Figure 5.1 shows RAIRS data for the sequential adsorption o f NO on Pt{211} at 307 K. A n initial absorption p eak at 1590 cm'^ is observed at an exposure o f 0.05 L. O n further exposure o f N O , this peak grows in intensity, and shifts up in frequency to 1613 c m '\ A t 1 L exposure, this peak shifts b ack dow n in frequency to 1601 cm'^ and falls in intensity, and a second absorption peak develops at 1584 c m '\ A t this exposure a third absorption feature at 1803 cm'^ is also observed. This peak increases in intensity up to a saturation exposure o f 5 L, w here the infrared spectra stop changing w ith increasing exposure o f N O. The absorption peak at 1601 cm'^ continues to fall in intensity w ith increasing N O exposure and the peak at 1580 cm'^ increases in intensity un til both features have a sim ilar intensity at saturation.
A fter the P t{ 2 1 1} sample was exposed to a saturation dose o f N O at 307 K , the sample w as annealed to 315 K for 40 seconds before being cooled back dow n and a R A IR spectrum taken (Figure 5.2). This w as repeated for increasing annealing tem peratures until no bands w ere observed in the R A IR spectrum . The resulting infrared spectra are show n in Figure 5.2. On initial heating to 315 K, the intensity o f the peaks at 1610 and 1580 cm'^ increases, and the peak at 1800 cm'^ loses intensity. B y 350 K, this species has disappeared from the spectrum . On further heating, the two peaks initially seen at 1610 and 1580 cm'^ m erge into one p eak at 1597 c m '\ This peak grow s to a m axim um intensity at 375 K, before falling in intensity and disappearing by 451 K.
F igure 5.3 show s TPD spectra record ed follow ing a satu ratio n exposure o f N O on P t{ 2 1 1} at 330 K, w ith a heating rate o f 8 K s '\ It can be seen from the spectra that there are tw o desorption features in the m ass 30 spectrum , labelled a and p, at 471 and 393 K respectively. D esorption peaks for m asses 28 and 44 are also observed. These desorption features are in coincidence w ith each other, w ith a desorption tem perature o f 482 K.
Chapter 5: The adsorption o f NO on P t{211} 0 .0 5 L 1590 0.1 L 1601 0.2 L 1 606 0.5 L 1613 A R /R 1584 1803 1601 2 L 1 609 1576 1801 0 .4 % 1801 2000 1 9 0 0 1 8 0 0 1 7 0 0 1 6 0 0 1 5 0 0 1 4 0 0 W a v e n u m b e r /c m
Figure 5.1: RA IR spectra for the sequential adsorption o f NO on the Pt(211} surface at 307 K. The NO exposures are indicated on the figure.
Chapter 5: The adsorption o f NO on P t{211} A R/R 307 K 1580 1610 1800 315 K 1607 1589 1801 330 K 1797 1607 1595 350 K 1608 1597 375 K 1598 421 K 0.5% 1597 451 K 2000 1900 1800 1700 1600 1500 1400 W aven u m b er/cm
Figure 5.2: R A IR spectra showing the sequential h eating o f the Pt{211} sam ple after a saturation dose o f NO at 307 K. The tem peratures to which the sam ple w as annealed are shown on the figure.
Chapter 5; The adsorption o f NO on P t{ 2 1 1} Mass 30 Mass 44 Mass 28 300 400 500 600 700 800 T e m p e ra tu re / K
Figure 5.3: TPD spectra follow in g a 5 L exposure of NO on Pt{211} at 330 K , at a h eating rate of 8 K s \ The spectra shown are for the desorption of m ass units 28, 30 and 44.
TPD experiments were also performed as a function o f increasing NO exposure at 330 K. In these experiments a single desorption peak at 470 K was observed following an NO exposure o f 0.05 L. On increasing the NO exposure, this peak did not shift in desorption temperature, however it increased in intensity with increasing coverage to reach a maximum intensity at an NO exposure o f 0.5 L. Following an NO exposure o f 0.2 L, a second desorption feature at -3 9 0 K is also seen in the spectrum. D esorption sequences as a function o f increasing NO exposure were also recorded for masses 28 and 44. In these spectra, desorption peaks for both masses were observed even at very low NO exposures. W ith increasing NO exposure the peaks due to mass 28 and 44 grew in intensity, until a saturation dose o f 5 L when the spectrum seen in Figure 5.3 was observed.
Further TPD experiments were also recorded at a heating rate o f 1 K s \ Figure 5.4 shows TPD spectra for masses 28, 30 and 44 for a saturation dose o f NO (20 L) at 300 K. These spectra show similar features to those seen in Figure 5.3, w here the heating rate was 8 K However, in the spectra shown in Figure 5.4 it is possible to
Chapter 5: The adsorption o f NO on P t{2 11}
resolve the first peak in the mass 30 spectrum into two peaks, at 321 K and 353 K. The mass 28 and 44 peaks are still in coincidence, with a peak at 440 K.
Mass 44 Mass 28 Mass 30
300 320 340 360 380 400 420 440 460 480 500
T e m p e ra tu re / K
Figure 5.4; TPD spectra o f a Pt{211} sam ple follow in g a 20 L exposure of NO at 300 K , at a heating rate of 1 K s'*. T he spectra shown are for the desorption of m ass units 2 8 ,3 0 and 44.
In all o f the TPD experiments, mass 32 was also monitored to check for the desorption o f O2. However, no signal w as observed for this species over all exposures o f NO, and
up to a desorption tem perature o f 1000 K.