Scanning electron microscopy study of platinum catalyst gauzes treated in air, ammonia and
NH3
in air
Authors: Salanov Aleksei , Suprun Evgenii , Serkova Alexandra , Sidelnikova Olga , Sutormina Elena , Isupova Lyubov , Parmon Valentin 
Published: December 2016

Ammonia oxidation with air on platinum catalyst gauzes is widely used in chemical industry for synthesis of nitric acid. It is well known that during this process the gauzes undergo deep structural rearrangement of surface layers (catalytic etching) leading to a loss of platinum and decrease of catalytic activity. To elucidate the contributions of individual reactions of O
2
and NH
3
with the platinum surface in the catalytic etching of platinum catalyst gauzes during the NH
3
oxidation, we carried out detailed investigation of the surface microstructure of platinum catalyst gauzes treated in air, ammonia, and in the reaction medium (NH
3
+O
2
). The platinum catalyst gauzes used in the study were made from a polycrystalline wire with d ≈ 82 μm, with the chemical composition (in wt.%) 81% Pt, 15% Pd, 3.5% Rh and 0.5% Ru. A laboratory flow reactor made of a quartz tube with the inner diameter of 11.2 mm was used at the feed (ca.10% NH
3
in air) flow rate 880–890 l/h, the gauze temperature 860±5 °C and total pressure ca.3.6 bar. The surface microstructure was studied using a scanning electron microscope (SEM) JSM‐6460 LV (Jeol) in the modes of secondary electrons (SE) and backscattered electrons (BSE) at beam energy 20 keV.

Different microstructure of the polycrystalline catalyst wires was observed by SEM after the treatment of the platinum gauzes at T ≈ 860 °C for 50 h in air, ammonia and the reaction medium (ca.10% NH
3
in air). A grid structure consisting of dark bands with the width 0.5–1.0 μm located along the grain boundaries separating the surface of bright grains was formed on the surface of the wires during the interaction of platinum gauzes with air (Fig. 1a,b). Increased concentrations of Rh, C and O (22.1, 31.6, 40.1 at.%, respectively) were observed in the dark areas. For comparison, the concentrations of these elements in the bright areas were 5.3, 0.3 and 13.7 at.%, respectively. The grid structure was registered on the wire surface by both the SE and BSE modes. This indicates that such the structure is spreading deep into the bulk of the wire because the depths of the analysis are significantly different in these modes, ≤10 and ~300–500 nm, correspondingly. The grid structure is formed during decomposition of the surface film including the graphite‐like layer on the Rh
2
O
3
oxide film covering the surface of the grains and grain boundaries in the bulk of the polycrystalline wire. The surface films are first removed from the surface of the grains and then from the grain boundaries as a result of carbon atom diffusion from these areas to the surface.

A micrograin structure with 1–30 μm grains was formed on the wires surface during the reaction of NH
3
with the platinum catalyst gauzes (Fig. 2a,b). No carbon was observed on the surface of this structure. Apparently, the reaction of ammonia with the carbon and oxide films is very fast leading to the destruction of the surface films on the grain surface and in the grain boundary areas. Fast destruction of the graphite‐like and oxide films in the grain boundaries leads to growing the grains to ca.30 μm. A dramatic structural transformation of the surface layer of platinum catalyst gauzes (catalytic etching) with the formation of a rough layer containing microcrystals and porous aggregates with the size as large as 10–20 μm separated by extended voids with the width ca.1–10 μm occurs during the catalytic NH
3
oxidation with air (Fig. 3a,b). At T ≈ 860 °C the reaction of gaseous NH
3
molecules with O
ads
atoms at the grain boundaries and other surface defects with the formation of gaseous NO leads to a sufficient local overheating of the surface whith initiates the release of metal atoms on the surface. Pits, pores and facets grow on the surface due to these processes. An intense release of metal atoms from the grain boundaries results in the formation of extended voids between the grains, which are gradually reconstructed into faceted crystalline agglomerates with through pores formed during the growing and merging of pits. A prolonged occurrence of these processes leads to the formation a rough corrosion layer on the wire surface in the platinum catalyst gauzes (Fig. 3a,b).