J. Phys. Chem. B 1999, 103, 10651-10656
10651
Geometry and Energetics of Pyridine Base Adsorption at Water-Zeolite Interfaces: A Simulation Study Masaharu Komiyama* and Mayumi Kobayashi Institute of Inorganic Synthesis, Yamanashi UniVersity, Kofu 400-8511, Japan ReceiVed: April 26, 1999; In Final Form: September 24, 1999
Array structures and molecular orientations within liquid-phase-adsorbed pyridine base (pyridine and β-picoline) adlayers on (010) surfaces of two natural zeolites (stilbite and heulandite) that have been observed by atomic force microscopy (AFM) were simulated by means of molecular mechanics (MM2′) calculations. For the β-picoline/stilbite system, present calculations indicated that an edge-on molecular orientation with the 6-C-N bond parallel to the surface is most likely, in accordance with previous AFM observations, but in contrast to a common expectation. While AFM was not able to determine molecular orientations within the adlayers of pyridine/heulandite system, the present simulation showed the N corner-on configuration to be the most stable one. Intralayer molecular interaction and molecule-substrate interaction energies were found to be of comparable value for these adsorption systems. Adsorption registries and adlayer-substrate distances, which were not determined by AFM, were also estimated.
1. Introduction Pyridine adsorption is often employed to probe surface acidity of zeolites and other oxide catalysts in the field of heterogeneous catalysis.1 The species is assumed to bond a surface acid site through its nitrogen lone pair, and from infrared absorption peak shifts and their intensities of C-C (or C-N) stretching bands, the amount and the strength of the acid sites are determined. Despite its widespread use, however, details of the adsorption geometry of pyridine on those surfaces are not known, largely due to the fact that there existed only a few techniques that yield such information. On metal surfaces, on the other hand, pyridine adsorption under ultrahigh vacuum conditions has been studied in detail,2-12 primarily motivated by the interests in surface-enhanced Raman spectroscopy of pyridine. On Ag(111) at ca. 140 K, it has been estimated with electron energy loss (EELS) and UV-photoemission (UPS) spectroscopies that at low coverage (0.5 L) it becomes nitrogen lone pair bonded with the molecular C2V axis inclined ca. 55° to the surface.2 For the inclined position an attractive π3-nitrogen lone pair interaction was proposed. Similar observations were made on Ni(001) at ca. 170 K.3 Various near-edge X-ray adsorption fine structure (NEXAFS), EELS, and other examinations of pyridine adsorption on metal surfaces followed. On Ir(111) angle-resolved UPS (ARUPS) and electron-stimulated desorption ion angular distribution (ESDIAD) found that pyridine is adsorbed via the N atom and the ring plane is tilted ca. 70° from the surface parallel.4 On Pt(111) NEXAFS found that below 240 K pyridine at saturation coverage is tilted 52 ( 6° to the surface and above that temperature the angle is 74 ( 10°.5 With temperatureprogrammed desorption (TPD) nitrogen corner-on bonding for the inclined and C-N edge-on bonding in the form of R-pyridyl for the perpendicular configurations were proposed.5,6 The formation of R-pyridyl is also reported on Ru(001).8 On Ag(111) at 100 K NEXAFS found that the tilt angle of pyridine ring from the surface plane is 45 ( 5° at low coverage and
changes to 70 ( 5° at a submonolayer coverage.7 On Ni(111) EELS indicated that at 120 K the pyridine molecular plane is flat at low coverage and nearly perpendicular at higher coverage,9 and a NEXAFS study at 120 K found the tilt angle to be ca. 70° to the surface for monolayer coverage.10 On Ni{111} at 125 K photoelectron diffraction found that the nitrogen atom of a pyridine molecule is located close to the Ni atop position, with the N-Ni bond ca. 20° off normal and the N-Ni distance being 1.97 Å.11 On clean Cu(111) EELS showed pyridine lying flat on the surface, and on oxygen-covered Cu(111) it showed pyridine lying perpendicular to the surface.12 Despite the abundance of experimental investigations for vacuum-metal interfaces, however, there exists only one work on the determination of the adsorbed layer structure of pyridine at a liquid-metal interface.13 Stern et al.14 adsorbed pyridine on Pt(111) electrode surface from its aqueous solution under various applied potentials and examined the adsorbed surface, ex situ, with Auger, LEED, and EELS. From the packing density of pyridine obtained by Auger, a tilt angle of 71° to the surface was estimated. A LEED analysis showed that the adsorbed pyridine lattice is incommensurate with the Pt surface and oblique, with lattice vector lengths of 3.32 and 4.74 Å with an inclined angle of 77°, and a rotation angle of 34.0°. The nearest commensurate structure was (2x3 × x21, 79°)R30°. Recently we have successfully observed, for the first time, molecular AFM images of aqueous-phase-adsorbed pyridine base layers on zeolite(010) surfaces, and determined their array structures for pyridine-on-heulandite15 and β-picoline-on-stilbite16 systems. On heulandite(010) pyridine molecules formed an almost perfect hexagonal array with the unit cell dimension of 5.5 Å. β-Picoline on stilbite(010) gave a quasi-hexagonal array with a 5.5 ( 0.4 Å unit cell length, with 5.8 Å separation often appearing. In both cases the pyridine rings were found to be tilted from the surface parallel, similar to many pyridine/ metal systems listed above. The registries for these systems were not determined due to the fast formation of the adsorbed layer. Nevertheless, relatively slow adsorption in β-picoline/stilbite system enabled us to estimate the adlayer orientation: one of
10.1021/jp9913618 CCC: $18.00 © 1999 American Chemical Society Published on Web 11/12/1999
10652 J. Phys. Chem. B, Vol. 103, No. 48, 1999
Komiyama and Kobayashi
TABLE 1: Structural Parameters of Pyridine Base Adlayers over Zeolite(010) Surfaces Obtained by AFM Observations15,16
adlayer (substrate)
most likely configurations
β-picoline (stilbite) pyridine (heulandite)
6-C-N edge-on or 5,6-C-C edge-on
ring tilt angle (deg)
2D hexagonal unit cell length (Å)
