CO and O2 interaction with kinked Pt surfaces

CO and O2In e ac ion wi h Kinked P Su aces Fe nando Ga cía-Ma ínez, Elia Tu co, F ede ik Schille , and J. En ique O ega* Ci e This: ACS Ca al. 2024, 14, 6319−6327 Read Online ACCESS Me ics & Mo e A icle Recommenda ions * sı Suppo ing In o ma ion ABSTRACT: We in es iga e he chemical in e ac ion o ca bon monoxide (CO) and oxygen (O2) wi h kink a oms on s eps o pla inum c ys al su aces using a specially designed P cu ed sample. We aim a desc ibing he undamen al s ages o he CO oxida ion eac ion, i.e., CO-co e ed/poisoned s age and O- co e ed/ac i e s age, a he poo ly known kinked P ace s by p obing CO up ake/sa u a ion and O2sa u a ion, espec i ely. Based on he sys ema ic analysis ha he cu ed su ace allows, and using high- esolu ion X- ay pho oemission, a di e si y o e ace and s ep/kink species a e s aigh o wa dly iden i ied and accu a ely quan i ied, de ining a smoo h s uc u al and chemical a ia ion ac oss di e en c ys al planes. In he CO-sa u a ed case, we obse e a p e e en ial adso p ion a s ep edges, whe e he CO co e age eaches a CO molecule pe s ep P a om, signi ican ly highe han hei close-packed analogous s eps wi h s aigh e ace e mina ion. Fo he O-sa u a ed su ace, a signi ican ly highe O co e age is obse ed in kinked planes compa ed o he P (111) su ace. While he s ong adso p ion o CO a he kinked edges poin s owa d a highe igni ion empe a u e o he CO oxida ion a kinks as compa ed o e aces, he la ge O co e age a s eps may lead o an inc eased eac i i y o kinked su aces du ing he ac i e s age o he CO oxida ion. KEYWORDS: CO adso p ion, O2adso p ion, pla inum, cu ed c ys al, kinked su ace, X- ay pho oemission spec oscopy ■INTRODUCTION The s udy o he ca bon monoxide (CO) oxida ion s ands as undamen al in comp ehending su ace ca aly ic eac ions, holding a p ominen place in su ace science esea ch. 1−4 Howe e , mos in es iga ions o da e ha e occu ed unde con olled condi ions, ei he using single c ys als o in ul ahigh acuum (UHV) en i onmen s, de ached om he eal condi ions o indus ial applica ions. Consequen ly, ex ending su ace science indings o eal ca aly ic sys ems, ope a ing a a mosphe ic p essu es (p essu e gap), and in ol ing powde / nanopa icle ca alys s (ma e ials gap), may lead o inaccu acies. 5 New expe imen al app oaches a e needed o allow he mos powe ul echniques o b idge bo h he p essu e gap, such as nea ambien p essu e x- ay pho oemission (NAP-XPS), and he ma e ials gap h ough no el sample designs. In he la e case, i is impo an o ealize ha me allic nanopa icles possess mul iple ace s, making i di icul o ack hei speci ic ac i i y and in e ac ions du ing he chemical eac ion. 6 The e o e, con en ional single-c ys al su aces o e limi ed in o ma ion as hey ep esen only one plane, ailing o mi o he complex, mul i ace ed s uc u e o ac ual ca alys s. 7 One po en ial app oach o b idge he s uc u al gap in ol es he u iliza ion o cylind ical sec o s o single c ys als. Thei cu ed su aces allow o a smoo h ansi ion in he c ys al o ien a ion, enabling a sys ema ic compa ison o a ious ace s unde iden ical eac ion condi ions. Mo eo e , wi h a p ope selec ion o he c ys al sec o , a cu ed su ace also o e s a sophis ica ed bu consis en means o analyzing icinal su aces, hence he e ec o unde coo dina ed, s ep a oms in su ace-ca alyzed eac- ions. 8−11 The combined use o cu ed su aces wi h (NAP-)XPS has success ully demons a ed i s po en ial o s aigh o wa dly assess he ole o s eps in he CO oxida ion on Pd, P , and Rh icinals 12−14 o Ag-oxida ion, 15 among o he ma e s. 16 The cu ed geome y allowed he iden i ica ion o su ace species a di e en eac ion s ages and an accu a e de e mina ion o he igni ion empe a u e ac oss di e en ace s. Su p isingly, species and igni ion empe a u es we e ound he same a A- ype ({100}-o ien ed mic o ace s) and B- ype ({111} mic o ace s) s epped P (111) su aces, 14 by con as o he expec ed A/B asymme ies obse ed in Pd and Rh. 12,13 Ye he ques ion a ises whe he such homogeneous and symme ic beha io in P occu s a beyond he (111) plane o ea u es di e en s ep geome ies in he icini y o he P (111) su ace, such as he mo e open kinked s eps (Figu e 1a). Thei complexi y is Recei ed: Janua y 19, 2024 Re ised: Ma ch 20, 2024 Accep ed: Ma ch 20, 2024 Published: Ap il 10, 2024 Resea ch A iclepubs.acs.o g/acsca alysis © 2024 The Au ho s. Published by Ame ican Chemical Socie y 6319 h ps://doi.o g/10.1021/acsca al.4c00435 ACS Ca al. 2024, 14, 6319−6327 This a icle is licensed unde CC-BY 4.0 Downloaded ia UNIV DEL PAIS VASCO on May 17, 2024 a 12:01:54 (UTC). See h ps://pubs.acs.o g/sha ingguidelines o op ions on how o legi ima ely sha e published a icles. p obably he eason why hey ha e been poo ly in es iga ed wi h XPS in he CO oxida ion con ex , e en unde UHV condi ions. 17,18 No e ha he UHV-XPS s udy is a e y ele an s ep 8,10,11 since i allows o de e mine co e-le el shi s o he a ie y o chemical species ha a ise in he p esence o s eps, he eby p o iding accu a e e e ence spec a o NAP-XPS, which gene ally exhibi s lowe esolu ion and poo e s a is ics. CO and O2adso p ion s udies using UHV-XPS a e abundan o P (111) and i s A/B icinal su aces; hence, he esul ing CO/P and O/P in e aces and adso p ion si es a e well es ablished. They a e ske ched in Figu e 1b,c. Exposing he (111) plane o CO a oom empe a u e (300 K) will ypically gi e ise o a c(4 ×2)-4CO pa e n wi h 0.5 monolaye s (ML, one adso bed molecule pe subs a e su ace a om), 19 while a mo e dense o e laye packing (0.68 ML) can be achie ed when exposing he su ace o CO in NAP condi ions. 20−22 In P (111) close-packed icinals, CO is known o adso b on op si es a he uppe edge (ST) o B- ype s eps. 23−25 In A- ype s eps, in addi ion o he SToccupa ion, a small amoun o CO adso bs a b idge posi ions (SB). 26−29 On he o he hand, exposing he P (111) su ace o O2abo e 77 K leads o he immedia e molecula dissocia ion, wi h O a oms a anged in a p(2 ×2) laye . 30−32 This s uc u e co esponds o 0.25 ML co e age wi h O a oms occupying h ee- old cc hollow si es. 33−35 Highe O co e ages lead o he o ma ion o one-dimensional (1D) oxygen s ipes, e en ually condensing in o a s uc u e whe e ows o P su ace a oms appea linked o ou oxygen a oms each (“4O” P su ace oxide). 36−39 A P -s epped su aces wi h (111) e aces and A/B- ype s eps, he dissocia i e O2 adso p ion and he subsequen chemiso p ion a e a o ed a s eps o e e aces, 40−44 esul ing in o oxygen a oms ancho ed o b idge si es a A- ype s eps, and hollow cc posi ions close o he edge a B- ype s eps. 41,45−47 Finally, a e he sa u a ion o he oxygen chemiso p ion, 1D “4O”-like chains along he s ep edges we e also obse ed o P icinal su aces wi h A/B s eps. 40,48,49 Adso p ion expe imen s on icinal P (111) su aces wi h kinked s eps a e much sca ce. Ea ly wo ks on he CO adso p ion on kinked P su aces epo ed STas he p e e en ial si e, al hough a small amoun o SBmay also be p esen . 17 Fo O2, while i is clea ha O binds s onge on he kinks as compa ed o e aces, he na u e o he adso bed species emains unknown. 50−52 An XPS s udy by Held e al. in P (531) 18 epo ed h ee di e en species a e annealing he su ace in ≈10−7mba O2, i.e., one chemiso bed s a e and wo ypes o oxide clus e s. Using a P cu ed c ys al wi h a special design and UHV-XPS, he e we in es iga e a wide ange o P icinal su aces wi h kinked s eps du ing CO and O sa u a ion a 300 K. They ep esen he wo undamen al s ages o he CO oxida ion eac ion, i.e., he CO-poisoned and he O-ac i e su aces. The sample is a cylind ical sec o o a P c ys al, wi h cylinde axis pa allel o he [112] di ec ion (see Figu e 1 and Me hods in he Suppo ing In o ma ion) and he (645) di ec ion a he cen e o he cu ed su ace [c-P (645) sample]. The e e ence (111) plane is loca ed close o one edge, allowing P icinal su aces o be spanned wi h kinked s eps up o a α= 28° icinal (o il ) angle wi h espec o he (111) plane. The la ge icinal angle ange allows us o each all icinal su aces om he (111) beyond he densely kinked (312) su ace (2-a om-wide e aces) a he opposi e sample edge. Th ough up ake and sa u a ion expe imen s on his sample, we iden i y a a ie y o CO and O species, which can also be quan i ied and discussed in model adso ba e s uc u es, hanks o he sys ema ic app oach ha only he cu ed geome y allows. ■EXPERIMENTAL METHODS Sample Cleaning and Cha ac e iza ion. The c-P (645) sample, ea u ing he (645) plane a i s apex, is desc ibed in Figu e 1. The sample is cleaned wi h A +spu e ing cycles a 1 kV, O2annealing (1 ×10−7mba , 800 K), and inal lashes (950 K) un il no con aminan s a e seen in he XPS spec um. Low- ene gy elec on di ac ion (LEED) was employed o p obe he su ace s uc u e o he cu ed su ace. Di ac ion pa e ns and he P 4 XPS egion o he clean c-P (645) c ys al a e shown in Figu e S1. X- ay Pho oemission Expe imen s and Peak Fi ing. XPS was ca ied ou a he Supe ESCA beamline o Ele a synch o on in T ies e (I aly) 53 in no mal emission geome y. The beam inge p in is ≈20 μm in he e ical di ec ion, i.e., along he pe pendicula di ec ion o he s eps and he cu a u e o he c ys al, allowing p obe o icinal su aces wi hin an accu acy below 0.1°. We obse ed beam damage e ec s du ing a long measu emen (>30 min) in P (111) a e CO sa u a ion, as a small ac ion o CO adso bed a TBdissocia ed owa d “C”. The e o e, we educed ou measu emen ime a he posi ions o he α-scan (<10 min) o minimize beam damage e ec s. The peak i ing was pe o med using he lm i package o Py hon. 54 Figu e 1. c-P (645) sample. (a) Side- iew desc ip ion o he sample, which is a cylind ical P c ys al sec o , wi h cylinde axis pa allel o he [112] di ec ion. The a omic models ske ch he cha ac e is ic (645) and (312) planes, ea u ing kinked s eps and (111) e aces o 5 and 2-a om- owwid h, espec i ely. In (b,c), we ske ch ele an adso p ion geome ies o CO and O a (111) e aces and s epped kinks. Fo CO, TTand TBs and o molecules adso bed in Te ace-Top and B idge posi ions, while ST6, ST8, and SB, espec i ely, e e o he CO adso bed on S ep-Top si es in he i s (six- old-coo dina ed P ), he second (eigh - old-coo dina ed P ) ow, and a S ep-B idge posi ions. Fo oxygen, O cc and OKco espond o O adso bed a h ee- old cc hollow and b idge kink-si es, espec i ely, while “4O” e e s o oxygen a anged in a local P chain s uc u e wi h P O4mo i s (see he main ex ). ACS Ca alysis pubs.acs.o g/acsca alysis Resea ch A icle h ps://doi.o g/10.1021/acsca al.4c00435 ACS Ca al. 2024, 14, 6319−6327 6320 Doniach−Sunjic line shapes 55 con olu ed wi h a Gaussian we e conside ed o he asymme ic peaks (adso bed CO in he C 1s, g aphi ic C, and a omic O), while Voig p o iles we e used o symme ic peaks (adso bed CO in he O 1s, “4O”-like s ipes), oge he wi h a Shi ley- ype backg ound. 56 Small peaks asc ibed o ib a ional exci a ions o CO molecules 57−59 could be dis inguished in he high- esolu ion C 1s spec a o Figu e 3; hence, hey we e included in he i ing p ocedu e. F om he i o he (111) su ace, he sa elli es we e ixed a 220 meV owa d highe binding ene gy o he main con ibu ion, and he in ensi y a io de i ed was 11.5% and 10% o TTand TBsi es, espec i ely. The o he pa ame e s we e cons ain o hose o he majo peak, and hei a ea was added o ha o he main ea u e o quan i ica ion. The in ensi y a io o he TTsa elli e was applied o he ST6 and ST8 sa elli es. In he case o he up ake and deso p ion expe imen s, spec a we e acqui ed conside ably as e han he high- esolu ion spec a. Fo his eason, ST6 + ST8 we e no esol ed; hence, a single peak STwas employed. Sa elli es we e nei he esol ed. Co e age Calib a ion. The co e age o high- esolu ion spec a o Figu e 3 was calib a ed using he c(4 ×2) pa e n obse ed by LEED as a e e ence. As desc ibed in e ., 25,29,60 a ac o ex ac ed om he (111) needs o be applied o CO adso bed in b idge si es o p ope ly accoun o hei co e age. Such a ac o was applied o he in ensi y o TBwhen con e ing peak a ea o co e age. In up ake expe imen s, co e age was calcula ed by assuming sa u a ion o TTsi es in he (111) plane measu emen . In he case o he deso p ion expe imen s, since he pho on ene gy is di e en , we also assumed CO sa u a ion a he beginning o he deso p ion amp a he (111) su ace. O 1s in ensi ies we e calib a ed using he p(2 ×2)-O supe s uc u e obse ed in he (111) plane by LEED. ■RESULTS CO Up ake. In o de o iden i y p e e en ial adso p ion si es, sepa a e CO up ake expe imen s we e pe o med a he (111), (645), and (312) su aces on he c-P (645) sample. These su aces we e exposed o 1 ×10−9mba o CO a 300 K while con inuously eco ding he C 1s pho oemission peak, he la e shown as colo plo s in Figu e 2a−c. In Figu es 2d− , we display indi idual spec a a 0.2 L and close o sa u a ion, oge he wi h hei espec i e peak i analyses, om which peak in ensi ies o each co e-le el line a e de e mined. The la e and he o al C 1s in ensi y a e ep esen ed in Figu e 2g−i o he h ee di e en su aces. As shown in Figu e 2d, in he (111) su ace, and sho ly a e he CO exposu e begins, a peak appea s a 286.7 eV, which belongs o CO ancho ed a op P (111) a oms (Te ace-Top, TT). 60 La e , and a a slowe pace, ano he ea u e de elops a 286.0 eV, which co esponds o CO adso bed a b idge si es (Te ace-B idge, TB). 31,60,61 The e olu ion o hese wo CO species is shown in Figu e 2g. While he TT-CO in ensi y s eadily g ows un il sa u a ion a a ound 0.6 L, he adso p ion a he TBsi es is slowe and app oaches sa u a ion a e he highes dose used he e (3 L o CO). Close o su ace sa u a ion, he a io TT/TBis close o 1. This is expec ed om he c(4 ×2) supe s uc u e and ag ees well wi h a p eceding epo on he CO adso p ion on P (111) a 200 K. 60 Finally, we no e he p esence o a sizable amoun o g aphi ic ca bon (“C”) in Figu e 2d, which becomes esidual in he (645) spec um in Figu e 2e, and comple ely disappea s in he (312) case in Figu e 2 . This poin s o CO e ace species as pa icula ly sensi i e o beam- induced dissocia ion. In ac , exposing he CO-sa u a ed (111) su ace o he beam o app oxima ely 30 min e eals a slow p e e en ial dissocia ion o CO-TB, since CO-TT emains cons an as “C” inc eases and TBdec eases wi h beam exposu e (no shown). The cen e column o Figu e 2 illus a es he up ake expe imen o he kinked (645) plane (α= 9.3°, 5-a om-wide e aces). As seen in Figu e 2e, a peak a 286.4 eV eme ges p io o any o he ea u e. Simila o P icinals wi h A- and B- ype s eps, 25,29 and in ag eemen wi h ea lie epo s on kinked P su aces, 17 we a ibu e his peak o CO adso bed in op posi ions a he kinked edge (S ep-Top, ST). A highe CO exposu es, adso p ion a he e ace TTand TBsi es is obse ed. The co e age e olu ion o Figu e 2h shows how adso p ion a ST si es slows down a a ound 0.15 L o CO, he poin a which adso p ion a TTsi es s a s. As in he P (111) su ace up ake, he occupa ion o TTposi ions is ollowed closely by ha a he TBsi es. TT-CO g ows as e han TB-CO and s ops a a ound 0.5 L o CO. Howe e , TB-CO con inues o inc ease a a qui e educed a e up o 0.7 L o CO. A he highes exposu e o his expe imen (1 L o CO), he su ace is no sa u a ed ye . The CO adso p ion kine ics on he (312) su ace (α= 22.2°, 2-a om-wide e aces) is depic ed in he igh column o Figu e 2. Again, ST-CO a ises immedia ely a e in oducing CO o he chambe , ollowed by adso p ion a TTsi es. As seen in he co e age e olu ion (Figu e 2i), ST-CO sa u a es a a ound 0.6 L o CO, in con as o TT-CO, which does no ye sa u a e a 1 L o CO. The inc ease in he sa u a ion dose o CO adso bed a ST si es is simply explained by i s highe s ep densi y as compa ed o he (645) plane. A he same ime, he e ec i e e ace leng h o he (312) su ace is s ongly educed wi h espec o he (645) plane. Acco dingly, he co e age o TT-CO and TB-CO dec eases om he (645) plane o he (312) plane. A he la e , he co e age o TT-CO is e y small, and TBis no e en obse ed. This ag ees wi h ea ly s udies on he CO adso p ion by elec on ene gy-loss spec oscopy on he e y same su ace 17 and by XPS on he ully kinked P (201) su ace plane (α= 39.2°, 1-a om-wide e aces). 62 A join analysis o he h ee up ake expe imen s allows us o es ablish he CO adso p ion sequence as ST→TT→TB, whe eas he CO deso p ion expe imen s shown in Figu es S2 and S3 o he Suppo ing In o ma ion con i m he expec ed e e se sequence o deso bing species, i.e., TB→TT→ST. The e o e, he impinging CO molecules adso b i s on kinks in he low co e age egime, and hey deso b om kinks only a e he CO om he e aces has almos anished. Such sequence is o impo ance o he CO oxida ion eac ion as i s ac i a ion is igge ed by he deso p ion o CO. In ac , a sligh ly la ge ac i i y o he (111) ace compa ed o o he su aces wi h A- and B-s eps was obse ed be o e he eac ion ligh -o . 14 Fu he mo e, P (111) was ound o igni e ea lie han P (557). 63 As desc ibed by Rempel e al. o he case o he NO educ ion, 64 and ollowing Saba ie ’s p inciple, a s ong adso p ion o eac an s may poison a su ace eac ion. The e o e, ope ando s udies a e needed o add ess he ac i a ion o he CO oxida ion in densely kinked su aces, such as (312) s udied in his wo k. C 1s α-Scan a CO Sa u a ion. To ully cha ac e ize he CO species ac oss he en i e cu ed su ace, we mo e he sample ela i e o he X- ay ligh spo in Δα≈2°s eps and a e sa u a ing he su ace wi h a 10 L o CO exposu e a 300 K. Figu e 3a shows he esul ing C 1s in ensi y map, called α-scan, acqui ed a 90 K o imp o e he ene gy esolu ion. The C 1s α- scan allows us o clea ly isualize he icinal-angle- a ia ion o species deduced om he up ake and deso p ion expe imen s a ACS Ca alysis pubs.acs.o g/acsca alysis Resea ch A icle h ps://doi.o g/10.1021/acsca al.4c00435 ACS Ca al. 2024, 14, 6319−6327 6321 selec ed angles in Figu e 2. The cu ed su ace was also p obed by LEED a e CO sa u a ion, as shown in Figu e S1 o he Suppo ing In o ma ion. A he (111) plane, one ob ains a sha p c(4 ×2) pa e n ha co esponds o he 0.5 ML CO sa u a ion co e age a 300 K. 19 The e o e, we can use he C 1s spec um in he (111) su ace o calib a e he CO co e age ac oss he en i e cu ed sample. The co esponding O 1s da a o he 10 L CO dose, as well as o a lowe dose o 0.25 L, can be ound in Figu e S4 o he Suppo ing In o ma ion. A he (111) su ace, peaks coming om TTand TBa e esol ed a 532.5 and 531.0 eV, espec i ely. Howe e , since he STline is oo close o he TT peak, hese a e e y di icul o esol e. 25 We he e o e eso ed o he C 1s egion in o de o s udy indi idual CO species on kinked su aces. Indi idual C 1s spec a co esponding o he (111), (645), and (312) su aces a e depic ed in Figu e 3b−d. A he (111) plane, TTand TBcomponen s a e nea ly obse ed, as well as he small shoulde s asc ibed o ib a ional exci a ions o CO. 57−59 A he (111) su ace, he a io TT/TBis close o 1, as expec ed om he c(4 ×2) supe s uc u e. 19 In he (645) and (312) planes, he imp o ed esolu ion allows us o un eil a double s uc u e in he kink- ela ed peak, which we asc ibe o CO adso p ion on op o six- old-coo dina ed P a oms a he kink edge (ST6, 286.17 eV) and a he eigh - old-coo dina ed P a oms o he second ow (ST8, 286.34 eV), bo h ske ched in Figu es 1c and 3g. ST8, ST6, and TBpeaks exhibi almos cons an binding ene gy all o e he α-scan, whe eas TTsmoo hly shi s om 286.67 eV a he (111) plane o 286.56 eV a he (312) su ace, likely e lec ing he change in he a e age s ain o P a oms wi hin (111) e aces as hey ge na owe . 8 Finally, he small low-binding ene gy ea u e a 285.8 eV has a simila ene gy and in ensi y in all h ee spec a and all ac oss he α-scan. Such lack o dependence on he s ep-densi y is he main eason o belie e ha his peak is ela ed o CO adso bed a de ec s, 29 disca ding CO ancho ed a e ace-hollow o s ep-b idge si es, wi h simila binding ene gy. 25,29,65 The co e age e olu ion o indi idual CO species as a unc ion o αa sa u a ion is shown in Figu e 3e. The shown da a a e he peak in eg als de e mined om he peak i s pe o med on indi idual spec a om he αscan. The linea a ia ion is cha ac e is ic o he cu ed su ace, 10,13 e lec ing he ac ha he numbe o s ep si es inc ease and e ace si es dec ease linea ly as a unc ion o α. The in ensi ies om he kink-CO peaks ST8 and ST6 ise wi h αa an almos iden ical a e, indica ing a simila occupa ion o he wo si es a he kinked edge, i espec i e o he s ep densi y. In con as , TT- and TB-CO Figu e 2. CO up ake a he (111), (645), and (312) P su aces. (a−c) C 1s colo plo s acqui ed du ing CO dosing a he (111), (645), and (312) su aces on he c-P (645) sample. Expe imen s on he (645) and (312) su aces we e ca ied ou a cons an CO p essu e o 1 ×10−9mba , whe eas o he (111) plane, a p essu e inc ease o 1 ×10−8mba CO was equi ed o speed up he p ocess. (d− ) Cha ac e is ic spec a and i ing lines a low ( op) and high (bo om) CO exposu es o each plane. (g−i) Co e age e olu ion o each CO species as a unc ion o CO exposu e. TT, TB, and ST s and o CO adso bed as Te ace-Top, Te ace-B idge, and S ep-Top si es, as ske ched in Figu e 1. The dashed black lines ep esen he g aphi ic ca bon (“C”) con ibu ion, while he solid black line co esponds o he o al CO co e age. The measu emen s we e pe o med a a pho on ene gy o 370 eV and 300 K. ACS Ca alysis pubs.acs.o g/acsca alysis Resea ch A icle h ps://doi.o g/10.1021/acsca al.4c00435 ACS Ca al. 2024, 14, 6319−6327 6322 dec ease a di e en pace. This beha io is also ound in close- packed A- ype icinal su aces 14 and will be discussed below. F om he co e ages de i ed om he α-scan, we may es ima e he e ec i e size o e aces and kinked s eps, i.e., he po ion o he su ace ha is co e ed by e ace-like (TT, TB) and s ep-like (ST6, ST8) CO species, using he so-called W-model (see e 10 and he Suppo ing In o ma ion o a ull desc ip ion and de ailed analysis). In he W-model, all s ep species a e con ined wi hin a s ipe o wid h Wa ound s ep edges, which emains cons an as he s ep densi y inc eases. As a consequence, he in ensi y o s ep-like species is expec ed o inc ease linea ly as a unc ion o α, while e ace-like species dec ease wi h i as he e aces na ow. The e o e, o he Wmodel, we will conside he o al s ep (ST6 + ST8) and e ace (TT+ TB) con ibu ions, ep esen ed in Figu e 3 , and i hei linea a ia ion, using ΘT0 = 0.5 ML, he sa u a ion co e age a he (111) su ace, as a single i ing pa ame e . The i e u ns he e ec i e s ep size W= 4.3 ± 0.1 Å, ske ched in Figu e 3g, as well as ΘS0= 0.71 ±0.02 ML, i.e., he local co e age wi hin he W- egion. F om he la e , we s aigh o wa dly deduce ha app oxima ely one CO molecule adso bs pe kink a om (see he Suppo ing In o ma ion), i.e., he e is a ull CO occupa ion o he kinked edge a all icinal angles. Full occupa ion a 300 K is also ound in, e.g., Ni(100) icinals, 66 and also in he close-packed P (332) and P (557) su aces bu wi h CO p essu es in he mba ange. 67 We conclude ha kinked s eps may accommoda e a signi ican ly highe amoun o CO as compa ed o close-packed s eps a a educed p essu e. Due o he high CO-co e age a he uppe kink edge, one could expec a la ge inc ease o he o al CO co e age wi h he icinal angle α. Howe e , he unde s ep egion emains CO-deple ed, which compensa es o he high CO co e age a kinks, esul ing in and leads o a sligh inc ease o he o al CO co e age wi h α. The CO-co e ed icinal planes ac oss he c-P (645) sample show no LEED spo s o he han hose a ising om o de ed mona omic s ep a ays, simila o B- ype bu con a y o A- ype P icinals. 29 Howe e , wi h he in o ma ion ex ac ed om he e olu ion o indi idual species o Figu e 3e and he W-model, we may s ill pos ula e he s uc u al adso p ion model o he (645) plane ske ched in Figu e 3g. P ows a e sequen ially illed by CO molecules om he uppe kink edge and owa d he inne e ace, i s in TTsi es and nex in TBposi ions, de eloping a c(4 ×2)-like supe s uc u e. The e o e, CO ancho ing a TTis a o ed o e ha a TB, hence explaining he as e decay o TB wi h αobse ed in Figu e 3e. In eali y, he sequen ial illing om he s ep edge and inside he same e ace p oposed in Figu e 3g does no change he c(4 ×2)-4CO densi y a each e ace, al hough i may sligh ly inc ease he o al α-dependen co e age due o he p oximi y o he i s occupied TTsi e in he lowe e ace. This small co e age inc ease is indeed de ec ed in he o al in ensi y cu e o Figu e 3 . Finally, bo h ΘT(α) and ΘS(α) de ia e om linea i y abo e α> 18°in Figu e 3e. This indica es ha local co e ages, hence adso ba e s uc u es a ound s eps and e aces, a y a densely kinked su aces ha ea u e na ow e aces, such as he (312) plane. O 1s α-Scan a O Sa u a ion. A e he igni ion o he CO oxida ion in P -like me als, he su ace o he ca alys is ypically co e ed by oxygen species ha a e eac i e owa d CO2 p oduc ion. 3 To cha ac e ize he O laye in he P -kinked planes, we sa u a e he c-P (645) sample wi h a 8 L O2dose a 300 K. The co esponding α-scan is shown in Figu e 4a, and selec ed O 1s spec a a ele an su aces appea in Figu e 4b. Measu emen s we e pe o med a 90 K, which led o a small buildup o CO om he esidual gas du ing he acquisi ion o he spec a. A such empe a u es, CO does no eac wi h he oxygen adso bed on P bu a he s icks o he su ace. 68 Howe e , i a simila expe imen is pe o med a 300 K (Figu e Figu e 3. C 1s α-scan a CO sa u a ion. (a) C 1s in ensi y colo plo ac oss he cu ed c-P (645) c ys al a e exposu e o 10 L CO a 300 K. (b−d) Selec ed spec a a he (111), (645), and (312) su aces, espec i ely, oge he wi h hei co esponding i s. Fo each species (TT, TB, ST6, and ST8), highe binding ene gy sa elli es a e added o accoun o ib a ional losses. The small low-binding ene gy peak (black illing) peak has a cons an in ensi y ac oss he cu ed c ys al and is assigned o CO adso bed a de ec s. Pho oemission spec a we e eco ded a 90 K and a a pho on ene gy o 370 eV. (e, ) CO co e age a ia ion o indi idual and o al CO species as a unc ion o α, ex ac ed om he i o indi idual spec a o he α-scan in (a). A small amoun o “C” a ound 284 eV, simila o ha o he de ec s, was also obse ed. (g) CO laye s uc u e a he (645) plane, as de i ed om he da a analysis. A s eps, a sligh shi om pu e a op si es is sugges ed, in o de o minimize epulsion. Lines in ( ) a e i s o da a using he W-model ha ende s he e ec i e size o he kinked s eps W, as indica ed in (g). ACS Ca alysis pubs.acs.o g/acsca alysis Resea ch A icle h ps://doi.o g/10.1021/acsca al.4c00435 ACS Ca al. 2024, 14, 6319−6327 6323 S5 o he Suppo ing In o ma ion), an O co e age educ ion is obse ed as compa ed o he esul s acqui ed a 90 K as he esidual CO emo es some o he oxygen a oms om he su ace by o ming CO2. The O 1s spec um o he (111) plane con ains a single, asymme ic peak a 529.7 eV, which co esponds o chemiso bed a omic O in cc hollow si es (O cc). 68 As con i med by LEED (no shown), O a oms a ange in a 0.25 ML, p(2 ×2)- O s uc u e a he P (111) su ace. 32,69 The e o e, he 1s in ensi y o he O a his c ys al posi ion is used o co e age calib a ion. A he (645) su ace, he in ensi y o O cc shi s om 529.7 o 529.6 eV and eaches i s maximum in ensi y. The binding ene gy posi ion keeps dec easing wi h αup o he (312) plane, exhibi ing a he same ime an in e media e in ensi y be ween hose achie ed a he (645) and (111) ace s. This sugges s ha O a oms ancho ed a h ee- old-coo dina ed su ace cc si es in bo h s ep edges and e aces (see Figu e 1c) con ibu e o he O cc peak, wi h i s binding ene gy shi e lec ing he s ep/ e ace occupa ion a io. In ac , ou da a ag ees wi h he O cc in ensi y inc ease and peak shi obse ed be ween P (111) and P (332), 40 and also wi h ea lie s udies epo ing he adso p ion o O a cc si es in B-s eps. 41,45−47 In addi ion o he p esence o O cc, wo small con ibu ions appea a 528.8 and 530.7 eV in he (645) plane, bo h g owing in in ensi y in he densely s epped (312) plane. The e o e, hese ea u es co espond o di e en oxygen species ancho ed a s eps. The lowe binding ene gy peak a 528.8 eV poin s o he p esence o chemiso bed oxygen. No simila ea u e was obse ed in UHV condi ions o he P (332) and P (553) su aces wi h close-packed B-s eps, 14,40,48 al hough i was de ec ed o he P (223) ace wi h squa e {100} mic o- ace s. 14,48 Since oxygen is epo ed o adso b a b idge si es a {100} s eps, 41,45−47 we he e o e asc ibe his peak o O chemiso bed a he b idge si es o he {100} mic o ace in he kinked s ep (OK). Howe e , OKexhibi s a shi owa d lowe binding ene gy as compa ed o O cc. This would be indica i e o an inc ease in he coo dina ion numbe o he chemiso bed oxygen, 70 esul ing in o adso p ion a he squa e si es o he squa e A-s ep ins ead o b idge si es. We canno con i m his scena io wi h ou expe imen al da a se and ollow he assigna ion o O adso bed a b idge si es. Howe e , gi en he p oximi y be ween squa e and b idge si es a he {100} mic o ace s, we belie e ha OKis likely displaced om a pu e b idge geome y owa d he posi ion o he squa e si e, in a b idge-like con igu a ion. On he o he hand, he 530.7 eV emission is assigned o oxygen si ing in 1D “4O”-like s ipes along he s ep edges. 40,48,49 Such O si e occupa ions occu igh a e he chemiso p ion p ocess. 40,48,49 The small amoun o “4O” implies ha only a small ac ion o he P kinked su aces de elops 1D “4O”-like chains ac oss he kinked edge, in ag eemen wi h obse a ions in P (332), 40 P (553), 48 and P (533). 49 The ene gy o “4O” is close o ha o adso bed OH. 71 Howe e , a peak simila o “4O” was obse ed du ing CO oxida ion a ound 1 mba in close-packed P icinal su aces 14 and in he kinked P (531) a e annealing in O2in UHV. 18 Unde such condi ions, he p esence o OH g oups is unlikely. Ne e heless, we do no disca d a small con ibu ion om OH g oups o he “4O” in ensi y, pa icula ly a 90 K. Bo h he alues o he OKand “4O” a e ske ched in Figu e 1. To gain mo e quan i a i e insigh s in o he di e en adso bed species, we i all indi idual spec a in he α-scan o Figu e 4a. The α-dependen O 1s in ensi y (exp essed in MLs) ob ained o each peak and he o al O co e age is shown in Figu e 4c. As expec ed o s ep-like species, he co e age o bo h he OKand he “4O”-like species g ows linea ly as a unc ion o |α|. In con as , O cc does no exhibi any α-dependen e ace-like dec easing end bu e eals a sha p co e age inc ease om 0.25 o 0.35 ML in he α= 0−10° ange, ollowed by a smoo h dec ease abo e α= 10°. Such nonlinea , α-dependen occupa ion o O cc si es e lec s he double s ep- e ace con ibu ion o he O cc peak discussed abo e. I is in e es ing o no e ha he s eep inc ease in he in ensi y o he O cc peak goes in pa allel wi h he apid a enua ion o he LEED pa e n away om he (111) di ec ion. The la e becomes uzzy and shows no signa u e o s ep-o de ing in icinal planes. This sugges s ha , in he p esence o O a oms, kinked s ep edges oughen, 72 allowing a concen a ion o O cc species (s eps + e aces), highe han 0.25 ML. The o al O co e age (O cc + “4O” + OK) almos doubles om he (111) plane (0.25 ML) o he (312) edge o he sample (0.46 ML) a α= 22°. I is in ac expec ed ha he oxygen co e age o open s epped su aces is la ge as compa ed o he (111) plane upon simila ea - men s, 18,40,44,48,49,73 e lec ing he highe eac i i y o s eps. Finally, he oxygen species obse ed o P (332) 40 and P (531) 18 we e epo ed o be signi ican ly ac i e owa d he CO oxida ion. Gi en ha we ound simila oxygen s ep species Figu e 4. O 1s α-scan sa u a ion o he c-P (645) su ace. (a) O 1s α-scan acqui ed a e O2dosing o 8 L a 300 K and subsequen cooling o 90 K. (b) Indi idual spec a a he (111), (645), and (312) su aces wi h line i ing. O cc, “4O”, and OKs and o chemiso bed O a cc hollow si es, he “4O”-like 1D s ipes along he s ep edges, and chemiso bed O a b idge si es o kinked edges (see he ex and Figu e 1). The high binding ene gy ea u es ( ed illed peaks) co espond o esidual CO, buil up o e he measu ing ime. (c) Co e age a ia ion as a unc ion o α o sepa a e O species and hei sum, as ex ac ed om he i o all indi idual spec a in he α-scan. Expe imen s we e ca ied ou a a pho on ene gy o 650 eV. ACS Ca alysis pubs.acs.o g/acsca alysis Resea ch A icle h ps://doi.o g/10.1021/acsca al.4c00435 ACS Ca al. 2024, 14, 6319−6327 6324 in hese kinked su aces, ou esul s p edic a highe ac i i y o he CO oxida ion a oxygen-co e ed P kinked planes compa ed o ha in he la (111) plane. ■CONCLUSIONS The CO and O2adso p ion on kinked P (111) icinals has been s udied using a cu ed c ys al su ace wi h he P (645) plane (α = 9.3°, 5-a om-wide e aces) in he cen e o he sample, allowing p obe o icinal su aces beyond he densely packed P (312) ace (α= 22.2°, 2-a om-wide e aces). The cu ed su ace allows o a sys ema ic analysis o he a ie y o species obse ed a s eps and e aces upon he sepa a e adso p ion o CO and O2. We demons a e ha CO p edomina ely adso bs a he kinked edges a low co e ages, and only a e he s eps a e nea sa u a ion, he adso p ion a he e aces begins. The con a y was obse ed du ing deso p ion expe imen s, which e eals a highe adso p ion ene gy o CO a kinks as compa ed o (111) e aces. In addi ion, ou da a shows how CO ully sa u a es he kinked edges wi h oughly one molecule pe s ep a om, which may u n in o a s ong poisoning o he s eps du ing CO oxida ion condi ions. O2adso p ion expe imen s e eal a signi ican ly highe oxygen co e age a icinal su aces wi h kinked s eps as compa ed o P (111) e aces. This e ec is pa ly due o s ep-edge oughening and pa ly due o he p esence o h ee addi ional s ep-like oxygen species, i.e., O chemiso bed a b idge-like P si es o he {100} mic o ace and cc posi ions o he {111} mic o ace a he kinks, and oxygen o ming 1D oxide chains, which g ow also along he s eps. The highe co e age and adso p ion ene gy o CO a kinks poin owa d a highe igni ion empe a u e, while he la ge O co e age a kinks may indica e an inc eased eac i i y unde CO oxida ion condi ions. ■ASSOCIATED CONTENT * sı Suppo ing In o ma ion The Suppo ing In o ma ion is a ailable ee o cha ge a h ps://pubs.acs.o g/doi/10.1021/acsca al.4c00435. LEED and P 4 co e le el o he clean and CO-sa u a ed (111), (645), and (312) P su aces; C 1s and O 1s e olu ion du ing CO deso p ion expe imen s a e su ace sa u a ion wi h 10 L CO a he same su aces; O 1s α- scans a 90 K a e a 0.25 and 10 L CO dose, and a 300 K a e 50 L O2dose; and addi ional de ails on he W-model (PDF) ■AUTHOR INFORMATION Co esponding Au ho J. En ique O ega −Cen o de Física de Ma e iales CSIC/UPV- EHU-Ma e ials Physics Cen e , San Sebas ián 20018, Spain; Depa amen o Física Aplicada, Uni e sidad del País Vasco, San Sebas ián 20018, Spain; Donos ia In e na ional Physics Cen e, San Sebas ián 20018, Spain; o cid.o g/0000- 0002-6643-806X; Email: [email p o ec ed] Au ho s Fe nando Ga cía-Ma ínez −Cen o de Física de Ma e iales CSIC/UPV-EHU-Ma e ials Physics Cen e , San Sebas ián 20018, Spain; P esen Add ess: Deu sches Elek onen- Synch o on DESY, No kes aβe 85, Hambu g, 22607, Ge many;o cid.o g/0000-0003-4299-3875 Elia Tu co −Cen o de Física de Ma e iales CSIC/UPV-EHU- Ma e ials Physics Cen e , San Sebas ián 20018, Spain; P esen Add ess: Nano ech@su aces Labo a o y, Empa− Swiss Fede al Labo a o ies o Ma e ials Science and Technology, Uebe lands asse 129, Dubendo , 8600, Swi ze land;o cid.o g/0000-0002-6437-9334 F ede ik Schille −Cen o de Física de Ma e iales CSIC/UPV- EHU-Ma e ials Physics Cen e , San Sebas ián 20018, Spain; o cid.o g/0000-0003-1727-3542 Comple e con ac in o ma ion is a ailable a : h ps://pubs.acs.o g/10.1021/acsca al.4c00435 No es The au ho s decla e no compe ing inancial in e es . ■ACKNOWLEDGMENTS We acknowledge inancial suppo om g an s PID2020- 116093RB-C44 and PID2019-107338RB-C6-3, unded by he Spanish MCIN/AEI/10.13039/501100011033 and by “ERDF A way o making Eu ope”, he Basque Go e nmen (G an IT- 1591-22), and he Dipu ación Fo al de Gipuzkoa (Gipuzkoa Nex p og am). 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