Área de funciones BNF

spans around 235K. 16 am u CH4/CH3NHj/R (110) .s 20L B I—I 300 400 500 600 700 800 900 Temperature (K)

Three desorption p eaks are o b served in the 2 8 amu spectrum . T h e first peak has a m axim u m at ~ 4 2 5 K and the third peak at ~ 8 18 K . A s sim ilar peaks are ob serv ed in the 14 am u spectrum , w e can co n clu d e that th ese peaks are due to N2 d esorption. T he sec o n d peak is b eliev ed to b e C O , fo llo w in g a com parison o f this peak w ith on e taken o f background C O d esorb in g from the P t(l 10) surface.

28 amu Nj/CHjNH^/PtCllO) •s B V B HH 300 400 SOO 600 700 800 900 Temperature (K)

F ig. 3 .6 Therm al desorption spectrum o f N2 fo llo w in g a 2 0 L exp osu re o f M A to P t(lIO ).

A sm all peak is o b served in the 3 0 am u spectrum at ~ 37 8 K .

(3

b 30 amu 30 amu/CH3NH2/Pt(110)

300 400 500 600 700 800 900

Temperature (K)

No desorption o f NH

3

Some o f the HCN spectra recorded in this work showed a prominent peak at ~ 390K which was not present in others. This peak was observed by Igoe [3] at 396K in the HCN/MA/Pt(331) spectra and at 383K in the H C N /M A /P t(lll) spectra. As this low temperature spike was not reproducible experiments were carried out to test the effect o f varying the exposure time and pressure used to achieve a certain coverage and o f varying the background pressure at which the spectra were recorded. Three additional HCN spectra were recorded (fig. 3.8) following exposure o f the crystal to 20L o f MA under differing conditions.

In the first o f these, (a), the exposure time and pressure was as before (200s at 10"^ torr) but the spectrum was taken when the background pressure had reached 5 x

10

10

For (b) the 20L coverage was achieved by exposing the crystal at 10'* torr for

2000

The crystal was exposed at 5 x 10'^ torr for 40G0s for the third exposure, (c). Again the spectrum was taken with the leak valve still open.

In all three spectra the 4 3 IK peak is reproduced with the lesser peak at ~ 540K. However, in this set o f spectra one can see the additional low temperature peak which was not observed in the HCN spectrum o f fig. 3.3. This peak reaches a maximum at ~ 388K in spectrum (a). In (b) the peak is less intense and has its maximum at ~ 395K. In (c) the additional peak is reduced to a shoulder at ~ 405K on the main peak.

HCN/CHjNHj/PtCl 10)

300 400 500 600 700 800 900

Temperature (K)

Fig. 3.8 Thermal desorption spectra o f HCN follow ing 20L exposures o f M A under differing exposure conditions.

E xposure o f the P t(llO ) surface to m ethylam ine resulted in a decrease in the intensity of the (1x2) Pt spots and an increase in background intensity as evidenced by LEED. M ethylam ine did not form any ordered overlayers on the crystal surface at any coverage and did not rem ove the (1x2) surface reconstruction. H owever, a (1x1) pattern was observed after heating the M A covered crystal to 600K. Further heating to 1200K returned the (1x2) LEED pattern.

3.1.2 Methylamine and Hydrogen/Deuterium Co-adsorption on Pt(llO)

M eth y lam in e and h ydrogen co-ad sorp tion on P t( llO ) y ield ed o n ly H C N w h o se spectrum sh o w ed tw o d istin ct peaks at ~ 4 3 0 K (P i) and ~ 5 1 0 K (P2) (fig- 3.9 ). N o C 2 N 2 , C H 4 , N a or C H 3 N H 2 w ere d etected. T h e exp erim en ts w ere carried out by first adsorbing the m eth y lam in e then back fillin g the cham ber w ith 5 x 10'^ torr o f H2 and taking the spectrum w ith the H2 tap open. U nder th ese co n d itio n s any H2 d esorb ing w o u ld not be d etected b eca u se o f the high background H2 pressure.

H C N /C H jN H j + H j/R (1 1 0 ) 27 am u

e

F ig. 3.9 Therm al desorption spectrum o f H C N fo llo w in g a 2 L exp osu re o f M A and H2to P t(llO ).

C o-ad sorp tion o f m eth ylam in e and deuterium resulted in three thermal d eco m p o sitio n products - H2, H C N and a peak at 28 amu representing D C N .