$Ni_3Al$(γ´) intermetallic is well known for its use as a high-temperature structural material due to its excellent high-temperature strength and good oxidation/corrosion resistance. Recently, its potential catalytic activity has been revealed for hydrogen production reactions such as methanol decomposition and methane steam reforming. Interestingly, this high activity is clearly found not only in the form of a fine powder, but also in the form of a flat foil. The γ´ foil is a thin sheet that has been successfully fabricated using a metallurgical process: cold rolling. With these two beneficial functions, i.e. high-temperature structural and catalytic properties, the foil can be used as a plate-type catalyst in a micro-channel reactor for hydrogen production.
For practical use, the activity of the γ´ foil must be further increased. As is well known, catalytic activity can be improved by increasing the surface area of the catalyst. Recently, Ma et al. proposed a method to enhance the catalytic activity using a NiAl(β´)/γ´ two-phase powder. The method was carried out by two steps; selective dissolution (acid leaching) and alkali leaching. The β´ phase was selectively dissolved in an acid solution, leaving a porous γ´ surface behind. And the alkali leaching made the porous γ´ surface Ni-enriched. That is, a porous and Ni-enriched surface was obtained by the two-step surface modification, enhancing the catalytic activity for methane steam reforming. In this study, an attempt is made to improve the activity using a binary Ni-Al two-phase alloy that consists of γ´ and a Ni solid solution (γ). With fine cuboidal γ´ particles in the γ matrix, the γ/γ´ two-phase structure is known as a fundamental structure of the Ni-base superalloys. This study attempts to selectively remove the γ from the alloy surface with the expectation of producing a rough, γ´-terminated surface. The advantage of the γ/γ´ alloy is that foil fabrication is possible, i.e. the γ/γ´ alloy can be cold rolled into thin foils more easily than the γ´ single-phase alloy; this advantage fits with the current study’s development plan of a plate-type catalyst.
The purpose of this paper is to enhance the catalytic activity of the γ´ foil using Ni(γ)/$Ni_3Al$(γ´) two-phase foil by the surface modification. In this study, the surface modification was a three-step process by selective dissolution of the γ phase, alkali leaching and additional hydrogen reduction. As is well known, hydrogen reduction should be accompanied before the catalytic property measurement. Namely, this hydrogen reduction is one of the surface modification methods. In present study, this three-step surface modification was carried out in Ni(γ)/$Ni_3Al$(γ´) two-phase foil and the catalytic property of the modified foil for methane steam reforming was examined. Based on the results of surface characterization, the effect of each surface modification method on catalytic activity was discussed.
In chapter 1, at first, the optimum condition for the selective dissolution of the γ phase in Ni(γ)/$Ni_3Al$(γ´) two-phase alloy was determined from the polarization curves of the γ and γ´ single-phase alloys in an aqueous electrolyte including 1wt.% $(NH_4)_2SO_4$ and 1wt.% citric acid. Secondly, the basic mechanism of the selective dissolution was proposed. The selective dissolution tests proved that γ was precisely removed above 1.7 $V_{SCE}$, resulting in the formation of a rough, γ´-terminated surface. Surface analyses revealed that a passive $AlO_x$, which retarded the dissolution, was preferentially formed on γ´, resulting in a successful selective dissolution.
In chapter 2, the selective dissolution method was applied to Ni(γ)/$Ni_3Al$(γ´) two-phase foil. Firstly, the selective dissolution was conducted for various dissolution times (0.5, 1.5, 3 and 5 h) in the γ/γ´ two-phase foils. In addition, the selective dissolution was also performed with the γ/γ´ two-phase foils having various two-phase structures. The γ/γ´ two-phase structure was controlled by 98% cold rolling and subsequent heat treatment at 600, 800 and 1000℃. The surface morphology after the selective dissolution reflected the original two-phase structure. The γ phase was selectively dissolved and the γ´ precipitates were exposed on the surface, obtaining various rough and γ´-terminated surfaces. Surface roughness increased with the dissolution time and the heat treatment temperature. Rougher surface was obtained increasing with the dissolution time and the foil dissolved for 5 h had the largest surface roughness. And coarser γ´ precipitate in the γ/γ´ two-phase structure made rougher surface and the foil heat-treated at 1000℃ had the largest surface roughness. Before the measurement of the catalytic activity, the alkali leaching and the hydrogen reduction were carried out on the various rough and γ´-terminated surfaces to make catalytically active sites. As a result, the catalytic activity was the highest in the foil having the largest roughness (the foil dissolved for 5 h and heat-treated at 1000 ℃).
In chapter 1 and 2, we focused on the selective dissolution method and found that the selective dissolution plays a role of surface roughening combining with the two-phase structure control. Therefore, in chapter 3, the role of the alkali leaching and the hydrogen reduction was examined. After the each surface modification steps: the selective dissolution, the alkali leaching and the hydrogen reduction, the surface morphologies were observed and the surface analyses were conducted. As a result, the alkali leaching made a Ni-enriched surface which mainly consists of $Ni(OH)_2$ and the hydrogen reduction changed some of $Ni(OH)_2$ to metallic nickel particles. In other words, the alkali leaching and the hydrogen reduction play a role of formation of catalytically active sites. That is, fine metallic nickel particles on the rough surface were attributed to increase the surface area, enhancing the catalytic activity.
In conclusion, the surface modification of Ni(γ)/$Ni_3Al$(γ´) two-phase foil by the selective dissolution, the subsequent alkali leaching and hydrogen reduction was effective to enhance the catalytic activity of the γ´ foil catalyst. In addition, the catalytic activity was enhanced by controlling the γ/γ´ two-phase structure. Rougher surface showed higher catalytic activity. In this method, the γ/γ´ two-phase structure having more equiaxed and larger γ´ precipitates were preferable to obtain the high catalytic activity.
본 연구에서는 금속간화합물 $Ni_3Al$(γ`) 촉매박판재의 촉매활성을 향상시키기 위하여, 박판재로서의 성형이 용이한 γ matrix에 γ` 석출물들이 분포되어있는 Ni(γ)/$Ni_3Al$(γ`) 2상박판재를 이용하여 선택부식을 통해 γ 상을 제거함으로써 rough하며 γ`-terminated한 박판재를 만들고자 하였다. 또한, 알카리처리와 수소환원처리를 통해 이러한 rough한 표면 위에 촉매활성점인 금속 Ni 입자를 형성시켜 그 촉매활성을 향상시키고자 하였다.
상대적으로 noble한 γ 상을 선택부식을 통해 제거하고자, Ni(γ)/$Ni_3Al$(γ`) 2상합금에서의 γ와 γ` 상과 동일한 조성을 갖는 γ와 γ` 단상합금을 제조하여 기본적인 전기화학적 거동을 알아보았다. 그 결과, active한 Al의 함량이 높은 γ` 상이 γ 상에 비해 일반적으로 active 함에도 불구하고, 이러한 성질은 transpassive 영역에서 전이되어 약 1.3 V 이상에서는 γ 상의 부식속도가 γ` 상에 비해 높은 것을 확인하였다. 이러한 γ와 γ` 상의 부식속도 차이를 이용하여, transpassive 영역에서의 potential에서 선택부식을 수행한 결과, 1.7 V 이상의 high transpassive 영역에서 정밀한 선택부식이 일어나는 것을 확인하였다. 이러한 부식속도의 차이는 선택부식 환경에서 active한 γ` 상의 표면에 선택적으로 안정한 Al-oxide가 형성되어 부식속도를 저하시켰기 때문으로 판단되었다. 또한, γ 상의 선택부식에 의해 표면에 γ` 석출물들이 노출됨으로써 표면이 rough해지고 γ`-terminated해지는 것을 확인하였다.
선택부식 후의 표면형상은 표면에 노출된 γ` 석출물들의 크기와 형태에 따라 달라질 것으로 판단되어, 선택부식시간을 달리하여 표면형상을 제어하였으며, γ matrix에 γ` 석출물들의 분포, 즉 2상 미세구조를 열처리를 통해 제어하여 다양한 표면형상을 얻고자 하였다. 선택부식 시간이 증가함에 따라 더 많은 양의 γ` 석출물들이 노출되어 박판재 표면을 rough하게 만드는 것을 확인하였으며, 조대한 γ` 석출물을 포함하고 있는 박판재일수록 표면이 더 rough해지는 것을 확인하였다. 본 연구의 실험조건에서는 선택부식 시간이 제일 긴 5시간의 선택부식을 수행한 박판재에서, 열처리 온도가 가장 높은 1000℃에서 열처리 된 박판재에서 가장 높은 촉매활성이 나타나는 것을 확인하였다. 이러한 결과는 일반적으로 bulk의 성질을 개선하고자 이용되는 미세구조제어가 촉매활성과 같은 표면성질을 개선하는 데에도 쓰일 수 있다는 가능성을 보여주는 결과이다.
선택부식을 통해 rough해지고 γ`-terminated해진 박판재를 이용하여 촉매활성점을 형성시키기 위하여 알카리처리, 수소환원처리를 수행하였다. Al의 선택적 용출에 의해 금속 Ni 입자들이 표면에 남는 일반적으로 알려진 알카리처리의 메커니즘과 달리, 본 연구에서는 알카리처리에 의해 표면에 flake형태의 Ni(OH)_2가 형성되고, 이러한 flake들이 수소분위기에서의 열처리를 통해 환원되어 미세한 금속 Ni 입자들로 변화되는 것을 확인하였다. 촉매활성은 이러한 알카리처리, 수소환원반응에 의한 촉매활성점 형성과 선택부식에 의한 roughening에 의해 향상되는 것을 확인하였다. 즉, 선택부식, 알카리처리, 수소환원처리에 의해 rough한 표면 위의 미세한 금속 Ni 입자가 형성되어 촉매활성향상에 큰 영향을 미친 것으로 판단되었다. 간략히 요약하자면, 본 연구에서는 선택부식, 알카리처리, 수소환원처리에 의한 표면개질방법에 의해 촉매특성이 우수한 금속간화합물 $Ni_3Al$(γ`) 촉매박판재의 제조에 성공하였다.
