1,720,985 research outputs found
The synthesis, modification and characterisation of metal incorporated H1 SIO2 mesoporous materials
No full textTwo approaches have been utilised for the modification of Hi SiOi mesoporous materials in order to produce well-defined active sites for catalysis: (i) the incorporation of transition metal salts into the synthesis gel in order to form Hi M/Si02 materials and (ii) the post- synthesis modification with Pt(acac)2 and various germanium additives (i.e. GeBu4, GeBugH, GePh4) and their subsequent decomposition/reduction to 673K under 10% hydrogen in nitrogen or pure nitrogen. The decomposition/reduction of Pt(acac)2 supported on amorphous silica and alumina has also been examined. The first part of this thesis describes the synthesis and characterisation of Hi M/Si02 (M = V, Cr, Fe, Co, Zn, Ga, Ge, Sn). It has been shown that the incorporation of metal sites via this methodology does not typically lead to direct incorporation into the silicate framework. However, the chromium incorporated material contains chromium as discrete species, with no formation of any oxide. Furthermore, characterisation suggests that the germanium has been successfully incorporated into the silicate 6amework and a new one-pot methodology for the incorporation of metal sites (i.e. Sn) has also been developed. A new methodology for the in situ study of heterogeneous systems utilising Energy Dispersive EXAFS (EDE) has been used to investigate the decomposition/reduction of Pt(acac)2 on various supports. Characterisation has shown that support choice has little influence on the 10% H2 in N2 reduction procedure, although the desorption of carbonaceous fragments was influenced, with the temperature of complete carbon removal shown to be Hi Si02<Si02-Al203. However, decomposition of supported Pt(acac)2 in a N2 atmosphere was shown to be influenced by support choice and follows a two stage decomposition mechanism, reducing over a broader temperature range (80K) and forming naked platinum particles at a much higher temperature (ca. 546K). The choice of support is observed to play an important role in defining platinum particle morphology. Most interestingly, the formation of platinum clusters, morphologically defined by the size and shape of the mesopores of the Hi Si02 material, show novel reactivity towards CO with the simultaneous appearance of three IR bands that are related to the terminal adsorption on CO. The incorporation of GeBu4 into the Pt(acac)2/Hi Si02 system leads to a significant modification in the H2/N2 reduction mechanism, with the EDE and TPR both indicating an increase in the reduction temperature range (ca. lOOK) and an increase in the fmal temperature in platinum particle formation (ca. 60K). At higher temperatures, the presence of germanium-platinum interactions indicates alloy formation. Ex situ characterisation of this system shows that Ge incorporation leads to significant modification of the platinum clusters with the formation of Pt-Ge alloys. The DRIFTS spectrum aAer CO adsorption exhibits only two bands, indicating that the Pt sites analogous to flat platinum surfaces are blocked, and the other platinum adsorption sites, proposed to relate to the high aspect ratio clusters, are unaffected by incorporation of GeBu4. However, further ex situ studies conducted using other germanium additives show that the choice of additive has signiGcant consequences on the reduction products. GeBusH influences only the high aspect ratio clusters, GePh4 impedes all CO adsorption by the formation of large arrays of Pt-Ge alloys (GePts, Ge2Pt3) and prior incorporation of the germanium into the mesoporous material appears to have no significant inGuence on the reduction products at all.</p
Combining diffuse reflectance infrared spectroscopy (DRIFTS), dispersive EXAFS, and mass spectrometry with high time resolution: Potential, limitations, and application to the study of NO interaction with supported Rh catalysts
No full textWe describe a new experiment that combines transmission based structural probes, such as dispersive EXAFS (EDE), with diffuse reflectance infrared spectroscopy (DRIFTS) and mass spectrometry (MS), at high time resolution. The potential and limitations of this experiment are discussed, and an example of its application to the study of fundamental steps occurring during gas–solid interactions is given; that of oxidation and reduction of alumina supported Rh at 573 K using NO and H2, and the structural-reactive role of linear (Rh(NO+)) Rh-nitrosyl species within these processes
The impact of phase changes, alloying and segregation in supported RhPd catalysts during selective NO reduction by H<sub>2</sub>
No full textIlluminating complexity: Time resolved, microreactor-based, EDE/mass spectrometry (EDE = energy dispersive extended X-ray absorption fine structure) was used to probe the behaviour of both Rh and Pd components of alloyed nanoparticles during NO reduction by H2. The surface enrichment in Pd is shown to curtail the rapid oxidation of the Rh component by NO and leads to a promotion of NO reduction
Structure-performance relationships of Rh and RhPd alloy supported catalysts using EDE/DRIFTS/MS
No full textEnergy dispersive extended X-ray absorption fine structure spectroscopy (ED-XAFS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry (MS), have been combined for the structure–function study of Rh and RhPd supported catalysts for the reduction of NO by CO. The combined results show that although alloying of Rh with Pd prevents the dissociative oxidation of the Rh by NO, it does not prevent the extensive disruptive oxidation of Rh by CO. The influence of oxidative disruption by molecular CO in such systems may therefore be far more pervasive and catalytically important than has been previously observed. The overall metal particle size observed in the RhPd alloy system during the CO/NO reaction is significantly larger than for the Rh-onlysystem for the entire temperature range employed. The catalytically active sites, however, are likely to be similar, with the overall activity of the alloy system to be reduced due to inactive RhPd alloy nanoparticles
Particle size effects in Rh/Al2O3 catalysts as viewed from a structural, functional, and reactive perspective: the case of the reactive adsorption of NO
No full textThe structural-dynamic behaviour of ?-Al2O3 supported Rh nanoparticles under He, H2/He, and NO/He has been investigated using a newly developed methodology that permits dispersive EXAFS (EDE), diffuse reflectance infra red spectroscopy (DRIFTS), and mass spectrometry (MS) to be applied simultaneously to the study of gas-solid interactions. This reveals a considerably variability in nanoparticle habit (for 11 Å diameter nanoparticles as a function of temperature), and between 8 Å and 11 Å particles in their response to NO. The selectivity (N2/(N2 + N2O)) of the reactive interaction between NO and the supported Rh shows essentially no particle size dependence above 473 K: it is apparent, however, that considerable differences in some aspects of the structural behaviour of the 8 Å and 11 Å Rh particles do nonetheless, exist. At 373 < T < 473 K a clear divergence in structural, functional, and reactive response of the different sized supported Rh nanoparticles toward NO is observed. These observations are discussed in terms of the ability of different sized Rh particles to change structure in response to the reactive environment, the subsequent effect this has on the nitrosyl functionality that different phases may support, and the reactive pathways for NO conversion that may therefore arise. <br/
Synchronous, time resolved, diffuse reflectance FT-IR, energy dispersive EXAFS (EDE) and mass spectrometric investigation of the behaviour of Rh catalysts during NO reduction by CO
No full textSynchronous, time resolved, infra-red, XAFS, and mass spectroscopies are simultaneously applied in situ to the investigation of the dynamic behaviour of Rh/Al2O3 catalysts during NO reduction by CO; NO conversion, and its kinetic character are closely correlated to the conversion of Rh(I) (predominantly Rh-I(CO)(2)) to Rh(0)
Oxidation/reduction kinetics of supported Rh/Rh2O3 nanoparticles in plug flow conditions using dispersive EXAFS
No full textThe kinetics of oxidation and reduction of Al2O3 supported Rh nanoparticles have been determined on a 50 millisecond timescale using energy dispersive EXAFS (EDE)
Identification of the surface species responsible for N2O formation from the chemisorption of NO on Rh/alumina
No full textEnergy dispersive EXAFS (EDE) and diffuse reflectance infrared spectroscopy (DRIFTS) are combined synchronously at high time resolution (17 Hz) to probe how NO(g) reacts with gamma-Al2O3 supported, metallic Rh nanoparticles of an average 11 A diameter; a bent nitrosyl species is considered to be the key to the formation of N2O
Extended X-ray absorption fine structure (EXAFS) characterisation of the hydroformylation of oct-1-ene by dilute Rh-PEt3 catalysts in supercritical carbon dioxide
No full textDilute EXAFS characterisation has been used to elucidate species involved during the course of the 3 mM Rh-catalysed hydroformylation of oct-1-ene in ScCO2; significant metal clustering occurs with a Rh:P ratio of 1:1 but at a 1:3 ratio, metal clustering is not detected, with the presence of monomer species only
Rapid phase fluxionality as the determining factor in activity and selectivity of highly dispersed, Rh/Al2O3 in deNO(x) catalysis
No full textThe nature of oxide-supported metal catalysts may change in oxidising conditions: Studies on the correlation between the Rh phase in the structure of the Rh/AL2O3 catalyst and the catalytic performance for the reduction of NO by H2 to N2 (on reduced, metallic sites) and N2O (on oxidised sites) reveal that the phases of the supported metal species can be interconverted on time scales that can be deterministic in temrs of the activity and selectivity of the catalysts
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