Enhanced Hydrogen Transport over Palladium Ultrathin Films through Surface Nanostructure Engineering

Abstract

Palladium ultrathin films (around 2 μm) with different surface nanostructures are characterized by TEM, SEM, AFM, and temperature programmed reduction (TPR), and evaluated in terms of H2 permeability and H2-N2 separation. A change in the characteristics of Pd seeds by controlled oxidation-reduction treatments produces films with the same thickness, but different surface and bulk nanostructure. In particular, the films have finer and more homogeneous Pd grains, which results in lower surface roughness. Although all samples show high permeo-selectivity to H2 , the samples with finer grains exhibit enhanced permeance and lower activation energy for H2 transport. The analysis of the data suggests that grain boundaries between the Pd grains at the surface favor H2 transfer from surface to subsurface. Thus, the surface nanostructure plays a relevant role in enhancing the transport of H2 over the Pd ultrathin film, which is an important aspect to develop improved membranes that function at low temperatures and toward new integrated process architectures in H2 and syngas production with enhanced sustainability.

Morphology and N2 Permeance of Sputtered Pd-Ag Ultra-Thin Film Membranes

“…Grain size is an important parameter to consider in the preparation of dense Pd-Ag membranes due to the possibility of defect formation in the space created between the grains. This is reflected in the hydrogen permeation properties of Pd-Ag thin films producing membranes with low hydrogen selectivity, as reported previously [19,20,21]. On the other hand, the grain size increases when increasing the substrate temperature [22].…”

“…Recently, Pereira et al [18] prepared Pd-Ag films (0.7–1.4-µm thick) by PVD-MS onto yttria-stabilized zirconia (YSZ)-γ-Al 2 O 3 (5-nm pore size) top layer support using two separate targets of Pd and Ag and at a substrate temperature of 473 K. The N 2 permeance of these membranes ranged from 8 × 10 −8 –9 × 10 −7 mol·m −2 ·s −1 ·Pa −1 at 200 kPa and room temperature depending on the Pd-Ag film thickness. On the other hand, some recent research showed that the microstructure and morphology of Pd-based layers (e.g., grain size, grain boundary effect) may play an important role in the hydrogen separation properties [19,20,21]. It is obvious that different membrane microstructures are obtained by using different preparation techniques (ELP, PVD, etc. )…”

Influence of Zeolite Protective Overlayer on the Performances of Pd Thin Film Membrane on Tubular Asymmetric Alumina Supports

“…Permeability tests of membranes were made using a stainless steel reactor (permeator) earlier described. , Both annular ends, between the membrane tube, and the permeator wall were sealed with molded graphite rings. The permeator was placed in an electrical furnace and heated to the desired temperatures (range 350–450 °C).…”