Platinum on Carbon Nanofibers as Catalyst for Cinnamaldehyde Hydrogenation

Abstract

The aim of the work described in this thesis was to investigate the role and nature of nanostructured carbon materials, oxygen surface groups and promoters on platinum-based catalysts for the selective hydrogenation of cinnamaldehyde. The selective hydrogenation of cinnamaldehyde to cinnamyl alcohol was chosen as a showcase. To establish structure-activity relations, it is crucial to prepare well-defined carbon nanofibers (CNF) supports and well-defined catalytic metal phases on these supports. In Chapter 2 an exploratory study on the selective hydrogenation of cinnamaldehyde is described. The influence of the reactant concentration, reaction solvent and nature of the metal and support was evaluated. Based on the results, it was decided to use 2-propanol/water as solvent, cinnamaldehyde concentration of 0.1 M and platinum as catalytic metal for further studies. In Chapter 3 the role of the nature and stability of oxygen surface groups in anchoring and stabilizing platinum particles was investigated. To that purpose CNF and platinum-containing CNF with different amounts and types of oxygen surface groups were prepared and analyzed. The decomposition temperature for acidic oxygen surface groups turned out to depend on the presence of platinum. Platinum anchoring occurs on both carboxyl and phenol type groups during catalyst synthesis. In Chapter 4 the influence of graphene orientations on platinum deposition and reduction behavior was analyzed. To that purpose, platinum was deposited on oxidized nanostructured carbon materials, i.e. CNF, carbon nanoplatelets (CNP) and carbon nanotubes (CNT). It was demonstrated that platinum deposited on CNT resulted in 20 K higher reduction temperature as compared to platinum on CNF and CNP. Application of the required higher reduction temperature resulted in complete reduction for that sample, accompanied by sintering of platinum to large agglomerates. The prepared catalysts were tested and Pt/CNP and Pt/CNF turned out to be active catalysts for the cinnamaldehyde hydrogenation, while fully reduced and sintered platinum on CNT was not very active for this reaction. In Chapter 5 particle size effects for cinnamaldehyde hydrogenation using platinum and ruthenium on CNF with similar amounts of acidic oxygen surface groups, i.e. before and after heat-treatment, were investigated. Different sized platinum and ruthenium particles were deposited on CNF and tested for the selective hydrogenation of cinnamaldehyde. Before heat-treatment, the platinum catalysts with larger metal particles turned out to be more selective. After heat-treatment, which resulted in removal of oxygen surface groups, smaller metal particles resulted in a higher selectivity for the platinum and ruthenium catalysts. The reversed particle size effect is ascribed to a changing adsorption mode of the reactant. In Chapter 6, we aimed to characterize platinum-promoter interactions and to relate this to catalytic performance. Therefore, CNF-supported platinum was combined with tin as promoter via incipient wetness impregnation (IWI) and via reductive deposition precipitation (RDP). Via detailed characterization, it was demonstrated that tin became partially reduced when deposited via RDP, thereby resulting in a close contact of the two metals. Via IWI synthesis such a close interaction was not obtained. The prepared catalysts were tested for the selective hydrogenation of cinnamaldehyde and enhanced selectivities towards cinnamyl alcohol were observed when close platinum-promoter interactions were present.