Influence of Carbon Vacancies on CO Chemisorption on TiC(001): A Theoretical Study

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1 Printed in the Republic of Kore Influence of Crbon Vcncies on CO Chemisorption on TiC(001): A Theoreticl Study De-Bok Kng Deprtment of Chemistry, Kyungsung University, Busn 48434, Kore E-mil: dbkng@ks.c.kr (Received October 10, 2016; Accepted November 7, 2016) ABSTRACT. The extended Hückel method is employed to nlyze the interction of crbon monoxide with the TiC(001) surfces, both perfect nd contining crbon vcncies. CO exhibits similr σ-dontion interction for both Ti 25C 25 nd Ti 25C 23 clusters, s deduced from the fct tht the popultions of the CO 5σ orbitl re identicl upon dsorption, but it bonds more strongly with the Ti 25C 23 thn with the Ti 25C 25 becuse the metl d electron density in Ti 25C 23 provides π bck-bonding interctions with CO tht re bsent in Ti 25C 25. This work suggests tht difference in rectivity towrd CO of stoichiometric TiC nd TiC with crbon defects is connected with the occupncy of 2π* orbitls tht leds to significnt wekening of the C-O bond. Key words: Titnium crbide, Adsorption, Electronic structure, CO INTRODUCTION Surfce chemicl rectivity of the trnsition metl crbides hs ttrcted interest for their ctlytic properties similr to mny metl surfces. 1,2 These properties result from the electronic structure of these mterils. In this pper, we focus on the electronic structure nd surfce chemicl interction of titnium crbides tht re criticl in their successful use s ctlysts. These substrtes exhibit difference in rectivity towrd CO, which hs been used s probe molecule of chemicl ctivity for both metl nd metl crbide ctlysts. In order to nlyze the bonding nd surfce rectivity with CO, we present the results of extended Hückel (EH) cluster nd bnd structure clcultions for the electronic properties of TiC(001) surfces, both perfect nd contining crbon vcncies. Contrry to the cse of metl surfces, limited theoreticl studies of CO dsorption hve been performed on TiC(001) surfce. Didziulis et l. used density functionl theory (DFT) to exmine interctions of CO with TiC(001). 3,4 They report tht CO dsorption on TiC(001) prefers on top Ti sites. Depending on the cluster model selected, the dsorption energies of CO re quite different. Work performed on smll clusters predicted wek dsorption energy of 0.55 ev, which is interpreted s rising from the σ-dontion bonding of the CO 5σ orbitl with the empty metl dz 2 orbitl. The strength of the surfce bond with CO nd the tendency for CO to dissocite re strongly ssocited with the extent of metl-to-co π bck-bonding interctions. Our clcultions show tht CO molecules form firly strong Ti-C π bckbonding with the surfce when dsorbed on top Ti sites of the TiC(001) contining C vcncies. We investigte in this pper how the bonding sitution is influenced by the vcncies on the crbon sublttice. The effect of C vcncies on the chemicl rectivity of the TiC(001) surfce is discussed in terms of the density of sttes (DOS) nd the frgment moleculr orbitl (FMO) popultions. We lso report quntittively the strength of metl-co nd C-O bonds by use of the integrted overlp popultions up to the Fermi level. All the clcultions in this work hve been performed with the CAESAR progrm pckge; 5 7 tomic prmeters re given in Tble 1. RESULTS AND DISCUSSION A two lyer thick Ti 25 C 25 slb is used to represent the TiC(001) surfce with stoichiometric composition, s depicted in Fig. 1. The Ti 25 C 23 slb with crbon vcncies is modeled by removing two crbon toms denoted by V in the first lyer of the TiC(001) surfce. The experimentl lttice prmeter ( = 4.32 Å) for the cubic rockslt-type TiC is employed in the clcultions. The Ti-C nerest neighbor distnce is 2.16 Å. No surfce reconstructions re considered. The C-O bond length of 1.14 Å is kept constnt for ll clcultions. For the surfce-co interctions, the Ti-CO bond distnce is 2.20 Å in both clusters. 3,4 Usully the metl-co bond is described in terms of the Blyholder model, 8 which involves electron dontion from the CO 5σ orbitl to the metl nd the bck-dontion from the metl to the CO 2π* orbitls. In the CO bonded clusters, crbon monoxide cn interct -7-

2 8 De-Bok Kng Tble 1. Prmeters for EH clcultions tom orbitl H ii, ev b ζ 1 b ζ 2 c 1 c 2 Ti 3d s p C 2s p O 2s p Coefficients used in double-ζ expnsion. b Slter-type orbitl exponents. Figure 1. Top view of the 2 2 supercell model for the TiC(001) surfce of the rock-slt structure. Big nd smll spheres represent Ti nd C toms, respectively. In the cse of TiC(001) with crbon defects, the two crbon toms re replced with vcncies denoted by V. with the substrtes s the σ-donor nd π-cceptor molecule, exhibiting similr interction s is expected for metl surfces. We use the model to exmine the interction of CO molecule with the metl site of both clusters. Moleculr orbitl clcultions were performed on the free CO molecule, the TiC(001) cluster, nd the cluster-co system. Adsorption energies (E d ) re obtined by subtrcting the energy of the combined cluster-co system from the sum of the energies of the free CO molecule nd the cluster. The EH clcultions produce dsorption energies tht re unrelistic nd provide only qulittive predictions for the dsorption energy. The results of clcultions on the bonding of CO with these clusters re summrized in Tble 2. Tble 2. Results from the TiC-CO cluster clcultions cluster Ti-CO bond (Å) C-CO bond (Å) E d (ev) Ti 25C Ti 25C Ti 25C Tken from refs. 9, 10. On the Ti 25 C 25 cluster, the dsorption energy of CO on Ti nd C sites is estimted to be 1.63 ev nd 1.72 ev, respectively. As might be expected, bonding of CO with crbon end down is more stble on titnium site thn crbon site of the cluster. For n on top dsorption of CO on Ti, we clculte much stronger dsorption energy for Ti 25 C 23 reltive to the Ti 25 C 25 cluster. The primry reson for this is due to the π bck-bonding of the defective cluster with the CO molecule. Thus the strength of CO bonding to surfces depends on the bck-dontion of electrons from surfce into the CO 2π*. Assuming the z-xis perpendiculr to the surfce, the occupied Ti 3d xz,yz orbitls in Ti 25 C 23 prticipte in the π bck-bonding interction with the empty 2π*. This interction is not possible in Ti 25 C 25, becuse this cluster hs no occupied Ti 3d orbitls. The occuption of CO 2π* orbitls is confirmed in Tble 3. The strengths of the CO σ-dontion bond to the unoccupied dz 2 level re comprble on both clusters, s deduced from the fct tht the popultions of the CO 5σ orbitl re identicl upon dsorption. Tble 3 lso contins the clculted Mulliken chrges on the CO molecule involved in the cluster-co bond. The chrge plys n importnt role in the reltive donor nd cceptor interctions of the cluster. The positive chrge on the CO molecule for the Ti 25 C 25 cluster hs been incresed slightly from zero chrge vlue of the free molecule, consistent with the σ-dontion bonding of CO. Adsorption of Tble 3. Relevnt overlp popultions, FMO occuptions, nd chrges for CO on TiC(001) TiC TiC 0.94 FMO occuptions 5σ π*/orbitl CO chrge Overlp popultions C-O Ti-CO

3 Influence of Crbon Vcncies on CO Chemisorption on TiC(001) 9 CO on the stoichiometric TiC (Ti 25 C 25 cluster) occurs only through its σ-dontion to Ti d 0 sites. On the other hnd, CO gins some electrons from the Ti 25 C 23 cluster nd is reduced significntly. The lrge negtive chrge of CO indictes its bck-bonding interction with the Ti tom of the cluster with crbon defects. The presence of electron density in the metl d orbitls of Ti 25 C 23 is criticl in understnding the difference in rectivity for CO between the two clusters. In the stoichiometric solid, the formlly ssigned chrges re Ti 4+ C 4-. The requirement of chrge neutrlity results in the decrese in the oxidtion stte of Ti from +4 to s C vcncies increse from the Ti 25 C 25 to Ti 25 C 23. The introduction of crbon vcncies in the TiC(001) structure fcilittes the formtion of reduced Ti 3+ ctions nd hence leds to the popultion of the metl d orbitls on the surfce. Consequently, Ti 25 C 23 hs more electrons t Ti 3d orbitls thn Ti 25 C 25. This occuption of Ti 3d orbitls strengthens the π bck-bonding interction between CO nd Ti of the Ti 25 C 23 cluster. Bnd structure clcultions were performed to move beyond the limittions of the cluster clcultions. In generl, this pproch for crystlline mterils genertes mny of electronic fetures tht re consistent with the cluster models. A two-dimensionl (2D) slb tht is four lyers deep is repeted in supercell geometry. This supercell is derived from the cubic unit cell contining four formul units. The 2 2 supercell model hs eight Ti nd eight C toms in ech lyer (64 toms in totl) for stoichiometric TiC, s shown in Fig. 1. To ddress the influence of C vcncies on the chemicl rectivity of the surfce, the TiC with crbon defects is lso considered. The defective 62 tom supercell with two C toms replced by vcncies cn be represented s nonstoichiometric TiC Only these repeting units re considered in the clcultions. Fig. 2 shows the totl DOS nd the crbon nd titnium contributions to the DOS clculted for TiC. The generl chrcteristics of the bnds, in order of incresing energy, is C 2s ( 21.5 to 23 ev), C 2p ( 11 to 13.5 ev), nd Ti 3d. The covlent nture of the bonding is mnifested by the significnt dmixture of the Ti 3d levels with C 2p orbitls. The Ti 3d levels hve smll mount of electron density in the C 2p bnd below the Fermi level nd thus re little populted. The C 2p levels re completely filled. We present the DOS for n ordered model structure of the crbon deficient TiC 0.94 in Fig. 2b nd discuss the interesting chnges which occur in comprison with stoichiometric TiC. The TiC 0.94 popultes low lying Ti 3d levels tht re formlly unoccupied in TiC. The totl DOS curves re similr, but the Fermi levels re quite different for the Figure 2. () Contributions of () Ti 3d (red line), (b) surfce Ti dz 2 (red line) nd d xz,yz (green line) orbitls to the totl DOS of () TiC(001) nd (b) TiC 0.94(001). The dshed verticl line indictes the Fermi level. two crbides. There is shift of the Fermi level to higher energy in TiC 0.94 compred with TiC. In stoichiometric TiC, the forml chrge of C is 4. The surfce crbon toms re electron-rich, since C is more electronegtive thn Ti. But crbon is neutrl s it is removed. As with C vcncies on the surfce, some metl sttes in the Ti 3d bnd become occupied nd become vilble for surfce-co bonding. The DOS from the TiC 0.94 shows the occupied metl d xz,yz sttes to prticipte in bonding interctions with CO 2π*. This presents significntly different environment for CO to gin electron density from the surfce. A single CO molecule is ssumed to occupy n on top Ti site t the center of the supercell model, resulting in 1/8 coverge with respect to surfce Ti toms. This provides n overll coverge of 1/16 for the perfect surfce nd n dsorbte-dsorbte seprtion of 8.64 Å. The repulsive lterl interction between neighboring CO molecules is not included in this clcultion. The 5σ orbitl of CO hs been stbilized by bout 0.9 ev by bonding to the surfce of either crbide. This is due to the σ-overlp between the CO 5σ orbitl nd the empty Ti 3dz 2 orbitl with which it intercts. As the 3dz 2 levels re unoccupied, only the bonding levels re occupied in the interction of CO with these surfces. Since the 5σ of CO is crbon lone-pir

4 10 De-Bok Kng orbitl, it cn prticipte effectively in σ-donor bonding to the surfce. The other mjor bonding interction for CO dsorption is the π bck-dontion from the metl d xz,yz orbitls to the CO 2π*. These interctions cn be observed in the DOS plots for the metl orbitls nd relevnt CO orbitls in Fig. 3. The d xz,yz -2π* bonding levels in TiC 0.94 re occupied to greter extent nd contribute significntly to the surfce bond of CO, wheres these levels in TiC re just bove the Fermi level nd do not contribute to the CO bonding. The energy stbiliztion of the 2π* orbitls is indictive of stronger surfce-co bond for the CO bound TiC The greter dispersion is lso observed for the 2π* bnd of CO on TiC This dispersion reflects more covlent bonding interction between the surfce Ti 3d xz,yz nd the 2π*. The bck-bonding from Ti 3d xz,yz into the CO 2π* contributes to wekening of the C-O bond, strengthening the Ti-CO bond. This interction cn be followed by lrge Ti- CO overlp popultions nd significnt popultions of the CO 2π* which my be n initil indictor of CO dissocition. The crbon deficient surfce displys n enhnced rectivity towrd CO, which is mnifested in stronger Ti-CO nd weker C-O bonding. The relevnt overlp popultions nd electron densities re presented in Tble 3. A significnt reduction is seen in the C-O overlp popultion reltive to tht of the free molecule. The overlp popultion flls from 1.35 in free CO to 0.43 t this surfce. However, no such π bck-bonding of CO is expected for the TiC becuse the d xz,yz -2π* bonding levels re unoccupied. Thus, the 2π* orbitls of CO re little populted. The smll occuption of the 2π* occurs through the Ti mixing into the crbon 2p bnd. There is no significnt reduction in the overlp popultion of the C-O bond on TiC. The s-dontion from the CO 5s orbitl into surfce Ti dz 2 orbitl does not ffect the bond strength of the molecule. Therefore, the (001) surfce of stoichiometric TiC does not weken the C- O bond significntly. Although no experimentl studies hve been reported for the bonding of CO to the surfce with C defects, the crbon deficient surfce whose metl d orbitls re prtilly occupied is expected to be more ctive for CO dsorption thn is the perfect surfce. The effect of creting crbon vcncies on TiC(001) surfce would be very promising wy to enhnce the ctivity towrd CO of the surfce. Figure 3. DOS with 5σ nd 2π* projections for CO on () TiC (001) nd (b) TiC 0.94(001). The dshed verticl line indictes the Fermi level. CONCLUSION A simple moleculr orbitl frmework ws used to understnd the chemicl ctivity towrd CO of the (001) surfce of TiC. This nlysis provides the detils of interctions of CO with the surfce which demonstrte differences in rectivity due to surfce composition. The effect of removing crbon toms to ctivte the surfce is explored in this study. The metl site of stoichiometric TiC is only cpble of wek σ-dontion bonding by CO due to lck of vilble d electrons for π bck-bonding interctions. The presence of metl d electron density in the TiC(001) with C defects dds greter stbility to the surfce-co bond by bck-bonding interction with CO 2π* nd hence increses n occupncy of the 2π* orbitls. This results in significnt wekening of the C-O bond nd enhnced ctlytic ctivity towrd CO of the titnium crbide. Acknowledgment. Publiction cost of this pper ws supported by the Koren Chemicl Society. REFERENCES 1. () Levy, R. L.; Boudrt, M. J. Science 1973, 181, 547. (b) Hwu, H. H.; Chen, J. G. Chem. Rev. 2005, 105, Oym, S. T. The Chemistry of Trnsition Metl Crbides nd Nitrides; Blckie Acdemic nd Professionl: Glsgow, U.K., Didziulis, S. V.; Butcher, K. D.; Perry, S. S. Inorg. Chem. 2003, 42, 7766.

5 Influence of Crbon Vcncies on CO Chemisorption on TiC(001) Didziulis, S. V.; Frntz, P.; Fernndez-Torres, L. C.; Guenrd, R. L.; El-bjeirmi, O.; Perry, S. S. J. Phys. Chem. B 2001, 105, Whngbo, M.-H.; Hoffmnn, R. J. Am. Chem. Soc. 1978, 100, Whngbo, M.-H.; Hoffmnn, R.; Woodwrd, R. B. Proc. R. Soc. A 1979, 366, Ren, J.; Ling, W.; Whngbo, M.-H. Crystl nd Electronic Structure Anlysis Using CAESAR; For detils, see: http// 8. () Blyholder, G. J. Phys. Chem. 1964, 68, (b) Sillrd, J.-Y.; Hoffmnn, R. J. Am. Chem. Soc. 1984, 106, Mnt, B. P.; Asr, G. G.; Anderson, J. A.; Homs, N.; Rmirez de l Piscin, P.; Rodriguez, S.; Ricrt, J. M.; Ills, F. Surf. Sci. 2013, 613, Asr, G. G.; Feri, L.; Florez, E.; Ricrt, J. M.; Liu, P.; Rodriguez, J. A.; Ills, F. J. Phys. Chem. C 2011, 115,