Chemical Physics Letters 556 (2013) 98–101

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Chemical Physics Letters

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Interaction of NO with the TiO2(110) surface: A quantum chemical study

Marie Arndt, Sukumaran Murali, Thorsten Klüner ⇑Institut für Reine und Angewandte Chemie, Theoretische Chemie, Carl von Ossietzky Universität Oldenburg, Postfach 2503, D-26111 Oldenburg, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 September 2012In final form 20 November 2012Available online 29 November 2012

0009-2614/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cplett.2012.11.057

⇑ Corresponding author. Fax: +49 441 798 3964.E-mail address: Thorsten.Kluener@uni-oldenburg.d

The adsorption of NO on the rutile (TiO2(110)) surface has been investigated by restricted open shell Har-tree–Fock (ROHF) and Møller–Plesset second order perturbation (ROMP2) theory. To mimic the TiO2

(110) surface, a Ti9O18Mg714+-cluster embedded in a field of point charges has been employed. The pre-

ferred orientation of NO on TiO2 (110) has been discussed in terms of 2D potential energy surfaces (PES)and a molecular orbital perspective. Adsorption energies of NO molecules in different electronic stateswere calculated by varying the distance coordinate and the polar angle. Our results reveal that a tiltedorientation of the NO molecule on the TiO2 (110) surface is energetically preferred.

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1. Introduction

The adsorption/interaction of organic molecules, nucleotides,nucleic acids, and amino acids on the titanium dioxide surface[1–5] has been gaining interest in the last few years as TiO2 playsan important role in various applications [6,7]. Among other struc-tural forms of titanium dioxide, rutile has been used for severalinvestigations as a prototype surface. In surface photochemistry,the adsorption and desorption of CO on the TiO2 (110) surfaceserved as a classical example of the adsorbate–surface interactionphenomenon [8] as CO is involved in many chemical processes.Since nitric oxide (NO) is an important molecule in chemical indus-try, automobile exhaust, and physiological process, the interactionof NO on the rutile surface is relevant in terms of its technologicaland gas sensor applications [9,10].

The adsorption of NO on TiO2 has been investigated experimen-tally by FT-IR studies [11] and TPD Spectroscopy [12]. Sorescu et al.[9] reported periodic DFT calculations of the adsorption propertiesof NO on a defective TiO2 (110) surface using the spin-polarizedPerdew–Wang 91 (PW91) exchange correlation functional. Theyfound that the most stable configuration of NO on the TiO2 (110)surface is in tilted form at 128� with respect to the surface normalwhere a linear arrangement N–O–Ti corresponds to a polar angleh = 0� which is in contrast to the present study, in which an N–O–Ti arrangement corresponds to h = 180�. The calculated adsorp-tion energy (�0.39 eV) was in close agreement with the thermaldesorption kinetics experiment (�0.36 eV) for NO desorption froma non-defective TiO2 (110) surface [13]. Owing to the importanceof NO in environmental research and chemical reactions, the stud-ies dealing with the interaction of NO on oxide and spinel surfaceshave been gaining increasing interest in recent years [14–16].

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e (T. Klüner).

However, highly accurate computational studies beyond standardDFT still remain an exception, mostly due to their enormous com-putational effort.

This Letter deals with adsorption behavior of NO on the TiO2

(110) surface at a high level of theory such as restricted open shellMøller–Plesset second order perturbation theory (ROMP2) which isbeyond conventional density functional theory (DFT). For compar-ison, the results of restricted open shell Hartree–Fock (ROHF) cal-culations are also presented. The adsorption energy (basis setsuperposition error [17], BSSE corrected) of NO on the TiO2 (110)surface has been calculated and potential energy surfaces (PES)of both ground and relevant excited states have been calculatedby varying the distance of the molecule from the surface and tiltangle of the molecular axis with respect to the surface normal (po-lar angle). In addition, the bonding and geometry of NO on the TiO2

(110) surface is discussed.

2. Computational details

In order to simulate the adsorption of NO on a rutile surface, theindividual components of both adsorbate (NO) and substrate (TiO2

surface) need to be meaningfully described. The TiO2 (110) surfaceis modeled by a cluster model which consists of a Ti9O18Mg7

14+

cluster embedded in a field of 4421 point charges (PCF), and thiscluster model has been investigated recently in a systematic studyof the adsorption and photo-induced desorption of CO from rutile[8]. The geometric structure of the cluster was adjusted to resultsof DFT slab calculations [18] and was kept frozen throughout thestudy. In all calculations, a basis set of triple zeta (TZ) qualitywas used.1 Details on the basis set can be found in the supporting

1 Basis sets notes: central Ti-atom: (14s11p6d3f) ? [8s6p4d1f]; peripherali-atom: (14s9p5d) ? [5s3p2d]; central O-atom: (9s5p1d) ? [4s2p1d]; peripheral-atom: (9s5p) ? [4s2p]; Mg-Atom: (10s6p) ? [2s1p]; O-Atom (NO):s5p) ? [6s3p]; N-Atom: (9s5p) ? [6s3p].

TO(9

Figure 1. Schematic illustration of distance coordinate (z) of the molecules centerof mass and the fivefold coordinated Ti-site at the surface, lateral coordinates (x andy), polar angle (h) and azimuthal angle u of the NO adsorbate on the Ti9O18Mg7

14+.The point charge field is not shown.

Table 1Calculated equilibrium distance values between NO and the Ti9O18Mg7

14+ cluster andadsorption energies for states X2B2 and A2B1 calculated at ROHF and ROMP2 level oftheory, respectively. The BSSE uncorrected values for the adsorption energies aregiven in parentheses.

Electronic states A2B1 X2B2 A2B1 X2B2

Methods Equilibrium distance [Å] Adsorption energies [eV]Linear adsorption geometryROHF 3.21 3.21 �0.16 (�0.49) �0.20 (�0.53)RMP2 3.00 3.00 �0.46 (�1.30) �0.51 (�0.30)

M. Arndt et al. / Chemical Physics Letters 556 (2013) 98–101 99

information. In the latter cluster, Ti9O18 was partially embedded inMg2+ ions in order to avoid spurious interaction of the oxygen anionswith the PCF. In addition, the cluster is built by obeying the followingcriteria: (a) the number of ‘Ti’ and ‘O’ atoms in the cluster should ful-fill the constraints of being stoichiometric and electro neutral (b) thecluster should be large enough to mimic the TiO2(110) surface, and(c) computationally not prohibitive. Using the NO–Ti9O18Mg7

14+/PCFcluster model as shown in Figure 1, the geometry of the NO-mole-cule on the surface was varied and potential energy curves were cal-culated as a function of two selected internal degrees of freedom.

The internal degrees of freedom contain the lateral coordinatesx and y, the desorption coordinate z, the inner NO distance r, theazimuthal angle u, which describes the rotation of the adsorbatearound itself, and the polar angle h, respectively. It should be noted,that the plane of rotation of the NO molecule was perpendicular tothe surface oxygen rows in case of the linear adsorption geometry.In the first part of our calculations, the distance (z) of the center ofmass of the NO molecule and the fivefold titanium adsorption sitewas varied from 2.0 to 13.0 Å, while keeping other degrees of free-dom such as the lateral coordinates (x and y) fixed. The polar angle(h) and the azimuthal angle (u) were set to a value of 0�. The inter-nal bond distance (r) of NO was fixed at 1.17 Å. In the second part,the equilibrium distance, calculated in the first part, was kept fixand the polar angle was varied from 0� to 180�. At a polar angleof 0� the NO molecule adsorbs with the nitrogen atom towardsthe surface. At h = 180� the oxygen atom points onto the fivefoldcentral titanium atom of the cluster.

The adsorption energies were calculated by varying the distancecoordinate (z) and polar angle (h) at ROHF and ROMP2 levels oftheory, using the MOLCAS package of programs [19]. The adsorptionenergy was calculated using the following equation:

Eads ¼ ENO—TiO2 � ENO � ETiO2

The basis set superposition error (BSSE) in the calculated totalenergies has been corrected by the counterpoise approach of Boysand Bernardi [17].

3. Results and discussion

In order to understand the bonding of NO on a rutile surface, weconstructed potential energy surfaces of various electronic statestaking into account the most relevant degrees of freedom. The re-sults are analyzed in a molecular orbital picture in order to illus-trate the underlying binding mechanism. The NO–TiO2 systemintrinsically has to be treated as an open shell system since theNO molecule is a radical with eleven valence electrons. The un-paired electron in nitric oxide (NO) is located in the 2p⁄ antibond-ing orbital. The rutile cluster employed in this letter belongs to theC2v point group with four irreducible representations i.e., A1, B1, B2

and A2. Hence, the electronic ground state in case of a linearadsorption of NO on the fivefold coordinated Ti site could be de-noted as 2A1, 2B1, 2B2 and 2A2, respectively. The calculations showthat the 2B1 and 2B2 states of the adsorbate–substrate system arenearly degenerate (DE = 0.05 eV) and the X2B2 state correspondsto the electronic ground state in this geometry. In case the polarangle is varied, the system exhibits Cs symmetry resulting in theirreducible representations A0 and A00, respectively. It is well estab-lished from experiments and DFT calculations that NO binds to therutile surface with a preferred orientation of the nitrogen atom ofthe NO molecule pointed towards the central titanium of the clus-ter surface, while the molecular axis is tilted with respect to thesurface by about 52� [13]. Furthermore, a recent study [20] hasshown that the adsorption geometry of molecular NO on thehydroxylated TiO2 (110) surface is also tilted.

To characterize the topology of the ground state potential en-ergy surface, we performed ROHF and ROMP2 calculations of theNO–TiO2 complex. The adsorption energies were calculated forthe states X2B2 and A2B1 for the linear adsorption. The results areshown in Table 1 where all adsorption energies have been cor-rected for the BSSE. The corresponding potential energy curvesare displayed in Figure 2.

It can clearly be seen that the X2B2 state is energetically pre-ferred by 0.04–0.05 eV with respect to the A2B1 state independentfrom the level of theory. In the X2B2 state the single electron of theNO molecule occupies the 2py

⁄-orbital, while the 2px⁄-orbital is

the highest occupied molecular orbital (HOMO) of the A2B1 state.Obviously, the occupation of the 2px

⁄-orbital is less favored dueto repulsive interaction with the surface oxygen rows of the rutilesurface. Since the difference between ROHF and ROMP2 bindingenergies is quite large, an accurate treatment of electron correla-tion is mandatory in this respect.

When the NO molecule on the rutile surface is in a tilted orien-tation, a symmetry reduction from C2v to Cs of the cluster will oc-cur. Thus, the resulting electronic states should be denoted as A0

and A00, respectively.To get more insight into the interaction of NO molecule with the

TiO2 (110) surface, the adsorption energies of the NO–Ti9O18Mg714+

complex in the states 2A00 and 2A0 were calculated at the ROMP2 le-vel with varying polar angles (h) at a fixed equilibrium distance of3.0 Å. The corresponding potential energy curves are shown in

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

2 2.5 3 3.5 4 4.5 5 5.5

)Ve(ygrene

noitpr osdA

NO - surface distance (A)

ΔE = - 0.16 eV

ΔE = - 0.20 eV

ΔE = - 0.51 eV

ΔE = - 0.46 eV

X B -ROMP222

A B -ROHF21

X B -ROHF22

A B -ROMP221

Figure 2. Adsorption energies of NO on the rutile surface for the X2B2 and A2B1

states calculated at the ROHF and ROMP2 level of theory for a linear adsorption ofNO on the fivefold coordinated Ti site of the rutile surface.

Figure 3. Potential energy curves of the NO–Ti9O18Mg714+ complex along Polar

angle (h) coordinate for both states A2A00 (dashed line) and X2A0 (solid line) at theROMP2 level of theory.

Table 2Adsorption energies of the NO–Ti9O18Mg7

14+ complex at various polar angles (h)calculated at the ROMP2 level of theory. The BSSE uncorrected values for theadsorption energies are given in parentheses.

States Polar angle(h)

Adsorption energy(eV)

2A0 0 �0.46 (�1.30)36 �0.57 (�1.33)145 �0.17 (�0.86)180 �0.77 (�0.79)

2A00 0 �0.51 (�1.30)180 �0.10 (�0.81)

Figure 4. Mixed orbital pattern of tilted adsorbate on the Ti9O18Mg714+ cluster for

the X2A0-state.

100 M. Arndt et al. / Chemical Physics Letters 556 (2013) 98–101

Figure 3, and the results are summarized in Table 2. The potentialenergy of 2A00 state shows two minima: (a) the global minimumcorresponds to the geometry, in which the nitrogen atom of theNO molecule points towards to the cluster (h = 0�); (b) the localminimum at h = 180� corresponding to the geometry in whichthe NO molecule adsorbs on the surface with its oxygen atominteracting with the fivefold coordinated Ti site. However, only in

case of linear adsorption (0� or 180�) the 2A00 state turns out tobe the ground state. Between 5� and 175� of the polar angle, itwas found that the 2A0 state was energetically favored. This stateshows two minima at h = 36� and h = 145�, respectively. The localminimum at h = 145� corresponds to the geometry of NO, in whichthe oxygen atom of NO is coordinated to the central titanium atomof the cluster, whereas the global minimum at h = 36� correspondsto the geometry, in which the nitrogen atom is coordinated to thefivefold titanium atom of the cluster. With the polar angle between0� and 36�, the interaction between the nitrogen atom of NO andthe central titanium of the cluster steadily increases. This interac-tion is lowered when the polar angle of NO changes from 36� up to90� where both oxygen and nitrogen atoms are not in a proper ori-entation to bind to titanium of the surface. If the polar anglereaches values above 90�, the proximity of oxygen over the surfaceleads to an enhanced interaction, finally resulting in the local min-imum at h = 145�.

The energetically most favorable geometry of the NO–Ti9O18-

Mg714+/PCF complex was obtained for the state, in which the NO

molecule is tilted by 36�, and the nitrogen atom is coordinated tothe central titanium of the Ti9O18Mg7

14+/PCF cluster. The tilt angleobtained from Sorescu et al. [13] was 46�, which slightly differsfrom the value calculated in our study, which could be attributedto the different methodology and approach employed in reference[13]. The corresponding interaction energy calculated in our studywas �0.57 eV, which is slightly smaller (strongly bound) than theexperimental value of about �0.39 eV [13], partially due to anomission of the zero point energy (ZPE) correction. Rodriguezet al. [21] found an adsorption energy of �0.50 eV which is in goodagreement with our value for the adsorption of NO on rutile. On theother hand, Stodt et al. reported an adsorption energy of �0.36 eVat an equilibrium distance of 2.47 Å and a polar angle of 50� at aB3LYP level of theory [22]. Future studies should also include elec-tron correlation effects at a higher level of theory such as CCSD(T)to elucidate the differences between the ROMP2 and B3LYP results.

In fact, the preference of a tilted geometry arises from the for-mation of a chemical bond between the NO-molecule on the TiO2

(110) surface as shown in Figure 4. This chemical bond could be

M. Arndt et al. / Chemical Physics Letters 556 (2013) 98–101 101

attributed to a HOMO–LUMO interaction of the dz2 -orbital of thecentral titanium atom with the 2px

⁄-orbital of the NO adsorbate,yielding a bonding linear combination (dz2 þ 2p�x) for the 2A0-state.On the other hand, the corresponding linear combination(dz2 þ 2p�y) can only result in non-bonding molecular orbitals dueto symmetry reasons, which in general disfavors the 2A00-stateenergetically. At a linear adsorption geometry, both states arenearly degenerate implying that the adsorbate–substrate interac-tion is dominated by electrostatic interactions and Pauli-repulsion.

4. Conclusions

The interaction of an NO molecule with a TiO2 (110) surface hasbeen investigated by ab initio ROHF and ROMP2 calculations. Thecounterpoise-corrected adsorption energy was found to be�0.57 eV, which is in reasonable agreement with experimental re-sults. The construction of potential energy curves with respect tothe distance coordinate and polar angle revealed further insightinto the mechanism of the chemical bond in this system. In fact,a genuine chemical bond is observed which resulted from theinteraction of the dz2 -orbital of the central titanium atom and the2px

⁄-orbital of the adsorbed NO molecule. A clear preference of atilted geometry of the NO on the surface was observed, yieldingan equilibrium polar angle of about 36� with the N-atom of themolecule pointing towards the surface. The adsorption energieswith respect to other degrees of freedom will be calculated in fu-ture studies which will lay the foundation for further quantumdynamical simulations of the photo-induced desorption processof NO on the TiO2 (110) surface.

Acknowledgements

We kindly acknowledge B. Meyer for providing the geometry ofthe substrate obtained within density functional theory (DFT) [18]

and the Deutsche Forschungsgemeinschaft (DFG) for financial sup-port (KL 1175/12-1 and KL 1175/11-2).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cplett.2012.11.057.

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