Editing Catalysis (section)

===Catalytic reaction mechanisms===
{{Main|catalytic cycle}}
In general, chemical reactions occur faster in the presence of a catalyst because the catalyst provides an alternative [[reaction mechanism]] (reaction pathway) having a lower [[activation energy]] than the noncatalyzed mechanism. In catalyzed mechanisms, the catalyst is regenerated.<ref name=LM82/><ref>{{cite book |last1=Laidler |first1=Keith J. |last2=Meiser |first2=John H. |title=Physical Chemistry |date=1982 |publisher=Benjamin/Cummings |isbn=0-8053-5682-7 |pages=424–425}}</ref><ref>{{cite book |last1=Atkins |first1=Peter |last2=de Paula |first2=Julio |title=Atkins' Physical Chemistry |date=2006 |publisher=W.H.Freeman |isbn=0-7167-8759-8 |page=839 |edition=8th |quote=The catalyst lowers the activation energy of the reaction by providing an alternative path that avoids the slow, rate-determining step of the uncatalyzed reaction}}</ref><ref name=Steinfeld>{{cite book |last1=Steinfeld |first1=Jeffrey I. |last2=Francisco |first2=Joseph S. |last3=Hase |first3=William L. |title=Chemical Kinetics and Dynamics |date=1999 |publisher=Prentice Hall |isbn=0-13-737123-3 |pages=147–150 |edition=2nd |quote=The catalyst concentration [C] appears in the rate expression, but not in the equilibrium ratio.}}</ref>

As a simple example occurring in the gas phase, the reaction 2 SO<sub>2</sub> + O<sub>2</sub> → 2 SO<sub>3</sub> can be catalyzed by adding [[nitric oxide]]. The reaction occurs in two steps: 
: 2{{nbsp}}NO + O<sub>2</sub> → 2{{nbsp}}NO<sub>2</sub> (rate-determining)
: NO<sub>2</sub> + SO<sub>2</sub> → NO + SO<sub>3</sub> (fast)
The NO catalyst is regenerated. The overall rate is the rate of the slow step<ref name=Steinfeld/> 
:v=2k<sub>1</sub>[NO]<sup>2</sup>[O<sub>2</sub>].

An example of [[heterogeneous catalysis]] is the reaction of [[oxygen]] and [[hydrogen]] on the surface of [[titanium dioxide]] (TiO{{sub|2}}, or ''titania'') to produce water. [[Scanning tunneling microscopy]] showed that the molecules undergo [[adsorption]] and [[dissociation (chemistry)|dissociation]]. The dissociated, surface-bound O and H atoms [[diffusion|diffuse]] together. The intermediate reaction states are: HO{{sub|2}}, H{{sub|2}}O{{sub|2}}, then H{{sub|3}}O{{sub|2}} and the reaction product ([[water dimer|water molecule dimers]]), after which the water molecule [[desorption|desorbs]] from the catalyst surface.<ref>{{cite news
 |last=Jacoby
 |first=Mitch
 |title=Making Water Step by Step
 |date=16 February 2009
 |newspaper=[[Chemical & Engineering News]]
 |page=10
 |url=https://pubsapp.acs.org/cen/news/87/i07/8707notw6.html?
}}</ref><ref>{{cite journal
 |vauthors=Matthiesen J, Wendt S, Hansen JØ, Madsen GK, Lira E, Galliker P, Vestergaard EK, Schaub R, Laegsgaard E, Hammer B, Besenbacher F
 |year=2009
 |title=Observation of All the Intermediate Steps of a Chemical Reaction on an Oxide Surface by Scanning Tunneling Microscopy
 |journal=[[ACS Nano]]
 |volume=3
 |issue=3
 |pages=517–26
 |issn=1520-605X
 |citeseerx=10.1.1.711.974
 |pmid=19309169
 |doi=10.1021/nn8008245
}}</ref>