Surface Science. The structure and reactivity of 2-butanol on Pd(111)

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1 Surfce Science 602 (2008) Contents lists ville t ScienceDirect Surfce Science journl homepge: The structure nd rectivity of 2-utnol on Pd(111) Feng Go, Yilin Wng, Luke Burkholder, Crol Hirschmugl, Dilno K. Sldin, Hin Cheuk Poon, Dvid Sholl c, Jonn Jmes d, Wilfred T. Tysoe, * Deprtment of Chemistry nd Biochemistry nd Lortory for Surfce Studies, University of Wisconsin-Milwukee, Milwukee, WI 53211, USA Deprtment of Physics nd Lortory for Surfce Studies, University of Wisconsin-Milwukee, Milwukee, WI 53211, USA c School of Chemicl nd Biomoleculr Engineering, Georgi Institute of Technology, Atlnt, GA 30332, USA d Deprtment of Chemicl Engineering, Crnegie-Mellon University, Pittsurgh, PA 12345, USA rticle info strct Article history: Received 31 Mrch 2008 Accepted for puliction 6 My 2008 Aville online 16 My 2008 Keywords: 2-Butnol Pd(111) Temperture-progrmmed desorption Low-energy electron diffrction Density functionl theory The structure, formtion nd decomposition pthwys of 2-utoxide species formed on Pd(111) surfce following the dsorption of 2-utnol is studied y comintion of density functionl theory (DFT), nlysis of the low-energy electron intensity versus em energy curves (LEED I/E) nd temperture-progrmmed desorption (TPD). Both DFT clcultions nd LEED I/E mesurements revel tht 2-utoxide dsors with the oxygen tom locted in the three-fold hollow sites on Pd(111) with the C O ond lmost perpendiculr to the surfce with the 2-utyl group in the trns configurtion. At coverges elow 1 monolyers, dsored 2-utoxide species completely thermlly decompose to desor hydrogen nd cron monoxide. The 2-utoxide species present t higher coverges either hydrogente to reform 2-utnol or undergo -hydride elimintion rection to form 2-utnone. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Pd(111) surfces modified y dsored chirl 2-utnol hve een shown to led to the enntioselective chemisorption of propylene oxide. Tht is, pre-covering Pd(111) surfce with either R- or S-2-utnol leds to preferentil dsorption of propylene oxide of the sme chirlity over nrrow 2-utnol coverge rnge [1,2]. This effect ws initilly scried to enntioselectivity induced y chirl 2-utoxide species formed y dehydrogention of 2-utnol [1]. However, it ws found more recently tht this effect ws in fct due to enntiospecific hydrogen-onding interctions etween 2-utnol nd propylene oxide [2]. Mesurements of the vrition in enntiospecificity with 2-utnol exposure suggested tht propylene oxide cn interct either with single dsored 2-utnol molecule or, t higher coverges, with two dsored 2-utnol species to form enntioselective sites [3]. More recently, 2-utnol hs een used s chirl proe for enntioselectivity on surfces modified y n mino cid [4]. Although the formtion of 2-utoxide is not directly relevnt to enntioselective chemisorption descried ove, it is nevertheless importnt to understnd surfce chemistry of 2-utnol (nd other C 4 + lcohols), which hve received much less ttention in the pst thn smller lcohols [5,6]. Reflection sorption infrred spectroscopy (RAIRS) reveled tht the dsored 2-utnol dehydrogented to form 2-utoxide, which underwent -hydride elimintion rection to yield 2-utnone [1]. The work descried elow focuses on the surfce chemistry of 2-utnol in greter detil using temperture-progrmmed desorption (TPD) nd y determining the structure of the 2-utoxide intermedite using low-energy electron diffrction (LEED) mesurements. Compred with X-ry diffrction structure mesurements, which my e modeled y single-scttering theory, direct methods for electron diffrction, strong multiple-scttering prolem, re still very much under development [7]. It is thus customry in determining surfce structures using LEED to clculte experimentl intensity versus em energy (I/E) curves for ll likely dsorte structures nd compre these with the experimentl dt to estlish the correct structure [8]. While this pproch is fesile for dsortes with reltively smll numer of degrees of freedom, it rpidly ecomes prohiitive for lrger molecules such s 2-utnol. In order to ddress this prolem, the 2-utoxide structure on Pd(111) surfce is clculted using density functionl theory (DFT). This provided limited numer of structures, which cn then e compred with the experimentl I/E curves to provide the correct surfce structure. 2. Experimentl methods * Corresponding uthor. Tel.: ; fx: E-mil ddress: wtt@uwm.edu (W.T. Tysoe). Temperture-progrmmed desorption (TPD) dt were collected in n ultrhigh vcuum chmer tht hs een descried /$ - see front mtter Ó 2008 Elsevier B.V. All rights reserved. doi:1016/j.susc

2 F. Go et l. / Surfce Science 602 (2008) in detil elsewhere [9] where desoring species were detected using Dycor qudrupole mss spectrometer plced in line of sight of the smple. The temperture rmp nd dt collection were controlled using LView softwre. All TPD spectr were recorded using heting rte of 6.5 K/s in this study. LEED I/E curves were mesured in low current system with n incident em current of 500 fa using system descried previously [10]. The diffrcted em ws mplified using microchnnel plte nd the position of the mplified electron pulse determined using wire grid system. This llowed full I/E curve to e otined for less thn one electron intercting with ech dsorte molecule. The Pd(111) smple ws clened using stndrd procedure which consisted of heting t 1000 K in Torr of oxygen nd then nneling t 1200 K in vcuo to remove ny remining oxygen. Since strong plldium pek effectively oscures the cron KLL Auger feture, Auger spectroscopy is insensitive to the presence of smll mounts of surfce cron. A sensitive guge of cron coverge ws to sturte the surfce with oxygen nd to perform temperture-progrmmed desorption experiment. The presence of surfce cron is mnifest y the desorption of CO. As the surfce ecomes depleted of cron, the CO yield decreses nd the yield of oxygen increses correspondingly. The complete sence of cron is indicted y the desorption of only O 2. 2-utnol (Aldrich, 99.5%) ws trnsferred to glss ottles nd ttched to the gs-hndling systems of the vcuum chmers nd ws further purified y severl freeze-pump-thw cycles. The clenliness of ll rectnts ws monitored mss spectroscopiclly. 3. Theoreticl methods The I/E curves from the 2-utoxide-covered smple were simulted for norml incidence y ssuming n ordered overlyer of the smllest possile (1 1) periodicity with frctionl occupncy of the sme mgnitude s the coverge H. This ide tht the intensities of integer-order ems my e clculted y considering their interctions with just other integer-order ems my e regrded s specil cse of the em set neglect method [11,12]. The dditionl simplifiction is tht the qusidynmicl [13] tretment of the dlyer llows it to e treted s literlly (1 1) overlyer with n dsorte scttering fctor reduced y fctor of H, so tht the coverge ppers s vrile in the LEED structure determintion [14 18]. Plne wve density functionl theory clcultions were performed using the Vienn initio Simultion Pckge (VASP) nd the ultrsoft pseudopotentils ville in this pckge [19]. VASP hs een shown to give results tht re in greement with other DFT pckges [20,21]. The results reported here re from clcultions with the generlized grdient pproximtion (GGA) using the Perdew Wng 91 functionl [22]. We used Monkhorst Pck [23] k-point smpling of the Brillouin zone nd plne wve expnsion with cutoff of 396 ev. To exmine the structure of isolted 2-utoxide on Pd(111), ll clcultions plced single dsored molecule in (3 3) surfce unit cell. The Pd (111) surfce ws represented y sl four lyers thick with vcuum spcing of 14 Å. The top two lyers of the sl were llowed to relx with the dsored molecule, since the dsorte is expected to exhiit some effect on the sustrte. The DFT-optimized lttice constnt for Pd ws used to define the surfce. This lttice constnt, 3.96 Å, is in good greement with the experimentl vlue of 3.89 Å [24]. All clcultions involved convergence of relxed tomic forces to within 0.03 ev/å nd include dipole corrections [22,25] in the direction norml to the surfce. 4. Results 4.1. Surfce Structure Determintion We hve shown previously tht the structure of disordered overlyers cn e determined from the chnge in the I/E curves of the sustrte (1 1) Brgg spots due to the presence of the overlyer [14 18]. In this cse, the overlyer structure is determined y simulting n ordered overlyer of the smllest possile (1 1) periodicity with frctionl occupncy of the sme mgnitude s the coverge H. This pproch hs een successful in estlishing the structures of numer of surfce species. Comprison of the structures mesured y LEED with those clculted y DFT yielded excellent greement in ll cses. Since it is prohiitively lrge tsk to compre the clculted structures of ll possile configurtions of reltively lrge dsortes such s 2-utoxide species, our pproch is to compre the structure of 2-utoxide clculted using DFT with the experimentl LEED I/E curves to estlish whether the clculted structure is indeed correct. To exmine the structure of isolted 2-utoxide on Pd(111), we initilly plced single dsored molecule in (3 3) surfce unit cell with the O C ond centered over fcc, hcp, top nd ridge sites. DFT clcultions showed tht the 2-utoxide dsors on the surfce with the oxygen tom locted ove the Pd(111) three-fold hollow site with the O C ond oriented pproximtely perpendiculr to the surfce. Other stle sites included those ner the hcp, top nd ridge sites, ut were less stle y 9 ev to 0.36 ev. To ensure tht these molecules were isolted nd tht there were no interctive effects in this study, we lso plced single dsored molecule in (4 3) surfce unit cell with the utyl group oriented long the length of the unit cell. These clcultions showed n identicl onding preference. The 2-utyl group is in the trns configurtion with its plne oriented pproximtely prllel to the surfce. This surfce structure is shown grphiclly in Fig. 1 for surfce coverge of 1/9 (H = 1) nd the intertomic distnces nd ngles re given in Tle 1. We lso exmined the zimuthl rottionl energy rrier using DFT y rotting the molecule y 10 increments through 120. Our clcultions indicte tht the energy rrier to zimuthl rottion is only 0.05 ev, Fig. 1. Schemtic depiction of the structure of 2-utoxide species on Pd(111) surfce with coverge of 1.

3 2266 F. Go et l. / Surfce Science 602 (2008) Tle 1 The structurl properties of 2-utoxide on Pd(111) clculted using density functionl theory 2-Butoxide/Pd(111) from DFT d(o C)/Å 1.45 d(c C)/Å 2, 3, 2 d(c H)/Å 1.10 d(o Pd)/Å 2.16, 2.18, 2.21 h(o C C)/ 109, 110 h(c O ngle to surfce norml)/ 8 suggesting tht dsored molecules cn rotte reltively freely, t lest t low coverges. Initil experiments were crried out to serch for ordered LEED ptterns. While none were found t intermedite exposures, very wek p 3 p 3R30 LEED pttern ws detected t 2-utoxide coverges close to sturtion (following 2-utnol exposure of 1.6 L, see elow). This implies tht the sturtion 2-utoxide coverge should e less thn 0.33 (where coverges re referenced to the plldium tom density on the (111) surfce). The LEED I/E curves were collected t norml incidence for ll spots visile on the LEED screen for surfce exposed to 1.6 L of 2-utnol nd then heted to 200 K to form the 2-utoxide species. This exposure ws selected to void the possiility of dsorption of contminnts from the ckground. There is therefore possiility of inducing rottion of the methyl group round the C 2 C 3 xis due to crowding of the dsored 2-utoxide species. Thus, the Pendry R-fctor [26] ws clculted for the structure determined from DFT (Fig. 1 nd Tle 1) for vrious coverges (H(2-utoxide) = 0.5, 0.3 nd 5) s function of rottion round the C 2 C 3 xis. The results re displyed in Fig. 2 where the Pendry R-fctor is plotted versus C 2 C 3 rottion ngle for H(2-utoxide) = 0.5 (N), 0.3 (d) nd 5 (j). The results show considerle vrition in the R-fctor with C 2 C 3 rottion ngle with the lowest vlues eing found for n ngle etween 250 nd 300. The vlue clculted from DFT is indicted y the verticl line on Fig. 2 nd is in good greement with the vlue otined from the LEED nlysis. The curves lso depend on the coverge used for the LEED clcultions with consistently lower vlues eing otined for H(2-utoxide) = 5, lthough the curve for H(2-utoxide) = 0.3 is not sustntilly worse. These results indicte tht the 2-utoxide structure determined y DFT clcultions is in excellent greement with tht from LEED I/E curves nd tht the sturtion 2-utoxide coverge is 5 0.3, consistent with the oservtion of fint p 3 p 3R30 LEED pttern Temperture-progrmmed desorption Fig. 3 displys series of 74 mu (2-utnol) TPD spectr collected s function of 2-utnol exposure, where 2-utnol ws dsored on the surfce t 100 K nd the heting rte ws set t 6.5 K/s. At 2-utnol exposures of L nd elow, no moleculr desorption ws detected, indicting complete 2-utnol decomposition. However, for exposures of L nd higher, desorption is detected t 285 K with desorption temperture tht decreses with incresing exposures so tht t n exposure of L, corresponding to sturted overlyer, 2-utnol desors t 235 K. An dditionl shrp feture ppering t 140 K for the highest 2-utnol exposure is due to dsorption in the second lyer. Ketone formtion ws detected previously y RAIRS [1]. There re no intense mss spectrometer ionizer frgments tht re dignostic for 2-utnone tht do not lso occur for 2-utnol. However, the 72 mu signl is selected to monitor 2-utnone since the I(72 mu)/i(74 mu) for 2-utnol is 3.0 with the mss spectrometer used for this work so tht 2-utnol contriutions to the 72 mu spectrum cn esily e sutrcted. The 72 mu spectr re plotted in Fig. 4 s function of 2-utnol exposure, where expo Plot of Pendry R-Fctor versus C 2 -C 3 Rottion Angle Pendry R-Fctor C 2 -C 3 Rottion Angle/degrees 74 mu MS Signl /.u. Fig. 2. Plot of the Pendry R-fctor versus the ngle of rottion out the C 2 C 3 ond of 2-utoxide species dsored on Pd(111) surfce using the results of density functionl theory clcultions s input into the LEED nlysis progrm, for H(2-utoxide) = 5 (j), 0.3 (d) nd 0.5 (N). Fig. 3. Temperture-progrmmed desorption dt monitoring 74 mu (2-utnol) following dsorption of 2-utnol on clen Pd(111) t 80 K t heting rte of 6.5 K/s s function of 2-utnol exposure, where exposures re mrked djcent to the corresponding spectrum.

4 F. Go et l. / Surfce Science 602 (2008) MS Signl /.u. 72 mu MS Signl /.u. L L mu 74 mu 2-utnone sures re mrked djcent to the corresponding spectrum. At 2- utnol exposure of L, where no moleculr 2-utnol desorption ws found (Fig. 3), wek 72 mu signl is detected t 290 K nd is ssigned to 2-utnone desorption. At higher 2- utnol exposures, the 72 mu signls re much wider thn the 74 mu signls (Fig. 3) suggesting tht these re not due predominntly due to 2-utnol desorption nd include frgments due to oth 2-utnol nd 2-utnone. It is strightforwrd to perform simple deconvolution to otin the 2-utnone desorption profiles y sutrcting the 2-utnol contriution to the 72 mu signls, nd the results re plotted in Fig. 4. This revels tht the 2-utnone desorption temperture decreses with incresing 2-utnol L Fig. 4. Spectr (): TPD spectr monitoring 72 mu (predominntly 2-utnone) collected following dsorption of 2-utnol on clen Pd(111) t 80 K t heting rte of 6.5 K/s s function of 2-utnol exposure, where exposures re mrked djcent to the corresponding spectrum. The spectr displyed in () show the desorption profiles due to 2-utnone lone fter the contriution due to 2-utnol frgmenttion hs een removed. exposure, indicting tht this is desorption-limited process. Second, t 2-utnol exposure of L, the 2-utnone yield decreses slightly compred with tht found t n exposure of L. This cn e understood y the proposing tht -hydride elimintion is hindered to some extent y crowded surfce. Finlly, it should e pointed out tht 2-utnol hs only very wek 74 mu frgment (only 0.5% of the strongest signl t 45 mu) while 2- utnone hs strong signl t 72 mu (25% of the strongest signl t 43 mu). Assuming the reltive sensitivity rtios of 45 mu signl for 2-utnol nd 43 mu signl for 2-utnone re in the sme order of mgnitude, it is estimted tht only few percent of dsored 2-utnol coverts to 2-utnone. As shown in Fig. 3, no moleculr desorption ws found t 2- utnol exposures of L nd lower nd the dt in Fig. 4 revel tht only limited mount of 2-utnone is generted. Therefore other msses were monitored to serch for other gseous products. Fig. 5 displys numer of msses t 2-utnol exposure of L (prior to monolyer sturtion). Note tht these re collected in severl TPD runs so tht slight line-shpe nd temperture vritions cnnot e voided. Nevertheless, the signl intensities correspond rther well with the frgmenttion of 2-utnol suggesting tht the formtion of hydrocrons nd oxygentes (except 2- utnone), if ny, is elow the detection limit. This rgument is further corroorted y performing similr deconvolution to tht displyed in Fig. 4 with other msses, nd no detectle desorption of other gseous products is reveled. Fig. 5 displys corresponding TPD spectr t 2-utnol exposure of L (fter monolyer sturtion). Agin, there is no indiction of the formtion of other gseous products. The 44 mu profile is lso plotted in this grph, which contins some contriution from ckground CO 2 t low tempertures. It is worth pointing out tht very wek methne desorption ws found etween 300 nd 400 K (dt not shown). Although wek, this desors t higher tempertures thn 2-utnol so tht cn still e esily resolved. The 2-utnol yield (otined y integrting the desorption profiles in Fig. 1) is plotted in Fig. 6 s function of exposure. At 2-utnol exposures of L nd greter, the 2-utnol yield increses lmost linerly with exposure, suggesting constnt sticking proility t ll exposures. The line intersects with the x-xis t 8 L. This suggests tht for exposures greter thn 8 L, even though 2-utnol cn still dsor on the surfce (monolyer sturtion occurs t L), this dditionl 2-utnol does not dissocite nd lso implies tht 45% of the sturted 2-utnol overlyer dissocites. Previous RAIRS investigtions suggest tht 2-utnol first deprotontes to form surfce 2-utoxide species [1], which decomposes y -hydride elimintion rection to form 2-utnone. The ove TPD results revel the formtion of limited mount of 2-utnone nd methne (dt not shown) nd no other hydrocrons or oxygentes. This suggests tht portion of dsored 2-utoxide species undergo complete decomposition. In order to explore this, 2 mu (H 2 ) desorption spectr were collected nd the results re displyed in Fig. 7 s function of 2- utnol exposure, where exposures re mrked djcent to the corresponding spectrum. At the lowest exposure ( L), single hydrogen desorption stte is found centered t 370 K. At higher exposures, in ddition to this desorption stte, nother less intense stte develops t 480 K. Fig. 5 plots the integrted H 2 desorption pek yield s function of exposure. This revels tht the H 2 yield sturtes following n exposure of L. Note tht this corresponds rther well with the dt presented in Fig. 4 indicting tht no 2- utnol desors t exposure lower thn 8. Fig. 8 presents the corresponding CO (28 mu) desorption dt. Severl desorption sttes re pprent. Desorption elow 300 K cn e ssigned to ckground CO dsorption for 2-utnol exposures elow L, nd frgmenttion of 2-utnol, s well s

5 2268 F. Go et l. / Surfce Science 602 (2008) L 2-utnol / Pd(111) 35 x L 2-Butnol / Pd(111) MS Signl /.u. /2 10 mu Butnol Yield 10-7 /.u Butnol Exposure / L L 2-utnol / Pd(111) Fig. 6. Plot of moleculr 2-utnol yield from temperture-progrmmed desorption experiments s function of 2-utnol exposure t 80 K. MS Signl /.u mu Fig. 5. TPD spectr collected t vrious msses following dsorption of 2-utnol on clen Pd(111) t 80 K t heting rte of 6.5 K/s where the detected msses re mrked djcent to the corresponding spectrum for exposures of () nd () L of 2-utnol t 80 K. ckground CO t higher exposures. The intense feture ove 400 K is ssigned to CO desorption due to 2-utnol decomposition, due minly to CO formed y dissocition of 2-utoxide. At the lowest 2-utnol exposure ( L), CO desors t 490 K. This decreses to 475 K t n exposure of L nd remins constnt t 460 K t 2-utnol exposures of L nd ove. Fig. 8 plots the high-temperture CO yield s function of 2-utnol exposure. / Similr to dt shown in Fig. 7, CO yield reches the mximum t n exposure of L. Note tht t 2-utnol exposures of nd L, there is slight decrese in the CO yield. Presumly this is ecuse the extent of dissocition decreses on crowded surfce. 5. Discussion It hs een demonstrted previously using reflection-sorption infrred spectroscopy tht 2-utoxide forms on clen Pd(111) following exposure to 2-utnol t 100 K. The structure of 2-utoxide, clculted y DFT is in excellent greement with structurl determintions from the LEED dt (Fig. 2) yielding sturtion 2-utoxide coverge of ML. The resulting structure is depicted in Fig. 1 nd shows tht the oxygen of the 2-utoxide species is onded t the three-fold hollow site with the C O ond perpendiculr to the surfce. The 2-utyl group is in trns conformtion nd lies lmost prllel to the (111) surfce. RAIRS dt lso revels tht this 2-utoxide species forms 2-utnone vi -hydride elimintion rection. This chemistry is relevnt to the oxidtion ctlysis in which oxygen rects with lcohols on gold-plldium lloys to form ketones implying tht the clen single crystl surfce cn reproduce the chemistry occurring under ctlytic conditions [27]. It is evident, however, tht dsorption on clen plldium results in complete therml decomposition of portion of the dsored 2-utoxide to finlly evolve hydrogen nd cron monoxide (Figs. 7 nd 8). Presumly lloying with gold will decrese the surfce rectivity to inhiit this totl decomposition pthwy. This effect is indeed found for 2-utnol chemistry on Au/Pd(111) lloy surfces [28]. The surfce chemistry cn e divided into to rther distinct regimes. Below 2-utnol exposure of 0.7 L, no 2-utnol desors from the surfce, while over the exposure rnge etween 0 nd 0.7 L the yield of hydrogen (Fig. 7) nd cron dioxide (Fig. 8) increse. Assuming reltively constnt 2-utnol sticking coefficient, sturtion coverge of 5 (Fig. 2) the coverge t n

6 F. Go et l. / Surfce Science 602 (2008) mu MS Signl /.u. 28 mu MS Signl /.u. H 2 Yield /.u. CO Yield /.u Butnol Exposure / L Butnol Exposure / L Fig. 7. () Temperture-progrmmed desorption dt monitoring 2 mu (hydrogen) following dsorption of 2-utnol on clen Pd(111) t 80 K t heting rte of 6.5 K/s s function of 2-utnol exposure, where exposures re mrked djcent to the corresponding spectrum. () displys the totl hydrogen yield, mesured from the TPD spectr displyed in (), s function of 2-utnol exposures. Fig. 8. () Temperture-progrmmed desorption dt monitoring 28 mu (cron monoxide) following dsorption of 2-utnol on clen Pd(111) t 80 K t heting rte of 6.5 K/s s function of 2-utnol exposure, where exposures re mrked djcent to the corresponding spectrum. () displys the totl cron monoxide yield, mesured from the TPD spectr displyed in (), s function of 2-utnol exposures. exposure of 0.7 L is 1. A structure with this 2-utoxide coverge is depicted schemticlly in Fig. 1 indicting tht the 2- utoxide species re rther isolted t this coverge. Addition of further 2-utnol will led to more closely pcked surfce with 2-utoxide species dsored djcent to ech other. At this point, oth 2-utnol (Fig. 3) nd 2-utnone (Fig. 4) desor from the surfce where the yield of 2-utnol increses essentilly linerly with 2-utnol exposure (Fig. 6). The 2-utnol desors etween 285 K nd 235 K depending on coverge (Fig. 3). Previous RAIRS results indicte the formtion of 2-utoxide t 200 K so tht the 2-utnol formtion detected in TPD likely rises due to the

7 2270 F. Go et l. / Surfce Science 602 (2008) rehydrogention of dsored 2-utoxide species. The decrese in pek temperture with incresing coverge could e indictive of second-order desorption kinetics due to surfce rection etween dsored hydrogen nd 2-utoxide species, or repulsive lterl interctions etween dsortes. At low exposures ( L), 2-utnone desors t pproximtely the sme temperture s 2-utnol (Fig. 4), ut t higher coverges desors t lower tempertures. Since -hydride elimintion is first-order process, the decrese in pek desorption temperture in this cse is clerly due to interctions with neighoring (2-utoxide) species. Since crowding of the surfce will likely inhiit dehydrogention rections y limiting ccess of the hydrogen to the surfce, such decrese in desorption temperture with incresing coverge implies tht the ctivtion energy for -hydride elimintion is lowered y interction with neighoring species. Thus, t low coverges, when there re very few djcent 2-utoxide species, there is sufficient spce for the 2-utoxide to completely dehydrogente. As neighoring 2-utoxide sites ecome occupied, totl decomposition is inhiited nd -hydride elimintion occurs with reltively high ctivtion energy. This cn e estimted using the Redhed eqution ssuming tht 2-utnone desorption in Fig. 4 reflects the -hydride elimintion kinetics s 62 kj/mol (tking pek temperture of 250 K nd ssuming pre-exponentil fctor of s 1 [29]). As the coverge increses to sturtion, the pek shifts to 200 K yielding n ctivtion energy of 49 kj/mol, due to lterl interctions. 6. Conclusion 2-utoxide formed on Pd(111) surfce following the dsorption of 2-utnol dsor with the oxygen tom locted in the three-fold hollow sites with the C O ond lmost perpendiculr to the surfce with the 2-utyl group in the trns configurtion. For 2-utoxide coverges less thn 1, dsored 2-utoxide completely thermlly decomposes. It is proposed tht, t such low coverges, the dsored 2-utoxide species re sufficiently isolted to llow them to decompose. At higher coverges, 2- utoxide either hydrogentes to reform 2-utnol or undergoes -hydride elimintion rection to form 2-utnone. It is suggested tht 2-utoxide therml decomposition is inhiited t coverges ove 1 due to surfce crowding. Acknowledgments We grtefully cknowledge support of this work y the US Deprtment of Energy, Division of Chemicl Sciences nd Division of Mterils Sciences nd Engineering, Office of Bsic Energy Sciences, under Grnt numers DE-FG02-00ER15091 nd DE-FG02-84ER-45076, respectively. References [1] D. Stcchiol, L. Burkholder, W.T. Tysoe, J. Am. Chem. Soc. 124 (2002) [2] D. Stcchiol, L. Burkholder, T. Zheng, M. Weinert, W.T. Tysoe, J. Phys. Chem. B 109 (2005) 851. [3] F. Go, Y. Wng, L. Burkholder, W.T. Tysoe, J. Am. Chem. Soc. 129 (2007) [4] F. Go, Y. Wng, W.T. Tysoe, J. Phys. Chem. C. 112 (2008) [5] J.L. Dvis, M.A. Brteu, Surf. Sci. 197 (1988) 123. [6] J.L. Dvis, M.A. Brteu, Surf. Sci. 235 (1988) 235. [7] D.K. Sldin, V.L. Shneerson, J. Phys.: Condens. Mtter, in press. [8] J.B. Pendry, Low Energy Electron Diffrction, Acdemic Press, London, [9] M. Kltchev, W.T. Tysoe, J. Ctl. 196 (2000) 40. [10] H.C. Poon, X.F. Hu, S.E. Chmerlin, D.K. Sldin, C.J. Hirschmugl, Surf. Sci. 600 (2006) [11] M.A. Vn Hove, R. Lin, G.A. Somorji, Phys. Rev. Lett. 51 (1983) 778. [12] D.K. Sldin, J.B. Pendry, M.A. Vn Hove, G.A. Somorji, Phys. Rev. B 31 (1985) [13] N. Bickel, K. Heinz, Surf. Sci. 163 (1985) 435. [14] T. Zheng, W.T. Tysoe, H.C. Poon, D.K. Sldin, Surf. Sci. 543 (2003) 19. [15] H.C. Poon, M. Weinert, D.K. Sldin, D. Stcchiol, T. Zheng, W.T. Tysoe, Phys. Rev. B. 69 (2004) [16] T. Zheng, D. Stcchiol, H.C. Poon, D.K. Sldin, W.T. Tysoe, Surf. Sci. 564 (2004) 71. [17] T. Zheng, D. Stcchiol, D.K. Sldin, J. Jmes, D.S. Sholl, W.T. Tysoe, Surf. Sci. 574 (2005) 166. [18] J. Jmes, D.K. Sldin, T. Zheng, W.T. Tysoe, D.S. Sholl, Ctl. Tody 105 (2005) 74. [19] G. Kresse, H. Hfner, Phys. Rev. B 47 (1993) 558. [20] F. Strrost, E.A. Crter, Surf. Sci. 500 (2002) 323. [21] P.J. Feielmn, B. Hmmer, J.K. Nørskov, F. Wgner, M. Scheffler, R. Stumpf, R. Wtwe, J. Dumesic, J. Phys. Chem. B 105 (2001) [22] J.P. Perdew, J.A. Chevry, S.H. Vosko, K.A. Jckson, M.R. Pederson, D.J. Singh, C. Fiolhis, Phys. Rev. B 46 (1992) [23] J.H. Monkhorst, J.D. Pck, Phys. Rev. B 13 (1976) [24] D.R. Lide, E.H.I.R. Frederikse (Eds.), CRC Hndook of Chemistry nd Physics, 83rd ed., CRC Press, New York, [25] J. Neugeuer, M. Scheffler, Phys. Rev. B 46 (1992) [26] J.B. Pendry, J. Phys. C 13 (1980) 937. [27] D.I. Enche, J.K. Edwrds, P. Lndon, B. Solson-Espriu, A.F. Crley, A.A. Herzing, M. Wtne, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006) 362. [28] F. Go, Y. Wng, Z. Li, O. Furlong, W.T. Tysoe, J. Phys. Chem. C 112 (2008) [29] P.A. Redhed, Vcuum 12 (1962) 203.