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Surfaces Gas Adsorption at Solid Surfaces

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Definitions Adsorption: A molecule (adsorbate) that forms a bond to the surface (adsorbent). Associative Adsorption: A gas molecule adsorbs without fragmentation. Dissociative Adsorption: A gas molecule adsorbs and fragmentation occurs. Fractional coverage of adsorbate (θ) where N s = number of surface sites occupied by adsorbate and N = total number of substrate adsorption sites – When θ = 1, a monolayer exists

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Langmuir Adsorption Isotherms Adsorption Isotherm: θ depends on, and is linearly related to, pressure (P) at a constant temperature (T). This relationship is used to look at equilibrium adsorption behavior, and to find the total surface area of a substrate (S A ), but the following assumptions must be made: – 1: The solid surface is uniform and each site (all are equivalent) can be occupied by 1 molecule. – 2: A dynamic equilibrium exists between the gas (at P) and the adsorbed layer at constant T. For associative adsorption: M(g) + S(surface site) M-S (k a and k d are the rate constants of adsorption and desorption, respectively) – 3: Adsorbates (g) continually collide with the surface. Upon impact of a vacant site, the adsorbate sticks. Upon impact of a filled site, they are reflected back into the gas phase. – 4: Once adsorbed, molecules are localized and the enthalpy of adsorption (ΔH AD ) per site is constant, independent of θ. kaka kdkd

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In the dynamic equilibrium of associative adsorption: – Rate Adsorption = k a P(1- θ) = Rate Desorption = k d θ – Upon rearrangement, we see that θ = N S /N = KP/(1+KP), where K = k a /k d. We can predict how θ changes with P. As P 0, θ = 0 and as P , θ = 1. If KP<<1, θ = KP. In the dynamic equilibrium of dissociative adsorption: – M 2 (g) + 2S(surface site) 2(M-S) θ = (K’P) / (1 + (K’P) ) where K’ = k’ a /k’ d Since N S is hard to determine, mass and volume may be used to find θ: θ = m/m = V/V , where m & V are the mass and volume of the adsorbed gas at constant P, and m & V are the mass and volume corresponding to all substrate sites being occupied. If N, m or V is known, S A can be calculated. k’ a k’ d 1/2 Langmuir Adsorption Isotherms continued

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Surface area, S A = N*A m, where A m = area molecule and N can be calculated using m : m /M = n M = total # of moles in 1 monolayer (ML) n M = N/L, where L = Avogadro’s number so N = (m L)/M, where M = molar mass or N can be calculated using V : PV = n M RT (at constant P & T), so N = (PV L)/RT And S A as specific surface area = S A /mass of substrate

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Heats of Adsorption Adsorption of gas on a solid is exothermic. ΔH AD is the isoteric (constant θ) enthalpy of adsorption. Determining the thermal energy evolved when a known amount of gas is allowed to adsorb onto a clean surface (T rise of solid) can help to derive ΔH AD. ΔG° = -RTlnK° = ΔH° AD – TΔS° AD lnK° = ΔH° AD /RT + ΔS° AD /R Differentiate with respect to T at constant θ [ / T(lnK°)] θ = ΔH° AD /RT Remember: KP = θ/(1-θ) lnK + lnP = ln(θ/(1-θ) [ / T(lnK)] θ + [ / T(lnP)] θ = 0 [ / T(lnK)] θ = [ / T(lnK°)] θ [ / T(lnP)] θ = ΔH AD /RT [ln(P 1 /P 2 )] θ = (ΔH AD /R)(T 1 – T 2 ) 2 2

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Adsorbate Bonding Physisorption: Adsorbate/Adsorbent bonding interaction is long range, weak,and reversible, associated with van der Waal interactions. There is a negligible exchange of electrons and a surface potential at the interface between two phases – gas and solid – where an “overspill” of electron charge from the solid into the gas phase results in an imbalance of electron density. Chemisorption: An exchange of electrons between absorbate and adsorbent occurs, associated with covalent, ionic and metallic bonding. The ΔH AD is larger than that of physisorption. As chemisorption proceeds, defects in ordered arrays and closer proximities of adsorbates destabilize the adsorbed layer, and this is seen in ΔH AD.

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Kinetics Sticking probability (S) is the probability of a molecule being associatively adsorbed from the gas phase into a chemisorbed state assuming Langmuir behavior. S = S 0 (1-θ) where S is the rate of adsorption of adsorbates over the rate of collision of molecules with the surface (Z), and S 0 is the sticking probability at θ = 0. Sometimes S is larger than predicted using Langmuir isotherm because if a molecule hits a filled site, it could form weak Van der Waal interactions with the surface, diffuse, and then move to a vacant spot where it will become chemisorbed. If adsorbate is initially physisorbed to a vacant site, it is referred to as an intrinsic precursor state, while if it is physisorbed to a filled site, it is an extrinsic precursor state.

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Kinetics continued The rate at which molecule collide with the surface is measured by a thermal accommodation coefficient (α) α = (T f – T i )/(T s – T i ) where T i is the T initial of the molecule in the gas phase, T f is the T final of the molecule after collision with the surface, and T s is the T surface. When T i = T f, α = 0 and the molecules are elastically scattered. When T s = T f, α = 1 and the adsorbates are all “accommodated”.

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Well-Defined Surfaces In order to make surfaces more comparable, controlled amounts of defects are added to surfaces. Typically, single crystal surfaces are used – cut using X-ray back scattering so that a particular crystal plane is exposed. The surfaces are then flat, with mostly large terraces, or vicinal, with short, flat terraces that are separated by atomic steps.

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The Miller Index The Miller index is used for notation of the crystal planes – (x,y,z) for simple cubic, face centered cubic (fcc), or body centered cubic (bcc) – (w,x,y,z) for hexagonal close packed surfaces (hcp) Rules – Find where the plane intersects the x, y, and z axes in multiples of unit cell dimensions (a) is used for a plane parallel to an axis, and –1 is written as Ī – Take the reciprocal of the numbers – If fractions result, make the ratio into whole numbers. – Example: A plane that runs parallel to the x-axis, but intersects the y- and z-axes at 1: , 1, 1 becomes (011). – Planes with a high Miller index are not flat, but have narrow planes separated by steps and are described with the following n(x,y,z) x (u,v,w), where n = the average number of atoms on a terrace, (x,y,z) is the Miller index of the plane and (u,v,w) is the Miller index of the step. – fcc: (331) = 3(111) x (111), which is 3-atom wide (111) terraces separated by (111) x (111) steps.

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(011) Surface Example

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Surface Relaxation Atoms in (111), (100), and (110) fcc have lost 3, 4 or 5 nearest neighbors of the original 12 and to compensate for the loss of bonding, the surface will relax, which is an oscillatory change in interplanar spacing (Δd). During surface relaxation, the 1 st layer of atoms contracts towards the 2 nd layer to increase coordination, and the 3 rd layer reacts by expanding away from the 2 nd layer. This continues until 5 or 6 layers deep (the selvedge) until the oscillations are damped. The largest relaxation occurs in high energy surfaces with low atomic density. – In fcc, (110) > (100) > (111) – In bcc (111) > (100) > (110) If the energy is large enough, the plane could reconstruct to enhance coordination of surface atoms and to lower the energy of the surface.

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Preparation of Clean Surfaces Cleaning the surface – Layered compounds (graphite, mica and some semiconductors) can be cleaved – Metal catalysts can be rid of excess oxygen by heating the sample in the presence of hydrogen. – Sputtering cleans the sample using high energy argon ions to bombard the surface. The energy transfer from the ions breaks bonds of the surface and adsorbates, but leaves the surface rough, so it is best to anneal the surface to flatten it after sputtering. Sputtering and annealing should be repeated several times because annealing the sample can bring impurities from the bulk to the surface.

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Surface bombardment (Z) by molecules Z = P/(2πMkT) where P is P ambient (Ncm ), M is in units of (kg/molecule), T is in Kelvin, and k is Boltzmann’s constant (J/K). The rate of contamination also depends on a molecule’s sticking ability. Example: CO – 300K, P = 10 torr, and assume worst, S = 1. Z = 3.82 x 10 cm s. If atomic density is typically 10 cm, Z/(cm/ML) = 0.382 ML/s And for 1 ML = 2.6 s to adsorb at an ambient P However, at 10 torr, it takes 7.3 hours for 1 ML to adsorb, so an ultra high vacuum is necessary to keep surfaces clean. Langmuir units (L) are used to describe gas exposures, and the definition is an exposure for 1 second at 10 torr. 1/2 -2 -6 14-2 15 -10 Maintenance of Clean Surfaces -6

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Mobility in Two Dimensions Surface Diffusion: atoms and molecules are mobile to equilibrate and minimize free energy states. The rate of diffusion is dependent on the direction in which the diffusion occurs, T and θ. Physisorbed molecules have low ΔH ° AD and low diffusion activation energies and therefore are very mobile in ambient T. Chemisorbed molecules, on the other hand, are immobile in ambient T, because of higher activation energies. The activation energy is larger for rough surfaces than for smooth surfaces. For the rate of diffusion: Stepped < (100) < (110) < (111).

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Lateral Interactions Direct Coulombic: (alkali metals) adsorbates and either adsorbent or oppositely charged adsorbates transfer charge and become co-adsorbed. Direct Covalent/Metallic: (transition metals) two adsorbates with partially filled valence orbits bond together Van der Waal: (self-assembling ML (SAM)) distortions of adsorbate’s electron density induces a temporary dipole moment in a neighboring adsorbate. These interactions are dominant in non-polar, large molecules. If substrate/adsorbate interactions are greater than adsorbate/adsorbate interactions, the overlayer is referred to as “commensurate” and the interadsorbate spacing is equal to the substrate/adsorbate spacing or a multiple. If substrate/adsorbate interactions are similar to adsorbate/adsorbate interactions, the overlayer is referred to as “incommensurate”, and the spacing may not be a multiple, and a wide range of sites will be occupied.

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