Metal-support Interaction Effect Research Articles

Effect of Annealing PtNi Nanophases on Extended TiO2 Nanotubes for the Electrochemical Oxygen Reduction Reaction

Titanium dioxide nanotube (TONT) arrays grown by electrochemical anodization were used as supports for alloyed PtNi catalysts which were developed by a cosputtering technique. The as-deposited PtNi on TONT had poor oxygen reduction reaction (ORR) activity with a significant overpotential, while the ORR activity of annealed PtNi∕TONT was improved. The maximum ORR activity, with a significant increase of the half-wave potential, was achieved for annealed PtNi∕TONT. This favorable enhancement was attributable to two factors. The first is the formation of a PtNi alloy catalyst, which resulted in the modification of the -electronic structure of Pt, leading to the favorable adsorption of . The second factor is a strong metal-support interaction (SMSI) effect. When the high-temperature thermal treatment was conducted in the reducing gas (hydrogen gas) on the PtNi∕TONT, there was electron transfer from the reduced support to the Pt catalyst. This caused a change of the work function that altered the electronic properties of surface Pt atoms (X-ray photoemission spectroscopy measurement), leading to an enhancement of the ORR activity. To investigate the SMSI effect further, the thin Pt∕TONT samples were prepared and used for the investigation of the interfacial region between the TONT support and the Pt catalyst.

Interactive supported electrocatalysts and spillover effect in electrocatalysis for hydrogen and oxygen electrode reactions

The aim of the present paper has been to introduce the electron conductive and d-d-interactive individual and composite hypo-d-oxides of the increased altervalent capacity, or their suboxides (Magneli phases), as catalytic supports and therefrom provide: (i) The Strong Metal-Support Interaction (SMSI) effect, and (ii) the Dynamic spillover interactive transfer of primary oxides (M-OH) for further electrode reactions, and thereby advance the overall electrocatalytic activity. The d-band has been pointed out as the bonding, adsorptive and catalytic orbital. In the same context, the phenomenon and significance of the d-d-correlations both in heterogeneous catalysis and electrocatalysis are displayed and inferred. Since hypo-d-oxides feature the exchange membrane properties, the higher the altervalent capacity, the higher the spillover effect. Potentiodynamic experiments have shown that the reversible peak of the primary oxide growth on Pt, Ru and Au supported upon hypo-d-oxides and suboxides becomes distinctly increased in the charge capacity and shifts to remarkably more negative potential values, so that it starts even within the range of H-adatoms desorption, while its reduction extends until and merge with the UPD of hydrogen atoms. With wet tungstenia doped titania supported Pt catalyst in membrane cells these peaks dramatically increase in their charge capacity and reversibly become shrunk with a decreased moisture content in the feeding inert gas mixture, and vice versa. Such distinct potentiodynamic scans, in conjunction with some broaden complementary kinetic electrocatalytic improvements rising from the same hypo-d-oxide and/or suboxide interactive support effects, have been proved to be the best and comparable experimental evidence for the spillover effect of primary oxides.

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 Influence Of The Support On The Deactivation Behaviour of Pt-Sn Reforming Catalysts

The effect of Al2O3-ZrO2, Al2O3-TiO2 and Al2O3-La2O3 mixed oxides on the deactivation of bifunctional Pt-Sn/ Al2O3 reforming catalysts has been investigated. The n-heptane reforming at 500°C was used as a test reaction. Changes in the catalytic behavior due to differences of the acidity and the support-metal interaction were observed. Levenspiel´s and Beltramini´s deactivation models were developed assuming a series fouling for the carbonaceous deposits. Through the Beltramini’s model it was possible to distinguish the amount of acidity that participates in the deactivation processes. Both models successfully correlated with the experimental profiles of n-heptane activity decay. The following deactivation decreasing order, Al2O3-TiO2-1>Al2O3-ZrO2-25>Al2O3-TiO2-2> Al2O3>Al2O3-La2O3-10 was found with both deactivation models. A quasi-linear correlation between the deactivation order and the coke formation kinetic constant (Levenspiel’s parameters) was observed. The catalyst acidity and the n-heptane conversion were correlated with Beltramini’s model. It was found that a high acidity (12 X 10-17 acid sites/g cat.) or metal dispersion (83%) increases the catalyst deactivation and it is necessary to have a balance of active sites in order to have a catalyst working as a bifunctional catalyst. On the other hand, the auto-regeneration Beltramini’s parameter suggests that the lowest deactivation of the Pt-Sn/ Al2O3-La2O3-10 catalyst is attributed to the cleaning capacity of the active sites. It was observed that the highest deactivation (80-92%) of the platinum-tin catalysts supported in alumina-titania mixed oxides were a result of the strong metal-support interaction (SMSI) effect. The Pt-Sn/Al2O3-La2O3-10 showed the best catalytic behavior with high initial and residual conversions (70 and 48%, respectively) and low deactivation (17 %) at a 50-minute reaction time. Furthermore, in the Pt-Sn/Al2O3 catalyst, the benzene yield was 1%, while the Pt-Sn/Al2O3-La2O3-10 showed a total inhibition of benzene production yield at residual conversions.

Interaction of Pt and Rh nanoparticles with ceria supports: Ring opening of methylcyclobutane and CO hydrogenation after reduction at 373–723 K

The catalytic properties of a Pt (4%) and a Rh (2.5%) catalyst on a low-surface area ceria support were determined as a function of hydrogen reduction in the temperature range between 373 and 723 K and compared to those of silica-supported Rh (3%) subjected to equivalent treatments. Two reactions were studied: the ring opening of methylcyclobutane (MCB) as representative for structure-sensitive hydrocarbon reactions and the hydrogenation of CO as an example for C O bond activation. After reduction in the low- and mid-temperature range the rates of either reaction decrease significantly on ceria-supported Pt and Rh whereas they are hardly affected on Rh–silica. Annealing in vacuum at 723 K before the reduction step has a beneficial effect on the reaction rates but annealing after reduction leads to a general activity decrease. Accompanying ex situ high-resolution electron microscopy (HREM) investigations largely exclude particle decoration and formation of noble metal–Ce intermetallic bonds as possible effects of metal-support interaction at low and medium reduction temperatures. Taking also into account parallel surface science studies it is concluded that the observed activity decrease originates mainly from electronic perturbations at the interface between the metal nanoparticles and the increasingly reduced ceria support.

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.

Effect of the strong metal-support interaction on hydrogen sorption kinetics of Pd-capped switchable mirrors

The morphology and electronic structure of Pd clusters grown on oxidized yttrium surfaces are investigated by scanning tunneling microscopy and ultraviolet photoelectron spectroscopy. The hydrogen sorption mediated by the Pd clusters is determined from the optically monitored switching kinetics of the underlying yttrium film. A strong thickness dependence of the hydrogen uptake is found. The electronic structure of the as-grown Pd clusters depends only weakly on their size. Strong changes of the photoemission spectra are found after hydrogenation, in particular the oxide peak shifts and the Pd peaks vanish. Both phenomena are due to a strong metal-support interaction (SMSI) state, characterized by a complete encapsulation of the clusters by a reduced yttrium oxide layer. Scanning tunneling spectroscopy confirms the SMSI state of small Pd clusters after hydrogen exposure. The SMSI effect is less important with increasing Pd thickness. This explains the critical thickness for the catalyzed hydrogen uptake by the Pd/ YOx / Y system. The results shed light on the mechanism of hydrogen absorption at the triple point gas-catalyst-oxide, which also plays an important role in today’s fuel cell technology.

Extended Brewer hypo–hyper- [formula omitted]-interionic bonding theory — I. Theoretical considerations and examples for its experimental confirmation

The Extended Brewer Interactive Interionic Bonding Theory (EBIIBT) has been developed to show the equivalence of interatomic and interionic bonding feature, as well as its effect upon electrocatalytic properties for the hydrogen electrode reactions. The equivalence of interionic hypo–hyper- d-interelectronic interaction in both metallic and any other ionic or their composite states has been proved and inferred. In such a context, the EBIIBT stays in the core of the Strong Metal-Support Interaction (SMSI) effect in heterogeneous catalysis, and the latter is an implied consequence of the former. Thermal Gravimetry (TG) analysis of Temperature Programmed Reduction (TPR) of mixed hypo–hyper- d-electronic oxides of transition elements was broadly employed to prove the extended Brewer theory along the phase diagrams of such constituents, as reflected in dramatically decreased temperatures of their mutual reduction versus the individual metallic ingredients entering into the resulting intermetallic phases or alloys. The same interionic Brewer (and/or intermetallic) bonding effect has also been confirmed both by Underpotential Deposition of hyper- d- upon hypo- d-electronic substrates and vice versa, and by the shift of the bonding peaks in XPS analysis. The former affords the basis for new trends in submonolayer hypo–hyper- d-interelectronic electrocatalysis of transition metals. Thus, it has been pointed out that the term SMSI has primarily a broader hypo–hyper- d-interelectronic interactive sense in both the bonding effectiveness and synergism in electrocatalysis, including bifunctional catalytic meaning as based upon the interionic interactive promotion.

Metal–support interaction effects on the growth of filamentous carbon over Co/SiO 2 catalysts

The addition of BaO, La 2O 3 and ZrO 2 to the SiO 2 support of a 12 wt.% Co/SiO 2 catalyst modifies the reduction behavior of Co species and leads to changes in metal dispersion. These changes are due to a modified metal–support interaction (MSI) between cobalt species and the Me x O y /SiO 2 (Me x O y : BaO, ZrO 2 and La 2O 3) support. Catalyst characterization by temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) have been used to determine the relative strength of the MSI and the results suggest an increasing MSI in the order Co/SiO 2≈Co/BaO/SiO 2<Co/La 2O 3/SiO 2<Co/ZrO 2/SiO 2. The rate of catalyst deactivation during methane decomposition (CH 4⇔C+2H 2) is shown to increase with increasing MSI. Analysis of the used catalysts also shows that an increasing rate of deactivation correlates with an increasing amount of graphitic carbon versus carbidic carbon on the used catalyst. It is suggested that an increase in graphitic carbon is a consequence of a strong MSI that limits carbon removal from the metal surface by filament formation. Consequently, graphitic, encapsulating carbon is formed from the carbon deposited during methane decomposition, leading to deactivation of the catalyst.

Kinetic study of liquid-phase hydrogenation of citral over Ir/TiO 2 catalysts

The kinetics of liquid-phase hydrogenation of citral (3,7-dimethyl-2,6-octadienal) on Ir/TiO 2 catalysts were studied in the temperature range of 303–363 K and a pressure range of 4.13–10.3 bar H 2. Geraniol and nerol were the only reaction products, generated through the hydrogenation of the carbonyl group of citral. Negative near second-order dependence on citral concentration and near first-order dependence on hydrogen pressure were observed for the initial rate of citral hydrogenation over the Ir/TiO 2-LTR and -HTR ( T red=473 and 773 K) catalysts. Furthermore, an increase in the catalytic activity as the reaction temperature, pressure and weight of catalysts was observed. Conversely, an increase in the citral concentration produces a drop in the reaction rate. Conventional Arrhenius behaviour was exhibited by all catalysts, Ir/TiO 2-LTR and -HTR showed activation energies of 15.9 and 4.6 kJ/mol, respectively. An important enhancement in the catalytic activity was observed in the HTR catalyst compared with the LTR counterpart, which is attributed to a strong metal support interaction effect. The reaction kinetics with Ir/TiO 2-LTR and -HTR catalysts under differential regimes was described by a conventional Langmuir–Hinshelwood treatment. It was found that under the conditions studied, the hydrogen adsorption was the rate-determining step.

Some contributions of electron microscopy to the characterisation of the strong metal–support interaction effect

The contribution of electron microscopy techniques to establishing the existence and actual nature of the metal–support interaction effects occurring in a variety of supported metal catalysts is reviewed. Data pertaining to systems based on both classic reducible supports like TiO 2, CeO 2 or some ceria-based mixed oxides, and several others generally considered as non-reducible oxides, like La 2O 3 or SiO 2, are presented and discussed. The specific temperature and chemical conditions under which strong metal–support interaction phenomena are onset or reverted in each case are also analysed. The whole set of data presented and discussed here clearly shows that the electron microscopy techniques have made an outstanding contribution to the characterisation of the strong metal/support interaction effects exhibited by different metal/oxide systems. Likewise, it is demonstrated that this powerful family of techniques has very much helped to discriminating between true SMSI-like phenomena, as originally defined by Tauster et al., and several other apparent effects which, though at a first sight show some of the chemical, nano-structural and/or catalytic characteristics of the SMSI effect, have a neatly different origin.

Reaction and surface characterization studies of titania-supported Co, Pt and Co/Pt catalysts for the selective oxidation of CO in H 2-containing streams

The selective, catalytic oxidation of CO in H 2-containing streams was investigated over titania-supported Co, Pt and Co/Pt catalysts. The work was motivated by the need to remove CO from the reformate fed to a polymer electrolyte fuel cell (PEFC) before it can poison the Pt anode. Pt/TiO 2 catalysts were active over a broad range of temperatures with a maximum at 120 °C. Co/TiO 2 catalysts were effective for CO oxidation at high temperatures (>100 °C). Mixed metal Co/Pt catalysts were quite effective, even at room temperature. The presence of H 2O and CO 2, major components of actual reformate, reduced the activity of all the catalysts tested, but less so for the Co/Pt catalyst. Activity decreased over long times but an intermittent, high-temperature “purge” regenerated the original activity. X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) were used to understand the synergism between metal components. The Pt either influenced activity by helping reduce the Co, or the Pt and/or Co stabilized a Ti suboxide state that may play a role in the reaction process. The latter is most likely attributable to the strong metal support interaction (SMSI) effect previously noted on Pt/TiO 2 catalysts.

1,3-Butadiene hydrogenation on pd-supported systems: geometric effects

A strong metal support interaction (SMSI) effect was observed on Pd/Nb2O5 and Pd/TiO2 catalysts, and it produces small, exposed Pd ensembles. A decrease in the trans/cis 2-butene ratio was observed after reduction at 773 K. Selectivity changes were ascribed to the decoration model. Theoretical models were developed based on semi-empirical molecular-orbital calculations for 1,3-butadiene and Pd n clusters. Experimental results are in agreement with our theoretical model, which proposes a greater stabilization of the cisoid intermediate on small Pd ensembles.

Bound-state Ni species — a superior form in Ni-based catalyst for CH 4/CO 2 reforming

The effects of nickel loading, calcination temperature, support, and basic additives on Ni-based catalyst structure and reactivity for CH 4 reforming with CO 2 were investigated. The results show that the structure of the nickel active phase strongly depends on the interactions of the metal and the support, which are related to the support properties, the additives and the preparation conditions. “Free” Ni species can be formed when the interaction is weak and their mobility makes them easily deactivated by coking and sintering. The effect of strong metal-support interaction (SMSI effect) is different for various supports. The formation of solid solution of Ni–Mg–O 2 and the blocking of TiO x by the partially reduced TiO 2 can both decrease the availability of Ni active sites in Ni/MgO and Ni/TiO 2. The spinel NiAl 2O 4 formed in Ni/γ-Al 2O 3 might be responsible for its high activity and resistance to coking and sintering because it can produce a highly dispersed active phase and a large active surface area as bound-state Ni species when the catalyst is prepared at high calcined temperatures or with low nickel loading. The addition of La 2O 3 or MgO as alumina modifiers can also be beneficial for the performance of the Ni/γ-Al 2O 3 catalyst.

Stable Ni/Al2O3 catalysts for methane dry reforming

To obtain insight into the effect of metal-support interactions in the CO2 reforming of CH4, a series of alumina-supported nickel catalysts were prepared by calcination of the catalyst precursors in air at different temperatures. The increase in the intensity of Ni-Al2O3 interactions with the calcination temperature was found to be unfavorable to the reduction of the catalyst. However, the catalyst with strong Ni-Al2O3 interactions suppressed carbon deposition effectively, which can be attributed to the formation of spinel, NiAl2O4, after calcination. When the reaction was carried out at temperatures higher than 973 K, all the catalysts tended to exhibit equilibrium activity.

Enhancement of the Low Temperature Activity in NO Reduction in Lean Conditions by SMSI Effect in Pt/CeO2–ZrO2 on Alumina Catalyst

A treatment with H2 at 800°C followed by mild reoxidation (<300°C) increases the low temperature (<200°C) steady-state activity in the reduction of NO by propene in the presence of O2 of a Pt(1.5%)/(ZrO2–CeO2 on Al2O3) catalyst. The effect is not present for a lower temperature treatment with H2 or for reoxidation above 300°C. Transient reactivity tests together with characterization data (H2-TPR, O2 uptake, H2 chemisorption) indicate that the high temperature reduction modifies the characteristics of ceria–zirconia and induced metal–support interaction effects, both responsible of the enhanced catalytic activity at low temperature.

Electron Microscopy (HREM, EELS) Study of the Reoxidation Conditions for Recovery of NM/CeO2 (NM: Rh, Pt) Catalysts from Decoration or Alloying Phenomena

The reversibility of metal–support interaction effects in NM/CeO2 catalysts (NM: Rh, Pt) is investigated using high-resolution electron microscopy and electron energy loss spectroscopy. Reoxidation treatments at 773 K followed by a mild reduction at 473 K are not effective in recovering ceria-based systems from the decorated or alloyed states observed upon high-temperature reduction (T red>973 K).

New evidence for the electronic nature of the strong metal-support interaction effect over a Pt/TiO 2 hydrogenation catalyst

Analysis of the Pt 4 f line asymmetry evidenced that the suppression of the hydrogenation activity of Pt/TiO 2 in the SMSI state is caused by a decrease in the d-electron density at the Fermi level of platinum particles, while the net charge of the metal particles remains unaltered.

Manifestation of the effect of strong metal-support interaction in a Pt/TiO2 rutile catalyst

Pt/TiO2 catalysts supported on ultradispersed rutile are prepared. The effect of strong metal-support interaction (SMSI) in these catalysts is more pronounced than in Pt/TiO2 catalysts with greater fractions of an anatase phase in the support. Mild oxidation of the rutile catalyst eliminates the SMSI effect.

Pt–TiO2–γ Al2O3Catalyst*1I. Dispersion of Platinum on Alumina-Grafted Titanium Oxide

Polymeric titanium oxides grafted on γ-Al2O3were used as supports for platinum catalysts containing 1 wt.% Pt. The supports were prepared from an interaction of alumina with titanium isopropoxide, impregnated with hexachloroplatinum acid (H2PtCl6) and calcined at 773 K. Pure alumina and titania were used for comparison. The solids were characterized by XRD, TPR, DRS-UV-vis, and TEM. Frontal hydrogen chemisorption was measured after TPR. CO adsorption isotherms were recorded after reduction at low (573 K) and high (773 K) temperature. The results indicate that the polymeric titanium oxide layer in the catalyst, although reducible under hydrogen at high temperature, does not promote a strong metal–support interaction effect. Platinum dispersions in the titania-modified catalysts are higher than in alumina catalysts.