Toward a Silver–Alumina Model System for NOx Reduction Catalysis

Article pubs.acs.org/JPCC

Toward a Silver−Alumina Model System for NOx Reduction Catalysis N. M. Martin,*,† E. Erdogan,† H. Grönbeck,‡ A. Mikkelsen,† J. Gustafson,† and E. Lundgren† †

Division of Synchrotron Radiation Research, Lund University, Lund, Sweden Competence Centre for Catalysis and Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden

‡

ABSTRACT: The growth and morphology of Ag deposited on NiAl(110) and on oxidized NiAl(110) have been investigated by a combination of scanning tunneling microscopy (STM) and high-resolution core-level spectroscopy (HRCLS). While the STM measurements reveal complete wetting and a bilayer growth on clean NiAl(110), Ag nanoparticles with a minimum size of 5 nm were obtained on the oxidized NiAl(110). The difference in Ag growth mode on clean and oxidized NiAl(110) is supported by Ag 3d HRCLS. The binding energy for Ag on clean NiAl(110) is the same as for bulk Ag, while the Ag 3d peak for particles on oxidized NiAl(110) shifts toward the bulk binding energy with increasing size. The adsorption properties at 100 K of CO and NO on oxidized NiAl(110) and on Ag particles on oxidized NiAl(110) were also investigated by probing the C 1s and N 1s core levels. In the case of oxidized NiAl(110), neither CO nor NO adsorbs. In the case of Ag particles on oxidized NiAl(110), CO does not adsorb, but a component at 397 eV is observed in the N 1s level after NO exposures. This component is tentatively assigned to silver nitride, suggesting NO dissociation in the presence of Ag particles on oxidized NiAl(110).

I. INTRODUCTION In 1993, Miyadera presented the silver−alumina based catalyst for hydrocarbon selective catalytic reduction (HC-SCR), which showed exceptional activity for reduction of NOx under lean conditions in the presence of water vapor.1 These findings triggered a series of structural and mechanistic studies of silver−alumina catalysts with a focus on automotive applications.2−12 For instance, a recent study reports on the importance of the interface between the Ag particle and the alumina support for the NOx reduction reaction.13 The authors suggest that the CO and NO dissociate on Ag and form a stable silver cyanide (AgCN), which momentarily occupies a bridge position with a very short lifetime (2 μs) between the Ag particle and alumina support (see Figure 1).

from their bulk counterpart. Metal nanoparticles may adhere more strongly to thin oxides on a metallic support and resist sintering at higher temperatures than on bulklike oxides.15 Therefore, conclusions based on observations from catalyst nanoparticles on a thin oxide support may not always reflect properties of the active particles in a real catalyst but, rather, differences between close-packed single crystal surfaces and nanoparticle model catalysts. Ag thin film growth has been investigated on various aluminum-containing alloy surfaces previously, and it has been concluded that Ag does not react with such surfaces at room temperature (rt).16 The lower surface energy of Ag usually leads to a wetting of the surface for the first few layers, and strain may enhance a 3D growth in thicker layers.17 The growth of Ag on NiAl(110) has previously been studied by a combination of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations,18,19 which showed an initial bilayer growth and wetting of the NiAl(110) surface. The growth mode of Ag/Al2O3 is, due to the low surface energy of the oxide, completely different, resulting in the formation of Ag nanoparticles.20−22 The interaction of CO and NO with Ag single crystals has also been studied previously. The results showed no interaction between CO or NO and the Ag(111) surface at 300 K under ultrahigh vacuum (UHV) conditions, while at low temperatures (below 80 K) NO was found to adsorb molecularly, forming NO, (NO)2, N2O, and O(a) species.23−26 At higher pressures, the partial decomposition of adsorbed NOx species resulted in formation of nitrites (NO2) and physisorbed NO in N2O3 structures over a preoxidized Ag surface.27

Figure 1. NOx removal over Ag−alumina catalyst as proposed in ref 13.

Since the detailed understanding of the reaction mechanism could promote the design of better silver−alumina SCR catalysts, a silver−alumina model system for fundamental surface science studies is well motivated. Nanoparticles supported on oxide thin films grown on a metal substrate overcome the electrical conductivity problem and structural complexity that a real catalyst has and open a way to apply the arsenal of surface science techniques for studies of structural, electronic, and adsorption details.14 It should be noted, however, that such thin oxide films have properties that differ © 2014 American Chemical Society

Received: July 31, 2014 Revised: September 16, 2014 Published: September 24, 2014 24556

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Figure 2. STM images for Ag deposited on NiAl(110) at 300 K as a function of Ag deposition time: (a) 15 s, 300 × 300 nm2, 1.5 V, 300 pA; (b) 30 s, 300 × 300 nm2, 1.5 V, 150 pA; (c) 60 s, 300 × 300 nm2, 1.6 V, 200 pA; (d) 120 s, 200 × 200 nm2, 2.8 V, 600 pA; (e) 240 s, 200 × 200 nm2, 1.0 V, 200 pA. A height of 2.04 Å was determined for the NiAl(110) step, as indicated in part f. The line profile shown in part g indicates a height of the 2D islands of 2.75 Å, indicative of bilayer Ag(110) formation. (h) Calibration curve for Ag on NiAl(110) calculated from STM images. (i) The Ag 3d peak area (circles) and binding energy shift (triangles) as a function of Ag deposition time on NiAl(110). (j) Ag 3d spectra after different Ag doses on a clean NiAl(110) surface. The center of the peak is indicated by a dotted line.

Figure 3. STM images of the clean Al2O3/NiAl(110) [(a) 200 × 200 nm2, 3 V, 25 pA] and for Ag deposited on alumina grown on NiAl(110) at 300 K as a function of Ag deposition time [(b) 30 s, 200 × 200 nm2, 3 V, 75 pA; (c) 60 s, 200 × 200 nm2, 3 V, 75 pA; (d) 120 s, 200 × 200 nm2, 3 V, 300 pA]. The clusters nucleate at the step edges and domain boundaries. The profiles shown on the bottom correspond to the lines drawn on each image on top.

In this work, we have used a well-defined model system, consisting of a NiAl(110) surface covered with a thin, wellordered Al2O3 film28 onto which Ag was deposited, to mimic the silver−alumina catalyst. The STM images show that the Ag clusters nucleate and grow at the step edges and at domain boundaries on the substrate. For the Ag on Al2O3 system, the high-resolution core-level spectroscopy (HRCLS) data show that there is a clear shift of the Ag 3d binding energy as a function of Ag coverage. For comparison and coverage calibration purposes, the results of Ag deposition on clean

NiAl(110) are also presented. The growth mode of Ag on clean NiAl(110) is completely different (Frank−van der Merwe growth): the Ag wets the surface and forms 2D islands and no shift of the Ag 3d as a function of coverage is observed. Further, the CO and NO adsorption on alumina and Ag/alumina was investigated at 100 K.

II. EXPERIMENTAL SECTION The STM measurements were carried out in Lund, Sweden, in a UHV chamber using a commercial Omicron STM1 operated 24557

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at rt. All STM images reported are recorded using a constant current mode. The HRCLS measurements were performed at beamline I311 at the MAX IV Laboratory in Lund, Sweden.29 For the HRCLS measurements, a normal emission angle was used, and the Al 2p, O 1s, N 1s, and Ag 3d levels were recorded with photon energies of 120, 650, 540, and 450 eV, respectively. The sample cleaning and oxidation have been described previously.30 Ag was deposited by evaporation of Ag metal from an commercial e-beam evaporator. The gas adsorption measurements were performed at a sample temperature of 100 K.

III. COMPUTATIONAL METHOD DFT was employed in the implementation with plane-waves and pseudopotentials.31,32 The spin-polarized Perdew−Burke− Ernzerhof (PBE) approximation was used for the exchange and correlation (xc) functional33 and ultrasoft scalar−relativistic pseudopotentials were used to describe the interaction between the valence electrons and the core.34 The number of electrons treated variationally for each element were N(5), O(6), and Ag(11). A plane-wave kinetic energy cutoff of 28 Ry was used to expand the Kohn−Sham orbitals. With this approach, the lattice constants of Ag was calculated to be 4.14 Å. Adsorption of nitrogen on Ag(111) was investigated in a p(2 × 2) surface cell. The surface was represented by five atomic layers. Repeated slabs were separated by 14 Å of vacuum. Reciprocal space integration over the Brillouin zone was approximated with a finite sampling of 13 special k-points. Structural optimization was performed without any constraints, and the structures were regarded as converged when the largest element of the gradient was lower than 0.05 eV/Å. The surface core level shifts were evaluated by the use of a pseudopotential that was generated with an electron hole in the 3d shell.35

Figure 4. (a) Ag 3d5/2 spectra after different Ag doses on alumina grown on a NiAl(110) surface. The binding energy value for bulk Ag 3d5/2 is indicated by the dotted line. (b) The Ag 3d peak area (circles) and binding energy shift (triangles) as a function of Ag deposition time on alumina/NiAl(110).

IV. RESULTS AND DISCUSSION A. Ag on NiAl(110). Previous studies18,19 of the growth of Ag on NiAl(110) revealed an initial bilayer growth [Ag(110) bilayer height between 0.29 and 0.32 nm] that encompasses at least the first two level islands with a face-centered cubic (fcc) Ag(110)-like structure. The Ag atoms in the first layer are located above the Ni atoms of the substrate and at the 4-fold hollow sites in the second layer, promoted by the almost perfect match between the surface unit cells for NiAl(110) and Ag(110). Figure 2a−e shows STM images after Ag deposition on NiAl(110) at 300 K for different deposition times. From these images it is clear that Ag forms large, flat 2D islands on NiAl(110). The Ag islands are observed to be oriented along the [001] direction of the substrate and grow outward from steps in a fingerlike fashion. A representative line profile is shown in Figure 2g, from which a step height of 2.75 Å was determined close to the value reported previously for the Ag(110) bilayer on NiAl(110) (∼3 Å). 18 Thus, our investigation confirms the growth of a bilayer of Ag(110) when deposited onto the NiAl(110) at rt. Figure 2h shows the calculated coverages of Ag on NiAl(110) as determined from the STM images presented above. Coverages between 0.1 and 1.7 BL were obtained for deposition times between 15 and 240 s. (1 BL equals twice the number of atoms in the NiAl(110) surface.) Above 150 s exposure time, the formation of a second bilayer is observed.

A series of Ag 3d5/2 spectra recorded during the Ag deposition on NiAl(110) at rt is shown in Figure 2j. No significant shift is observed in the binding energy of the Ag 3d5/2 peak for different Ag doses, as indicated in Figure 2i (triangles). The area underneath the Ag 3d5/2 peak as a function of Ag dose is also shown in Figure 2i (circles). There is a clear change in the increase of the Ag 3d5/2 area as a function of deposition time, indicating the formation of 1 BL film coverage wetting the substrate. B. Ag on Al2O3/NiAl(110). The Al2O3 thin film grown on NiAl(110) single crystal has been described in detail previously.28,30 Briefly, the film has a thickness of about 5 Å and consists of a double oxide layer with NiAl−Ali−Oi−Als−Os stacking and Al10O13 stoichiometry. For historical reasons the film is referred to as Al2O3. Line defects are clearly visible on the film, as illustrated by the STM image in Figure 3a, which shows antiphase and reflection domain boundaries.36 In Figure 3b−d is shown STM images after Ag deposition on the Al2O3 at 300 K for different deposition times. The Ag atoms form clusters with a height of 20−40 Å, corresponding to 9−18 Ag layers and a typical width of 5−10 nm. Clearly, the growth mode for Ag on the oxide is completly different from the growth mode for Ag on the clean NiAl(110). The Ag clusters are found to start to nucleate at step edges and higher coverages continue to nucleate along the domain boundaries. The cluster nucleation and growth at the step sites have previously been reported in the literature for other model systems.37,38 24558

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Figure 5. (a) O 1s, (b) Al 2p, (c) N 1s, and (d) Ag 3d5/2 spectra for the clean Al2O3 sample (black) and after the Al2O3 sample was exposed to 110 L of NO at 100 K (red), Ag deposition for 60 s at rt (green), and 110 L of NO exposure on the Ag/Al2O3 at 100 K (blue). Note the presence of a N 1s component only when Ag particles are present on the surface.

or related to a lattice strain that could exist due to shorter bond distances in small clusters as compared to in the bulk.41 Some groups have also argued that the screening is more efficient for a large particle as compared to a small particle.42−44 Regardless of the origin of the shift, the exact binding energy is a fingerprint of the size of the Ag particle on the thin Al2O3 film. The area underneath the Ag 3d5/2 peak as a function of Ag dose is also plotted in Figure 4b for Ag on Al2O3 (circles). The Ag on Al2O3 behavior is typical for the growth of 3D clusters, as observed in the STM images shown in Figure 3. C. CO and NO Adsorption on Ag/Al2O3/NiAl(110) at 100 K. Having established the main structural characteristics of the Ag/Al2O3/NiAl(110) model system, the system was tested for adsorption of CO and NO at 100 K. Starting with the Al2O3/NiAl(110) film, the O 1s and Al 2p levels are shown in Figure 5 (black). The deconvolution of the spectra has been presented in detail in ref 30. Upon exposing the Al2O3 film to CO or NO at 100 K, no clear change is observed in the O 1s and Al 2p levels (red), suggesting that neither CO nor NO adsorb on alumina under these conditions. Ag was deposited on the alumina film for 60 s at rt as described above (corresponding to Figure 3c), resulting in a component in the Ag 3d5/2 binding energy region at a binding energy of about 368.35 eV, and a lowering of the oxide signals is observed in both Al 2p and O 1s (green). When the sample was exposed to CO, no C 1s component and no change in the Al 2p, Ag 3d5/2, or O 1s levels could be observed, indicating that CO does not adsorb (not shown). However, when exposing the system to 110 L of NO at 100 K, the O 1s and Al 2p levels (blue) show a slight increase of the oxide components, while the Ag 3d signal is slightly reduced, suggesting that NO adsorbed. In addition, a weak peak is observed in N 1s at about 397 eV. Previous reports have indicated molecular NO adsorption on Ag(111) and formation of NO, (NO)2, N2O, and O(a) species

Figure 6. Model representation of nitride on Ag(111). Dark blue is N and light blue represents the Ag atoms.

The Ag particle size is an important parameter for Ag-based catalysis, and studies have reported an increased activity for small Ag particles.6 Experimentally, the size is difficult to determine, since the Ag atoms and Ag clusters are relatively mobile on the Al2O3 surface at 300 K and are also sensitive to the interaction with the STM tip. Moreover, the apparent tip height introduces an error in the estimation of the cluster size.14 A detailed study of the shape of Ag clusters (7−50 nm in size) on MgO nanocubes has been presented in ref 22 combining transmission electron microscopy (TEM) and atomistic simulations. The silver clusters were observed to have the shape of a truncated octahedron irrespective of the support morphology, and (100), (111), and (110) facets were identified. A similar shape is expected for the clusters in this study, even though no atomic resolution could be obtained on the clusters. A series of Ag 3d5/2 spectra taken during Ag deposition on Al2O3 is shown in Figure 4a. In this case, we find that the binding energy of the Ag 3d peak shifts to lower binding energy with increasing Ag dose. This phenomenon has been observed previously for small particles deposited on oxide supports,39,40 and the reason for the shift has been discussed in the literature. It has been argued to be due to the size change of the particles 24559

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at low temperatures,24−26 while no NO adsorption was observed at room temperature. The reported binding energy values for N 1s and O 1s for the NO/Ag(111) system has always been found to be higher than 401.9 eV. The present binding energy in the N 1s level of 397 eV is not compatible with the above assignments. Further, the increase of the O 1s and Al 2p oxide components demonstrate an increased Al oxidation after NO adsorption, similar to that found when exposing a Pd/Al2O3/NiAl(110) model system to O2.14 Thus, we conclude that the NO have dissociated and formed adsorbed N and O species on the surface. An additional effect contributing to the observed changes in the HRCLS signal could be due to a reshaping of the Ag particles upon NO absorption as the silver nitride is formed, although we expect this effect to be small. Nitrides are known to exhibit low binding energies in the N 1s,45 suggesting that the atomic N, resulting from the NO dissociation, is used in the formation of silver nitride. Additional confirmation of the ability of the Ag/Al2O3/ NiAl(110) model system to dissociate NO comes from calculations of the expected binding energy from N adsorbed in a hollow fcc site on Ag(111) (see Figure 6). In a previous study27 we established the core level shift for NO, and by calculating the N 1s shift between elemental N and NO, we obtain a theoretical binding energy on adsorbed N of 399 eV. This is in fair agreement with the experimental value of 397 eV. Improvement of the agreement between experiment and calculations is expected by varying of the theoretical structural model. Furthermore, as the nitride signature is absent on Ag(111), our measurements suggest that the dissociation and nitride formation are promoted either by another Ag facet, by the edges of the cluster, or by the Ag/Al2O3 interface. From the experiments and the calculations, the following model for the NO adsorption can be envisioned. The NO dissociates in the presence of the Ag particles on the Al2O3/ NiAl(110). While the N atoms form silver nitride with the Ag atoms in the Ag island, the oxygen atoms diffuse to the Al2O3 film, reacting with the Al and resulting in a increased thickness of the film. The fact that a nitride may form on the Ag clusters may also have consequences for the NO reduction, since the Ag surface properties change significantly. In fact, it may be that CO and hydrocarbon more easily dissociate on such a silver nitride surface, providing the opportunity for more facile reactions. The results presented here do not disqualify the model proposed in ref 13, on the contrary, since our results show that upon NO adsorption nitrogen becomes available for further reactions with other atoms.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +46 46 22 287 21. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Max IV staff is gratefully acknowledged. This work was financially supported by the Foundation for Strategic Research (SSF), the Swedish Research Council, the Crafoord Foundation, the Knut and Alice Wallenberg Foundation, the Anna and Edwin Berger Foundation, and NordForsk.

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REFERENCES

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V. SUMMARY We have characterized the deposition of Ag onto the NiAl(110) and the Al2O3/NiAl(110) surfaces. Our results show that when Ag is deposited on the metal substrate, it wets the surface while on the oxide it forms Ag clusters with a minimum size of 5 nm. The HRCLS measurements show a shift of the Ag 3d peak from high binding energy to lower energies as the Ag dosage is increased on the Al2O3/NiAl(110), ultimately reaching the value of bulk Ag. CO and NO adsorption studies on the Ag clusters combined with theoretical calculations indicate that NO dissociates in the presence of the Ag clusters and that the N reacts with the Ag, forming a silver nitride while the oxygen diffuses and reacts with the Al in the NiAl substrate. 24560

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