High-temperature H2 Reduction Research Articles

Dynamic Behavior of Platinum Atoms and Clusters in the Native Oxide Layer of Aluminum Nanocrystals.

Strong metal-support interactions (SMSIs) are well-known in the field of heterogeneous catalysis to induce the encapsulation of platinum (Pt) group metals by oxide supports through high temperature H2 reduction. However, demonstrations of SMSI overlayers have largely been limited to reducible oxides, such as TiO2 and Nb2O5. Here, we show that the amorphous native surface oxide of plasmonic aluminum nanocrystals (AlNCs) exhibits SMSI-induced encapsulation of Pt following reduction in H2 in a Pt structure dependent manner. Reductive treatment in H2 at 300 °C induces the formation of an AlOx SMSI overlayer on Pt clusters, leaving Pt single-atom sites (Ptiso) exposed available for catalysis. The remaining exposed Ptiso species possess a more uniform local coordination environment than has been observed on other forms of Al2O3, suggesting that the AlOx native oxide of AlNCs presents well-defined anchoring sites for individual Pt atoms. This observation extends our understanding of SMSIs by providing evidence that H2-induced encapsulation can occur for a wider variety of materials and should stimulate expanded studies of this effect to include nonreducible oxides with oxygen defects and the presence of disorder. It also suggests that the single-atom sites created in this manner, when combined with the plasmonic properties of the Al nanocrystal core, may allow for site-specific single-atom plasmonic photocatalysis, providing dynamic control over the light-driven reactivity in these systems.

Structural Evolution of Anatase-Supported Platinum Nanoclusters into a Platinum-Titanium Intermetallic Containing Platinum Single Atoms for Enhanced Catalytic CO Oxidation.

Strong metal-support interactions characteristic of the encapsulation of metal particles by oxide overlayers have been widely observed on large metal nanoparticles, but scarcely occur on small nanoclusters (<2 nm) for which the metal-support interactions remain elusive. Herein, we study the structural evolution of Pt nanoclusters (1.5 nm) supported on anatase TiO2 upon high-temperature H2 reduction. The Pt nanoclusters start to partially evolve into a CsCl-type PtTi intermetallic compound when the reduction temperature reaches 400 °C. Upon 700 °C reduction, the PtTi nanoparticles are exclusively formed and grow epitaxially along the TiO2 (101) crystal faces. The thermodynamics of the formation of PtTi via migration of reduced Ti atoms into Pt cluster is unraveled by theoretical calculations. The thermally stable PtTi intermetallic compound, with single-atom Pt isolated by Ti, exhibits enhanced catalytic activity and promoted catalytic durability for CO oxidation.

Engineering metal-oxide interface by depositing ZrO2 overcoating on Ni/Al2O3 for dry reforming of methane

Zirconium oxide (ZrO2) was deposited onto Ni/Al2O3 catalyst as overcoating by atomic layer deposition (ALD) for dry reforming of methane (DRM). High-temperature heating during H2-reduction could transform the ALD-prepared ZrO2 thin film to tetragonal phase and crack the encapsulating layer on Ni sites, which constructed a beneficial Ni-ZrOx interface. Interfacial surface oxygen vacancies on ZrO2 overcoating were induced by the partial reduction of ZrO2 surface during high-temperature H2 reduction, with the assistance of Ni. During DRM, the interfacial oxygen vacancies enhanced CO2 activation by dissociating CO2 and releasing active O, thereby limiting carbon formation. For DRM at 700 °C and 800 °C, Ni/Al2O3 with 5 cycles of ZrO2 ALD overcoating enhanced both activity and stability significantly. For a 100-h DRM test at 600 °C, no deactivation was observed for the Ni/Al2O3 catalyst with 10 cycles of ZrO2 ALD overcoating, as compared to 59% relative activity loss of Ni/Al2O3.

The Superior Electrical Conductivity and Anodic Stability of Vanadium-Doped Ti4O7

The effectiveness of Magnéli phase titanium oxides (Ti n O2n−1, 4 ≤ n ≤ 10), promising alternatives to carbon-based materials in aqueous electrochemical technologies, is limited by their instability at strongly oxidizing electrode potentials. Ti4O7 and Ti5O9, the most electrically-conductive Ti n O2n−1, passivate in the harsh conditions demanded by fuel cells, batteries, and devices for the electrochemical treatment of water. Among other properties relevant to electrodes, doping of Ti n O2n−1 with transitional metals vanadium, chromium, and iron improves the oxidation stability as demonstrated by higher onset temperatures in the thermograms of doped Ti4O7 heated in air. To investigate the influence of doping on their anodic stability, disk electrodes of Ti4O7 doped with V, Cr, and Fe were investigated by electrochemical accelerated life testing (ALT). These disks, prepared by high-temperature H2 reduction of nominally 2 at% doped titanium(IV) oxide (TiO2), were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), the 4-point probe method, cyclic voltammetry (CV), Tafel plot analysis, and electrochemical impedance spectroscopy (EIS). V- and Fe-doping are shown to improve the electrical conductivity and V-doping is demonstrated to improve the anodic stability as measured by the mean time-to-failure (TTF) during electrochemical ALT.

The Thermal-Oxidation Behavior of Pristine and Doped Magnéli Phase Titanium Oxides

Magnéli phase titanium oxides (Ti n O2n−1, 4 ≤ n ≤ 10) are important materials for solid state and electrochemical technologies such as memristors, batteries, fuel cells, and electrochemical devices for water treatment. Developing an understanding of transitions between Ti n O2n−1 and its product of oxidation, titanium(IV) oxide (TiO2), as well as strategies such as doping to modulate the conditions for such changes will enable the development of more effective devices. To elucidate a mechanism for their thermal oxidation and investigate the influence of doping, the thermal-oxidation behavior in air of Ti4O7 doped with vanadium, chromium, and iron were investigated by thermogravimetric analysis (TGA). These powders prepared by high-temperature H2 reduction of dopant-containing TiO2 were characterized by scanning electron microscopy (SEM), gas adsorption analysis, X-ray fluorescence (XRF), energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (PXRD). V- and Fe-‍doping improved the thermal stability of Ti4O7 as evidenced by higher onset temperatures in their thermograms. Three-dimensional diffusion reaction models adequately describe the solid-state kinetics of thermal oxidation of Ti4O7 in air as demonstrated by linear model-fitting. Doping shows a mixed influence on the kinetics for thermal oxidation in air reducing both the Arrhenius pre-exponential factor and the activation energy.

Role of Metal Oxides in Cu-Based Catalysts with NaBH4 Reduction for the Synthesis of Methanol from CO2/H2

Cu/ZnO catalyst was prepared with NaBH4 reduction instead of high temperature H2 reduction to relieve the aggregation of Cu species. With the purpose of strengthening the interaction between Cu species and ZnO, some Al2O3, Y2O3, ZrO2 promoters are introduced to promote the synthesis of methanol from CO2/H2. The introduction of metal oxide promoters improves the dispersion of Cu species, decreases the onset reduction temperature and increases the quantity of basic sites. The Cu/ZnO/ZrO2 catalyst exhibited the highest CH3OH yield of 0.20 g mL−1 h−1 with CO2 conversion of 22.8% and CH3OH selectivity of 56.2% at 543 K. The catalytic activity is closely correlated to the exposed Cu surface area and a synergistic effect between the Cu species and metal oxides was proposed, which promoted the catalytic conversion of CO2 as well as H2 and simultaneously facilitated the migration of atomic hydrogen from Cu sites to metal oxides through spillover.

A novel plasma reduction for the preparation of AuPd bimetallic nanocatalyst and its application in selective oxidation of benzyl alcohols

In this work, a low temperature plasma reduction technology is developed for the preparation of AuPd alloy nanocatalyst. The HAuCl4 and PdCl2 mixture as catalyst precursor was directly decomposed on MgO support by plasma to form AuPd nanoalloy. Results demonstrated that the final particles states of nanoalloys prepared by this technique including size, morphology, and chemical valence are highly dependent on temperature of plasma treatment. The catalytic performance of AuPd/MgO in the selective oxidation of benzyl alcohols can be tailored by optimizing the synthesis parameter. A relative low temperature in plasma process prevents particles migration and helps to maintain the small size that favors high activity. The AuPd nanoparticles with diameter about 3.5 nm by optimizing the plasma treatment condition exhibit superior conversion and TOF compared to catalyst prepared by the traditional high temperature H2 reduction method. It is believed that the low temperature plasma technology has a unique advantage for the preparation of high-quality alloy nanocatalyst and could be extended for fabricating the other alloy nanocatalysts.

Isolated Sn on mesoporous silica as a highly stable and selective catalyst for the propane dehydrogenation

Tin oxide supported on silica (SnO2/SiO2) were prepared by incipient-wetness impregnation method and examined for propane dehydrogenation at 600 °C. The physicochemical properties of SnO2/SiO2 were characterized by N2-physisorption, XRD, UV–vis-DRS and H2-TPR. The results indicate that, at Sn loading ≤ 1.78 wt%, SiO2 contains a uniform distribution of Sn as isolated species in tetrahedral coordination, while crystalline SnO2 together with isolated Sn species are found for the higher Sn loadings. The reducibility of various Sn species was studied by a combination of XPS, H2-TPR, and reduction-reoxidation-TPR techniques. The results reveal that after high-temperature H2 reduction, crystalline SnO2 is fully reduced to metallic Sn, whereas isolated Sn are stabilized in the Sn2+ state with high dispersion due to a strong interaction between isolated Sn and support. Catalytic activity tests showed that these Sn2+ species exhibits both high activity and stability for propane dehydrogenation.

Thermostable Rh Metal Nanoparticles Formed on Al2O3 by High-Temperature H2 Reduction and Its Impact on Three-Way Catalysis

The influence of high-temperature H2 reduction treatment on Rh and Pd catalysts supported on Al2O3 was studied in relation to thermal aging in air. After air-aging at ≥900 °C, the Rh/Al2O3 catalyst was more strongly deactivated compared with the Pd/Al2O3 catalyst. As has been widely recognized, the solid-state reactions between Rh oxide and Al2O3 decreased the active surface area and stabilized inactive Rh3+ species. The activity was restored by the postreduction treatment with 20% H2/He at 200 °C, whereas a striking enhancement of activity was achieved by the reduction at 800–1100 °C, where substantial deactivation occurred for Pd/Al2O3. A mechanistic interpretation is proposed based on local structural characterization, which explains these contrasting thermal behaviors. The high-temperature reduction treatment produced active and thermostable Rh metal nanoparticles, which were highly dispersed on Al2O3. The observed dispersion (as high as ∼20% after reduction at 1000 °C) is among the highest for suppor...

Highly dispersed Pt-based catalysts for selective CO2 hydrogenation to methanol at atmospheric pressure

Hydrogenation of CO2 into methanol is promising for achieving the sustainable energy economy, but still has some problems, e.g. low methanol selectivity and high operation pressures (>10 atm). Herein, we prepared a Pt/film hybrid with highly dispersed Pt nanoparticles, and combined Pt/film with In2O3 to form a Pt/film/In2O3 catalyst. By using a dielectric barrier discharge (DBD) plasma reactor, a CO2 conversion of 37.0% and a methanol selectivity of 62.6% are achieved in the hydrogenation of CO2 with H2 on Pt/film/In2O3 at 1 atm and 30 °C. These are higher than those on Pt/In2O3 prepared by the conventional high-temperature H2 reduction (24.9% and 36.5%) and commercial Cu/ZnO/Al2O3 (25.6% and 35.1%). The high-energy electrons of the DBD plasma trigger the CO2 hydrogenation at 1 atm and 30 °C. The higher Pt nanoparticles dispersion, film and In2O3 promote the adsorption of CO2 on Pt/film/In2O3, thus enhancing the hydrogenation of CO2 into methanol. These results are helpful for efficient methanol production from CO2 hydrogenation under atmospheric pressure.

In-situ immobilization and pyrolysis of metal-organic framework supported on biomorphic layered double hydroxides as highly active and stable heterogeneous catalyst

Abstract In recent years, the pyrolysis of metal-organic frameworks (MOFs) has been widely employed to prepare advanced heterogeneous catalysts. However, large size or agglomeration caused by pyrolysis severely limits the performance of the catalysts due to the high mass transfer resistance. In this work, MOF crystals (Cu-BTC) in-situ grown on the biomorphic hierarchical layered double hydroxides (LDHs) surface (ZnAl-LDH/Cu-BTC) and it as a pyrolytic precursor to prepare layered double oxides/metal (ZnAl-LDO/Cu) composite catalyst were reported. ZnAl-LDH support induced the uniform distribution of nano-sized Cu-BTC on the nanosheets surface, and this unique method mediated Cu-BTC precursor for the in-situ construction of highly stable Cu nanoparticles under a high-temperature H2 reduction. ZnAl-LDO/Cu exhibited excellent catalytic activity and stability for the reduction 4-nitrophenol (4-NP) in the presence of NaBH4, which could be recycled for fifteen successive cycles with a high reduction efficiency (over 93%). Meanwhile, the ZnAl-LDO/Cu demonstrated high catalytic efficiencies on rhodamine B, congo red, methyl orange and methylene blue dyes. Hence, the concept presented in this work can be further expanded to synthesis of other MOFs derived high-efficiency non-noble metal catalysts, which have a great potential as a cost-effective and environmentally-friendly reusable catalyst to replace noble metals catalyst in large scale catalytic application.

Constructing Multifunctional Metallic Ni Interface Layers in the g-C3N4 Nanosheets/Amorphous NiS Heterojunctions for Efficient Photocatalytic H2 Generation.

The construction of exceptionally robust and high-quality semiconductor-cocatalyst heterojunctions remains a grand challenge toward highly efficient and durable solar-to-fuel conversion. Herein, novel graphitic carbon nitride (g-C3N4) nanosheets decorated with multifunctional metallic Ni interface layers and amorphous NiS cocatalysts were fabricated via a facile three-step process: the loading of Ni(OH)2 nanosheets, high-temperature H2 reduction, and further deposition of amorphous NiS nanosheets. The results demonstrated that both robust metallic Ni interface layers and amorphous NiS can be utilized as electron cocatalysts to markedly boost the visible-light H2 evolution over g-C3N4 semiconductor. The optimized g-C3N4-based photocatalyst containing 0.5 wt % Ni and 1.0 wt % NiS presented the highest hydrogen evolution of 515 μmol g-1 h-1, which was about 2.8 and 4.6 times as much as those obtained on binary g-C3N4-1.0%NiS and g-C3N4-0.5%Ni, respectively. Apparently, the metallic Ni interface layers play multifunctional roles in enhancing the visible-light H2 evolution, which could first collect the photogenerated electrons from g-C3N4, and then accelerate the surface H2-evolution reaction kinetics over amorphous NiS cocatalysts. More interestingly, the synergetic effects of metallic Ni and amorphous NiS dual-layer electron cocatalysts could also improve the TEOA-oxidation capacity through upshifting the VB levels of g-C3N4. Comparatively speaking, the multifunctional metallic Ni layers are dominantly favorable for separating and transferring photoexcited charge carriers from g-C3N4 to amorphous NiS cocatalysts owing to the formation of Schottky junctions, whereas the amorphous NiS nanosheets are mainly advantageous for decreasing the thermodynamic overpotentials for surface H2-evolution reactions. It is hoped that the implantation of multifunctional metallic interface layers can provide a versatile approach to enhance the photocatalytic H2 generation over different semiconductor-cocatalyst heterojunctions.

Atomic Layer Deposition Route To Tailor Nanoalloys of Noble and Non-noble Metals.

Since their early discovery, bimetallic nanoparticles have revolutionized various fields, including nanomagnetism and optics as well as heterogeneous catalysis. Knowledge buildup in the past decades has witnessed that the nanoparticle size and composition strongly impact the nanoparticle's properties and performance. Yet, conventional synthesis strategies lack proper control over the nanoparticle morphology and composition. Recently, atomically precise synthesis of bimetallic nanoparticles has been achieved by atomic layer deposition (ALD), alleviating particle size and compositional nonuniformities. However, this bimetal ALD strategy applies to noble metals only, a small niche within the extensive class of bimetallic alloys. We report an ALD-based approach for the tailored synthesis of bimetallic nanoparticles containing both noble and non-noble metals, here exemplified for Pt-In. First, a Pt/In2O3 bilayer is deposited by ALD, yielding precisely defined Pt-In nanoparticles after high-temperature H2 reduction. The nanoparticles' In content can be accurately controlled over the whole compositional range, and the particle size can be tuned from micrometers down to the nanometer scale. The size and compositional flexibility provided by this ALD-approach will trigger the fabrication of fully tailored bimetallic nanomaterials, including superior nanocatalysts.

In situ synchrotron powder diffraction for an ionic conductor transition

In situ synchrotron X-ray powder diffraction can be one of the most powerful probes to investigate the structure evolution by a chemical reaction thanks to simultaneity of data collection. It is not, however, with ease to produce a homogeneous chemical reaction in the limited spaces, which is essential to see an atomic-scale structure evolution. We have developed an in situ capillary cell for both high-temperature H2reduction and precise humidity control at the SPring-8 BL44B2. We successfully applied this in situ system to an electronic conductor LaSr3Fe3O10, which is transformed into an ionic conductor by the two-step chemical treatments [1]. LaSr3Fe3O10has a triple-layer structure with a FeO6octahedral unit. One triple layer is bonded with another layer through van der Waals interaction. Structure refinements with in situ synchrotron powder diffraction data revealed that the H2reduction at 613 K produced in-plane oxygen vacancies, which resulted in suppression of the interlayer interaction. We found from charge density studies and Raman spectroscopy measurements that the following humidification intercalated H2O and OH-into the interlayer and intralayer, respectively. That means that H2O plays a role for suppression of three-dimensional electronic conductivity, stabilizing the intercalation structure. On the other hand, the OH-ions behave as carriers for ionic conductivity, maintaining the charge neutrality in the intralayer. Finally we determined the composition of the ionic conductor to be LaSr3Fe3O8.0(OH)1.2·2H2O, which indicates a transformation of LaSr3Fe3O10into an OH-ionic conductor. In the presentation, I will discuss the OH-ionic conduction channel based on electrostatic potentials obtained from charge densities.

The activation of supported Au catalysts prepared by impregnation

In this study, Au/TiO2, Au/γ-Al2O3, and Au/C catalysts were prepared by an incipient-wetness impregnation method. The CO oxidation activity after different pretreatment was analyzed. Two pretreatments are found possible to activate these catalysts for CO oxidation: (i) high-temperature hydrogen reduction and (ii) aqueous base treatment using NH3(aq). The high-temperature hydrogen reduction is effective for Au/TiO2 and Au/C, but not Au/Al2O3. The base treatment is effective for Au/Al2O3 and Au/TiO2, but not Au/C. Small Au particles of ca. 2 nm size were observed in activated Au catalysts from both pretreatments; however, the high-temperature H2 reduction procedure also resulted in large Au particles of ca. 25 nm size which makes it a less efficient pretreatment than the base treatment. The activated Au/TiO2 catalysts show comparable turnover frequency as the Au/TiO2 catalyst prepared by a deposition precipitation method. The effective pretreatments were found to be accompanied by a reduced acidity, which is measured by the pH of aqueous catalyst suspension. Reasons for such activation are discussed.

HREM study of strong-metal interaction in Pt/TiO2

The chemisorption ability and catalytic properties of metal particles supported on reducible oxides are often altered by high temperature reduction(HTR) in a process known as strong metal-support interaction(SMSI). Different models have been proposed to explain the SMSI mechanism. In recent years, experimental evidences have favored the "decoration model", which suggests that SMSI is due to the encapsulation of the metal particles by oxide overlayer species dial have migrated from the support. HREM profile imaging was the most useful tool to directly relate these surface decorations to the SMSI effects. The profile imaging can provide atomic-scale information about supported particles and Uieir surfaces without image being obscured by overlapping contrast from the support.In the present work, the SMSI effect in Pt/TiO2 and Pt/CeO2 model catalysts have been studied using HREM profile imaging and multislice simulations. HREM observations were made with a JEM-4000EX microscope, operated at 400 kV. Fig. 1(a) shows a typical profile image of TiO2 after HTR in H2 at 923K.

Nature of the metal-support interface in supported metal catalysts: Results from X-ray absorption spectroscopy

X-ray absorption spectra characterizing the metal-support interface in supported metal complexes and supported metal catalysts are summarized and evaluated. Single-metal-atom transition metal complexes on non-reducible metal oxide supports are bonded with metal-oxygen bonds with metal-oxygen distances of approximately 2.15 A; the bonding distance is only weakly sensitive to the oxidation state of the metal. Nearly this same metal-oxygen distance is characteristic of the metal-support interface in metal-oxide-supported metal clusters following high temperature reduction in H2 (HTR:T > 450 °C). The metals at the interface may be polarized sufficiently that they bond with the oxygen of the support much as the cations in mononuclear complexes bond with it. When the supported metals are treated in H2 at low temperatures (LTR:T < 350 °C) or are prepared under He with partially hydroxylated supports, a longer metal-support oxygen distance is observed, typically 2.5–2.7 A. This distance is suggested to characterize interactions between zero-valent metals and support oxygen. Changes in the performance of supported metal catalysts resulting from differences in the temperature of pretreatment in H2 are attributed to changes in the electronic properties and/or morphology of the metal clusters, which are suggested to be related to the concomitant changes in the structure of the metal-support interface.

Ethylene hydrogenation and ethane hydrogenolysis on a Rh foil with titania overlayers

The effects of submonolayer deposits of titania on the hydrogenation of ethylene and the hydrogenolysis of ethane on a Rh foil have been investigated. Titania has been found to suppress the activity of Rh for both reactions, and the rate of each reaction is proportional to the exposed Rh area. Titania induces no change in the activation energy of either reaction. Furthermore, high temperature reduction in H2 does not change the activation energy for ethane hydrogenolysis on a titania-covered Rh foil. It appears that the titania overlayer has no effect on the activation of H2, ethylene, or ethane, and that the suppression of activity for these reactions is purely the result of titania blocking active sites.

A new TiO2-attached rhodium metal catalyst. Catalyst characterization and non-SMSI behaviour

TiO2-attached Rh metal cluster catalysts have been prepared by the reaction between Rh(η3-C3H5)3 and surface OH groups of TiO2, followed by H2 treatments at different temperatures. This class of Rh/TiO2 catalysts, which are very active for ethene hydrogenation and ethane hydrogenolysis, show no SMSI phenomena, unlike conventional Rh/TiO2 catalysts prepared by a traditional impregnation method using aqueous RhCl3. The structural properties of these catalysts, such as a shortening of the Rh–Rh distance, their raft-like structure and their positively charged character, have been studied in relation to non-SMSI behaviour by means of EXAFS together with other physiochemical techniques. These properties of structure, electronic state, chemisorption and catalytic activity may be ascribed to the Rh—O—Ti bonds which are formed in during attachment of the Rh–allyl complexes onto TiO2 and are partially retained after high-temperature H2 reduction.

Structure and Reactivity of RH in the SMSI State

AbstractHydrogenolysis of n-butane and CO hydrogenation were used to charac-terize the behavior of TiO2supported Rh in the SMSI state. The Rh was supported on a model support consisting of nonporous spherical particles of TiO2. The simple geometry of the support permits the metal crystallites to be imaged ‘edge-on’ in a transmission electron microscope. This allows detailed examination of the metal surface and the metal-oxide interface as a function of pretreatment. After high temperature reduction in H2, there is no significant change in the morphology of the Rh crystallites. However, the presence of amorphous overlayers 0.2–0.4 nm thick is clearly evident on the Rh surface. These overlayers can only be partially removed after oxidation at 473 K. The presence of these overlayers is well correlated with the drop in hydrogen chemisorption and altered reactivity. While the butane hydrogenolysis activity of Rh is considerably suppressed in the SMSI state, the CO hydrogenation activity is increased by a factor of 3. The H2uptake and the butane hydrogenolysis activity can be only partially restored after oxidation at 473 K. Treatment at 773 K in oxygen is necessary for restoring the pre-SMSI behavior.