C. Muryn, D. Purdie, P. J. Hardman, Allen L. Johnson, N. S. Prakash, G. N. Raiker, G. Thornton, D. Law
Surface EXAFS, NEXAFS and photoemission have been used to study the structure of surface phases resulting from reaction of H2S with NiO(100) and SO2 with TiO2(110). S K-edge SEXAFS data, in conjunction with the LEED pattern symmetry, suggest that the NiO(100)/H2S reaction at 570 K results in reduction of the substrate to form an azimuthally aligned Ni(100)c(2 × 2)S raft, with S occupying the fourfold Ni(100) hollow site. S K-edge NEXAFS results for SO2 adsorption on low-temperature TiO2(110) identify two surface SOx species: chemisorbed SO2 and SO2–4. The polarisation dependence of the NEXAFS suggests that the SO2 molecular plane lies close to parallel to the surface. Photoemission data also evidence chemisorption at 110 K and show that this precursor phase reacts further in a thermally activated process to form a stable surface sulphate-like species. A mechanism is suggested which involves SO2 bonding to a terrace Ti site, with an activated hop to an adjacent site in which sulphur bonds to two oxygen atoms on the raised row.
{"title":"Sulphur-induced structural chemistry of oxide surfaces","authors":"C. Muryn, D. Purdie, P. J. Hardman, Allen L. Johnson, N. S. Prakash, G. N. Raiker, G. Thornton, D. Law","doi":"10.1039/DC9908900077","DOIUrl":"https://doi.org/10.1039/DC9908900077","url":null,"abstract":"Surface EXAFS, NEXAFS and photoemission have been used to study the structure of surface phases resulting from reaction of H2S with NiO(100) and SO2 with TiO2(110). S K-edge SEXAFS data, in conjunction with the LEED pattern symmetry, suggest that the NiO(100)/H2S reaction at 570 K results in reduction of the substrate to form an azimuthally aligned Ni(100)c(2 × 2)S raft, with S occupying the fourfold Ni(100) hollow site. S K-edge NEXAFS results for SO2 adsorption on low-temperature TiO2(110) identify two surface SOx species: chemisorbed SO2 and SO2–4. The polarisation dependence of the NEXAFS suggests that the SO2 molecular plane lies close to parallel to the surface. Photoemission data also evidence chemisorption at 110 K and show that this precursor phase reacts further in a thermally activated process to form a stable surface sulphate-like species. A mechanism is suggested which involves SO2 bonding to a terrace Ti site, with an activated hop to an adjacent site in which sulphur bonds to two oxygen atoms on the raised row.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"31 1","pages":"77-89"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81227384","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The use of specular reflection of X-rays to study the structure of the liquid/vapour interfaces along the direction normal to the surface is described. If RF(θ) is the theoretical Fresnel reflection law for X-rays incident on an ideal flat surface at an angle θ, and R(θ) is the measured reflectivity from the true surface, the ratio R(θ)/RF(θ) is a measure of the electron density along the surface normal; i.e.R(θ)//RF(θ)≈|1//p∞∫∂〈p(z)〉//∂z exp (iQzz) dz|2. where p∞ is the electron density far from the surface, ∂〈p(z)〉/∂z is the gradient of the average electron density along the surface normal and Qz=(4π/λ) sin (θ). For simple liquids p–1∞∂〈p〉/∂z≈[1/√(2πσ2)] exp (–z2/2σ2), and R(θ)/RF(θ)≈ exp (–Q2σ2), where σ2 is dominated by the mean-square average of thermally excited fluctuations in the height of the surface. For liquid crystals and for lyotropic miceller systems temperature-dependent structure in R(θ) is due to surface-induced layering in 〈p(z)〉. Other experimental results from thin layers of liquid 4He and monolayers, of amphiphathic molecules on the surface of H2O will be described. The possibility of complementing specular reflectivity measurements of surface roughness by studying diffuse scattering at small angles off of the specular condition will also be illustrated with results from the H2O surface.
{"title":"Structure of surfaces and interfaces as studied using synchrotron radiation. Liquid surfaces","authors":"P. Pershan","doi":"10.1039/DC9908900231","DOIUrl":"https://doi.org/10.1039/DC9908900231","url":null,"abstract":"The use of specular reflection of X-rays to study the structure of the liquid/vapour interfaces along the direction normal to the surface is described. If RF(θ) is the theoretical Fresnel reflection law for X-rays incident on an ideal flat surface at an angle θ, and R(θ) is the measured reflectivity from the true surface, the ratio R(θ)/RF(θ) is a measure of the electron density along the surface normal; i.e.R(θ)//RF(θ)≈|1//p∞∫∂〈p(z)〉//∂z exp (iQzz) dz|2. where p∞ is the electron density far from the surface, ∂〈p(z)〉/∂z is the gradient of the average electron density along the surface normal and Qz=(4π/λ) sin (θ). For simple liquids p–1∞∂〈p〉/∂z≈[1/√(2πσ2)] exp (–z2/2σ2), and R(θ)/RF(θ)≈ exp (–Q2σ2), where σ2 is dominated by the mean-square average of thermally excited fluctuations in the height of the surface. For liquid crystals and for lyotropic miceller systems temperature-dependent structure in R(θ) is due to surface-induced layering in 〈p(z)〉. Other experimental results from thin layers of liquid 4He and monolayers, of amphiphathic molecules on the surface of H2O will be described. The possibility of complementing specular reflectivity measurements of surface roughness by studying diffuse scattering at small angles off of the specular condition will also be illustrated with results from the H2O surface.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"158 1","pages":"231-245"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"86358816","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
P. Johnston, R. Joyner, P. Pudney, E. S. Shpiro, B. Williams
Rhodium catalysts have been prepared, supported on γ-alumina, vanadium(III) oxide, chromia and molybdena. The activity and selectivity of these materials in the conversion of synthesis gas, (H2/CO: 1/1, 523 K and 5 bar pressure), to methanol, ethanol and hydrocarbons has been studied. All catalysts showed similar activity except for chromia, which was almost inactive at the temperature chosen. Selectivity to higher oxygenates followed the trend: V2O3 > MoO3 > Al2O3 > Cr2O3. The alumina supported catalysts had the highest selectivity to methanol. Catalysts have been characterised in situ by extended X-ray absorption fine structure (EXAFS) and the alumina-supported material has been examined in the greatest detail. On reduction at 473 K, rhodium particles with diameter 30 A diameter, on the chromia support.The relation between catalyst structure and performance is discussed.
{"title":"In situ studies of supported rhodium catalysts","authors":"P. Johnston, R. Joyner, P. Pudney, E. S. Shpiro, B. Williams","doi":"10.1039/DC9908900091","DOIUrl":"https://doi.org/10.1039/DC9908900091","url":null,"abstract":"Rhodium catalysts have been prepared, supported on γ-alumina, vanadium(III) oxide, chromia and molybdena. The activity and selectivity of these materials in the conversion of synthesis gas, (H2/CO: 1/1, 523 K and 5 bar pressure), to methanol, ethanol and hydrocarbons has been studied. All catalysts showed similar activity except for chromia, which was almost inactive at the temperature chosen. Selectivity to higher oxygenates followed the trend: V2O3 > MoO3 > Al2O3 > Cr2O3. The alumina supported catalysts had the highest selectivity to methanol. Catalysts have been characterised in situ by extended X-ray absorption fine structure (EXAFS) and the alumina-supported material has been examined in the greatest detail. On reduction at 473 K, rhodium particles with diameter 30 A diameter, on the chromia support.The relation between catalyst structure and performance is discussed.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"41 1","pages":"91-105"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89831305","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
An X-ray absorption spectroscopy (XAS) cell has been developed to allow the loading of air-sensitive catalyst precursors in a glove box and their in situ activation and operation to be monitored. This has been used to investigate two alumina-supported propene metathesis catalysts derived from Mo2(OAc)4 and Li4[Mo2Me8]. The EXAFS data so derived has been analysed by spherical wave methods using ab initio phase shifts and back-scattering factors. The detailed structural features obtained are sample-history dependent, but the following general observations can be drawn about the mean surface species present. In all cases the quadruple Mo—Mo bond is cleaved on the surface; some samples show evidence of one (or fewer) metal atoms at longer distances. For the latter sample activation in vacuo at 70°C yielded sites approximating to [MoO2(Me)2(O-)]n. The addition of propene caused a change in the XAS spectra. There was evidence of a shell with MoC distance of ca 1.8 A that can be attributed to a terminal carbene ligand. For one of the Mo2(OAc)4-derived catalysts there was also evidence for a molybdacyclobutane unit. The Li4[Mo2Me8]-based materials also showed evidence of residual MoO bonds after exposure to propene. The Zr K-edge EXAFS of Zr(allyl)4/oxide (oxide = silica and alumina) were very similar for a range of sample conditions, showing evidence for a residual allyl group and Zr-O bonds.
{"title":"Characterisation of oxide-supported alkene conversion catalysts using X-ray absorption spectroscopy","authors":"J. Evans, J. Gauntlett, J. Mosselmans","doi":"10.1039/DC9908900107","DOIUrl":"https://doi.org/10.1039/DC9908900107","url":null,"abstract":"An X-ray absorption spectroscopy (XAS) cell has been developed to allow the loading of air-sensitive catalyst precursors in a glove box and their in situ activation and operation to be monitored. This has been used to investigate two alumina-supported propene metathesis catalysts derived from Mo2(OAc)4 and Li4[Mo2Me8]. The EXAFS data so derived has been analysed by spherical wave methods using ab initio phase shifts and back-scattering factors. The detailed structural features obtained are sample-history dependent, but the following general observations can be drawn about the mean surface species present. In all cases the quadruple Mo—Mo bond is cleaved on the surface; some samples show evidence of one (or fewer) metal atoms at longer distances. For the latter sample activation in vacuo at 70°C yielded sites approximating to [MoO2(Me)2(O-)]n. The addition of propene caused a change in the XAS spectra. There was evidence of a shell with MoC distance of ca 1.8 A that can be attributed to a terminal carbene ligand. For one of the Mo2(OAc)4-derived catalysts there was also evidence for a molybdacyclobutane unit. The Li4[Mo2Me8]-based materials also showed evidence of residual MoO bonds after exposure to propene. The Zr K-edge EXAFS of Zr(allyl)4/oxide (oxide = silica and alumina) were very similar for a range of sample conditions, showing evidence for a residual allyl group and Zr-O bonds.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"39 1","pages":"107-117"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89974133","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A water-soluble carbodiimide has been used to end-graft aminated poly (ethylene oxide)(PEO) chemically onto colloidal polystyrene particles. Two particle sizes (115 and 347 nm diameter) and two PEO molecular weights (112 000 and 615 000 g mol–1) were combined to give suspensions with four different ratios of polymer layer thickness to particle radius. Electrophoresis demonstrated that the PEO was grafted, not just adsorbed. Dynamic light scattering showed that the adsorbed and grafted layers had similar structures and that non-ionic surfactant perturbed the PEO configurations. Steady shear and oscillatory rheometry indicated that long-ranged polymeric forces between particles governed the variation of viscosity and storage modulus with applied stress and PS volume fraction. Hard-sphere and effective hard-sphere scaling helps rationalize the rheological behaviour in terms of the variation of the polymeric force among the different suspensions and hydrodynamic deformation of the polymer layers.
用水溶性碳二亚胺将胺化聚环氧乙烷(PEO)端接枝到胶体聚苯乙烯颗粒上。两种粒径(直径115和347 nm)和两种PEO分子量(112,000和615,000 g mol-1)相结合，得到四种不同聚合物层厚度与颗粒半径比的悬浮液。电泳表明PEO是接枝的，而不仅仅是吸附的。动态光散射表明，吸附层和接枝层具有相似的结构，非离子表面活性剂干扰了PEO的构型。稳态剪切和振荡流变表明，颗粒间的长期聚合力决定了黏度和储存模量随外加应力和PS体积分数的变化。硬球和有效的硬球结垢有助于根据不同悬浮液之间聚合物力的变化和聚合物层的水动力变形来合理化流变行为。
{"title":"Rheology of aqueous suspensions of polystyrene latex stabilized by grafted poly(ethylene oxide)","authors":"H. Ploehn, J. W. Goodwin","doi":"10.1039/DC9909000077","DOIUrl":"https://doi.org/10.1039/DC9909000077","url":null,"abstract":"A water-soluble carbodiimide has been used to end-graft aminated poly (ethylene oxide)(PEO) chemically onto colloidal polystyrene particles. Two particle sizes (115 and 347 nm diameter) and two PEO molecular weights (112 000 and 615 000 g mol–1) were combined to give suspensions with four different ratios of polymer layer thickness to particle radius. Electrophoresis demonstrated that the PEO was grafted, not just adsorbed. Dynamic light scattering showed that the adsorbed and grafted layers had similar structures and that non-ionic surfactant perturbed the PEO configurations. Steady shear and oscillatory rheometry indicated that long-ranged polymeric forces between particles governed the variation of viscosity and storage modulus with applied stress and PS volume fraction. Hard-sphere and effective hard-sphere scaling helps rationalize the rheological behaviour in terms of the variation of the polymeric force among the different suspensions and hydrodynamic deformation of the polymer layers.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"57 1","pages":"77-90"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"90958488","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The use of viscoelastic measurements for studying stability/flocculation of concentrated dispersions is discussed. With electrostatically stabilised dispersions, the system becomes predominantly elastic when significant double-layer overlap occurs. This was demonstrated using polystyrene latex dispersions in 10–5 and 10–3 mol dm–3 NaCl. Plots of the elastic modulus G′versus surface-to-surface separation distance h showed a rapid increase when h was less than twice the double-layer thickness. The experimental G′ values were compared with theoretical values calculated from the second differential of the interaction energy vs. distance relationship. With sterically stabilised latex dispersion [containing grafted poly(ethylene oxide) chains], predominantly elastic response was also obtained when h became less than twice the adsorbed layer thickness (2δ). The G′vs. volume fraction curves were converted to G′vs. h and this was compared with the values of G′ calculated from direct force, F, vs. distance curves. In both cases there was a rapid increase in G′ with decrease in h when h < 2δ.Viscoelastic measurements could also be applied for flocculated dispersions. With weakly flocculated systems such as those obtained by addition of a free non-adsorbing polymer (depletion flocculation), the systems showed pronounced non-Newtonian behaviour above a critical volume fraction of free polymer (ϕ+P). The occurrence of this behaviour decreased with increasing molecular weight of the free polymer. The extrapolated yield stress was used to calculate the energy of separation between particles, Esep, in the flocculated dispersion. Esep was compared with the theoretical value of the free energy of depletion, Gdep, that was calculated using available theories. With strongly flocculated dispersions, scaling concepts could be applied and the power exponent in ϕ(G′∼ϕn) could be used as a measure of the strength of flocculation. From G′ and the critical strain γc, above which non-linear response is obtained, the cohesive energy of the flocculated structure, Ec, was calculated. log–log plots of Ecversus ϕ were used to obtain the power exponent in ϕ and this should give a measure of the strength of flocculation.
{"title":"Use of viscoelastic measurements for investigating the stability/flocculation of concentrated dispersions","authors":"T. Tadros, A. Hopkinson","doi":"10.1039/DC9909000041","DOIUrl":"https://doi.org/10.1039/DC9909000041","url":null,"abstract":"The use of viscoelastic measurements for studying stability/flocculation of concentrated dispersions is discussed. With electrostatically stabilised dispersions, the system becomes predominantly elastic when significant double-layer overlap occurs. This was demonstrated using polystyrene latex dispersions in 10–5 and 10–3 mol dm–3 NaCl. Plots of the elastic modulus G′versus surface-to-surface separation distance h showed a rapid increase when h was less than twice the double-layer thickness. The experimental G′ values were compared with theoretical values calculated from the second differential of the interaction energy vs. distance relationship. With sterically stabilised latex dispersion [containing grafted poly(ethylene oxide) chains], predominantly elastic response was also obtained when h became less than twice the adsorbed layer thickness (2δ). The G′vs. volume fraction curves were converted to G′vs. h and this was compared with the values of G′ calculated from direct force, F, vs. distance curves. In both cases there was a rapid increase in G′ with decrease in h when h < 2δ.Viscoelastic measurements could also be applied for flocculated dispersions. With weakly flocculated systems such as those obtained by addition of a free non-adsorbing polymer (depletion flocculation), the systems showed pronounced non-Newtonian behaviour above a critical volume fraction of free polymer (ϕ+P). The occurrence of this behaviour decreased with increasing molecular weight of the free polymer. The extrapolated yield stress was used to calculate the energy of separation between particles, Esep, in the flocculated dispersion. Esep was compared with the theoretical value of the free energy of depletion, Gdep, that was calculated using available theories. With strongly flocculated dispersions, scaling concepts could be applied and the power exponent in ϕ(G′∼ϕn) could be used as a measure of the strength of flocculation. From G′ and the critical strain γc, above which non-linear response is obtained, the cohesive energy of the flocculated structure, Ec, was calculated. log–log plots of Ecversus ϕ were used to obtain the power exponent in ϕ and this should give a measure of the strength of flocculation.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"26 1","pages":"41-55"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80723351","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The combination of glancing angle X-ray absorption fine structure (XAFS) and X-ray reflectivity measurements provide detailed structural information about interfaces. Results from transition-metal/Al interfaces are used to illustrate the capabilities of the techniques. Reflectivity measurements are used to determine the thickness and density profile of the films, along with the amount of interfacial or surface roughness. The XAFS measurements provide information about the local bonding at the interface, and are well suited for studying the interfacial chemistry. Of interest is the role of impurities such as oxygen in modifying interface structure and thermal stability. For Cu/Al, Ni/Al and Cr/Al a small amount of interfacial oxygen is found to affect strongly both the initial interface structure, and the response of the interface to thermal annealing. For Ni/Al it is also found that interfacial oxygen affects primarily the interface reaction, while oxygen within the Al layer suppresses the grain boundary reaction channel.
{"title":"Glancing angle XAFS and X-ray reflectivity studies of transition-metal/aluminium interfaces","authors":"S. Heald, E. Barrera, H. Chen","doi":"10.1039/DC9908900021","DOIUrl":"https://doi.org/10.1039/DC9908900021","url":null,"abstract":"The combination of glancing angle X-ray absorption fine structure (XAFS) and X-ray reflectivity measurements provide detailed structural information about interfaces. Results from transition-metal/Al interfaces are used to illustrate the capabilities of the techniques. Reflectivity measurements are used to determine the thickness and density profile of the films, along with the amount of interfacial or surface roughness. The XAFS measurements provide information about the local bonding at the interface, and are well suited for studying the interfacial chemistry. Of interest is the role of impurities such as oxygen in modifying interface structure and thermal stability. For Cu/Al, Ni/Al and Cr/Al a small amount of interfacial oxygen is found to affect strongly both the initial interface structure, and the response of the interface to thermal annealing. For Ni/Al it is also found that interfacial oxygen affects primarily the interface reaction, while oxygen within the Al layer suppresses the grain boundary reaction channel.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"4 1","pages":"21-30"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76006251","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Desorption of hydrogen at high temperature leads to a contraction of the Ir–Ir coordination distance of highly dispersed iridium metal particles. The long metal–support oxygen distance (2.55 A) observed if the metal crystallites are covered with chemisorbed hydrogen disappears after treatment in vacuum at 623 K. Instead, a metal–oxygen coordination in the metal–support interface is now detected with a distance of 2.19 A. It is concluded that the long metal-oxygen distances found for oxide–supported metal catalysts after reduction in H2 originate from an M0–(OH)– interaction or from the presence of chemisorbed hydrogen in the metal–support interface. Admission of O2 at 77 K does not lead to a corrosive oxidation of the metal particles. The adsorption of O2 resulted in the same type of Ir—O bonds as present in the metal–support after evacuation.
{"title":"EXAFS study of the influence of hydrogen desorption and oxygen adsorption on the structural properties of small iridium particles supported on Al2O3","authors":"F. Kampers, D. Koningsberger","doi":"10.1039/DC9908900137","DOIUrl":"https://doi.org/10.1039/DC9908900137","url":null,"abstract":"Desorption of hydrogen at high temperature leads to a contraction of the Ir–Ir coordination distance of highly dispersed iridium metal particles. The long metal–support oxygen distance (2.55 A) observed if the metal crystallites are covered with chemisorbed hydrogen disappears after treatment in vacuum at 623 K. Instead, a metal–oxygen coordination in the metal–support interface is now detected with a distance of 2.19 A. It is concluded that the long metal-oxygen distances found for oxide–supported metal catalysts after reduction in H2 originate from an M0–(OH)– interaction or from the presence of chemisorbed hydrogen in the metal–support interface. Admission of O2 at 77 K does not lead to a corrosive oxidation of the metal particles. The adsorption of O2 resulted in the same type of Ir—O bonds as present in the metal–support after evacuation.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"26 1","pages":"137-141"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"75348338","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Total internal reflection microscopy has been used to monitor the submicroscopic distance separating a microscopic sphere and a transparent plate. Light scattered from an evanescent wave by a single sphere is exponentially sensitive to the elevation of the sphere above the plate. Changes in separation as small as 1 nm are detectable. The technique is virtually instantaneous, non-intrusive, and easily automated. Two applications are discussed: from an equilibrium distribution of separations, a direct measure of the mean potential of surface forces acting on the sphere can be obtained; from an autocorrelation of the separation as a function of time, the sphere's diffusion coefficient, D, can be obtained. Forces as small as 10–13 N can be accurately measured. Owing to its close proximity to the wall, a 10 µm polystyrene sphere in 0.5 mmol dm–3 NaCl has a value of D of ca. 2% of the bulk value. The fractional reduction in D suggests that the sphere resides ca. 100 nm above the glass plate.
{"title":"Brownian motion of a single microscopic sphere in a colloidal force field","authors":"D. Prieve, S. Bike, Nasser A. Frej","doi":"10.1039/DC9909000209","DOIUrl":"https://doi.org/10.1039/DC9909000209","url":null,"abstract":"Total internal reflection microscopy has been used to monitor the submicroscopic distance separating a microscopic sphere and a transparent plate. Light scattered from an evanescent wave by a single sphere is exponentially sensitive to the elevation of the sphere above the plate. Changes in separation as small as 1 nm are detectable. The technique is virtually instantaneous, non-intrusive, and easily automated. Two applications are discussed: from an equilibrium distribution of separations, a direct measure of the mean potential of surface forces acting on the sphere can be obtained; from an autocorrelation of the separation as a function of time, the sphere's diffusion coefficient, D, can be obtained. Forces as small as 10–13 N can be accurately measured. Owing to its close proximity to the wall, a 10 µm polystyrene sphere in 0.5 mmol dm–3 NaCl has a value of D of ca. 2% of the bulk value. The fractional reduction in D suggests that the sphere resides ca. 100 nm above the glass plate.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"23 18 1","pages":"209-222"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"91258860","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Mechanisms of nucleation and growth of uniform micrometre and submicrometre inorganic particles are discussed. Precipitation reactions involving the hydrolysis and condensation of silicon and titanium alkoxides are studied and evidence is presented that the short nucleation time model of LaMer fails to describe the unniformity of the resulting precipitates. Drawing on analogies with emulsion and dispersion polymerization, a model is postulated where aggregation plays a central role in determining final number densities. Results of numerical calculations where growth is assumed to occur solely by aggregation are described.
{"title":"Colloidal interactions during the precipitation of uniform submicrometre particles","authors":"J. Look, G. Bogush, C. Zukoski","doi":"10.1039/DC9909000345","DOIUrl":"https://doi.org/10.1039/DC9909000345","url":null,"abstract":"Mechanisms of nucleation and growth of uniform micrometre and submicrometre inorganic particles are discussed. Precipitation reactions involving the hydrolysis and condensation of silicon and titanium alkoxides are studied and evidence is presented that the short nucleation time model of LaMer fails to describe the unniformity of the resulting precipitates. Drawing on analogies with emulsion and dispersion polymerization, a model is postulated where aggregation plays a central role in determining final number densities. Results of numerical calculations where growth is assumed to occur solely by aggregation are described.","PeriodicalId":12210,"journal":{"name":"Faraday Discussions of The Chemical Society","volume":"1 1","pages":"345-357"},"PeriodicalIF":0.0,"publicationDate":"1990-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83228297","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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