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1.
Ressler Thorsten Wienold Julia Jentoft Rolf E. Neisius Thomas Günter Marco M. 《Topics in Catalysis》2002,18(1-2):45-52
The potentials of X-ray absorption spectroscopy (XAS) (quantitative phase composition and average valence together with a short-range order structure analysis) combined with a time-resolution in the second range make time-resolved (TR-) XAS a powerful tool for investigating the reactivity of solids in catalysis and solid-state chemistry. General aspects of TR-XAS investigations are discussed (i.e., instrumentation, data analysis). In addition, some experiments illustrate how the kinetics of solid-state reactions in heterogeneous catalysis can be elucidated from TR-XAS studies. 相似文献
2.
Isomerization and disproportionation of n-pentane were catalyzed by sulfated zirconia, Fe- and Mn-promoted sulfated zirconia,
and Pt-, Fe-, and Mn-promoted sulfated zirconia in a flow reactor at temperatures of −25 to 50°C and n-pentane partial pressures
of 0.005–0.01 atm. Incorporation of the Fe and Mn promoters increased the activity of the sulfated zirconia by two orders
of magnitude at 50°C; addition of Pt to the latter catalyst increased the activity only slightly. The primary reactions, disproportionation
(to give butanes and hexanes) and isomerization, occurred in parallel; secondary disproportionation reactions gave heptanes,
propane, butanes, and pentanes. The data are consistent with acid-base catalysis and carbenium ion intermediates, and the
isomerization is inferred to proceed both by unimolecular and bimolecular mechanisms. H2 in the feed stream and Pt in the catalyst both led to reductions in the rate of catalyst deactivation.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
3.
4.
The H–D exchange reaction between CD4 and each of a family of solid acids (the zeolite HZSM-5, sulfated zirconia, iron- and manganese-promoted sulfated zirconia, and AlCl3/sulfonic acid resin) was investigated with a batch recirculation flow reactor; the data determine initial rates of the exchange reaction giving CD3H at temperatures ranging from 440 K for AlCl3/sulfonic acid resin to 688 and 703 K for the zeolite and promoted sulfated zirconia, respectively. Extrapolated results show that the reaction is three orders of magnitude faster with the AlCl3/sulfonated resin (an analogue of the very strongly acidic combination of AlCl3 and H2SO4) than with HZSM-5 or promoted sulfated zirconia and two orders of magnitude faster with the latter than with sulfated zirconia. 相似文献
5.
This review is a summary of the mechanisms of catalytic cracking of small (C3-C6) alkanes. Most of the evidence has arisen from product distributions and kinetics of cracking of these alkanes, interpreted
on the basis of solution carbocation chemistry and theoretical chemistry. Cracking of small alkanes catalyzed by solid acids
such as the zeolite HZSM-5 proceeds by two mechanisms: (1) The unimolecular (protolytic cracking) mechanism, which proceeds
via an alkanium ion formed by protonation of the alkane by the catalyst. This supposed transition state collapses to give
either H2 and a carbenium ion or an alkane and a carbenium ion; the carbenium ions give up protons to the catalyst to form alkenes.
The cracking products include methane and ethane as well as H2. (2) The classical (bimolecular) cracking mechanism, which involves carbenium ion chain carriers that react with the alkane
reactant to abstract hydrides and generate carbenium ions that undergo β-scission. The products include alkanes and alkenes,
but not methane, ethane, or H2. Because protolytic cracking gives alkene products, which are much stronger bases than alkanes, the alkenes become the predominant
proton acceptors as conversions increase, and thus bimolecular cracking prevails at all but the lowest conversions. Protolytic
cracking in the near absence of secondary reactions has been observed only for propane and n-butane at low conversions; secondary
reactions appear to be generally significant for other alkanes. Although the product distributions are qualitatively understood,
there are still inconsistencies in the literature of quantitative product distributions and kinetics, and more experimental
work is needed with standard catalysts such as HZSM-5. Theoretical chemistry is leading to deeper understanding of the transition
states, showing that cracking mechanisms involving bare carbocations are oversimplified. Rather, the catalyst surface must
be included, and it has been simulated by clusters that are zeolite fragments. Surface alkoxides are more stable than surface
carbenium ions, and cracking takes place by concerted bond breaking and formation. Theoretical activation energies for protolytic
cracking of alkanes are close to experimental activation energies that have been corrected for the adsorption energy of the
reactant, but it appears that more theoretical work (as well as better data) is required for satisfactory agreement of theory
and experiment.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
6.
Schlögl R. Knop-Gericke A. Hävecker M. Wild U. Frickel D. Ressler T. Jentoft R.E. Wienold J. Mestl G. Blume A. Timpe O. Uchida Y. 《Topics in Catalysis》2001,15(2-4):219-228
The mode of operation of selective oxidation reactions is described by a series of chemical rules defining the catalyst and some reaction intermediates. In contrast to catalytic processes over metallic elements, little is known, however, about the atomistic details of selective oxidation. In particular, the participation of the subsurface region of the catalyst in the kinetically relevant elementary steps (Mars–van Krevelen mechanism) is not positively verified. Using in situ X-ray absorption techniques to study binary and ternary molybdenum oxides the present contribution shows that it is possible to tackle some of the problems in selective oxidation by direct experimental observation. The modification of the Mo–O local bonding interaction upon thermal reduction of MoO3to MoO3-x
is illustrated. This was also found for mixed Mo–V oxides in which the chemical state of the vanadium seemed unaffected by the reaction but the surface Mo:V ratio varied substantially with the gas phase composition. It is further shown that the solid-state phase transformation between reduced and oxidised forms of molybdenum oxides occur so rapidly, that possibly relevant suboxide cannot be identified by ex situ phase analysis. Observation of the time-law of redox transformations showed that lattice oxygen is only available for selective oxidation if the associated solid-state transformation occurs in the kinetic regime of reaction control and not in that of diffusion control. 相似文献
7.
Limitations in modern sensing technologies result in large errors in sensed target object geometry and location in unstructured
environments. As a result, positioning a robotic end-effector includes inherent error that will often lead to unsuccessful
grasps. In previous work, we demonstrated that optimized configuration, compliance, viscosity, and adaptability in the mechanical
structure of a robot hand facilitates reliable grasping in unstructured environments, even with purely feedforward control
of the hand. In this paper we describe the addition of a simple contact sensor to the fingerpads of the SDM Hand (Shape Deposition
Manufactured Hand), which, along with a basic control algorithm, significantly expands the grasp space of the hand and reduces
contact forces during the acquisition phase of the grasp. The combination of the passive mechanics of the SDM Hand along with
this basic sensor suite enables positioning errors of over 5 cm in any direction. In the context of mobile manipulation, the
performance demonstrated here may reduce the need for much of the complex array of sensing currently utilized on mobile platforms,
greatly increase reliability, and speed task execution, which can often be prohibitively slow. 相似文献
8.
Zirconia/surfactant composites were hydrothermally synthesized in aqueous sulfuric acid at 373 K using Zr(O-nPr)4 as oxide precursor and hexadecyl-trimethyl-ammonium bromide as template. Mesostructural features similar to those of MCM-41 were detected by X-ray diffractometry, with d=4.6 nm. A sample obtained from a starting mixture with Zr:S:CTAB = 2:2:1 was stable enough for removal of occluded organics. After calcination at 813 K, the d-value was 3.6 nm, the surface area 200 m2/g, and the mean pore diameter estimated by the BJH method 2.2 nm. Extended X-ray absorption fine structure analysis suggests Zr to be in a short-range structure (<4 Å) similar to that of Zr in monoclinic ZrO2. Scanning electron microscopy including energy dispersive X-ray analysis showed 1-5 m sulfur-containing ZrO2 spheres. The material catalyzes the isomerization of n-butane to i-butane at 378 K with a steady activity in the order of magnitude of commercial sulfated ZrO2. 相似文献
9.
Barbara S. Klose-Schubert Rolf E. Jentoft Friederike C. Jentoft 《Topics in Catalysis》2011,54(5-7):398-414
The stability of a series of sulfated zirconia catalysts, promoted with up to 2 wt% iron or manganese, in their calcined state was investigated. Phase composition, nature of surface sulfate species, degree of hydroxylation, and butane isomerization activity changed during aging over months in various atmospheres and during milling. The metastability of small oxide particles is discussed, including literature data on alumina, titania and other oxides. Catalytically active fractions of a material easily transition into more stable, less active forms. 相似文献
10.
Sabine Wrabetz Xiaobo Yang Genka Tzolova-Müller Robert Schlögl Friederike C. Jentoft 《Journal of Catalysis》2010,269(2):351-358
A method to characterize the surface sites of catalysts in their active state by adsorption microcalorimetry was developed. A calorimeter cell was used as a flow-type reactor, and the skeletal isomerization of n-butane (1 kPa) at 378 K and atmospheric pressure proceeded at comparable rates and with the same states of induction period, maximum and deactivation phase as in a tubular reactor. The reaction was run for selected times on stream and after the removal of weakly adsorbed species, n-butane or isobutane were adsorbed at 313 K. The surface of activated sulfated zirconia was characterized by at least two different sites for n-butane adsorption, a small group of sites (about 20 μmol g?1) that yielded heats of 50–60 kJ mol?1 and sites that were populated at higher pressures (above about 5 hPa n-butane) and yielded heats of about 40 kJ mol?1. The strongly interacting sites disappear during the induction period and are proposed to be the sites where the isomerization reaction is initiated. 相似文献