Immobilization of nanocatalysts on cordierite honeycomb monoliths for low temperature NOx reduction

Noble metal nanocatalysts such as Pd, Pt, and Au were strongly immobilized on the inside walls of monolithic honeycomb-structured cordierite, in which bi-functional molecules were used as linkers for anchoring noble metal nanoparticles (NPs) on the cordierite surface. The supported nanocatalysts were characterized by ICP-MS, TEM, and X-ray powder diffraction. The efficiencies of the immobilized nanocatalysts for the removal of harmful nitrogen oxides (NO_x) have been investigated by measuring the deNO_x capability as a function of temperature. The catalytic activities depend mainly on the compositions of the nanocatalysts. The Pd/Pt bi-metal catalyst anchored on the cordierite surface shows higher NO_x conversion and better activity than the commercial emission catalyst at low temperature region, which could be due to the large portion of active surface areas of the catalysts with nanometer scale.     

Dendrimer templates for heterogeneous catalysts: Bimetallic Pt–Au nanoparticles on oxide supports

Polyamidoamine (PAMAM) dendrimers were used to template Pt, Au, and bimetallic Pt–Au dendrimer encapsulated nanoparticles (DENs) in solution. Adjusting the solution pH allowed for slow, spontaneous adsorption of the nanoparticles onto silica, alumina, and titania. After dendrimer removal, the catalysts were characterized with infrared spectroscopy of adsorbed CO and tested with CO oxidation catalysis. Infrared spectroscopy of the monometallic Pt catalysts showed a slight shift in the CO stretching frequency for the different supports. For the bimetallic catalysts, infrared spectra showed CO adsorbed on both Pt and on Au sites. Spectra collected during CO desorption showed substantial interactions between the two bands, confirming the presence of bimetallic particles on all the supports. The bimetallic catalysts were found to be more active than the monometallic catalysts and had lower apparent activation energies. The titania supported Pt–Au catalyst was resistant to deactivation during an extended treatment at 300 °C. Correlations between IR spectra and catalytic activity showed differences between the mono- and bimetallic materials and implicated a bimetallic Pt–Au ensemble at the catalytic active site. This is the first study to show that DENs are appropriate precursors for studying support effects on catalysis by metal nanoparticles, although the magnitude of the effects were small.     

Nanocatalysis on Supported Oxides for CO Oxidation

Active gold and palladium nanoparticles supported on a variety of oxides (CeO₂, ZrO₂, Al₂O₃, SiO₂, MgO and ZnO) were synthesized using laser vaporization and microwave irradiation methods. The catalytic activities for CO oxidation on the nanoparticle catalysts were evaluated and compared among different oxide supports. The effect of shape on the catalytic activity is demonstrated by comparing the activities of the Au and Pd catalysts deposited on MgO nanocubes and ZnO nanobelts. The Au/CeO₂ nanoparticles deposited on MgO nanocubes exhibit high catalytic activity and stability. The enhanced catalytic activity is attributed to the presence of a significant concentration of the corner and edge sites in MgO nanocubes. The Au- and Pd-doped Mn₂O₃ nanoparticles show promising results for the low temperature CO oxidation. Several approaches for incorporating the Au and Pd nanocatalysts within mesoporous oxide supports are presented and discussed.     

Nanomechanical characterization of chemical interaction between gold nanoparticles and chemical functional groups

Core/shell nanostructured carbon materials with carbon nanofiber (CNF) as the core and a nitrogen (N)-doped graphitic layer as the shell were synthesized by pyrolysis of CNF/polyaniline (CNF/PANI) composites prepared by in situ polymerization of aniline on CNFs. High-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared and Raman analyses indicated that the PANI shell was carbonized at 900°C. Platinum (Pt) nanoparticles were reduced by formic acid with catalyst supports. Compared to the untreated CNF/PANI composites, the carbonized composites were proven to be better supporting materials for the Pt nanocatalysts and showed superior performance as catalyst supports for methanol electrochemical oxidation. The current density of methanol oxidation on the catalyst with the core/shell nanostructured carbon materials is approximately seven times of that on the catalyst with CNF/PANI support. TEM tomography revealed that some Pt nanoparticles were embedded in the PANI shells of the CNF/PANI composites, which might decrease the electrocatalyst activity. TEM-energy dispersive spectroscopy mapping confirmed that the Pt nanoparticles in the inner tube of N-doped hollow CNFs could be accessed by the Nafion ionomer electrolyte, contributing to the catalytic oxidation of methanol.     

Mesoporous silica supports for improved thermal stability in supported Au catalysts

In this work, we present a comprehensive review of our research on the role of mesoporous silica pore architecture, composition of the pore walls (addition of Co or Al), and silica surface chemistry (surface modification by TiO₂) to improve the hydrothermal stability of Au particles. We have found that mesoporous silica architecture plays an important role in improving Au stability, with three dimensional mesoporous architectures being less effective than one dimensional (1-D) pores. The tortuous 1-D pores in aerosol silica were found to be most effective at controlling Au particle size. Since Au particles continue to grow larger than the pore diameter, we conclude that Ostwald ripening must be the dominant sintering pathway for these Au catalysts. These catalysts are active for CO oxidation even after the Au particles have grown large enough to block the pores, suggesting that the thin walls of mesoporous silica provide easy access to gas phase molecules. Further improvements in Au stability and reactivity were obtained by surface modification of the aerosol and MCM-41 silica with TiO₂. After TiO₂ modification of the silica, the Au particles remained smaller than the pore size (< 3 nm) even after three cycles of CO oxidation at temperatures up to 400 °C.     

Mesoporous Au/TiO2 Catalysts for Low Temperature CO Oxidation

The activity and stability of structurally well defined mesoporous Au/TiO₂ catalysts with different support morphologies and pore sizes for low temperature CO oxidation was investigated by kinetic measurements and in-situ IR spectroscopy. The resulting catalysts with Au particle sizes of ∼3 nm exhibit a high activity for CO oxidation, similar to or exceeding that of highly active standard Au/TiO₂ catalysts with similar size Au nanoparticles and loading, and a significantly lower tendency for deactivation. Possible reasons for the improved performance of these catalysts are discussed.     

Effect of heat treatment on stability of gold particle modified carbon supported Pt-Ru anode catalysts for a direct methanol fuel cell

Carbon supported Au-PtRu (Au-PtRu/C) catalysts were prepared as the anodic catalysts for the direct methanol fuel cell (DMFC). The procedure involved simple deposition of Au particles on a commercial Pt-Ru/C catalyst, followed by heat treatment of the resultant composite catalyst at 125, 175 and 200 °C in a N₂ atmosphere. High-resolution transmission electron microscopy (HR-TEM) measurements indicated that the Au nanoparticles were attached to the surface of the Pt-Ru nanoparticles. We found that the electrocatalytic activity and stability of the Au-PtRu/C catalysts for methanol oxidation is better than that of the PtRu/C catalyst. An enhanced stability of the electrocatalyst is observed and attributable to the promotion of CO oxidation by the Au nanoparticles adsorbed onto the Pt-Ru particles, by weakening the adsorption of CO, which can strongly adsorb to and poison Pt catalyst. XPS results show that Au-PtRu/C catalysts with heat treatment lead to surface segregation of Pt metal and an increase in the oxidation state of Ru, which militates against the dissolution of Ru. We additionally find that Au-PtRu/C catalysts heat-treated at 175 °C exhibit the highest electrocatalytic stability among the catalysts prepared by heat treatment: this observation is explained as due to the attainment of the highest relative concentration of gold and the highest oxidation state of Ru oxides for the catalyst pretreated at this temperature.     

The effects of stoichiometry on the properties of exsolved Ni-Fe alloy nanoparticles for dry methane reforming

The dry reforming of methane has received notable attention as a chemical process to convert natural gas into value-added chemicals and fuels. Ni-based exsolution catalysts using perovskite oxides supports have been used for their attractive sinter-resistance and coke-resistance properties. The perovskite oxide in itself has unique defect chemistry that can be used to manipulate and control the properties of the catalyst nanoparticles exsolved on the surface, therefore influencing both the nanoparticle and support characteristics. In this study, the La:Fe ratio of Ni-doped LaFeO₃ was used to manipulate and control the properties of exsolved Ni-Fe alloy nanoparticles. The Ni-Fe nanoparticles consisted of different sizes ranging from 10 to 380 nm. Temperature programmed surface reaction studies along with materials characterization with SEM, STEM-HAADF, XRD, and BET showed that the Ni-Fe nanoparticles from different solid precursors have the same active sites for methane activation but differ in performance and stability because of size effects, metal-support strength, composition and support basicity. A mechanism is proposed to decipher the merits of the Ni-Fe nanoparticles with the best activity, selectivity, and stability in this study.     

Au/TS‐1 catalyst for propene epoxidation with H2/O2: A novel strategy to enhance stability by tuning charging sequence

For propene epoxidation with H₂ and O₂, the catalytic performance of Au/TS‐1 catalyst is extremely sensitive to preparation parameters of deposition‐precipitation (DP) method. In this work, effect of charging sequence in DP process on catalyst structure and catalytic performance of Au/TS‐1 catalyst is first investigated. For different charging sequences, the compositions of Au complexes (e.g., [AuCl(OH)₃]^?) and pore property of TS‐1 (i.e., with or without H₂O prefilling micropores) could affect the transfer of Au complexes into the micropores, resulting in different Au locations and thus significantly different catalytic performance. Notably, when TS‐1 is first filled with H₂O and then mixed with Au complexes, the reduced Au/TS‐1 catalyst could expose Au nanoparticles on the external surface of TS‐1 and show high stability. The results provide direct evidence showing that micropore blocking is the deactivation mechanism. Based on the results, a simple strategy to design highly stable Au/Ti‐based catalysts is developed. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3963–3972, 2016     

Boosting the performance of Pt electro-catalysts toward formic acid electro-oxidation by depositing sub-monolayer Au clusters

CO poisoning is the main obstacle to the application of Pt nanoparticles as anode catalysts in direct formic acid fuel cells (DFAFCs). Significant types of Pt alloys have been investigated, which often demonstrate evidently improved catalytic performance governed by difference mechanisms. By using a well-known electrochemical technique of under potential deposition and in situ redox replacement, sub-monolayer Au clusters are deposited onto Pt nanoparticle surfaces in a highly controlled manner, generating a unique surface alloy structure. Under optimum conditions, the modified Pt nanoparticles can exhibit greatly enhanced specific activity (up to 23-fold increase) at potential of −0.2 V vs. MSE toward formic acid electro-oxidation (FAEO). Interestingly, the mass specific activity can also be improved by a factor of 2.3 at potential of −0.35 V vs. MSE although significant amount of surface Pt atoms are covered by the overlayer Au clusters. The much enhanced catalytic activity can be ascribed to a Pt surface ensemble effect, which induces change of the reaction path. Moreover, the sub-monolayer Au coating on the surface also contributes to the enhanced catalyst durability by inhibiting the Pt oxidation. These results show great potential to rationally design more active and stable nanocatalysts by modifying the Pt surface with otherwise inactive materials.     

Au/Ce1−xZrxO2 as effective catalysts for low-temperature CO oxidation

The physico-chemical properties and activity of Ce-Zr mixed oxides, CeO₂ and ZrO₂ in CO oxidation have been studied considering both their usefulness as supports for Au nanoparticles and their contribution to the reaction. A series of Ce_1−xZr_xO₂ (x = 0, 0.25, 0.5, 0.75, 1) oxides has been prepared by sol–gel like method and tested in CO oxidation. Highly uniform, nanosized, Ce-Zr solid solutions were obtained. The activity of mixed oxides in CO oxidation was found to be dependent on Ce/Zr molar ratio and related to their reducibility and/or oxygen mobility. CeO₂ and Ce_0.75Zr_0.25O₂, characterized by the cubic crystalline phase show the highest activity in CO oxidation. It suggests that the presence of a cubic crystalline phase in Ce-Zr solid solution improves its catalytic activity in CO oxidation. The relation between the physico-chemical properties of the supports and the catalytic performance of Au/Ce_1−xZr_xO₂ catalysts in CO oxidation reaction has been investigated. Gold was deposited by the direct anionic exchange (DAE) method. The role of the support in the creation of catalytic performance of supported Au nanoparticles in CO oxidation was significant. A direct correlation between activity and catalysts reducibility was observed. Ceria, which is susceptible to the reduction at the lowest temperature, in the presence of highly dispersed Au nanoparticles, appears to be responsible for the activity of the studied catalysts. CeO₂-ZrO₂ mixed oxides are promising supports for Au nanoparticles in CO oxidation whose activity is found to be dependent on Ce/Zr molar ratio.     

Fabrication of plasmonic Au/TiO2 nanotube arrays with enhanced photoelectrocatalytic activities

Uniformly dispersed Au nanoparticles (NPs) deposited on the surface of highly ordered TiO₂ nanotube arrays (Au/TiO₂ NTs) were synthesized through a two-step process including anodization method and microwave-assisted chemical reduction route. The investigation indicated that Au NPs grew uniformly on the walls of TiO₂ NTs. Au/TiO₂ NTs exhibited excellent visible light absorption due to the LSPR effect of Au NPs. Au/TiO₂ NTs exhibited much higher photocurrent density and the photoconversion efficiency of Au decorated TiO₂ NTs was about 2.05 times greater than that of bare TiO₂ NTs. Besides, the PL intensity of Au/TiO₂ NTs was much lower than that of TiO₂ NTs, revealing a decrease in charge carrier recombination. The prepared Au/TiO₂ NTs exhibited superior photoelectrocatalytic activity and stability in the degradation of MB under simulated solar light irradiation. The synergy effect between nanotubular structures of TiO₂ and uniformly dispersed Au nanoparticles, as well as the small bias potential and strong interaction between Au and TiO₂, facilitated the Au plasmon-induced charge separation and transfer, which lead to highly efficient and stable photoelectrocatalytic activity.     

Influence of the catalyst surface area on the activity and stability of Au/CeO2 catalysts for the low-temperature water gas shift reaction

The influence of the support surface area on the activity and stability/deactivation of Au/CeO₂ catalysts (2.7 wt% Au) in the water gas shift reaction in dilute water gas were investigated by kinetic measurements and in situ Diffuse Reflectance IR spectroscopy. For ceria support surface areas between 24 and 284 m² g⁻¹, the gold particle size is independent on the catalyst surface area (about 2.1 nm) up to 188 m² g⁻¹, and we found increased amounts of (i) Auⁿ⁺, (ii) Ce³⁺, (iii) OH groups, and (iv) carbon containing adsorbed side products such as formates and carbonates for increasing surface area supports. Consequences of these results on the mechanistic understanding of the reaction are discussed.     

Uncalcined TS-2 immobilized Au nanoparticles as a bifunctional catalyst to boost direct propylene epoxidation with H2 and O2

Developing stable yet efficient Au–Ti bifunctional catalysts is important but challenging for direct propylene epoxidation with H₂ and O₂. This work describes a novel strategy of employing uncalcined titanium silicalite-2 (TS-2-B) to immobilize Au nanoparticles as a bifunctional catalyst for the reaction. Under no promoter effects, the Au/TS-2-B catalyst compared to the referenced Au/TS-1-B catalyst delivers outstanding catalytic performance, that is, exceptionally high stability over 100 hr, propylene oxide (PO) formation rate of 118 g_PO·hr⁻¹·kg_cat⁻¹, PO selectivity of 90% and hydrogen efficiency of 35%. The plausible relationship of catalyst structure and performance is established by using multiple techniques, such as UV–vis, high-angle annular dark-field scanning transmission electron microscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy. A unique synergy of Au–Ti⁴⁺–Ti³⁺ triple sites is proposed for our developed Au/TS-2-B catalyst with the higher stable PO formation rate and hydrogen efficiency. The insights reported here could shed new light on the rational design of highly stable and efficient Au–Ti bifunctional catalysts for the reaction.     

Construction of Au/TiO2 Heterojunction with high photocatalytic performances under UVA illumination

Anatase TiO₂ nanoparticles (NPs) were successfully prepared through a hydrothermal approach, and Au NPs at various Au (0.1–2 wt%) contents were photodeposited onto the TiO₂ NPs surface. The photocatalytic efficiency for the Au/TiO₂ NPs for resorcinol photodegradation throughout UVA illumination was assessed. The TEM images and XPS findings indicated that the Au NPs are highly distributed onto TiO₂ surface in the metallic state. The 0.1%Au/TiO₂ NPs exhibited the highest photocatalytic efficiency of about 95.34%; however, 72.36% is given by pure TiO₂ NPs. It was found that the photodegradation rate of 0.1% Au/TiO₂ NPs exhibited 1.5 times of magnitude higher than pure TiO₂ NPs. 0.1%Au/TiO₂ NPs was considered to be the outstanding photoactive due to the ultimate efficient charge-carriers separation through charge transfer between Au and TiO₂ NPs. The Au NPs sizes, its dispersity on TiO₂ surface and surface plasmon resonance (SPR) were believed the critical factors for the higher photocatalytic performance of 0.1% Au/TiO₂ NPs. The prepared photocatalysts are found to be the promising materials for toxic organic compounds remediation and solar conversion.     

Paper-structured fiber composites impregnated with platinum nanoparticles synthesized on a carbon fiber matrix for catalytic reduction of nitrogen oxides

Platinum nanoparticles (PtNPs) were synthesized on surface-activated carbon fibers with high thermal conductivity, and paper-structured composites were fabricated by a papermaking technique, using the PtNPs-supported carbon fibers and ceramic fibers as matrix materials. As-prepared materials, denoted paper-structured PtNPs catalyst, possessed a unique porous microstructure derived from entangled inorganic fiber networks on which PtNPs were well dispersed. In catalytic reduction of nitrogen oxides (NO_X) in the presence of methane (CH₄), both of which are model exhaust gas components of combustion engines, paper-structured PtNPs catalyst demonstrated excellent NO_X and CH₄ removal efficiency and rapid thermal responsiveness by comparison with the PtNPs-supported carbon fibers, commercial Pt catalyst powders and a monolithic Pt-loaded honeycomb. These features of the new catalyst material are thought to arise from synergistic effects of the highly active PtNPs in association with the unique paper-like microstructure, in promoting effective transfer of heat and reactants to the active sites of the Pt nanocatalysts. The paper-structured PtNPs catalyst with paper-like practical utility is expected to be a promising catalytic material for efficient NO_X gas purification.     

High Electrochemical Performance and Stability of Co-Deposited Pd–Au on Phase-Pure Tungsten Carbide for Hydrogen Oxidation

Co-deposited Pd and Au nanoparticles were loaded on phase-pure tungsten mono-carbide (WC) prepared by a polymer-induced carburization method. Among the electrocatalysts, Pd₃Au/WC displayed an excellent electrochemical activity for the hydrogen oxidation reaction comparable to the state-of-the-art Pt/C catalyst both in half-cell tests and single cell tests under proton exchange membrane fuel cell (PEMFC) conditions. This unique and strong synergistic effect was not observed on the carbon support, and thus the crucial role of WC was demonstrated as a strongly interacting support as well as an active component of the electrocatalyst. Dissolution of Pd observed on the carbon support was suppressed in this Pd₃Au/WC electrocatalyst, which showed good stability in a continuous operation for 3,000?min. Thus the proposed electrocatalyst could be a potential alternative anode catalyst of lower cost for PEMFC replacing Pt/C.     

活性纳米金颗料在介孔分子筛SBA-15上的分散

Chemical modification （CM） and deposition-precipitation （DP） methods were used for the dispersion of active Au nanoparticles on mesoporous silica materials in this work. XRD, TEM, N2 adsorption isotherms and UV-Vis absorption spectra were used to characterize in detail Au-SBA-15 materials prepared by the two methods. The analysis results showed that high loading （1.7%, by mass） and uniform Au nanoparticles （approximately 3 nm） were dispersed in the channels of mesoporous SBA-15 by the CM method. While for the DP method, most of Au nanoparticles with the size of 10—15nm were aggregated outside of the channels of SBA-15 and the actual loading of Au was only 0.38% （by mass）.     

Catalytic activity of ethylene oxidation over Au,Ag and Au–Ag catalysts: Support effect

Au, Ag and Au–Ag catalysts on different supports of alumina, titania and ceria were studied for their catalytic activity of ethylene oxidation reactions. An addition of an appropriate amount of Au on Ag/Al₂O₃ catalyst was found to enhance the catalytic activity of the ethylene epoxidation reaction because Au acts as a diluting agent on the Ag surface creating new single silver sites which favor molecular oxygen adsorption. The Ag catalysts on both titania and ceria supports exhibited very poor catalytic activity toward the epoxidation reaction of ethylene, so pure Au catalysts on these two supports were investigated. The Au/TiO₂ catalysts provided the highest selectivity of ethylene oxide with relatively low ethylene conversion whereas, the Au/CeO₂ catalysts was shown to favor the total oxidation reaction over the epoxidation reaction at very low temperatures. In comparisons among the studied catalysts, the bimetallic Au–Ag/Al₂O₃ catalyst is the best candidate for the ethylene epoxidation. The catalytic activity of the gold catalysts was found to depend on the support material and catalyst preparation method which govern the Au particle size and the interaction between the Au particles and the support.     

The effect of mesoporous scale defects on the activity of Au/TS-1 for the epoxidation of propylene

In an effort to examine the role of crystal morphology on activity, mesoporous scale defects were introduced into titanium silicalite-1 (TS-1) support materials through the addition of carbon pearls during the synthesis procedure. Catalysts prepared using these support materials were consistently active and stable, with a 0.33 wt% Au catalyst producing 132 g_PO/h/kg_cat at 200 °C, the highest propylene oxide (PO) rate thus reported, despite relatively high gold loadings and considerable contamination with octahedral titanium species. While activity is high per gram catalyst, activity per gram gold was still relatively low implying that a significant portion of gold deposited on the support surface may be inactive for epoxidation.