Article
Hydrothermal treatment of metallic-monolith catalyst support with self-growing porous anodic-alumina film

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Abstract

Metallic-monolith catalyst support with self-growing porous anodic alumina (PAA) film was prepared by anodizing Al plate. The effect of hydrothermal treatment (HTT) on the crystalline state and textural properties of PAA film was investigated by XRD, BET, SEM and TG. The HTT treatment above 50 °C and the subsequent calcination above 300 °C could convert the amorphous skeleton alumina into γ-alumina and increase the specific surface area (SBET). However, SEM images showed the HTT modification was a non-uniform process along the thickness of PAA film. The promotion effect of HTT on SBET was non-linear, and the slope of SBET gradually decreased with the HTT temperature or time increased. The limited HTT effect should be attributed to a changed pore structure caused by an unfavorable pore sealing limitation. Pore widening treatment (PWT) before HTT could break the pore sealing limitation, because of the reduced internal diffusion resistance of hydrothermal reaction. The synergistic combination of PWT and HTT developed a PAA support with a large SBET comparable to commercial γ-alumina. In the catalytic combustion of toluene, the Pt-based catalyst prepared by using the PWT and HTT co-modified PAA support gave higher Pt dispersion and more favorable catalytic activity than that treated by HTT alone. The presence of a bimodal pore structure was suggested to be a decisive reason.

Introduction

Over the last decade, metallic-monolith catalyst (MMC) support developed based on stainless steel or FeCrAl is considered to be a promising alternative to the conventional cordierite-monolith catalyst supports because of the thinner wall thickness, stronger mechanical strength, and higher thermal conductivity [1,2]. However, smooth metal surface and high thermal expansion coefficient of the metal substrate easily cause peeling of the catalytic coating layer (prepared by dip coating) from the metal substrate. In order to improve the adhesion of coating layer on metal substrate, recent research focused on surface pretreatment of the metal substrate by acid/base corrosion to increase the surface roughness, or on formation of a thin and rough interlayer prepared by high-temperature oxidation (especially FeCrAl substrates [3]) to alleviate a thermal expansion mismatch. Nevertheless, it is commonly accepted that the adhesion of coating layer on MMC is inferior to that of the cordierite-monolith catalyst supports.

Some research groups attempted to use self-growing film of metal substrate as catalytic support layer instead of the conventional coating layer, as seen in the α-alumina whisker layer prepared by the high-temperature oxidation [3]. However, due to the thin thickness (typically less than 1 μm) and inert alumina, this α-alumina whisker layer is more commonly used as an interlayer between the wash-coating layer and the alloy substrate to improve the adhesion of the wash-coating layer [4]. Another self-growth film is a self-growing alumina film prepared by steam-oxidizing an Al substrate. For example, Han et al. [5] used Al-mesh as a substrate, treated with steam at 120 °C for 12 h, and then calcined in air at 600 °C for 4 h to obtain an Al2O3@Al-mesh monolith catalyst support. This in-situ self-growing layer typically has a thickness of about 1 μm and a specific surface area of about 15 m2·g−1 [[5], [6], [7]]. Desired catalysts can be obtained by a wetness impregnation method. In addition to the aforementioned self-growing film, another type of metal self-growing film also received considerable attention, namely porous anodic-alumina (PAA) film. The PAA film is derived from the in-situ self-growth of the Al substrate, thus bringing about a high adhesion between the porous layer and the unoxidized Al substrate. For more than half a century, the adhesion of PAA film has frequently been verified in many fields such as corrosion protection and metallic coloration (including coloration of iPhone mobile phone shell). Using the PAA film on Al substrate as the catalytic support layer is deemed to be an approach of developing a new MMC catalyst, and therefore, has received increasing attention. Some relevant research groups [[8], [9], [10], [11], [12]] and our group [[13], [14], [15], [16]] applied the novel MMC catalyst support to various fields such as hydrogenation reaction [8], steam reforming of methanol [9], Suzuki cross-coupling reaction [10], Fischer-Tropsch synthesis [11], VOCs combustion [12] and steam reforming of methane [16].

However, unmodified PAA support typically has a specific surface area of 5–40 m2·g−1 [[8], [9], [10],[17], [18], [19], [20], [21], [22], [23], [24], [25], [26]], which is much lower than that of conventional alumina powders or pellets. Furthermore, alumina in the PAA film is amorphous [14,15,21], whereas commercial alumina-based catalysts commonly use γ-alumina with a high specific surface area. For this issue, some literature [12,21,22] and our previous study [14,15,23] found that hydrothermal treatment (HTT, namely soaking the unmodified PAA support in hot water) could convert the amorphous skeleton alumina to γ-alumina and increase its specific surface area. Zhang et al. [21,26] clearly detected the presence of γ-alumina and the increased specific surface area from 5–15 m2·g−1 to 80–100 m2·g−1, by immersing the PAA support in hot water at 80–90 °C for 1–2 h and then calcining at 500 °C. Some researchers [[24], [25], [26]] achieved similar conclusions by using almost identical conditions. On the other hand, Zhang et al. [21] also reported that a larger specific surface area became absent when the HTT time was further prolonged. The phase-change saturation of amorphous alumina into γ-alumina was proposed to be the main reason for the limited effect of HTT.

Furthermore, in order to improve the application temperature of the PAA support, our previous research [15] used an Al/Fe-Cr alloy/Al clad plate to prepare a high-temperature type PAA support. We found that a PWT treatment (pore widening treatment, immersing the unmodified PAA support in oxalic acid solution) ahead of HTT could significantly enhance the adhesion of this PAA support [15]. It was also observed that a larger specific surface area can be obtained when the PAA support was subjected to the PWT treatment before HTT. However, our previous work has not addressed sufficient evidence and interpretations for the increased specific surface area. Our latest work confirms that the HTT treatment is a non-uniform process along the thickness of PAA film, and the limited promotion effect of HTT on specific surface area (as reported in literature [21]) should be attributed to the changed pore structure rather than phase-change saturation to γ-alumina, which is rarely discussed comprehensively in literature. In this work, based on our latest research results, the HTT effect on the crystalline state and textural properties of PAA support is investigated to reveal fully the HTT mechanism. Finally, the complete oxidation of toluene is chosen as a probe reaction to examine the synergistic effect of PWT and HTT.

Section snippets

Catalyst support preparation

Commercial Al plate (JIS A1050, thickness 0.3 mm) was anodized in 4.0 wt% of oxalic acid for 8 or 16 h at an electric current density of 50 A·m−2 and a temperature of 20 °C. The anodized plate was then immersed in a 4.0 wt% oxalic acid solution at 25 or 30 °C for 180 or 300 min (PWT). The plate was then calcined at 350 °C to remove residual oxalic acid. Subsequently, hydrothermal treatment (HTT) was carried out in deionized water at 40–90 °C for 0–180 min. Finally, the plate was calcined in air

Effect of HTT on crystal structure of PAA film

Results of XRD (Fig. 1) show that the anodic alumina in the AD(16) sample (without PWT and HTT modification) is amorphous. No obvious changes in crystal structure were observed after the HTT treatment at 40 °C. However, when the HTT temperature is elevated to 50 °C, several broad diffraction peaks are clearly detected, which may be attributed to boehmite (α-AlOOH) or pseudo-boehmite (α′-AlOOH). The presence of hydrated alumina is considered to result from the hydrothermal reaction of amorphous

Conclusions

In this work, the effect of hydrothermal treatment (HTT) on the crystalline state and textural properties of PAA support is investigated to reveal the HTT mechanism.

The PAA film formed during the anodization consists of a large number of parallel tubes with relatively smooth walls. Skeleton alumina in the PAA film is amorphous. When unmodified PAA support is soaked in hot water above 50 °C (HTT treatment), the amorphous skeleton alumina reacts with hot water to produce pseudo-boehmite

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    These authors contributed equally to this work.

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