section_path: General principles/Catalytic reaction mechanisms
section_name: Catalytic reaction mechanisms
paragraph_text: Catalytic reaction mechanisms
  
  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 non-catalyzed mechanism. In catalyzed mechanisms, the catalyst is regenerated.
  
  As a simple example occurring in the gas phase, the reaction 2 SO + O → 2 SO can be catalyzed by adding nitric oxide. The reaction occurs in two steps:
  
  The NO catalyst is regenerated. The overall rate is the rate of the slow step
  
  An example of heterogeneous catalysis is the reaction of oxygen and hydrogen on the surface of titanium dioxide (TiO, or titania) to produce water. Scanning tunneling microscopy showed that the molecules undergo adsorption and dissociation. The dissociated, surface-bound O and H atoms diffuse together. The intermediate reaction states are: HO, H O, then H O and the reaction product (water molecule dimers), after which the water molecule desorbs from the catalyst surface.