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Monolayer Detection of Supported Fe and Co Oxides on Ceria to Establish Structure-Activity Relationships for Reduction of NO by CO Torin C. Peck, Gunugunuri K Reddy, Michael Jones, and Charles Alexander Roberts J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00398 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on April 3, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

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Monolayer Detection of Supported Fe and Co Oxides on Ceria to Establish Structure-Activity Relationships for Reduction of NO by CO Torin C. Peck, Gunugunuri K. Reddy, Michael Jones, Charles A. Roberts* Toyota Research Institute – North America, 1555 Woodridge Ave., Ann Arbor, MI 48105, United States


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ABSTRACT

A series of ceria-supported iron and cobalt oxide catalysts were synthesized with incremental metal loadings to determine within a narrow range the monolayer surface coverage. Onset of monolayer formation was monitored utilizing X-ray diffraction and Raman spectroscopy to identify the presence of microcrystalline Fe-oxide and Co-oxide phases. Formation of Fe2O3 and Co3O4 microcrystalline phases were found on the 5.40 Fe/nm2 FeOx/CeO2 and 2.58 Co/nm2 CoOx/CeO2 catalysts, respectively, and thus, those surface densities approximate the monolayer coverage. The surface sensitivity of this approach was confirmed by corroborating the results with surface characterization by X-ray photoelectron spectroscopy. Catalytic activity testing was performed to illustrate the importance of establishing the monolayer coverage of each independent ceriasupported oxide system, i.e. maximizing the number of metal-oxygen-support interfacial bonds. Steady-state activities for reduction of NO by CO in the presence of a stoichiometric amount of oxygen were monitored at 275 °C. Indeed, the maximum areal activity of the catalysts (0.0116 and 0.0386 μmol NO/m2/s) were found at or slightly above the approximated monolayer coverages for both supported iron or cobalt oxide catalysts, respectively.

These results represent a

broadening of the knowledge of monolayer oxide systems on the catalytically interesting CeO2 support, and are expected to guide rational design of future improved catalysts for the industrially useful reaction of reduction of NO by CO in the presence of oxygen.

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1.0 Introduction Supported metal and metal oxide catalysts are among the most widely applied heterogeneous catalyst materials. Supported metal oxides have been utilized for a wide variety of applications, such as oxidative dehydrogenation of propane, olefin polymerization, and selective catalytic reduction (SCR) of NOx with NH3.1-3 This catalyst configuration claims significant advantages in comparison to unsupported catalysts, and the advantages can be classified into two categories: strong metal support interaction (SMSI) and electronic metal support interaction (EMSI).4 As implied by the nomenclature, SMSI describes a physical or structural interaction between the catalyst and support, i.e. effective surface area and sintering resistance, 5,6 and EMSI describes properties that emerge from the unique electronic structure that results at metal-support interfaces.4 The formation of interfacial bonds between the catalyst and support can give rise to EMSI, modifying the electron exchange properties of the catalyst and significantly affecting the adsorption properties of the catalyst or even introduce resistance to catalyst poisoning.4,7 Additionally, interfacial sites have unique redox properties which can facilitate reduction or oxidation reactions, respectively.8-10 To take advantage of electronically induced support effects, it is desirable to maximize the dispersion of the supported metal oxide, thereby maximizing the potential number of metal-oxygen-support (M-O-S) interfacial bonds. The largest population of accessible interfacial bonds occurs at the monolayer loading of the metal oxide on the support surface. Exceeding the monolayer loading leads to the formation of three-dimensional microcrystals, which do not contribute additional interfacial bonds. Rather the bulk oxide characteristics begin to dominate and the opportunity for significant EMSI effects diminishes.11 The benefit of designing monolayer supported metal oxide catalysts has been well demonstrated for supported Group V oxide catalysts,

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where the maximum activity for oxidative dehydrogenation of propane with respect to vanadium oxide surface density on SiO2 was determined to be at the point of monolayer loading.1 Similar reports have been published for Group VI oxide catalysts, such as methanol oxidation over supported molybdenum oxide.12

The successful determination of the structure-activity

relationships related to monolayer coverage for the studies on supported Group V/VI oxides was largely dependent on the utilization of Raman spectroscopy to determine the surface density at which bulk-like oxide microcrystal formation occurs, signaling the point at which the monolayer was exceeded.1,12-14 Similar to Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), has also been utilized to determine monolayer coverage of supported Group V/VI oxides.15,16 While Raman spectroscopy can easily identify the presence of microcrystalline species, it lacks surface sensitivity, which is necessary for identifying surface monolayer species. It is, therefore, desirable to confirm the species detected by Raman spectroscopy are indeed on the support surface using a spectroscopy such as XPS that is intrinsically surface sensitive (~1-3 nm depth resolution). Utilization of XPS to confirm results from Raman spectroscopy has been utilized previously, as similar values for monolayer coverage have been reported for supported VOx and NbOx catalysts.17,18 Further confirmation of the accuracy of the Raman spectroscopy method on additional supported oxides systems such as FeOx/CeO2 and CoOx/CeO2 is an important advancement in supported metal oxide characterization because the method requires less data analysis, manipulation, and linear fitting than the XPS method. The above method utilizing Raman spectroscopy lends itself well to the determination of Group V/VI monolayers due to a strong Raman signal from their M-O-S and terminal M=O bonds, however, such a rigorous application of the method to determine supported transition metal oxide 4 ACS Paragon Plus Environment


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monolayers is seldom reported in the literature. It has been previously reported that a 10 wt.% cobalt oxide supported on zirconia maintained a higher NO oxidation rate compared to loadings of 1 wt. %, 5 wt.% and 15 wt.%, however microcrystalline Co3O4 was already present at the 10 wt.% loading.19 Based on the evidence presented, it is unclear which Co loading corresponded to the onset of the monolayer, therefore the link between structure and activity requires further exploration. Cobalt oxide catalysts supported on ceria have been prepared by wet impregnation, with Co contents of 20, 30, and 60 wt.%, however, the XRD indicates the presence of bulk Co3O4 for each sample.11 Regarding other reports of cobalt oxides supported on CeO2, bulk Co3O4 crystal formation has been reported for surface densities of ~7 Co/nm2 and higher.20,21 Additional studies exist with characterization of both very low (0.071-2.86 Co/nm2) and very high (7.1-71.7 Co/nm2) surface densities, and results indicated the presence of non-crystalline and crystalline species, respectively, but intermediate loadings were not characterized, so a precise surface density of cobalt oxide corresponding to monolayer formation on CeO2 remains unknown.21 Published work regarding iron oxide supported on ceria (FeOx/CeO2) contain limited loadings, and similar to the CoOx/CeO2 case, the range of surface densities studied are not sufficient to distinguish the submonolayer to monolayer transition.22,23 Previous results from the authors’ research group have examined the performance of FeOx/CeO2 for NO reduction by H2 and CO, and it was discovered that the interfacial bond between FeOx and CeO2 (Fe-O-Ce) plays a critical role in the rate-determining-step of the reaction mechanism, however, these studies only covered highly dispersed, sub-monolayer FeOx/CeO2, and it remains necessary to determine if the activity can be further optimized by achieving a precise monolayer loading of FeOx on CeO2.24-26 Bulk Co3O4 has been studied as a promising catalyst for the direct decomposition of NOx,27 and it was concluded that there was some improvement in the intrinsic

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activity of cobalt oxide when supported on CeO2 for N2O decomposition.28 Additionally, H2-TPR studies of cobalt oxide supported on ceria, as well as ceria cobalt mixed oxides, indicate a decreased temperature for reduction of Co3O4 species, and the decreased temperature for reduction of Co may prove useful in increasing the rate of NO reduction at lower temperatures.29,30 A deeper investigation into monolayer coverage effects was not reported, but the CoOx/CeO2 system remains of interest for other NOx removal reactions. Thus, this study will utilize Raman spectroscopy, X-ray diffraction (XRD), and XPS to study FeOx/CeO2 catalysts spanning all three critical regions of supported metal oxides: sub-monolayer, monolayer, and above monolayer. It will, therefore, extend the work previously performed on the highly dispersed, sub-monolayer FeOx/CeO2 by creating more fully developed structure-activity relationship for the important NO reduction by CO reaction. The approach is also applied to a supported cobalt oxide (CoOx/CeO2) catalyst system at sub-monolayer, monolayer, and above monolayer coverages, as CoOx has also shown activity for NO reduction reactions. The activity for NO reduction by CO will be evaluated in the presence of O2 and as a function of the various surface densities to examine the effect of monolayer coverage. This study will demonstrate the robustness of the Raman spectroscopy approach to monolayer determination and yield a structureactivity relationship of ceria-supported transition metal oxides which will guide rational design of future improved NOx reduction catalyst systems.

2.0 Experimental 2.1. Catalyst Synthesis, BET Surface Area, and Elemental Analysis Catalysts were prepared using incipient wetness impregnation of an aqueous solution with varying concentrations of Fe(NO3)3 •9H2O (Sigma Aldrich, 99.99%) or Co(NO3)2 •6H2O (Sigma Aldrich, 6 ACS Paragon Plus Environment


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99.99%) onto a nonporous CeO2 support (C.I. Kasei Co., Ltd. NanoTek powder) with an asreceived specific surface area of 72.3 m2/g. After impregnation, the samples were dried overnight at 60 °C. Due to precursor solubility, it was necessary to perform multiple impregnations of a nitrate solution when the desired Fe or Co surface densities were in excess of ~2 Fe/nm2 and ~4 Co/nm2, respectively. Once the nominal metal loading was achieved, the sample was further dried at 120 °C for 24 hours, followed by calcination in air at 550 °C for 30 minutes (2 °C/min), yielding the as-prepared catalysts. Nitrogen physisorption isotherms were collected at 77 K of the as-prepared samples using a Micromeritics ASAP 2020 instrument. Prior to analysis, approximately 300 mg of sample was degassed at 120 ˚C for 4 hours at 50

none none Co3O4 Co3O4 Co3O4 Co3O4

15 Current Study Current Study 15 14 30

Metal Oxide

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The final issue related to the surface insensitivity of Raman spectroscopy can be resolved by corroborating the results with a technique that is intrinsically surface sensitive, such as XPS (~1-3 nm depth resolution). The ratio of the XPS peak areas of the Fe3p to Ce3d (see Supporting Information Table S2) was plotted as a function of Fe surface density (Figure 3a). The Fe3p/Ce3d ratio increases linearly up to 3.87 Fe/nm2. Upon reaching a surface density of 5.40 Fe/nm2, there is a clear decrease in the slope, indicative of monolayer formation followed by growth of crystalline Fe-oxide species.16 Thus, the XPS results are in good agreement with the results from Raman spectroscopy. The ratio of the XPS peak areas of the Co2p to Ce3d (see supporting information Table S3) was plotted as a function of Co surface density (Figure 3b). In this case, there are fewer sub-monolayer points than in the Fe3p/Ce3d series, making it more difficult to determine the cobalt surface density 18 ACS Paragon Plus Environment


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at which a change in slope is observed. Upon close inspection, a non-linear region appears to exist at the lowest cobalt surface densities (Figure 3b, inset) when the 2.58 Co/nm2 sample deviates from the expected trend. The linearity of Figure 3b starting from a cobalt surface density of 3.57 Co/nm2 is due to the consistent growth of the Co3O4 bulk crystals, which is in agreement with both the Raman spectra (Figure 2b) and the XRD (Figure 1b). The non-linearity at the highest cobalt surface density of 11.6 Co/nm2 cannot be attributed to monolayer identification as both the Raman and XRD have clearly identified the presence of bulk Co3O4 crystal species at this loading. Thus, if one assumes that the low cobalt surface density region of Figure 3b is, indeed, non-linear, the XPS data is in good agreement with the Raman spectroscopy which indicates Co3O4 microcrystalline formation as low as 2.58 Co/nm2. It is important to point out that the same result was achieved using Raman spectroscopy, however, fewer data points and fitting were required to assess the monolayer loading range.

Figure 3: Ratio of XPS peak areas of a) Fe3p and Ce3d in FeOx/CeO2 and b) Co2p and Ce3d in CoOx/CeO2 samples as a function of metal atom surface densities. Figure 2b Inset: Co2p/Ce3d ratio of samples with 0.52, 1.22, 2.58, and 3.57 Co/nm2.

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3.4 Effect of Monolayer on the Areal Activity for Reduction of NO by CO The monolayer represents the coverage with the largest population of highly dispersed, twodimensional, non-crystalline species, thus containing the greatest number of Fe-O-Ce interfacial bonds and the greatest opportunity to benefit from the EMSI.11,29 Therefore, the maximum activity for reduction of NO by CO over FeOx/CeO2 catalysts is expected at the monolayer coverage due to the critical role of the Fe-O-Ce in the NO reduction mechanism.25,26 Indeed, when the catalysts are evaluated for steady-state reduction of NO by CO at 275 °C in the presence of a stoichiometric quantity of O2, areal activity (rate normalized by the total catalyst surface area loaded in the reactor) increases monotonically with increasing Fe surface density until a maximum activity of 0.0116 μmol NO/m2/s was observed over the 5.40 Fe/nm2 FeOx/CeO2 catalyst (Figure 4a). Beyond this Fe surface density, due to the three-dimensional nature of the Fe2O3 microcrystal formation (see Figure 2a), no additional Fe-O-Ce interfacial sites are formed and areal activity decreases. The decrease in areal activity from the loss of the interfacial sites as bulk character is recovered has previously been attributed to a decrease in the effect of the EMSI.11 The increase in activity starting from bare CeO2 support (0.0 Fe/nm2) to the monolayer and subsequent decrease in activity above monolayer coverage is additional proof of the critical role of the Fe-O-Ce interfacial site and any resulting EMSI.29 The initial increase in areal activity coincides with the growing population of accessible Fe-O-Ce interfacial sites introduced by sub-monolayer, two-dimensional FeOx species. The decrease in activity above monolayer coverage is attributed to a loss of accessibility of the NO molecules to the Fe-O-Ce interfacial sites that comes with the formation of three-dimensional Fe2O3 microcrystals.



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Figure 4: Areal activity of (a) FeOx/CeO2 and (b) CoOx/CeO2 catalysts of various metal surface densities for steady-state reduction of NO by CO in the presence of a stoichiometric amount of O2 (NO:CO:O2 = 1:3:1) at 275 °C. An analogous situation is presented for steady-state reduction of NO by CO over CoOx/CeO2 at 275 °C (Figure 4b). The introduction of sub-monolayer surface CoOx species leads to the rapid increase in areal activity over the bare CeO2 support, as a result of the introduction of Co-O-Ce interfacial sites. The maximum activity of 0.0386 μmol NO/m2/s was observed over the 3.57 Co/nm2 CoOx/CeO2 catalyst. Higher Co surface densities lead to a decrease in activity as threedimensional Co3O4 microcrystals form and grow. It must be noted, however, that the maximum areal activity occurs at a higher Co surface density than the monolayer surface density reported above. As noted above, it is possible that the discrepancy is a result of poor interaction of the nitrate precursor with the CeO2 support during synthesis, leading to the early formation of threedimensional crystalline Co3O4 prior to the true monolayer coverage. The increasing areal activity beyond the monolayer identified in Figure 2b (2.58 Co/nm2) is a result of continued formation of accessible CoOx surface species with Co-O-Ce interfacial sites in parallel with the formation of

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the Co3O4 crystals, as unoccupied CeO2 support sites remained available. The XRD patterns for CoOx/Co3O4 (Figure 1b) contained reflections characteristic of the presence of Co3O4 starting at only 4.84 Co/nm2, indicating the propensity of the cobalt nitrate precursor to form Co3O4 crystals on CeO2 at much lower surface densities, especially compared to the FeOx/CeO2 system. The rapidly increasing Co2p/Ce3d ratio above 2.58 Co/nm2 in Figure 3b also indicates the rapid formation of bulk cobalt oxide crystals. By combining the results from Raman spectroscopy and catalytic activity, it is more accurate to state the surface density range for monolayer coverage of CoOx on CeO2 is 2.58 – 3.57 Co/nm2. It is suggested that additional studies utilizing a precursor other than nitrate, such as an acetate, could more narrowly define this range. While the precise molecular structure of the active site for NO reduction by CO on FeOx/CeO2 and CoOx/CeO2 is unknown, it has been demonstrated above and previously that the metal M-O-S interfacial site plays a critical role in the mechanism.11,25,26,47 Furthermore, corroborating evidence for the role of the M-O-S bond is found in the supported Group V/VI oxide literature, which has identified this bond as the likely active site in a variety of reactions.13,14 The current results strongly support the hypothesis that the M-O-S bond is the active site. To test this hypothesis, it would be desirable to define a turnover frequency (TOF) by normalizing the rate by the number of surface accessible M-O-S sites. If this TOF were found to be independent of the surface density, it would be strong evidence for identification of the NO reduction active site. A method to quantify the accessible M-O-S sites in FeOx/CeO2 and CoOx/CeO2 has yet to be found. Unfortunately, the molecular Fe-O-Fe, Co-O-Co, Fe-O-Ce, and Co-O-Ce features were not detectable in the Raman spectra.

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A series of cobalt oxide or iron oxide catalysts supported on ceria were prepared by incipient wetness impregnation, ranging in surface densities from 0.52-11.6 Co/nm2 and 1.11-9.36 Fe/nm2. The characterization by XRD revealed the highly dispersed nature of the Fe species, devoid of crystalline iron oxide features. Detection of a weak Co3O4 reflection began from 4.84 Co/nm2. Raman spectroscopy was utilized to detect the Fe or Co surface densities where oxide microcrystals formed on the CeO2 support as 5.40 Fe/nm2 and 2.58 Co/nm2, respectively, and these surface densities approximate the monolayer coverage. Similar results were obtained from analysis of the relevant Fe, Co, and Ce XPS peak ratios, which confirmed that ability of Raman spectroscopy to yield surface sensitive results for supported oxide catalyst systems.

As

hypothesized, the monolayer FeOx/CeO2 catalysts yielded the maximum areal activity for NO reduction by CO in a stoichiometric amount of O2. For CoOx/CeO2 the maximum areal activity was found at 3.57 Co/nm2, a surface density that was above the monolayer coverage identified by Raman spectroscopy, which suggests microcrystalline Co3O4 formation prior to complete coverage of the ceria support. Regardless, the maximized areal activities were determined at or around the monolayer loading for two separate transition metal oxides supported on CeO2, which confirms the catalytic importance of the metal-support interfacial M-O-S bond and any resulting EMSI. The above results showcase the utility of Raman spectroscopy to quickly produce structureactivity relationships for supported metal oxides and these relationships can be utilized to design supported transition metal oxide catalyst with increased activity for a variety of catalytic applications.

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Supporting Information. Table S1, Table S2, and Table S3 and Figure S1 and Figure S2. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author *E-mail: [email protected]; Tel: +1-734-995-3625; Fax: +1-734-995-2549. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements The authors are grateful for the support and discussion from the additional members of Catalytic Materials research team at the Toyota Research Institute of North America: Hongfei Jia, Michael Rowe, Kimber Stamm Masias, and Chen Ling. Hirohito Hirata and the catalyst researchers of Toyota Motor Corporation are thanked for their continued support of North American catalyst research.



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Monolayer Detection of Supported Fe and Co ... - ACS Publications

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