In-Situ Detection of NO Chemisorbed on Platinum Using Infrared-Visible Sum-Frequency Generation (SFG)

Abstract

NO Adsorption / Catalysis / Sum-Frequency Generation / Low-Energy Electron DiffractionIn-situ sum-frequency generation (SFG) surface vibrational spectroscopy for different polarisation arrangements has been employed along with low-energy electron diffraction (LEED) measurements and kinetic modelling calculations to study NO adsorption on Pt(111) as a function of NO gas-phase pressure (p NO ϭ 10 Ϫ9 Ϫ10 Ϫ4 mbar) and substrate temperature (T s ϭ 300Ϫ400 K). The observation of a NO stretching vibrational band with a frequency of 1724 cm Ϫ1 with different signal intensities for ssp and ppp polarisation combinations indicated the presence of tilted NO species at high NO coverages (θ Ն 0.5). At lower coverages (0.5 Ͼ θ Ͼ 0.25) the adsorption geometry was found to change towards upright NO giving rise to a vibrational band with slightly lower frequencies of 1716Ϫ1720 cm Ϫ1 . LEED studies demonstrated that under adsorption/desorption equilibrium conditions up to saturation coverage NO adsorption leads to the formation of ordered adsorbate structures with 2ϫ 2 periodicity. The detailed analysis of the pressure and temperature dependence of the LEED and SFG surface vibrational data is consistent with a NO adsorption mechanism involving the successive formation of Pt(111)ϩp(2ϫ2)-NO (θ ϭ 0.25), Pt(111)ϩ(2ϫ2)-2 NO (θ ϭ 0.5) and Pt(111)ϩ(2ϫ2)-3 NO (θ ϭ 0.75) adsorbate structures as the NO pressure is increased from 10 Ϫ9 to 10 Ϫ4 mbar. A tentative binding site assignment for the two higher-coverage structures with θ ϭ 0.5 and 0.75 is proposed and discussed in the light of the present and previous experimental data.

8 Surface vibrational spectroscopy on noble metal catalysts from ultrahigh vacuum to atmospheric pressure

“…The advantages of SFG spectroscopy have been outlined in a number of previous publications covering a wide range of substrate (interface)-adsorbate combinations 17,[76][77][78][79] First applications of SFG to catalysis research were reported in the 1990s by groups in Annandale (Exxon), 77 Berkeley 80 and Ju ¨lich. 81 In these studies singlecrystal surfaces such as Ni(100)), 77 Pt (111) and Pt(110) [81][82][83] were utilized, since these ''mirror-like'' surfaces facilitated the optical experiment.…”

“…185 The absence of bridge-bonded CO (expected around 1850 cm 21 ) in the SFG spectra may have different reasons (several groups have reported difficulties in detecting this species on Pt (111), Pt foil, and Ni (111)). It may be due to a low Raman polarizability of bridge-bonded CO, 17,70,79,83 an inherently broad linewidth of bridged CO at 300 K 81 and/or may originate from an interference effect between different SFG susceptibility tensor elements as discussed by Baldelli et al 186 Increasing the pressure to 1 mbar and subsequently to 200 mbar further increased the dipole-coupling and shifted the frequency to 2097 cm 21 but the spectra were very similar to those taken under UHV with on-top CO being the only species detected (Fig. 8(b)).…”

“…28(b) and (c) show an example from Volpp, Wolfrum et al comparing CO and NO on Pt (111). 79,310 The ssp spectrum of CO shows no resonance indicating the upright orientation of both on-top and bridged CO. By contrast, the stretching mode of NO also appears in the ssp spectrum which indicates that NO adsorbs in a tilted geometry (at high coverages). By analyzing the amplitudes of the ppp and ssp signals the tilt angle can be determined.…”

Electrocatalytic Reduction of HNO2 by a Silicomolybdate Polyanion: A Differential Electrochemical Mass Spectrometry Study

“…From the inset of Figure 2, it is obvious that the addition of nitrites leads to an increase of the electrode Voc. This effect could be attributed to NO chemisorption onto the Pt electrode [29,30]. As one can notice from Figure 2, when a PPy film deposited on a Pt electrode is in contact with a nitrous acid solution, the open circuit potential of the electrode increases, too.…”

A modeling analysis of molecular orientation at interfaces by polarization-dependent sum frequency generation vibrational spectroscopy

“…SFG has already been utilized as a powerful tool to probe the surface molecular structure and orientation of adsorbates on solid surfaces relevant for catalytic reactions, including the adsorption of CO and NO on single crystals [9][10][11][12][13][14], metal oxides [12], polycrystalline electrodes [9], and nanoparticles [15], as well as methanol dissociation (e.g. coverage-dependent, aerobic and anaerobic conditions) on TiO2 [16,17].…”