Surface properties of binary and ternary alloys under the presence of CO, CO2 and H2S

LAURA M. CORNAGLIA; ANA M. TARDITI; AGUSTINA DALLA FONTANA

Santa Fe

Conferencia; VI San Luis School and Conference on Surfaces, Interfaces and Catalysis; 2018

Pd-based membranes are the most promising technology to be employed in hydrogen purification processes, due to their high permeability and infinite selectivity. The permeation mechanism through Pd film is the solution-diffusion which consists of the following stages: (i) molecular hydrogen adsorption on the surface, (ii) molecular dissociation, (iii) atomic diffusion through the alloy, (iv) recombination and (v) desorption of H2 from the permeate side. Although their excellent properties, Pd and most of its alloys are very sensitive to sulfur compounds even at low ppm levels1,2. Along with H2S, CO is another important contaminant normally present when generating H2 from fossil fuels. For this reason, different binary and ternary Pd-based alloys have been studied.The study of surface properties of the membranes exposed to operation conditions is a necessary step for a better understanding of the phenomena involved in the hydrogen permeation and for the optimization of their performance. With the aim to analyze the effect of CO, CO2 and H2S on the surface properties of PdAu and PdAgAu alloys, XPS and LEIS experiments were performed after exposure to different treatments: i) H2/Ar, ii) CO/He, iii) CO2/Ar and iv) after exposure to H2S(100 ppm)/H2. The surface composition of the PdAu binary membrane did not change after treatments, showing an average Au relative composition of about 4% at the surface. On the contrary, the ternary alloy showed a higher silver surface composition (35 at. %) with respect to the bulk composition (15 at. %) after reduction in H2 flow, indicating a silver surface enrichment under these conditions. After exposure to a CO2(20%)/Ar stream, a higher silver surface composition was observed (42 at. %). A high concentration of silver on the surface could decrease the dissociation sites on the alloy surface or change the reactivity for hydrogen dissociation and consequently, the hydrogen permeation decreases. This result could explain the higher suppression on the hydrogen flux upon exposure to CO2 observed on the PdAgAu membranes prepared by electroless deposition. It is important to note that the gold composition remained practically constant. On the other hand, after exposure the ternary alloy to a CO(5%)/He atmosphere, a palladium surface enrichment occurred.