Ag in methanol oxidationThe formation of oxygen species in the frame of oxidation reactions over Ru

and Ag catalysts was investigated. It is known, that the formation of subsurface oxygen plays an important role in the oxidation of methanol over Cu [1]. The aim of present investigations is to show, that the formation of subsurface oxygen is a more common approach, which is realized in other metals like Ag and Ru under selective oxidation conditions as well.

The silver catalyzed partial oxidation of methanol and ethylene are two important catalytic reactions in the chemical industry [2,3]. Over the last 30 years, a wealth of information about mechanistic details of these reactions has been obtained and different oxygen surface species have been identified and assigned as active sites [3,4]. However, recent experimental results and calculations have pointed out that this picture might be oversimplified and the role of different oxygen species as spectators or active site is not clear [5].

Ag foils were investigated by in situ X-ray photoelectron spectroscopy (XPS) in the selective methanol oxidation to formaldehyde. The reaction conditions were selected as follows: T = 200 – 600°C, p=0.5 mbar, reactant ratio CH3OH:O2 = 1-5. Four different oxygen species were identified by in situ XPS as shown in Figure 1.

2,5

3,0

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536 534 532 530 5285,0

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370 369 368 3670

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Ag3dO1s

Intensity (10 4 CP

S)

Inte

nsity

(10

4 CP

S)

Binding Energy (eV)

O2 200° C

O2 600° C

UHV RT

O2+CH3OH 600° C

Fig. 1 : O1s and Ag3d core level spectra of a Ag foil in different gas atmospheres at 200 and 600 °C.

The peak with binding energy of 528.5 eV is assigned to the p(4x4) reconstruction on the Ag(111) surface, which is stable in an oxygen atmosphere just in the temperature range from 400 – 500 K [6]. A nucleophilc oxygen species at 529.2 eV O1s binding energy is related to the formation of ionic Ag as well. This species is surface located as well. A less surface related species was observed at the binding energy of 529.7eV. This species is more covalently bonded than the species at lower binding energy, since the formation of ionic silver is much less pronounced compared to the nucleophilic species. This species is identified as Oγ [2]. At a binding energy of 530.6 eV a bulk located oxygen species was observed. The species is more stable against switching from pure oxygen to the reactant mixture compared to the surface located species. The nucleophlic species at 529.2 eV decomposes after the switching of the reaction atmosphere from pure oxygen to methanol : oxygen mixture of 1:5, 2:1 and 5:1. The measurements have shown that the appearance of the Oγ species is preceded by an accumulation of the bulk related oxygen species, this suggested that the bulk species is “transformed” to the Oγ.

The abundance of different oxygen species on the surface of the working catalyst for different mixing ratios was compared with the product distribution and conversion determined by MS. The resulting correlations are shown in Figure 2 [7]. It

536 534 532 530 5281.5

2.0

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6.0In

tens

ity (1

04 CP

S)

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0 1 2 3 4 5 60

5

10

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0

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CH

3OH

Con

vers

ion

(%)

Gas feed mixing ratio (CH3OH:O2)

O@

529.

7 (%

)

0 1 2 3 4 5 60.0

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0

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mal

ized

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duct

Rat

io

(CH

2O/C

O2)

Gas feed mixing ratio (Ch3OH:O2)

O@

529.

7 (%

)

O2 only

1:5

2:1

5:1

Fig. 2: XP spectra under reaction conditions for different mixing ratios. Correlation of the MS data for CH3OH conversion and product ratio with the abundance of Oγ

is obvious that the increase in activity and reduction in “selectivity” are both related to an increase in the amount of the Oγ species. This indicates that a higher activity is obtained when more Oγ is present in the Ag surface but this leads to a smaller selectivity.

Cu-Ag alloy in ethylene epoxidationCombining first-principles calculations an in situ photoelectron spectroscopy,

we show how the composition and structure of the surface of an alloy catalyst is affected by the temperature and pressure of the reagents. The Ag-Cu alloy, recently proposed as an improved catalyst for ethylene epoxidation, forms a thin Cu-O surface oxide, while a surface alloy is found not to be stable[8]. Several possible surface structures are identified, among which the surface is likely to dynamically evolve under reaction conditions [9].

Ru in CO oxidation

Commercial RuO2, the starting material is always not single phase RuO2 as

seen by XRD or EDX. Metallic Ru can be found as “impurity” even in XRD.

Therefore commercial RuO2 was re-oxidized at 1023 K. This produces single crystals

in the µm range evidenced by XRD and EBSD. These single crystals exhibit in

contrast to the expected habit (four pronounced lateral surfaces) eight pronounced

lateral surfaces. This may be caused by a reconstruction of the {100} surfaces into a

c(2x2) structure and of the {110} surfaces into microfacetted c(2x2)-(100) surfaces.

Fig. 3: Left, SEM: One of the particles rarely observed with one rough lateral facet to be seen in the magnified micrograph. Right: Comparison of the consecutive experiments with varying feed composition, the temperature program is indicated.

These RuO2 crystals are initially inactive in CO oxidation under ambient pressure

flow conditions. The samples activate during an induction period of more than 24 h

either in net-oxidizing or net-reducing conditions. Under net-oxidizing conditions

kinetic oscillations occur as shown in Figure 3 right [10] The apical surfaces are

strongly facetted but still smooth after becoming active. After activation under net-

reducing conditions the apical faces of the crystals are roughened. In both cases the

sample is still RuO2 in XRD but the rough faces are partly reduced (From EDX point

analyses of morphologies seen in Figure 3 left) Under net-oxidizing conditions the

sample stays RuO2 in all cases. In net-reducing conditions the phase evolves either

into defective oxide or into metal plus sub-surface oxygen depending on the sample

temperature (or synonymously on the heat of reaction).

The large data set under atmospheric reaction conditions showed that heat and mass

transport controls the phase evolution. The heated debate in the literature about the

nature of the active phase is revolving around boundary conditions: the most active

phase is metal plus sub-surface oxygen, followed by defective oxide and by pure

metal. The frequently observed co-existence of phases is caused by the slow dynamic

responses of the well-ordered Ru-O system to the local chemical potential and

explains the apparently conflicting results in the literature.

Ru in methanol oxidation

In contrast to CO oxidation, the identification of possible reaction pathways

for CH2O production from CH3OH is more complex. The present study of CH3OH

oxidation on model Ru(0001), Ru(10-10) and polycrystalline Ru catalyst interrogates

with in-situ photoemission the catalytically active states under different reaction

conditions. The experiments were performed at the BESSY synchrotron facility in

Berlin at the high-pressure XPS station ISISS of FHI-AC.

Ru 3d and O 1s core level spectra, measured with photon energies of 450eV and

650eV, respectively, were used as fingerprints for the dynamic response of the

catalyst surface to changes in temperature and molecular mixing ratios. The gas phase

products were monitored by mass spectrometry as a measure of the catalytic activity.

The investigations were carried out in the pressure range of 10-2 to 10-1 mbar using

different CH3OH:O2 molecular mixing ratios and catalyst temperatures.

Our experiments reveal that the reaction pathways are identical on the (0001),

(10-10) and polycrystalline Ru surface supporting a nano-structured catalytically

active state without long-range order (structure insensitivity). This state was identified

as a non-stoichiometric oxygen-doped metal RuxOy. It is formed by interaction with

the reactants in the gas phase independently of the initially present material be it oxide

or metal [11].

Both activity and selectivity are very sensitive to the CH3OH:O2 mixing ratio

at a given temperature. Three different CH3OH oxidation pathways, partial oxidation

CO+H2+H2O (a) and CH2O+H2O (b) for ratios of 2.3 and 1.5, and full oxidation to

CO2+H2O (c) at a ratio of 0.75 were found to be exclusively determined by the

amount of accumulated oxygen (the y parameter in the RuxOy formula) controlling

the interaction of the catalyst surface with the reactants CH3OH and O2. The pure

oxide phase RuO2 frequently advertised as the most active state is inactive in all 3

reaction pathways under all conditions used here. The dramatic changes in the

selectivity relating to relatively small differences in the chemical state of RuxOy

demonstrate the critical relevance of the dynamic response of the catalyst to changes

in the surrounding chemical potential as generalized in Figure 1. It is assumed that the

variation in the y parameter changes the nature of the surface-adsorbed oxygen

species between electrophilic and nucleophilic reactivity as evidenced in the Ag-O

system.

References:[1] Bluhm H., Havecker M., Knop-Gericke A., Kleimenov E., Schlögl R., Teschner D., Bukhtiyarov VI., Ogletree DF., Salmeron M., Journal of Physical Chemistry B. 108(38):14340-14347, 2004

[2] X. Bao, M. Muhler, B. Pettinger, R. Schloegl and G. Ertl, Catalysis Letters 22 (1993) 215-225.[3] V.I. Bukhtiyarov, A.I. Nizovskii, H. Bluhm, M. Haevecker, E. Kleimenov, A. Knop-Gericke, R. Schögl, J. Catal. 238 (2006) 260.[4] X. Bao, M. Muhler, Th. Schedel-Niedrig, R. Schlögl, Phys. Rev. B 54 (1996) 2249-2262

[5] C. Stegemann, N. C. Schiodt, C. T. Campbell, P. Stoltze, J. Catal. 221 (2004) 630-649[6] M. Schmid, A. Reicho, A. Stierle, I. Costina, J. Klikovits, P. Kostelnik, O. Dubay, G. Kresse,J.Gustafson, E. Lundgren, JN. Andersen, H. Dosch, P. Varga; Phys. Rev. Lett. 9614 (2006) 6102

[7] T. Rocha, A. Oestereich, M. Hävecker, R. Blume, D. Teschner, A. Knop-Gericke, R. Schlögl in Preparation[8] S. Linic, J. Jankowiak and M. A. Barteau, J. Catal. 224, 489 (2004)[9] S. Piccinin, S. Zafeiratos, C. Stampfl, T. Hansen, M. Hävecker, D. Teschner,A. Knop-Gericke, R. Schlögl and M. Scheffler, in preparation[10] D. Rosenthal, F. Girgsdies, O. Timpe, R. Blume, G. Weinberg, D. Teschner, R. Schlögl; Z. Phys. Chem. 223 (2009) 183-207[11] R. Blume, M. Hävecker, S. Zafeiratos, D. Teschner, A. Knop-Gericke, R. Schlögl, P. Dudin, A. Barinov, M. Kiskinova; Catal. Today 124 (2007) 71-79