Microscopy study of the front and back sides of platinum catalyst gauzes used in ammonia oxidation

Abstract

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 the platinum loss and catalytic activity decrease. To reveal the mechanism of the catalytic etching of platinum catalyst gauzes during the NH 3 oxidation, we studied in detail the surface microstructure of the front and back sides of platinum catalyst gauzes used in ammonia oxidation. 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 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. A pack of four gauzes was loaded into the reactor to maintain standard conditions of the catalytic process. The surface microstructure was studied using a scanning electron microscope (SEM) JSM‐6460 LV (Jeol). Substantially different surface layer microstructure of the front and back sides of the polycrystalline wire in the first gauze relative to the gas flow after the treatment in the reactor at T ≈ 860 °C for 50 h in the reaction medium (ca.10% NH 3 in air) was observed after the SEM study. SEM images of the wire surface for the front and back sides of the gauze are shown in Figures 1 and 2, respectively. Images b and c in Figures 1 and 2 were obtained from the central part of the wires shown in Fig. 1a and Fig. 2a, respectively. The SEM images demonstrate that the front side of the wire was etched much more significantly than the back side due to the strong effect of the gas feed on this side of the wire. The wire surface on the front side of the gauze was covered by a continuous corrosion layer consisting of crystalline agglomerates with the sizes 5–15 μm separated by deep voids with the width 1–10 μm (Fig. 1a). The agglomerates had different shapes, crystalline faceting and contained through pores with the diameter 1–5 μm (Fig. 1b). The surface of these agglomerates consisted of crystalline facets without large defects (Fig. 1c). On the surface of the wire from the back side of the gauze weak etching was observed only at the wire interweaving places whereas the major part of the wire surface looked relatively smooth (Fig. 2a). Granular structure with 1–13 μm grains (Fig. 2b) separated by grain boundaries containing 200–400 nm pores was observed in the central part of the wire (Fig. 2a). Crystalline planes with the height ~ 100 nm and many dark spots with the diameter 50–150 nm were observed on the surface of the grains (Fig. 2c). Some of them had pyramidal shape resembling the shape of etching pits at the places where screw dislocation exit to the surface. The concentration of these pits is 4.0 x 10 8 cm ‐2 , which is close to the dislocation density in platinum. The obtained data indicate that the size of agglomerates on the front side of the gauze (5–15 μm) is close to that of grains observed on the back side (1–13 μm). This result seems to suggest that the etching develops in the course of gradual growth and transformation of the grains into crystalline agglomerates during the growth and merging of etching pits at the grain boundaries. Through pores with the size of 1–5 μm inside the agglomerates may be formed during merging of growing etching pits on the surface and in the bulk of the grains. The emergence and growth of the pits can be related to the reaction of ammonia molecules with oxygen atoms absorbed at the grain boundaries, dislocations and other surface defects. The reaction of gaseous NH 3 molecules with absorbed oxygen atoms O abs with the formation of gaseous NO results in local overheating of the surface initiating the release of metal atoms to the surface. Intense release of metal atoms from pits at the grain boundaries forms extended voids between the grains. Metal atoms released from the defects quickly migrate over the grain surface and are gradually incorporated at the energetically most favorable sites. As a result, the grains are gradually reconstructed into faceted crystalline agglomerates with through pores formed due to the growth and merging of pits. When these processes go on for a long time, a rough corrosion layer including crystalline agglomerates with through pores separated by deep extended void is formed (Fig. 1).