Themed Issue on characterization of adsorbed species

Species adsorbed on ordered or disordered surfaces can be characterized by many ever-improving experimental techniques and at the same time they can be modeled by a variety of combinations of models and methods. One of the major developments in characterization of adsorbed species in the last decade was undoubtedly a greatly enhanced overlap between the experimental and the theoretical methods used. Such an enhanced overlap is based on increasing computational power and software development on the theoretical side, and on a parallel increase of the sensitivity and resolution of experimental techniques. As a result, many of the experimental measurements can be directly compared with corresponding values calculated, in a relatively reliable manner, on a model that is a reasonable representation of the real system. Such a combination of experimental and computational investigations can reveal new atomistic details about the adsorbed species, and also about the adsorbing surface itself.

Recent developments in computational hardware and software make it now possible to use significantly more realistic models and more reliable methods for calculating the properties of molecules adsorbed on surfaces and of the surfaces themselves. A realistic model of a surface must consist of tens or even hundreds of atoms; therefore, traditional wave-function based post Hartree-Fock methods cannot be used in the majority of cases. Instead, methods based on the density functional theory (DFT) have become standard techniques used in the modeling of surfaces. Application of DFT to adsorption complexes on surfaces has been established as a valuable enhancement of experimental investigation in this field, in particular for the description of chemically adsorbed complexes. DFT calculations on physically adsorbed complexes have been questioned in the past due to the fact that applied exchange-correlation functionals did not account for the dispersion interaction between adsorbate and adsorbent. Several strategies to account for the weak intermolecular interactions have recently been proposed and their reliability was tested on small intermolecular complexes in the gas phase. Extension and application of these strategies to adsorbate–adsorbent interactions are just appearing in the literature, and that is clearly apparent from this PCCP themed issue where such strategies were used in ten contributions.

On the experimental side, recent years have seen the development and increasing use of instrumental techniques that, under favorable conditions, allow very precise information on both, solid surfaces and adsorbed species, to be obtained. Among the examples, imaging of surfaces and adsorbed species by using scanning tunneling microscopy (STM), and increasing development of in situ (and operando) surface spectroscopy techniques constitute the main foci of current development. STM and related techniques (among them AFM) can give a very clear picture not only of the solid surface (at the atom size scale) but also of the location, motion and (sometimes) interaction energy of surface-adsorbed species. Developments on operando spectroscopic methods heavily lean on a combination of dedicated experimental design and availability of fast scanning spectrometers having rapid data acquisition electronics; increasing understanding of the kinetics of surface mediated chemical process and identification of reaction intermediates are among the aims of those studies. On the thermodynamic side, recent developments on variable temperature infrared (VTIR) spectroscopy, which allows identification of adsorbed species and simultaneous measurement of the corresponding adsorption enthalpy and entropy can be mentioned.

Valuable as the above summarized development may be, we wish to remark that recent years have also seen increasing synergy between the theoretical and experimental approach to studying surfaces and adsorbed species; and indeed a combined approach involving knowledge and expertise from both sides should pave the path leading to future development and further understanding. Articles by Lyubinetsky (DOI: 10.1039/C000250J), Kuroda (DOI: 10.1039/C000967A), Maurin (DOI: 10.1039/c001173h), Čejka (DOI: 10.1039/C001950J), Reuter (DOI: 10.1039/C001978J), Wöll (DOI: 10.1039/C002215M), and Blasco (DOI: 10.1039/C002427A) (and coauthors) in this issue demonstrate the combined theoretical and experimental approach for studying a variety of systems, including O₂ dissociation on TiO₂, acetylene interaction with Cu⁺ ions in zeolites, adsorption of light hydrocarbons in MOFs, CO₂ adsorption in zeolites, interaction of substituted azobenzene with a gold surface, self-assembled monolayers of organic molecules on gold, and Beckmann rearrangement reaction on boron-containing zeolites.

The role of dispersion interactions in adsorption is clearly demonstrated in several contributions to this PCCP themed issue; examples include adsorption on noble metal surfaces (Reuter et al. (DOI: 10.1039/C001978J) and Wöll et al. (DOI: 10.1039/C002215M)), metal oxides (Civalleri et al. (DOI: 10.1039/C001192D)), silica and hydroxyapatite (Ugliengo et al. (DOI: 10.1039/C000009D and DOI: 10.1039/C002146F)), graphite (Jordan et al. (DOI: 10.1039/C000988A), Bludský et al. (DOI: 10.1039/C001155J), and Bichoutskaia et al. (DOI: 10/1039/C000370K)), and zeolites (Rauhut et al. (DOI: 10.1039/B921531J) and Čejka et al. (DOI: 10.1039/C001950J)). The corresponding calculations were all performed with the DFT method extended for the description of weak intermolecular interactions: (i) combining DFT with symmetry-adapted perturbation theory (Jordan et al. (DOI: 10.1039/C000988A)), (ii) employing a hybrid DFT–ab initio DFT/CC approach (Bludský et al. (DOI: 10.1039/C001155J), Čejka et al. (DOI: 10.1039/C001950J)), and (iii) using an empirically corrected DFT-D method (Reuter et al. (DOI: 10.1039/C001978J), Bichoutskaia et al. (DOI: 10/1039/C000370K), Ugliengo et al. (DOI: 10.1039/C000009D), Civalleri et al. (DOI: 10.1039/C001192D), Rauhut et al. (DOI: 10.1039/B921531J), and Wöll et al. (DOI: 10.1039/C002215M)). The effect of dispersion interactions on calculated properties is also discussed. Several DFT investigations relevant to the catalytic properties of surfaces are presented: CO activation on Ru and Co surfaces (Shetty et al. (DOI: 10.1039/B926731J)), nature of surface carbonates and bicarbonates on RuO₂ (Schneider et al. (DOI: 10.1039/C001683G)), the role of surface geometry (surface metal cation coordination) on surface properties (Illas et al. (DOI: 10.1039/C000405G)), formation of PtAu clusters on MgO/Ag (Pacchioni et al. (DOI: 10.1039/C000841A)) and dissociative H₂ adsorption on Cu(111) surface investigated using quasi-classical and quantum dynamics (Diaz et al. (DOI: 10.1039/C001956A)). Enantioselectivities of a homologous series of chiral compounds on homochiral metal–organic frameworks was investigated computationally by Snurr et al. (DOI: 10.1039/C000809E).

A combination of instrumental techniques (FTIR, EPR, EXAFS, and UV-vis spectroscopy) was used by Roduner et al. (DOI: 10.1039/C000750A) to investigate an adsorption of oxygen on Cu-exchanged zeolites. A related aspect to the characterization of surface species is the use of probe molecules for investigating the nature of adsorption sites, as demonstrated by Hadjiivanov et al. (DOI: 10.1039/C000949K) and by Cairon (DOI: 10.1039/C000991A) who investigate Cu-BTC MOF and silica–alumina, respectively, using IR spectroscopy of adsorbed CO. The use of VTIR spectroscopy to investigate both, adsorbed species and adsorption sites, is discussed by Zecchina et al. (DOI: 10.1039/C002031C) and by Tsyganenko et al. (DOI: 10.1039/C003942J) who report on CO and H₂ adsorbed on alumina and on CO, and cyanide adsorbed on metal oxides and zeolites, respectively.

In summary, the range of articles in this Themed Issue should give a flavour of both, the current state of the art in the characterization of adsorbed species and the challenges ahead; and hopefully they will provide a broad perspective of the field for the non-specialist reader and a wealth of valuable details for experts. We wish to thank our colleagues who kindly contributed their articles and the many referees who generously gave their time and expertise. Finally, we sincerely thank the Staff of the Journal and, in particular, Philip Earis, Anna Roffey, Tanya Smekal and Jane Hordern for their continuous support at every stage of the issue development—from conception to editing.

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