DE19843411A1 - Production of surface decorated substrates - Google Patents

Abstract

Ordered nanometric surface structures made of metal and/or metal oxide clusters can be deposited on substrates. Production of surface decorated substrates comprises: (a) placing a polymer in a suitable solvent, forming a dissolved core shell polymer system; (b) charging of the polymer core with metal compounds; (c) applying the polymer system, as a film, to one side of a substrate, so that the system is arranged in a regular structure in the film; and (d) subjecting the substrate to reactive ion etching, ion sputtering and/or wet chemical process, in which the film is removed. The regular structure is converted into a relief structure based on and depending upon the charge of the micelle cores, and the duration of the etching or wet chemical processes. An Independent claim is also included for the substrate comprising clusters of metal atoms or compounds on its surface, the clusters being arranged regularly on the substrate surface.

Description

Finely dispersed precious metal catalysts form the basis of important chemical processes such as hydrogenation [1], the selective oxidation of alkenes, alkynes and aromatics to alcohols, epoxides, ketones, aldehydes and carboxylic acids [2], the oxidation of CO [3], the decomposition of Nitrogen oxides (NO _x ) in nitrogen and a corresponding oxidation product (H ₂ O, CO ₂ ) [4], the conversion of methane with carbon dioxide to synthesis gas (CO / H ₂ ) [5]. An important criterion for the activity and selectivity of the catalysts is that the metal particle is applied to a suitable, mostly oxidic support. For example, in contrast to the metals of the platinum group, pure gold is catalytically inert, however, in combination with an oxidic carrier, one has a catalytically highly active system. The choice of the oxidic carrier plays a crucial role in the activation and restructuring of the catalytically active particle surface during the catalytic cycle. The structure of the support and its chemical interaction with the precious metal has a lasting effect on the activity and selectivity of the catalyst [6]. The deposition of platinum layers on sulfidic or oxidic supports with a well-defined surface structure leads to tailor-made, highly active catalyst systems. Zeolites with a variable but precisely defined pore structure lead to an increase in the active surface area and, in conjunction with the "right" noble metal, to an increase in both the selectivity and the activity [7]. In modern three-way catalysts, in addition to the noble metal (Pt, Pd, Rh) and the support (SiO ₂ , Al ₂ O ₃ ), CeO _{2 is used} as a promoter, which stores and releases the oxygen necessary for the reaction [8]. Compared to catalysts without promoters, this significantly improves the catalytic process.

In addition to the surface quality and type of carrier, plays with regard to Improvement of the catalytic process on the size and distance of Precious metal particles play an extremely important role. Using lithographic Methods can order metal particles of 10-100 nm in size in an orderly manner Substrate can be applied [9].

A high catalytic activity is achieved in particular with particle sizes smaller than 5 nm. The regular arrangement of 1-3 nm large precious metal particles on a carrier in A cost-effective process is an important goal for further development  heterogeneous catalyst systems. Another goal is the controlled display of Compound clusters from the catalytically active metal component on the metal oxide, wherein the relative proportions can be varied systematically.

The amphiphilic micellar systems described in the present invention Block copolymers allow the orderly separation of the smallest, more catalytically active Metal particles on almost any substrate.

For the deposition of "bare" metal / metal oxide clusters, those containing metal salts are used Micelles applied in the form of an ultra-thin film on the selected substrate, then the polymer shell is treated by treatment with an oxygen plasma Burn off in an oxygen-rich atmosphere, by pyrolysis or under reducing conditions, e.g. B. selectively removed in a hydrogen plasma (Scheme 1).

In this treatment, the transition metal compound is converted to the elemental metal or, under oxidizing conditions, to the metal oxide particle. The thermal reduction or conversion of noble metal compounds in an oxygen atmosphere produces a similarly narrow particle distribution as the burning of the polymer by means of the plasma process described above ( Fig. 1).

According to the process described above, gold, platinum, palladium, titanium dioxide, Iron oxide and cobalt oxide clusters of different sizes can be produced. The The density is determined by the molecular weight of the amphiphilic block copolymers Size of the particles both by the degree of polymerization of the micelle nucleus-forming block and also adjusted by the amount of loading with a metallic precursor in solution. Here one moves between particle diameters of 1 and 20 nm and one Particle-particle distance of 20 and 200 nm. The uniformity of the clusters and the regular arrangement is due to the homogeneous distribution of the salts in solution on the Micelles and the uniform size distribution of the two-block copolymer micelles achieved.

The nanoparticles produced in this way stand out in addition to their high Uniformity is also characterized by its temperature resistance. At the tempering of 13 nm high gold particles, no decrease in height is observed up to 800 ° C. At T = 800 ° C a reorganization of the particles is initially observed, the nanoparticles become apparent  smaller (8 nm). After that, the height of the particles increases at four, eight, and twelve hours Annealing does not decrease further. This could be shown by SFM investigations.

Similar behavior was observed in the case of platinum nanoparticles, only that here a reorganization of the particles is already found at T = 600 ° C.

In addition to the high chemical resistance, a high chemical resistance of the Nanoparticles observed. When the particle formation is complete, the nanoclusters are opposite sulfuric acid or chloride-containing solutions stable. This opens up for chemical Implementations on these nano-precious metal clusters new perspectives.

Block copolymer micelles can also be loaded with different metallic salts in solution. These mixing systems are built into the micelles in a highly dispersed manner, as has been demonstrated by means of electron energy loss spectroscopy. Each micelle is loaded homogeneously with both inorganic components. The separation of the hybrid systems and the subsequent gas plasma treatment succeed as described and the result is "bare", highly disperse mixed clusters such as. B. Au / TiO ₂ , Au / Fe ₂ O ₃ or Au / Co ₃ O ₄ on different substrates. Force micrographs of monomicellar films after gas plasma treatment regularly show ordered and equally sized nanoclusters, as shown in Fig. 5.

Task

An important area of application for precious metals applied to metal oxides (Au / TiO ₂ , Au / Fe ₂ O ₃ , Pt / Al ₂ O ₃ or Au / Co ₃ O ₄ ) is the oxidation catalysis of CO to CO ₂ [12]. This process is important, among other things, in fuel cell technology [13]. The oxygen required for the reaction is supplied by the oxidic carrier material, which thus acts as a promoter. The catalysis takes place at the precious metal / oxide support interface. Particularly in the case of fuel cells which are operated with methanol, the hydrogen is obtained by the so-called "steam reforming process" [14]. It produces 75% hydrogen, 25% carbon dioxide and traces of carbon monoxide. The remaining carbon monoxide poisoned the anode catalysts. Therefore the CO content has to be reduced by selective oxidation to CO ₂ [15].

In general, high catalytic activity with low precious metal consumption is sought. This is due to the use of tiny particles and thus an increased active Surface and in heterogeneous catalysis by a maximum interface between Precious metal and metal oxide reached.  

table

Activation parameters for the reaction CO → CO ₂ heterogeneous systems

The specific catalyst has a great influence on the activation energy of the oxidation. Au / Fe ₂ O ₃ or Au / TiO ₂ show the same catalytic activity and selectivity at approx. 80 ° C as Pt / Al ₂ O ₃ at 150-200 ° C [16]. In particular with methanol fuel cells, an activation temperature below 100 ° C is essential.

Precious metal particles also act as oxidation catalysts after the hydrogenation important role. The oxidation of ethylene with molecular oxygen on silver colloids is used industrially for the production of ethylene oxide [17]. Salts are used as promoters of alkali (preferably cesium compounds) or alkaline earth metals [18]. This The process is limited to the ethylene → ethylene oxide system and has not so far been restricted to longer-chain alkenes can be used. This only appears with type catalyst systems Metal / metal oxide also promising for long-chain alkenes [19].

With the combination of nanodispersed gold on TiO _2, numerous saturated hydrocarbons to alcohols / ketones and unsaturated hydrocarbons to epoxides were converted in high yield and selectivity. In the listed patent, the preparation of the catalyst was carried out conventionally using the coprecipitation method [19].

Solution of the task

When the micelle-stabilized noble metal particles are applied to an oxidic support (e.g. SiO ₂ ), heterogeneous catalysts are obtained which are distinguished by higher stability, activity and selectivity compared to homogeneous systems.

Basically new are metal / metal oxide composite particles based on block copolymer micelles, in particular Au / TiO ₂ and Au / Fe ₂ O ₃ with the objective of epoxidation

as well as the oxidation reaction of CO to CO ₂ , which is of high technical and ecological relevance, especially in fuel cells and internal combustion engines. The aim is high catalytic activity with low consumption of precious metals. The approach with block copolymer micelles allows the particle size, the particle-particle distance and the resulting active surface to be precisely adjusted. The residue-free removal of the polymer and simultaneous conversion of the metal salts through the gas plasma process result in well-defined metal / metal oxide composite systems on various substrates, such as mica, glass or quartz.

To achieve good film formation, the micelles are placed on an atomic planar Substrate applied. This is shown in the following scheme.

In model experiments, micro glass or micro quartz spheres with a smoother surface are used as carriers Surface used. For the microspheres, the specific surface area of the carrier and the The amount of precious metal is predictable. However, the proportion by weight of precious metal is smaller than usual for conventional powder catalysts.

Loading of glass balls with Pd clusters (5 nm size)

Depending on the size of the glass balls, there are different values for the Amount of precious metal that is available for the catalytic process. Usually are values of 1-10 wt% noble metal standard for catalysts based on palladium / support. In the approach with block copolymer micelles, there is no such high content of precious metal achievable, but also not absolutely necessary, because the size and distance of the nanoparticles with this concept set exactly. This will also lower it Amount of noble metal expected an identical or at least similar catalytic activity.  

Examples

1. In a mini-reactor, the epoxidation should be investigated, starting from C ₆ to C ₈ alkenes. Gold chloride / iron chloride-filled block copolymer micelles act on pearlescent pigments, which can be converted into a highly active hybrid system (gold / iron oxide) by the gas plasma process described above. The pigments have a very smooth outer surface and a small diameter, which leads to a sufficiently high specific surface. The catalyst is slurried and molecular oxygen is blown directly into the educt / solvent / catalyst system. The selectivity of the reaction is to be demonstrated by infrared spectroscopy.

2. The oxidation catalysis from CO to CO ₂ is to be investigated with a composite system deposited on micro glass balls or beads. Analogous to point 1), gold / iron oxide or gold / cobalt oxide hybrid systems act as catalysts. Here, too, substrates with a smooth surface and small diameter (approx. 100 µm) are necessary, since in contrast to porous powder materials used in conventional catalyst processes, the micelles can be separated in a well-defined manner. The catalyst is to be enclosed in a temperature-controlled glass tube and the reaction gas is to be led past it. The detection takes place via a gas chromatograph connected downstream.

requirements

• (i) representation of catalytically active particles such as Pt, Pd and Au in connection with a suitable metal oxide such as TiO ₂ , ZrO ₂ , Fe ₂ O ₃ or Co ₃ O ₄ with a defined size distribution in a size range from 1 to 20 nm diameter by etching, Pyrolysis, oxidation or reduction of polymeric core-shell particles, in particular block and graft copolymers, which are associated to micellar core-shell systems in solution, the core of which is loaded with suitable inorganic starting compounds, such as, for. B. HAuCl ₄ , Me ^{n +} Cl _n (Me = titanium, zirconium, iron or cobalt), Zeise salt (K [Pt [C ₂ H ₄ ] Cl ₃ ]) or Pd (OAc) ₂ .
• (ii) Use of block copolymers, graft copolymers, in suitable solvents associate to micellar core shell systems, as well as from highly branched dendritic Molecules with a core-shell structure for the objectives mentioned under (i).  
• (iii) The use of starting compounds whose core is loaded by systems from (ii) can be and which by the pyrolysis, etching, oxidation or under (i) Reduction process in catalytically active particles such as platinum metals or catalytically active Compound systems (precious metal / metal oxide) with a defined size distribution can be transferred.
• (iv) The deposition of the loaded core-shell particles described under (ii) - (iv) in mono and multi-layers by immersion, casting, spin-spin processes or by Adsorption from dilute solution on planar flat substrates such as mica flakes (Length <15 µm) and on planar curved surfaces such as micro glass and Micro quartz balls (diameter 1 to 50 µm).
• (v) The etching of the substrates described under (iv) and coated with polymer films by reactive plasmas such as oxygen or hydrogen, and by pyrolysis.
• (vi) Testing the metal clusters or metal / metal oxide composite clusters resulting from the etching processes listed in (v) with regard to their catalytic activity in selected model reactions.
  • a) Epoxidation, starting from C ₆ -C ₈ alkenes
  • b) Oxidation catalysis (CO → CO ₂ )

Literature:

[1] McPherson, RW; Starks, CM; Fryar, GJ; Hydrocarbon Proc. 75, (March 1979)
[2] Gonzalez, RG; Petrochemical Handbook Hydrocarbon Proc. 166, (March 1991) Weissermel, K .; Arpe, HP; Industrial Org. Chemistry, 3rd edition, Weinheim, VCH publishing company, 1988
[3] Hardacre, C .; Rayment, T .; Lambert, RM; J. Catal. 158, 102 (1996) Nibbelke, RH; Campman, MAJ; Hoebink, JHBJ; Marin, GB; J. Catal. 171, 358, (1997)
[4] Mergler, YJ; Nieuwenhuys, BE; J. Catal. 161, 292 (1997) Ranga Rao, G .; Fornasiero, P .; Di Monte, R .; Kaspar, J .; Vlais. G.; Balducci, G .; Meriani, S .; Gubitosa, G .; Cremona, A .; Graziani, M .; J. Catal. 162, 1. (1996)
[5] Zhang, ZL; Tsipouriari, VA; Verykios, XE; Catal. 158, 51 (1996)
[6] Karpinski, Z .; Zhang, Z .; Sachtler, WMH; Catal. Lett. 13, 123, (1992)
[7] Rabo, JA; Zeolite Chemistry and Catalysis, ACS Monograph, Vol. 171, ACS, Washington, DC, 1976
[8] Stubenrauch, J .; Vohs, J. Catal. 159, 112, (1996) Cordatos, H .; Gorte, RJ; J. Catal. 159, 112, (1996) Trovarelli, A .; Catal. Rev. Sci. Closely. 38, 439 (1996) Kondarides, DJ; Verykios, XE; J. Catal. 174, 52 (1998)
[9] Gabor A. Somorjai, Surface Chemistry and Catalysis, Wiley Interscience, 1994
[10] Tuzar, Z .; Kratochvil, P .; Surface and Colloid Science, Vol. 15, Plenum Press, New York (1993)
[11] Spatz, JP; Roescher A .; Möller, M .; Adv. Mater. 8, 337, (1996) Spatz, JP; Moessmer, S .; Möller, M .; Chem. Eur. J. 2. 1552, (1996) Möller, M .; Spatz, JP; Roescher, A .; Moessmer, S .; Tamil Selvan, S .; Klok, HA; Macromol. Symp. 117, 207, (1997)
[12] Haruta, M .; Tsubota, S .; Kobayashi, T .; Kageyama, H .; Genet, MJ; Delmon, B .; J. Catal. 144, 175 (1993)
[13] Mann, RF; Amphlett, JC; Peppley, BA; Frontiers Sci. ser. 7, 613, (1993)
[14] Daimler Benz AG, "Necar II-Driving without Emissions" Daimler Benz AG, Stuttgart, 1996
[15] Oetjen, HF; Schmidt, VM; Stimmig, U .; Trita, F .; J. Elektrochem. Soc. 143, 3838, (1996)
[16] Kahlich, MJ; Gasteiger, HA; Behm, RJ; J. Catal. sub.
[17] Van Santen, RA; Kuipers, HPCE; Adv. Catal. 35, 265, (1987)
[18) Minahan, DM; Hoflund, GB; Epling, WS; Schoenfeld, DW; J. Catal. 168, 393 (1997)
[19] European Patent: EP 0 709 360 A1

Reference list

Fig. 1: Force micrograph of 8 nm high Au nanoclusters on glass after thermal treatment of a micellar polymer film in an oxygen atmosphere (T = 200 ° C, p (O ₂ ) = 0.5 bar, t = 120 min). The length of an image edge corresponds to 1 µm.

Fig. 2: Force micrographs of bare precious metal clusters on different substrates:

• a) ∅ _Pt = 7 nm on quartz glass;
• b) ∅ _Pd = 5 nm on mica;
• c) ∅ _Au = 8 nm on glass

Fig. 3: Force micrographs of gold clusters on quartz glass.

• a) 13 nm high Clusters after oxygen plasma treatment;
• b) 13 nm high clusters after four hours Tempering under normal atmosphere at 600 ° C;
• c) 8 nm high gold particles after four-hour tempering under normal atmosphere at 800 ° C.

The length of a picture edge corresponds 1.75 µm.

Fig. 4: Force micrographs of bare platinum clusters on quartz glass.

• a) 12 nm high platinum cluster after the oxygen plasma treatment;
• b) 12 nm high Platinum nanoparticles after tempering for four hours under normal atmosphere at 400 ° C;
• c) 6 nm high particles after tempering for four hours under normal atmosphere at 600 ° C.

The length of an image edge corresponds to 1.75 µm.

Fig. 5: Force micrographs of gold / metal oxide hybrid systems on glass.

• a) 5 nm high gold / titanium dioxide;
• b) 5 nm high gold / cobalt oxide;
• c) 5 nm high Gold / iron oxide cluster after oxygen treatment.

The length of an image edge corresponds to 1 µm.

Fig. 6:

Fig. 7: schematic representation of the dimensions micelle (metal cluster) ⇔ micro glass ball (upper ball: before removal of the polymer shell (lower ball: after removal of the polymer shell).

Claims (1)

Nanostructured substrates for use in catalysis, characterized in that salt mixtures are introduced into micelles from block copolymers, these loaded micelles are applied to a substrate and the block copolymer is removed after the micelles have self-organized.