Bifunctional earth-abundant phosphate/phosphide catalysts prepared via atomic layer deposition for electrocatalytic water splitting

The development of active and stable earth-abundant catalysts for hydrogen and oxygen evolution is one of the requirements for successful production of solar fuels. Atomic Layer Deposition (ALD) is a proven technique for conformal coating of structured (photo)electrode surfaces with such electrocatalyst materials. Here, we show that ALD can be used for the deposition of iron and cobalt phosphate electrocatalysts. A PE-ALD process was developed to obtain cobalt phosphate films without the need for a phosphidation step. The cobalt phosphate material acts as a bifunctional catalyst, able to also perform hydrogen evolution after either a thermal or electrochemical reduction step

Catalytic functionalization of nanoporous films and powders using Atomic Layer Deposition

Atomic layer deposition (ALD) is a film growth technique to cover solid surfaces with ultrathin films of oxides. ALD proceeds in vapor phase at reduced pressure and involves chemisorption of a precursor compound and reaction in a cyclic process. ALD is appreciated for its excellent conformality on flat surfaces as well as high aspect ratio structures. ALD seldom had been applied to materials with channels narrower than 50 nm. In this work the application of ALD in materials with still narrower pores was explored. The aim was to investigate the possibility to generate BrØnsted acid sites for adsorption and catalysis by deposition of alumina, and of TiO2 for photocatalytic purposes. Nanopores according to IUPAC are classified as mesopores with diameters from 2 to 50 nm and micropores being smaller than 2 nm. This study was divided in two parts dealing with the investigation of nanoporous powders and films. The powders comprised ordered mesoporous silica materials such as Zeotile-4 composed of a mosaic of zeolitic nanoslabs and presenting complex tridimensional mesoporosity, and COK-12 material with hexagonal arrays of tubular mesopores. Combinations of micro- and mesopores are present in hierarchical zeolite powders such as ultrastable Y zeolite. Silica thin films with very high porosity were prepared using an original procedure by linking zeolitic nanoslabs into open mesoporous networks adapting the concept for synthesis of Zeotile-4 powder. Calcined films characterized by ellipsometric porosimetry (EP) presented pore sizes in the range of 6-18 nm and unprecedented porosities of 70-90% depending on the temperature of assembly of the nanoslabs. Unlike Zeotile-4, the films were disordered according to SAXS and TEM. These mesoporous thin films were extremely flexible, and exhibited a reversible shrinkage of up to 30% of the original thickness upon capillary condensation of adsorbate vapor. ALD of alumina was achieved using alternating pulses of trimethyl aluminum and water vapor. Titania deposition was achieved using tetrakis(dimethylamino)titanium (TDMAT) and water. The introduction of amorphous TiO2 into the pores of Zeotile-4 was successful. Zeotile-4 particles were partially filled and a TiO2 penetration front was observed. The penetration depth of the TiO2 deposit to the interior of individual Zeotile-4 particles and the alteration of porosity by the introduced TiO2 was determined using TEM of particle cross sections and N2 physisorption. The TiO2 penetration depth into individual particles was dependent on mesopore architecture, pore width and shape and ALD pulse length. The use of ALD leads to patterned TiO2 deposition in the anisotropic Zeotile-4 support material. In COK-12 with mono dimensional pore system the ALD process worked less well and lead to pore clogging.Aluminum was successfully introduced via ALD on silicate-based ordered mesoporous materials, hierarchical materials and mesoporous zeolites. The incorporation of aluminum via ALD gave rise to aluminum in four-, five- and six fold coordination detected via 27Al MAS NMR. The ALD treatment of these materials was found to enhance the catalytic activity in decane hydrocracking by creation of additional BrØnsted acid sites. The hydrocracking activity of ultrastable Y zeolites was significantly enhanced. ALD is particularly attractive for materials in which the incorporation of aluminum through direct synthesis is difficult to achieve, such as e.g. Zeotile-4. ALD of TiO2 in mesoporous silica films was also successful. TiO2 was deposited throughout the film as revealed with Rutherford Backscattering Spectroscopy and EP. The residual porosity and mechanical flexibility of the film evolved with the number of applied ALD cycles. Upon calcination the deposited amorphous TiO2 layer was fragmented into anatase nanoparticles useful in photocatalysis.Micro-ring resonators were coated with nanoporous aluminosilicate films synthesized via sol-gel method and with a mesoporous silicate film on which alumina ALD was applied. The acid sites in these aluminosilicate coatings enabled adsorption and sensing of ammonia vapor. A reversible response to ammonia with selectivity relative to CO2 was obtained with these sensors. The ammonia detection limit was estimated at about 5 ppm. The work opens perspectives on development of nano-photonic sensors for real-time, non-invasive, low cost and light weight biomedical and industrial sensing applications.nrpages: 100status: publishe

Mixed matrix membranes comprising of matrimid and mesoporous COK-12: Preparation and gas separation properties

© 2015 Elsevier B.V. Novel ordered mesoporous COK-12 type silica particles were used as fillers in polyimide (Matrimid) based mixed-matrix membranes (MMMs). The highly ordered 2D hexagonal structured filler materials with short and straight pores were synthesized at room temperature under quasi-neutral pH. The gas permeation and SEM results of the MMMs confirmed a homogenous filler dispersion and defect free synthesis of membranes. The presence of large mesopores in the fillers leading to faster diffusion of penetrant gas resulted in higher gas permeabilities in comparison to pristine Matrimid membrane. The potential of the membrane under mixed-gas conditions and different operating temperatures was also evaluated. The membranes showed decreasing trend of activation energy of permeation with the addition of filler. The ease of synthesis, highly ordered structure with short and straight pore channels and improved gas permeation properties makes COK-12a promising filler for industrial gas separations.status: publishe

In-situ growth of platinum with hierarchical porosity for low impedance biomedical microelectrode fabrication

We report a novel process that allows depositing hierarchically porous platinum, i.e. platinum with structured pores from about 1 micron down to 3 nm in porosity, in situ on flexible microelectrodes. The process is based on molecular self-assembly of a porous silica zeotile, which is then deposited on the electrode contacts of flexible platinum electrode arrays fabricated with classic lithographic techniques. The pores are then Pt filled using atomic layer deposition (ALD) and the silica mold is removed chemically. Compared to our earlier work, the in-situ ALD step improves adhesion and allows a lower temperature budget as no sintering is required. An up to 1000x reduction in electrode impedance was measured as well as a 12x increase in charge injection capacity.publisher: Elsevier articletitle: In-situ Growth of Platinum with Hierarchical Porosity for Low Impedance Biomedical Microelectrode Fabrication journaltitle: Procedia Engineering articlelink: http://dx.doi.org/10.1016/j.proeng.2016.11.366 content_type: article copyright: © 2016 The Author(s). Published by Elsevier Ltd.status: publishe

Tailoring nanoporous materials by atomic layer deposition

Atomic layer deposition (ALD) is a cyclic process which relies on sequential self-terminating reactions between gas phase precursor molecules and a solid surface. The self-limiting nature of the chemical reactions ensures precise film thickness control and excellent step coverage, even on 3D structures with large aspect ratios. At present, ALD is mainly used in the microelectronics industry, e. g. for growing gate oxides. The excellent conformality that can be achieved with ALD also renders it a promising candidate for coating porous structures, e.g. for functionalization of large surface area substrates for catalysis, fuel cells, batteries, supercapacitors, filtration devices, sensors, membranes etc. This tutorial review focuses on the application of ALD for catalyst design. Examples are discussed where ALD of TiO(2) is used for tailoring the interior surface of nanoporous films with pore sizes of 4-6 nm, resulting in photocatalytic activity. In still narrower pores, the ability to deposit chemical elements can be exploited to generate catalytic sites. In zeolites, ALD of aluminium species enables the generation of acid catalytic activity

Creation of gallium acid and platinum metal sites in bifunctional zeolite hydroisomerization and hydrocracking catalysts by atomic layer deposition

Atomic layer deposition (ALD) is a vacuum technology for the deposition of a small number of atoms on surfaces. Its use in catalysis is growing. Here, we explored the use of ALD for introducing acid and metal sites in zeolites for performing bifunctional catalysis. Plasma-enhanced ALD involving cyclic exposure of a sample to tris(2,2,6,6-tetramethyl-3,5-heptanedionato)gallium (Ga(TMHD)(3)) vapor and O-2 plasma (Ga-ALD) was used for introducing acid sites. Interestingly, Ga-ALD was found to cause preferential deposition of Pt nanoparticles via incipient wetness impregnation on the edges of COK-14 crystal plates, in contrast to previously published results on Al-ALD. Benefiting from the optimum proximity between the Ga acid and Pt metal sites, it is shown here that Ga-ALD is a way to introduce sufficient acidity into all-silica zeolite COK-14 for obtaining bifunctional catalytic behavior. Hydrogenation-dehydrogenation activity in bifunctional catalysts is typically provided by trace amounts of platinum dispersed on the zeolite. Pt-ALD was applied for finely dispersing platinum on ZSM-5 zeolite. Pt-ALD involved alternating exposure to the trimethyl(methylcyclopentadienyl)platinum(iv) (MeCpPtMe3) precursor and ozone. The Pt-ALD method proved to be an efficient way to uniformly disperse ultra-small Pt nanoparticles onto the zeolite. The bifunctional catalytic behavior of ALD-functionalized zeolites was confirmed in the hydroconversion of the n-decane model molecule