Inﬂuence of Dipole−Dipole Interactions on Coverage-Dependent Adsorption: CO and NO on Pt(111) Prashant Deshlahra, † Jonathan Conway, † Eduardo E. Wolf, † and William F. Schneider* ,†,‡ † Department of Chemical and Biomolecular Engineering and ‡ Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States * S Supporting Information ABSTRACT: Density functional theory (DFT) calculations of energetic, geometric, vibrational, and electrostatic properties of diﬀerent arrangements of CO and NO at quarter and half monolayer coverage on Pt(111) are presented. Diﬀerences in the extents of electron back-donation from the Pt surface to these molecules cause the low-coverage adsorbate dipoles to have opposite signs at atop and more highly coordinated bridge or fcc sites. These dipoles of opposite sign occupy adjacent positions in the experimentally observed atop−bridge or atop−fcc high -coverage arrangements, leading to attractive electrostatic interactions and concomitant changes in dipole moments, bond lengths, and vibrational frequencies. The interaction energies are estimated by charge partitioning to extract individual dipoles from the mixed arrangement and by calculations of ﬁeld−dipole interactions. These estimated dipole interactions contribute signiﬁcantly (20−60%) to the DFT-calculated relative stability of mixed arrangements over atop-, bridge-, or fcc-only arrangements and thus play an important role in coverage-dependent adsorption. We further extend these analyses to a range of molecules with varying dipole moments and show that the general nature of these interactions is not limited to CO and NO. 1. INTRODUCTION Interactions between adsorbates at a heterogeneous surface introduce coverage dependence into adsorption energies, 1−4 inﬂuence adsorbate site preferences and the distribution of adsorbates on a surface, 5,6 and can contribute to poisoning and the promotion of chemical reactions. 7 Electronic interactions that arise from competition between adsorbates for surface states tend to be short-ranged and can be described in terms of the d-band center models. 8−10 Strain interactions arise from adsorbate-induced expansion or compression of the surface and can be longer-ranged. 11 When adsorption creates signiﬁcant surface dipoles, longer-ranged electrostatic interactions can appear. In this work, we consider the interactions of such dipoles and their inﬂuence on adsorbate binding, vibrational spectroscopy, and site preferences. Gas molecules with or without a net intrinsic dipole moment exchange charge density with the surface on which they adsorb, leading to a modiﬁed adsorbate dipole. 12,13 The interactions of these adsorbate dipoles with external electric ﬁelds can have structural and energetic consequences relevant to catalysis. Field−dipole interactions arise in electrochemical or electro- catalytic systems, 14−16 electric ﬁelds induced by charge transfer at metal support interfaces of supported catalysts, 17−19 and model catalytic reaction studies in ﬁeld-ion microscopes 20 as well as on single-crystal surfaces using electroreﬂectance vibrational spectroscopy. 21 These interactions can signiﬁcantly aﬀect the adsorbate stability 12,13,15,22,23 and reaction bar- riers 13,22 on the catalyst surface. Electrochemical ﬁeld−dipole eﬀects have been studied in several experiments and simulations. Shifts in the vibrational frequency of adsorbates with the electric ﬁeld, known as the vibrational Stark eﬀect, are well known. 16 However, the eﬀect of coverage on the energetics of these interactions and the eﬀect of the local environment leading to diﬀerent Stark tuning rates on electrochemical and UHV systems is not well understood. Received: March 6, 2012 Revised: April 22, 2012 Published: April 30, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 8408 dx.doi.org/10.1021/la300975s | Langmuir 2012, 28, 8408−8417