COMMENT pubs.acs.org/JPCC
Reply to “Comment on ’Operando DRIFTS and XANES Study of Deactivating Effect of CO2 on a Ce0.8Cu0.2O2 CO-PROX Catalyst’” Arturo Martínez-Arias* and Marcos Fernandez-García* Instituto de Catalisis y Petroleoquímica, CSIC, C/Marie Curie 2, Cantoblanco, 28049 Madrid, Spain
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n a recent comment, Bazin and Rehr analyzed the size sensitivity of X-ray absorption near edge spectroscopy (XANES) in the case of metallic clusters.1 They show, in agreement with previous experimental and theoretical works, that XANES shape or, in other words, the energy position and intensity of the continuum resonances is strongly dependent on size and shape for metal clusters having less than ca. 50 atoms. As shown in the literature, this has both geometric and electronic grounds.2 5 The authors paid particular attention to the fact that principal component analysis (PCA) of XANES data can be biased by such size-related effects. In this sense, a small part of the comment is dedicated to one of our articles; it is concretely mentioned as an example of the use of a copper metallic foil as a component for the quantitative analysis of XANES results by PCA:2 “These numerical calculations show that the choice of the copper metallic foil is definitely not valid for Cu0 (55)” (this latter reference corresponds to ref 6 here). As mentioned, authors are critical of such type of practice on the basis of important differences found between the spectra of metal components as a function of their crystal size.1 Although we agree that choice of the most adequate components for the analysis requires consideration of mentioned changes as a function of metal particle size, no such mentioned use of a copper metallic foil reference is done in our work since all components employed for the analysis were extracted from the set of experiments carried out with the catalysts themselves (as specified in Figure 7 of our article)6 and no external reference spectrum is employed during the corresponding PCA analysis. In any case, we recognize that even our “internal/self-made” Cu0 reference extracted from the experiments may not be adequate to simulate particles of a very small size if important sintering effects occur during the treatment followed by XANES and subjected to PCA analysis. So, as mentioned by the authors of the comment, details of the very first steps of metallic copper genesis could be missed in this particular case. Nonetheless, in the case of the Ce0.8Cu0.2O2 catalyst with relatively high copper loading employed in our work, relatively large metallic copper particles are already formed from the first stages of its reduction, according to a recent work combining XAFS and XRD.7 Therefore the error in Cu0 evolution obtained by PCA analysis in our work is most likely relatively small. In addition, it may be noted that such Cu0 evolution is not relevant to the conclusions taken in our work.6 As a side, more general, comment, we would also like to mention that very small metal particles are usually supported on other materials, and thus the separation of “support” and “size/shape” effects on structural/electronic properties of metal nanoparticles is not straightforward from an experimental point of view. Thus, r 2011 American Chemical Society
comparison with theoretical analyses as those reported in the comment must await implementation of novel preparation and/or spectroscopic methods. To finish, speaking in a general way, we would like to emphasize that PCA analyses using iterative transformation factor analysis or any other iterative self-resolving or multivariate curve resolution procedure do not make use of external (e.g., outside the sample itself) references at any step of the fitting procedure and thus are not necessarily significantly biased by size and/or shape effects. This corresponds to references 2, 6, 8, and 9 here mentioned as well as some references (for example, 10, 11, and 12) also mentioned throughout the comment in the context of PCA analysis application to XANES data. On the contrary, this is not the case of many other examples presented in the literature where a linear fitting of the XANES spectrum using external references is performed. This is the case either (i) after a PCA analysis of the number of components and no subsequent use of selfresolving and/or multivariate methods or (ii) in fittings without a priori knowledge of the number of chemical species present in the sample and thus without connection with the PCA method. Note that this latter is the most frequent case concerning the use of external XANES references, typically metal foils, and is not described in the comment.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected].
’ REFERENCES (1) Bazin, D.; Rehr, J. J. J. Phys. Chem. C 2011, DOI: 10.1021/ jp2047773. (2) Fernandez-García, M. Catal. Rev. Sci. Eng. 2002, 44, 59. (3) Ankudinov, A. L.; Rehr, J. J.; Low, J. J.; Bare, J. S. J. Chem. Phys. 2002, 116, 1911. (4) Bazin, D.; Rehr., J. J. J. Phys. Chem. C 2003, 107, 12398. (5) Chen, H. M.; Liu, R. S.; Asakura, K.; Jang, L.-T.; Lee, J.-F. J. Phys. Chem. C 2007, 111, 18550. (6) Gamarra, D.; Fernandez-García, M.; Belver, C.; Martínez-Arias, A. J. Phys. Chem. C 2010, 114, 18576. (7) Ciston, J.; Si, R.; Rodriguez, J. A.; Hanson, J. C.; Martínez-Arias, A.; Fernandez-García, M.; Zhu, Y. J. Phys. Chem. C 2011, 115, 13851. lvarez, C.; Haller, G. L. J. Phys. (8) Fernandez-García, M.; Marquez-A Chem. 1995, 99, 12565. (9) Conti, P.; Zamponi, S.; Giorgetti, M.; Berrettoni, M.; Smyrl, W. H. Anal. Chem. 2010, 82, 3629. Received: July 13, 2011 Published: September 14, 2011 23237
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(10) Planer-Friedrich, B.; Suess, E.; Scheinost, A. C.; Wallschlager, D. Anal. Chem. 2010, 82, 10235. (11) Piovano, A.; Agostini, G.; Frenkel, A. I.; Bertier, T.; Prestipino, C.; Ceretti, M.; Paulus, W.; Lamberti, C. J. Phys. Chem. C 2011, 115 1311. (12) Iglesias-Juez, A.; Kubacka, A.; Fernandez-García, M.; Di Michiel, M.; Newton, M. A. J. Am. Chem. Soc. 2011, 133, 4484.
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