Strong Metal-support Interaction State Research Articles

Strong Metal‐Support Interaction Triggered by Molten Salts

Strong metal‐support interaction (SMSI) plays a vital role in tuning the geometric and electronic structures of metal species. Generally, a high‐temperature treatment (>500 °C) in reducing atmosphere is required for constructing SMSI, which may induce the sintering of metal species. Herein, we use molten salts as the reaction media to trigger the formation of high‐intensity SMSI at reduced temperatures. The strong ionic polarization of the molten salt promotes the breakage of Ti‐O bonds in the TiO2 support, and hence decreases the energy barrier for the formation of interfacial bonds. Consequently, a high‐intensity SMSI state is achieved in TiO2 supported Ir nanoclusters, evidenced by a large number of Ir‐Ti bonds at the interface, at a low temperature of 350 °C. Moreover, this method is applicable for triggering SMSI in various supported metal catalysts with different oxide supports including CeO2 and SnO2. This newly developed SMSI construction methodology opens a new avenue and holds significant potential for engineering advanced supported metal catalysts toward a broad range of applications.

Strong Metal-Support Interaction Triggered by Molten Salts.

Strong metal-support interaction (SMSI) plays a vital role in tuning the geometric and electronic structures of metal species. Generally, a high-temperature treatment (>500 °C) in reducing atmosphere is required for constructing SMSI, which may induce the sintering of metal species. Herein, we use molten salts as the reaction media to trigger the formation of high-intensity SMSI at reduced temperatures. The strong ionic polarization of the molten salt promotes the breakage of Ti-O bonds in the TiO2 support, and hence decreases the energy barrier for the formation of interfacial bonds. Consequently, a high-intensity SMSI state is achieved in TiO2 supported Ir nanoclusters, evidenced by a large number of Ir-Ti bonds at the interface, at a low temperature of 350 °C. Moreover, this method is applicable for triggering SMSI in various supported metal catalysts with different oxide supports including CeO2 and SnO2. This newly developed SMSI construction methodology opens a new avenue and holds significant potential for engineering advanced supported metal catalysts toward a broad range of applications.

Unraveling the Unique Strong Metal-Support Interaction in Titanium Dioxide Supported Platinum Clusters for the Hydrogen Evolution Reaction.

Strong metal-support interaction (SMSI) is crucial to modulating the nature of metal species, yet the SMSI behaviors of sub-nanometer metal clusters remain unknown due to the difficulties in constructing SMSI at cluster scale. Herein, we achieve the successful construction of the SMSI between Pt clusters and amorphous TiO2 nanosheets by vacuum annealing, which requires a relatively low temperature that avoids the aggregation of small clusters. In situ scanning transmission electron microscopy observation is employed to explore the SMSI behaviors, and the results reveal the dynamic rearrangement of Pt atoms upon annealing for the first time. The originally disordered Pt atoms become ordered as the crystallizing of the amorphous TiO2 support, forming an epitaxial interface between Pt and TiO2. Such a SMSI state can remain stable in oxidation environment even at 400 °C. Further investigations prove that the electron transfer from TiO2 to Pt occupies the Pt 5d orbitals, which is responsible for the disappeared CO adsorption ability of Pt/TiO2 after forming SMSI. This work not only opens a new avenue for constructing SMSI at cluster scale but also provides in-depth understanding on the unique SMSI behavior, which would stimulate the development of supported metal clusters for catalysis applications.

Unraveling the Unique Strong Metal‐Support Interaction in Titanium Dioxide Supported Platinum Clusters for the Hydrogen Evolution Reaction

AbstractStrong metal‐support interaction (SMSI) is crucial to modulating the nature of metal species, yet the SMSI behaviors of sub‐nanometer metal clusters remain unknown due to the difficulties in constructing SMSI at cluster scale. Herein, we achieve the successful construction of the SMSI between Pt clusters and amorphous TiO2 nanosheets by vacuum annealing, which requires a relatively low temperature that avoids the aggregation of small clusters. In situ scanning transmission electron microscopy observation is employed to explore the SMSI behaviors, and the results reveal the dynamic rearrangement of Pt atoms upon annealing for the first time. The originally disordered Pt atoms become ordered as the crystallizing of the amorphous TiO2 support, forming an epitaxial interface between Pt and TiO2. Such a SMSI state can remain stable in oxidation environment even at 400 °C. Further investigations prove that the electron transfer from TiO2 to Pt occupies the Pt 5d orbitals, which is responsible for the disappeared CO adsorption ability of Pt/TiO2 after forming SMSI. This work not only opens a new avenue for constructing SMSI at cluster scale but also provides in‐depth understanding on the unique SMSI behavior, which would stimulate the development of supported metal clusters for catalysis applications.

Control of metal-support interaction for tunable CO hydrogenation performance over Ru/TiO2 nanocatalysts.

The catalytic behavior of CO hydrogenation can be modulated by metal-support interactions, while the role of the support remains elusive. Herein, we demonstrate that the presence of strong metal-support interactions (SMSI) depends strongly on the crystal phase of TiO2 (rutile or anatase) and the treatment conditions for the TiO2 support, which could critically control the activity and selectivity of Ru-based nanocatalysts for CO hydrogenation. High CO conversion and olefin selectivity were observed for Ru/rutile-TiO2 (Ru/r-TiO2), while catalysts supported by anatase (a-TiO2) showed almost no activity. Characterization confirmed that the SMSI effect could be neglected for Ru/r-TiO2, while it is dominant on Ru/a-TiO2 after reduction at 300 °C, resulting in the coverage of Ru nanoparticles by TiOx overlayers. Such SMSI could be suppressed by H2 treatment of the a-TiO2 support and the catalytic activity of the as-obtained Ru/a-TiO2(H2) can be greatly elevated from almost inactive to >50% CO conversion with >60% olefin selectivity. Further results indicated that the surface reducibility of the TiO2 support determines the SMSI state and catalytic performance of Ru/TiO2 in the CO hydrogenation reaction. This work offers an effective strategy to design efficient catalysts for the FTO reaction by regulating the crystal phase of the support.

Enhanced Methanol Synthesis over Self-Limited ZnOx Overlayers on Cu Nanoparticles Formed via Gas-Phase Migration Route.

Supported metal catalysts are widely used for chemical conversion, in which construction of high density metal-oxide or oxide-metal interface is an important means to improve their reaction performance. Here, Cu@ZnOx encapsulation structure has been in situ constructed through gas-phase migration of Zn species from ZnO particles onto surface of Cu nanoparticles under CO2 hydrogenation atmosphere at 450 °C. The gas-phase deposition of Zn species onto the Cu surface and growth of ZnOx overlayer is self-limited under the high temperature and redox gas (CO2 /H2 ) conditions. Accordingly, high density ZnOx -Cu interface sites can be effectively tailored to have an enhanced activity in CO2 hydrogenation to methanol. This work reveals a new route for the construction of active oxide-metal interface and classic strong metal-support interaction state through gas-phase migration of support species induced by high temperature redox reaction atmosphere.

Operando induced strong metal-support interaction of Rh/CeO2 catalyst in dry reforming of methane

Strong metal-support interaction (SMSI) is an important concept in heterogeneous catalysis that has a profound effect on the structure and activity of the supported metal catalyst. However, the catalyst with an uncontrolled SMSI state significantly prevents the accessibility of metal surface to reactants by encapsulation process which inevitably limits their practical application. Herein, we demonstrate that under the reaction condition of dry reforming of methane (DRM), the occurrence of SMSI (CO2-SMSI) can be operando induced between Rh and CeO2 which significantly improves the catalytic activity in DRM reaction. Detailed study has revealed that the formation of suitable Rhδ+/Rh0 species in CO2-SMSI is critical in improving CH4 activation, while the presence of permeable encapsulation layer is helpful to provide more active sites. This discovery provides a new approach to overcome the limitation of classical SMSI catalyst in DRM reaction.

Strong Metal-Support Interaction (SMSI) in Au/TiO2 photocatalysts for environmental remediation applications: Effectiveness enhancement and side effects

Strong Metal−Support Interaction (SMSI) is a well-known phenomenon of heterogeneous catalysis that have not been extensively investigated in photocatalytic applications. Moreover, the reactions previously studied for photocatalysts under SMSI state are mainly restricted to energy related uses. The present work seeks to explore the effect of SMSI induced by soft wet-chemistry in a Au/TiO2 photocatalyst with specific focus on photocatalytic environmental remediation. With this aim, the developed photocatalyst has been evaluated considering liquid, gas and solid pollutants in order to represent the wide range of environmental photocatalysis applications. These photooxidation scenarios were methylene blue dissolved in water, gaseous NO, and soot directly deposited on the photocatalyst. The results revealed that the SMSI induction has a generally positive effect on photoactivity promoting the MB and soot removal by 53% and 60%, respectively. However, the SMSI did not provide any additional benefit in the NOx elimination compared to the non-SMSI Au/TiO2 photocatalyst, because the enveloping of AuNPs limits the gold-pollutant interaction.

GuaiacolHDOon La‐modified Pt/Al2O3: Influence of rare‐earth loading

AbstractThe stability of the catalyst used in hydrodeoxygenation (HDO) of biomass‐derived oils needs improvement. La has been applied in delaying Al2O3phase‐change under reaction conditions. Lanthanum (0.5–8 wt.%)‐γ‐alumina was studied as Pt (1 wt.%) carrier aimed at guaiacol (GUA) HDO. Materials characterization included N2physisorption, X‐ray diffraction (XRD), thermal analysis, FTIR, UV–vis, and TPR. Solids pore size (~8–10 nm) was suitable for GUA (kinetic diameter~0.668 nm) hydrotreating. Mixed carriers were amorphous (XRD), suggesting well‐dispersed La domains; meanwhile, carbonates/bicarbonates were formed (from CO2) due to the basic surface properties of modified supports (FTIR). That could impart catalyst stability by inhibiting coking through the passivation of Lewis acidity on Al2O3. Pt reducibility increased with La loading in various formulations. However, that was not reflected in enhanced GUA HDO (T = 488 K and P = 3.2 MPa, batch reactor), presumably due to the strong metal–support interaction (SMSI), where LaOxcovered the metallic Pt particle surface. GUA HDO on various catalysts was approximated by pseudo‐first‐order kinetics (integral regime,k), where deviations were observed as La loading increased, presumably by an SMSI state that could affect the rate‐determining step of the reaction mechanism. Basic sites provided by rare‐earth could contribute to altering HDO reaction pathways as well. At 1 wt.% rare‐earth, GUA HDO was maximized (k~25% higher than that on Pt/Al2O3), with that material also exhibiting similar deoxygenation (85%–90% at total GUA conversion) to the latter Pt over pristine alumina. Conversely, both parameters significantly diminished over the catalyst of the highest La content. Materials at low rare‐earth concentrations deserve further studies focused on catalyst stability under HDO conditions.

Selective oxidation of methane to CO on Ni@BOx via reaction-induced vapor migration of boron-containing species onto Ni

Dynamic migration of support-derived species and subsequent encapsulation of supported metal nanoparticles (NPs) are the well-known features of the Strong Metal-Support Interaction (SMSI) effect. Here, we report occurrence of the classical SMSI state in physical mixture of Ni NPs and porous hexagonal boron nitride powder (Ni|pBN) during methane oxidation reaction at 600 ℃, in which Ni NPs are encapsulated by BOx overlayers derived from the pBN component. The SMSI state can switch the methane oxidation selectivity from CO2 on the fresh Ni|pBN catalyst to CO on the used catalyst. The encapsulation of Ni NPs by BOx overlayers has also been observed in Ni/Al2O3 catalyst which is physically mixed with pBN. Comprehensive characterizations confirm that the SMSI state is induced by vapor migration of boron-containing species e.g. BOxHy from pBN to metal surfaces, in which both high reaction temperature and water product play an important role.

Dynamic interplay between metal nanoparticles and oxide support under redox conditions.

The dynamic interactions between noble metal particles and reducible metal-oxide supports can depend on redox reactions with ambient gases. Transmission electron microscopy revealed that the strong metal-support interaction (SMSI)-induced encapsulation of platinum particles on titania observed under reducing conditions is lost once the system is exposed to a redox-reactive environment containing oxygen and hydrogen at a total pressure of ~1 bar. Destabilization of the metal-oxide interface and redox-mediated reconstructions of titania lead to particle dynamics and directed particle migration that depend on nanoparticle orientation. A static encapsulated SMSI state was reestablished when switching back to purely oxidizing conditions. This work highlights the difference between reactive and nonreactive states and demonstrates that manifestations of the metal-support interaction strongly depend on the chemical environment.

Overturning CO2 Hydrogenation Selectivity with High Activity via Reaction-Induced Strong Metal-Support Interactions.

Encapsulation of metal nanoparticles by support-derived materials known as the classical strong metal-support interaction (SMSI) often happens upon thermal treatment of supported metal catalysts at high temperatures (≥500 °C) and consequently lowers the catalytic performance due to blockage of metal active sites. Here, we show that this SMSI state can be constructed in a Ru-MoO3 catalyst using CO2 hydrogenation reaction gas and at a low temperature of 250 °C, which favors the selective CO2 hydrogenation to CO. During the reaction, Ru nanoparticles facilitate reduction of MoO3 to generate active MoO3-x overlayers with oxygen vacancies, which migrate onto Ru nanoparticles' surface and form the encapsulated structure, that is, Ru@MoO3-x. The formed SMSI state changes 100% CH4 selectivity on fresh Ru particle surfaces to above 99.0% CO selectivity with excellent activity and long-term catalytic stability. The encapsulating oxide layers can be removed via O2 treatment, switching back completely to the methanation. This work suggests that the encapsulation of metal nanocatalysts can be dynamically generated in real reactions, which helps to gain the target products with high activity.

Strong Metal–Support Interactions between Pt Single Atoms and TiO2

Strong metal-support interaction (SMSI) has gained great attention in heterogeneous catalysis field. However, whether single - atom catalyst can exhibit SMSI remains unknown. Here, we demonstrated that the SMSI can occur on TiO 2 supported Pt single atoms but at a much higher reduction temperature than that on Pt nanoparticles (NPs). In SMSI state Pt single atoms neither are covered by, nor sink into the subsurface of, TiO 2 support. The suppression of CO adsorption on Pt single atoms stems from the coordination saturation (18 electron role) rather than the physical coverage by support. Based on the new finding it is revealed that in 3-nitrostyrene hydrogenation reaction single atoms are the real active sites while Pt NPs barely contribute to the activity through selectively encapsulating the NP sites. The finding in this work provides a new approach to study the active sites by tuning the SMSI state.

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.

Carbide-Supported Au Catalysts for Water-Gas Shift Reactions: A New Territory for the Strong Metal-Support Interaction Effect.

Strong metal-support interaction (SMSI) has been regarded as one of the most important concepts in heterogeneous catalysis, which has been almost exclusively discussed in metal/oxide catalysts. Here, we show that gold/molybdenum carbide (Au/MoC x) catalysts feature highly dispersed Au overlayers, strong interfacial charge transfer between metal and support, and excellent activity in the low-temperature water-gas shift reaction (LT-WGSR), demonstrating the active SMSI state. Subsequent oxidation treatment results in strong aggregation of Au nanoparticles, weak interfacial electronic interaction, and poor LT-WGSR activity. The two interface states can be transformed into each other by alternative carbonization and oxidation treatments. This work reveals the active SMSI effect in metal/carbide catalysts induced by carbonization, which opens a new territory for this important concept.

Classical strong metal-support interactions between gold nanoparticles and titanium dioxide.

Supported metal catalysts play a central role in the modern chemical industry but often exhibit poor on-stream stability. The strong metal-support interaction (SMSI) offers a route to control the structural properties of supported metals and, hence, their reactivity and stability. Conventional wisdom holds that supported Au cannot manifest a classical SMSI, which is characterized by reversible metal encapsulation by the support upon high-temperature redox treatments. We demonstrate a classical SMSI for Au/TiO2, evidenced by suppression of CO adsorption, electron transfer from TiO2 to Au nanoparticles, and gold encapsulation by a TiO x overlayer following high-temperature reduction (reversed by subsequent oxidation), akin to that observed for titania-supported platinum group metals. In the SMSI state, Au/TiO2 exhibits markedly improved stability toward CO oxidation. The SMSI extends to Au supported over other reducible oxides (Fe3O4 and CeO2) and other group IB metals (Cu and Ag) over titania. This discovery highlights the general nature of the classical SMSI and unlocks the development of thermochemically stable IB metal catalysts.

Interface-Confined FeOx Adlayers Induced by Metal Support Interaction in Pt/FeOx Catalysts.

Active oxide nanolayers can be stabilized on noble metal surfaces through interface confinement effect in oxide/metal inverse catalysts. Here, using normal metal/oxide catalysts we show that Fe oxide nanolayers can be confined on Pt nanoparticles (NPs) when treating a Pt/FeOx catalyst in Ar or H2/O2 atmospheres at elevated temperatures. Pt NPs partially covered with Fe oxide nanopatches are more active in CO oxidation than bare Pt NPs, while those with fully encapsulated Fe oxide shells in strong metal-support interaction (SMSI) state show much lower activity. Characterization results indicate that three steps play an important role in the formation of Fe oxide overlayers: Pt-aided reduction of interfacial Fe oxide, Pt alloying of interfacial Fe atoms, and surface segregation of alloyed Fe atoms onto surface of Pt NPs. Active surface oxides in so-called support-metal interface confinement (SMIC) state and fully encapsulated oxide layers in the SMSI state can be sequentially produced which depend on the treatment conditions.

Effects of Support on Sulfur Tolerance and Regeneration of Pt Catalysts Measured by Ethylene Hydrogenation and EXAFS

The effect of support on sulfur tolerance and regenerability under reducing environments was investigated by rate measurements for ethylene hydrogenation, hydrogen chemisorption, and extended X-ray absorption fine structure (EXAFS). Catalysts, 1 % Pt/Al2O3 and 1 % Pt/P25 (TiO2), were tested after sulfidation in H2S/H2 at 250 °C followed by regeneration treatments in H2 at 250, 350 and 450 °C. Our combined results showed a 20–27 times decrease in the rate of ethylene hydrogenation on both sulfided catalysts, accompanied by a 4–6 times drop in the Pt surface area. Regenerations up to 450 °C were unable to remove all the sulfur, as evidenced by the presence of Pt–S bonds by EXAFS at about 2.25–2.33 A, characteristic lengths for chemisorbed sulfur and bulk-type PtS. However, a partial recovery of the hydrogenation rate per mole of Pt was observed on sulfided Pt/Al2O3 after reduction at 450 °C, while the induction of strong metal support interactions (SMSI) at reduction temperature above 350 °C was observed on Pt/P25, regardless of the presence of sulfur. For Pt/P25, the reversal of the SMSI state together with sulfur removal by mild oxidation suggests that sequential reduction/oxidation treatments may be more effective in restoring the S-free state of TiO2-supported catalysts. Pt/Al2O3 and Pt/TiO2 (P25) sulfur tolerance and regenerability were evaluated after reduction treatments in H2. Both catalysts were equally poisoned by sulfur based on C2H4 hydrogenation. Reduction treatments up to 450 °C were not able to remove sulfur on either catalyst. Sulfur on Pt may inhibit the formation of the SMSI state on Pt/TiO2, especially below 350 °C.

Strong Metal–Support Interactions between Gold Nanoparticles and ZnO Nanorods in CO Oxidation

The catalytic performances of supported gold nanoparticles depend critically on the nature of support. Here, we report the first evidence of strong metal-support interactions (SMSI) between gold nanoparticles and ZnO nanorods based on results of structural and spectroscopic characterization. The catalyst shows encapsulation of gold nanoparticles by ZnO and the electron transfer between gold and the support. Detailed characterizations of the interaction between Au nanoparticles and ZnO were done with transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), electron paramagnetic resonance (EPR), and FTIR study of adsorbed CO. The significance of the SMSI effect is further investigated by probing the efficiency of CO oxidation over the Au/ZnO-nanorod. In contrast to the classical reductive SMSI in the TiO(2) supported group VIII metals which appears after high temperature reduction in H(2) with electron transfer from the support to metals, the oxidative SMSI in Au/ZnO-nanorod system gives oxygen-induced burial and electron transfer from gold to support. In CO oxidation, we found that the oxidative SMSI state is associated with positively charged gold nanoparticles with strong effect on its catalytic activity before and after encapsulation. The oxidative SMSI can be reversed by hydrogen treatment to induce AuZn alloy formation, de-encapsulation, and electron transfer from support to Au. Our discovery of the SMSI effects in Au/ZnO nanorods gives new understandings of the interaction between gold and support and provides new way to control the interaction between gold and the support as well as catalytic activity.

Imaging of low temperature induced SMSI on Pd/TiO2 catalysts

Pd/TiO2 catalysts were found to enter an strong metal support interaction (SMSI) state after reduction at temperatures as low as 473 K. This was identified as a significant loss in the CO uptake as monitored by low temperature FTIR. Electron microscopy provides direct evidence of the presence of ordered, reduced titanium oxide layers over palladium (SMSI state) for Pd/TiO2 catalysts following reduction at temperature 623 K. The crystal phase was identified as Ti4O7 and this phase, once formed, was found to be stable even after exposure to atmospheric conditions.