In Situ Characterization of Highly Dispersed, Ceria-Supported Fe Sites

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In Situ Characterization of Highly Dispersed, CeriaSupported Fe Sites for NO Reduction by CO Charles Alexander Roberts, Dario Prieto-Centurion, Yasutaka Nagai, Yusaku Nishimura, Ryan D Desautels, Johan van Lierop, Paul T. Fanson, and Justin M Notestein J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5126975 • Publication Date (Web): 29 Jan 2015 Downloaded from http://pubs.acs.org on February 3, 2015

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The Journal of Physical Chemistry

In Situ Characterization of Highly Dispersed, CeriaSupported Fe Sites for NO Reduction by CO Charles A. Roberts,1,* Dario Prieto-Centurion,2,† Yasutaka Nagai,3 Yusaku F. Nishimura,3 Ryan D. Desautels,4 Johan van Lierop,4 Paul T. Fanson,1 Justin M. Notestein2 1

Toyota Motor Engineering & Manufacturing North America, Inc., 1555 Woodridge Ave., Ann Arbor, MI 48105, United States.

2

Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208, United States. 3

4

TOYOTA Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan.

Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada.

*E-mail: [email protected]; Tel: +1-734-995-3625; Fax: +1-734-995-2549 †Present Address: SABIC Americas, Houston, TX 77042.

ACS Paragon Plus Environment

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Abstract

Highly dispersed FeOx was impregnated onto CeO2 using the unique precursor Na/Feethylenediaminetetraacetate (NaFeEDTA) at varying Fe surface density (0-1.5 Fe/nm2). These catalysts were used for NO reduction by CO, and were compared to a more traditional Fe(NO3)3 precursor impregnated on Na-promoted CeO2. Extensive characterization and spectroscopic measurements showed that NaFeEDTA produced a narrower distribution of smaller, noncrystalline, surface FeOx species with excellent redox cyclability (Fe3+↔Fe2+).

The

NaFeEDTA catalysts exhibited corresponding higher steady-state activity for NO reduction by CO. In situ x-ray absorption spectroscopy with simultaneous gas phase monitoring indicated that NO conversion began concurrent with reduction of Fe and Ce, suggesting that NO reduction occurred at a reduced Fe-O-Ce interface site. These results illustrate a new synthesis-structureactivity relationship for NO reduction by CO over redox-cycling FeOx sites that may support future rational design of emission control catalysts without Pt-group metals or zeolites.

KEYWORDS NOx Reduction; Iron Oxide; In Situ Spectroscopy; Mössbauer Spectroscopy; Carbon Monoxide


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1.0 Introduction The activity and selectivity of supported oxide heterogeneous catalysts are largely affected by the dispersion, particle size, and catalyst structure.1 Linking activity and selectivity to catalyst structure is aided by synthesis methods that produce supported surface oxide structures that are as homogenous as possible. To this end, many studies have been done to characterize and quantify the active supported oxide surface structures,1,2-8 and obtain true catalytic turnover frequencies.9 Unfortunately, even below monolayer coverage, incipient wetness impregnation (IWI) using traditional precursors such as metal-nitrates, fails to produce the homogeneity necessary to identify structure-function relationships so as to rationally design highly selective active sites.10-15 Significant efforts have been made to improve uniform nanostructuring of supported catalytic active sites.1,16-18 Specifically for IWI of supported metal oxide, it has been demonstrated that better control over the formation of highly dispersed isolated FeOx species on a SiO2 support could be achieved using a Na/Fe-ethylenediaminetetraacetate (NaFeEDTA) precursor.19 Such isolated Fe or low-coordinated FeOx species are generally attributed to the activity in Feexchanged zeolites utilized for selective catalytic reduction (SCR) of nitrogen oxides (NOx) for automotive exhaust purification.20-29 The Fe-exchanged zeolites have garnered much attention as they are free of Pt-group metal (PGM) and active for a variety of SCR reactions,20-28,30,31 especially NH3-SCR.28,29,32-34 There have been attempts to use IWI on more common oxide supports (CeO2, ZrO2, SiO2) to reproduce the zeolite active sites and reactivity,35-37 but the difficulties regarding synthesis of homogeneous supported surface oxide structures remained. Utilizing the above NaFeEDTA precursor for IWI, Prieto-Centurion et al. synthesized highly dispersed Fe-species on a CeO2 support.38 Evaluating


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NO reduction by H2, the NaFeEDTA catalysts were shown to be more active than those synthesized via a corresponding nitrate precursor. The increased activity was attributed to a higher density of redox-cycling Fe sites as quantified by temperature programmed reduction and in situ x-ray absorption near edge structure (XANES) measurements. This study builds on the research of Prieto-Centurion et al. by utilizing the same class of NaFeEDTA catalysts to develop quantifiable relationships between the catalyst structure and activity for NO reduction by CO; a more common and challenging reaction in automobile exhaust catalysis. Furthermore, deeper insight into the relationships between synthesis, the homogeneity and nature of the resulting redox-cycling sites, and NO reduction activity is sought by additional characterization with Mössbauer spectroscopy and x-ray absorption spectroscopy (XAS). Specifically, in situ XANES with simultaneous product monitoring will be utilized to probe the role and location of Fe redox centers during NO reduction by CO under active working conditions. The results support future rational design of emission control catalysts without PGM or zeolites.

2.0 Experimental Methods 2.1 Catalyst Synthesis, BET Surface Area, and Elemental Analysis Catalysts were prepared by incipient wetness impregnation (IWI) of different Fe-containing precursors onto a non-porous CeO2 (C.I. Kasei Co., Ltd. NanoTek powder) with specific surface area of 101 m2/g. The precursor sodium Fe(III) ethylenediaminetetraacetate (NaFeEDTA) was prepared following the method of Prieto-Centurion, et al.,19 which is based on that by Meier and Heinemann.39 A more traditional Fe(NO3)3•9H2O precursor was used to synthesize a set of control catalyst materials for comparison. Catalyst samples were synthesized with Fe surface


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densities from 0.36-1.53 Fe/nm2. Due to solubility limits of the EDTA-containing precursor, synthesis of catalysts with greater than 0.6 Fe/nm2 via NaFeEDTA required multiple cycles of impregnation, drying overnight, and re-impregnation (1-3 cycles prior to calcination) to achieve the desired total Fe surface density. Using the NaFeEDTA necessarily also impregnates Na onto the CeO2 support in a 1:1 atomic ratio of Fe:Na. Control catalyst materials were synthesized by impregnation of NaHCO3 and drying overnight to yield the Na-modified CeO2. The modified CeO2 support was then impregnated with Fe(NO3)3•9H2O at a Fe:Na atomic ratio of 1:1. The promotion effect of alkali content and identity is studied elsewhere for NO reduction in H 2.38 Finally, the catalyst samples were dried and calcined in air at 550 °C for ~1 hr to remove the precursor ligand and form the highly dispersed Fe species. Nitrogen physisorption isotherms were obtained using a Micromeritics ASAP 2010 instrument. Samples were degassed for 6 hours at 1.0 Fe/nm2) showing pseudo-absorbance was found above 500 nm (2.48 eV), indicating the formation of three-dimensional aggregates.

It is reiterated that these three-

dimensional aggregates do not appear to be crystalline in nature. The presence of isolated Fe3+ species cannot be ruled out, so it is likely that the catalysts in the current study consist of a distribution of isolated Fe3+ and two-dimensional dimers or oligomers at Fe loadings less than ~1.0 Fe/nm2, with small amounts of non-crystalline three-dimensional aggregates at the higher Fe loading. Unfortunately, DRUV-vis spectroscopy does not facilitate the quantification of the


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distribution of sites as the absorbtivity of the various FeOx species is not known a priori. Thus, it was not possible to determine how the distributions differed between samples prepared via NaFeEDTA or Fe(NO3)3. The finding that the Fe species on CeO2 formed by both NaFeEDTA and Fe(NO3)3 have an average oxidation falling below Fe3+ indicates that both Fe3+ and Fe2+ species are present, which is unusual for FeOx supported on metal oxides. Previously, a SiO2 support was used with the same precursors and XANES revealed that the Fe K-edge energy was identical to the Fe2O3 standard containing Fe3+.19 Unfortunately, the EXAFS analysis found no change in average Fe-O bond distance or average Fe coordination amongst the various samples. Mössbauer spectroscopy, however, is a short range local probe, and can be more sensitive to changes in Fe-O bond lengths or small changes in molecular species. For perfect octahedral symmetry, orbitals retain sufficient degeneracy so that the EFG is zero, yielding QS = 0. Lower symmetries, such as trigonal or tetragonal, remove the degeneracy so that a non-zero EFG and QS results. Typically for bulk Fe-oxide compounds, this behavior results in QS being strongly dependent on the oxidation state and electronic spin multiplicity of the Fe. However, for FeOx decorating the CeO2 support, the EFGs are generated by the molecular orbitals of the species, and in view of the large variety of orbitals involved, QS value interpretation becomes problematic, such that QS is more akin to a distortion parameter.45 With this in mind, the double Gaussian distribution of QS values required to fully describe the spectral lineshapes (see σQS- and σQS+ in Table 3) show that there is a clear shift from the ̅̅̅̅ to more negative with increasing Fe surface majority of the QS values being larger than 𝑄𝑆 density. This in turn indicates that the Fe coordination changes from being more 4- to 6coordinated as the Fe surface density increases.


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The EXAFS analysis did yield distinct trends in Debye-Waller factor with Fe surface density and precursor identity (Figure 2b and Table 2). The Debye-Waller factor in the EXAFS equation is the root-mean-squared deviation of interatomic distances. Therefore, increasing Debye-Waller factor while the average Fe-O bond length is unchanging is an indication that the distribution of FeOx species is broadening. This interpretation may at first seem at odds with the Mössbauer interpretation that the shift from 4- to 6-coordinated in accompanied by a decrease in distortion of the EFG, since this implies ordering. However, for Mössbauer spectroscopy, the ordering that occurs is related to the local bonding environment, which is exposed to less distortion from proximal Ce and/or Na as Fe surface density increases. Thus, the fact that the average QS increases while fit distributions (σQS- and σQS+, Table 3) change with Fe surface density and differing precursor are also indicators that a broadening of FeOx species present is occurring (see Figure S7 and the Supporting Information for additional details). Most importantly, the catalyst prepared by Fe(NO3)3 exhibits the greater Debye-Waller factor and average QS for a NaFeEDTA catalyst of the same Fe surface density (~1 Fe/nm2). Therefore, the NaFeEDTA precursor is best suited for maintaining a more narrow distribution of FeOx species supported on CeO2. The findings from the above combination of techniques support the conclusions that the FeOx species are non-crystalline and highly dispersed (Raman and XRD), are largely Fe3+ with a fraction of Fe2+ (XANES and Mössbauer), have increasing size and coordination with increasing Fe surface density (UV-vis and Mössbauer), and have a narrower distribution of FeOx species when synthesized by NaFeEDTA (EXAFS and Mössbauer). 4.2 Reduction Behavior of the Fe-oxide Species Prieto et al. obtained H2-TPR cycles to quantify the number of Fe sites that could perform redox cycles between Fe3+ and Fe2+ (Fe3+↔Fe2+ sites). The quantities were found to vary with


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Fe surface density and precursor identity (see Table 1). The initial H2-TPR profiles (first cycle) indicated the presence of significantly more low-temperature reduction (below that of Fe2O3) over NaFeEDTA/CeO2 catalysts than Fe(NO3)3/Na-CeO2. The higher percentage reduction of the NaFeEDTA/CeO2 is proposed to be a result of the more uniform interactions with the reducible CeO2 surface that we have identified here.

In contrast, EXAFS analysis and

Mössbauer spectroscopy suggest the highest Fe loading or the Fe(NO3)3-prepared materials are more fully coordinated, and yield aggregates that interact less with the CeO2 surface and are, thus, less reducible. Regardless, the quantified Fe3+↔Fe2+ sites provided a useful metric to correlate with the activity of the dispersed FeOx catalysts as will be demonstrated below. 4.3 Activity vs. Synthesis Method The current results for NO reduction by CO demonstrated that the NaFeEDTA/CeO 2 catalysts outperform the Fe(NO3)3/Na-CeO2 catalysts with nearly twice the activity at ~1.0 Fe/nm2 (Figure 4b). By comparison of Figure 4b and Table 1, NO reduction activity appears to be correlated to the fraction of Fe that participates in redox cycles, including the plateau beginning ~1.0 Fe/nm 2. To validate this hypothesis, activities over all materials were fit to a single correlation against the number of FeOx centers able to undergo reversible Fe3+↔Fe2+ redox cycles (Figure 7), as determined by H2-TPR.38 All materials collapse to a single trendline and materials derived from Fe(NO3)3 give lower rates because of their lower redox active Fe surface density (see Table 1). A similar plot reported for NO reduction by H2 over a related set of catalysts also reported a single trendline.38

The single trendline from the previous work and herein (Figure 7) for

different total Fe surface densities, different synthesis methods, and for different reductants (H2 or CO) suggests that the redox cyclability of the Fe site is critically important for NO reduction activity. Although the different reaction conditions preclude a direct comparison of rates, the


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fact that neither reaction is structure sensitive leads to the proposal that the rate-determining step involves both NO and a reduced site. Therefore, because synthesis via the NaFeEDTA precursor yields a higher percentage of Fe3+↔Fe2+ sites that facilitate the NO reduction activity, it is the preferred synthetic method for well-dispersed FeOx species on CeO2 for this reaction.

Figure 7. Mass normalized activity at 250 °C as a function of Fe3+↔Fe2+ active surface density for NaFeEDTA/CeO2 () and Fe(NO3)3/Na-CeO2 (). The 0.0 Fe/nm2 data point is taken from the CeO2 support-only. 4.4 In situ XAS Fe Reduction Behavior Although the above results identify the reducibility and redox cyclability of Fe sites as critical for NO reduction activity, the structure of the reduced site and/or the active site remains unknown. The results of in situ XAS with simultaneous product monitoring allow deeper understanding of the nature of such sites by facilitating direct correlations between the activity and the accompanying physical and/or electronic structural changes to the catalyst. Although the


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geometry and design of the in situ cell (flow over the pressed pellet) did not permit quantitative correlations with Fe surface density or precursor identity, it was still possible to correlate the onset of NO conversion and reduction of Fe and Ce as a function of temperature. In situ XAS with simultaneous product monitoring identified a ~1.0 eV decrease (reduction) in Fe K-edge energy of the dispersed NaFeEDTA/CeO2 and Fe(NO3)3/Na-CeO2 catalysts during NO reduction by CO. The relatively small shift is evidence for the increased presence of Fe2+ surface species (Figure 5b), but not complete reduction to Fe2+ (>4.0 eV difference from Fe2O3 to FeO, see Table 2). Furthermore, the onset of Fe reduction and the onset of NO conversion occurred at the same temperature (Figure 6), thus the formation of Fe2+ is likely related to the formation of the active site structure for NO reduction by CO. The formation of Fe2+ was previously suggested to be related to formation of Fe active sites for SCR of NO by NH3 over Fe-ZSM-5.32 In situ XAS of both Fe and Ce K-edges makes it possible to comment on the mechanism by which the Fe2+ is formed. Figure 6 revealed that reduction of Ce4+ to Ce3+ also occured concomitant with the onset of NO reduction. A similar finding was previously made for Cuoxide species supported on CeO2, which showed simultaneous reduction of Cu2+ to Cu0 and Ce4+ to Ce3+ with the onset of NO conversion under typical 3-way catalyst automotive exhaust conditions.52

The relatively small shift in Ce K-edge energy of ~1.0 eV is indicative of

incomplete reduction and, given the temperature is below that of bulk CeO2 reduction (~600 °C), the shift is likely due to reduction of surface CeO2. The physical interpretation of a similar finding in a related system by Nagai et al. was that reduction occurs at the CuOx-CeOx interface, and the same interpretation is employed here: reduction occurs at the FeOx-CeOx interface and involves the breaking of the surface Fe-O-Ce interfacial bond to form Fe2+ and Ce3+.


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It is of interest to note that in the case of Cu-oxide on a CeO2 support, the reduction from Cu2+ to Cu0 metal was necessary for reduction of NO.52 The function of the Cu0 metal on a CeO2 support is, therefore, similar to that of supported PGM such as Pt, Pd, or Rh, as these metals must also avoid oxidation and remain metallic to serve as active NO adsorption sites. Based on this interpretation, it would seem more difficult to mimic the active sites for NO reduction predicted for partially reduced Cu in the active sites of Cu-zeolites53,54 by means of simple impregnation of Cu salts onto an oxide support. On the other hand, because the highly dispersed Fe-oxides in the current study remain as partially-reduced FeOx species during active reduction of NO by CO, it appears the dispersion of Fe-oxide on ceria is a feasible route to mimic potential active sites of Fe-zeolites. Despite the lack of direct evidence from the in situ XAS, the increased activity demonstrated in the steady-state experiments implies that the NaFeEDTA precursor affords a degree of control over the structure and reactivity of the resulting FeOx domains. 4.5 Synthesis-Structure-Activity Relationship The above data all suggest that synthesis via NaFeEDTA produces Fe species that are more reducible. The fraction of reducible Fe3+↔Fe2+ sites is maximized at ~1.0 Fe/nm2, which directly correlates with the maximum achievable activity of NO reduction by CO for NaFeEDTA/CeO2 catalysts in this range of Fe loadings. Fitting to a single correlation against the redox active Fe (Figure 7) and in situ XAS with simultaneous product monitoring demonstrated that reduction of these sites is critical for the onset of NO conversion and are likely intimately related to the active site formation, if not the actual active sites. The synthesisstructure-activity relationship (illustrated in Figure 8a) can be established: The maximum relative NO reduction activity over dispersed Fe-oxide supported on CeO2 is achieved by maximizing the


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number of Fe3+↔Fe2+ sites at a given Fe surface density by using the NaFeEDTA precursor. Figure 8b further illustrates that when Fe(NO3)3 is used for synthesis, the percentage of Fe3+↔Fe2+ sites is maximized at the lowest Fe surface density loading. Therefore, only subtle increases in activity are achieved with increasing Fe surface density.

Figure 8. Percentage of Fe redox sites activity for NO reduction by CO as a function of Fe surface density for (a) NaFeEDTA/CeO2 and (b) Fe(NO3)3/Na-CeO2 catalysts.

5.0 Conclusions Two synthetic methods were utilized to prepare the dispersed Fe-oxides on CeO2 (NaFeEDTA and Fe(NO3)3) precursors.

The characterization of catalysts prepared by both methods

demonstrated that the NaFeEDTA precursor was preferred for the synthesis of dispersed FeOx of smaller domain size and redox active Fe3+↔Fe2+ centers for NO reduction.

Therefore, a

synthetic method was developed to improve the control over Fe-oxide speciation in the range 01.5 Fe/nm2. Evaluation of NO reduction by CO yielded a synthesis-structure-activity relationship for the highly dispersed FeOx supported on CeO2 over all Fe surface densities and preparation methods.


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It was found that Fe3+↔Fe2+ active centers are responsible for the steady-state activity by fitting correlating activities over all materials against redox active Fe. The same conclusion was made for NO reduction by H2, thus, the independence from reductant identity leads to the conclusion that interaction of NO with the Fe3+↔Fe2+ sites is likely rate-determining. In situ XAS with simultaneous product monitoring suggested that the redox-cycling active centers are at the interfacial Fe-O-Ce and this bond is essential for NO reduction activity. Development of new, controllable synthetic routes to desirable surface oxide structures is predicted to be useful for future rational design of improved automotive exhaust catalysts. For future application, SCR of NO under increasingly lean conditions will require a high degree of control over active center speciation. To compete with and eventually replace the currently employed PGM or zeolite-based catalysts, increased synthetic control during IWI, as demonstrated herein, will be essential for FeOx/CeO2 or any other supported metal oxide.


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Johan van Lierop gratefully acknowledges funding from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation (CFI). Acknowledgements The authors are grateful to Hirohito Hirata and the catalyst researchers of Toyota Motor Corporation for their beneficial discussion and support of this collaborative research. Naoki Takahashi and the Toyota Central R&D Labs catalyst researchers and in situ XAS team of Masatoshi Sakai, Hideki Takagi, and Kazuhiko Dohmae are thanked for their support in executing the experiments and their discussion. Torin Peck of TEMA is greatly thanked for his efforts to support reactor evaluation. Thanks are given to Louisa Savereide of Northwestern University for her support of the technical discussion. Supporting Information. Table S1 and Figure S1, Figure S2, Figure S3, Figure S4, Figure S5, Figure S6, and Figure S7. This information is available free of charge via the Internet at http://pubs.acs.org.


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References (1)

Wegener, S. L.; Marks, T. J.; Stair, P. C. Design Strategies for the Molecular Level Synthesis of Supported Catalysts. Acc. Chem. Res. 2012, 45, 206-214.

(2)

Wachs, I. E. Raman and IR Studies of Surface Metal Oxide Species on Oxide Supports: Supported Metal Oxide Catalysts. Catal. Today 1996, 27, 437-455.

(3)

Burcham, L. J.; Briand, L. E.; Wachs, I. E. Quantification of Active Sites for the Determination of Methanol Oxidation Turn-over Frequencies Using Methanol Chemisorption and in Situ Infrared Techniques. 1. Supported Metal Oxide Catalysts. Langmuir 2001, 17, 6164-6174.

(4)

Weckhuysen, B. M.; Keller, D. E. Chemistry, Spectroscopy and the Role of Supported Vanadium Oxides in Heterogeneous Catalysis. Catal. Today 2003, 78, 25-46.

(5)

Wachs, I. E.; Jehng, J. –M.; Ueda, W. Determination of the Chemical Nature of Active Surface Sites, Present on Bulk Mixed Metal Oxide Catalysts. J. Phys. Chem. B 2005, 109, 2275-2284.

(6)

Nair, H.; Baertsch, C. D. Method for Quantifying Redox Site Densities in Metal Oxide Catalysts: Application to the Comparison of Turnover Frequencies for Ethanol Oxidative Dehydrogenation Over Alumina-Supported VOx, MoOx, and WOx Catalysts. J. Catal. 2008, 258, 1-4.

(7)

Eaton, T. R.; Campos, M. P.; Gray, K. A.; Notestein, J. M.; Quantifying Accessible Sites and Reactivity on Titania-Silica (Photo) Catalysts: Refining TOF Calculations. J. Catal. 2014, 309, 156-165.


32

Page 33 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8)

Eaton, T. R.; Boston, A. M.; Thompson, A. B.; Gray, K. A.; Notestein, J. M. Counting Active Sites on Titanium Oxide-Silica Catalysts for Hydorgen Peroxide Activation through In Situ Poisoning with Phenylphosphonic Acid. Chem. Cat. Chem. 2014, 6, 3215-3222.

(9)

Boudart, M. Heterogeneous Molecular Catalysis: Oxymoron or Reality? J. Molec. Catal. A 1997, 120, 271-280.

(10) Arena, F.; Frusteri, F.; Spadaro, L.; Venuto, A.; Parmaliana, A. Design, Preparation and Testing of Effective FeOx/SiO2 Catalysts in Methane to Formaldehyde Selective Oxidation. Stud. Surf. Sci. Catal. 2000, 143, 1097-1105. (11) Arena, F.; Gatti, G.; Coluccia, S.; Martra, G.; Parmaliana, A. Preparation Method and Structure of Active Sites of FeOx/SiO2 Catalysts in Methane to Formaldehyde Selective Oxidation. Catal. Today 2004, 91-92, 305-309. (12) Arena, F.; Gatti, G.; Martra, G.; Coluccia, S.; Stievano, L.; Spadaro, L.; Famulari, P.; Parmaliana, A. Structure and Reactivity in the Selective Oxidation of Methane to Formaldehyde of Low-Loaded FeOx/SiO2. J. Catal. 2005, 231, 365-380. (13) Gervasini, A.; Messi, C.; Ponti, A.; Cenedese, S.; Ravasio, N. Nanodispersed Fe Oxide Supported Catalysts with Tuned Properties. J. Phys. Chem. C 2008, 112, 4635-4642. (14) Gervasini, A.; Messi, C.; Carniti, P.; Ponti, A.; Ravasio, N.; Zaccheria, F. Insight into the Properties of Fe Oxide Present in High Concentrations on Mesoporous Silica. J. Catal. 2009, 262, 224-234.


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(15) Nozaki, C.; Lugmair, C. G.; Bell, A. T.;Tilley, T. D. Synthesis Characterization, and Catalytic Performance of Single-Site Iron(III) Centers on the Surface of SBA-15 Silica. J. Am. Chem. Soc. 2002, 124, 13194-13203. (16) van Dillen, A. J.; Terörde, R. J. A. M.; Lensveld, D. J.; Geus, J. W.; de Jong, K. P. Synthesis of Supported Catalysts by Impregnation and Drying Using Aqueous Chelated Metal Complexes. J. Catal. 2003, 216, 257-264. (17) Eggenhuisen, T. M.; den Breejen, J. P.; Verdoes, D.; de Jongh, P. E.; de Jong, K. P. Fundamentals of Melt Infiltration for the Preparation of Supported Metal Catalysts. The Case of Co/SiO2 for Fischer-Tropsch Synthesis. J. Am. Chem. Soc. 2010, 132, 1831818325. (18) Zaera, F. Nanostructured Materials for Applications in Heterogeneous Catalysis. Chem. Soc. Rev. 2013, 42, 2746-2762. (19) Prieto-Centurion, D.; Boston, A. M.; Notestein, J. M. Structural and Electronic Promotion with Alkali Cations of Silica-Supported Fe(III) Sites for Alkane Oxidation. J. Catal. 2012, 296, 77-85. (20) Chen, H. -Y.; Sachtler, W. M. H. Activity and Durability of Fe/ZSM-5 Catalysts for Lean Burn NOx Reduction in the Presence of Water Vapor. Catal. Today 1998, 42, 73-83. (21) Voskoboinikov, T. V.; Chen, H. -Y.; Sachtler, W. M. H. On the Nature of Active Sites in Fe/ZSM-5. Appl. Catal. B 1998, 19, 279-287.


34

Page 35 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Joyner, R.; Stockenhuber, M. Preparation, Characterization, and Performance of Fe-ZSM-5 Catalysts. J. Phys. Chem. B 1999, 103, 5963-5976. (23) Gao, Z. -X.; Kim, H.-S.; Sun, Q.; Stair, P. C.; Sachtler, W. M. H. UV-Raman Characterization of Iron Peroxo Adsorbates on Fe/MFI Catalyst with High Activity fo NOx Reduction. J. Phys. Chem. B 2001, 105, 6186-6190. (24) Heinrich, F.; Schmidt, C.; Löffler, E.; Menzel, M.; Grünert, W. Fe-ZSM-5 Catalysts for the Selective Reduction of NO by Isobutane – The Problem of the Active Sites. J. Catal. 2002, 212, 157-172. (25) Choi, S. -H.; Wood, B.R.; Ryder, J.A.; Bell, A.T. X-ray Absorption Fine Structure Characterization of the Local Structure of Fe in Fe-ZSM-5. J. Phys. Chem. B 2003, 107, 11843-11851. (26) Kumar, M. S.; Schwidder, M.; Grunert, W.; Bruckner, A. On the Nature of Different Iron Sites and their Catalytic Role in Fe-ZSM-5 DeNOx Catalysts: New Insights by Combined EPR and UV/VIS Spectroscopic Approach. J. Catal. 2004, 227, 384-397. (27) Xia, H.; Sun, K.; Liu, Z.; Feng, Z.; Ying, P.; Li, C. The Promotional Effect of NO on N2O Decomposition over the Bi-Nuclear Sites in Fe/ZSM-5. J. Catal. 2010, 270, 103-109. (28) Fierro, G.; Moretti, G.; Ferraris, G.; Andreozzi, G. B. A Mössbauer and Structural Investigation of Fe-ZSM-5 Catalysts: Influence of Fe Oxide Nanoparticle Size on the Catalytic Behaviour for the NO-SCR by C3H8. Appl. Catal. B 2011, 102, 215-223.


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

(29) Schwidder, M.; Kumar, M. S.; Klementiev, K.; Pohl, M. M.; Brückner, A.; Grünert, W. Selective Reduction of NO with Fe-ZSM-5 Catalysts of Low Fe Content: Part I. Relations between Active Site Structure and Catalytic Performance. J. Catal. 2005, 231, 314-330. (30) Heck, R. M.; Farrauto, R. J.; Gulati, S. T. Catalytic Air Pollution Control: Commercial Technology, Second Edition; John Wiley & Sons, Inc., New York, 2002. (31) Centi, G.; Perathoner, S. Introduction: State of the Art in the Development of Catalytic Processes for the Selective Catalytic Reduction of NOx into N2. Stud. Surf. Sci. Catal. 2000, 171, 1-23. (32) Kumar, M. S.; Schwidder, M.; Grünert, W.; Bentrop, U.; Brückner, A. Selective Reduction of NO with Fe-ZSM-5 Catalysts of Low Fe Content: Part II. Assessing the Function of Different Fe Sites by Spectroscopic In Situ studies. J. Catal. 2006, 239, 173-186. (33) Brandenberger, S.; Kröcher, O.; Wokaun, A.; Tissler, A.; Althoff, R. The Role of Brønsted Acidity in the Selective Catalytic Reduction of NO with Ammonia over Fe-ZSM-5. J. Catal. 2009, 268, 297-306. (34) Brandenberger, S.; Kröcher, O.; Tissler, A.; Althoff, R. The Determination of the Activities of Different Iron Species in Fe-ZSM-5 for SCR of NO by NH3. Appl. Catal. B 2010, 95, 348-357. (35) Chen, K.; Dong, L.; Yan, Q.; Chen, Y.; Dispersion of Fe2O3 Supported on Metal Oxides Studies by Mössbauer Spectroscopy and XRD. J. Chem. Soc., Faraday Trans. 1997, 93, 2203-2206.


36

Page 37 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(36) Parmaliana, A.; Arena, F.; Frusteri, F.; Martinez-Arias, A.; Granados, M. L.; Fierro, J. L. G. Effect of Fe-Addition on the Catalytic Activity of Silicas in the Partial Oxidation of Methane to Formaldehyde. Appl. Catal. A 2002, 226, 163-174. (37) Reddy, A. S.; Chen, C. -Y.; Chen, C. -C.; Chien, S. -H.; Lin, C. -J.; Lin, K. -H.; Chen, C. L.; Chang, S. -C. Synthesis and Characterization of Fe/CeO2 Catalysts: Epoxidation of Cyclohexane. J. Molec. Catal. A 2010, 318, 60-67. (38) Prieto-Centurion, D.; Eaton, T. R.; Roberts, C. A.; Fanson, P. T.; Notestein, J. M. Catalytic Reduction of NO with H2 Over Redox-Cycling Fe on CeO2. Appl. Catal. B 2015, 168-169, 68-76. (39) Meier, R.; Heinemann, F. W. Structures of the Spontaneously Resolved Six-Coordinate Potassium Chloro-(Ethylenediaminetriacetato Acetic Acid) Iron(III) Monohydrate and the Seven-Coordinate Potassium (Athylenediaminetetracetato) Iron(III) Sesquihydrate. Inorg. Chim. Acta 2002, 337, 317-327. (40) Rodríguez-Carvajal, J. Recent Advances in Magnetic Structure Determination by Neutron Powder Diffraction, Physica B 1993, 192, 55-69. (41) Pushkarev, V. V.; Kovalchuk, V. I.; d’Itri, J. Probing Defect Sites on the CeO2 Surface with Dioxygen. J. Phys. Chem. B 2004, 108, 5341-5348. (42) Wilke, M.; Farges, F.; Petit, P. -E.; Brown, G. E.; Martin, F. Oxidation State and Coordination of Fe in Minerals: An Fe K-XANES Spectroscopic Study. Am. Mineral. 2001, 86, 714-730.


37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 40

(43) Czjzek, G.; Fink, J.; Götz, F.; Schmidt, H.; Coey, J. M. D.; Rebouillat, J. –P.; Liénard, A. Atomic Coordination and the Distribution of Electric Field Gradients in Amorphous Solids. Phys. Rev. B 1981, 23, 2513-2531. (44) Niemantsverdriet, J. W. Spectroscopy in Catalysis: An Introduction, VCH Publishers, New York, 1995. (45) Greenwood, N. N.; Gibb, T. C. Mössbauer Spectroscopy, Chapman and Hall Ltd., London, 1971. (46) Burns, R. G. Mineral Mössbauer Spectroscopy: Correlations between Chemical Shift and Quadrupole Splitting Parameters. Hyper. Interact. 1994, 91, 739-745. (47) Dyar, M. D.; Agresti, D. G.; Schaefer, M. W.; Grant, C. A.; Sklute, E. C. Mössbauer Spectroscopy of Earth and Planetary Materials. Annu. Rev. Earth Planet. Sci. 2006, 34, 83125. (48) Nagai, Y.; Dohmae, K.; Teramura, K.; Tanaka, T.; Guilera, G.; Kato, K.; Nomura, M.; Shinjoh, H.; Matsumoto, S. Dynamic In Situ Observation of Automotive Catalysts for Emission Control Using X-ray Absorption Fine Structure. Catal. Today 2009, 145, 279287. (49) de Smit, E.; Weckhuysen, B. M. The Renaissance of Iron-Based Fischer-Tropsch Synthesis: On the Multifaceted Catalyst Deactivation Behaviour. Chem. Soc. Rev. 2008, 37, 2758-2781.


38

Page 39 of 40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(50) Bordiga, S.; Buzzoni, R.; Geobaldo, F.; Lamberti, C.; Giamello, E.; Zecchina, A.; Leofanti, G.; Petrini, G.; Tozzola, G.; Vlaic, G. Structure and Reactivity of Framework and Extraframework Iron in Fe-Silicalite as Investigated by Spectroscopic and Physicochemical Methods. J. Catal. 1996, 158, 486-501. (51) Gao, X.; Wachs, I. E. Investigation of Surface Structures of Supported Vanadium Oxide Catalysts by UV-vis-NIR Diffuse Reflectance Spectroscopy. J. Phys. Chem. B 2000, 104, 1261-1268. (52) Nagai, Y.; Dohmae, K.; Nishimura, Y. F.; Kato, H.; Hirata, H.; Takahashi, N. Operando XAFS Study of Catalytic NO Reduction over Cu/CeO2: the Effect of Copper-Ceria Interaction under Periodic Operation. Phys. Chem. Chem. Phys. 2013, 15, 8461-8465. (53) Kispersky, V. F.; Kropf, A. J.; Ribeiro, F. H.; Miller, J. T. Low Absorption Vitreous Carbon Reactors for Operando XAS: A Case Study on Cu/Zeolites for Selective Catalytic Reduction of NOx by NH3. Phys. Chem. Chem. Phys. 2012, 14, 2229-2238. (54) Kwak, J. H.; Tran, D.; Burton, S. D.; Szanyi, J.; Lee, J. H.; Peden, C. H. F. Effects of Hydrothermal Aging on NH3-SCR Reaction over Cu/Zeolites. J. Catal. 2012, 287, 203209.


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Table of Contents Graphic


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In Situ Characterization of Highly Dispersed, Ceria-Supported Fe Sites

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