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Nick Hutzler

I am a second year Physics graduate student in the Doyle Lab. I am working on an experiment to measure the electron electric dipole moment using a cryogenic molecule beam.

Final Project: Applications of Gas Adsorption

Introduction

Adsorption is the physical process where molecules (or atoms, though we shall use the word "molecule" to include those as well) in a fluid phase stick become bound to the surface of another solid or liquid. Adsorption is a very broad term that can include a gas or liquid adsorbing onto a liquid or solid through electrostatic attractions, chemical bonds, or some combination of both. In this discussion we will focus primarily on the adsorption of gases onto solid.

Adsorption terminology. (From [Ke])

Before proceeding, let's introduce some terminology. In adsorption, the fluid-phase adsorptive molecules sticks onto the adsorbent and become adsorbate, or admolecules. When adsorbate unsticks from the adsorbent, whether spontaneously or induced, it is called desorption.

Brief Explanation of Adsorption

The fundamental reason why adsorption occurs is because surfaces have energy. The surface at the interface of a solid or liquid necessarily has some energy density associated with it that is higher than the energy density of bulk material. This is an empirical fact, because materials (both solid and liquid) tend to sacrifice surface area for bulk.

In liquids, surface energy arises from the fact that in the bulk, molecular (van der Waals) attractions between some molecule and all its neighbors balance out, resulting in a net force of 0. However, a molecule at a surface will feel a net attraction inwards, which gives rise to a surface energy. This force pulling the surface inward is balanced by the internal pressure of the liquid.

In solids, surface energy arises from disruption of the solid's lattice structure.

Types of Adsorption

Adsorption can be classified by whether the mechanism is "physical" (physisorption) or "chemical" (chemisorption) in nature. Because we will be focusing on physisorption, we then discuss several different adsorption mechanisms that would be classified as physical. The lists presented in this section are adapted from [Ke].

Physisorption vs. Chemisorption

  • Physisorption is classified by weakly bound adsorbate, usually by van der Waals or London dispersion forces. An important feature of this type of adsorption is that it is reversible; specifically, desorption can be induced by raising the temperature of the adsorbent, or by decreasing the pressure of the adsorptive. The adsorptive does not suffer any type of chemical change when being adsorbed.
  • Chemisorption is classified by strongly bound adsorbate, usually as a result of chemical bonds. Because chemical reactions occur, this process is typically irreversible, and the characteristics of the mechanism are strongly dependent on the species involved. Therefore, we will not focus on this type of adsorption.

Types of Physisorption

  • Monolayer/multilayer adsorbates: When the adsorbent is uniformly covered with sites on which the adsorptive can stick, the adsorbate will generally be a roughly uniformly distributed layer. When the adsorption sites are homogeneous and the adsorptive pressure is much smaller than the saturation pressure, this layer is typically one molecule thick. When the adsorptive pressure approaches the saturation pressure, there are typically multiple layers of adsorbate. The admolecules in monolayer often form a lattice gas, where there is typically one admolecule per site but they can hop from site-to-site (see for example [Mo])
  • Pore fluids: An adsorbent can have holes or troughs ("pores") that can become filled with adsorbate, and under the right conditions can become a gas or liquid. In fact, some very interesting things can happen because of the complicated thermodynamics of these systems; for example, liquid water a few molecular layers thick has been observed at the surface of certain porous solids... at 77K! [Ro]
  • Steric Adsorbates: Adsorbents can have site that can sterically attract certain atomic groups. This mechanism plays is important, for example, in the adsorption of large biomolecules onto activated carbon.
  • Ionic Adsorbates: Adsorbent surfaces can be covered with ions that can exchange with ionic parts of a molecule, binding the molecule to the adsorbate.
  • Quantum Adsorbates: Adsorption can occur due to purely quantum mechanical effects. One example is when an adsorbent has pores that are comparable to the de Broglie wavelength of the adsorptive, allowing them to enter the pores.

In real life, adsorbents typically have many of the above features.

Adsorption Thermodynamics

Basics

In this section we present some basic quantities and calculations useful for understanding adsorption.

Vapor Pressure

Frenkel Equation

The Frenkel Equation is given by [cite]

<math>\tau = \tau_0 e^{Q_d/RT}</math>

where <math>Q_d</math> is the heat of desorption (kcal/mol), R is the gas constant, T is the temperature (K), and <math>\tau_0</math> is some constant. From Frenkel's Law we can see why lowering the temperature of the adsorbent yields lower equilibrium pressures, since the admolecules will spend more time on the adsorbent after each collision.

The heat of desorption is dependent on the adsorbent. There is some minimum value (the heat of vaporization) that exists for any surface, which is the usual energy cost of the phase change. However, real surfaces can have other features (described earlier) that can enhance adsorption, yielding much higher binding energies. For heavier gases this enhancement is typically a factor of 2-3, but for the light gases Hydrogen and Helium, this factor can be as high as 10 and 30 (resp) for certain porous surfaces! [cite]

Equilibrium

Adsorption is often a rate-equation governed process, so "true" equilibrium would only occur at <math>t=\infty</math>. However, for real-life industrial applications, [Ke] defines a technical equilibrium as follows: an adsorbtion process is at equilibrium if <math>\Delta m/m < \varepsilon</math>, where <math>\Delta m</math> is the mass change in the total mass <math>m</math> over some time <math>\Delta t</math>, and <math>\varepsilon</math> is some parameter that depends on the application. Typical values are <math>\varepsilon=10^{-5}</math> and <math>\Delta t = </math> 30 minutes.

Industrial processes are often cyclical, and involve adsorption/desorption cycles on some time scale <math>t_c</math>. The distance from equilibrium of a process can then be characterized by a Deborah number:

<math>De = \frac{\Delta m/m}{\Delta t/t_c}</math>

For <math>De\approx 0</math> the system is at equilibrium.

Adsorption Isotherms

One very important tool for studying adsorption is an adsorption isotherm, a curve of surface coverage <math>\theta</math> versus pressure (or some related variables) at a constant temperature. An application of these models is calculating the internal surface area of the adsorbent.

Langmuir Isotherm

A simple yet powerful model was developed by Langmuir. This model assumes that the adsorptive forms a monolayer on the adsorbent, and that adsorbent covered with a monolayer is passivated. A quick derivation can be worked out by considering the equilibrium coefficient K of the adsorption reaction <math>M+S\leftrightharpoons MS</math> between molecules M and adsorption sites S:

<math>K = \frac{[MS]}{[M][S]}</math>

If <math>\theta</math> is the fraction of adsorbent that is covered by a monolayer, then <math>[S]\propto 1-\theta</math> and <math>[MS]\propto\theta</math>. In the gas phase, the concentration is proportional to the pressure P, so

<math>const\propto\frac{\theta}{P(1-\theta)}\quad\Rightarrow\quad \theta = \frac{bP}{1+bP}</math>

where b is the constant given in [Ho] as

<math>b = \frac{e^{-Q/kT}/\tau_0}{N\sigma_0/\sqrt{2\pi MPT}}</math>

here T is the temperature in K, Q is the activation energy in eV, k is the Boltzmann constant, N is the number of sites, <math>\sigma_0</math> is the adsorption site area, M is the adsorptive molecular weight, and <math>\tau_0</math> is the adsorption time.

The Langmuir Isotherm is too much of a simplification to find much practical use. Instead, it is more common to use the BET isotherm, which allows for multiple layers.

BET Isotherm

The BET isotherm is given by [Ho]:

<math>\frac{P}{P_0-P}\times\frac{1}{\theta} = \frac{1}{C\theta_m}+\frac{C-1}{C\theta_m}\times\frac{P}{P_0}</math>

where P is the adsorptive pressure, <math>P_0</math> is the saturation pressure, <math>\theta_m</math> is the surface area covered by a at least one layer, Q is the heat of adsorption, and <math>C\propto\exp(Q/kT)</math>. Here we allow multiple layers to form, so <math>\theta</math> is the total surface coverage counting each layer separately. For example, if half of the adsorbent was covered by a single layer we would have <math>\theta=0.5</math>, if the entire adsorbent was covered by 3 layers, we would have <math>\theta=3</math>, and so on.

The BET isotherm is a much more powerful tool because it can yield two desirable quantities: the head of adsorption, and the surface area of the adsorbent. Define

<math>Y = \frac{P}{P_0-P}\times\frac{1}{\theta}, \quad X = \frac{P}{P_0} \quad \Rightarrow \quad Y = \frac{1}{C\theta_m}+\frac{C-1}{C\theta_m}X </math>

The slope and intercept of this line then yield <math>\theta_m</math> and Q. Since <math>\theta</math> is proportional to the total gas adsorbed, which can be determined from pressure and flow measurements, this is a common experimental method to measure the surface area, which is sometimes called the BET area. Adsorbent materials can have BET areas of several thousands of square meters per gram. [Ke]

Other Isotherms

While the Langmuir and BET isotherms may be the most common, there are a few more that appear every now and then [We]. In each of the following, <math>C</math> represents some constant, but not necessarily the same constant.

  • Henry's Law: <math>\theta=C P</math>
  • Freundlich Isotherm: <math>\theta=CP^m</math>, where <math>m\leq 1</math>
  • DR Isotherm: <math>\log\theta=\log\theta_m-C(\log(P_V/P))^2</math>, where <math>P_V</math> is the vapor pressure of the liquid form of the adsorptive gas.

All of the isotherms we have discussed here have their own regimes where they work, though none of them are universal.

Enthalpy

Application: Cryopumping

Introduction

Cryopumping refers to the condensation or adsorption of gas onto a cooled surface. Cryopumping has a molecular explanation that does not rely on surface energies: when a molecule collides with a sufficiently low-temperature surface, the molecule can lose enough of its energy so that it can become bound by weak van der Waals/dispersion forces with the surface. Because cryopumping generally occurs (unless something is not going well) in low-pressure regimes, it is useful to consider the adsorptive gas in the molecular picture. The adsorption binding energies of the molecules arises from the minimum in the van der Waals interaction with the surface:

[Figure from [Da]]

Some values of this adsorption binding energy <math>\varepsilon</math>for various gases on activated carbon is found in [Da]:

Gas <math>\quad\varepsilon</math> (kJ/mol)
He 0.5
H2 1.5
Ne 3
Ar 10
Kr 15

A typical, commercially available cryopump consists of metal panels covered with a porous adsorbent (coconut charcoal is very common) that is cooled by either a cryorefrigerator or by liquid cryogens.

Cryocondensation vs. Cryosorption

Cryocondensation and cryosorption are not the same thing, though they both usually fall under the term "cryopumping." Cryocondensation refers to using gas condensing on a cold surface until the vapor pressure (for that temperature) is reached. This is the dominant process for gas sticking to a smooth, clean surface. Cryosorption refers to using a cold, porous material so that the adsorptive gas is adsorbed, as opposed to simply condensed. Cryosorption can achieve much lower pressures, so we will focus on it for this discussion.

Random Facts Yet To Be Classified

  • In addition to the commonly used BET isotherm method mentioned earlier, there is a large selection of advanced methods for finding pore sizes. See [Ke 1.4] for more details.

References

[Ke] Keller & Staudt, Gas Adsorption Equilibria

[We] Welch, Capture Pumping Technology

[Ro] Robens, E. "Some Remarks on the Interface Ice/Water." Proceedings VIII Ukrainian-Polish Symposium: Theoretical and Experimental Studies of Interfacial Phenomena and their Technological Applications (2004)

[Mo] Morgenstern, K. and Rieder, K. H. PRL 93 (2004) 056102

[Ho] Hoffman, Singh, and Thomas, Handbook of Vacuum Science and Technology.

[Da] Day, Use of Porous Materials for Cryopumping (link?)