electrocatalysis on surfaces modified... - Brookhaven National ...

pathway that involves H 2 O 2 as the intermediate [145]. For acid solutions, the reaction in a
dir ect f our-electr on pathway is:
O 2 + 4H + + 4e - → 2H 2 O E 0 = 1.229 V vs . N HE ( 7 )
Tw o electrons are exchanged in a “per oxide” pathw ay:
O 2 + 2H + + 2e - → H 2 O 2 E 0 = 0.67 V (8)
Peroxide can undergo f ur ther reduction,
H 2 O 2 + 2H + + 2e - → 2H 2 O E 0 = 1.77 V (9)
or it can catalytically decompos e into O 2 and H 2 O. P eroxide is the final reduction pr oduct on
some less active surfaces. On some surf aces, the two pathways can occur in parallel. The
same pathways are oper ative in alkaline solutions but at different set of potentials involving
intermediates and products deter mined by the solution pH [145].
16.3.5.1 Alkalin e solut ion s
Foreign metal adatoms caus e r emarkable catalytic ef fects on oxygen reduction on
several electr ode s urf aces. The adatoms of P b [ 146, 147], Bi [ 148], and Tl [149,150] cause
a shift of the half -wave potential of the O 2 r eduction to mor e pos itive values and an
appr oximate doubling of the diff us ion limited cur rent density. The latter is due to the change
of the reaction mechanis m f rom a two- electron r eduction on Au into a vital four -electron
reduction on A u covered with UPD metal adlayers . Catalytic eff ects were obser ved als o f or
Pt [ 151], glas sy carbon and graphite [2], and Ru [152] surfaces w ith P b and Tl ads orbates.
The latter thr ee cases are somew hat dif fer ent f rom the UP D adlayers on metal surfaces. For
carbons , it appears that the UPD occurs through an interaction of metal with carbon sur face
gr oups. For oxidized Ru surf aces, cations interact with surface oxides.
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