SURFACE CHEMISTRY-1 BY- A.P. SINGH PGT-CHEM, KVS

1. SURFACE CHEMISTRY-1BY- A.P. SINGHPGT-CHEM, KVS

2. Adsorption The phenomenon of higher concentration of any molecular species at the surface than in the bulk Adsorbent The substance on the surface of which adsorption takes place is called adsorbent Adsorbate The substance which is being adsorbed on the surface of another substance. Desorption The process of removal of an adsorbed substance from the surface on which it is absorbed

3. Adsorbent Materials • Activated Carbon • Activated Alumina • Silica Gel • Molecular Sieves (Zeolites) • Polar and Non-polar adsorbents

4. Activated carbon • Made from nutshells, wood, and petroleum, bituminous coal by heating in the absence of oxygen to dehydrate and carbonize (remove volatile components), • "Activation" is the process that produces the porous structure essential for effective adsorption by oxidation of carbon with water vapor or CO2. • Activated carbon attracts non-polar molecules such as hydrocarbons. • Typical surface areas are 300 to 1500 m2/g.

5. Adsorption vs absorption

6. Types of Adsorption Positive adsorption occurs when the concentration of adsorbate is higher on the surface of adsorbent than in the bulk. Negative adsorption occurs when the concentration of adsorbate is less on the surface of adsorbent than in the bulk.

7. Types of adsorption • Physical adsorption • Chemical adsorption

8. Factors affecting adsorption Effect of adsorbate: The easily liquifiable gases like NH3, HCl, CO2 etc. are adsorbed to a greater extent than the permanent gases such as H2 ,O2, N2, etc. Effect of specific area of the absorbent: The greater the specific area of the solid, the greater would be its adsorbing capacity. Effect of temperature:adsorption decreases with increase in temperature. Effect of pressure: An increase in pressure causes an increase in the magnitude of adsorption of an adsorbent.

9. Freundlich Isotherm A graph between the amount (x/m) adsorbed by an adsorbent and the equilibrium pressure of the adsorbate at constant temperature is called adsorption isotherm At low pressure the graph is nearly straight line At high pressure x/m becomesindependent of p Over a narrow range of p k p1/n

10. Freundlich Isotherm logk + log p For adsorption from a solution having conc. Of solute C mol/L , the above eqns will be modified to k C1/n logk + log C

11. Application of Adsorption • In clarification of sugar • In gas masks • In catalysis • In adsorption indicators • In chromatographic analysis • In softening of hard water • In preserving vacuum • In paint industry • In removing moisture from air in the storage of delicate instruments

12. Catalysis Catalysts - Substances, which alter the rate of a chemical reaction and themselves remain chemically and quantitatively unchanged after the reaction. Promoters - Substances that enhance the activity of a catalyst. For example, in Haber‘s process for manufacture of ammonia, molybdenum acts as a promoter for iron which is used as a catalyst. Poisons– Substances that decrease the activity of a catalyst.

13. Homogeneous catalysis – When the reactants and the catalyst are in the same phase (i.e.,liquid or gas. Eg:- Oxidation of sulphur dioxide into sulphur trioxide with dioxygen in the presence of oxides of nitrogen as the catalyst in the lead chamber process. 2SO2(g) + O2(g)  NO(g) 2SO3(g) Heterogeneous catalysis- The catalytic process in which the reactants and the catalyst are in different phases. is known as heterogeneous catalysis. Eg:- Oxidation of sulphur dioxide into sulphur trioxide in the presence of Pt. 2SO2(g)→ 2SO3(g)

14. Mechanism of heterogeneous catalysis – It involves five steps: (i) Diffusion of reactants to the surface of the catalyst. (ii) Adsorption of reactant molecules on the surface of the catalyst. (iii) Occurrence of chemical reaction on the catalyst‘s surface through formation of an intermediate (iv) Desorption of reaction products from the catalyst surface. (v) Diffusion of reaction products away from the catalyst‘s surface.

16. Important features of solid catalysts Activity – The activity of a catalyst depends upon the strength of chemisorptions to a large extent. The reactants must get adsorbed reasonably strongly on to the catalyst (but not so strongly) to become active. Eg:- 2H2 (g) + O2 (g) Pt→ 2 H2O(l)

17. (b) Selectivity The selectivity of a catalyst is its ability to direct a reaction to yield a particular product. For example, starting with H2 and CO, and using different catalysts, we get different products.

18. Shape-Selective Catalysis by Zeolites The catalytic reaction that depends upon the pore structure of the catalyst and the size of the reactant and product molecules is called shape-selective catalysis. Zeolitesare good shape-selective catalysts because of their honeycomb-like structures. They are microporousAluminosilicates . Zeolites are being very widely used as catalysts in petrochemical industries for cracking of hydrocarbons and isomerisation. An important zeolite catalyst used in the petroleum industry is ZSM-5. It converts alcohols directly into gasoline (petrol) by dehydrating them to give a mixture of hydrocarbons.

19. Enzyme Catalysis Enzymes are complex nitrogenous organic compounds which are produced by living plants and animals. They catalyse the reactions that occur in the bodies of animals and plants to maintain the life process. The enzymes are, thus, termed as biochemical catalysts and the phenomenon is known as biochemical catalysis. E.g. a) Conversion of glucose into ethyl alcohol: The zymase enzyme converts glucose into ethyl alcohol and carbon dioxide. b)Decomposition of urea into ammonia and carbon dioxide: The enzyme ureasecatalyses this decomposition.

20. c) In stomach, the pepsin enzyme converts proteins into peptides while in intestine, the pancreatic trypsin converts proteins into amino acids by hydrolysis. d) Conversion of milk into curd: It is an enzymatic reaction brought about by lacto bacilli enzyme present in curd. Some Enzymatic Reactions

21. Characteristics of enzyme catalysis (i) Most highly efficient: One molecule of an enzyme may transform one million molecules of the reactant per minute. (ii) Highly specific nature: one catalyst cannot catalyse more than one reaction. (iii) The rate of an enzyme reaction becomes maximum at a definite temperature, called the optimum temperature. Its range for enzymatic activity is 298-310K. (iv) Highly active under optimum pH: it is between pH values 5-7. (v) Increasing activity in presence of activators and co-enzymes: Activators are generally metal ions such as Na+, Mn2+, Co2+, Cu2+, etc.

22. (vi) Enzymes are also inhibited or poisoned by the presence of certain substances. The inhibitors or poisons interact with the active functional groups on the enzyme surface and often reduce or completely destroy the catalytic activity of the enzymes. The use of many drugs is related to their action as enzyme inhibitors in the body. Mechanism of enzyme catalysis There are a number of cavities present on the surface of colloidal particles of enzymes. These cavities are of characteristic shape and possess active groups such as -NH2, -COOH, -SH, -OH, etc. These are actually the active centres on the surface of enzyme particles. The molecules of the reactant (substrate), which have complementary shape, fit into these cavities just like a key fits into a lock & an activated complex is formed which then decomposes to yield the products.

23. Thus, the enzyme-catalysed reactions may be considered to proceed in two steps. Step 1: Binding of enzyme to substrate to form an activated complex. E + S → ES* Step 2: Decomposition of the activated complex to form product. ES* → E + P

24. Some Industrial Catalytic Processes

25. Automotive Emission Control Automobile catalytic converters need to catalyze several reactions – CHx + O2CO2 + H2O – CO + O2 CO2 – NOx +CHx N2 + H2O + CO2 – This is achieved by the use of a supported precious metal catalyst like platinum, palladium etc. – Catalyst needs O2 to operate, CeO2 acts as a temporary regenerable source of O2. CeO2 Ce2O3 + 1/2O2

26. Thank you