TY - JOUR

T1 - Catalytic Etching of Platinoid Gauzes During the Oxidation of Ammonia by Air. Reconstruction of the Surface of a Platinoid Gauze Backside in the Course of Ammonia Oxidation at 1133 K

AU - Salanov, A. N.

AU - Suprun, E. A.

AU - Serkova, A. N.

AU - Kochurova, N. M.

AU - Sidel’nikova, O. N.

AU - Sutormina, E. F.

AU - Isupova, L. A.

AU - Parmon, V. N.

N1 - Publisher Copyright: © 2018, Pleiades Publishing, Ltd.

PY - 2018/11/1

Y1 - 2018/11/1

N2 - Abstract: Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to study microstructure, morphology and chemical composition of the surface and near-surface layers of polycrystalline wire of commercial platinoid gauzes containing Pt (81 wt %), Pd (15 wt %), Rh (3.5 wt %), and Ru (0.5 wt %) after ammonia oxidation (10 vol % NH 3 ) by air at 1133 K for 50 h in the presence of these gauzes. Upon the completion of the catalytic reaction of ammonia oxidation, reconstruction (catalytic etching) of the surface layer on the backside of gauze wire (in the direction of the gas flow) was observed, in which the regions with different degrees of etching were identified. The analysis of these regions showed that the catalytic etching of the platinoid wire is initiated by etching the surface layer in the region of grain boundaries and dislocations in the course of highly exothermic catalytic reaction of ammonia oxidation by oxygen penetrated in the regions of defects. The regions with minimal etching contain smooth grains with crystalline terraces, 50 nm high, and with etching pits with size of ~72 nm in a concentration of 4.2 × 10 8 cm –2 . The region with medium etching includes rough grains with etching pits with size of ~85 nm in a concentration of 2.5 × 10 8 cm –2 . The regions with maximal etching consist of recrystallized grains with large pores with sizes of 350–400 nm in concentration of 8.9 × 10 6 cm –2 . These grains are separated by voids with a width of 1–5 μm and a depth of 10 μm, which increases the specific surface area in the surface layer of wire. The growth of the specific surface area of the platinoid wire is accompanied by an increase in the volume rate of ammonia oxidation and, as a result, local overheating due to the high exothermicity of the reaction. With increasing temperature, the rate of diffusion of metal atoms increases, which, in turn, accelerates etching in this region. These processes lead to increasing the region of etching along the wire, which points to the autocatalytic regime of etching of platinoid gauzes in ammonia oxidation by oxygen.

AB - Abstract: Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to study microstructure, morphology and chemical composition of the surface and near-surface layers of polycrystalline wire of commercial platinoid gauzes containing Pt (81 wt %), Pd (15 wt %), Rh (3.5 wt %), and Ru (0.5 wt %) after ammonia oxidation (10 vol % NH 3 ) by air at 1133 K for 50 h in the presence of these gauzes. Upon the completion of the catalytic reaction of ammonia oxidation, reconstruction (catalytic etching) of the surface layer on the backside of gauze wire (in the direction of the gas flow) was observed, in which the regions with different degrees of etching were identified. The analysis of these regions showed that the catalytic etching of the platinoid wire is initiated by etching the surface layer in the region of grain boundaries and dislocations in the course of highly exothermic catalytic reaction of ammonia oxidation by oxygen penetrated in the regions of defects. The regions with minimal etching contain smooth grains with crystalline terraces, 50 nm high, and with etching pits with size of ~72 nm in a concentration of 4.2 × 10 8 cm –2 . The region with medium etching includes rough grains with etching pits with size of ~85 nm in a concentration of 2.5 × 10 8 cm –2 . The regions with maximal etching consist of recrystallized grains with large pores with sizes of 350–400 nm in concentration of 8.9 × 10 6 cm –2 . These grains are separated by voids with a width of 1–5 μm and a depth of 10 μm, which increases the specific surface area in the surface layer of wire. The growth of the specific surface area of the platinoid wire is accompanied by an increase in the volume rate of ammonia oxidation and, as a result, local overheating due to the high exothermicity of the reaction. With increasing temperature, the rate of diffusion of metal atoms increases, which, in turn, accelerates etching in this region. These processes lead to increasing the region of etching along the wire, which points to the autocatalytic regime of etching of platinoid gauzes in ammonia oxidation by oxygen.

KW - catalytic ammonia oxidation

KW - catalytic etching

KW - dislocations

KW - energy-dispersive X-ray spectroscopy

KW - etching pits

KW - grain boundaries

KW - platinoid gauzes

KW - scanning electron microscopy

KW - METAL WOOL

KW - DYNAMICS

UR - http://www.scopus.com/inward/record.url?scp=85062594209&partnerID=8YFLogxK

U2 - 10.1134/S0023158418060137

DO - 10.1134/S0023158418060137

M3 - Article

AN - SCOPUS:85062594209

VL - 59

SP - 792

EP - 809

JO - Kinetics and Catalysis

JF - Kinetics and Catalysis

SN - 0023-1584

IS - 6

ER -