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 met-al-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. SiO 2 , γ-Al 2 O 3 , TiO 2 , and ZrO 2 ) were prepared by the impregnation method. The catalysts were characterized by N 2 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 Sb 2 O 3 and also the influence of the nature of the support were investigated. SiO 2 supported Pd catalyst exhibited the highest surface area (192.6 m 2 /g) and pore volume (0.542 cm 3 /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 attri-buted by SbOx species by a typical SMSI effect as known for other reducible supports such as TiO 2 , ZrO 2 , CeO 2 , and Nb 2 O 5 .


Introduction
The catalysis is considered one of the most important research areas in chemistry, related fields, industry and technology which provides new material and conditions to reduce the cost, as well as time and besides that looking for a new and alternative solution for treatment of wastes, moreover catalytic materials have massive contribution to the environmental impact which influence in cleaning the air from toxic gases such as CO and NO x , water wastes, etc.
At the end of 1970s Tauster and coworkers are connected to the strong metal-support interaction (SMSI), they perceived that the adsorption of H 2 and CO to descend after some treatment of oxide-supported noble metal catalysts, most apparently for Pt supported on TiO 2 [1] [2]. Modern advanced electron spectroscopy studies showed that the metal confined which is now regarded as the major demonstration of strong metal-support interaction (SMSI) [3] [4]. SMSI is sometimes used in a more general sense by including support effects on metal dispersion, spatial distribution, and particle shape. Although the Pt/TiO 2 system remains the classic example of SMSI via encapsulation, several other combinations of reducible transition metal oxides (TMO) and metals have shown similar behavior [5] [6] [7] [8]. Despite enormous efforts to directly visualize the encapsulation process, the precise mechanism of encapsulation remains unknown.
Thermodynamic considerations favor oxide spillover onto the metal surface rather than migration through the metal particle and subsequent segregation to the surface. For the past 20 years, a large number of publications have dealt with different aspects of the SMSI effect [1]. This research effort may easily be justified in terms of a fundamental understanding of such a special phenomenon, but also because of its implications in the design of specific preparation, characterisation, or catalytic applications of the metal/support system exhibiting this effect.
Since the very first studies [2] [7] [9], the characteristics of the SMSI effect could be established in a rather precise way. Typically associated with reducible supports, it is characterised by the following features: 1) the catalyst reduced at low temperature show a conventional chemical behaviour, 2) Upon increasing T redn , the chemisorption capability of the dispersed metal phase against the classic probe molecules like H 2 or CO is heavily disturbed (SMSI state). Likewise, substantial modifications of its catalytic behaviour are observed [7]. The phenomena are reversible, i.e. the re-oxidation at an appropriate temperature followed by a mild re-reduction treatment allows recovering the catalyst from SMSI state.

A. Benhmid et al.
In the decade of 1980, the majority of the studies dealing with this effect were focused on M/TiO 2 systems [7]. The experimental conditions allowing both the induction of the SMSI state and the recovery from it were soon established. Regarding the onset of the effect, reduction, usually with flowing hydrogen, at 773 K, may be considered as a typical treatment. As far as the regeneration of the catalyst is considered, re-oxidation in a flow of O 2 at 773 K, followed by reduction at T ≤ 573 K, is generally accepted to induce the reversion of the phenomenon.
SMSI plays a critical role in determining catalytic activity and stability. Gold particles supported on metal oxides, such as TiO 2 , Fe 2 O 3 , and CeO 2 possess significant oxidation activity when compared to unsupported gold particles [10]. SMSI was originally explained as an electron transfer between the support and the metal [11], or by the formation of intermediate phases [12]. Current thinking with respect to the fundamental concepts underlying strong metal-support interactions additionally attaches importance to interfacial and transport phenomena, together with charge redistribution during metal-support interface formation. SMSI gives rise to three major effects (electronic, geometric and bifunctional), as shown in Scheme 1. The change in the electronic properties of the metal catalyst originates from the strong interactions between the cluster and the oxide support [13] [14] [15]. The geometric effect originates from the physical covering of a thin layer of a reducible oxide species blocking active catalytic sites on the metal's surface [7] [16]- [24]. The bifunctional effect provides dual active sites at the perimeter between the metal and the support leading to significantly improved catalytic activity/selectivity. The reaction species can migrate or spillover either from metal or support to the boundary or edge where the chemical reactions occur. Recognition of the importance of SMSI has attracted many researchers in diverse heterogeneous catalytic applications such as chemical synthesis [25] water splitting [26] and environmental engineering [27] [28] [29] [30] [31].
The origin of SMSI was widely discussed. Two major factors were considered [7]. For some authors, the perturbations associated with the electronic interaction Scheme 1. A model of a supported metal particle, illustrating the effects of metal-support interfacial interactions [32]. occurring between the dispersed metal phase and the reduced oxide support would be the determined factor. For some others, the effect had a geometric origin, i.e. upon high temperature reduction; the metal particles would become partly covered by a thin layer of the reduced support, thus blocking the chemisorption active centres at the metal surface. Regarding the latter interpretation, a number of experimental techniques, and particularly high resolution electron microscope (HREM) [16] [33] [34] has clearly shown the occurrence of metal covering (decoration).
Though to a much lesser extent than titania, several other reducible oxides have also been investigated in relation to the SMSI effect. Such is the case of vanadia or niobia-based catalysts [7] [9]. Moreover, the SMSI effect has been reported to occur on Pt/SiO 2 and Pd/La 2 O 3 [35]. These two cases are particularly striking because the supports, SiO 2 and La 2 O 3 are generally considered to be hardly reducible oxides. On Pt/SiO 2 , the metal deactivation effects start to be observed on the catalyst after 10 h at 823 K, they becoming much stronger after a prolonged reduction treatment at 973 k. Severe reduction conditions are thus required to induce the effect [34].
The aim of the present study was to carry out CO-Chemisorption of Pd-Sb/MeOx (to measure the dispersion of Palladium) and to check the SMSI phenomena.

Catalyst Preparation
Preparation of catalysts involves mainly two steps as described below: Step 1: Commercial TiO 2 (anatase, Powder, Kronos, BET-SA-315 m 2 /g) was impregnated with aq. solution of SbCl 3 (Alfa Produkte, Karlsruhe, Germany) by keeping 8 wt% Sb with respect to the total amount of the catalyst and kept it aside for 1 hour followed by precipitation with (NH 4 ) 2 SO 4 and kept at 70˚C for 1 hour on a water bath. After cooling to room temperature the solution was neutralized with 25% ammonia (adjusted to pH of 7) and heated on the water bath for another hour. Afterwards the slurry was filtered and dried on a rota-vapor to remove excess water; the resulting solid mass was further dried in an oven at 120˚C for 16 h, followed by calcination at 400˚C for 3 h in air (50 ml/min).
Step 2: The above mentioned Sb impregnated TiO 2 sample was again impregnated with desired amount of acidified aqueous solution of PdCl 2 (99.8%, Alfa Produkte, Karlsruhe, Germany) to get the desired amount of Pd (10 wt%) the excess solvent was removed by rota vapor, and the sample was dried in an oven at 120˚C for 16 h. In a similar fashion, all of the catalysts were prepared using four different oxide supports such as ZrO 2 , SiO 2 , and γ-Al 2 O 3 . More details on catalyst preparation have been reported elsewhere [36]. The denotation and composition of different samples prepared are presented in Table 1. The composition of the materials was checked by inductively coupled plasma-optical emission spectrometry (ICP-OES), and the values were found to be close to the theoretical ones.

Characterization of Catalysts
The BET surface areas and pore size distribution of the catalysts were determined using a Micromeritics Gemini III 2375 instrument by N 2 physisorption at −196˚C.
Before the measurements, the known amount of catalyst was evacuated for 2 h at 150˚C to remove physically adsorbed water.
Samples for transmission electron microscopy (TEM) were prepared by depositing the sample on a copper grid (300 mesh) coated with lacy carbon film. TEM analysis was performed using a CM-20 microscope (twin) (Philips, The Netherlands) at 200 kV with EDAX PV9900.
Catalyst pretreatment: calcination in air at 300˚C for 2 h, catalyst sample weight 0.12 to 0.14 g.

BET Surface Areas and Pore Size Distribution
BET surface areas and pore volumes of the fresh catalysts prepared with four different supports are presented in Table 1. Of the four supports used, the surface area of the TiO 2 (anatase)-supported catalyst decreased drastically (from 315 to Open Journal of Metal 78 m 2 /g) after impregnation of both Sb (8 wt%) and Pd (10 wt%). However, no such drastic decrease in surface area was seen in the catalysts prepared with other supports. These decreases in surface area and pore volume after the deposition of Sb and Pd is due mainly to penetration of dispersed Pd and Sb species into the pores of the support, as well as to solid-state reactions between the dispersed components and the support. But the extent of decrease in surface areas depends on the nature of the support applied. Table 1 shows that of all of the supports studied in this series, SiO 2 -supported Pd catalyst exhibited the highest surface area (193 m 2 /g) and pore volume (0.542 cm 3 /g); ZrO 2 had the lowest surface area (47 m 2 /g) and pore volume (0.107 cm 3 /g).
Pore size distributions of fresh catalysts are portrayed in Figure 1.

Transmission Electron Microscopy (TEM)
Electron micrographs of the fresh (activated) 10%Pd8%Sb catalysts with different supports are depicted in Figure 2. All of these catalysts showed a narrow particle size distribution of Pd, but with varying sizes, depend on the type of support used. It can be seen from Figure 2   Pd particles (1.5 -2 nm) with a narrow size distribution, more or less comparable to the γ-Al 2 O 3 -supported solid. The decreasing order of Pd particle size in these fresh 10%Pd8%Sb catalysts over the different supports is as follows:

Combined TPR and CO Chemisorption
High temperature reduction (HTR): Figure 5 shows the plot obtained with Pd/Sb 2 O 3 /Al 2 O 3 . The first negative peak is due to H 2 desorption by decomposition β-Pd hydride. That means that a portion of Pd(II) is already reduced to metallic Pd at room temperature (r.t.) during the isothermal segment of the TPR procedure which is not monitored. It is well known that metallic Pd absorbs H 2 into its bulk forming β-Pd hydride. The decomposition peak is superimposed by the further reduction of Pd(II). 3 peaks in the temperature range up to ca. 250˚C point to the reduction of different Pd(II) species or Pd(II) species of different size. The additional peak with maximum at the final temperature should be at- tributed to a reduction of Sb 2 O 3 since reduction of supported PdO or PdCl 2 is usually finished at temperatures below 300˚C. Probably, hydrogen activated on Pd reduces Sb(III) at lower temperature than in case of the Pd free sample. The peak maximum at final temperature is an apparent maximum again, because the applied isothermal holds in the TPR procedure.
A similar HTR-plot was obtained with the Pd/Sb 2 O 3 /ZrO 2 sample ( Figure 6).
Pd hydride decomposition was very distinctive. Again a reduction of Sb 2 O 3 besides Pd(II) was observed.
The β-Pd hydride decomposition is completely superimposed by the H 2 consumption in the case of the Pd/Sb 2 O 3 /TiO 2 sample (Figure 7).    250˚C. This temperature has to be also applied at least for elution of the TPR gas to obtain a hydrogen free Pd surface for a subsequent CO pulse chemisorption measurement.

Low Temperature Reduction (LTR)
TPR Plots for all catalysts are shown in Figures 8-10 and Figure 11. At least the different CO chemisorption can be compared or related to properties of the various supports and to the catalytic activity. The results of the CO chemisorption measurements after LTR are summarised in Table 2. Most surprisingly, a very low CO chemisorption was reproducibly obtained with Pd/Sb 2 O 3 /TiO 2 .
However, in case of large Pd particles also bridged bound CO (CO: Pd = 1/2) is present. Consequently dispersity and surface area could be up to twice and particle size up to a half of the values given in Table 2.     The result has to be considered to correct the values of the dispersity and particle size.
D corr = D × 10%Pd/r%Pd d corr = d × r% Pd/10%Pd Table 3 demonstrates the reversibility of the blocking of the CO chemisorption after HTR and subsequent TPO/LTR, respectively. This behaviour is written in the literature as typical for a SMSI effect [41]. TPO was performed up to 400˚C, subsequent reduction only up to 250˚C. and 60 Å (γ-Al 2 O 3 ), respectively. TEM images display 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.

Conclusions
TPR studies confirmed a close neighbourhood between the catalysts components Pd and Sb. 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 (LTR), almost completely restored the normal chemisorptive properties of the samples, this suppression was attributed by SbOx species by a typical SMSI effect as known for other reducible supports such as TiO 2 , ZrO 2 , CeO 2 , and Nb 2 O 5 . We believe that this is the first report for such SMSI effect caused by the Sb 2 O 3 .