So-called Strong Metal-support Interaction Research Articles

Direct Observation of Atomistic Reaction Process between Pt Nanoparticles and TiO2 (110)

The catalytic activity and selectivity of heterogeneous catalysts are governed by atomic and electronic structures at the heterointerface between noble metal nanoparticles (NPs) and oxide substrates. In specific chemical reactions, it is well-known that the catalytic activity is strongly suppressed by annealing in a reducing atmosphere, so-called strong metal-support interaction (SMSI). However, it is still unclear the formation process and atomistic origin of the SMSI. By preparing well-defined platinum (Pt) NPs supported on atomically flat TiO2 (110) substrate, we directly show the formation of chemically ordered Pt-Ti intermetallic NPs and impregnation of NPs into TiO2 substrate at high temperatures by using atomic-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Furthermore, we observed negative charge transfer from the Pt-Ti intermetallic NPs to the TiO2 surface, which would strongly affect the catalytic activities.

Density functional theory and kinetic Monte Carlo simulation study the strong metal–support interaction of dry reforming of methane reaction over Ni based catalysts

Abstract Oxide supports modify electronic structures of supported metal nanoparticles, and then affect the catalytic activity associated with the so-called strong metal–support interaction (SMSI). We herein report the strong influence of SMSI employing Ni4/α-MoC(111) and defective Ni4/MgO(100) catalysts used for dry reforming of methane (DRM, CO2 + CH4 → 2CO + 2H2) by using density functional theory (DFT) and kinetic Monte Carlo simulation (KMC). The results show that α-MoC(111) and MgO(100) surface have converse electron and structural effect for Ni4 cluster. The electrons transfer from α-MoC(111) surface to Ni atoms, but electrons transfer from Ni atoms to MgO(100) surface; an extensive tensile strain is greatly released in the Ni lattice by MgO, but the extensive tensile strain is introduced in the Ni lattice by α-MoC. As a result, although both catalysts show good stability, H2/CO ratio on Ni4/α-MoC(111) is obviously larger than that on Ni4/MgO(100). The result shows that Ni/α-MoC is a good catalyst for DRM reaction comparing with Ni/MgO catalyst.

A New Type of Strong Metal-Support Interaction Caused by Antimony Species

Interactions between metals and supports are of fundamental interest in heterogeneous catalysis, Noble metal particles supported on transition metal oxides (TMO) may undergo a so-called strong metal-support interaction via encapsulation. This perspective addresses catalytic properties of the metal catalysts in the SMSI state which can be explained on the basis of complementary studies. The electronic geometric and bifunctional effects originating from strong metal-support interactions (SMSI) that are responsible for the catalyst’s activity, selectivity, and stability are key factors that determine performance. A series of Pd-Sb supported on different metal oxide (i.e. SiO2, γ-Al2O3, TiO2, and ZrO2) were prepared by the impregnation method. The catalysts were characterized by N2 adsorption (BET-SA and pore size distribution), TEM (transmission electron microscope), TPR (temperature-programmed reduction), CO-chemisorption, the structural characterization of Pd (dispersity, surface area), interaction between Pd and Sb2O3 and also the influence of the nature of the support were investigated. SiO2 supported Pd catalyst exhibited the highest surface area (192.6 m2/g) and pore volume (0.542 cm3/g) compared to the other supported oxides catalysts. The electron micrographs of these catalysts showed a narrow size particle distribution of Pd, but with varying sizes which in the range from 1 to 10 nm, depending on the type of support used. The results show almost completely suppressed of CO chemisorption when the catalysts were subjected to high temperature reduction (HTR), this suppression was overcome by oxidation of a reduced Pd/MeOx catalysts followed by re-reduction in hydrogen at 453 K low temperature reduction (LTR), almost completely restored the normal chemisorptive properties of the catalysts, this suppression was attributed by SbOx species by a typical SMSI effect as known for other reducible supports such as TiO2, ZrO2, CeO2, and Nb2O5.

Strong Metal\u2013Support Interaction and Reactivity of Ultrathin Oxide Films

Noble metal particles supported on transition metal oxides (TMO) may undergo a so-called strong metal–support interaction via encapsulation. This perspective addresses catalytic properties of the metal catalysts in the SMSI state which can be explained on the basis of complementary studies performed on well-defined, metal-supported TMO films. In particular, the results of low temperature CO oxidation revealed the key role of weakly bound oxygen species provided by a two-dimensional (“monolayer”) oxide film. The binding energy of such oxygen atoms can be used as a descriptor for oxidation activity. Reaction rate enhancement often observed for TMO films partially covering the metal surface is rationalized within a mechanism in which the metal acts as a promoter to create the most active “oxidered/oxideox” interface formed by reduced and oxidized phases in the film. Although only low temperature CO oxidation is considered, it is tempting to generalize these ideas to other oxidation reactions following the Mars–van Krevelen type mechanism. In addition, metal-supported ultrathin TMO films may serve as well-defined model systems to examine different aspects of the “electron theory of catalysis” which was proposed long ago and which is based on electron transfer mechanisms.Graphical

Electron transfer-induced catalytic enhancement over bismuth nanoparticles supported by N-doped graphene

Catalytic reduction of toxic 4-nitrophenol (4-NP) to useful 4-aminphenol over appropriate nanocatalysts is environmentally desired. Herein, bismuth nanoparticles (NPs) supported by N-doped reduced graphene oxide (Bi/NG) and carbon black (Bi/C) have been synthesized using hydrazine as the reductant. The resulting Bi/NG shows a superior catalytic activity towards the reduction of 4-nitrophenol (4-NP) with a high normalized rate constant (32.1 min−1·mg−1), which is 2.9 times and 18.5 times higher than that of Bi/C and unsupported Bi NPs, respectively. The 4-NP conversion is as high as 99.7% and the yield of 4-AP is estimated to be 99.4%. The electronic structure of Bi NPs is modified through the electron transfer from Bi NPs to NG nanosheets due to so-called strong metal/support interaction (SMSI) effect. Such electron transfer results in abundant adsorption of reactants and rapid generation of active H species, both of which account for the promising catalysis of Bi/NG. Catalyst reuse shows no apparent deactivation or poisoning during the catalytic and separation processes, exhibiting good recyclability and reusability. This work may shed light on the design of high-efficiency catalysts for the 4-NP reduction by virtue of the electronic modification between active metals and carbon-like support.

Influence of the strong metal support interaction effect (SMSI) of Pt/TiO2 and Pd/TiO2 systems in the photocatalytic biohydrogen production from glucose solution

Abstract Two different catalysts consisting of Pt/TiO2 and Pd/TiO2 were submitted to diverse oxidative and reductive calcination treatments and tested for photocatalytic reforming of glucose water solution (as a model of biomass component) in H2 production. Oxidation and reduction at 850 °C resulted in better photocatalysts for hydrogen production than Degussa P-25 and the ones prepared at 500 °C, despite the fact that the former consisted in very low surface area (6–8 m2/g) rutile titania specimens. The platinum-containing systems prepared at 850 °C give the most effective catalysts. XPS characterization of the systems showed that thermal treatment at 850 °C resulted in electron transfer from titania to metal particles through the so-called strong metal-support interaction (SMSI) effect. Furthermore, the greater the SMSI effect, the better the catalytic performance. Improvement in photocatalytic behavior is explained in terms of avoidance of electron–hole recombination through the electron transfer from titania to metal particles.

Direct interactions between metal nanoparticles and support: STM studies of Pd on TiO 2(1 1 0)

We have fabricated ultra-nanoparticulate model catalysts of Pd on TiO 2(1 1 0) using metal vapour deposition (MVD) to form particles in the size range 1–50 nm, which can be imaged at very high spatial resolution (and in some cases at atomic resolution) using scanning tunnelling microscopy (STM). Using these methods we are able to identify the atomic level mechanism responsible for certain phenomena in catalysis, for which molecular level models have previously been proposed from macroscopic measurements. In this paper we address two such phenomena, namely spillover and the so-called strong metal–support interaction (SMSI) effect. Oxygen spillover from Pd particles to the titania support occurs due to the fast adsorption of oxygen on Pd compared with titania, and is driven by reaction with Ti 3+ ions in the vicinity of the particles. The SMSI state is identified at atomic resolution as being due to the appearance of Ti at the surface of the Pd particles. These Ti layers are partially oxidised and form very well defined structures of two main types—a rectangular lattice and hexagonal unit cells of large dimension. These layers passivate the surface for the adsorption of CO.

Effect of the redox treatment of Pt/TiO 2 system on its photocatalytic behaviour in the gas phase selective photooxidation of propan-2-ol

Several titania systems were synthesized by the sol–gel method using two different titanium precursors (titanium isopropoxide or tetrachloride) and diverse ageing methods (magnetic stirring, sonication, reflux and microwave radiation). Screening of such different synthetic conditions led us to choose titanium isopropoxide as the titanium precursor and sonication as the method of choice for ageing the gel. Application of the method to the synthesis of a platinum-doped system resulted in a solid with a BET surface area of 57 m 2/g and consisting of 100% anatase titania. The system was submitted to different oxidative and reductive treatments in order to study the effect of such treatments on catalytic performance in gas-phase selective photooxidation of propan-2-ol. Interestingly, both oxidation and reduction at 850 °C led to an increase in molar conversion and selectivity to acetone as compared to calcination at 500 °C. So much so, that oxidation at 850 °C either in synthetic air flow or in static air resulted in better catalytic performance than Degussa P25, despite the fact that our catalysts consisted in very low surface area (6–8 m 2/g) rutile titania specimens. XPS analyses of the systems showed that thermal treatment at 850 °C resulted in electron transfer from titania to Pt 0 particles through the so-called strong metal-support interaction (SMSI) effect. Furthermore, the greater the SMSI effect, the better the catalytic performance. Improvement in photocatalytic activity is explained in terms of avoidance of electron–hole recombination through the electron transfer from titania to platinum particles.

The SMSI between supported platinum and CeO 2–ZrO 2–La 2O 3 mixed oxides in oxidative atmosphere

CeO 2–ZrO 2–La 2O 3 (CZL) mixed oxides were prepared by citric acid sol–gel method. The as-received gel was calcined at 500, 700, 900 and 1050 °C to obtain the so-called C5, C7, C9 and CK, respectively. The C5, C7 and C9 powders were impregnated with H 2PtCl 6 and then calcined at 500 °C to prepare P5C5, P5C7 and P5C9, respectively. The impregnated CK powders were calcined at 500, 700 and 900 °C to prepare P5CK, P7CK and P9CK, respectively. The XRD and XPS analyses show that the surface distribution of Pt is evidently influenced by the structural and textural properties of the support. The CO adsorption followed by FTIR reveals that the dispersion and the chemisorption sites of Pt are reduced as the calcination temperature of CZL support increases. The chemisorption ability of the CK samples is even completely deactivated. The encapsulation mechanism, which has been applied to explain the so-called strong metal–support interaction (SMSI) after reductive treatment, is introduced here to demonstrate the abnormal observations though the samples were prepared in oxidative atmosphere. The HRTEM results also confirm this explanation. The effects of oxygen vacancies, the chemisorption sites on the Pt surface and Pt/Ce interfacial sites on the three-way catalytic activities are discussed.

High-throughput concept for tailoring switchable mirrors

The optical properties, the switching kinetics and the lifetime of hydrogen switchable mirrors based on Mg–Ni alloys are determined with particular regard to the composition of the optically active metal-hydride layer in combination with the thickness of the catalytic capping layer. For this, a high-throughput experiment is introduced. The switching kinetics and the reversibility of switchable mirrors are strongly thickness dependent, though the details hinge on the fine structure of the clustered capping layer. Therefore, the kinetics is correlated with the surface structures of Pd on Mg y Ni 1− y as investigated by scanning tunneling microscopy. The results are explained by the so-called strong metal–support interaction (SMSI) state, characterized by a complete encapsulation of the capping layer clusters by oxidized species originating from the support. The SMSI-effect is less important with increasing Pd-layer thickness, and is suppressed by a good wetting of the Pd-clusters on the optically active film. This explains the critical thickness for the catalyzed hydrogen uptake observed in many switchable mirror systems. Moreover, the degradation of the kinetics during cycling is found to depend on the Pd-layer thickness and on the gas environment. Only films, covered with at least 15 nm Pd, show small degradation caused by the SMSI-effect. The SMSI-effect is partly reversible: after changing the gas environment from hydrogen to oxygen, the oxide on the Pd-clusters can be partly removed.

Mesosynthesis of ZnO−Silica Composites for Methanol Nanocatalysis

Methanol catalysis meets chemistry under confined conditions. Methanol is regarded as one of the most important future energy sources. ZnO/Cu composite materials are very effective in heterogeneous catalysis for methanol production due to the so-called strong metal-support interaction effect (SMSI). Therefore, materials of superior structural design potentially representing model systems for heterogeneous catalysis are highly desired. Ultimately, such materials could help to understand the interaction between copper and zinc oxide in more detail than currently possible. We report the preparation of nanocrystalline, size-selected ZnO inside the pore system of ordered mesoporous silica materials. A new, liquid precursor for ZnO is introduced. It is seen that the spatial confinement significantly influences the chemical properties of the precursor as well as determines a hierarchical architecture of the final ZnO/SiO(2) nanocomposites. Finally, the ability of the materials to act as model systems in methanol preparation is investigated. The materials are characterized by a variety of techniques including electron microscopy, X-ray scattering, solid-state NMR, EPR, EXAFS, and Raman spectroscopy, and physisorption analysis.

Encapsulated Pd Nanocrystals Supported by Nanoline-Structured SrTiO3(001)

Palladium nanocrystals were grown on a nanostructured SrTiO(3)(001) surface and annealed in ultrahigh vacuum at 620 degrees C. This leads to the so-called strong metal-support interaction (SMSI) state, characterized by encapsulation of the metal clusters with an oxide layer. Scanning tunneling microscopy (STM) of the oxide adlayer on the Pd(111) cluster surface reveals two superstructures with different lattice parameters and crystallographic rotations. Interpretation of the STM images is most readily achieved via noncommensurate TiO(x)() surface layers which result in two distinct Moiré patterns.

?Catalytically active Au on Titania:? yet another example of a strong metal support interaction (SMSI)?

In light of the many similarities between previous studies of the so-called strong metal support interaction (SMSI) involving Gr. VIII metals, and catalytically active Au, it is apparent that these two phenomena must be closely related. That active Au on titania spreads to form a bilayer structure, is electron-rich as determined by theory and experiment, nucleates on reduced Ti defects created by annealing to temperatures > 750 K, and is deactivated via sintering in oxygen, is convincing evidence that the same basic principles responsible for activation of Au on titania are operative for SMSI involving Gr. VIII metals.

Imaging cluster surfaces with atomic resolution: the strong metal-support interaction state of Pt supported on TiO2(110)

Nanosized platinum clusters were grown on a TiO2(110) surface and annealed in ultrahigh vacuum at high temperatures. This leads to the so-called strong metal-support interaction (SMSI) state, characterized by a complete encapsulation of the clusters with a reduced titanium oxide layer. We present atomically resolved scanning tunneling microscopy measurements of the cluster surfaces and an atomic model of the SMSI state. The ability to resolve the cluster surface geometry with atomistic detail may help to identify the active sites responsible for the SMSI.

Influence of strong metal–support interaction on exchange with deuterium and other reactions of hydrocarbons. Part 2.—Studies with Pt/Nb2O5and Rh/Nb2O5

The changes in the catalytic properties of Pt/Nb2O5 and Rh/Nb2O5 caused by raising the reduction temperature from 523 to 773 K have been investigated for the hydrogenolysis of 2-methylbutane and n-pentane. The exchange of deuterium with 2-methylbutane was also studied over the Pt catalyst. The so-called strong metal–support interaction (SMSI) brought about by the high-temperature reduction decreased the rates of hydrogenolysis by factors of 104 or 105 with both catalysts, despite the comparatively low metal dispersions which were measured mainly by CO adsorption. The effects of SMSI were reversible and eliminated by oxidation at 673 K followed by reduction at 523 K. The use of a static reactor system enabled small amounts of initial products, e.g. alkene, to be determined during exchange or hydrogenolysis of 2-methylbutane over Pt/Nb2O5 reduced at 773 K and in all cases, gave detailed information about changes in the patterns of products as the contact time is increased. The relative rates of exchange, dehydrogenation, isomerisation and hydrogenolysis at 572 K over the Pt/Nb2O5 reduced at 773 K were 1000 : 100 : 4 : 1. The alkene formed initially from the reaction of D2 and 2-methylbutane was highly exchanged. The results are used to discuss possible effects of SMSI on the catalysts.

Temperature Programmed Reduction and Spectroscopic Studies of Reduction Behavior of fe/tiO2 Catalyst

AbstractFe/TiO2 catalyst was prepared by incipient wetness impregnation of TiO2 with aqueous solution of ferric nitrate. The reduction behavior of the catalyst was studied by temperature programmed reduction profiles, Mössbauer spectroscopy, x‐ray diffraction and x‐ray photoelectron spectroscopy. The results show that the reduction of Fe/TiO2 was accompanied by a phase transition of anatase to rutile titania. α‐Fe2O3 was reduced to Fe3O4 in the initial reduction stage. Due to the strong support effect of TIO2, FeTiO3 was gradually formed as the reduction temperature reached 450°C. Complete reduction to the metallic Fe° particles occurred at temperatures higher than 670°C. The anatase‐rutile transition was initiated by the reduced Ti3+ ions and led to the formation of TiOx. At higher reduction temperature, TiOx migrated to the surface of metallic Fe° particles forming FeTiOx in the so‐called strong metal‐support interaction (SMSI) state.