Catalyst stabilization by stoichiometrically limited layer-by-layer overcoating in liquid media

The use of metal oxide overcoats over supported nanoparticle catalysts has recently led to impressive improvements in catalyst stability and selectivity. The deposition of alumina is especially important for renewable catalysis due to its robustness in liquid-phase conditions. However, there are limited reports of work on alumina deposition and stabilization that goes beyond atomic layer deposition (ALD). Here, we present a layer-by-layer deposition technique for the controlled formation of conformal alumina overcoats in the liquid phase. This technique is easy to perform in common wet chemistry conditions. Alternated exposure of the substrate to stoichiometric amounts of aluminum alkoxide and water in liquid-phase conditions leads to the formation of a porous overcoat that was easily tunable by varying synthesis parameters. The deposition of 60 Al2O3 layers onto Al2O3-supported copper nanoparticles suppressed irreversible deactivation during the liquid-phase hydrogenation of furfural – a key biomass-derived platform molecule. The porous overcoat leads to highly accessible metal sites, which significantly reduces the partial site blocking observed in equivalent overcoats formed by ALD. We suggest that the ease of scalability and the high degree of control over the overcoat’s properties during liquid-phase synthesis could facilitate the development of new catalyst overcoating applications.

Structured Non-Noble Metal Catalysts for the Liquid-Phase Reduction of Nitroaromatics

Oliver Anthony Beswick

The industrial production of aromatic amines, essential intermediates for the synthesis of several drugs, dyes, pigments and agrochemicals, requires the use of heterogeneous catalysis. These molecules were formerly produced by the Béchamp reduction, a non-catalytic batch process requiring costly downstream separation steps. Catalytic approaches afford the implementation of continuous processes producing less waste. While it is generally not the primary goal when improving processes to make them more environmentally friendly, it is often one of the consequences, thereby improving the image of the chemical industry. The development of new catalysts is one of the major axes for optimizing industrial production of chemicals. Often multidisciplinary, it involves a better understanding of existing systems, advances in characterization and modelling techniques for the discovery of new catalytic materials. This thesis aims at designing new catalysts based on non-noble metals (e.g. Fe, Co, Ni) for the chemoselective hydrogenation of nitroaromatics in the liquid phase. The use of activated carbon fibres (ACFs) as a structured support has been at the centre of the strategy applied during this work in order to develop new catalysts capable of surpassing the best systems based on Fe, Co or Ni. The high porosity of ACFs, composed of very thin pores (~ 2 nm) and a high specific surface area, has been exploited to generate and stabilize highly dispersed crystallites of active phase. The formation of ~2 nm metal oxides (MOx) nanoparticles (NPs) resulting from the impregnation/pyrolysis of Fe, Co or Ni precursors within the ACFs was performed. The reduction of nitroaromatics with different functional groups was carried out by catalytic hydrogenation with molecular hydrogen and by catalytic transfer hydrogenation (CTH) with hydrazine as reducing agent. The MOx/ACF catalysts were shown to be active and selective under mild conditions in the CTH of para-chloronitrobenzene (p-CNB) to para-chloroaniline (p-CAN). The FeOx/ACF was tolerating many substituents of nitroarenes during the -NO2 hydrogenation. The CoOx/ACFHNO3 catalyst was less active compared to the Fe-based catalyst but afforded a meta-vinylaniline (m-VA) yield above 99% during the reduction of the meta-nitrostyrene (m-NS), exceeding, to the best of our knowledge, the best performance reported to date. The very low activity of the NiOx/ACF catalyst in CTH has prompted us to test Ni-based catalyst in the hydrogenation of nitroarenes with gaseous hydrogen. Raney Nickel is a well-known catalyst used in industry for this reaction. First, the existence of particles size effects was evaluated using unsupported Ni PVP-stabilized NPs (2¿14 nm). An antipathetic structure sensitivity of the meta-dinitrobenzene (m-DNB) hydrogenation to meta-nitroaniline (m-NAN) was demonstrated, showing a 4.6-fold increase in the turnover frequency over the 14 nm NPs. Once supported on ACFs and cleaned from PVP by a UV-ozone treatment, the 2 nm Ni NPs afforded an increase of selectivity towards m-NAN from 34 to 96% at close to full conversion (Xm-DNB = 99%). The selective transformation favouring m-NAN has been attributed to reactions taking place at the edges and vertices of the NPs. The Langmuir-Hinshelwood two-site kinetic model was consistent with the experimental data. The incorporation of gold to form Ni-Au (1:1) bimetallic NPs led to a significant increase in the selectivity (99%), possibly by mimicking the electronic

EPFL2017

Atomic Layer Deposition on Dispersed Materials in Liquid Phase by Stoichiometrically Limited Injections

Benjamin Pierre Le Monnier, Jeremy Luterbacher, Farzaneh Talebkeikhah, Frederick Georges Wells

Atomic layer deposition (ALD) is a well‐established vapor‐phase technique for depositing thin films with high conformality and atomically precise control over thickness. Its industrial development has been largely confined to wafers and low‐surface‐area materials because deposition on high‐surface‐area materials and powders remains extremely challenging. Challenges with such materials include long deposition times, extensive purging cycles, and requirements for large excesses of precursors and expensive low‐pressure equipment. Here, a simple solution‐phase deposition process based on subsequent injections of stoichiometric quantities of precursor is performed using common laboratory synthesis equipment. Precisely measured precursor stoichiometries avoid any unwanted reactions in solution and ensure layer‐by‐layer growth with the same precision as gas‐phase ALD, without any excess precursor or purging required. Identical coating qualities are achieved when comparing this technique to Al2O3 deposition by fluidized‐bed reactor ALD (FBR‐ALD). The process is easily scaled up to coat >150 g of material using the same inexpensive laboratory glassware without any loss in coating quality. This technique is extended to sulfides and phosphates and can achieve coatings that are not possible using classic gas‐phase ALD, including the deposition of phosphates with inexpensive but nonvolatile phosphoric acid.

2019

Structured Non-Noble Metal Catalysts for the Liquid-Phase Reduction of Nitroaromatics

Oliver Anthony Beswick

EPFL2017