Scanning electron microscopy study of rhodium, palladium and platinum foils treated in NH3‐air flow

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 a loss of platinum and decrease of catalytic activity. To determine the role of individual metals: Pt, Pd and Rh in the catalytic etching of platinum catalyst gauzes during the NH 3 oxidation, we carried out detailed investigation of the surface microstructure of platinum, palladium and rhodium catalysts treated in the reaction medium (NH 3 +O 2 ). Polycrystalline Pt, Pd and Rh foils with the size of 10 x 5 x 0.04 mm were used as the catalyst samples. Each sample was assembled into a package with four platinum gauzes required to maintain standard conditions of the NH 3 oxidation process. The platinum catalyst gauzes were made from a polycrystalline wire (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 about 3.6 bar. The surface microstructure was studied using a scanning electron microscope (SEM) JSM‐6460 LV (Jeol) in the mode of secondary electrons at beam energy 25 keV. The SEM study of polycrystalline Pt, Pd and Rh foils after treatment at T~860°C for 5 h in the reaction medium (~10% NH 3 in air) revealed differences in the surface microstructure of these samples. The O 2 reaction with Rh during the catalytic oxidation of NH 3 over rhodium foil results in deep rhodium oxidation followed by the formation of a continuous layer of Rh 2 O 3 crystals with the size 1‐2 μm. Fast reaction of gaseous NH 3 molecules with O atoms of rhodium oxide leads to the formation of oxygen vacancies and movement of Rh atoms to the surface of the oxide crystals. Rh atoms quickly migrate over the oxide surface and desorb into the gas phase. Increased concentration of Rh atoms in the near‐surface gas layer initiates the formation and gradual growth of elongated pyramidal Rh crystals with low concentration of defects. Such continuous processes lead to the formation of a solid layer of pyramidal Rh crystals with the sizes 0.3‐0.5 μm at the base, 0.05‐0.1 μm at the end and the length 1‐2 μm (Fig. 1a,b). The O 2 interaction with Pd during the NH 3 oxidation on palladium foil leads to intense dissolving of oxygen atoms at defects and in the metal lattice, whereas the resulting oxide PdO quickly decomposes under these conditions. 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 the grain boundaries leads to the formation of extended voids between the grains. Adsorbed Pd atoms quickly migrate over the metal surface and get incorporated into energetically the most favorable sites. Due to these processes, pits, pores and crystalline facets grow on the surface, whereas grains are gradually reconstructed into faceted crystalline agglomerates with through pores formed due to the growth and merging of pits. So, dramatic structural reconstruction of the foil surface layer (catalytic etching) with the formation of a rough layer takes place during the catalytic NH 3 oxidation with air on Pd. This rough layer contains microcrystals and porous agglomerates with the size of ~10‐20 μm containing pores with the diameter 1‐2 μm separated by voids with the width ~1‐10 μm (Fig. 2a,b). The O 2 interaction with platinum during the NH 3 oxidation over Pt foil results in removal of the surface carbon impurities followed by dissociative chemisorption of oxygen on the surface. It is well known that oxygen dissolution in the Pt lattice with the formation of oxide phases is substantially slower than on Pd and Rh. Small amount of oxygen atoms can be absorbed at the grain boundaries and other defects. The NH 3 reaction with O abs at these defects initiates release of a few Pt atoms to the surface leading to weak etching of the platinum foil surface. The catalytic NH 3 oxidation on Pt at T~860°C for 5 h results in minor structural reconstruction of the foil surface layer related to the formation of grain boundaries and shallow parallel furrows with the width 1‐2 μm covered with crystalline facets (Fig. 3a,b).