Plasma Synthesis of Highly Dispersed Metal Clusters Confined in Nanosized Zeolite

Abstract

Metal-containing zeolites (and, more generally, porous materials) are very important for catalysis, electronics, and sensors, but their preparation is still a problem. Reproducibility and control of the properties of the metal cluster is difficult. Broad size distribution of the metal particles together with a low stability of the clusters is often observed. New methods have been reported, such as UV photolysis and g irradiation of hydrated metal complex ions incorporated within the microporous molecular sieves. Organic templates such as tetraalkylammonium cations can stabilize separate metal clusters and prevent agglomeration and also limit the metal loading in the material. However, such solids still need to be heat treated (calcined), which may result in a slow agglomeration of the clusters and a possible migration to the outer surface of the matrix, with pore blocking and low energy efficiency. Postsynthetic treatment by nonthermal plasma was recently applied to metal-loaded templated mesoporous materials (MCM and SBA-15). The plasma is believed to dissociate and ionize residual water molecules in the mesoporous matrix, creating hydrated electrons e ðaqÞ and radicals H· for reduction of the metal ions to clusters. The authors however still needed to use additional water and apply a final thermal treatment to remove residual impurities, resulting in partial sintering of the metal clusters. We report herein a one-step environmentally friendly approach for the preparation of active and highly dispersed metal clusters confined predominantly in the channels of nanosized BEA type zeolite (diameter of individual particles of 10 nm) by plasma treatment. This approach, which was discovered using operando FTIR in the plasma reactor, leads to a fast and simultaneous calcination of the nanosized molecular sieve (removal of the organic template) and reduction of the metal cations to clusters. Prior to calcination, the nanosized pure BEA and Pt-containing BEA (Pt/BEA, 0.75 wt% Pt) zeolites were directly subjected to Ar or O2 cold plasma treatments at low pressure (350 Pa). The changes in the samples under plasma treatment followed by operando FTIR spectroscopy are shown in Figure 1. The initial spectra of the pure BEA and Pt/BEA samples exhibit the methyl stretching modes at 3100–2800 cm 1 originating from the tetraalkylammonium (TEA) organic template (Figure 1). From the beginning of the plasma ignition, the intensity of the CH bands decreased due to the removal of the TEA template. In oxygen plasma, the IR feature of acidic Bronsted sites appeared at 3600 cm 1 (Figure 1b, c). This phenomenon is better pronounced in the Pt-containing BEA samples (Figure 1b). The degree of degradation of the organic template (D%) could be calculated by integrating the total absorbance, AHC, in the region of 3100–2800 cm 1 as a function of time according to the Equation (1):