DE102004046108A1 - Nanostructures and coherent spatial structures for adsorbing chemicals (e.g. hydrogen in fuel cells) has all of its surfaces adapted for adsorption - Google Patents

Abstract

A Nanostructure for the adsorption of chemical substances, in particular of hydrogen, has a maximum outer dimension from 2000 nm up. Every surface The nanostructure is a curved outer surface with a Gaussian curvature less than zero.

Description

The Invention relates to a spatial Structure for the adsorption of chemical substances, in particular of hydrogen, with the features of the preamble of claim 1.

One example for the use of such a spatial Structure consists in a hydrogen storage in which hydrogen reversible to the spatial Structure is attached. There are many other applications as well such spatial Structures for Adsorbing chemical substances, and in particular the cases here are of interest in which it is a reversible Adsorbieren the chemical substances, that is to say the temporary and detachable connection of the chemical substances to the respective spatial structure. But there is also useful applications where the connection is essentially should be permanent, for example, if certain substances to be filtered out of a gas stream.

STATE OF TECHNOLOGY

The Future of fuel cell technology, for example in the automotive sector, depends on Progress in the storage of hydrogen in hydrogen storage with as possible low volume, if possible low weight but still possible high reliability.

The Simple storage of hydrogen in pressure tanks holds a high Danger potential, here for a significant filling pressures in the range of 500 to 1,000 bar are necessary. Besides, they are corresponding pressure tanks bulky and heavy and therefore difficult to accommodate in motor vehicles.

One known alternative to hydrogen storage pressure tanks is based on porous solid bodies of metals. These hydrogen storage systems are based on the effect that certain metals, such as palladium or magnesium, and intermetallic compounds, such as ZrMn ₂ , absorb hydrogen under moderate pressure and bind it within their lattice structure as metal hydride. When the temperature increases, the hydrogen is released again. These chemical reactions are not only reversible but easy to control and allow a largely lossless storage of hydrogen over a longer period of time, since the gas is chemically bonded and therefore can not escape. Also from a safety point of view metal hydride storage is almost harmless. However, the disadvantage is the low mass-specific storage density of known metal hydride storage, which is only between 1.7 and 5 percent by weight of hydrogen, based on the mass of the storage medium. Metal hydride storage is very difficult. An additional disadvantage is that depending on the metal hydride to release the hydrogen very high temperatures are required. The previously achieved maximum storage density of 5 percent by weight was achieved with the aid of complex metal hydrides, such as sodium aluminum hydride (sodium allanate) in which, despite reducing kinetic barriers with suitable catalysts very long loading times are required and only relatively low Wiederabgaberaten of hydrogen can be realized.

It is therefore reinforced looking for alternative hydrogen storage. A high potential in this regard have nanostructures, i. nanoscale spatial structures that due to their enlarged surface principal Offer advantages in the absorption of chemical substances. Also the Accessibility of the surface of nanostructures is compared with the accessibility of absorption sites in the Inside of a solid attractively priced.

In Recent years have reports of high storage capacities of Nanostructures in the form of carbon tubes with a diameter of up to 20 nanometers and a length of 100 nm up to a few Millimeters, attracted a lot of attention. The initially published However, high storage densities could not be detected experimentally become. Serious Storage densities are between 2 and 4.5% by weight, However, it is still unclear which sizes and other parameters of Carbon tubes have the best storage properties. So the walls of the tubes can be different Structures, i. Arrangements of the individual atoms, which have for example, as "armchair", "zigzag" and "chiral". The necessary loading times for carbon tubes to those specified above Storage densities are still in the range of some Hours and therefore tend to be too long for one To be used, for example, in the field of motor vehicles, in which a faster recharging of a hydrogen storage be possible should.

Curved and helical collimator tubes have also been considered as nanostructures to store hydrogen. As a result, but the basic characteristics of such Can not improve carbon tubes for hydrogen storage.

Next Carbon tubes are also nanostructures in the form of hollow spheres and arranged one inside the other Ball cups (nano onions) for hydrogen storage has been considered. In this However, the nestability worsens increasingly the reachability the then interior surfaces.

The Accessibility of the surfaces for the to be accumulated hydrogen is at a known spatial Structure having the features of the preamble of claim 1 largest, the is called nanocone and in which an inner and an outer curved surface of a single cone-shaped atomic single layer of graphitic carbon are formed. However, the question is whether, e.g. Hydrogen also on the inner Tip of a nanocone can attach, or if not really very unfavorable potential conditions especially for the reversible attachment of chemical substances are present.

It are also planar graphitic carbon layers for storage Hydrogen has been tested. Even at relatively high pressures during the Loading, however, were only relatively low storage densities achieved.

In Terrones, H. et al .: "Curved nanostructured materials "in New J. Phys. 5 (2003) 126, PII: S1367-2630 (03) 65766-6 are various curved Nanostructures, for example based on hexagonal carbon, in principle described and ways for their production shown.

TASK OF THE INVENTION

It the object of the present invention is a spatial Structure, the fundamental Benefits for has the adsorption of chemical substances on their curved surface.

SOLUTION

According to the invention this Task by a spatial Structure solved with the features of claim 1. preferred embodiments this spatial Structure are in the subclaims 2 to 10 and 12 to 14 described. The dependent claim 11 relates a bulk material consisting of a multitude of nanoscale new spatial Structures, and the dependent claim 15 relates to a hydrogen storage with at least one new spatial Structure.

DESCRIPTION THE INVENTION

The curved surface in the form of a saddle, here being the mathematical-geometric Definition of such a saddle is meant and in particular through a Gaussian curvature is less than zero, has a particularly homogeneous Distribution of potential relationships at the approach of one chemical substance to the surface. At a saddle are the bends the surface so evenly distributed, that there are no areas over the surface comes in which strongly divergent potential conditions arise, such as at the inner tip of a nanocone. At the same time is a surface in the form of a saddle everywhere easily accessible. If only a portion of this surface of the adsorbed chemical substance can be achieved, the chemical substance can reach every other part of the saddle-shaped surface.

Preferably the saddle has a minimum radius of curvature of more than 1 nm, more preferably, the minimum radius of curvature is greater than 10 nm. This limit also means a limitation on the size of the whole Saddle down, which ensures that the saddle-shaped surface in total for the adsorbing chemical substance is to be achieved and not too small radii of curvature of the saddle a potential course for the approach of the chemical substance to the surface which gives such an approximation or an associated, in particular reversible adsorption the substance to the surface in principle opposes.

typical Dimensions of the saddle are in the range of a few tens of nanometers up to a few hundred nanometers, taking its maximum dimensions usually not more than 1000 nm.

Of the Saddle coming from the curved surface the spatial Structure may be a hyperbolic paraboloid. However, preference is given to saddles which are minimal surfaces, Saddles in the form of minimal surface areas are particularly preferred where the saddle has a mean curvature of zero.

If every surface the saddle a minimal area This also facilitates its manufacture because this is synonymous with that, the saddle is a geometric shape of minimal strain energy has, i. is energetically stable. The saddle can therefore, for example be prepared by a flat starting structure, such as For example, a hexagonal array of graphitic carbon is deformed by placing at defined points by storage from foreign atoms tensions are introduced. In the endeavor The structure to break down these stresses, the desired curved shape of the saddle then almost automatically.

Furthermore it is preferred if the saddle has a symmetry parameter s greater than 1 has. Particularly preferred is a symmetry parameter s of Second

The new spatial Structure may be adjacent to one or more contiguous saddle-shaped surfaces also other surfaces exhibit. However, it is particularly preferred that it has exclusively saddle-shaped surfaces.

Concrete it may be at the new spatial Structure around a nanostructure acting through two complementary surfaces in Form of a saddle is limited. The structure itself can do this from an atomic single layer, for example from graphitic Consist of carbon. The spatial However, structure may also be made of other substances, e.g. zeolites (Silicate minerals), boron nitride or organometallic complexes consist. But it is in any case preferred if it is is an atomic or at least molecular single layer. The typical number of atoms involved in the saddle is in between about 50 and a few hundred.

If the spatial Structure is a nanostructure, it is preferred if they are not is stackable. This prevents the use of spatial Structure in a bulk material consisting of a variety of individual spatial structures that are the nanostructures on top of each other arrange that by a very small distance of their surfaces unfavorable potential conditions for the Intermediate occur of the chemical substance to be adsorbed.

The new spatial Structure may also have a multiplicity of saddle-shaped surfaces merging with one another. Every single one of these surfaces has the shape of a saddle, and the individual surfaces are steadily possibly with the interposition of transition areas with each other connected.

The spatial Structure with a plurality of merging saddle-shaped surfaces can an open-pore sponge structure, whereby hereby not on the Deformability of a sponge but on the course of the surfaces of his Pores is turned off. With an open-pore sponge structure are due to the openness of the pores all surfaces of the spatial structure for the chemical easy to reach to adsorbing chemical substance. A corresponding Sponge structure is for example by curing a so-called microemulsion formable. This is a known method, over which Information can be taken from S. Burauer: "Microemulsions: Internal interface and monomeric solubility "Diploma thesis, University of Cologne, 1997, and O. Lade: "Characterization, Modeling and Polymerization of Microemulsions from Alkyl Methacrylates ", Cuvillier Verlag, Göttingen 2001. These sections also contain information on other sources. The characterization of the interfaces of microemulsions as saddle-shaped surfaces with the for the new spatial Structure characteristic parameters is also T. Sottmann: "Microemulsions: Properties of Internal Interfaces ", Dissertation, University of Göttingen, 1997, can be seen. The manufacturability of the saddle-shaped nanostructures is for example by crushing a cured microemulsion in smaller units, ideally one saddle each correspond.

Of the large number of uses of the new spatial structure for the particular reversible adsorption of chemical substances is of particular importance in which the reversibly adsorbed chemical substance is hydrogen. In other words, it relates to the present invention in one concrete embodiment, a hydrogen storage with at least one of the above-described spatial structures. If the spatial structure is a nanostructure, it will be used in large numbers.

Next The storage of hydrogen is the new spatial structure also for others Applications suitable to adsorb chemical substances are. This is about other storage applications as well to applications in which the chemical substances are permanently, i. irreversible, bound to, for example, their chemical Reactivity to degrade or filter them out of a current.

BRIEF DESCRIPTION OF THE FIGURES

in the The invention is described below with reference to the figures preferred embodiments further explained and described.

1 shows the course of the potential energy according to Lennard-Jones.

2 shows a saddle with the symmetry parameter s = 1.

3 shows a saddle with the symmetry parameter s = 2.

4 shows a saddle with the symmetry parameter s = 3.

5 shows a saddle in the form of a hyperbolic paraboloid.

DESCRIPTION OF THE FIGURES

introduction

mit den empirischen Größen ε (Tiefe der Potenzialmulde) und σ (Kollisionsabstand) bewährt. Der Vorteil dieser vereinfachenden Darstellung ist die Möglichkeit, das Wechselwirkungspotenzial für eine Mischung von Teilchen zu verallgemeinern. Während der erste Term in der linken Klammer die Abstoßung zwischen den abgeschlossenen Elektronenhüllen der Teilchen wiedergibt, beschreibt der zweite Ausdruck die Anziehung auf Grund der zwischenmolekularen Kräfte. Das Minimum der potenziellen Energie (Potenzialmulde) liegt bei diesem Modell stets bei

To describe the physical adsorption of gases on solid surfaces, the empirical interaction potential according to Lennard-Jones has been determined according to

proven with the empirical quantities ε (depth of potential well) and σ (collision distance). The advantage of this simplistic representation is the ability to generalize the interaction potential for a mixture of particles. While the first term in the left bracket represents the repulsion between the closed electron shells of the particles, the second term describes the attraction due to the intermolecular forces. The minimum of potential energy (potential well) is always included with this model

This Equilibrium state is thus at a distance that just a little bigger than the collision distance is.

Quality Solutions of the Hydrogen storage problems should therefore be the fundamental and allow quick access to the potential well. Because the attractive forces correspond to the negative gradient of the potential field, however even with values r / σ> 2 negligible becomes small, nanostructures had to be found, from which none unnecessary Potential barriers for the hydrogen result.

• 1. Die Kohlenstoffatome werden derart angeordnet, dass ihre Modifikation eine maximale Oberfläche aufweist und gleichzeitig die Kohlenstoffmenge minimiert. Diese Maßnahme führt zwangsläufig zu einer Gewichtsersparnis und zu einer sehr effizienten Speicherfähigkeit.
• 2. Die Kohlenstoffatome werden derart angeordnet, dass sich die Oberflächen der Molekülstrukturen nicht gegenseitig abdecken (wie z.B. bei Multi-Wall CNT) und der Wasserstoff durch die entstehende Potenzialbarriere nur unter Aufwendung zusätzlicher Energien und Kräfte (z.B. durch höhere Drücke) die inneren Flächen erreichen kann. Diese Maßnahme sollte zu einer Maximierung an „verfügbaren" Oberflächen und zu kurzen Beladungszeiten führen.

The nanostructures described below take this requirement into account, with the aim of providing the greatest possible number of "docking points" for the hydrogen, which can basically be achieved by two measures:

• 1. The carbon atoms are arranged so that their modification has a maximum surface area while minimizing the amount of carbon. This measure inevitably leads to a Ge saving weight and a very efficient storage capacity.
• 2. The carbon atoms are arranged in such a way that the surfaces of the molecular structures do not cover each other (as in Multi-Wall CNT) and the hydrogen reaches the inner surfaces through the resulting potential barrier only by using additional energies and forces (eg higher pressures) can. This measure should lead to a maximization of "available" surfaces and to short loading times.

nanostructure in the form of a saddle

A spatial structure that meets the above requirements is a nanoscale saddle, also referred to as a nanosattel, that fits into the nano saddle 2 to 5 is reproduced in various concrete embodiments.

lautet. Es handel sich hierbei um Ennepersche Minimalflächen, deren Symmetrieparameter s ist. Bei dem in 3 für s = 2 dargestellten Nanosattel haben die krummlinigen Parameter u und v beispielsweise die Werte u ∊ [0, 1.2], v ∊ [–π, π] A saddle is a minimal surface in the differential geometric sense whose mean curvature disappears and its parametric form

reads. These are Enneper minimal surfaces whose symmetry parameter is s. At the in 3 For nano saddles shown for s = 2, the curvilinear parameters u and v have the values, for example u ∈ [0, 1.2], v ε [-π, π]

The parameter v provides the completeness of the area from the center. The parameter u defines the inner and outer edges of the surface. With u _min = 0 there is no central hole. Otherwise they are stripes on the minimal surface of Enneper.

Further variants of the nanostructure in the form of a saddle result from the consideration of hyperbolic paraboloids (cf. 5 ), which in contrast to Enneper 's minimal surfaces have straight edges and those of parametrization x (u, v) = a · (u - v) y (u, v) = b · (u + v) z (u, v) = u · v to obey. Design parameters here are the scaling variables a and b.

The implicit form of the saddle is

mit der komplexen Variablen z := r·ej·ϕ,der analytischen Funktion f

und der meromorphen Funktion g

The geometry of the saddle can also be displayed using the Weierstrass parameterization:

with the complex variable z: = r · e j · φ . the analytic function f

and the meromorphic function g

With the value ranges r ε [0, 1] and φ ε [-π, π] the representation is consequently

Will the complex variable according to z: = α + j · β With the linear parameters α (real part, eg α ε [-1, 1]) and β (imaginary part, eg β ε [-1, 1]) substituted, the saddle obeys the representation

die stets negative Gaußsche Krümmung

und die mittlere Krümmung H = 12 (κ1+ κ2) = 0. The saddle according to the invention has the main curvatures

the always negative Gaussian curvature

and the mean curvature H = 1 2 (κ 1 + κ 2 ) = 0.

A differential surface element of the saddle is through dA = (1 + α 2 + β 2 ) dα∧dβ given. The value ranges α ε [α _min , α _max ] β ε [β _min , β _max ] result in the (one-sided) surface area

These area is the hydrogen (at least one side) for "docking" available.

The saddle therefore fulfills as minimal surfaces the d'Alembert differential equation y = x · f (y ') + g (y') with the given functions f and g or the Lagrangian differential equation (1 + f v 2 ) · F uu + 2 · f u f v f uv + (1 + f u 2 ) · F vv = 0.

Of the Saddle can therefore also be characterized as an area the one special solution for given Boundary conditions is (plateausches problem).

In the case of a nanostructure in the form of a saddle, only open surfaces that are easily accessible to the hydrogen atoms are realized, which do not intersect each other, as long as the design parameter u remains small enough. The mean curvature is zero, which ultimately suggests a low power-to-weight ratio. If the integer symmetry parameter s = 1 (cf. 6 ), the minimal surface takes on a shape in which several nanostructures can cover each other, similar to so-called "crisps" in their packaging. Consequently, undesirable potential barriers with regard to the addition of hydrogen to the carbon atoms are to be expected for this case. For all symmetry values s ≥ 2 (compare the 3 and 4 ), however, the surfaces of the individual components can no longer cover each other.

Of the nanosattel according to the invention exists in the area preferably from a strained by the incorporation of impurities and thereby spatially deformed monolayer of graphitic carbon; but he can also build on other suitable substances, e.g. on zeolites (Silicate minerals), boron nitride or from MOF's.

• – Medikamentwirkstoffe zu transportieren oder
• – Chemikalien zu binden bzw. chemische Reaktivitäten herabzusetzen oder
• – Katalysatoren zu miniaturisieren.

Due to their special physical and chemical properties, which are due to their geometric shape, in addition to the storage of hydrogen and other gaseous substances, the possibilities arise with the new nanostructure in the form of a saddle

• - to transport drug substances or
• - to bind chemicals or reduce chemical reactivities or
• - miniaturizing catalysts.

The for one geometric saddle bounded by two complementary surfaces in the form of a saddle is described, properties are also spatial Given structures containing one or more surfaces in the form of a nanosattel exhibit.

Claims (13)

Spatial structure for the adsorption of chemical substances, which has at least one curved surface with a maximum radius of curvature of 1000 nm, characterized in that the surface has the shape of a saddle with a Gaussian curvature of less than zero. spatial Structure according to claim 1, characterized in that the saddle a minimum radius of curvature of x nm. spatial Structure according to claim 1 or 2, characterized in that the Saddle is formed by a single atomic layer, at the back the spatial Structure another saddle-shaped Surface has. Spatial structure according to one of claims 1 to 3, characterized in that the saddle is a hy is perbolic paraboloid. spatial Structure according to one of the claims 1 to 3, characterized in that the saddle is a minimal surface. spatial Structure according to claim 5, characterized in that the saddle a mean curvature of has zero. spatial Structure according to claim 5 or 6, characterized in that the Saddle has a symmetry parameter s greater than 1. spatial Structure according to one of the claims 1 to 7, characterized in that it has exclusively saddle-shaped surfaces. spatial Structure according to claim 8, characterized in that it has a Nanostructure is through two complementary surfaces in the form of a saddle is limited. bulk existing from a variety of spatial structures according to claim 9 or 10. spatial Structure according to one of the claims 1 to 8, characterized in that they have a plurality of merge into one another saddle-shaped surfaces having. spatial Structure according to claim 11, characterized in that it has a having open-porous sponge structure. Hydrogen storage with at least one spatial Structure according to one of the claims 1 to 9 or 11 and 12.