Surface and Interface Chemistry  Solid/gas Interface (part two) Valentim M. B. Nunes Engineering Unit of IPT 2014

The physical adsorption may involve the formation of multilayers on the surface. Adsorption of gases in solids The extension of the theory of Langmuir for the adsorption treatment in multilayers was made by Brunauer, Emmett and Teller  EQUATION BET. For the first layer,  H ads   H ads of Langmuir. For the other layers,  H   H liquefaction = -  H vap.

Considering ΔH 1 the heat of adsorption for the first layer and ΔH 2 the heat of liquefaction for the adsorption of the 2 nd and following layers, we obtain: P s – saturation pressure V m – volume of the monolayer

Considering z = p/p s If c >> 1 Non reactive gases in polar surfaces, since c  10 2, because  H des >>  H vap

For c < 1 isotherms of type III

The V m parameter is of particular importance as it is used to calculate the surface area of an adsorbent, from the actual area occupied by each molecule of adsorbate BET surface area The most widely used adsorbate, even in industrial terms, is the nitrogen at 77 K. Determination of the area of finely divided solids!

AdsorbateEffective occupied area N 2 at 77 K 16.2  m 2 Kr, Xe, Ar at 77 K ~17 a 27  m 2 Ar at 90 K 14 a 17  m 2

Isosteric heat of adsorption ( constant  ) Considering the Langmuir Isotherm

Chemical Adsorption Unlike physical adsorption, this process is rarely reversible.  H ads,quim >>  H ads,física It is a highly selective process: for example the H 2 is adsorbed chemically by W and Ni, but not for alumina or Cu. It is very important in heterogeneous catalysis, since E a is smaller in the catalyzed process.

Langmuir – Hinshelwood mechanism A + B  P v = k  A  B

Examples of heterogeneous catalyzes CatalyzerFunctionExample MetalsHydrogenation Dehydration Fe, Ni, Pt, Ag Oxides and semiconductors Oxidation Dessulfuration Dehydration NiO, ZnO, MgO Isolating oxidesDehydrationAl 2 O 3, SiO 2 AcidsPolymerization Isomerization Cracking Alkylation

Adsorption in porous solids Several phenomena Filling of micro porous Adsorption in monolayer Adsorption in multilayer Capillary condensation IUPAC, 1986 Micro porous: d < 2 nm Meso porous: 2 < d < 50 nm Macro porous: d > 50 nm

In the micro pores filling of cavities can occur at very low pressures, not being appropriate the models studied previously. In intermediate sized pores (meso porous) we must consider first mono and multilayer adsorption, followed by capillary condensation from a given pressure. In macro pores, as in non-porous surfaces, the multilayer adsorption can be extend up to a very high number of layers.

Capillary condensation For interpreting quantitatively the effects of capillary condensation we use the Kelvin equation, adapted to the phenomenon: r p –pore radius; p – equilibrium pressure; p s – saturation pressure of gas or vapor; V m –molar volume of liquid;  - surface tension;  - contact angle; T – temperature.

This equation is only valid for mesoporous (spherical meniscus). If θ <π/2, then p < p s, and adsorbate condensation can occur at a pressure lower than the saturation pressure. p/p s 1 0 n ads /mol.g -1 p’p’ nAnA nBnB

Macro porosity Porosimetry with mercury;   140º for most of the solids. It is necessary to apply an excess of pressure to force the Hg to penetrate the pores of the adsorbent. The method consists in determining the volume of mercury which penetrates a solid due to the hydrostatic pressure applied. For each value of pressure, Δp i it may be assumed that the mercury penetrates into every pore (cylindrical) with radii exceeding r i, value obtained from:

Pressurerprp  MPa 7500 nm  200 MPa 3.5 nm  400 MPa 1.8 nm (+) Pore diameter (-) p/atm volume /cm 3.g -1 “ink-bottle”