As one of devices for converting NOx contained in diesel exhaust gas into harmless substances, urea selective catalytic reduction (hereinafter abbreviated as SCR) is practically applied.
The SCR is a system of hydrolyzing aqueous urea jetted on an inlet side of an SCR reactor vessel by heat of exhaust gas to produce ammonia, and reducing nitrogen oxides (NOx) in the exhaust gas and the ammonia using an SCR catalyst in the SCR reactor vessel to convert them into harmless substances such as nitrogen and water.
A diesel particulate filter (DPF) for capturing a diesel particulate matter (hereinafter referred to as PM) and a diesel oxidation catalyst (DOC) for oxidizing unburned fuel are connected to an upstream side of the SCR. The temperature of exhaust gas flowing through the SCR reactor vessel varies in a wide range of 200° C. during low loading (at the time of starting of an engine, etc.) to 600° C. during reproduction of DPF.
In recent years, it is investigated that a catalyst carrier of a honeycomb structure that carries a catalyst is applied to SCR, DOC, and the like. As a catalyst carrier, a catalyst carrier for purifying exhaust gas has been known (see, for example, Patent Literature 1). In the catalyst carrier, a honeycomb structured body having a roll shape in which a band-shaped corrugated metal foil and a band-shaped flat metal foil are wound in a multilayer shape is inserted into an outer jacket, and a catalytic material is attached to the corrugated metal foil and the flat metal foil. The corrugated metal foil has many fins having different phases between the front and rear in an axial direction in which the foils are wound (i.e., an offset structure).