§8.5 adsorption at gas / solid interface Levine: pp. 397– 402 section 13.5 adsorption of gases on solids

8.3.1 Specific area Si Suspending bond Surface layer Matter that is at or near an interface is not in the same state as matter in the interior of a phase. Owing to the suspending bond, material is apt to react with other substances. 1) Surface atoms

arrisSurface areaSurface atom % 1 cm6 cm 2 3   m6  10 4 cm 2 30% 1 nm 6  10 7 cm 2 ~ 100 % 2) Degree of subdivision 3) Specific area: The surface-to-volume / mass ratio Metal catalyst: Pt, Pd, Ni, Fe Oxide: Al 2 O 3, SiO 2 Activated carbon: charcoal. Catalysts or catalyst support

Owing to the strong interatomic action, the surface tension of solid is usually much larger than that of liquids LiquidHg(l)NaCl(s)MgO(s)Mica < 80 mNm Powered solids have very high surface energy!

Micropowder explosion: micropowder is thermo- dynamically unstable. 400 mJ  m -2  6  10 3 m 2 = 2400 J 4) Surface energy and micropowder explosion

河北抚宁淀粉厂燃爆事故 ，河北省秦皇岛骊骅淀粉 股份有限公司淀粉四车间 ( 包装车间 ) 发生爆炸事故

Solid can not lower surface energy by shrinking its surface. How can solid lower its surface energy? gas solid Adsorbed molecules Adsorption of gas molecules 2. The way for solids to lower their surface energy

8.3.2 Adsorption Sorption: movement of a material from one phase to another. only on the surface: adsorption distributing throughout: absorption Adsorbate: the material being adsorbed. Adsorbent: the material doing the adsorbing.

Adsorption on a solid surface: a : under HCOOH vapor; b: under evacuation; c: heated under evacuation Infrared spectrum of HCOOH adsorbed on Al 2 O 3 suggested that some part of HCOOH dissociate into HCOO  + H + on the absorbent surface. chemical adsorption, physical adsorption chemisorption, physisorption

Chemisorption involves the formation of chemical bonds between adsorbed molecule and solid surface, and often the breaking of preexisting bonds in the adsorbed molecule. In some cases the chemisorption step requires an activation energy. physisorption involves forces similar to the van der Waals’ forces that lead to condensation of vapors to liquid. b.p.

Transition from physisorption to chemisorption The potential curve of adsorption By following the transition from physical absorption to chemical adsorption, catalysts can activate reactants molecules at relatively mild conditions. This is the basic principal for heterogeneous catalysis. Physical adsorption Transition state Chemical adsorption 2Ni + H 2 2Ni + 2H  Ni  Ni  H  Ni  Ni  HHHH

sorptionphysisorptionchemisorption interaction Thermal effect temperature Adsorbed layer reversibility selectivity Comparison between chemical and physical adsorption Van der Vaals’ force Chemical bond Condensation heat < 40 kJ mol -1; -4~-40 Chemical reaction heat > 80 kJ mol ~ -800 Around b.p. Relatively higher multilayermonolayer Reversible. equilibrium established rapidly. Less reversible poor good

Conditions for chemisorption 2W + N 2 = 2 W-N ad  ads H m = D(N 2 ) – 2X (W-N) < 0 Thermodynamic discussion of adsorption Adsorption is spontaneous, Entropy decreases during adsorption adsorption is a exothermic process Therefore:

8.3.3 description of adsorption The adsorption of gas depends on the nature of the solid and the gas being adsorbed, the state of subdivision of the solid, the temperature and the pressure of the gases. Therefore, the amount of adsorption (a) of a certain kind of solid for certain gases is a function of T and p. constant pressure : isobar constant temperature : isotherm constant adsorption : isochore

isobar As temperature increases, adsorption decrease. Adsorption heat can be determined from isochore on the base of Clausius- Clappeyron equation equation T/K a/ml  g p isochore T/K 20 p/kPa a

Adsorption increases with increase of pressure under low pressure. While at high pressure, adsorption attains maximum value. isotherm an expression that gives the fraction  of a surface that is covered by adsorbed molecules in equilibrium at constant temperature as a function of pressure or concentration. adsorption isotherm: T/K a/ml  g -1 T

1) Types of adsorption isotherm: Brunauer found that the isotherms of substances can be subdivided into five types. 78 K, N 2 / active carbon 78 K N 2 / silica gel or iron catalyst I VmVm a p pcpc II a p pcpc

352 K Br 2 / silica gel 323 K benzene / Fe 2 O K water / active carbon III a p pcpc IV a p pcpc V a p

8.3.4 Langmuir adsorption isotherm 1) The basic assumptions (1) The solid surface is uniform (2) monolayer adsorption (3) no intermolecular interaction The uniform surface contains a fixed number of adsorption sites. Each site holds only one adsorbed molecule. The heat of adsorption is the same for all sites and does not depends on the fraction coverage  Noble Prize USA, 1881/01/31 – 1957/08/16 for his discoveries and investigations in surface chemistry. Irving Langmuir

S: the total adsorption sites S 1 : sites occupied by molecules 2) Degree of coverage

b (adsorption coefficient) : the kinetic equilibrium constant of the adsorption and desorption 3)Adsorption and desorption equilibrium

at low pressure, bp << 1, at high pressure, bp >> 1, p 

For monolayer adsorption, the specific area can be thus estimated according to For Langmuir adsorption, when plotting p/V against p, straight line can be obtained.

When dissociation occur during adsorption: Adsorption of mixed gases For gas A and B:  A,  B, P A, P B Dissociatvie / nondissociatvie (molecular)

Adsorption of hydrogen on different metal surface. 4) Problems with Langmuir isotherm Adsorption heat depends on the coverage, which suggests that the surface of adsorbent is not uniform. Some sites are more active than the other. Adsorption of nitrogen on graphitized carbon black. Condensation heat

8.3.5 BET adsorption isotherm 1938, Brunauer-Emmett-Teller 1) Basic assumptions (1) uniform surface; (2) multilayer adsorption; (3) the heat of the layer other than the first layer is the condensation heat; (4) desorption only occur at the layer exposed to the gas Multilayer adsorption = condensation ? Brunauer, S., Emmett, P. H. & Teller, E. (1938) Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60, pp

p/p0p/p0 V/VmV/Vm 1 2 c = 1 c = 10 c = 100 c = 1000 I II III BET absorption isotherm is valid for type I through type III.

Measurement of specific area of solid with BET

8.3.6 Other isotherms Both Langmuir and BET isotherms are based on the assumption that the surface of the solid is uniform and the adsorption heat is independent of coverage. 1) Freundilich isotherm Freundilich adsorption isotherm can be derived on the assumption that:

2) Temkin adsorption isotherm Temkin adsorption isotherm can be derived on the assumption that: Valid for adsorption in solution

Conversion of CO to CO 2 on nanoporous copper catalysts

Discussion: 1. Give explanation to the difference of the figures presented in Levine, p Figure Discuss about the behavior of the adsorption of CO on Pd as shown in Figure