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Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
High Pressure Scanning Tunneling Microscopy (HP-STM) and Ambient Pressure X-ray Photoelectron Spectroscopy were used to study the structural properties and catalytic behavior of noble metal surfaces at high pressure. HP-STM was used to study the structural rearrangement of the top most atomic surface layer of the metal surfaces in response to changes in gas pressure and reactive conditions. AP-XPS was applied to single crystal and nanoparticle systems to monitor changes in the chemical composition of the surface layer in response to changing gas conditions. STM studies on the Pt(100) crystal face showed the lifting of the Pt(100)-hex surface reconstruction in the presence of CO, H2, and Benzene. The gas adsorption and subsequent charge transfer relieves the surface strain caused by the low coordination number of the (100) surface atoms allowing the formation of a (1 x 1) surface structure commensurate with the bulk terminated crystal structure. The surface phase change causes a transformation of the surface layer from hexagonal packing geometry to a four-fold symmetric surface which is rich in atomic defects. Lifting the hex reconstruction at room temperature resulted in a surface structure decorated with 2-3 nm Pt adatom islands with a high density of step edge sites. Annealing the surface at a modest temperature (150 C) in the presence of a high pressure of CO or H2 increased the surface diffusion of the Pt atoms causing the adatom islands to aggregate reducing the surface concentration of low coordination defect sites. Ethylene hydrogenation was studied on the Pt(100) surface using HP-STM. At low pressure, the lifting of the hex reconstruction was observed in the STM images. Increasing the ethylene pressure to 1 Torr, was found to regenerate the hexagonally symmetric reconstructed phase. At room temperature ethylene undergoes a structural rearrangement to form ethylidyne. Ethylidyne preferentially binds at the three-fold hollow sites, which are present on the Pt(100) hex reconstructed phase, but not the (100)-(1x1) surface. The increase in ethylene pressure caused the adsorbate interactions to dominate the crystal morphology and imposed a surface layer structure that matched the ethylidyne binding geometry. The STM results also showed that the surface was reversibly deformed during imaging due to increases in Pt mobility at high pressure. The size dependence on the activity and surface chemistry of Rh nanoparticles was studied using AP-XPS. The activity was found to increase with particle size. The XPS spectra show that in reaction conditions the particle surface has an oxide layer which is chemically distinct from the surface structure formed by heating in oxygen alone. This surface oxide which is stabilized in the catalytically active CO oxidation conditions was found to be more prevalent on the smaller nanoparticles. The reaction-induced surface segregation behavior of bimetallic noble metal nanoparticles was observed with APXPS. Monodisperse 15 nm RhPd and PdPt nanoparticles were synthesized with well controlled Rh/Pd and Pd/Pt compositions. In-situ XPS studies showed that at 300 C in the presence of an oxidizing environment (100 mTorr NO or O2) the surface concentration of the more easily oxidized element (Rh in RhPd and Pd in PdPt) was increased. Switching the gas environment to more reducing conditions (100 mTorr NO and 100 mTorr CO) caused the surface enrichment of the element with the lowest surface energy in its metallic state. Using in-situ characterization, the redox chemistry and the surface composition of bimetallic nanoparticle samples were monitored in reactive conditions. The particle surfaces were shown to reversibly restructure in response to the gas environment at high temperature. The oxidation behavior of the Pt(110) surface was studied using surface sensitive in-situ characterization by APXPS and STM. In the presence of 500 mTorr O2 and temperatures between 25 and 200 C, subsurface oxygen was detected in the surface layer. STM images show that these conditions were found to cause a roughened surface decorated with 1 nm islands. The formation of this surface oxide is a high pressure phenomenon and was not detected in 50 mTorr O2. After forming the surface oxide at high pressure, its chemical activity was measured through the reaction with CO at low pressure while continuously monitoring the oxygen species with XPS. The subsurface oxygen was removed by CO oxidation at a comparable rate to the chemisorbed oxygen at 2 C. Repeating the experiment at -3 C reduced the reaction rate, but not the relative activity of the two chemical species suggesting that neither species is significantly more active for the CO oxidation reaction. These studies use molecular level surface characterization in the presence of gases to show the structural changes induced by gas adsorption at high pressure.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
High Pressure Scanning Tunneling Microscopy (HP-STM) and Ambient Pressure X-ray Photoelectron Spectroscopy were used to study the structural properties and catalytic behavior of noble metal surfaces at high pressure. HP-STM was used to study the structural rearrangement of the top most atomic surface layer of the metal surfaces in response to changes in gas pressure and reactive conditions. AP-XPS was applied to single crystal and nanoparticle systems to monitor changes in the chemical composition of the surface layer in response to changing gas conditions. STM studies on the Pt(100) crystal face showed the lifting of the Pt(100)-hex surface reconstruction in the presence of CO, H2, and Benzene. The gas adsorption and subsequent charge transfer relieves the surface strain caused by the low coordination number of the (100) surface atoms allowing the formation of a (1 x 1) surface structure commensurate with the bulk terminated crystal structure. The surface phase change causes a transformation of the surface layer from hexagonal packing geometry to a four-fold symmetric surface which is rich in atomic defects. Lifting the hex reconstruction at room temperature resulted in a surface structure decorated with 2-3 nm Pt adatom islands with a high density of step edge sites. Annealing the surface at a modest temperature (150 C) in the presence of a high pressure of CO or H2 increased the surface diffusion of the Pt atoms causing the adatom islands to aggregate reducing the surface concentration of low coordination defect sites. Ethylene hydrogenation was studied on the Pt(100) surface using HP-STM. At low pressure, the lifting of the hex reconstruction was observed in the STM images. Increasing the ethylene pressure to 1 Torr, was found to regenerate the hexagonally symmetric reconstructed phase. At room temperature ethylene undergoes a structural rearrangement to form ethylidyne. Ethylidyne preferentially binds at the three-fold hollow sites, which are present on the Pt(100) hex reconstructed phase, but not the (100)-(1x1) surface. The increase in ethylene pressure caused the adsorbate interactions to dominate the crystal morphology and imposed a surface layer structure that matched the ethylidyne binding geometry. The STM results also showed that the surface was reversibly deformed during imaging due to increases in Pt mobility at high pressure. The size dependence on the activity and surface chemistry of Rh nanoparticles was studied using AP-XPS. The activity was found to increase with particle size. The XPS spectra show that in reaction conditions the particle surface has an oxide layer which is chemically distinct from the surface structure formed by heating in oxygen alone. This surface oxide which is stabilized in the catalytically active CO oxidation conditions was found to be more prevalent on the smaller nanoparticles. The reaction-induced surface segregation behavior of bimetallic noble metal nanoparticles was observed with APXPS. Monodisperse 15 nm RhPd and PdPt nanoparticles were synthesized with well controlled Rh/Pd and Pd/Pt compositions. In-situ XPS studies showed that at 300 C in the presence of an oxidizing environment (100 mTorr NO or O2) the surface concentration of the more easily oxidized element (Rh in RhPd and Pd in PdPt) was increased. Switching the gas environment to more reducing conditions (100 mTorr NO and 100 mTorr CO) caused the surface enrichment of the element with the lowest surface energy in its metallic state. Using in-situ characterization, the redox chemistry and the surface composition of bimetallic nanoparticle samples were monitored in reactive conditions. The particle surfaces were shown to reversibly restructure in response to the gas environment at high temperature. The oxidation behavior of the Pt(110) surface was studied using surface sensitive in-situ characterization by APXPS and STM. In the presence of 500 mTorr O2 and temperatures between 25 and 200 C, subsurface oxygen was detected in the surface layer. STM images show that these conditions were found to cause a roughened surface decorated with 1 nm islands. The formation of this surface oxide is a high pressure phenomenon and was not detected in 50 mTorr O2. After forming the surface oxide at high pressure, its chemical activity was measured through the reaction with CO at low pressure while continuously monitoring the oxygen species with XPS. The subsurface oxygen was removed by CO oxidation at a comparable rate to the chemisorbed oxygen at 2 C. Repeating the experiment at -3 C reduced the reaction rate, but not the relative activity of the two chemical species suggesting that neither species is significantly more active for the CO oxidation reaction. These studies use molecular level surface characterization in the presence of gases to show the structural changes induced by gas adsorption at high pressure.
Author: Zhongwei Zhu Publisher: ISBN: Category : Languages : en Pages : 133
Book Description
Surface structure, mobility, and composition of transition metal catalysts were studied by high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at high gas pressures. HP-STM makes it possible to determine the atomic or molecular rearrangement at catalyst surfaces, particularly at the low-coordinated active surface sites. AP-XPS monitors changes in elemental composition and chemical states of catalysts in response to variations in gas environments. Stepped Pt and Cu single crystals, the hexagonally reconstructed Pt(100) single crystal, and Pt-based bimetallic nanoparticles with controlled size, shape and composition, were employed as the model catalysts for experiments in this thesis. Surface reconstruction at low-coordinated step sites at high gas pressures was first explored on a stepped Pt(557) single crystal surface under O2. At 298 K, 1 Torr of O2 is able to create nanometer-sized clusters that are identified as surface Pt oxide by AP-XPS, which covers the entire Pt(557) surface. On the flat Pt(111) surface under 1 Torr of O2, Pt oxide clusters can form but are mostly accumulated within 2 nm from the steps. The hexagonal oxygen chemisorption pattern is observed on the terraces. At lower pressures such as 10−7 Torr, O2 only adsorbs at the step edges on Pt(557). The majority of the Pt oxide clusters disappear on both Pt(557) and Pt(111) surfaces after O2 is evacuated to the 10−8 Torr range. Quantitative XPS analysis with depth profiles indicates that the Pt oxide formed on Pt(557) is less than 0.6 nm thick and that the Pt oxide concentration at surface together with oxygen coverage varies reversibly with the O2 pressure. The disappearance of Pt oxide clusters upon O2 evacuation is ascribed to reactions of Pt oxide towards H2 and CO in the vacuum background gases. The structure and surface chemistry of the Pt(557) surface was therefore studied under H2-O2 and CO-O2 mixtures. After exposing Pt(557) to approximately 1 Torr of O2 to induce the formation of Pt oxide clusters, H2 was slowly added into the system. Both HP-STM and AP-XPS results show that the Pt oxide coverage decreases with the H2 partial pressure and that all the Pt oxide disappears at H2 partial pressures above 43 mTorr. Pt steps are restored with the removal of Pt oxide clusters. Water is produced in the gas-phase, which co-adsorbs with hydroxyl species on Pt(557). Detailed analysis shows that the consumption of surface Pt oxide is exclusively responsible for the decrease of oxygen coverage on Pt(557). In the coexistence of 1 Torr of CO and 1 Torr of O2, Pt oxide clusters are not observed like under the H2-O2 mixture. Instead, triangular Pt clusters and double-sized terraces induced by CO are observed. Influences of step configuration on the surface restructuring processes were studied on Pt(557) and Pt(332) that differ only in the step orientation. 500 mTorr of CO creates Pt clusters shaped as triangles and parallelograms on Pt(557) and Pt(332), respectively. When 500 mTorr of C2H4 was introduced afterwards, Pt clusters are removed on Pt(332) but preserved on Pt(557). The three-fold hollow sites at the (111) steps enable the Pt(332) surface to accommodate ethylidyne even covered by CO. As a result, kink Pt atoms at the cluster edges are driven to diffuse to form straight steps, so as to admit more ethylidyne at steps. In contrast, Pt(557) has (100) steps on which ethylidyne does not adsorb, therefore keeping the island structure after the introduction of C2H4. When 500 mTorr of C2H4 was added first into the high-pressure cell, a periodic pattern is resolved at step edges on Pt(332). In contrast, some bright species separated by more than 1 nm are observed on Pt(557). Further introducing 500 mTorr of CO does not facilitate the formation of Pt clusters. The structure and mobility under C2H4, H2, and CO were also studied on the Pt(100) surface, whose topmost layer is rearranged into a hexagonal overlayer in vacuum. Under 1 Torr of C2H4, the hexagonal reconstruction is preserved on Pt(100), which is covered by highly mobile adsorbates. Pt atoms on the hexagonal layer can also move as a result of the weakened interaction between the surface layer and the bulk. The mobility is enhanced under 1 Torr of 1:1 C2H4-H2 mixture because the Pt(100)-hex surface is active in ethylene hydrogenation. The surface mobility along with the catalytic reaction is quenched after introducing 3 mTorr of CO. Meanwhile, the hexagonal reconstruction is lifted by the adsorption of CO. At 5 × 10−6 Torr of C2H4, CO from background gases can also adsorb on Pt(100), creating Pt islands that do not revert to the hexagonal surface when the C2H4 pressure was further increased to 1 Torr. In order to understand the effect of substrates on surface reconstruction, the structure of the stepped Cu(557) surface was monitored in equilibrium with high pressures of gases. Cu generally binds to the reducing gases such as CO, H2, and C2H4 weaker than Pt, leading to a lower coverage on Cu than on Pt at the same gas pressure. Accordingly, 12 Torr of CO is required to induce clusters on Cu(557), because higher CO pressures are needed to keep a sufficient amount of CO that can stabilize clusters. At 1 Torr, large terraces with an average width of 23 nm are observed on Cu(557), because of the low diffusion barrier for Cu atoms both on terraces and along the steps. 500 mTorr of H2 results in step coalescence on Cu(557), giving rise to 6 nm wide terraces. C2H4 adsorption at 500 mTorr results in 5 nm large clusters. CO does not change the Cu(557) surface structure while adding into C2H4, but causes the appearance of large terraces while co-adsorbing with H2. Under oxidizing gases, for example 1 Torr of O2, the Cu(557) surface is significantly oxidized, forming thick layers of Cu oxide. Pt-based bimetallic nanoparticle catalysts were also investigated with AP-XPS under reaction conditions to study their surface chemistry. PtFe nanoparticles do not undergo any surface segregation at 298 K when the gas environment changes, but surface Fe atoms are partially reduced under the C2H4-H2 mixture and partially oxidized under O2. Neither does the surface composition of Pt9Co-Co core-shell nanoparticles change while heating under H2 even to 673 K nor do oxidation states. In Pt-Ni systems, at 393 K, Ni is oxidized under O2 and migrates to the surface because Ni is more susceptible to oxidation than Pt. In contrast, when the surface is reduced by H2, Pt segregates to the surface since the surface free energy of Pt is lower. Such segregation does not occur at 353 K owing to the low atomic mobility in lattice.
Author: Publisher: ISBN: Category : Languages : en Pages : 134
Book Description
Surface structure, mobility, and composition of transition metal catalysts were studied by high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at high gas pressures. HP-STM makes it possible to determine the atomic or molecular rearrangement at catalyst surfaces, particularly at the low-coordinated active surface sites. AP-XPS monitors changes in elemental composition and chemical states of catalysts in response to variations in gas environments. Stepped Pt and Cu single crystals, the hexagonally reconstructed Pt(100) single crystal, and Pt-based bimetallic nanoparticles with controlled size, shape and composition, were employed as the model catalysts for experiments in this thesis.
Author: Masaru Ishii Publisher: ISBN: Category : Automobiles Languages : en Pages :
Book Description
"The work in this thesis is a study of the study of the structure and NOx storage reactivities of precious metal and oxide support model catalysts by using surface science techniques. The introduction details the current situation of automobile catalysts and NOx storage reduction (NSR) catalysts. The experimental gives details of surface microscopy techniques, the preparation of clean TiO2 (110) and Pt (111) surface and the construction of a barium source. The main body of data presented here uses scanning tunnelling microscopy (STM) technique" - introduction.
Author: Franklin Tao Publisher: Royal Society of Chemistry ISBN: 1782621032 Category : Technology & Engineering Languages : en Pages : 285
Book Description
Catalysis is a central topic in chemical transformation and energy conversion. Thanks to the spectacular achievements of colloidal chemistry and the synthesis of nanomaterials over the last two decades, there have also been significant advances in nanoparticle catalysis. Catalysis on different metal nanostructures with well-defined structures and composition has been extensively studied. Metal nanocrystals synthesized with colloidal chemistry exhibit different catalytic performances in contrast to metal nanoparticles prepared with impregnation or deposition precipitation. Additionally, theoretical approaches in predicting catalysis performance and understanding catalytic mechanism on these metal nanocatalysts have made significant progress. Metal Nanoparticles for Catalysis is a comprehensive text on catalysis on Nanoparticles, looking at both their synthesis and applications. Chapter topics include nanoreactor catalysis; Pd nanoparticles in C-C coupling reactions; metal salt-based gold nanocatalysts; theoretical insights into metal nanocatalysts; and nanoparticle mediated clock reaction. This book bridges the gap between nanomaterials synthesis and characterization, and catalysis. As such, this text will be a valuable resource for postgraduate students and researchers in these exciting fields.
Author: Nathan Musselwhite Publisher: ISBN: Category : Languages : en Pages : 135
Book Description
Model materials consisting of metal nanoparticles loaded onto oxide supports were synthesized, characterized, and investigated in a number of catalytic chemical reactions. By varying the size, shape, and composition of nanoparticle, as well as the material used to support the nanoparticles, it was found that small changes to the catalyst can have enormous changes to the reaction activity and selectivity. Investigation of these carefully synthesized catalysts via in situ characterization, and reaction studies, leads to a deeper understanding of the molecular level parameters that govern catalysis. Through study of the properties of the nanoparticles it was discovered that nanoparticle size and shape have a dominant role in the chemoselective catalysis of furfural over platinum nanoparticles. When vapor phase furfural and hydrogen gas were passed over Pt nanoparticles ranging in size from 1.5 to 7.1 nm, the catalytic selectivity was found to be dominated by the size of the nanoparticle. Large nanoparticles promoted hydrogenation of furfural to furfuryl alcohol, while smaller nanoparticles favored decarbonylation to furan. The same size specific selectivity was found in the hydrogenative reforming (the transformation of hydrocarbons to branched isomers) of C6 hydrocarbons, in which Pt nanoparticle size controls isomerization selectivity. Methylcyclopentane was found to be extremely size dependent at lower temperatures (553 K). It was found that smaller sized nanoparticles favored isomer formation, while larger sizes catalyzed the aromatization reaction more efficiently. n-hexane was found to be much less dependent on particle size, but still showed an increase in isomerization with small particles over larger sized Pt nanoparticles. The composition of PtxRh1-x bimetallic nanoparticles was also studied. These catalysts were characterized under hexane reforming conditions with Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS), in order to find the actual surface atomic composition under real catalytic working conditions. By using AP-XPS and catalytic data in tandem, it was found that an optimum Rh loading occurred when the surface ensemble statistically favored one Rh atom surrounded by Pt atoms. By utilizing different oxide materials for catalytic supports the flow of charge can play a role in the reaction at the surface or interface in a phenomenon known as the strong metal-support interaction (SMSI). When Pt nanoparticles were loaded onto mesoporous supports made of Co3O4, NiO, MnO2, Fe2O3, and CeO2 it was found that their activity for carbon monoxide oxidation was greatly enhanced relative to the support alone or Pt loaded onto inert mesoporous silica. This finding demonstrates that the interface of the metallic Pt nanoparticle and the oxide support is able to produce turnovers that are orders of magnitude higher than the two materials separately. When the same type of experiments were investigated with n-hexane as the reactant and macroporous Al2O3, TiO2, Nb2O5, Ta2O5, and ZrO2 were utilized as supports, it was found that the reaction selectivity was greatly altered depending on the catalytic support material. TiO2, Nb2O5, and Ta2O5 (all of which are strong Lewis acids) were found to be much more selective for isomer production than the standard SiO2 mesoporous silica supported Pt nanoparticle catalyst. Finally, an acidified mesoporous silica material was utilized as the support. This material was synthesized by using AlCl3 to modify the surface of mesoporous silica. This support was found to have no activity for hexane isomerization alone. However, when Pt nanoparticles were supported on the material, the activity and isomer selectivity in hexane reforming was increased several orders of magnitude as compared to the same nanoparticles supported on unmodified mesoporous silica. This dissertation builds on the existing knowledge of known concepts in catalysis science such as structure sensitive reactions, the metal-support interaction, and acid-base chemistry. The results show how small changes in the active sites of a catalyst can create large changes in the catalytic chemistry. This research demonstrates how careful material control, characterization and reaction study can help to elucidate the molecular level components necessary to design efficient catalysts.
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Author: Zhongwei Zhu
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Author: Yao-En Li
Author: Masaru Ishii
Author: Franklin Tao
Author: Brian James McIntyre
Author: Tsanshao Joseph Wang
Author: Nathan Musselwhite
Author: Rong He