Synthesis and Characterisation of Platinum Nanoparticles Capped with Isolated Zinc Species in SBA-15 channels: the Wall Effect

The strong directing effects and difficulties in the removal of organic based surfactants makes the templated synthesis of nanoparticles in solid porous structures of defined molecular sizes such as SBA-15, without the use of surfactants, considerable attractive. However, the effects of their internal surface structures, adsorption affinities and lattice mismatch on the particle morphology grown therein have not been fully appreciated. Here, we report the internal surface of the silica preferentially hosts isolated tetrahedrally coordinated oxidic Zn species on the molecular walls of the SBA-15 channels from wet impregnated Zn2+ and Pt2+ species. This leads to less thermodynamic stable but kinetic controlled configuration of atomic zinc deposition on core platinum nanoparticles with unique confined lattice changes and surface properties to both host and guest structures at the interface upon reduction of the composite. This method on templated nanoparticles may generate interests to form new catalytic tunable materials. With increasing world population1, diminishing access to fossil fuel reserves2 and a rapidly developing world, the need for new or more efficient routes of energy and chemicals provisions has never been more vital.3 Tailoring catalyst structures and surfaces for specific chemical reactions to improve catalytic efficiencies is continuously of interest to the catalysis community. Many bimetallic catalysts have recently been found to show superior activity or selectivity than their monometallic counterparts due to surface site blocking and electronic modulation.4,5 For example, bimetallic nanoparticles containing platinum are often used in catalytic reactions with low activation barriers and enhanced activity or selectivity.6 Platinum zinc species are reported to be active for hydrogenation and dehydrogenation reactions.7–9 However, size control in bimetallic nanoparticles is commonly achieved in the presence of surfactants or polymers which may impair working catalysts and are not easy to remove for most catalytic applications.10 There are some recently reported surfactant-free syntheses methods in catalyst preparation.11–14 Synthesis of these bimetallic nanoparticles inside thermally stable porous structures as supports, without separating the templates, perhaps represents one of the most attractive methods.15,16 Supported nanoparticles have been well researched along with the introduction of the smallest particle possible, single-atom catalysts.17–19 Surface decoration by single atoms on monometallic particles are anticipated to give high catalytic performances.20–22 For example, it has been seen that zinc deposited on a copper nanoparticle has increased catalytic ability with enhanced formate binding.23 One method of synthesizing single isolated sites is through the use of ion-exchange Bronsted proton of Al sites in zeolites. It has been seen that Zn2+ species can be exchanged with the protonic sites in the zeolite surface leaving isolated Zn ions immobilised in the zeolite walls.24,25 The introduction of these dopant ions into the zeolite walls can lead to the formation Lewis acid sites.24,26 The nature of these acidic sites can depend on the synthesis method, as well as the dopant type and concentration. Various methods of binding Zn2+ onto the surfaces of zeolites and silicas have been discussed, many proposing a tetrahedral coordinated species with the surface oxygens binding to the Zn2+, allowing it to be capped 2 by other reagents.25,27 These zinc modified zeolites have been shown to have enhanced catalytic behaviour for methane activation as well as propylene hydrogenation.28,29 Further investigation was performed on zincosilicate CIT-6 leading to enhanced Diels-Alder reactivity which was not previously possible with alternative dopants.30 However, the small size of internal cavities of zeolites do not allow the synthesis of metal nanoparticles to be decorated with these isolated species. There has been a previous attempt utilising the mesoporous channels of SBA-15 silica as a porous support to promote a core-shell growth of AgNi particles31 without the use of a surfactant. The 1-dimensional channels not only restrict the particle growth ensuring a small particle size distribution, but are thought to encourage the two metal precursors to grow separately forming a phase boundary under mild reduction temperatures as benign hosts. The core and shell thickness can be tuned by using different precursor ratios. The incorporation of Ni to Ag particles of defined particle size may generate new catalysts for selective hydrogenation.32–34 It is, however, the effects of the internal surface structure of SBA-15, surface affinities and lattice mismatches on the metallic particle morphology grown therein and vice versa to the SBA-15 host structure have not been fully appreciated. Here, we report the synthesis of the PtZn bimetallic system in SBA-15 channels using the same methodology. Instead of obtaining thermodynamic more stable Pt-Zn core-shell or their homogeneous alloy structure, it is interesting to see the surface atomic doping of Zn atoms on the core Pt nanoparticles with kinetic controlled lattices at the materials interface, in this porous template after mild reduction. We attribute this interesting observation to the high affinity of silica walls as the main influencing factor for hosting Zn2+, allowing preferential homogeneous wetting on silica internal surface during preparation, presumably due to comparable bonding environment and hydrophilic properties. In addition, the structural mismatches of host and guest can also affect their final lattice parameters, giving the fundamental basis for new catalytic properties of the composites. Thus, we also show that the effects on the porous template structure in molecular dimensions, internal surface structure and chemical affinity play important roles to the growth of nanoparticles therein and could not be ignored. It is anticipated that the rationalizations may be further developed as a new synthesis method for atomic deposition of metal nanoparticles within porous template structures. Results and discussion Platinum zinc of differing compositions (Pt2Zn8, Pt5Zn5, Pt8Zn2) along with monometallic Pt and Zn were synthesized without the use of surfactants within the 1dimensional channels of the mesoporous silica support, SBA-15, in order to encourage small nanoparticle growth therein. These bimetallic PtZn/SBA-15 particles after reduction, along with the Zn/SBA-15 and Pt/SBA-15 monometallic comparison samples, are extensively characterized by Transmission Electron Microscopy (TEM), Low Energy Ion Scattering (LEIS), X-Ray Absorption Spectroscopy (XAS), Wide Angle X-ray Scattering (WAXS), Small Angle X-ray Scattering (SAXS) and Atom Probe Tomography (APT) for a full structural and spatial understanding. Transmission Electron Microscopy (TEM) images were taken of the samples prepared through the reported synthesis in order to confirm the nanoparticle formation inside the silica channels and ensure no larger particles were aggregating on the silica surface. To control the particle size and distribution of the two metals, the particles must form inside the channels rather than on the external silica surface. Figure 1A, which shows the particles are well distributed inside the SBA-15 channels and with no larger aggregated particles on the outside of the porous silica. This was confirmed by secondary electron imaging, Figure 1C, showing that there are no particles on the surface of the silica in the same region where particles are seen in the bright field image, Figure 1B. The Pt2Zn8/SBA-15 nanoparticles have a mean diameter of 4.2 nm (±1.9 nm), which is narrower than the average SBA15 channel diameter of 6.5 nm (S1) meaning that the channels are not blocked for any reactants. This is also valid for the Pt5Zn5/SBA-15 and the Pt8Zn2/SBA-15 which have mean diameters of 3.2 nm (±2.3 nm) and 3.1 nm (±1.5 nm) respectively (S2&S3). Pt/SBA-15 nanoparticles show a similar particle size of 3.1 nm (±1.5 nm) (S4). Interestingly, in the direct comparison with Pt nanoparticles, no particles or areas of material aggregation could be seen in the Zn/SBA-15 sample, indicating much smaller Zn species were present (Figure 2), indicative of the strong wetting properties of silica walls towards Zn species. Scanning EDX was performed in conjunction with the secondary electron imaging in order to understand the elemental distribution of these samples. Figure 1 D, E & F shows the presence of both platinum and zinc in the nanoparticles, with zinc located fully across the silica support, indicating the zinc spreads across the walls of the silica rather than being only deposited at the platinum particles. The higher density regions of zinc near the platinum suggest that there is a favourable interaction between the zinc and the platinum with platinum particles capped with zinc species on the silica surface (see further TEM and EDX in SI S1-S5).