Editing Catalysis (section)

==Inhibitors, poisons, and promoters{{anchor|Catalyst modifiers}}==
An added substance that lowers the rate is called a [[reaction inhibitor]] if reversible and [[catalyst poisoning|catalyst poisons]] if irreversible.<ref name="IUPACc"/> Promoters are substances that increase the catalytic activity, even though they are not catalysts by themselves.<ref>{{cite book |last1=Dhara SS |title=A Textbook of Engineering Chemistry |last2=Umare SS |publisher=S. Chand Publishing |year=2018 |isbn=9789352830688 |location=India |pages=66 |url=https://books.google.com/books?id=fF1jDwAAQBAJ}}</ref>

Inhibitors are sometimes referred to as "negative catalysts" since they decrease the reaction rate.<ref name=Laidler78/> However the term inhibitor is preferred since they do not work by introducing a reaction path with higher activation energy; this would not lower the rate since the reaction would continue to occur by the noncatalyzed path. Instead, they act either by deactivating catalysts or by removing reaction intermediates such as free radicals.<ref name=Laidler78>Laidler, K.J. (1978) ''Physical Chemistry with Biological Applications'', Benjamin/Cummings. pp. 415–17. {{ISBN|0-8053-5680-0}}.</ref><ref name=LM82>Laidler, K.J. and Meiser, J.H. (1982) ''Physical Chemistry'', Benjamin/Cummings, p. 425. {{ISBN|0-618-12341-5}}.</ref> In [[heterogeneous]] catalysis, [[coking]] inhibits the catalyst, which becomes covered by [[polymer]]ic side products.

The inhibitor may modify selectivity in addition to rate. For instance, in the hydrogenation of [[alkyne]]s to [[alkene]]s, a [[palladium]] (Pd) catalyst partly "poisoned" with [[lead(II) acetate]] (Pb(CH{{sub|3}}CO{{sub|2}}){{sub|2}}) can be used ([[Lindlar catalyst]]).<ref name=Lindlar>{{OrgSynth |author1=Lindlar H. |author2=Dubuis R. |title=Palladium Catalyst for Partial Reduction of Acetylenes |collvol=5 |collvolpages=880 |year=2016 |doi=10.15227/orgsyn.046.0089}}</ref> Without the deactivation of the catalyst, the alkene produced would be further hydrogenated to [[alkane]].<ref>Jencks, W.P. (1969) ''Catalysis in Chemistry and Enzymology'' McGraw-Hill, New York. {{ISBN|0-07-032305-4}}</ref><ref>Bender, Myron L; Komiyama, Makoto and Bergeron, Raymond J (1984) ''The Bioorganic Chemistry of Enzymatic Catalysis'' Wiley-Interscience, Hoboken, U.S. {{ISBN|0-471-05991-9}}</ref>

The inhibitor can produce this effect by, e.g., selectively poisoning only certain types of active sites. Another mechanism is the modification of surface geometry. For instance, in hydrogenation operations, large planes of metal surface function as sites of [[hydrogenolysis]] catalysis while sites catalyzing [[hydrogenation]] of unsaturates are smaller. Thus, a poison that covers the surface randomly will tend to lower the number of uncontaminated large planes but leave proportionally smaller sites free, thus changing the hydrogenation vs. hydrogenolysis selectivity. Many other mechanisms are also possible.

Promoters can cover up the surface to prevent the production of a mat of coke, or even actively remove such material (e.g., rhenium on platinum in [[catalytic reforming|platforming]]). They can aid the dispersion of the catalytic material or bind to reagents.