Surface Chemistry of 2Propanol on Clean and Oxygen Precovered Ir(111

Surface Chemistry of 2-Propanol on Clean and Oxygen Precovered Ir(111)

Son Hoang, Ming Pan, and C. Buddie Mullins*Department of Chemical Engineering, Center for Nano- and Molecular Science and Technology, Center forElectrochemistry, and Texas Materials Institute, UniVersity of Texas at Austin, 1 UniVersity Station C0400,Austin, Texas 78712-0231

ReceiVed: July 30, 2009; ReVised Manuscript ReceiVed: NoVember 18, 2009

The decomposition and oxidation of 2-propanol on clean and O-precovered Ir(111) were studied usingtemperature-programmed desorption and molecular beam reactive scattering. On the clean surface, 2-propanolreacts via a 2-propoxide intermediate, followed by �-hydride elimination (the reaction-limiting step), producingacetone and adsorbed hydrogen at 225 K. Reactive scattering measurements show that the acetone productionfrom the reaction of 2-propanol reduces with increasing surface temperature from 333 to 600 K. At 600 K,2-propanol reacts mainly via nonselective decomposition, producing CO, H2, and adsorbed C. No acetoneproduction was observed when a beam of 2-propanol was impinged on the sample at 600 K. We speculatethat at high surface temperature (600 K) the formation of acetone in the η1 (O) binding configuration isinhibited, leaving only η2 (C, O) acetone species, which tend to nonselectively decompose to CO, H2, andCad. On the O-covered Ir(111) surface, the thermal-induced reaction of 2-propanol also occurs via a 2-propoxideintermediate with �-C-H bond activation. The reaction mechanism changes with surface concentration ofoxygen, causing changes in the kinetics of acetone and water evolution. Interestingly, acetone is formedupon impinging a 2-propanol beam on O-Ir(111) at 600 K. We propose that the role of surface oxygenincludes (1) acting as a Brønsted base, (2) acting as a precursor of a nucleophilic attacker in the �-C-Hcleavage step, and (3) altering the surface interaction with the produced acetone.

Introduction

There has been renewed interest in the chemistry of smallalcohols because of their potential application as fuel for directalcohol fuel cells (DAFCs).1–8 The direct methanol fuel cell(DMFC) is the most popular DAFC and on the way to beingcommercialized. However, there are still some problems withthe DMFC, namely, toxicity, slow methanol oxidation reactionkinetics, and methanol crossover.8,9 A number of groups haveinvestigated alternatives for methanol including 2-propanolwhich has been proven to be a viable candidate for replacingmethanol for DAFCs, especially for cells that operate in thelow current density region.2,7 First, 2-propanol is much less toxicthan methanol, and, second, 2-propanol is less prone to crossoverthan methanol. Qui et al. showed that the crossover of2-propanol through the membrane can be seven times smallerthan that for methanol.8 In addition, a 2-propanol cell can havea higher open circuit voltage than a methanol cell. However,Qui and other authors claim that the circuit voltage of a fuelcell using 2-propanol is not stable in the high current densityregion (>200 mA/cm2) due to anode poisoning by the deepoxidation products of 2-propanol and/or acetone buildup.2,8

Additionally, significant effort has been expended regardingthe chemistry of ethanol and heavier alcohols since selectiveoxidation of alcohols to aldehydes and ketones plays animportant role in the chemical industry.10–15 Among heavieralcohols, 2-propanol has received considerable attention becauseit is one of the most common probes for acid-base propertiesof the metal oxide surfaces.16–18 The acid-base properties ofsurfaces would be represented in the dehydration (to propene)versus dehydrogenation (to acetone) selectivity. The selectivity

of the 2-propanol reaction is toward dehydration on acidic oxidesurfaces such as γ-alumina, whereas basic oxides are more activetoward 2-propanol dehydrogenation. We also find interest inthe surface chemistry of 2-propanol on the Ir(111) surfacebecause it can serve as a reverse model of Fischer-Tropschsynthesis on the microscopic scale.17,19,20 Further, the surfacechemistry of 2-propanol has been studied on several transitionmetal surfaces such as Pd(111),21 Pt(111),22 Rh(111),23

Mo(110),20 Ni(100),24 and Au(111).25 The reaction of 2-pro-panol, like other alcohols on those surfaces, often starts withthe cleavage of the O-H bond, producing a 2-propoxide species.The reactivity of the 2-propoxide species, which dependsstrongly on the reactivity of the C-O bond, determines thereaction pathway. On mid- and late-transition metal surfacessuch as Pd(111)21,26 and Pt(111),22 the C-O bond is generallyretained and the reactions follow the dehydrogenation pathway,producing CO, hydrogen, and/or acetone. On the other hand,on early transition metal surfaces, e.g., Mo(110),20 due to astrong interaction between Mo and O, the C-O bond is readilycleaved and the reaction produces propene, propane, hydrogen,and oxygen. It is also well-known that surface oxygen has aninfluence on the selectivity of alcohol conversion on metalsurfaces. As reported for the Pd(111) surface by Barteau et al.,21

the role of oxygen adatoms in the reaction of alcohols include(1) acting as a Brønsted base in the reaction with proton donors;(2) stabilizing surface alkoxides species; (3) altering theinteraction between carbonyl compounds and the surface; (4)nucleophilic oxidation of adsorbed aldehydes to carboxylates;and (5) reacting with surface hydrogen, thus inhibiting thehydrogenation reaction. The reactions of 2-propanol on Au(111)and Rh(111) show the great influence of oxygen adatoms. Gonget al. reported that 2-propanol desorbs intact on clean Au(111);however, it converts to acetone with 100% selectivity on the

* Author to whom correspondence should be addressed. E-mail:[email protected].

J. Phys. Chem. C 2009, 113, 21745–21754 21745

10.1021/jp907324n 2009 American Chemical SocietyPublished on Web 12/03/2009

oxygen-covered surface.25 On the other hand, 2-propanol totallydecomposes into gaseous hydrogen, adsorbed carbon, and carbonmonoxide on clean Rh(111).23 Saturating Rh(111) with oxygenshows great changes in surface reactivity toward dehydroge-nation products with 80% selectivity to acetone production withrespect to combustion.

In order to study how the oxygen concentration and thesurface temperature affect the reaction mechanism of 2-propanolon Ir(111), we started with an investigation of 2-propanolreactions on the clean surface followed by experiments on theoxygen-covered surface. We found that the reaction of 2-pro-panol occurs mainly via selective �-hydride elimination of2-propoxide species on both clean and oxygen-covered Ir(111)surfaces. Surface oxygen alters the reaction kinetics by produc-ing various nucleophilic attackers in the �-C-H bond activationstepsthe rate-limiting step. To further study the roles of oxygenadatoms and surface temperature, we performed reactive scat-tering measurements in which 2-propanol beams were impingedon clean or oxygen-covered Ir(111) surfaces which were heldat constant temperature. The selectivity toward acetone and thereaction probability of 2-propanol decreases significantly on theclean surface with increasing surface temperature. We proposethat high surface temperature inhibits the formation of adsorbedacetone in an end-on, η1 (O) configuration, leaving side-on η2

(C, O) acetone which has a higher binding energy and is proneto nonselectively decompose to Cad, CO, and H2. In contrast,the selectivity and the reaction probability of 2-propanol on theoxygen-covered surface is not strongly affected by the surfacetemperature. We propose that surface oxygen acting as aBrønsted base abstracts H from the hydroxyl group thereforeenhancing the reaction probability of 2-propanol at hightemperature. Moreover, the addition of oxygen to the surfaceincreases the work function of the surface, thereby suppressingthe formation of acetone in an η2 (C, O) configuration andpromoting the formation of η1 (O) acetone species.

Experimental Section

All experiments were conducted in a supersonic molecularbeam scattering apparatus, in which the scattering subchamberis kept under ultrahigh vacuum (base pressure is ∼1.5 × 10-10

torr). A complete description of the apparatus can be foundelsewhere;27–30 however, some details are described here thatare particularly relevant for the current study. In brief, thescattering chamber is equipped with a quadrupole mass spec-trometer (QMS), an Auger electron spectrometer (AES), low-energy electron diffraction optics (LEED), and an ion-sputteringgun. The single-crystalline iridium (111) sample is mounted bytwo 0.5 mm diameter tantalum wires and can be heatedresistively up to 1550 K and cooled down to 77 K in ∼3 minby a liquid nitrogen reservoir. The heating rate can be controlledaccurately by a proportional-integral-derivative (PID) controller.The heating rate used in all experiments is 2 K/s unless notedotherwise. Carbon is the main impurity on the Ir(111) surfaceand was removed using Ar+ sputtering and oxygen adsorption/annealing cycles. The cleanliness of the sample was checkedby reproducible O2 desorption from TPD spectra and theintensity of the C (271 eV) peak in the AES spectra.

2-Propanol (Fisher Scientific, 99.9%), acetone (Fisher Sci-entific, 99.9%), oxygen (Matheson Trigas, 99.99%), hydrogen(MG Industries, 99.99%), (CD3)2CHOH (Sigma Aldrich, 99atom % D), (CH3)2CDOH (Cambridge Isotope Laboratories,98%), and carbon monoxide (Matheson Trigas, 99.998%) wereused without further purification. The purity of all reactants wasconfirmed by mass spectrometry.

Supersonic molecular beams of all species used in TPDexperiments (B beam) were generated by expansion from anozzle with a 1000 µm diameter aperture. The 2-propanol beamsused in molecular beam reactive scattering (MBRS) measure-ments (A beam) were generated by expansion from a differentnozzle with a 203 µm diameter aperture. Both the A beam andB beam were quadruply differentially pumped before impingingon the sample.

The saturation oxygen coverage (0.5 ML) creates a p(2 × 1)overlayer structure on Ir(111). Other coverages of oxygen canbe calibrated with the saturation coverage by comparing theintegrals for oxygen desorption spectra from TPD. Similarly,the coverages of CO were calibrated with the saturation coverageof CO (0.33 ML) by comparing integration of their desorptionspectra. The absolute coverage of 2-propanol was then deter-mined by comparing the Auger intensity of the C (271 eV) peakwith that of a known CO coverage. One advantage of usingAES to measure the absolute coverage of 2-propanol is thatthe AES intensity of carbon (or other elements) does not dependon the chemical bonding of carbon on the surface. In otherwords, AES measures the total concentration of surface carbonregardless of whether the carbon is from 2-propanol, 2-pro-poxide, CO, or C, etc. Therefore, the AES measurement is notaffected by the decomposition induced by the electron beamfrom the Auger spectrometer. Unfortunately, a drawback of themethod is that the electron beam can induce the desorption ofsurface species (i.e., 2-propanol, CO, etc.), leading to uncertain-ties in the measurements. In order to evaluate the uncertaintiesin measurements caused by desorption stimulated by the Augerelectron beam, we performed experiments in which the Augerspectra were taken three times (each measurement was ∼5 minafter the previous) for the same surface covered by 0.3 ML of2-propanol. The C (271 eV)/Ir (171 eV) ratio was 0.387 (0.023. However, the C (271 eV)/Ir (171 eV) ratio was highestfor the third measurement and was lowest for the secondmeasurement. This suggests that the uncertainty caused bydesorption stimulated by the electron beam is smaller than thesystematic error for the Auger technique. Additionally, Seetset al.31–33 employed the Auger technique quantitatively tomeasure adsorption probabilities of carbon-containing molecules(from the adsorption of CH4 on Ir(111) and Ir(110)) with highaccuracy without evidence of desorption of adsorbed carbon orhydrocarbon fragments.

Acetone can be unambiguously distinguished from 2-propanolby monitoring the mass-to-charge ratio of the parent ion by theQMS. For example, the daughter ion m/e ) 43 amu appears inthe cracking patterns of both acetone-d0 ((CH3)2CdO) and2-propanol-d0 ((CH3)2CHOH). Therefore, we monitored theparent ion mass 58 amu to unequivocally distinguish acetone-d0 from 2-propanol-d0 (m/e ) 58 is only ∼1% of the m/e ) 45signal for 2-propanol) even though the signal of ion of mass 43amu is the biggest signal in the cracking pattern of acetone-d0

(four times larger than the intensity of the 58 amu ion). Otherspecies having one or several overlapping cracking fragmentswith acetone or 2-propanol can be ruled out by comparing theratio of the overlap fragment intensity with other mass fragmentintensities.

Results and Discussion

2-Propanol Decomposition on Ir(111). The thermal chem-istry of 2-propanol on the clean Ir(111) surface was studied usingtemperature-programmed desorption (TPD) and reactive scat-tering measurements. On the clean Ir(111) surface, 2-propanoldecomposes via two competitive reaction mechanisms, selective

21746 J. Phys. Chem. C, Vol. 113, No. 52, 2009 Hoang et al.

dehydrogenation and nonselective decomposition. 2-Propanolselectively dehydrogenates to produce acetone and adsorbedhydrogen. This pathway was not observed for the reaction of1-propanol on clean Ir(111) as there is no propanal productionfollowing adsorption of 1-propanol on Ir(111) at 77 K (Sup-porting Information Figure S1). The nonselective decompositionpathway for 2-propanol occurs via carbon-carbon bond cleav-age and produces CO, H2, and hydrocarbon species adsorbedon the surface which in turn decompose into H2 and Cad. Uponcompleting TPD experiments, Cad was detected directly via AESspectra and indirectly via the observation of CO (the oxidationproducts of Cad) in the TPD spectra following the adsorptionof oxygen on the surface. We did not observe other hydrocar-bons or any products other than 2-propanol and acetonedesorbing from the surface during TPD experiments from 77to 1550 K.

2-Propanol TPD spectra following adsorption of variouscoverages of 2-propanol on clean Ir(111) are shown in Figure1. At high coverages of 2-propanol, there are two desorptionfeatures at 150 and 215 K labeled as R and �, respectively, anda higher temperature tail from 225 K up to 325 K. The R peakis assigned to multilayer adsorption on top of the chemisorbedlayer because it grows indefinitely with increasing 2-propanolexposure.

The properties of the � feature are most relevant to the surfacechemistry of 2-propanol. The � feature shows characteristicsof first-order desorption in that the peak temperature remainsconstant with increasing coverage, and therefore we initiallybelieved that this feature corresponded to the molecular de-sorption of 2-propanol. However, the TPD spectra from thecoadsorption of 0.3 ML of (CH3)2CHOD and 1 ML of H onIr(111) at 77 K show that at least some of the 2-propanol fromthe � feature is due to the reformation of 2-propanol from2-propoxide and adsorbed hydrogen: the (CH3)2CHOH (45 amu)signal is 30% of the (CH3)2CHOD (46 amu) signal in the TPDspectra (Figure 2a). Obviously, the (CH3)2CHOH desorption isdue to the bimolecular reaction of (CH3)2CHO intermediatesproduced from the O-D bond cleavage of (CH3)2CHOD andhydrogen adatoms. In addition, as shown in Figures 2a and 2b,the desorption of (CH3)2CHOH and (CH3)2CHOD from thecoadsorption of 0.3 ML of (CH3)2CHOD and 1 ML of H has asimilar peak shape with the desorption of (CH3)2CHOH fromthe adsorption of 0.3 ML of (CH3)2CHOH. Therefore, wepropose that the � feature is due to the recombination of

2-propanol from 2-propoxide and a hydrogen adatom but showscharacteristics of pseudo-first-order desorption. The coverageof 2-propanol corresponding to a saturated � feature is ∼0.3ML. The inset to Figure 1 shows the uptake measurementsobtained from the TPD spectra. Note that in the low-coverageregion, almost all adsorbed 2-propanol decomposes. More detailsregarding the thermally induced surface reactions of 2-propanolwill be discussed later in this paper. In the higher coverageregion, the uptake increases linearly indicating that the stickingprobability is coverage independent.

As mentioned earlier, 2-propanol decomposes via selectivedehydrogenation or nonselective decomposition. No other majorC3 species such as 1-propanol, propaldehyde, acrolein, 2-propen-1-ol, propanoic acid, propene, or propane or C1 or C2 speciessuch as methane, ethane, ethylene, acetaldehyde, or formalde-hyde were observed. Figure 3 shows the TPD spectra of productsfrom the reaction of 2-propanol on clean Ir(111) adsorbed at77 K. Acetone molecules adsorbed on the clean Ir(111) surfacedesorb molecularly between 160 and 220 K with the peak at

Figure 1. TPD spectra following adsorption of various coverages of2-propanol at 77 K. The spectra were acquired at a heating rate of 2K/s.

Figure 2. TPD spectra following the coadsorption of (a, top) 1 ML ofH and 0.3 ML of (CH3)2CHOD and (b, bottom) 1 ML of H and 0.3ML of (CH3)2CHOH at 77 K. The heating rate is 2 K/s. The evolutionof (CH3)2CHOH at 225 K shown in panel a (red curve) is attributed tothe recombination of 2-propoxide ((CH3)2CHOad) intermediate andadsorbed H, therefore corresponding to second-order desorption.However, all three peak shapes in panel a and panel b are similar,suggesting that (CH3)2CHOH evolution in panel b is also attributed tothe reformation of (CH3)2CHOad and Had.

Figure 3. TPD spectra following adsorption of 0.3 ML of 2-propanolat 77 K. The heating rate is 2 K/s. CO mostly comes from thebackground. The coverage of CO from the reaction is estimated to be0.05 ML (after subtracting the contribution from the background).

Surface Chemistry of 2-Propanol on Ir(111) J. Phys. Chem. C, Vol. 113, No. 52, 2009 21747

180 K (Figure 4b). However, acetone produced from theadsorption of 2-propanol desorbs between 200 and 250 K andpeaks at 225 K. This implies that acetone produced from thereaction of 2-propanol on Ir(111) is either stabilized bycoadsorbates and/or that the process is reaction limited. Asshown in Figure 4a, the kinetics for acetone evolution showisotope effects reflecting the smaller amount and higher de-sorption temperature for acetone produced from the TPDreaction of (CH3)2CDOH compared to the reaction of other2-propanol species ((CH3)2CHOD, (CD3)2CHOH, and(CH3)2CHOH). Therefore, we believe that the observed kineticsfor acetone production from the reaction of 2-propanol onIr(111) are due more to reaction limitations than from stabiliza-tion from coadsorbates. H2 desorbs with its peak at 285 K andis ascribed to the recombination of Had on the surface.34 Had

may come from three sources of hydrogen, namely, HR fromthe OH group, H� from the �-C-H (R to the O atom) bond,and Hγ from the γ-C-H (� to the O atom) bond.

The prevalent pathway for acetone production is via O-Hbond activation followed by �-C-H bond activation. In orderto prove this, we employed a strategy similar to that used byXu and Friend in their study of the surface chemistry of2-propanol on Rh(111).23 We conducted TPD experiments inwhich the parent ions of acetone species with masses rangingfrom 58 to 64 amu were monitored following the adsorption of0.3 ML of isotopically labeled 2-propanols, namely,(CH3)2CDOH, (CH3)2CHOD, and (CD3)2CHOH on Ir(111) at77 K. The reactions of (CH3)2CDOH and (CH3)2CHOD produce90% and 95%, respectively, (CH3)2CO (58 amu) of the totalamount of desorbing acetone (58-64 amu) and the reactionsof (CD3)2CHOH produce 98% (CD3)2CO (64 amu) of the totalamount of desorbing acetone (58-64 amu) strongly indicatingthat the O-H and �-C-H bonds are activated. These resultsalso show that there is only very little or no reversible γ-C-Hbond cleavage in selective dehydrogenation. If the γ-C-H bondwere activated, we would expect H-D exchange at the Cγ

resulting in partially deuterated products of (CH3)2CO orpartially hydrogenated products of (CD3)2CO, which were notobserved. A notable amount of (CH3)2CHOH (∼30% of(CH3)2CHOD) was produced at 215 K from the coadsorptionof 0.3 ML of (CH3)2CHOD and 1 ML of H at 77 K, whilethere was only an insignificant amount of (CH3)2CHOH (5%of (CH3)2CDOH) produced at 215 K from the coadsorption of0.3 ML of (CH3)2CDOH and 1 ML of H, indicating that theO-D bond is activated at temperatures below 215 K and the�-C-D bond is activated at temperatures above 215 K. Inaddition, proton exchange water products, HD18O (21 amu) andD2

18O (22 amu), are produced at 150 K and at 230 K from thecoadsorption of H2

18O and (CH3)2CHOD and the coadsorptionof H2

18O and (CH3)2CDOH, respectively. This evidence mightsuggest that O-D bonds are activated at ∼150 K while the�-C-D bonds are activated at 230 K (note that acetone evolvesfrom the reaction of (CH3)2CDOH at 230 K, 5 K higher thanthat of (CH3)2CHOH due to differences in the kinetics of �-C-Hcleavage which is discussed in detail later). We did not observeany 2-propanol products in which one or several H atoms replaceD in (CD3)2CHOH or (CD3)2CO in the TPD spectra of thecoadsorption of H2 and (CD3)2CHOH at 77 K. This evidencefurther confirms no reversible γ-C-H bond cleavage as wasobserved on Rh(111)23 and on surfaces in which the reactionslead to hydrocarbon products such as Mo(110).35

Observation of a kinetic isotope effect in acetone evolution(e.g., from the TPD reaction of (CH3)2CDOH versus(CH3)2CHOH) supports the notion that �-C-H (�-C-D)activation is the rate-limiting step (Figure 4a). As expected, thedesorption feature of acetone from �-C-D cleavage((CH3)2CDOH) is broader and peaks at ∼5 K higher than thatfrom �-C-H activation ((CH3)2CHOD, (CD3)2CHOH, and(CH3)2CHOH)) even though the onsets for acetone desorptionfor all of the various 2-propanols commence at the sametemperature (Figure 4a). If the O-H bond cleavage were therate-limiting step, we would expect differences in the kineticsof acetone evolution produced from the reactions of(CH3)2CHOD and (CH3)2CHOH which were not observed. Inaddition, the reaction of (CH3)2CDOH produced a smalleramount of acetone with respect to the reaction of (CH3)2CHOD,(CD3)2CHOH, and (CH3)2CHOH, reflecting the slower rate of�-C-D cleavage versus �-C-H cleavage.

It has been well established that HR from the hydroxyl groupis abstracted by metal surfaces at low surface temperature,producing alkoxide intermediates (RO-).11,19,23,26,35–39 The re-activity of the alkoxide species will determine the reaction trends

Figure 4. TPD spectra of acetone following adsorption of (a) variousisotope labeling 2-propanol ((CH3)2CDOH, (CH3)2CHOD,(CH3)2CHOH, and (CD3)2CHOH) and (b) various coverage of acetoneat 77 K. The spectra were acquired at a heating rate of 2 K/s. Acetonefound after the adsorption of 2-propanol desorbs at higher temperaturethan acetone on the clean surface. This may be due to a stabilizationof acetone by coadsorbates or because acetone desorption is reactionlimited. In panel a, the acetone evolution following the adsorption of2-propanol on Ir(111) shows isotope kinetic effects reflecting a smallerand shifted toward higher temperature of acetone desorption peak fromTPD reaction of (CH3)2CDOH. This indicates not only that acetonedesorption is reaction limited, but not due the stabilization bycoadsorbates, but also that �-C-H (D) activation is the rate-limitingstep for the 2-propanol reaction.


and therefore the products. Friend et al. showed that on Mo(110), 2-propanol decomposes via cleavage of the C-O bond,followed by γ-C-H bond scission producing propene andpropane.20,35 On the Ir(111) surface, like other late transitionmetal surfaces such as Rh(111)-p(2 × 1)-O,23 Pd(111),21,26,39

Pt(111),22 and Au(111)-O,25 the C-O bond is retained and thereaction occurs via �-hydride elimination producing acetone.

We have discussed the �-hydride elimination reaction path-way above. Next, we begin discussing the nonselective decom-position pathway and finally propose a reaction mechanism for2-propanol on the clean Ir(111) surface. Some adsorbed 2-pro-panol nonselectively decomposes on Ir(111) producing Cad, CO,and H2. Using AES, Cad was detected upon completing the TPDreaction of 2-propanol. The appearance of Cad is furtherconfirmed by the appearance of CO (after subtracting CO fromthe background) in the TPD spectra following the adsorptionof oxygen on the surface left from the TPD reaction of2-propanol. The evolution of H2 from 400 to 800 K alsoconfirms the nonselective decomposition pathway. H2 evolutionfrom 2-propanol decomposition is similar to that shown forethanol decomposition on Ir(111) by Weststrate et al.11 UsingTPD and XPS, Weststrate claimed that H2 evolution at 530 Kis due to CHad decomposition which is produced from nonselec-tive surface reaction of ethanol.11 The same conclusion wasobtained for benzene decomposition on Ir(111).40 Acetone,2-propanol, and CO are the only oxygen-containing specieswhich we observed in TPD spectra following adsorption of2-propanol on Ir(111) at 77 K. We conclude that nonselectivedecomposition is relatively small since there was only a smallamount of CO (0.05 ML) produced from the decomposition of0.3 ML of 2-propanol on clean Ir(111). Clearly, it indicates thatnonselective decomposition is a minor pathway. Moreover, theshift toward higher temperatures (∼3 K) and the smaller amountof CO evolution from the reaction of (CD3)2CHOH comparedwith those from the reaction of (CH3)2CHOH and (CH3)2CHODreflect the slower rate of γ-C-D activation compared to γ-C-Hactivation for nonselective decomposition. The appearance ofD2 (4 amu) desorption features at 290 and 310 K in thetemperature-programmed reaction of (CH3)2CDOH and(CD3)2CHOH on Ir(111), respectively, shows that some �-C-Dand γ-C-D bond cleavage occurs at 290-310 K and thekinetics of �-C-D cleavage is faster than that of γ-C-Dcleavage. The fact that the �-C-H bond is more labile than theγ-C-H bond is reasoned by the withdrawal of electron densityfrom the carbon bound to oxygen by oxygen (high electrone-gativity). We propose a reaction mechanism of 2-propanol onclean Ir(111) as shown in the scheme to the right.

2-Propanol Oxidation on Oxygen-Covered Ir(111). Onoxygen-covered Ir(111), like the clean surface, 2-propanol alsodehydrogenates, forming acetone together with water, hydrogen,carbon monoxide, and carbon dioxide (at high oxygen cover-ages). However, we did not observe any oxygen evolution inthe TPD spectra from 77 to 1550 K following the coadsorptionof 0.3 ML of 2-propanol and coverages of oxygen from θO )0.12 to 0.5 ML. Additionally, no other major C3 species suchas 1-propanol, propaldehyde, acrolein, 2-propen-1-ol, propanoicacid, propene, or propane or C1 or C2 species such as methane,ethane, ethylene, acetaldehyde, or formaldehyde were observed.

Figure 5 shows the acetone TPD spectra following adsorptionof 0.3 ML of 2-propanol on the Ir(111) surface precovered withoxygen (θO ) 0-0.5 ML). The acetone desorption temperaturepeak shifts to lower temperature (180 K) with increasing oxygencoverage and then shifts back to 200 K when oxygen saturatesthe surface suggesting that the reaction mechanism may depend

on the surface concentration of oxygen. Interestingly, nearlythe same amount of 2-propanol reacts on Ir(111) for variousoxygen precoverages, as suggested by the TPD desorptionfeatures with θO ) 0.12-0.34 ML (data not shown) for whichthe maximum acetone yield occurs at θO ) 0.12 ML. Saturatingthe surface with oxygen (θO ) 0.5 ML) results in a decrease inthe surface reactivity, reflected in the minimum for 2-propanolreactivity (maximum of 2-propanol desorption) and a minimumin the acetone yield. We believe that oxygen modifies the surfacereactivity by producing various nucleophilic attackers (Oad andOHad) in the �-hydride elimination step. More details regardingthe reaction mechanism for each oxygen coverage will bediscussed in the next section.

We have also investigated reversible C-O bond cleavage byprobing oxygen exchange between a 2-propanol molecule or2-propoxide and the oxygen-covered surface. We created asurface with adsorbed surface 18O via several cycles of adsorp-tion/annealing of H2

18O and 16Oa. The first cycle of coadsorption

Figure 5. TPD spectra of acetone following adsorption of 2-propanolon oxygen-covered Ir(111) surface at 77 K with various oxygencoverages.


of H218O and 16Oa produces 18OH and 16OH. All adsorbed

hydroxyl groups reform to water, which desorbs off the surfaceupon heating to 400 K, leaving one oxygen atom, 18O or 16O,with a statistical distribution. Thus, we can enrich the 18O surfaceconcentration by several cycles of adsorption/annealing of H2

18Oafter the initial cycle of coadsorption/annealing of H2

18O and16Oa. After 3 cycles we obtain a surface covered with 0.085ML of 18O and 0.068 ML of 16O. Following the adsorption of(CH3)2CHOH at 77 K on the 18O-enriched surface (created asdescribed immediately above), no (CH3)2CH18OH (47 amu) or(CH3)2Cd18O (60 amu) were detected in the TPD spectra,indicating no oxygen exchange and no reversible C-O bondcleavage.

Although the reaction mechanisms are different for surfaceswith different precoverages of atomic oxygen, the �-hydrideelimination step is still the rate-limiting step for all surfaceconcentrations of oxygen. Additionally, the desorption peak foracetone evolution from oxygen-covered Ir(111) from �-C-Dcleavage ((CH3)2CDOH) is always ∼5-7 K higher than thatfrom �-C-H cleavage ((CH3)2CHOH, (CH3)2CHOD, and(CD3)2CHOH).

θO ) 0.12 ML. Figure 6a shows TPD spectra following theadsorption of 0.3 ML of 2-propanol on Ir(111) precovered by0.12 ML of O. Evolving acetone has two desorption featureswith peaks at 185 and 225 K. Water desorbs with a sharp peakat 180 K. Interestingly, CO evolution is almost the same asfrom the reaction on the clean surface suggesting that little2-propanol is combusting to create additional CO. The reactionsof isotopically labeled 2-propanol such as (CH3)2CDOH,

(CH3)2CHOD, and (CD3)2CHOD on this surface confirm thathydrogen in this water production comes primarily from HR

(Supporting Information Figure S3). The reaction of(CH3)2CDOH produces 85% H2O, 5% HDO, and 10% D2O,indicating no or little H� (D) in desorbing water. Indeed, H�

(D) adatoms recombine with another hydrogen adatom, H orD, forming HD (3 amu) and D2 (4 amu), respectively, whichdesorb from the surface at 290 K. No D2O or HDO wasproduced from the reaction of (CD3)2CHOH, indicating littleor no Hγ in desorbing water. In addition, the water productiondistribution 60% D2O, 35% HDO, and 5% H2O from thereaction of (CH3)2CHOD further confirms the assumption. Thesharp water evolution feature at 185 K leads us to the conclusionthat water formation is not via OH reformation because onIr(111), OH groups (produced by coadsorption of H2O and O2)reform at 220 K. Based on those observations, we propose areaction mechanism for low surface oxygen concentration which,in the initial stage, involves an O adatom abstracting HR from2-propanol to form OHad and the 2-propoxide intermediate at150 K. Then, OHad abstracts another HR atom from another2-propanol molecule forming 2-propoxide (step 3) or the H�

atom of 2-propoxide forming acetone which desorbs at 180 K.Barteau and Madix reported the participation of surface OHgroups in the reaction with deuterated methanol (CH3OD) toproduce methoxide intermediate on Ag(110).41 In both cases,water is produced and readily desorbs from the surface at 180K. Some of the 2-propoxide species are �-dehydrogenated ason the clean surface to produce acetone that evolves at 225 K(see reaction scheme at top of next page).

Figure 6. TPD spectra following adsorption of 15 s 2-propanol on Ir(111) precovered by (a) 0.12 ML of O, (b) 0.23 ML of O, (c) 0.34 ML of O,and (d) 0.5 ML of O at 77 K. The heating rate is 2 K/s.


θO ) 0.23-0.34 ML. The high-temperature desorption featureof acetone at 225 K disappears with increasing θO from 0.12 to0.23 ML (Figures 6b and 6c). The lower temperature featureshifts to 180 K with θO ) 0.23 ML and then shifts back towardhigh temperature (∼190 K) with θO ) 0.34 ML (Figures 6band 6c). Water desorbs in a broad feature with a “bump” at250 K which may be due to γ-C-H bond cleavage. The HDOand D2O bumps appear in the TPD reaction of (CD3)2CHOHbut do not appear in the TPD reaction of (CH3)2CHOD. Thereis also a bump in the H2O desorption at ∼250 K in the reactionof (CH3)2CDOH (Supporting Information Figure S4).

Analysis of the desorption of water suggests that there is directparticipiation of Oad in the �-hydride elimination process. Infact, water can come from three sources: (1) produced fromthe direct oxidation reaction of 2-propanol and/or 2-propoxide,(2) recombination of Oad and Had produced from the directhydride elimination by the Ir(111) surface, and (3) reformationfrom OHad groups produced from the O-H cleavage and/or thehydride elimination by Oad. The water desorption featurecommences at ∼190 K, peaks at 220 K, and stops at 260 K,well above the temperature at which acetone is produced (from150 to 225 K). Water evolves with a peak at 180 K on theclean Ir(111) surface. Therefore, H2O is not a product from thedirect 2-propanol or 2-propoxide oxidation reaction. Addition-ally, no water was observed following the coadsorption of H2

and O2 on Ir(111) at 77 K (Supporting Information Figure S3),suggesting that there is no water produced from Oad and Had

produced from the direct hydride elimination by Ir(111).Therefore, water appears to come from the reformation ofhydroxyl groups. Moreover, more HDO (up to 42% of the totaldesorbing water) and D2O (up to 15% of the total desorbingwater) are produced from the reaction of (CH3)2CDOH, indicat-ing the participation of H� in making water (note that there isnot much H� involved in producing water for a low surfaceconcentration of oxygen as mentioned in the previous section).We propose a reaction mechanism in which Oad directlyparticipates in the �-hydride elimination step as follows:

θO ) 0.5 ML. The Ir(111) surface is saturated at θO ) 0.5ML, creating a p(2 × 1) overlayer structure. In fact, Ir(111)-p(1× 2)-O is the least reactive surface of all oxygen-coveredIr(111) surfaces, as reflected in the smallest amount of 2-pro-panol reacting (maximum amount of 2-propanol desorption).In addition, this surface produces the largest amount ofcombustion products, namely, CO and interestingly CO2 (Figure6d). Indeed, no CO2 was produced from the reaction of2-propanol on Ir(111) with θO ) 0.12-0.34 ML. Additionally,

for the oxygen-saturated Ir(111) surface, a new sharp featurein acetone desorption appears at 205 K. On the contrary, waterdesorbs in multiple features that overlap each other, starting at190 K. Unlike Rh(111),23 the addition of O to the Ir(111) surfacedoes not inhibit γ-C-H bond scission. The reaction of(CD3)2CHOH on oxygen-saturated surface produced even moreHDO and D2O than on clean and oxygen-unsaturated surfaces.

On oxygen-saturated Ir(111), Weststrate reported the forma-tion of acetate and formate from insertion of oxygen to ethoxide(CH3CH2Oad) and methoxide (CH3Oad), respectively.11,38 Friendand her co-workers also claimed that oxygen insertion into2-propoxide formed a bioxygen bridge structure, (CH3)2COOad,in which the molecule binds to the surface via two �-C-Obridges on Rh(111)-p(2 × 1)-O.23 We propose that the samestructure forms on Ir(111), leading to an acetone desorptionfeature at 205 K by the scission of one �-C-O bond. Carbondioxide which appears at 430 K may be due to the completeoxidation of carbon-containing species such as CxHy and Cad

by oxygen on the surface or the decomposition (CH3)2COOad

remaining on the surface (see scheme at top of next page).Reactive Scattering Measurements. Reactive scattering

experiments were used to further study thermally inducedchemistry of 2-propanol on clean and oxygen-covered Ir(111).

Figure 7. Reactive scattering measurements (MBRS) of 2-propanolon the clean Ir(111) surface. Panels a and b show the MBRS of2-propanol on the clean surface at 333 and 600 K, respectively. In theseexperiments, a continuous beam of 2-propanol with the flux of 9.8 ×1012 molecules cm-2 s-1 was impinged on the clean surface which waskept at the desire temperature (i.e., 333 and 600 K). For more clarity,a blank experiment for each measurement is shown from t ) 10 to20 s in which 2-propanol impinges on an inert flag in front of the Ir(111)sample. The movement of flag caused spikes in CO and H2 signals att ) 25 s.


Figures 7a and 7b display the formation of acetone and H2 inreactive scattering experiments of 2-propanol at 333 and 600K, respectively, on the clean Ir(111) surface. In these experi-ments, 2-propanol beams with a flux of ∼ 9.8 × 1012 moleculescm-2 s-1 were impinged on the sample after the sample washeated to and held at 333 and 600 K, respectively. For moreclarity, blank experiments are shown from t ) 10 to 20 s inwhich a 2-propanol beam is impinged on an inert flag in frontof the Ir(111) sample.

At 333 K, acetone was produced together with H2 when thebeam impinges on the Ir(111) surface. The formation ratereaches a maximum and decreases slowly due to the loss ofactive sites by carbon adsorbing on the surface due to nonselec-tive partial decomposition of impinging 2-propanol. The reactionprobability of 2-propanol and the maximum rate of acetoneproduction decrease with increasing surface temperature from333 to 600 K (data not shown).

At 600 K, little to no acetone production was observed;however, significant H2 was still produced. This indicates thatat high temperature 2-propanol tends to nonselectively decom-pose (combustion pathway) rather than selectively dehydroge-nate. 2-Propanol can decompose via two general pathways: (1)a more direct decomposition (e.g., γ-C-H bond cleavage, etc.)and (2) decomposition of acetone intermediate species; however,our data do not allow discrimination between the two pathways.Only a very small portion of the impinging 2-propanol beamdecomposes since, as shown in Figure 7b, the m/e ) 45 signalfrom scattering off of the Ir(111) sample is roughly the sameas from scattering from the inert flag. Indeed, nearly all2-propanol molecules are trapped at this low impingement energy(∼2.3 kcal/mol) and then desorb off the surface due to the highsurface temperature which kinetically favors desorption.31–33,42–50

2-Propanol molecules that chemisorb on the surface may undergodecomposition via 2-propoxide and acetone intermediate species.The combustion products produced at high surface temperaturemay result from direct decomposition of 2-propanol or changesin the binding configuration of adsorbed acetone produced fromthe 2-propoxide intermediate on the surface. Acetone binds tometal surfaces in two configurations η1 (O) and η2 (C, O).36,51,52

The η1 (O) configuration is created via the donation of the lonepair of electrons on the O atom. The η2 (C, O) configuration,involving both C and O, has a higher binding energy and tendsto nonselectively decompose.22 Using RAIRS and EELS to studythe surface chemistry of 2-propanol on Pt(111), Vannice andco-workers22 claimed that flashing to a surface temperature of200 K leaves only strongly bound η2 (C, O) acetone. Perhapsat high temperature acetone which is produced from 2-propanoldehydrogenation binds to the Ir(111) surface predominantly viathe η2 (C, O) configuration, leading to increased nonselectivedecomposition. Additionally, as mentioned above, multiplepathways for combustion are possible (e.g., γ-C-H bondcleavage, etc.), and our data do not allow us to distinguishbetween the various mechanisms.

Figure 8a shows the reactive scattering measurements of2-propanol on oxygen-saturated Ir(111) at 333 K. In the acetonespectra, bimodal features appear which are attributed to the

reaction of 2-propanol and surface oxygen (t ) 40-50 s) andthe dehydrogenation of 2-propanol by the oxygen-depletedsurface (from t ∼ 50 s). From t ) 40 to 50 s, 2-propanolmolecules impinging on the oxygen-covered surface react withoxygen adatoms, producing acetone and H2O, and note that nohydrogen is produced during this period. The reaction ratereaches a maximum at t ∼ 45 s and decreases due the depletionof oxygen. From t ∼ 50 s, when almost all surface oxygen isconsumed, 2-propanol molecules impinging on the surface,which can now be considered as a nearly “clean surface”, aredehydrogenated by the surface to produce acetone and H2 (whichstarts desorbing at t ∼ 55 s) with a similar mechanism to thatwhich occurs on the clean Ir(111) surface at 333 K (shown inFigure 7a).

Similarly, at 600 K H2 is produced and starts to desorb at t∼ 55 s when H2O evolution decreases. In the period of t )40-50 s, the reaction occurs at 600 K in the same way as itdoes at 333 K with almost the same amount of 2-propanolreacting and the same amount of acetone and water produced(Figure 8b). Here (i.e., Figure 8b) a second peak in the acetone

Figure 8. Reactive scattering measurements (MBRS) of 2-propanolon the oxygen-saturated Ir(111) surface. Panel a and b display theMBRS of 2-propanol on the oxygen-saturated surface at 333 and 600K, respectively. In these experiments, the surface was saturated withoxygen by adsorbing molecular oxygen at 77 K. The sample is thenheated to and held at the desired temperature (i.e., 333 and 600 K)while being impinged by a beam of 2-propanol with the same flux asused in experiments on the clean surface (9.8 × 1012 molecules cm-2

s-1). For more clarity, a blank experiment for each measurement isshown from t ) 10 to 20 s in which 2-propanol impinges on an inertflag in front of the Ir(111) sample. The movement of the flag causedspikes in CO and H2 signals at t ) 25 s.


spectra is not observed as in Figure 8a. We believe that from t∼ 50 s, after all of the oxygen has been consumed, the surfacereaction of 2-propanol occurs via decomposition of intermediatespecies just as on the clean surface at 600 K in Figure 7b (i.e.,no acetone is produced).

As discussed above, the reaction pathway on the oxygen-covered surface is relatively unaffected by the increase in thesurface temperature, in contrast to the reaction on the cleansurface. We believe that surface oxygen plays two roles here.First, it acts as a Brønsted base, by abstracting a hydrogen atomfrom the hydroxyl group. Therefore, the reaction probability of2-propanol at 600 K is much larger compared to the cleansurface at 600 K. The second role of surface oxygen is topromote the production of η1(O)-acetone which is prone todesorb without decomposition while suppressing the productionof acetone in the η2 (C, O) configuration which tends todecompose. The roles of surface temperature and oxygenadatoms in the surface chemistry of 2-propanol will be furtherdiscussed in the following section.

Further Discussion. The reaction of 2-propanol on clean andoxygen-covered Ir(111) primarily occurs via O-H bond and�-C-H bond cleavage to produce acetone, H2, and H2O (onthe oxygen-covered surface). The reaction pathway changes withoxygen coverage although �-C-H bond activation is still therate-limiting step. The role of oxygen adatoms has been studiedon several surfaces including Pd(111),21,26,39 Au(111),25

Rh(111),23 and Ni(110).24 Davis and Barteau21 concluded thatthe role of oxygen adatoms in the oxidation of alcohols onPd(111) included (1) acting as a Brønsted bases in direct reactionwith proton donors; (2) acting as a stabilizer for surface alkoxidespecies; (3) altering carbonyl compound-surface interactions;(4) nucleophilic oxidation of adsorbed aldehydes; and (5)preventing surface hydrogen from participating in hydrogenationreactions. On clean Rh(111),23 Xu and Friend found that2-propanol decomposes to CO, H2, and surface C with no otherproducts detected. On the contrary, on Rh(111)-p(2 × 1)-O,adsorbed oxygen inhibits nonselective C-H bond scission andpromotes �-C-H bond cleavage since the �-C-H bond is theweakest bond of all C-H bonds in 2-propanol and 2-propoxide.2-Propanol interacts weakly with clean Au(111) and desorbsintact after adsorption on the surface at 77 K.25 Surface oxygenon Au(111) participates in both O-H bond and �-C-H bondactivation steps, leading to acetone with ∼100% selectivity. Byusing isotopically labeled molecules in our study of the surfacechemistry of 2-propanol on Ir(111), we were able to show thatsurface oxygen acts as a nucleophilic oxidant and as a Brønstedbase. We propose that changing oxygen coverage leads tovarious nucleophilic attackers (Oad or OHad) in the �-C-Hactivation step. Therefore, we believe that tuning the reactivityof the surface can be done by controlling the surface concentra-tion of oxygen. The reactive scattering measurements of2-propanol on clean and oxygen precovered Ir(111) confirmsthe role of the oxygen adatom as a Brønsted base. The reactionprobability of 2-propanol on the clean surface decreases withincreasing surface temperature. Surface oxygen reacts with HR

from the OH group of 2-propanol as a Brønsted base, thusincreasing the reaction probability of 2-propanol at hightemperature.

On the clean surface, high surface temperature inhibits theformation of acetone in the η1(O) binding configuration, leavingthe stronger binding configuration η2(C, O) which leads to morenonselective decomposition products. Using EELS to study thebinding configuration of acetone on Ru(001), Anton et al.53

claimed that upon heating the surface the η1(O) acetone species

desorbs molecularly; however, the η2(C,O) acetone species hasa stronger binding energy and is an intermediate for dissociation.The same phenomenon was observed on Pt(111)36,52 by Vanniceand Avery and on Rh(111)51 by Houtman and Barteau. Onoxygen-covered Ir(111), the selective dehydrogenation products,i.e., acetone, H2, and H2O, are produced even at high temperature(600 K). This may be explained by the addition of electroneg-ative oxygen adatoms which modify the electronic structure ofthe surface, resulting in the shifts in binding configuration ofproduced acetone from η2(C, O) to η1(O). Anton et al.53

proposed that surface oxygen on metal surfaces increases thework function, thus increasing the energy gap between theoccupied d band level of metals and the π*CO acceptor level ofacetone. As a consequence, the back-donation of the electronsfor back-bonding into the π* orbitals of the carbonyl bound inthe η2(C, O) configuration is more difficult. On the other hand,the addition of oxygen promotes surface-acceptor propertiestoward electrons from the lone pair on the carbonyl, leading toformation of the η1(O) configuration.

Conclusions

The surface chemistry of 2-propanol on the clean and oxygen-covered Ir(111) surfaces was studied using TPD and MBRS.Most adsorbed 2-propanol undergoes O-H bond cleavage at150 K and �-C-H bond cleavage to form acetone, H2, and H2O(on the oxygen-covered surface). Isotopic labeling experimentsreveal that �-hydride elimination is the rate-limiting step forall surface concentrations of oxygen, including the clean surface,although different nucleophilic attackers in the �-C-H bondactivation step alter the reaction mechanism.

Some adsorbed 2-propanol follows a nonselective decomposi-tion pathway to produce CO, H2, and surface carbon, which islikely via decomposition of the acetone product in a η2(C, O)configuration. On the clean surface, the sticking probability of2-propanol is very low at high temperature (600 K) and2-propoxide intermediate conversion to acetone in a η2(C, O)configuration is favored at this temperature, leading to combus-tion products. The MBRS of 2-propanol on Ir(111)-p(2 ×1)-O at high temperature shows that oxygen adatoms promoteselective dehydrogenation production of acetone, H2, and H2Oby modifying the electronic structure of the surface, thusenhancing acetone production from a η1(O) configuration andminimizing acetone production from the η2(C, O) configuration.

Acknowledgment. The authors thank the Welch Foundation(F-1436), Department of Energy (DE-FG02-04ER15587), andNational Science Foundation (CTS-0553243) for their generousfinancial support. S.H. acknowledges the Vietnam EducationFoundation (VEF) for support via a graduate fellowship.

Supporting Information Available: TPD spectra followingadsorption of 0.3 ML of 1-propanol on clean Ir(111) at 77 K(Figure S1), TPD spectra of H2 and H2O following thecoadsorption of O and H on Ir(111) at 77 K (Figure S2), TPDspectra of isotopic water species following the coadsorption ofvarious isotopic 2-propanol species and 0.12 ML of O on Ir(111)at 77 K (Figure S3), and TPD spectra of isotopic water speciesfollowing the coadsorption of various isotopic 2-propanolspecies and 0.34 ML of O on Ir(111) at 77 K (Figure S4). Thisinformation is available free of charge via the Internet at http://pubs.acs.org.

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Surface Chemistry of 2Propanol on Clean and Oxygen Precovered Ir(111

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Transcript of Surface Chemistry of 2Propanol on Clean and Oxygen Precovered Ir(111