SURFACE AND INTERFACE ANALYSIS Surf. Interface Anal. 2002; 34: 105–111 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sia.1263 XPS characterization of adsorbed reaction intermediates on automotive exhaust gas catalysts: NO and CO + NO interactions with Pd A.-S. Mamede, G. Leclercq, E. Payen, P. Granger, L. Gengembre and J. Grimblot ∗ Universit ´ e des Sciences et Technologies de Lille, Laboratoire de Catalyse, UPRESA 8010, B ˆ at. C3, 59655 Villeneuve d’Ascq Cedex, France Received 16 July 2001; Revised 13 November 2001; Accepted 22 December 2001 Nitrous oxide (NO) adsorption and the CO + NO reaction have been investigated on bulk and alumina- supported Pd catalysts by XPS at various temperatures between 25 and 300 ◦ C with P NO in the 0.5–5.0 (×10 −2 ) atm range and P CO =5 × 10 −3 atm. A catalytic reactor settled in the preparation chamber coupled to the spectrometer allows surface changes occurring under catalytic conditions on these catalysts to be characterized. Various nitrogen-containing species have been detected, which depend on the oxidation state of Pd. The results have been explained tentatively in the light of a mechanism proposed for NO transformation. Copyright  2002 John Wiley & Sons, Ltd. KEYWORDS: Pd-based catalysts; XPS; NO adsorption; CO C NO reaction; three-way catalytic converters INTRODUCTION Three-way catalytic converters (TWCs) are now widely used to control NO x emissions from automotive exhaust gases in a wide range of air/fuel ratios. 1,2 Typical TWCs contain essentially Pt and Rh, supported over Ce-doped alumina, which significantly improves their oxygen storage capacity. The constant restrictions set by the legislation on NO x emissions imply a better optimization of the catalytic performances of noble metals in TWCs, particularly during the cold start when NO is not completely reduced into N 2 but mainly produces N 2 O. Among the different strategies that have been explored in the last two decades, the substitution of Pt and Rh by less expensive materials led to unsuccessful attempts from practical viewpoints. 3,4 Recently, the development of Pd-only three-way catalysts has received great interest 5,6 because the number of active sites can be potentially higher when Pt is replaced by Pd using similar loadings. Additionally, higher metal dispersion is usually obtained with Pd. The most undesirable factor that could further limit its development is the poor selectivity towards the transformation of NO into N 2 . Chafik et al. 7 found that the oxidation state of noble metals could play a key role in determining their catalytic performances. These authors suggest that the formation of N 2 mainly involves reduced sites whereas oxidized sites would be at the origin of N 2 O formation. In the literature, numerous studies can be found on the interaction of selected molecules (hydrocarbons, CO, NO x ) potentially present in automotive exhaust gases with noble L Correspondence to: J. Grimblot, Universit´ e des Sciences et Technologies de Lille, Laboratoire de Catalyse, UPRESA 8010, Bˆ at. C3, 59655 Villeneuve d’Ascq Cedex, France. E-mail: jean.grimblot@univ-lille1.fr metal surfaces. In particular, we can note the works of Ertl and co-workers, who extensively studied the oxidation of CO on single-crystal surfaces of metals such as Pd, Pt, Ir, Ru and Rh. 8 By contrast, in the present study we examined by XPS the reactivity of three different commercial Pd-containing catalysts whose surface morphology is very different from that of single-crystal planes. The originality of the present study concerns also the coupling of the spectrometer with a preparation chamber in which a small catalytic reactor has been settled. This allows the surface changes occurring on the catalysts after controlled exposure to NO under various partial pressures at room temperature or to the CO C NO mixture at different temperatures to be followed. Such a coupling permits practical catalytic conditions to be simulated, even if the catalysts are characterized under ultrahigh vacuum as required for XPS. EXPERIMENTAL Catalysts The investigated catalysts were Pd black (58 m 2 g 1 , Sigma) and alumina-supported palladium—Pd(10 wt.%)/Al 2 O 3 (100 m 2 g 1 , Fluka product) and Pd(1 wt.%)/Al 2 O 3 (74 m 2 g 1 , Alfa Aesar)—denoted bulk Pd, Pd 10 and Pd 1 , respec- tively. The metal dispersions estimated from adsorption and electron microscopy measurements are 0.10, ¾0.25 and ¾0.40 on bulk Pd, Pd 10 and Pd 1 , respectively. The powders were pressed mechanically into thin pellets and fitted on the sample block holder. Some small amounts of impuri- ties have been found on these commercial catalysts, i.e. Mo, K, Na, Cl and S. Their binding energies (BEs) and inten- sities were dependent on the treatment of the catalysts. Their presence will not be considered for interpreting the interaction between the molecules (NO, CO) and Pd. Note, Copyright  2002 John Wiley & Sons, Ltd.