Preferential CO oxidation in hydrogen (PROX) on ceria-supported catalysts, part I: Oxidation state and surface species on Pt/CeO2 under reaction conditions

Abstract

The CO content of hydrogen feed to proton exchange membrane fuel cells (PEMFC) must be kept under 1-100 ppm for their proper operation. This can be achieved by using catalysts able to selectively oxidize CO in the presence of excess hydrogen (PROX). The present study reports on the mechanism of PROX reaction on Pt/CeO 2 catalyst, by using catalytic tests, in-situ DRIFTS, high-pressure XPS, HRTEM and TDS techniques. Bulk metallic, pronounced adsorbate-induced surface Pt and a small amount of oxidized Pt sites were detected by in-situ, high pressure XPS under PROX conditions. The pre-oxidized ceria surface was strongly reduced in pure H 2 but significantly re-oxidized under PROX conditions (i.e. O 2 +CO in excess hydrogen) at T=358 K. The remaining small amount of Ce 3+ decreased with increasing temperature. HRTEM found well-crystallized CeO 2 particles (8-10 nm) in the case of activated (pre-oxidized) sample that transformed in a large extent to an oxygen deficient ceria super-cell structure after PROX reaction. Metallic Pt particles (2-3 nm) and small (0.5-0.6 nm) Pt clusters were found by HRTEM. These findings were in accordance with the variations in relative intensity of the corresponding Pt-CO bands (DRIFTS). Different types of carbonate and formate species were detected (XPS and DRIFTS). Their possible role in the reaction mechanism is discussed. Resolved OH bands could not be found by DRIFT in the PROX reaction mixture indicating significant amount of adsorbed water in a hydrogen-bonded structure. Its presence seems to suppress hydrogen oxidation while CO oxidation still takes place, as the metallic particles are covered by CO (DRIFTS). The direct contribution of surface water in a low-temperature water-gas-shift (LTWGS) type reaction in the PROX mixture is proposed.

Influence of the metal precursor on the catalytic behavior of Pt/Ceria catalysts in the preferential oxidation of CO in the presence of H2 (PROX)

“…As a result, the oxygen concentration over the catalyst would be virtually constant up to a certain oxygen inlet concentration, when the maximum oxygen storage capacity (OSC) of ceria is exceeded [11]. This result is in agreement with the values reported in the literature [10,13,24,36,53] which show a similar dependence of the CO oxidation reaction with the λ value. Mariño et al [10] studied the influence of the gas composition for platinum and iridium catalysts.…”

“…The differences between the DRIFT spectra for both catalysts may be related with the differences observed in catalytic activity. As described in the literature [13,56,57], the surface -OH groups may facilitate the formation of carbonate species by reaction with CO. Föttinger et al [56] suggested that the formation of carbonates may be associated with two models, the first one being related with the dissociation of CO on the noble metal followed by the reaction of CO 2 with the support to form the surface carbonates, and the second one being related with 18 the direct reaction between CO and -OH groups of the support, followed by CO 2 reaction with additional -OH groups. As described above, at 333 K, the Cl-free catalyst presented a higher catalytic activity and a large deactivation rate with time on stream (Fig.…”

“…In agreement with these results, this shift was due to the formation of an active electronegative species by chlorine. The characteristic bands for gas phase 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 19 CO was observed in the range 2120-2170 cm -1 [13,55]. In this region, bands corresponding to CO adsorbed on sites belonging to a CeOCl phase, where Ce (IV) was reduced to Ce (III), can be found.…”

“…For both systems, the CO and H 2 oxidation rates were increased with an increase in the  value, while the selectivity towards CO 2 formation fell. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 cm -1 , corresponding to polydentate carbonate [13,41,53,55,56]. The presence of formate was confirmed by peaks at 2926 cm -1 and 2840 cm -1 , which are characteristic of -CH stretching vibrations, these bands being more pronounced for the Cl-free catalyst.…”

“…Pozdnyakova et al [13] suggested that the reaction mechanism in Pt/CeO 2 catalysts involves -OH groups on the surface. Their presence seems to suppress hydrogen oxidation while CO oxidation still takes place, as the metallic particles are covered by CO.…”