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Reactions on surfaces are reactions in which at least one of the steps of the
reaction mechanism In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage o ...
is the
adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
of one or more reactants. The mechanisms for these reactions, and the
rate equation In chemistry, the rate law or rate equation for a reaction is an equation that links the initial or forward reaction rate with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial reac ...
s are of extreme importance for
heterogeneous catalysis In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. ...
. Via
scanning tunneling microscopy A scanning tunneling microscope (STM) is a type of microscope used for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer, then at IBM Zürich, the Nobel Prize in Physics in 1986. ...
, it is possible to observe reactions at the solid gas interface in real space, if the time scale of the reaction is in the correct range. Reactions at the solid–gas interface are in some cases related to catalysis.


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 and the respective
rate constant In chemical kinetics a reaction rate constant or reaction rate coefficient, ''k'', quantifies the rate and direction of a chemical reaction. For a reaction between reactants A and B to form product C the reaction rate is often found to have the ...
s for the adsorption, desorption and reaction are ''k''1, ''k''−1 and ''k''2, then the global reaction rate is: :r=k_2 C_\mathrm=k_2 \theta C_\mathrm where: * ''r'' is the rate, mol·''m''−2·s−1 *C_Ais the concentration of
adsorbate Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a f ...
, ''mol·m−3'' *C_\mathrm is the surface concentration of occupied sites, ''mol·m''−2 *C_\mathrm is the concentration of all sites (occupied or not), ''mol·m''−2 *\theta is the ''surface coverage'', (i.e. C_/C_) defined as the fraction of sites which are occupied, which is dimensionless *t is time, ''s'' *k_2 is the rate constant for the surface reaction, ''s''−1. *k_1is the rate constant for surface adsorption, ''m3·mol−1·s''−1 *k_is the rate constant for surface desorption, ''s''−1 C_\mathrm is highly related to the total surface area of the adsorbent: the greater 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 a hundred ''m''2/gram) If we apply the
steady state In systems theory, a system or a process is in a steady state if the variables (called state variables) which define the behavior of the system or the process are unchanging in time. In continuous time, this means that for those properties ''p' ...
approximation to AS, then: : \frac = 0 = k_1 C_\mathrm C_\mathrm (1-\theta)- k_2 \theta C_\mathrm -k_\theta C_\mathrm so \theta =\frac and :r=\frac . The result is equivalent to the
Michaelis–Menten kinetics In biochemistry, Michaelis–Menten kinetics is one of the best-known models of enzyme kinetics. It is named after German biochemist Leonor Michaelis and Canadian physician Maud Menten. The model takes the form of an equation describing the rat ...
of reactions catalyzed at a site on an
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
. The rate equation is complex, and the reaction
order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
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 In chemical kinetics, the overall rate of a reaction is often approximately determined by the slowest step, known as the rate-determining step (RDS or RD-step or r/d step) or rate-limiting step. For a given reaction mechanism, the prediction of the ...
can be: *Limiting step: adsorption/desorption :k_2 \gg \ k_1C_\mathrm, k_,\textr \approx k_1 C_\mathrm C_\mathrm. The order respect to A is 1. Examples of this mechanism are N2O on gold and HI on
platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...
*Limiting step: reaction of adsorbed species :k_2 \ll \ k_1C_\mathrm, k_\text\theta =\frac =\frac The last expression is the
Langmuir isotherm The Langmuir adsorption model explains adsorption by assuming an adsorbate behaves as an ideal gas at isothermal conditions. According to the model, adsorption and desorption are reversible processes. This model even explains the effect of pressu ...
for the surface coverage. The adsorption equilibrium constant K_1=\frac, and the numerator and denominator have each been divided by k_. The overall reaction rate becomes r= \frac . Depending on the concentration of the reactant the rate changes: :* Low concentrations, then r= K_1 k_2 C_\mathrm C_\mathrm, that is to say a first order reaction in component A. :* High concentration, then r= k_2 C_\mathrm. It is a zeroth order reaction in component A.


Bimolecular reaction


Langmuir–Hinshelwood mechanism

In this mechanism, suggested by
Irving Langmuir Irving Langmuir (; January 31, 1881 – August 16, 1957) was an American chemist, physicist, and engineer. He was awarded the Nobel Prize in Chemistry in 1932 for his work in surface chemistry. Langmuir's most famous publication is the 1919 ar ...
in 1921 and further developed by
Cyril Hinshelwood Sir Cyril Norman Hinshelwood (19 June 1897 – 9 October 1967) was a British physical chemist and expert in chemical kinetics. His work in reaction mechanisms earned the 1956 Nobel Prize in chemistry. Education Born in London, his parents we ...
in 1926, two molecules adsorb on neighboring sites and the adsorbed molecules undergo a bimolecular reaction:
Keith J. Laidler Keith James Laidler (January 3, 1916 – August 26, 2003), born in England, was notable as a pioneer in chemical kinetics and authority on the physical chemistry of enzymes. Education Laidler received his early education at Liverpool College. H ...
and John H. Meiser ''Physical Chemistry'' (Benjamin/Cummings 1982) p.780
: A + S ⇌ AS : B + S ⇌ BS : AS + BS → Products The rate constants are now k_1,k_,k_2,k_ and k for adsorption/desorption of A, adsorption/desorption of B, and reaction. The rate law is: r=k \theta_\mathrm \theta_\mathrm C_\mathrm^2 Proceeding as before we get \theta_\mathrm=\frac, where \theta_E is the fraction of empty sites, so \theta_\mathrm+\theta_\mathrm+\theta_E=1. 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 \theta_\mathrm=K_1C_\mathrm\theta_E, with K_i=k_i/k_, which is nothing but Langmuir isotherm for two adsorbed gases, with adsorption constants K_1 and K_2. Calculating \theta_E from \theta_\mathrm and \theta_\mathrm we finally get ::r=k C_\mathrm^2 \frac. The rate law is complex and there is no clear order with respect to either reactant, 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 1 \gg K_1C_\mathrm, K_2C_\mathrm, so r=k C_\mathrm^2 K_1K_2C_\mathrmC_\mathrm. The order is one with respect to each reactant, and the overall order is two. *One molecule has very low adsorption In this case K_1C_\mathrm, 1 \gg K_2C_\mathrm, so r=k C_\mathrm^2 \frac. The reaction order is 1 with respect to B. There are two extreme possibilities for the order with respect to A: :# At low concentrations of A, r=k C_\mathrm^2 K_1K_2C_\mathrmC_\mathrm, and the order is one with respect to A. :# At high concentrations, r=k C_\mathrm^2 \frac. The order is minus one with 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. K_1C_\mathrm \gg 1, K_2C_\mathrm, so r=k C_\mathrm^2 \frac. The reaction order is 1 with respect to B and −1 with respect to A. Reactant A inhibits the reaction at all concentrations. The following reactions follow a Langmuir–Hinshelwood mechanism: * 2 CO + O2 → 2 CO2 on a
platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...
catalyst. * CO + 2H2CH3OH on a ZnO catalyst. * C2H4 + H2C2H6 on a
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
catalyst. * N2O + H2N2 + H2O on a platinum catalyst. * C2H4 + ½ O2CH3CHO on a
palladium Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself ...
catalyst. * CO + OH → CO2 + H+ + e on a platinum catalyst.


Eley–Rideal mechanism

In this mechanism, proposed in 1938 by D. D. Eley and E. K. Rideal, only one of the molecules adsorbs and the other one reacts with it directly from the gas phase, without adsorbing (" nonthermal surface reaction"): : A(g) + S(s) ⇌ AS(s) : AS(s) + B(g) → Products Constants are k_1, k_ and k and rate equation is r = k C_\mathrm \theta_\mathrm C_\mathrm. Applying steady state approximation to AS and proceeding as before (considering the reaction the limiting step once more) we get r=kC_\mathrm C_\mathrm\frac. The order is one with respect to B. There are two possibilities, depending on the concentration of reactant A: :* At low concentrations of A, r=kC_\mathrm K_1C_\mathrmC_\mathrm, and the order is one with respect to A. :* At high concentrations of A, r=kC_\mathrm C_\mathrm, and the order is zero with respect to A. The following reactions follow an Eley–Rideal mechanism: * C2H4 + ½ O2 (adsorbed) → (CH2CH2)O The dissociative adsorption of oxygen is also possible, which leads to secondary products
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
and
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
. * CO2 + H2 (ads.) → H2O + CO * 2 NH3 + 1½ O2 (ads.) → N2 + 3H2O on a platinum catalyst * C2H2 + H2 (ads.) → C2H4 on
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...
or
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
catalysts


See also

*
Diffusion-controlled reaction Diffusion-controlled (or diffusion-limited) reactions are reactions in which the reaction rate is equal to the rate of transport of the reactants through the reaction medium (usually a solution). The process of chemical reaction can be considered ...


References

{{Reflist
Graphic models of Eley Rideal and Langmuir Hinshelwood mechanisms
Surface science Chemical kinetics Chemical reaction engineering