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Reactions on surfaces
 
 
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By reactions on surfaces it is understood reactions in which at least one of the steps of the reaction mechanism
Reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs .Although only the net chemical change is directly observation for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in a reaction mechanism....
 is the adsorption
Adsorption

Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid , forming a film of molecules or atoms ....
 of one or more reactants. The mechanisms for these reactions, and the rate equation
Rate equation

The rate law or rate equation for a chemical reaction is an equation which links the reaction rate with concentrations or pressures of reactants and constant parameters ....
s are of extreme importance for heterogeneous catalysis
Heterogeneous catalysis

Heterogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in a different phase to the reactants. Heterogeneous catalysts provide a surface for the chemical reaction to take place on....
.

reaction occurs through these steps:

A + S AS ? Products

Where A is the reactant and S is an adsorption site on the surface.






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By reactions on surfaces it is understood reactions in which at least one of the steps of the reaction mechanism
Reaction mechanism

In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs .Although only the net chemical change is directly observation for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in a reaction mechanism....
 is the adsorption
Adsorption

Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid , forming a film of molecules or atoms ....
 of one or more reactants. The mechanisms for these reactions, and the rate equation
Rate equation

The rate law or rate equation for a chemical reaction is an equation which links the reaction rate with concentrations or pressures of reactants and constant parameters ....
s are of extreme importance for heterogeneous catalysis
Heterogeneous catalysis

Heterogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in a different phase to the reactants. Heterogeneous catalysts provide a surface for the chemical reaction to take place on....
.

Simple decomposition

If a reaction occurs through these steps:

A + S AS ? Products

Where A is the reactant and S is an adsorption site on the surface. If the rate constants for the adsorption, desorption and reaction are k1, k-1 and k2 then, the global reaction rate is:

where is the concentration of occupied sites, is the surface coverage and is the total number of sites (occupied or not).

is highly related to the total surface area of the adsorbent: the bigger the surface area, the more sites and the faster the reaction. This is the reason why heterogeneous catalysts are usually chosen to have great surface areas (in the order of hundred m2/gram)

If we apply the steady state
Steady state (chemistry)

In chemistry, a steady state is a situation in which all thermodynamic variable are constant in spite of ongoing processes that strive to change them....
 approximation to AS, then

so and . Please notice that, with , the formula was divided by .

The result is completely equivalent to the Michaelis-Menten kinetics
Michaelis-Menten kinetics

File:Michaelis-Menten.pngMichaelis?Menten kinetics approximately describes the enzyme kinetics of many enzymes. It is named after Leonor Michaelis and Maud Menten....
. The rate equation is complex, and the reaction order is not clear. In experimental work, usually two extreme cases are looked for in order to prove the mechanism. In them, the rate-determining step
Rate-determining step

The rate-determining step is a chemistry term for the slowest reaction step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which water is poured in....
 can be:

  • Limiting step: Adsorption/Desorption
, so . The order respect to A is 1. Examples of this mechanism are N2O
Nitrous oxide

Nitrous oxide, commonly known as "laughing gas", is a chemical compound with the chemical formula Nitrogen2Oxygen. At room temperature, it is a colorless Flammability gas, with a pleasant, slightly sweet odor and taste....
 on gold and HI on platinum
Platinum

Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is in Group 10 of the periodic table of elements....


  • Limiting Step: Reaction
so which is just Langmuir isotherm
Langmuir equation

The Langmuir equation or Langmuir isotherm or Langmuir adsorption equation relates the coverage or adsorption of molecules on a solid surface to gas pressure or concentration of a medium above the solid surface at a fixed temperature....
 and . Depending on the concentration of the reactant the rate changes:
  • Low concentrations, then , that is to say a first order reaction.
  • High concentration, then . It is a zeroth order reaction.

Bimolecular reaction


Langmuir
Langmuir

Langmuir is a scientific unit used in surface science.It is also the name of:* Irving Langmuir, a Nobel Prize-winning chemist and physicist, developer of the Langmuir probe; see also Irving Langmuir House....
-Hinshelwood
Hinshelwood

Hinshelwood is a surname, and may refer to:* Adam Hinshelwood, English footballer* Ben Hinshelwood, rugby player* Cyril Norman Hinshelwood, chemist...
 mechanism

This mechanism proposes that both molecules adsorb and the adsorbed molecules undergo a bimolecular reaction:

A + S AS

B + S BS

AS + BS ? Products

The rate constants are now ,,, and for adsorption/desorption of A, adsorption/desorption of B, and reaction. The rate law is:

Proceeding as before we get , where is the fraction of empty sites, so . Let us assume now that the rate limiting step is the reaction of the adsorbed molecules, which is easily understood: the probability of two adsorbed molecules colliding is low. Then , with , which is nothing but Langmuir isotherm for two adsorbed gases, with adsorption constants and . Calculating from and we finally get
.


The rate law is complex and there is no clear order respect to any of the reactants but we can consider different values of the constants, for which it is easy to measure integer orders:

  • Both molecules have low adsorption
That means that , so . The order is one respect to both the reactants

  • One molecule has very low adsorption
In this case , so . The reaction order is 1 respect to B. There are two extreme possibilities now:
  1. At low concentrations of A, , and the order is one respect to A.
  2. At high concentrations, . The order is minus one respect to A. The higher the concentration of A, the slower the reaction goes, in this case we say that A inhibits the reaction.


  • One molecule has very high adsorption
One of the reactants has very high adsorption and the other one doesn't adsorb strongly.

, so . The reaction order is 1 respect to B and -1 respect to A. Reactant A inhibits the reaction at all concentrations.

The following reactions follow a Langmuir-Hinshelwood mechanism :
  • 2 CO
    Carbon monoxide

    Carbon monoxide, with the chemical formula CO, is a colorless and odorless, tasteless, yet highly toxic gas. Its molecules consist of one carbon atom covalent bond to one oxygen atom....
     + O2
    Oxygen

    Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
     ? 2 CO2
    Carbon dioxide

    Carbon dioxide is a chemical compound composed of two oxygen atoms covalent bond to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state....
     on a platinum
    Platinum

    Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is in Group 10 of the periodic table of elements....
     catalyst.
  • CO + 2H2
    Hydrogen

    Hydrogen is the chemical element with atomic number 1. It is represented by the chemical symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustion and explosive Diatomic molecule gas with the molecular formula H2....
     ? CH3OH
    Methanol

    Methanol, also known as methyl alcohol, carbinol, wood alcohol, wood naphtha or wood spirits, is a chemical compound with chemical formula carbonhydrogen3oxygenhydrogen ....
     on a ZnO catalyst.
  • C2H4
    Ethylene

    Ethylene is the chemical compound with the formula C2H4. It is the simplest alkene. Because it contains a carbon-carbon double bond, ethylene is called an unsaturated hydrocarbon or an olefin....
     + H2 ? C2H6
    Ethane

    Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane, that is, an aliphatic hydrocarbon....
     on a copper
    Copper

    Copper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity....
     catalyst.
  • N2O
    Nitrous oxide

    Nitrous oxide, commonly known as "laughing gas", is a chemical compound with the chemical formula Nitrogen2Oxygen. At room temperature, it is a colorless Flammability gas, with a pleasant, slightly sweet odor and taste....
     + H2 ? N2
    Nitrogen

    Nitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674?. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere....
     + H2O on a platinum catalyst.
  • C2H4 + ½ O2 ? CH3CHO on a palladium
    Palladium

    Palladium is a rare and lustrous silvery-white metal that was discovered in 1803 by William Hyde Wollaston, who named it palladium after the 2 Pallas, which in turn, was named after the epithet of the Greek mythology goddess Athena, acquired by her when she slew Athena#Pallas_Athena....
     catalyst.
  • CO + OH ? CO2 + H+ + e- on a platinum catalyst.


Eley-Rideal mechanism

This mechanism proposes that only one of the molecules adsorbs and the other one reacts with it directly, without adsorbing:

A + S AS

AS + B ? Products

Constants are and and rate equation is . Applying steady state approximation to AS and proceeding as before (considering the reaction the limiting step once more) we get . The order is one respect to B. There are two possibilities, depending on the concetration of reactant A:

  • At low concentrations of A, , and the order is one with respect to A.


  • At high concentrations of A, , and the order is zero with respect to A.


The following reactions follow a Eley-Rideal mechanism :
  • C2H4 + ½ O2 (adsorbed) ? H2COCH2
    Ethylene oxide

    Ethylene oxide is the organic compound with the chemical formula C2H4O. This colorless flammable gas with a faintly sweet odor is the simplest epoxide, a three-membered ring consisting of two carbon and one oxygen atom....
     The dissociative adsorption of oxygen is also possible, which leads to secondary products carbon dioxide and water
    Water

    Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
    .
  • CO2 + H2(ads.) ? H2O + CO
  • 2NH3
    Ammonia

    Ammonia is a chemical compound with the chemical formula nitrogenhydrogen. It is normally encountered as a gas with a characteristic pungent odor....
     + 1½ O2 (ads.) ? N2 + 3H2O on a platinum catalyst
  • C2H2
    Acetylene

    Acetylene is the chemical compound with the symbol carbonhydrogen. It is the simplest alkyne.As an alkyne, acetylene is Saturation because its two carbon atoms are Chemical bond together in a triple bond....
     + H2 (ads.) ? C2H4 on nickel
    Nickel

    Nickel is a chemical element, with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge....
     or iron
    Iron

    Iron is a chemical element with the symbol Fe and atomic number 26. Iron is a Group 8 element and period 4 element. Iron is lustrous and silvery in color....
     catalysts