Applied Catalysis A: General 478 (2014) 21–29 Contents lists available at ScienceDirect Applied Catalysis A: General jou rn al hom ep age: www.elsevier.com/locate/apcata Crystalline structure refinements and properties of Ni/TiO 2 and Ni/TiO 2 -Ce catalysts and application to catalytic reaction of “CO + NO” Adolfo Romero-Galarza, Kevin A. Dahlberg, Xiaoyin Chen, Johannes W. Schwank ∗ Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA a r t i c l e i n f o Article history: Received 18 December 2013 Received in revised form 18 March 2014 Accepted 23 March 2014 Available online 30 March 2014 Keywords: Anatase Nickel Ceria Rietveld refinement Structural defects CO + NO reaction a b s t r a c t Catalysts containing 7 wt% Ni were prepared by impregnation of anatase-phase TiO 2 and Ce-doped TiO 2 that were synthesized by sol–gel techniques. The catalysts were extensively characterized by a variety of methods including BET surface area measurements, temperature programmed H 2 reduction (H 2 -TPR), scanning transmission electron microscopy (STEM), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for CO adsorption. The crystallography, crystallite size, microstrain in the crys- talline structure and concentration of all phases in the catalysts were determined by X-ray diffraction and refinement of the crystalline structure with the Rietveld method. As catalytic probe reaction, the stoichiometric oxidation of CO by NO was used. Doping with ceria affected the crystallography of the anatase-phase titania, increased the structural defects, and decreased the anatase crystallite size. This, in turn, led to more extensive contact of nickel particles with the support facilitating the reduction of nickel and of the surface of the Ce-doped TiO 2 . Among the various catalysts investigated, the Ce-doped Ni/TiO 2 catalyst with a Ce/(Ce + Ti) ratio of 0.1 was found to be the most active for CO oxidation by NO. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The emission of nitrogen oxides (NO x ) and CO from vehicles is a great worldwide environmental concern. One possible reaction for eliminating NO x and CO emitted from gasoline engines is via the catalytic stoichiometric oxidation of CO by NO (“CO + NO”) over a three-way catalyst (TWC) to form CO 2 and N 2 . The high cost of the precious metals Pt, Pd, and Rh that are widely used in TWC [1–10] has motivated the search for less expensive metals and oxides, including for example Cu [11–15], Ag [16–18], and Ni [19–21]. Wang et al. [19] studied ceria-supported NiO for the reduction of NO in stoichiometric NO/CO and NO/CO/O 2 . In a follow-up study on Ni/CeO 2 catalysts [20], DRIFTS-coupled with MS was used to inves- tigate the reaction mechanism of CO and NO under two different conditions, namely NO reaction with CO pre-treated catalyst and CO reaction with NO pre-treated catalyst. CeO 2 is an interesting support material as it has electrical conductivity and high oxygen storage capacity. Some authors [1,22,23] have proposed that the presence of oxygen vacancies in CeO 2 imparts higher activity in the reaction of “CO + NO” by increasing the oxygen mobility and regen- erating the active sites. It is well known that supports such as CeO 2 ∗ Corresponding author. Tel.: +1 734 763 3374. E-mail address: schwank@umich.edu (J.W. Schwank). or TiO 2 can have quite interesting effects on the reactivity of cata- lysts compared to alumina or silica [22,24–38]. But to the best of our knowledge, no studies have been reported on the “CO + NO” reac- tion over nickel catalysts supported on anatase-phase TiO 2 doped with cerium. Cerium can be dissolved in titania [39,40], improving its thermal stability by hindering the thermal sintering of titania particles [41,42]. Most previous studies using TiO 2 as support were focused on commercial titania, such as Degussa P25, or titania synthesized by sol–gel technique with different phase ratios of anatase to rutile [19,28–30,43–46]. Studies where titania polymorph anatase or pure rutile phase were used as the catalyst support are rare. Nev- ertheless, there is evidence that the TiO 2 phase can play a significant role. Previous work [47] has shown that the reduction temperature of CuO nanoparticles supported on TiO 2 depends on its allotropic form, i.e. CuO is reduced at lower temperatures when CuO is sup- ported on the anatase phase of titania than its counterpart rutile phase. Yan et al. [48] observed higher activity for CO oxidation when gold was deposited on the brookite phase. Bokhimi and Zanella [39] studied in detail the crystallography and microstructures of gold catalysts on titania polymorphs (anatase, brookite, and rutile). The study showed that the gold crystallite size and morphology depends on the titania polymorph used as support. So it is impor- tant to realize that each titania phase affects in different ways the properties of a catalyst in terms of interaction between metal http://dx.doi.org/10.1016/j.apcata.2014.03.029 0926-860X/© 2014 Elsevier B.V. All rights reserved.