O did not adsorb on Pt(331) at room temperature.

Weak adsorption on P t ( l l l ) at 78K was observed by Avery [90], Thermal desorption spectra revealed N2O peaks at 9 IK at 0 = 0.02L increasing to 97K at 0 = 0.2L (saturation o f the monolayer). No residual adsorbed oxygen was detected in the EELS spectra after desorption indicating that decomposition to N2 and 0(a) does not occur. A shift in the v(N N ) mode by 80cm’’ strongly suggests that bonding occurs through the nitrogen end o f the molecule. N2O forms a donor bond with either its

2n

3n

Umbach and Menzel [91] concluded from UPS data that N2O adsorbed on clean Ru(OOl) bonds through the 7 a orbital which moves to higher binding energies and overlaps the

In

2n

5.2.4 Nitric Oxide and Carbon Monoxide Co-adsorption on Platinum

N itric o x id e did not react w ith carbon m o n o x id e on P t(3 3 1 ) accord in g to the eq u ation ;

NO(a) + CO(a) N2(g) + C02(g)

+

+

(1)

as w as ob serv ed on P t ( l l l ) and P t( llO ) by L am bert and C om rie [28]. T his is not surprising, h o w ev er, in light o f the fact that N O adsorption and desorption is m olecu la r on P t(3 3 1 ) w hereas sig n ifica n t d eco m p o sitio n o f (3-NO occurred on the (1 1 1 ) and (1 1 0 ) su rfaces p rovidin g N(a) and 0(a) for reaction. T h ese authors also o b served that large C O ex p o su res w ere m ore e ffe c tiv e at su p p ressin g reaction (1) than large N O ex p o su res d esp ite com parable stick in g p rob ab ilities and their results s u g g ested this w as by a m ech an ism other than sim p ly d en y in g adsorption sites to N O . Further in v e stig a tio n s in w h ich the surface w as d o sed w ith (3-NO o n ly fo llo w e d by C O rev ealed tw o concurrent p ro cesses w ere taking p lace - (i) C O cau sed the transform ation (3-NO to a -N O and (ii) it also d isp la ced (3-NO from the surface with co n sid era b le e ffic ie n c y . CO had n o e ffe c t on a -N O . T h e transform ation reaction w a s ob serv ed in this w ork w here step -site N O (Tp = 4 4 3 K ) w a s con v erted to terrace site N O (Tp = 3 70 K ). T h e d isp lacem en t o f N O by C O m ay be a co n seq u en ce o f their electro n ic structures. W ork fu n ction m easurem en ts reveal that N O tends to donate electron s to the m etal thereby in creasin g the ch arge d en sity in the surrounding sites. T h e se sites w ou ld b e en erg etica lly favourable to adsorb in g C O m o le c u le s w h ich tend to abstract electron s from the m etal. Lam bert and C om rie su g g est (3-NO to a -N O m ay in v o lv e som eth in g lik e a bridge bond ed N O b eco m in g a linear N O due to the adsorption o f C O on a n eigh b ou rin g site but th ey had no direct e v id e n c e to support this.

T h e CO results o f fig . 5 .1 3 are prelim inary in nature. W h ile th ey sh o w a d efin ite d ecrease in the ratio o f the terrace to step peak in ten sities it has not been esta b lish ed w hether this is due to a d ecrease in the terrace site peak inten sity, an in crease in the step site peak in ten sity or a com b in atio n o f the tw o. C O2 is a very

5.3

CONCLUSIONS

B oth C O and N O adsorb and desorb m o lecu la rly on P t(3 3 1 ). Initial adsorption is on to the step sites w here the m o le c u le s are m ore strongly bound. Therm al desorp tion occurs as tw o p h ases - a lo w tem perature terrace peak and a higher tem perature step peak. T he tem perature o f the peak m axim a d ecrease as co vera g e in creases ind icatin g a d ecrease in the activation en erg y o f desorption.

A n a ly ses o f the area under the peaks reveal that at saturation 57% o f CO is adsorbed on the terraces w h ereas 57% o f N O is adsorbed on the steps. T he CO surface con centration is 1.5 tim es greater on the steps than on the terraces. W ith N O this valu e increases to 2.6.

A R U P S exp erim en ts reveal a 0 .4 e V shift in the bin d ing en er g ie s o f the 4 a , 5 o and Iti orbitals o f C O on g o in g from a saturated su rface to a su rface com p risin g o n ly step site CO.

N2O and O2 do not adsorb on P t(3 3 1 ) at room tem perature.

C o-ad sorp tion o f step site N O w ith C O on P t(3 3 1 ) d o es not result in any sig n ific a n t am ounts o f ch em ical reaction products. H o w ev er, a transform ation reaction is o b served in w h ich the p resen ce o f the C O ca u ses a d ecrease in the N O desorp tion tem perature from 4 4 3 to 3 70 K . T h is su g g ests CO is adsorbing onto the step sites and cau sin g N O to m o v e to the terraces. T h e inten sity o f the N O peak d oes not decrease.

CHAPTER SIX