The Unique Properties of the Oxide-Metal Interface: Reaction of

Article pubs.acs.org/JPCC

The Unique Properties of the Oxide-Metal Interface: Reaction of Ethanol on an Inverse Model CeOx−Au(111) Catalyst S. D. Senanayake,*,† K. Mudiyanselage,† A. Bruix,‡ S. Agnoli,§ J. Hrbek,† D. Stacchiola,† and J. A. Rodriguez† †

Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973 United States Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark § Department of Chemical Sciences, University of Padova, Via F. Marzolo 1, 35131 Padova, Italy ‡

S Supporting Information *

ABSTRACT: The metal and oxide interface has been implicated through rigorous investigation as being pivotal to catalytic processes such as the production of H2 by reforming of alcohols. In this work, using high resolution X-ray photoelectron spectroscopy (XPS), we extend the study further by looking at the interaction of an oxygenate composed of the simplest C−O, O−H and C−C containing functionalities, ethanol (CH3CH2OH) with a model metal-oxide interface. We have discovered that this reactant can adsorb molecularly on Au(111) (Au0), while on O−Au(111) (Au+) and on inverse CeOx−Au(111) (Ce4+/Ce3+/Au+) surfaces it forms ethoxy (CH3CH2O−) species. Decomposition temperatures for the ethoxy on Au(111) surface are low (∼200 K) but much higher on CeOx covered surfaces (>500 K). Neither scission of the C−C or C−O bond of ethanol, nor surface aldehyde/acetate species was observed on these surfaces. However, O was lost from CeOx after reaction showing a clear reduction from Ce4+ to Ce3+. Most interestingly, the fractionally covered Au(111) surface with CeOx showed evidence for ethoxy being bound to both Au and CeOx at 300 K, which is not observed on either Au(111) or O−Au(111) at this temperature. Based on these results we hypothesize that the interface between CeOx and Au is providing a site for either (1) direct deprotonation of ethanol and adsorption of ethoxy, (2) adsorption for the spillover of ethoxy from the oxide to the interface, or (3) spillover of O from CeOx to Au(111) followed by direct deprotonation of ethanol/adsorption of ethoxy. We discuss the implication of these results involving inverse catalysts at dynamic steady state conditions.

1. INTRODUCTION The fundamental investigations into the role of metals and oxides in heterogeneous catalysis have taken a long and evolving road that has yielded important insights toward the understanding of how the structure of catalytic materials affects selectivity and activity.1−12 The geometry, electronic structure, and the redox properties of both metals and oxides have come to play a significant role in numerous catalytic processes often in combination and co-operatively.7−10,13−16 Systematic studies have unraveled catalytic phenomena strongly coupled to catalyst structure, which have aided the control of reactions tailored toward desired products (selectivity) and yield (activity). The concept of the “active site” has advanced to include the dynamic and collective effects of multiple components of catalysts.4,13,15,17 Of these, the most significant discoveries have to do with the understanding of how to tune the chemical and electronic properties of metals and oxides with respect to direct conversion of reactants to products. Today, we know that on the nanometer scale there are unique properties for both metals and oxides that can enhance chemical reactivity, including the size, shape, surface termination, electronic structure and atomic arrangement. One relatively clear way to tune the chemistry of © 2014 American Chemical Society

nanoparticles is by interactions with the substrate they are supported on. With this knowledge, catalysts can be constructed that specifically target the electronic and redox interactions between the active metal or oxide nanoparticle and their support.15,18,19 Through the years, we have also learned that the interfaces where both metals and oxides meet can also have the vital ingredients necessary for chemical reactions (active perimeter and interfacial sites).15 This particular phenomenological paradigm has now been well demonstrated with both conventional (metals supported on oxides)9 and inverse (oxides supported on metals) catalyst configurations.8,10,13,16 The inverse model catalysts are an ideal benchmark to test the effect of structure on catalytic activity, whereby a model is formed by placing islands of oxide (CeOx) onto a well-defined metal surface. We have studied these inverse catalysts previously to test the influence of the interface and the catalytic effects of the oxide nanoparticles8,10,13,16 However, details associated with bond breaking and forming steps at this Received: August 6, 2014 Revised: October 3, 2014 Published: October 3, 2014 25057

dx.doi.org/10.1021/jp507966v | J. Phys. Chem. C 2014, 118, 25057−25064

The Journal of Physical Chemistry C

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interface are yet to be revealed. In this study, we explore one inverse configuration (CeOx−Au(111)) where an oxide nanoparticle (CeOx) is supported on a well-defined metal (Au(111)) surface to probe the propensity of the interfacial site to dissociate the O−H (435 kJ mol−1), C−O (395 kJ mol−1), and C−C bonds (∼330 kJ mol−1) of a simple oxygenate, ethanol(CH3CH2 OH). The Au−Ce interface has been demonstrated to have interesting chemistry related to alcohol chemistry on conventional supported powder catalysts (Au− CeO2).20,21 The interaction between Au nanoparticles and CeO2 support were implicated in yielding a high propensity for oxidation of alcohols in the extraction of H2 (and CO/CO2), with predominantly surface ethoxy(CH3CH2O−) and CO32−/ CO species observed under reaction conditions.20,21 Previously, we have investigated this model interface (Au− Ce) for the water−gas shift reaction (CO+H2O → CO2+H2), where the most critical step involving the breaking of the O−H bond of H2O to yield OH’s was readily achievable.13 We also implicated the interface of this inverse catalyst as having the ability to hold onto the carboxyl species (HOCO) that is likely to be the most probable intermediate in the mechanism for this reaction. In this work we have asked ourselves the question if the bond breaking applies to a similar deprotonation functionality in ethanol (acidic H−OH) as was prevalent in H2O, and if identical sites may be responsible for this process. With this in mind we carried out several systematic investigations on clean Au(111), oxygen covered (0.2 ML) Au(111), covered (>1 ML), and fractionally covered (1 ML)/Au(111)

O (eV)

CeOx (eV)

CH3CH2OH (eV)

CH3CH2O− + OH (eV)

530.1

532.5 532.7 533.6

532.2 531.8

529.9

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Figure 4. XP data after adsorption of 2 Langmuirs of ethanol (86 K) on O (0.2 ML)/Au(111) in the O 1s region, followed by annealing up to 300 K (solid red line). Dotted red line corresponds to O/Au(111) surface prior to ethanol exposure.

participating in the desorption. The quantitative analysis of the C and O containing species (in Supporting Information Figure S3) show that the desorptions are not uniform with a more gradual removal of C containing species as compared to the O species, with the removal of O in particular being a slow process up to 200 K followed by a more rapid desorption at higher temperatures. This collection of data suggests the following adsorption and desorption pathways over O−Au(111) (Au+/ Au0): CH3CH 2OH(g) → CH3CH 2OH(a)

rapid desorption of C containing adsorbates and at 300 K, two peaks appear at lower (ca. −0.3 eV) binding energies of 286.6 and 287.7 eV. These two peaks are likely to be the contributions of only the CH3− and −CH2−O groups of the ethoxy species.26 This ethoxy species remains on the surface with continued heating up to 500 K, and all C containing species have desorbed by 600 K. A small feature at ∼290 eV was a result of beam damage and can be attributed to the formation of formates, acetates, or carboxylates as previously observed on a ceria surface.26 The O 1s region is shown in Figure 6 for the same experiment described above. The lattice O of CeOx is clearly visible at 530.1 eV, which with adsorption of 2 L ethanol attenuates (∼60% vs 100% for O/Au(111)) and we observe the formation of an additional peak at 533.6 eV corresponding to multilayers of ethanol. With annealing to 200 K a large amount of the multilayer ethanol has desorbed and we see only small contributions of the 533.6 eV peak alongside an additional feature at 531.8 eV corresponding to a combination of monolayer of ethanol, ethoxy and −OH from steps 3 and 4 in section 3.2. By 300 K, the OH and ethoxy contributions remain on the surface. With further annealing the lattice O (530.1 eV) recovers, and we observe the slow disappearance of the OH and ethoxy species. By 600 K all oxygen containing surface species has desorbed leaving a slightly attenuated (∼90%) lattice O feature (CeOx), indicating that some of the O from the ceria has participated in the desorption process. In Supporting Information Figure S4 we have attempted to separate the desorption of C and O containing species from the surface, and we observe that the rate of desorption is

(3)

CH3CH 2OH(a) + O(s) → CH3CH 2O(a) + HO(a)

∼ 200 K (4)

CH3CH 2O(a) + OH(a) → CH3CH 2OH(g) + O(s)

< 300 K (5)

HO(a) + HO(a) → H 2O(g) + O(a)

Figure 5. XP data after adsorption of 2 Langmuirs of ethanol (100 K) on CeOx (>1 ML)/Au(111) in the C 1s region, followed by annealing to 600 K. Dotted red line at 100 K corresponds to O/Au(111) surface prior to ethanol exposure. Dotted red line at 300 K corresponds to beam damage check.

(6)

3.3. Ethanol Reaction on CeOx (>1 ML)−Au(111). In order to obtain insights on the interactions of ethanol solely on the CeOx sites, first we performed experiments with a ceria film on Au(111). Ceria was deposited to cover the surface of the Au(111) to the extent that approximately 1 ML or greater was prevalent leaving no exposed Au sites on the surface. Subsequently, 2 L of ethanol were dosed to this surface at 86 K. The results are depicted in Figures 5 and 6 for the C 1s and O 1s regions, respectively and in Supporting Information Figure S4. In the C 1s region, two broad peaks at 286.9 and 288.0 eV appear at 86 K. These peaks are less resolved compared with those for the Au(111) surface (section 3.1) and appear slightly unequal in intensity, with a slight favor to the lower binding energy peak. These two peaks are likely to be the multilayer (∼5−6 layers) and also possibly a monolayer of ethanol (CH3− and −CH2−O) that has bound to the surface with a small concentration of ethoxy. With annealing there is a 25060



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Figure 7. XP data after adsorption of 2 Langmuirs of ethanol (at 100 K) on CeOx (0.5 ML)−Au(111) in the C 1s region, followed by annealing to 600 K. Solid red line corresponds to 300 K.

acetates, or carboxylates coming from beam damage in this experiment.26 From 400−500 K only the two higher BE peaks (287.7 and 286.6 eV) are left. All C-containing species have desorbed from the surface by 600 K. The fitting of the C 1s peaks for the ethoxy species at 300 K is shown in Figure 8. Figure 6. XP data after adsorption of 2 Langmuirs of ethanol (100 K) on CeOx (>1 ML)/Au(111) in the O 1s region, followed by annealing to 600 K. Solid red line corresponds to 300 K. Dotted red line at 100 K corresponds to CeOx/Au(111) surface prior to ethanol exposure. Dotted red line at 300 K corresponds to beam damage check.

nonuniform. The desorption of ethanol is complete by 300 K and gives rise to the slow desorption of ethoxy. The likely pathways that prevail when only Ce4+/Ce3+ sites and no ceriaAu interface is present can thus be summed very similar to that observed in section 3.2, with steps 3−6. 3.4. Ethanol Reaction on CeOx (0.5 ML)−Au(111). Upon exploring the surface chemistry of Au(111), O−Au(111) and CeOx covered Au(111) surfaces, the final variant is to create an interface with half the surface covered (0.5 ML) with ceria, where CeOx in contact with Au at an interface. The C 1s spectra obtained following exposure of this CeOx (0.5 ML) covered surface to ethanol and subsequent annealing are depicted in Figure 7 and Supporting Information Figure S5. O 1s and Au 4f were also collected but not shown here as they do not reveal any further insights. Initially, a set of overlapping peaks appear with ethanol exposure (2 L) at 100 K, several of which are notably visible at 286.3 and 284.9 eV; however, these are very broad and difficult to interpret without fitting. These features are likely contributions of multilayers and monolayers of ethanol intermixed with any ethoxy and species on both CeOx and Au(111) sites. With annealing to 200 K, some of the multilayers have desorbed, and the attenuation of the C feature starts. At 300 K, three peaks at 287.6, 286.6, and 284.9 eV are clearly distinguishable. Small contribution of a peak at ∼291 eV can be attributed to the formation of traces of formates,

Figure 8. XP data after adsorption of 2 Langmuirs of ethanol (at 100 K) on CeOx (0.5 ML)− Au(111) in C 1s region, followed by annealing to 300 K.

Based on the previous sections, we can assign two of the features at high temperatures (287.7 and 286.6 eV) to ethoxy bound to ceria, while the lower energy features (285.7 and 284.6 eV) can be attributed to ethoxy bound either at the interfacial ceria or Au(111) sites. Based on the comparison of binding energies of ethoxy species formed on ceria and O/ 25061



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CeOx−Au(111) inverse catalyst. In particular, we have systematically unraveled the role of the metal (Au0), oxidized metal (Au+) and an interface between Ce−Au toward surface reaction with a prototypical oxygenate (ethanol). These components make up in combination the most important parts of the inverse catalyst system. The CeOx−Au(111) system is catalytically remarkable as both Au(111) and CeO2(111) alone are catalytically inactive for most carbon based reactants. However, the addition of a small concentration of CeOx helps to activate this model catalyst drastically.8,10,13,16 The interface was thought to be the most important part of the catalyst with an ability to provide sites that are uniquely different from those available on Au(111) or on the CeOx islands. In the results presented above, we observed that both CeOx and O−Au(111) enables the deprotonation of ethanol and adsorption of ethoxy; however, Au(111) does not do this at all. The activation of ethanol on O−Au(111) has been studied previously.27,28 Our results show that on both Au(111) and O− Au(111) the adsorbates on the surface only prevail up to 250 K. The ethoxy remains considerably more strongly bound on CeOx than that on O−Au(111) up to 600 and 250 K, respectively. With the stabilization of ethoxy at room temperature on the interface sites between CeOx−Au, we have a uniquely different site that may have a strong propensity to do one of three processes (Figure 10): (1) the direct

Au(111) sites, the peaks observed at 285.7 and 284.6 eV can be assigned, most likely, to ethoxy species on the interfacial Au (111) sites. These ethoxy species at the interfacial Au(111) sites decompose at 400 K hence their thermal stability is in between that of ethoxy species on ceria and Au sites of O/ A(111). These results show that ethoxy species at the interface are neither bound too strongly as those on ceria or too weakly as on Au(111) and on O/Au(111). These results also show the importance of the metal-oxide interface for tuning the stability of ethoxy species. Since ethanol does not dissociate on Au(111) without O, the ethanol should dissociate at the metal−oxide interface using O associated with the ceria particles and, then, spill over/migrate to the Au(111) sites. The participation of O on ceria particles during the formation of ethoxy is further confirmed by the annealing of the pre ethanol exposed surface CeO1.94 (rich in Ce4+) resulting, after reaction with ethanol in reduction to CeO1.65 with predominantly Ce3+, as shown by the Ce 4d XP spectra in Figure 9. This opens the possibility to the

Figure 9. Pre (black line) and post (red line) ethanol (2 L) reaction on CeOx (0.5 ML)−Au(111) in the Ce 4d region. Figure 10. Possible mechanism for deprotanation and adsorption sites of ethoxy on CeOx−Au(111). [1] Direct dissociation at interfacial site. [2] Spillover of ethoxy from CeOx to interfacial site. [3] spillover of O from CeOx to interfacial site followed by [1].

participation of surface O in the decomposition of the ethoxy to gaseous products such as CO/CO2 and H2/H2O (step 12), in a Mars−van Krevelen-type process. Based on these measurements we proposed the following scheme for the reactions of ethanol on CeOx (0.5 ML)/Au(111) 7−12: CH3CH 2OH(g) → CH3CH 2OH(a)

∼ 100 K

(7)

CH3CH 2OH(a) → CH3CH 2OH(g)

< 200 K

(8)

CH3CH 2OH(a) → CH3CH 2O(a) + OH(a) HO(a) + HO(a) → H 2O(g) + O(s)

> 200 K

∼ 300 K

CH3CH 2O(a)−Au + OH → CH3CH 2OH(g)

(9) (10)

∼ 300 K

CH3CH 2O(a)−Ce−V0 → CO/CO2 (g) + H 2 /H 2O(g)

dissociation of the H−O bond in ethanol followed by adsorption of ethoxy (step 9); (2) accommodate spillover of ethoxy from the CeOx to the interface; (3) spillover of O that accommodates either process 1 or 2, subsequently. It is worth noting that the spillover of O (process 3) from CeOx to Au(111) has not been observed at either ultrahigh vacuum conditions or at higher pressures. Furthermore, these scenarios are difficult to separate experimentally, however, remain probable pathways for most catalytic processes at steady state associated with these systems. For instance, in the water gas-shift (CO+H2O → CO2+H2), the H2O is dissociated (to OH+H) on the CeOx, while the CO is bound to Au(111), and the meeting of the two reactants occurs at the interface in the formation of a carboxyl.13 In this instance, a dynamic catalytic reaction (water gas-shift) requires migration of adsorbed reactant to meet at the interface between CeOx−Au after being adsorbed either on the oxide or metal. In

(11)

∼ 550 K

(12)

4. DISCUSSION We have used ethanol and adsorbed ethoxy species as probe molecules to deconstruct the surface sites for adsorption on a 25062



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addition to this the third possible scenario may also prevail at catalytic conditions involving the migration of O from the CeOx to Au(111) likely to be stabilized at the interface and providing a site composed of O−Au(111), which we have demonstrated has the capability to break the O−H bonds of ethanol easily but not sufficiently to hold on to the intermediate beyond low temperatures. In reactions such as ethanol steam reforming, the strong metal support interactions between metal nanoparticles and oxide support are often attributed to the ability to break the C− C bond.29,30 Our results show no evidence for this mechanism on the surface, but there may be bond breaking via the incorporation of surface O while forming CO/CO2. Furthermore, our results implicate that at least in the case of Au− Ce this is likely to be specific to the geometry or electronic structure and not simply the interface between Au−Ce. The bonding geometry of ethoxy species at the interfacial sites of the inverse catalyst can be different from that on CeOx or Au (monodentate vs bridge-bonded bidentate) as reported previously for the ethoxy on Au/CeO2 powder catalyst systems.20,21 Ability to overcome the strength of the C−C bond is clearly not within the strength of this interaction.

ASSOCIATED CONTENT

S Supporting Information *

Quantitative analysis of XP data including Au 4f, O 1s, and C 1s of figures provided in manuscript. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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5. CONCLUSIONS Ethanol adsorbs molecularly on Au(111) (Au 0 ), and dissociatively on O−Au(111) (Au+) and on inverse CeOx− Au(111) (Ce4+/Ce3+/Au+) surfaces as an ethoxy species. Desorption temperatures for the ethoxy on Au(111) surface are low (∼200 K) but higher on CeOx covered surfaces (∼600 K). No scission of the C−C or C−O bond was observed on these surfaces. However, O was lost from CeOx after reaction showing a clear reduction from Ce 4+ to Ce 3+ . Most interestingly, the fractional covered CeOx−Au(111) surface showed evidence for the presence of ethoxy bound not only to CeOx but also Au(111) at 300 K, which is not observed either on Au(111) or O−Au(111) at 300 K. Based on these results, we hypothesized that the interfacial sites have unique properties for bond breaking of O−H, which is likely an important factor in catalytic processes.

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AUTHOR INFORMATION

Corresponding Author

*Address: Bldg. 555A, Brookhaven National Laboratory, P.O. Box 5000, Upton, NY 11973-5000. Phone: 631-344-4343. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work performed at Brookhaven National Laboratory, supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and Catalysis Science Program under contract No. DE-AC0298CH10886. This work used resources of the National Synchrotron Light Source, which is a DOE Office of Science User Facility. 25063



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The Unique Properties of the Oxide-Metal Interface: Reaction of

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