Comment on “Direct Decomposition of NO x over TiO 2 Supported Transition Metal Oxides at Low Temperatures”

In a recent paper entitled "Direct decomposition of NO x over TiO 2 supported transition metal oxides at low temperatures", 1 Damma et al. report highly promising results on the catalytic direct decomposition of NO to N 2 and O 2 at temperatures as low as 100 °C, with excellent tolerance to O 2 , H 2 and SO 2 , making the tested catalysts unique in the field of gas exhaust treatment for pollution control. Progress in this field is scarce, 2 and the development of efficient formulations for this reaction would be of the utmost interest to meet the ever more stringent emission regulations. However, as we attempted to reproduce the catalytic data of the above-cited paper, 1 we ended up with NO 2 as the product of NO conversion rather than N 2 and O 2 , which is consistent with previous reports highlighting materials similar to those prepared by Damma et al. as NO oxidation catalysts. 3,4 In our attempts to reproduce the catalytic data, we tested two samples of the Mn(20wt%)/TiO 2 Hombikat catalyst prepared according to the method reported by Damma et al., as this material exhibited the highest catalytic activity in the above-mentioned paper. 1 The first sample (here referred to as "sample 1") was prepared at LRS laboratory (France) using Hombikat TiO 2 (UV 100, Sachtleben Chemie GmbH). The second sample was kindly and diligently provided by one of the authors of the above-cited paper, 1 namely Prof. Smirniotis (University of Cincinnati, USA), following our request after we had informed him about the apparent discrepancy between our catalytic results obtained with sample 1 and those reported in his recent paper. 1 Prof. Smirniotis indicated that this second sample was not the one used to obtain the data reported in the original paper, 1 which was no longer available, but a newly-prepared batch (here referred to as "sample 2"). We wish to underline the constructive manner in which Prof. Smirniotis interacted with us as he prepared a fresh sample and sent it to us at his earliest convenience. As shown in Figure A, both of the repeat Mn/TiO 2 samples 1 and 2 exhibit powder X-ray diffraction (PXRD) patterns similar to that of the original sample reported by Damma et al., 1 displaying both peaks of anatase TiO 2 (JCPDS 21-1272) and of a Mn oxide phase. We actually identified the latter phase as MnO 2 (JCPDS 72-1984) and not Mn 2 O 3 , as indicated in the original paper by Damma et al. 1 The relative intensity of peaks was somewhat different, possibly due to a prefe