Inhibition of Ammonia Poisoning by Addition of Platinum to Ru/α‐Al₂O₃ for Preferential CO Oxidation in Fuel Cells

Abstract

In polymer electrolyte fuel cell (PEFC) systems, small amounts of ammonia (NH3 ) present in the reformate gas deactivate the supported ruthenium catalysts used for preferential oxidation (PROX) of carbon monoxide (CO). In this study, we investigated how the addition of a small amount of platinum to a Ru/α-Al2 O3 catalyst (Pt/Ru=1:9 w/w) affected the catalyst's PROX activity in both the absence and the presence of NH3 (130 ppm) under conditions mimicking the reformate conditions during steam reforming of natural gas. The activity of undoped Ru/α-Al2 O3 decreased sharply upon addition of NH3 , whereas Pt/Ru/α-Al2 O3 exhibited excellent PROX activity even in the presence of NH3 . Ruthenium K-edge X-ray absorption near-edge structure (XANES) spectra indicated that in the presence of NH3 , some of the ruthenium in the undoped catalyst was oxidized in the presence of NH3 , whereas ruthenium oxidation was not observed with Pt/Ru/α-Al2 O3 . These results suggest that ruthenium oxidation is retarded by the platinum, so that the catalyst shows high activity even in the presence of NH3 .

Operando Spectroscopic Study of the Dynamics of Ru Catalyst during Preferential Oxidation of CO and the Prevention of Ammonia Poisoning by Pt

“…We previously reported that the conversion of 130 ppm of NH 3 in the gas mixture was less than 30% and that the effluent gas included trace amounts of NO x over Ru/α-Al 2 O 3 . 31 A trace amount of NO 2 was detected in the effluent gas in our previous study; therefore, we surmised that NO 2 is related to deactivation of the Ru catalyst. However, during the operando IR investigation in the present study, no adsorption peaks assignable to NO 2 were observed.…”

“…Previously, we reported that the deactivation of Ru/α-Al 2 O 3 in the presence of NH 3 is enhanced with increasing O 2 /CO ratio. 31 Therefore, to emphasize the deactivation behavior of Ru/Al 2 O 3 and the difference between Ru/α-Al 2 O 3 and Pt/Ru/α-Al 2 O 3 , we carried out the operando study under flowing gas with CO/O 2 = 1/4 mol/mol. First, Ru/α-Al 2 O 3 was examined ( Figure 4 ).…”

“…Pt was sequentially impregnated into Ru/α-Al 2 O 3 , and the loading amount was set to 0.2 and 1.8 wt % for Pt and Ru, respectively (Pt/Ru = 1/17 mol/mol). Details of the preparation procedures are described in a previous report 31 and the Supporting Information .…”

“…The influence of impurities in the reformate on the catalytic activity and durability of PROX catalysts is an important topic of research. − For example, ammonia (NH 3 ) deactivates the conventional Ru-based PROX catalyst Ru/α-Al 2 O 3 . In domestic PEFC systems, natural gas is subjected to steam reforming, also over an Ru catalyst; however, these catalysts catalyze the synthesis of NH 3 from hydrogen and trace amounts of nitrogen that are also present in natural gas. , This results in the reformate containing NH 3 (usually 10–80 ppm) that deactivates the catalyst. , …”

“…Previously, we studied the activities of two Ru-based PROX catalysts, the conventional Ru-based PROX catalyst Ru/α-Al 2 O 3 and a Pt–Ru bimetallic catalyst (Pt/Ru/α-Al 2 O 3 ), in the presence of NH 3 . In the deactivated Ru/α-Al 2 O 3 catalyst, we found that the Ru was partially oxidized.…”