Mo2N/gamma-Al2O3, both Modelta+ (0<δ<2) and N sites, are probed by CO adsorption. Two characteristic IR bands were observed at 2045 and 2200 cm(-1), due to linearly adsorbed CO on Mo and N sites, respectively. The surface N sites are highly reactive and can react with adsorbed CO to form NCO species. Unlike adsorbed CO on reduced passivated one, the adsorbed CO on fresh Mo2N/gamma-Al2O3 behaves similarly to that of group VIII metals, suggesting that fresh nitride resembles noble metals. It is found that the surface of Mo nitrides slowly transformed into sulfide under hydrotreating conditions, which could be the main reason for the activity drop of molybdenum nitride catalysts in the presence of sulfur-containing species. Some surface reactions, such as selective hydrogenation of 1,3-butadiene, isomerization of 1-butene, and hydrodesulfurization of thiophene, were studied on both fresh and reduced passivated Mo2N/gammaAl(2)O(3) catalysts using IR spectroscopy. The mechanisms of these reactions are proposed. The adsorption and reaction behaviors of these molecules on fresh molybdenum nitride also resemble those on noble metals, manifesting the unique properties of fresh molybdenum nitride catalysts. Mo and N sites are found to play different roles in the adsorption and catalytic reactions on the fresh Mo2N/gammaAl(2)O(3) catalyst. Generally, Mo sites are the main active sites for the adsorption and reactions of adsorbates; N sites are not directly involved in catalytic reactions but they modify the electronic properties of Mo sites.

In situ FT-IR spectroscopic studies of CO adsorption on fresh Mo2C/Al2O3 catalyst

The surface sites of supported molybdenum carbide catalyst derived from different synthesis stages have been studied by in situ FT-IR spectroscopy using CO as the probe molecule. Adsorbed CO on the reduced passivated Mo2C/Al2O3 catalyst gives a main band at 2180 cm(-1), which can be assigned to linearly adsorbed CO on Mo4+ sites. The IR results show that the surface of reduced passivated sample is dominated by molybdenum oxycarbide. However, a characteristic IR band at 2054 cm-1 was observed for the adsorbed CO on MoO3/Al2O3 carburized with CH4/H-2 mixture at 1033 K (fresh Mo2C/Al2O3), which can be assigned to linearly adsorbed CO on Modelta+ (0 < delta < 2) sites Of Mo2C/Al2O3, Unlike adsorbed CO on reduced passivated Mo2C/Al2O3 catalyst, the IR spectra of adsorbed CO on fresh Mo2C/Al2O3 shows similarity to that on some of the group VIII metals (such as Pt and Pd), suggesting that fresh carbide resembles noble metals. To study the stability Of Mo2C catalyst during H-2 treatment and find proper conditions to remove the deposited carbon species, H-2 treatment of fresh Mo2C/Al2O3 catalyst at different temperatures was conducted. Partial amounts of carbon atoms in Mo2C along with some surface-deposited carbon species can be removed by the H, treatment even at 450 K. Both the surface-deposited carbon species and carbon atoms in carbide can be extensively removed at temperatures above 873 K.

Microcalorimetric and IR spectroscopic studies of CO adsorption on molybdenum nitride catalysts

The adsorption of CO on both nitrided and reduced passivated Mo(2)N catalysts in either alumina supported or unsupported forms was studied by adsorption microcalorimetry and infrared (IR) spectroscopy. The CO is adsorbed on nitrided Mo(2)N catalysts on three different surface sites: 4-fold vacancies, Mo(delta+) ( 0 < delta < 2) and N sites, with differential heats of CO adsorption decreasing in the same order. The presence of the alumina-support affects the energetic distribution of the adsorption sites on the nitrided Mo(2)N, i.e. weakens the CO adsorption strength on the different sites and changes the fraction of sites adsorbing CO in a specific form, revealing that the alumina supported Mo(2)N phase shows lower electron density than pure Mo(2)N. On reduced passivated Mo(2)N catalysts the CO was found to adsorb mainly on Mo(4+) sites, although some slightly different surface Mo(delta+) d (0 < delta < 2) sites are also detected. The nature, density and distribution of surface sites of reduced passivated Mo(2)N/gAl(2)O(3) were similar to those on reduced MoO(3)/gamma-Al(2)O(3).

The Synthesis and Characterization of a Novel (E,E)-Dioxime and its Mono- and Heterotrinuclear Complexes Containing an 18-Membered Dioxadithiadiazamacrocycle

The novel (E,E)-dioxime, 5,6:17,18-dibenzo-11,12-(4-nitrobenzo)-2,3-bis(hydroxyimino)-7,16-dithia-10,13-dioxa-1,4-diazacyclooctadecane) (H2L), has been synthesized from reaction of (E,E)-dichloroglyoxime (1) with 2,3:14,15-dibenzo 8,9-(4-nitrobenzo)-4,13-dithia-7,10-dioxa-1,16-diazahegzadecane (2). The mononuclear Co(III) complex (4) of this dioxime was prepared by oxidation of the cobalt (II) complex. The -capped Co(III) complex (5) was synthesized by using a precursor Co(III) complex and boron trifluoride dietherate. The heterotrinuclear complexes (6) and (7) were prepared by reaction of (5) with NiCl2·6H2O and CdCl2·H2O, respectively. In addition, the homotrinuclear Cu(II) complex (8), has also been prepared by the reaction of this dioxime with CuCl2·H2O. The structures of the dioxime and its complexes were identified by using elemental analysis, 1H- and 13C-NMR, IR, and mass spectral data.

Positive interactions in communities

Current concepts of the role of interspecific interactions in communities have been shaped by a profusion of experimental studies of interspecific competition over the past few decades. Evidence for the importance of positive interactions — facilitations — in community organization and dynamics has accrued to the point where it warrants formal inclusion into community ecology theory, as it has been in evolutionary biology.

黄土丘陵区土壤水资源评价指标与分析

在总结土壤水研究现状和对土壤水的资源特性认识的基础上,重新定义了土壤水资源,土壤水资源是指植物根系可吸收利用的范围,其补给主要来源于大气降水,即能被植物(包括农作物)及土壤微生物所利用并可以恢复和更新的土壤水的数量;并阐述了土壤水资源的特征,它具有对降水依赖性与相关性,不易保存性或易耗散性,不可开采性与就地利用性,有效性和无效性,可调控性的特征;提出了土壤水资源的评价指标,包括土壤水资源最大存储量、可利用量、无效水资源量和实际存储量;根据评价指标初步计算了黄土丘陵区典型地区的土壤水资源。

a dynamic model to handle defect-oriented processes

Tongji Univ, Inst Dev Study, Syst Engn Soc China, Comm Syst Dynam, Syst Dynam Soc, China Chapter, Shanghai Inst Foreign Trade, Syst Dynam Soc, Chapters Asia Pacific Area