T h e lo w tem perature hydrogen peak at ~ 3 7 5 K ob serv ed on the
C sH sN s/PtC llO ) sy stem is sim ilar in tem perature to reco m b in ative H2. T he rate o f
d esorption o f the m ain high tem perature peak at ~ 4 5 0 K is thought to b e dependant on the rate o f d eco m p o sitio n o f the s-triazine m o lec u le s. T w o h ydrogen peaks (at 3 9 0 K and 5 0 5 K ) w ere a lso obtained by S om ers [1 6 ] fo llo w in g room tem perature adsorption o f s-triazine on P t(l 11).
H C N desorption occurs as tw o phases from P t ( llO ) - a sharp m ain peak at ~ 4 6 0 K fo llo w e d by a p ron oun ced high tem perature shoulder. O n ly on e H C N peak is ob serv ed from the CsH sNs/PtCl 11) system , h o w ev er, this peak is fo llo w e d by a lo n g high tem perature tail w h ich co u ld co n cea l a sec o n d feature. A s w ith H C N produced from m eth yla m in e this product co u ld b e the result o f d eco m p o sitio n lea v in g intact H - C N bonds or the result o f H(a) and CN(a) recom b ination (this p oin t w ill be d iscu ssed further in sectio n 3 .2 .4 ).
A sin g le sharp peak at ~ 5 9 0 K is ob served for C2N2 d esorp tion on P t(llO ).
T h e tem perature o f the peak m axim um d oes not vary w ith in creasin g exp osu re
su g g estin g a first-order reaction. On P t ( l l l ) C2N2 d esorb ed as tw o p eak s. Pi and P2,
at 7 0 0 and 72 0 K . A s o n ly a set o f saturation co v e ra g e spectra w ere recorded on P t(l 11) any co v e ra g e d ep en d en cies o f the ind ividu al products are not know n.
A R U P S exp erim en ts [16] carried out on the (1 1 1 ) surface su g g est that s- triazine adsorption at 3 0 0 K is at least partially d isso c ia tiv e as p hotoelectron spectra o f the adsorbed m o le c u le co u ld not be align ed w ith gas phase H e l p h otoelectron spectra. T h is author further fou n d that the sp e c ie s form ed on adsorption w a s stab le up to 4 0 5 K as there w as little d ifferen ce ob served b etw een spectra taken on adsorption and th o se taken fo llo w in g a 4 0 5 K anneal (ev en though o n e third o f the h ydrogen has desorb ed ). T h e adsorbate ind u ced features at both 3 0 0 K and 4 0 5 K are seen at 4 .8 , 8.8 , 11.2 and 1 3 .1 eV . V ery different spectra (w ea k adsorbate in d u ced features at 6 .3 ,
9.1 and 1 l.O eV ) are ob serv ed fo llo w in g heatin g to 6 2 5 K w here C2N2 (C N ) is the on ly
s-T riazin e can bond to a m etal surface by o n e o f three m o d es as su g g ested by S o m ers. It cou ld bond w ith the arom atic ring parallel to the surface, w ith the ring perpendicular to the surface (through the n itrogen lo n e pair) or w ith the ring at any a n g le other than 0 or 9 0 ° to the surface. H o w ev er, the exp erim en ts carried out in that w ork or the present w ork co u ld not id en tify w h ich m o d e w as in effect.
3.2.4
s-Triazine and Hydrogen/Deuterium Co-adsorption
T he H C N spectra ob tained fo llo w in g co-adsorption o f s-triazine and H2 on
P t ( llO ) sh o w tw o H C N peaks, Pi and P2, at ~ 4 6 0 K and ~ 5 1 2 K (correspon d in g in
tem perature to the m ain H C N peak and the high tem perature sh ou ld er in the s-triazine spectra). S im ilar H C N d esorption spectra w ere o b serv ed fo llo w in g co-ad sorp tion o f
m eth yla m in e and H2 but in the present ca se P2 is by far the dom inan t peak. D euterium
co -a d sorp tion rev eals the 4 6 0 K peak at 27 amu (H C N ) and the 5 1 2 K peak at 28 amu (D C N ) again su g g estin g that the low tem perature peak is the result o f d eco m p osition lea v in g intact H -C N bonds and the high tem perature peak is the result o f H(a) and CN(a) recom b ination .
O n ly n e g lig ib le traces o f C2N2 w ere detected. T his su g g ests that the s-triazine
ring has co m p letely broken dow n to H2T, H C N T and CN(a) b e lo w 5 00 K . H(a) -i- CN(a)
recom b ination is favored o v er CN(a) -t- CN(a) recom b ination in the p resen ce o f e x c e s s h ydrogen as w as the ca se with m eth ylam in e d eco m p o sitio n .