State-resolved reactivity of CH4 on Pt(110)-(1×2): The role of surface orientation and impact site

Abstract

The reactivity of methane (CH(4)) on Pt(110)-(1 x 2) has been studied by quantum state-resolved surface reactivity measurements. Ground state reaction probabilities, S(0)(v=0) congruent with S(0)(laser-off), as well as state-resolved reaction probabilities S(0)(2nu(3)), for CH(4) excited to the first overtone of the antisymmetric C-H stretch (2nu(3)) have been measured at incident translational energies in the range of 4-64 kJ/mol. We observe S(0)(2nu(3)) to be up to three orders of magnitude higher than S(0)(v=0), demonstrating significant vibrational activation of CH(4) dissociation on Pt(110)-(1 x 2) by 2nu(3) excitation. Furthermore, we explored the azimuthal and polar incident angle dependence of S(0)(2nu(3)) and S(0)(v=0) for a fixed incident translational energy E(t)=32 kJ/mol. For incidence perpendicular to the missing row direction on Pt(110)-(1 x 2) and polar angles theta>40 degrees, shadowing effects prevent the incident CH(4) molecules from impinging into the trough sites. Comparison of this polar angle dependence with reactivity data for incidence parallel to the missing rows yields state-resolved site specific reactivity information consistent with a Pt(110)-(1 x 2) reactivity that is dominated by top layer Pt atoms located at the ridge sites. A comparison of S(0)(v=0) measured on Pt(110)-(1 x 2) and Pt(111) yields a lower average barrier for Pt(110)-(1 x 2) by 13.7+/-2.0 kJ/mol.

Precursor-mediated reactivity of vibrationally hot molecules: Methane activation on Ir(111)

“…They noted that their data did not rule out the possibility of a precursor-mediated reaction channel involving vibrationally hot methane molecules, but suggested that at low E trans , CH 4 molecules might be steered into a particularly low-barrier or barrier-less reaction path that could dramatically enhance reactivity. More recent state-resolved experiments of CH 4 dissociation on Pt(1 1 0)-(1 × 2) confirm the low-E trans reactivity reported by Walker and co-workers for molecules without laser excitation, but S 0 for molecules in the 2 3 vibrational state (2 quanta of C-H stretch) appear to react via a direct reaction mechanism at all E trans between 4 and 62 kJ/mol [21].…”

“…Observation of a collision-induced reaction pathway implicated methane's bending vibrations in promoting reaction [15]. Since that time, the range of surfaces explored with conventional beam-surface scattering techniques has expanded to include Pt(1 1 1) [16][17][18], Pt(1 1 0)-(1 × 2) [19][20][21], Pt(5 3 3) [22], Ni(1 0 0) [23], Ru(0 0 0 1) [24,25], Ir(1 1 1) [26,27], Ir(1 1 0) [28,29] Pd(1 1 0) [30] and mixed metal systems such as Cs/Pt(1 1 1) [31], Co/Cu(1 1 1) [32] and Au/Ni(1 1 1) [33]. The majority of these studies reveal that S 0 increases monotonically and nearly exponentially with increasing E trans over the range of E trans explored.…”

Mode-specific reactivity ofCH4onPt(110)−(1Bisson1, Sacchi2, Beck3 2010Phys. Rev. BSelf Cite553310View full textAdd to dashboardCite

“…To determine absolute reaction probabilities for CH 4 , we perform a timed exposure of the clean crystal surface to the molecular beam, measuring both the incident dose of CH 4 molecules per unit area with the calibrated QMS as well as the resulting coverage of carbon reaction products on the surface by performing calibrated AES measurements across the surface in a computer-controlled scan. 16 We measured the state-resolved reactivity of CH 4 , prepared in the four different vibrational states shown in Table I; each containing different amounts of C-H stretch and bend excitation given approximately by their zero-order normalmode labels ͑ 1 and 3 are the symmetric and antisymmetric stretch modes and 2 and 4 are IR-inactive and IR-active bend vibrations, respectively͒. Because of the near degeneracy of the stretch fundamentals and bend overtones, the vibrational states of CH 4 can be grouped into polyads of nearly isoenergetic vibrational states 17 ͑Fig.…”

“…The state-resolved reactivity for E t Ͻ 10 kJ/ mol has been discussed elsewhere. 16 While the 2 2 + 4 , 1 + 4 , and 3 + 4 states have similar vibrational energy ͑Table I͒, they result in different reactivities. Furthermore, the 1 + 4 and 2 3 states have similar reactivities even though their vibrational energy is different.…”

Quantum State–Resolved Studies of Chemisorption Reactions

“…For example, on Ni(111) and Pt(111), the minimum barrier is calculated for dissociation over the top site (90). Bisson et al (91) exploited shadowing effects on a corrugated Pt(110)-(1×2) surface to control the impact site of incident methane in order to measure the relative reactivity of ridge and trough sites. To do this, they determined the polar-angle dependence of the CH 4 (ν 3 ) reactivity for incidence parallel and perpendicular to the close-packed rows of Pt atoms.…”