A299

Surface Science 143 (1984) 411-422 North-Holland, Amsterdam

HYDROGEN ADSORPTION ON PLATINUM

C M SAYERS

Materials Physics Dwlston, AERE Harwell, Oxfordshtre, 0Xl l OR.A, UK

Received 28 September 1983, accepted for pubhcatlon 24 Aprd 1984

411

The amphtude weighted vibrational density of states of hydrogen adsorbed at two- and three-coordinated s~tes on P t ( l l l ) have been computed Electron energy loss spectroscopy shows peaks at 550, 860 and 1230 cm -1 [6] The peaks at 550 and 1230 cm -1 are assigned to vibrations

t,o) - 1 93 ,~ and force of hydrogen at a two-coordinated site, w~th nearest ne~ghbour d~stance ~ H P t - ~O) • l 93 A compares with the value t.cl) ~ 0 8 6 3 1 × 1 0 5 dyn cm - I The value "'HPt constant ~HPt

RO> 84 ( + 0 06) A. determined by LEED for hydrogen on Nl ( l l l ) , and the force constant is m H N t ~ 1 good agreement with the average value k ~ ffi 1 08 × 105 dyn cm-1 obtained for bridged metal hydrides Inelastic neutron scattering shows, m addition to modes at 500 and 1296 cm -1, peaks at 856 and 936 cm -1 corresponding to the weak feature observed at 860 cm -1 with EELS These vtbraUons are assigned to hydrogen motion perpendicular and parallel to the surface at a

(1 ) three-coordinated s~te with bond length ..~O)apt --- ~ 905 A and force constant kHp t = 0 4866 × 105 dyn cm -1 This bond length compares with the value .-apt~O) =1 9 A obtained from hehum dtffracuon data for hydrogen at a three-coordinated site on P t ( l l l )

Surface Science 143 (1984) 423-441 423 North-Holland, Amsterdam

AN XPS STUDY OF THE INTERACTION OF SO 2 WITH CaO(100): EFFECT OF TEMPERATURE AND METAL ADSORBATES (Fe, Na) Y.C LEE and P A. MONTANO

Department of Physws, West Virginia Umoerszty, Morgantown, West Vtrgtma 26506-6023, USA

and

J M. COOK

Morgantown Energy Technology Center, Morgantown, West Virginia 26505, USA

Received 28 December 1983, accepted for publication 15 March 1984

Using X-ray photoelectron spectroscopy (XPS), the chemlsorptlon of SO 2 on CaO(100) has been studied in the temperature range from 25 to 400 o C The chemical state of sulfur has been identified as that of sulfate The kinetics of SO 2 chermsorptlon on CaO are discussed The binding states of Fe and Na on CaD were determined to be Fe 2+ and Na + respectively The effect of the metal adsorbates (Fe, Na) on the chemlsorptton of SO 2 was also studied In the presence of Fe (coverage ___ 0 5 monolayer), both sulfate and sulfide were observed, m the presence of Na, however, only sulfate was observed The saturated amount of sulfate formed depends on the Fe coverage, but tt is independent of the Na coverage At low Fe coverages ( < 0 5 monolayer), there is a large increase in the rate of sulfate formauon at low SO 2 exposures ( < 3 L) This increase is explained by the "activation" of SO 2 chenusorpt~on sites by Fe adatoms At increased Fe coverage, the rate of sulfate formation decreases due to the blocking effect of the Fe islands formed on the surface Slmdarly, the effect of Na adatoms on SO 2 uptake can also be explmned by the same mechanism However, the mabdlty of the Na islands to block the SO 2 chenusorptlve sites and the lack of chermsorptlve bond formation between SO 2 and Na are explained by the diffusion of SO 2

through the Na adlayer