Eeva-Liisa Lakomaa

Atomic layer epitaxy (ALE) on porous substrates

Abstract Atomic layer epitaxy (ALE), commonly used for growing single crystals and thin films, has been applied to process catalysts on porous high surface area substrates. Unlike many compound growth methods ALE growth is not controlled by the dose of the reactant but rather by the surface itself. The self-controlling feature of ALE allows the growth of compounds on porous, heterogeneous surfaces as well. The desired surface structures are formed in the chemisorption reactions, and no heat treatments afterwards are needed. The basic ALE reactions with porous high surface area substrates, including the chemisorption and surface saturation, will be presented. The surface densities of metal compounds on alumina and silica can be controlled by various means. The number of bonding sites can be regulated by heat treatment or by using reactants able to block selected bonding sites before binding the active metal species. The reaction temperature can sometimes be used for controlling the densities of the metal compounds. The size of the reactant molecule or its chemical character may control the saturation level obtained. The growth of compound layers can be used to increase the metal concentration in the catalyst. In addition to the binding of one metal compound on the surface, two or several metal compounds can be bound on the substrates in a controlled way.

Chemisorption of chromium acetylacetonate on porous high surface area silica

Abstract Atomic layer epitaxy (ALE) reactions (i.e. saturating gas-solid reactions) of chromium acetylacetonate (Cr(acac)3) at 200–280°C with silica preheated at 200–820°C were studied by determining chromium and carbon concentrations, recording FTIR spectra, and reacting Cr(acac)3 with the silylated silica surface. Cr(acac)3 was found to be selectively chemisorbed to silica through reaction with the isolated OH groups, leading to release of one acac ligand. The relatively large size of the supported chromium complex that formed had a highly controlling effect on the amount of chromium atoms bound. In addition to this steric hindrance, the saturation density of chromium could be further regulated by the preheat temperature of the silica, which determines the number of OH groups, and by the reaction temperature. The reaction with the silylated silica surface provided a means for achieving an even lower saturation density of chromium and confirmed that the strongly H-bonded OH groups present on silica preheated at 200°C were only partly reactive. The ligands of the surface complex could be removed by water vapor and air treatment.

Atomic layer growth of TiO2 on silica

Abstract Study was made of the initial stages of TiO2 on porous high-surface-area silica support by atomic layer epitaxy (ALE). The effects of the number and types of surface bonding sites and the reaction temperature on the growth rate and structure of TiO2 were determined. The reactants were TiCl4 and H2O. At 175°C binding of TiCl4 takes place to one or to two hydroxyl groups, but at 450°C only to two isolated hydroxyl groups. The number and type of binding sites available and the reaction temperature selected determine how the growth will proceed. XRD peaks at the 2θ values of anatase and rutile were detected after one reaction cycle of TiCl4 and H2O at 450°C.

Surface reactions in Al2O3 growth from trimethylaluminium and water by atomic layer epitaxy

The layer by layer growth of Al 2 O 3 from trimethylaluminium (TMA) and water vapour by atomic layer epitaxy (ALE) has been studied on porous large surface area silica. The use of this support enables the study of each reaction sequence brought to surface saturation by using elemental determinations, Fourier transform infrared (FTIR) and solid state nuclear magnetic resonance (NMR) spectroscopy. The chemisorption of TMA as a function of the preheat temperature of silica shows that bonding takes place to isolated OH groups and siloxane bridges. Increasing the preheat temperature of silica creates an increased amount of siloxane bridges. Increasing the chemisorption temperature of TMA leads to the reaction of double and triple siloxane bridges as measured by 29 Si NMR. This type of reaction has not been presented earlier. The first five reaction cycles of TMA and water on silica show that, after the first TMA reaction, 2.8 Al/nm 2 are present whereas the following reaction cycles increase Al by 0.5 and 0.3 Al/nm 2 alternatively in the cycles 2-5. Water removes Al-CH 3 groups already at 120°C, but Si-CH 3 groups are not affected so much and are responsible for the slow progress of growth for the next layers.

Surface coverage of ALE precursors on oxides

The work with single molecular layers on porous materials adds to the atomic level understanding of the film growth in ALE. In this paper the primary factors affecting the surface coverage of three different ALE precursors (TiCl4, Cr(acac)3 and hexamethyldisilazane (HMDS) on porous, high surface area SiO2 were studied by element determinations and FTIR spectroscopy. Also a preliminary low-energy ion scattering (LEIS) study on Ti/silica samples was carried out. First, the surface coverage of metal species achievable at a certain reaction temperature on an oxide is determined by the number of specific reactive sites for the precursor. TiCl4 utilizes both isolated and H-bonded OH groups while Cr(acac)3 and HMDS react preferentially with isolated OH groups. Second, the bonding mode affected the surface coverage of titanium species. The bifunctional reactivity of TiCl4, on the one hand, consumed two OH groups per one TiCl4 molecule lowering the surface coverage on silica preheated at elevated temperatures, and on the other hand, enabled the utilization of the H-bonded OH groups present on silica preheated at lower temperatures thereby increasing the surface coverage. Third, the steric hindrance between the precursor and the already chemisorbed metal complexes was shown to have an effect on the surface coverage of Cr(acac)3 because of the size of two remaining acac ligands of bound chromium complex. Hence, at saturating reaction conditions of compound precursors, where undecomposed, reproducible and selectively chemisorbed surface species are obtained, the full monolayer coverage on oxides is seldom achieved.

The utilization of saturated gas-solid reactions in the preparation of heterogeneous catalysts

Abstract Saturated gas-solid reactions known from Atomic Layer Epitaxy (ALE) were used to processvarious catalysts. Good homogeneity of metal species was verified both along the entire catalyst bed and inside the particles. A variety of volatile metal compounds including metal chlorides, alkoxides and β-diketonates were successfully used as reactants. The ALE processing is described with reference to examples demonstrating the achievement of surface saturation, reproducibility of processes, selection of process parameters, growth of oxides to modify the support and the binding of two metal compounds.

Processing of catalysts by atomic layer epitaxy : modification of supports

Abstract Different supports were modified with titania, zirconia and chromia by the atomic layer epitaxy technique (ALE). In ALE, a metal precursor is bound to the support in saturating gas-solid reactions. Surface oxides are grown by alternating reactions of the metal precursor and an oxidizing agent. Growth mechanisms differ depending on the precursor-support pair and the processing conditions. In this work, the influences of the support, precursor and reaction temperature were investigated by comparing the growth of titania from Ti(OCH(CH 3 ) 2 ) 4 on silica and alumina, titania from TiCl 4 and Ti(OCH(CH 3 ) 2 ) 4 on silica, and zirconia from ZrCl 4 on silica and alumina. The modification of porous oxides supported on metal substrates (monoliths) was demonstrated for the growth of chromia from Cr(acac) 3 .