Silica-modified Pt/TiO2 catalysts with tunable suppression of strong metal-support interaction for cinnamaldehyde hydrogenation

Abstract

Tuning Strong Metal-support Interactions (SMSI) is a key strategy to obtain highly active catalysts, but conventional methods usually enable TiOx encapsulation of noble metal components to minimize the exposure of noble metals. This study demonstrates a catalyst preparation method to modulate a weak encapsulation of Pt metal nanoparticles (NPs) with the supported TiO2, achieving the moderate suppression of SMSI effects. The introduction of silica inhibits this encapsulation, as reflected in the characterization results such as XPS and HRTEM, while the Ti4+ to Ti3+ conversion due to SMSI can still be found on the support surface. Furthermore, the hydrogenation of cinnamaldehyde (CAL) as a probe reaction revealed that once this encapsulation behavior was suppressed, the adsorption capacity of the catalyst for small molecules like H2 and CO was enhanced, which thereby improved the catalytic activity and facilitated the hydrogenation of CAL. Meanwhile, the introduction of SiO2 also changed the surface structure of the catalyst, which inhibited the occurrence of the acetal reaction and improved the conversion efficiency of C=O and C=C hydrogenation. Systematic manipulation of SMSI formation and its consequence on the performance in catalytic hydrogenation reactions are discussed.

1. Introduction

Supported nano-sized metal catalyst plays an important role in the catalytic processes in the chemical industry, including the production of fine chemicals and fuels from renewable and sustainable raw materials [1,2]. The supports of inorganic oxides like TiO2 and Al2O3 were usually considered as inert materials for loading, fixing and dispersing active metal components [3]. However, the researchers have discovered that electron transfer between support and metal can be triggered during the high-temperature reduction by hydrogen treating, leading to the wrapped Pt group metal nanoparticles (NPs). This wrapping thus alters the catalytic activity and stability, as well as the product selectivity during catalytic reactions. This effect is known as strong metal-support interaction (SMSI) [3e5].

Studies on the formation mechanism of SMSI effect have been of interest in recent years. One prevailing view is that the removal of lattice oxygen during high-temperature reduction can afford a substructure on support surface that migrates to the low-electron metal region due to defects in the arrangement of its electronic layers, forming a stable encapsulation layer [6,7]. On the one hand, it is important for heterogeneous catalysis to develop the preparation method for improving the anti-sintering stability and better adsorption of reactant on metal NPs [7]. For example, Li et al. [8] investigated the stability of metal NPs against sintering and proposed Sabatier's principle that very strong metal-support interactions trigger Ostwald ripening, while very weak interactions lead to particle migration and agglomeration. Jong et al. [6] showed a 15-fold increase in the productivity of CO catalysis can be achieved by modulating the SMSI between the metal oxide support and Pt group NPs. However, SMSI also leads to the suppression of the adsorption of small molecules and the coverage of active metal components during the wrapping process. It can explain the phenomenon reported first by Tauster [9] in 1978 that the chemisorption capacity of metals for small gas (H2, CO2) molecules was substantially reduced. In this case, a part of TiO2 is also encapsulated on the surface of the active metal NPs during the generation of the SMSI effect, which blocks the active catalytic sites and thus reduces the catalyst activity.

In recent years, considerable attention has been paid to the SMSI effect on the catalytic reactions of organic compounds [10e14]. One typical example is the hydrogenation of cinnamaldehyde (CAL), whose products are important intermediates or raw materials for the productions of fragrances, cosmetics, pharmaceuticals and fungicides [13,14]. As is known, CAL can generate hydrocinnamic aldehyde (HCAL) by a C=C bond hydrogenation, cinnamic alcohol (COL) by a C=O bond hydrogenation, and hydrocinnamyl alcohol (HCOL) by both of the C=C and C=O hydrogenation.