Surface Science of Catalysis - American Chemical Society

Chapter 21

Photoinduced Pathways to Dissociation and Desorption of Dioxygen on Ag(110) and Pt(111) 1

Downloaded by NORTH CAROLINA STATE UNIV on October 12, 2012 | http://pubs.acs.org Publication Date: January 23, 1992 | doi: 10.1021/bk-1992-0482.ch021

S. R. Hatch and Alan Campion Department of Chemistry, University of Texas, Austin, TX 78701

The rates for photodissociation and photodesorption of dioxygen on Ag(110) under UV irradiaton have been measured with TPD and HREELS. The bonding of O to Ag(110) permits polarization measurements at normal incidence. The rates are found to be independent of the azimuthal orientation of the electricfieldof the light. Dependences on angle of incidence are consistent with the angle dependence of metal absorption predicted by Fresnel's equations. These observations rule out direct intra-adsorbate excitation and suggest a substrate mediated mechanism. Similar photoexcitation thresholds suggest that both channels are activated by a common process. A model is proposed whereby substrate charge transfer induces dissociation. Dissociating adatoms, in competition with molecular oxygen for adsorption sites, force the molecular desorption. This model reproduces the dependences on initial O coverage and extent of irradiation for both channels and allows calculation of the photodissociation cross-section. In contrast, angle and wavelength dependences for O on Pt(111) suggest the participation of direct excitation in the photochemistry. 2

2

2

The study of the photochemistry of surface adsorbates has enjoyed tremendous growth in recent years (1-4) Surface mediated photochemistry is interesting, both practically and fundamentally. From a catalytic standpoint, surface photochemistry provides an effective means of synthesizing at low temperatures, molecular fragments which may be important reaction intermediates normally found at higher temperatures. The adsorption of oxygen on Ag(l 10) and Pt(111) comprise two systems which have been extensively studied, due in part to their importance as models of silver catalyzed selective ethylene epoxidation and platinum catalyzed oxidation of NH3, CO, and NO. We report here the results of mechanistic investigations of the photoinduced dissociation and desorption of dioxygen on Ag(110) and Pt(111). 1

Current address: Exxon Research and Engineering Company, Annandale, NJ 08801 0097-6156/92/0482-0316$06.75/0 © 1992 American Chemical Society

In Surface Science of Catalysis; Dwyer, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


21. HATCH & CAMPION

Dissociation and Desorption of Dioxygen

317

With regard to the mechanisms responsible for photochemistry on metallic substrates, two limiting case models have been proposed. In the first, dissociation is induced by direct absorption of a photon by a bond within the adsorbate-substrate complex(4-6). In the second, dissociation results from adsorbate capture of a photogenerated hot carrier (substrate electron or hole) (7-11). Substrate-mediated photochemistry may involve photoelectrons, excited above the vacuum level, and/or hot electrons, excited above the Fermi energy but below the vacuum level. Polarization measurements would allow determination of the photochemical mechanisms in a system with aligned transition dipoles. For direct absorption, the photochemical rate will be determined by Fermi's Golden Rule, with the rate being sensitive to the angle between the relevant transition dipole and the electric vector of the exciting light. For substrate mediated photochemistry, the rate will be relatively insensitive to the orientation of the electric field vector and should follow light absorption by the metal. We have performed such a polarization experiment on Ag(ll.O), a surface on which oxygen molecules adsorb with their internuclear axes aligned along the [110] azimuth (12-17). This orientation of the molecules permits alignment of the transition dipoles, a situation highly unlikely on surfaces of threefold or higher symmetry where the existence of domains scrambles the orientations. The results of the polarization measurements for Ag(110) provide strong evidence for substrate mediated oxygen photodissociation and desorption. In contrast, the importance of direct intra-adsorbate excitation is suggested in the oxygen photochemistry we observe on Pt(l 11). Experimental The experiments were performed in a multipurpose ultrahigh vacuum system equipped with temperature programmed desorption (TPD), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and high resolution electron energy loss spectroscopy (HREELS). The apparatus has been described in detail previously (18). Irradiation was provided by a PTI100 W high pressure Hg arc lamp mounted outside the vacuum chamber. The light was directed through a polarizing prism, followed by a variable diameter aperature, before being refocussed through a fused silica biconvex lens and transmitted to the sample through a quartz window. By measuring the depth of the null in cross polarization, we determined the degree of polarization to be 0.98. For the wavelength, coverage, and time dependence studies, the polarizing prism was replaced by a narrow band interference filter centered at the indicated wavelength with a full width at half maximum of 10 ± 2 nm. The Pt(lll) work was done before that on Ag(l 10) and used a simpler configuration. The unfiltered lamp output was directed through a circulating water quartz cell to reduce infrared heating, before being directed into the vacuum chamber. In all the experiments, photoelectrons should not contribute to the photochemistry since excitation energies are well below the relevant work function. The samples were prepared by sputter and anneal cycles until clean and well ordered as judged by AES, HREELS, and LEED. Gases were reproducibly dosed through a calibrated pinhole doser. We report molecular oxygen coverages in ML [02] and atomic coverages in ML [O], unless otherwise indicated, where one monolayer [ML] coverage is defined as the density of surface silver atoms (8.4 x 10 cm" ). Detailed error analyses have not been performed. On the basis of selected repeated measurements, we estimate our data reproduciblity to be ± 10 %. 1 4

2

Results O2/Ag(110). Figure 1(a) shows the HREELS spectrum following a 350 L dose of 02 at 100 K. Two losses are seen: v (Ag-02) at 241 cm"* and the peroxo v (O-O) at


318

SURFACE SCIENCE OF CATALYSIS -1

638 c m , in excellent agreement with literature values (72,77). Figure 1(b) is the HREELS spectrum of 0.33 ML of atomic oxygen (where the coverage was determined by the appearance of a 3 x 1 LEED pattern) adsorbed on Ag(110). The spectrum is characterized by a single loss, v (Ag-O) at 320 cm'*. Figure 1(c) shows a typical HREEL spectrum of 0.25 ML 02 at 100 K following 30 seconds of normal incidence UV irradiation with a 315 nm cutoff filter. The frequency of the 638 cm * loss remains unchanged but the intensity decreases with increasing photon fluence. Another loss is evident at approximately 275 c m . This peak shifts.to higher frequency with increasing fluence. We attribute this shift to an increase in the 320 cm"* Ag-O stretch associated with photo-induced dissociation of oxygen overlapping with the Ag-02 stretch. The peak height of the 638 cnr* loss was taken as the measure of the molecular oxygen coverage and the height of the 320 cm"* peak was used for the atomic oxygen coverage after the molecular contribution at 241 cm"* was spectrally subtracted. Previous work has shown that this loss is proportional to O coverage up to 0.25 ML (79). Atomic oxygen coverages were well below 0.25 ML. TPD measurements provided an independent check of the coverages. -


-1

Figure 2(a) shows the thermal desorption spectrum following a 350 L dose of 02 at 100 K. In agreement with previous studies of oxygen on Ag(l 10),we see two peaks: a low temperature peak at 214 K arising from molecular 02 desorption and a high temperature peak at 550 K arising from recombinative desorption of atomic oxygen. The low temperature state is reported to have a desorption temperature of 190 K while the high temperature state desorbs at 590 K (72,20,27). We attribute the discrepency between our desorption temperatures and those in the literature to poor thermocouple contact and have therefore plotted our TPD spectra versus time. Since we are using TPD only to measure coverage, this procedure is entirely satisfactory. In accord with previous work, we take the observation that both TPD and HREELS peaks saturate at 350 L to indicate a total oxygen coverage (0) of 0.25 ML O2. Figure 2(b) is a thermal desorption spectrum following 30 seconds of normal incidence UV irradiation through a 315 nm cutoff filter. The area of the low temperature peak decreases, as observed on both Pt(l 11) (18) and Pd(l 11) (6), due to photo-induced desorption and dissociation. Interestingly, the area of the high temperature peak does not increase as observed for Pt and Pd. On Pt and Pd the increase in the area of the high temperature peak is attributed to thermal dissociation during the temperature ramp, followed by recombinative desorption at higher temperatures. Apparently, thermal dissociation during TPD is suppressed on Ag compared to Pt and Pd. A possible explanation for this observation is displacementby oxygen adatoms. On Ag(110) oxygen adsorbs in the troughs oriented in the [110] direction whereas it may adsorb in a number of different orientations Pt(lll) and Pd(l 11). Since oxygen molecules and adatoms compete for the same sites on the silver surface and since dissociation and adsorption are confined to one dimension, it is easy to see how site blocking could be more effective than on Pt(l 11) or Pd(l 11) where the dissociating adatoms could take other directions in search of adsorption sites. This possibility makes determination of the relative amounts of photodissociated and thermally dissociated oxygen from TPD areas difficult. We have chosen, instead, to measure only the difference in total peak areas for both desorption states before and after photolysis to determine the number of photodesorbed molecules and desorption rates. Angle Dependences. The alignment of the oxygen molecules on Ag(110) permits polarization measurements at normal incidence by rotation of the incident plane of polarization. For a direct absorption mechanism, a transition moment aligned along the surface normal would result in no photochemistry. A transition moment lying in



21. HATCH & CAMPION


0 250 500 750 ELECTRON ENERGY LOSS (cm" )

319

1000

1

Figure 1. On Ag(110), HREELS spectra of (a) 0.25 mL [OJ at 100 K; (b) 0.33 mL [O] calibrated using LEED; (c) 0.25 mL [0 ] following 30-s irradiation through a 315-nm cutoff filter at normal incidence to the 0.25 mL [OJ saturated surface. (Reproduced from ref. 11. Copyright 1991 American Chemical Society.) 2

Z w

2 a

TIME

Figure 2. The TPD spectra (a) of 0.25 mL [OJ dosed at 100 K on Ag(110); (b) following 30-s irradiation through a 315-nm cutoff filter. (Reproduced from ref. 11. Copyright 1991 American Chemical Society.)


320

SURFACE SCIENCE OF CATALYSIS

the plane of the surface, on the other hand, would dictate that the photochemical rate should follow a cos < > | dependence where < > | is the angle between the transition moment 2

and the electric vector. If the photochemistry is substrate-mediated then no dependence is expected. Figure 3 shows the average rates (30 sec.) derived for photoinduced desorption and dissociation as a function of the azimuthal angle , for polarized light at normal incidence with A, > 315 nm. The relevant angles are shown in Figure 12. For this experiment, the Ag(l 10) crystal was oriented using LEED so that the angle between the 0-0 bond axis and the incident electric vector could be accurately measured(± 2°). The power density was 0.09 ± 0.01 W cm . Since, at normal incidence the perpendicular component of the electric field is zero and since significant photochemistry is observed for both channels we conclude that electric field strength along the surface normal is not required. Neither the average photodissociation rate, derived from the atomic HREELS peak intensity, nor the photodesorption rate, derived from HREELS and TPD, show any dependence on the azimuthal angle of the incident electric vector, ruling out a direct intra-adsorbate excitation mechanism. These results are consistent with a substrate excitation mechanism. To ensure that there was no azimuthal anisotropy in the surface fields we measured the azimuthal dependence of the Ag(llO) reflectance for s-polarized light incident at 30° off-normal and found less than a 5% deviation for all angles at wavelengths of 289,313, and 365 nm. That the reflectance is independent of azimuthal orientation of the surface indicates that the metal absorption and tangential electric field strengths are azimuthally invariant. Average rates (1 min.) of photo-induced dissociation and desorption of 02 are given in Figure 4 as a function of the angle of incidence (8) of p-polarized light. Wavelengths shorter than 315 nm were eliminated during irradiation with a cutoff filter. The photodissociation and photodesorption rates are normalized to the corresponding rate for normal incidence photolysis and are compared with the Ag metal absorbance calculatedfromFresnel's equations and the optical constants of Ag, indicated by the solid line (and the right hand ordinate). The rates in Figure 4 are in reasonable agreement with the metal absorbance curve. Thus, both the azimuthal angle dependence and the angle of incidence dependence are consistent with substrate excitation.


-2

Wavelength Dependence. Figure 5 shows the wavelength dependences of the photoyields for both reaction channels deduced from HREELS intensity measurements. Unpolarized irradiation (bandpass filters) at normal incidence was used. A constant photon fluence of 1.25 x 10 photons/cm was maintained by changing the irradiation time to compensate for variations in throughput and wavelength of the bandpass filters. Figure 5 makes the important point that the cross sections for both photodesorption and photodissociation have similar wavelength dependences, extending to approximately 440 nm before dropping below our detection limits. The photolysis yields for both channels increase by a factor of 4 for decreasing wavelengths between 400 and 300 nm. For comparison, the silver absorbance increases by a factor of 7 with excitation energy from 400 nm to 320 nm (22). Also evidentfromFigure 5 and as noted in the inset, the ratio of desorption to dissociation remains constant at approximately 2from250 to 360 nm. The similarities in the optical responses suggest that a common mechanism describes both processes. These results contrast with the wavelength dependences of oxygen photochemisry on Pt(lll) and Pd(lll). For Pt(lll), the threshold for oxygen dissociation is observed to be approximately 315 nm while that for desorption is approximately 460 nm. (Figure 11). 18

2


21. HATCH & CAMPION


321


0.05

1

1

1

1

1

1

1

0.00'— — —'— —'— —'—'—'— — — 0 60 120 180 240 300 360 0 (degrees)

Figure 3. For 0 on Ag(110), average rates (30 s irradiation of 0.25 mL [0 ]) for photoinduced desorption by HREELS (open squares) and TPD (solid squares); and dissociation, by HREELS (open circles) as a function of azimuthal angle (). 0* is parallel to the 0 bond axis. (Reproduced from ref. 11. Copyright 1991 American Chemical Society.) 2

2

2

Angle (degrees) Figure 4. For 0 -saturated Ag(110), average rates for photoinduced desorption by HREELS (open squares) and TPD (solid squares); and dissociation (open circles) as a function of angle of incidence (0) for p-polarized light. The solid curve is the calculated p-polarized metal absorption angle dependence. (Reproduced from ref. 11. Copyright 1991 American Chemical Society.) 2



322


WAVELENGTH (nm) Figure 5. The wavelength dependences of the photoinduced desorption (squares) and dissociation (circles) yields of 0.25 mL [0 ] determined from HREELS. The inset shows the ratio of photodesorption to photodissociation yields. The lines are guides to the eye. (Reproduced from ref. 11. Copyright 1991 American Chemical Society.) 2


21. HATCH & CAMPION


323


On Pd(lll), photodissociation and photodesorption long wavelength cross sections are reported at 334 and 437 nm, respectively (6). Also consistent with there being no direct excitation, we find that the threshold for photodissociation is significantly red-shifted (1.32 eV) from that for gas phase hydrogen peroxide. The hydrogen peroxide 0-0 bond dissociates upon photoexcitation below 300 nm via excitationfromthe non-bonding O electrons into a repulsive o* orbital, antibonding with respect to the 0-0 bond (23). Time and Coverage Dependence. The dependences of the photodissociation yield and adsorbed dioxygen coverage on irradiation time are shown in Figure 6. The 02-saturated surface was irradiated at normal incidence with unpolarized light through a 313 nm bandpass filter. Also shown in Figure 6 are the results, represented by the solid lines, of a numerical simulation based upon a reaction mechanism described below. Figure 7 shows how the yield depends on initial 02 coverage under the same conditions as in Figure 6. Unpolarized light was directed through a 313 nm bandpass filter to the crystal at normal incidence for 1 min. giving a constant fluence of 0.91 x 10 photons cm for each coverage. Oxygen coverages were determined from the HREELS loss intensities. The solid line shown in Figure 7 is the initial coverage dependence of the photoyields extractedfroma numerical simulation of the proposed reaction model (see below). 18

-2

02/Pt (111). Figure 8(a) shows TPD spectra for 0.45 ML of 02 adsorbed on Pt(l 11) before and after photolysis. As in previous reports, two desorption peaks are observed: a sharp peak at 150 K and a broad peak between 600 and 1000 K (24-26). Isotope mixing, hydrogentitration,and HREELS results have shown that dissociation occurs significantly only above 150 K where molecular desorption is also rapid. In previous work with oxygen on Pt(lll) we have shown that an increase in the high temperature peak area correlates with an increase in surface atomic oxygen coverage (18). This is shown in Figure 8(c) where, after full-arc irradiation of oxygen on a Pt(l 11) crystal at 97 K with unpolarized light incident at 57° off-normal and having a power density of 0.07 ± 0.01 W/cm , the area of the high temperature peak increases. Also shown, Figure 8(c), is a decrease in the low temperature peak area and a shift of this peak to lower temperature following irradiation. Both effects increase with increasing fluence. These observations have been attributed to depletion of the molecular species from photoinduced dissociation and desorption of molecular oxygen and rearrangement of the molecular species to another surface site, respectively. Figure 8(b) shows that resistively heating the crystal with the same time/temperature profile as during irradiation results in some thermal dioxygen desorption. Thermal desorption varied between 5% and 15% with irradiation conditions, although the crystal temperature increase was typically less than 2 K. This effect was not observed on Ag(llO), because the desorption temperature was higher, nor was it observed during polarized experiments because the intensity of the radiation was lower. Both the photodesorption and photodissociation rates reported here for the 02/Pt(lll) system are derived from TPD. To do so requires a knowledge of the relative contributionsfromphotoinduced and thermally induced dissociation in the high temperature peak. As was noted for Ag(110), one can envision photodissociated O adatoms sterically hindering dioxygenfromthermally dissociating during TPD. Thus, the increase in the high temperature peak area would not be a true indication of the contribution of photoinduced dissociation. To calibrate the increase in area of the high temperature peak, we thermally dissociated a known coverage of atomic oxygen on the surface. To this, we co-dosed a known quantity of molecular oxygen following which a TPD was performed. This procedure was repeated, varying the initial coverages of atomic oxygen and codosed dioxygen in the ranges we expected to see following 2



324


IRRADIATION TIME (sec)

Figure 6. For 0 -saturated Ag(llO), the dependence of the 0 (squares) and O (circles) coverages on irradiation time. Unpolarized light was passed through a 313-nm bandpass filter and impinged at normal incidence. The lines are fits based on the mechanism described in the text. (Reproduced from ref. 11. Copyright 1991 American Chemical Society.) 2

2

0.15

. ^

O J

2 Z O

0.15

•

Desorption

•

Dissociation

o

2

•

0.10 z

o.io •

i

•

u o

O

B

0.05

"•

•

Q

o

O 0.00 0.11

0.14

0.17

0.20

0.23

0.26

INITIAL 02 COVERAGE (ML[02l)

Figure 7. The dependence of the photodissociation (circles) and photodesorption (squares) yield as a function of initial coverage. The solid line is the fit based on the mechanism described in the text. (Reproduced from ref. 11. Copyright 1991 American Chemical Society.)



21. HATCH & CAMPION


325

1000

TEMPERATURE (K) Figure 8. On Pt(lll), the TPD spectra of (a) 0.45 mL [OJ adsorbed at 95 K, (b) following resistive annealing at 97 K for 9 min, (c) following 6-min irradiation with no UV filtering. (Reproduced from ref. 11. Copyright 1991 American Chemical Society.)


326


photolysis. By varying the coverages of atomic and molecular oxygen, we correlate the sensitivity of thermal dissociation to the coverage of predissociated oxygen and obtain a sensitivity factor that gives the ratio of the measured to actual coverage of pre-TPD atomic oxygen. Typical values for this factor were between 0.2 and 0.3. The TPD determined rates we report are derivedfromthe corrected coverages. HREELS, used to monitor surface species before and after photolysis, showed increases and decreases following photolysis in the 480 and 860 cm~l losses, respectively. Figure 9 contains typical spectra for: (a) 0.45 ML O2; (b) 0.25 ML O; (c) 0.45 ML O2 following 6 min of full arc irradiation (0.05 ± 0.01 W/cm ). These spectra have been been previously examined in detail and will not be further discussed except to note that the increasing intensity of the 480 cm peak corresponds to an increase in atomic oxygen coverage while the diminution of the 860 cm peak is indicative of dioxygen depletion (18). 2


-1

-1

Unpolarized Angle-of-Incidence Dependence. Figure 10 shows, for Pt (111), the dependence of the average rates of photodesorption and photodissociation on angle of incidence for unpolarized light The full-arc output, measured in a separate experiment outside the chamber, was 0.07 ± 0.01 W/cm . Shown in Figure 10 are the average of the rates following irradiation times of 2 and 5 minutes. For this measurement, irradiation time, not fluence, was constant. Unlike Ag(110), the dependences of the photoinduced desorption and dissociation rates on angle of incidence are strikingly different. Whereas photodesorption is observed to increase with angle, the photodissociation rate decreases. That the photodesorption rate does 2

not appear to go to zero at 90° is probably a consequence of a nonuniform transverse intensity profile of the lamp. These data clearly demonstrate, however, that the two processes have different physical origins and suggest that at least one of the channels involves direct excitation of the adsorbate-substrate complex. Also shown in Figure 10 are results obtained by measuring the initial 0 2 pressureriseupon irradiation. These results are obtained by measuring the background pressure, insensitive to crystal angle, of m/e = 32 in the vacuum chamber. Mass spectrometry showed that 62 was the only species desorbingfromthe surface during irradiation. We observed and subtracted significant backgrounds arising from excitation of oxygen adsorbed on a molybdenum heat sink near the crystal. The averages of 3 measurements are shown and are in reasonable agreement with the TPD and HREELS derived rates. The pressurerisedetermined rates are normalized to the value at 0° angle of incidence . Wavelength Dependence. Figure 11 shows the cut-off wavelength dependence for photodissociation and photodesorption. The rates are averaged over a 6 min. photolysis interval. The initial coverage at each wavelength was 0.45 ML and the angle of incidence was 57° off normal. At the highest energies, the desorption rate is approximately twice the dissociation rate. As the average photon energy is lowered with successively higher cut-off filters, both rates rapidly decrease. Furthermore, the ratio of desorption to dissociation does not remain constant as on Ag(l 10). The ratio shows a dramatic increase at lower energy. The dissociation rate displays a threshold near 315 nm while the desorption rate threshold is above 480 nm. The desorption rate dependence may qualitatively be described by two components, one which controls the desorption above 350 nm and one which dominates at shorter wavelengths. DISCUSSION O2/Ag(110). The photochemistry we observe could result from a number of excitations; thermal, plasmon, photoelectron, substrate hot carrier, or direct excitation.



21. HATCH & CAMPION


327

s

P 2

u >

Surface Science of Catalysis - American Chemical Society

Recommend Documents