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

Journal Pre-proof Hydrothermal treatment of metallic-monolith catalyst support with self-growing porous anodic-alumina film

ChuanQi Zhang, YuanJing Pu, Feng Wang, HeCheng Ren, Hua Ma, Yu Guo PII:

S1004-9541(20)30038-0

DOI:

https://doi.org/10.1016/j.cjche.2020.01.012

Reference:

CJCHE 1631

To appear in:

Chinese Journal of Chemical Engineering

Received date:

18 January 2019

Revised date:

29 May 2019

Accepted date:

17 January 2020

Please cite this article as: C. Zhang, Y. Pu, F. Wang, et al., Hydrothermal treatment of metallic-monolith catalyst support with self-growing porous anodic-alumina film, Chinese Journal of Chemical Engineering(2020), https://doi.org/10.1016/j.cjche.2020.01.012

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© 2020 Published by Elsevier.

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Hydrothermal treatment of metallic-monolith catalyst support with self-growing porous anodic-alumina film ChuanQi Zhang, YuanJing Pu, Feng Wang, HeCheng Ren, Hua Ma*, Yu Guo* College of Chemical Engineering, State key Laboratory of Materials-oriented Chemical Engineering, NanJing Tech University, 30 Puzhunan Road, NanJing, 211816, P.R. China

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*Corresponding authors: E-mail address: [email protected] (Y. GUO), [email protected] (H. MA).

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These authors contributed equally to this work.

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Abstract

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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

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PAA film was investigated by XRD, BET, SEM and TG. The HTT treatment above 50 °C and the subsequent

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calcination above 300 °C could convert the amorphous skeleton alumina into γ-alumina and increase the specific

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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 increasing the HTT temperature or time. 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.

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Keywords: Alumina; Catalyst support; Hydrothermal; Pore widening treatment; Anodization;

1. 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

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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

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peeling of the catalytic coating layer (prepared by dip coating) from the metal substrate. In order to improve the

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adhesion of coating layer on metal substrate, recent research focused on surface pretreatment of the metal

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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

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mismatch. Nevertheless, it is commonly accepted that the adhesion of coating layer on MMC is inferior to that of the cordierite-monolith catalyst supports.

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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 [5–7]. Desired catalysts can be obtained by a wetness impregnation method. In addition to

Journal Pre-proof 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 brings 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

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therefore, has received increasing attention. Some relevant research groups [8–12] and our group [13–16] applied the novel MMC catalyst support to various fields such as hydrogenation reaction [8], steam reforming of

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methanol [9], Suzuki cross-coupling reaction [10], Fischer-Tropsch synthesis [11], VOCs combustion [12] and

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steam reforming of methane [16].

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However, unmodified PAA support typically has a specific surface area of 5–40 m2·g-1 [8–10, 17–26], which is much lower than that of conventional alumina powders or pellets. Furthermore, alumina in the PAA film is

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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

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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 [24–26] 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]

Journal Pre-proof 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

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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

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discussed comprehensively in literature. In this work, based on our latest research results, the HTT effect on the

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crystalline state and textural properties of PAA support is investigated to reveal fully the HTT mechanism.

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Finally, the complete oxidation of toluene is chosen as a probe reaction to examine the synergistic effect of PWT

2. Experimental

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and HTT.

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2.1. 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/m2 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 at 500 °C for 180 min. Hereinafter, the resultant samples are denoted as AD(a)-PWT1(or 2)-HTT(b/c), where a, b and c are anodization time (h), HTT treatment temperature (°C) and HTT treatment time (min), respectively. In the case of PWT1, the PWT temperature and time was controlled at 30 °C and 300 min, respectively. In the case of PWT2, the PWT temperature and time was

Journal Pre-proof controlled at 25 °C and 180 min, respectively. Unless otherwise specified, the calcination at 500 °C for 180 min was conducted for all samples. A plate-type Pt-based PAA catalyst was synthesized by the aqueous impregnation of Pt onto the resulting PAA support in a solution of Pt(NO3)2. Pt loading was controlled at 0.25 wt% for all samples.

2.2 Catalyst characterization

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PAA film thickness was measured using a digital microscope (VH-8000, Keynece Corp.). Powder XRD spectra was obtained by a D8 advance X-ray diffractometer (BRUKER Corp.). Morphology of the PAA sample

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was examined by a scanning electron microscope (SEM, S-4800, Hitachi Ltd.). Surface pore diameter was

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measured based on the SEM images of sample surface. BET specific surface area (SBET) was determined using

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the nitrogen adsorption method (SA3100, Beckman Coulter, Inc.). Thermogravimetric (TG) analyses were performed on a TGA-51 (Shimadzu Corp.). After a pretreatment in pure dry air at 120 °C for 300 min, the

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sample was heated up to 600 °C at a ramping rate of 10 °C·min -1 in a dry air flow. The quality of PAA sample after the pretreatment at 120 °C was used as a reference quality. Pt loading in the Pt-based PAA catalyst was

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determined by using an inductively coupled plasma spectrometer (ICPS-7510, Shimadzu Corp.). Pt dispersion was measured by the CO-pulse adsorption method (ChemBET 3000, Quantachrome Instruments, Co.), as reported in our previous work [27].

Note that the plate-type sample containing the Al substrate was used in the analyses of BET, TG, ICP, and CO-pulse adsorption, while the powder sample (by scraping off the anodic alumina film from the plate-type sample) was used in the XRD analysis. Additionally, the data of SBET, TG weight loss and Pt loading reported here were based on the alumina film quality (excluding the quality of Al substrate).

2.3 Catalyst activity test

Journal Pre-proof Details regarding the activity test of the Pt-based PAA sample for the toluene catalytic combustion have been given elsewhere [28]. In this work, the plate-type catalyst was cut to small pieces (ca. 4 mm2), and packed into a fixed bed quartz reactor (i.d., 6 mm) by using quartz sand for dilution. The total gas flow was set at 300 mL·min-1, containing 500 ppm Toluene/21% O2/N2 base. The space velocity is controlled at 250000 mL·h-1·g-1 (excluding the quality of Al substrate). A commercial Pt-based cordierite honeycomb catalyst (Ningbo Depu First Environmental Protection

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Technology Co., Ltd.) was used as a reference catalyst for the catalytic combustion reaction of toluene. Pt

3. Results and Discussion

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3.1. Effect of HTT on crystal structure of PAA film

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loading is about 0.75% in the cordierite catalyst (excluding the quality of cordierite honeycomb support).

Results of XRD (Fig. 1) show that the anodic alumina in the AD(16) sample (without PWT and HTT

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modification) is amorphous. No obvious changes in crystal structure is observed after the HTT treatment at 40 °C. However, when the HTT temperature is elevated to 50 °C, several broad diffraction peaks are clearly

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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 alumina. A further increase in the HTT temperature does not bring about a remarkable change in the relative intensity or 2θ of the diffraction profile, indicating that no new species are formed other than the hydrated alumina species of α-AlOOH or α'-AlOOH. Zhang et al. [21] assigned the hydrated alumina species to boehmite. As early as 1953, Calvet et al. [29] first proposed the concept of pseudo-boehmite, because the product obtained during the low-temperature synthesis of boehmite has a broadened diffraction peak, excessive bound water and higher surface area. Since the diffraction peak location of pseudo-boehmite is almost the same as that of the boehmite, Tettenhorst et al. [30] reported that

Journal Pre-proof the main difference between boehmite and pseudo-boehmite was the grain size. Similar conclusions were also made in the [31] and [32]. In this work, XRD result obtained over a commercial pseudo-boehmite reference sample (produced by Guizhou MoRui New Material Technology Co., Ltd.) is also added to Fig. 1 (g). It is found that the location and relative intensity of the diffraction peaks associated with the HTT-treated samples (without the subsequent calcination) is almost identical to that of the pseudo-boehmite reference sample. Moreover, it is reported that the hydrated alumina should be attributed to pseudo-boehmite when the grain size is less than 10

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nm, and is thought to be boehmite when the grain size larger than 50 nm [32]. In the case of the grain size ranging from 10 to 50 nm, it can be considered to be a transitional state of pseudo-boehmite and boehmite

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(which can also be regarded as pseudo-boehmite). The grain size of the hydrated alumina species formed during

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HTT is calculated by using Scherrer Equation. The grain size is 3.3 nm for AD(16)-HTT(50/90), 3.7 nm for

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AD(16)-HTT(90/90), and 5.2 nm for the pseudo-boehmite reference sample. The relatively broad diffraction peaks and small grain size shown in Fig. 1 illustrate that hydrated alumina formed during HTT should be

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pseudo-boehmite, rather than boehmite.

AD(16)-HTT(70/90) treated with HTT (but without subsequent calcination) is used to investigate the effect

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of calcination temperature on α'-AlOOH. The calcination at 200 °C does not significantly change the crystalline state of α'-AlOOH. When the sample is subjected to the calcination of 300 °C, diffraction peaks attributable to γ-alumina appeared at 2θ=45.8° and 67.0°, indicating an initial conversion of α'-AlOOH into γ-alumina. As the calcination temperature increases to 325 °C, the γ-alumina diffraction peaks at 45.8° and 67.0° are intensified. A calcination temperature higher than 400 °C makes almost all diffraction peaks attributable to γ-alumina. In addition, it is also reported that boehmite could not be converted into γ-alumina after dehydration, but was directly converted into δ-alumina [32]. However, Fig. 1 clearly shows that the hydrated alumina species are converted into γ-alumina (but not δ-alumina) when the HTT-treated sample is subjected to a calcination treatment above 300 °C. This result also supports the above conclusion that hydrated alumina formed during

Journal Pre-proof HTT is pseudo-boehmite.

3.2. Effect of HTT on textural properties of PAA film In Fig. 2, AD(16)-HTT(b/90) (b=50 or 70 °C) treated at different HTT temperatures are used to investigate the effect of HTT temperature on the PAA microstructure. As shown in Fig. 2(a1)–2(a4), it can be thought that the PAA film of AD(16) after anodization is composed of a large number of parallel tubes, and the pore walls are

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relatively smooth. From Fig. 2(a1), the surface pore diameter of AD(16) is determined to be 72 nm. The SBET of 10.5 m2·g-1 associated with AD(16) is much smaller than that of the conventional alumina powders or pellets.

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macropore responsible for its poor SBET.

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For example, the commercial γ-alumina powder produced by Aladdin Industrial has a SBET of 169 m2·g-1. It is the

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When AD(16) is modified by the HTT treatment at 50 °C for 90 min and the subsequent calcination at 500 °C, the surface pore diameter shrinks from 72 nm to 53 nm, but the ordered porous surface structure is still

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observable (Fig. 2(b1)). Cross-section images (Figs. 2(b2)–2(b4)) show that the skeleton is still an ordered cylindrical pore structure, but a large number of fine particles adheres to the main pore walls, making the pore

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walls relatively rough. Along the film thickness, the number of particles gradually decreases from top to bottom. At the bottom of PAA film, the HTT-treated sample (Fig. 2(b4)) is similar to the untreated sample (Fig. 2(a4)), indicating that the HTT effect should be non-uniform along the PAA film thickness. Figs. 2(c1)–2(c4) display the SEM images of AD(16)-HTT(70/90) treated at a higher HTT temperature. In comparison with AD(16)-HTT(50/90), the surface of AD(16)-HTT(70/90) becomes wrinkled, and the ordered porous surface structure is barely observed. Figs. 2(c2)–2(c4) show that more particles adhere to the upper and middle portions of the main pore walls. Furthermore, the adhesion of particles can be clearly observed on the bottom pore walls, but is not found in the case of AD(16)-HTT(50/90) treated at 50 °C. Fig. 3 shows the morphology change of AD(16)-HTT(70/c) with the different HTT time (c=0–90 min). After

Journal Pre-proof 10 min of the HTT treatment, some fine particles appear on the smooth pore walls. Surface and cross-section images illustrate that the surface pore diameter becomes slightly smaller, but the ordered pore structure is still clearly visible. As the HTT time is lengthened to 15 min, the number of particles adhering to the main pore walls increases sharply, and the wrinkled surface makes it difficult to discern the ordered surface structure. When the HTT treatment is carried out for 90 min, the surface wrinkles become large and rough, and the main pore skeleton seemingly is fully packed with the particles. These phenomena are similar to the HTT temperature

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effect shown in Fig. 2, indicating that the influence mechanism of HTT time is consistent with that of HTT temperature.

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Considering that a large number of particles adheres to the pore walls, it is believed that the HTT treatment

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will obviously affect the SBET of PAA sample, which is generally an important feature of catalyst support. Fig.

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4(a) gives the SBET of PAA sample as a function of HTT temperature. PAA samples with different film thickness were prepared by varying the anodization time. AD(8) and AD(16)

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were HTT-treated at different temperatures for 90 min (and then calcined at 500 °C) to prepare AD(8)-HTT(b/90) and AD(16)-HTT(b/90) (where b=50–90 °C). As shown in Fig. 4(a), as the HTT temperature rises, SBET of all

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samples gives a gradual increase tendency. The maximum SBET of 126.7 m2·g-1 is achieved on AD(16)-HTT(90/90), which is about 12 times that of the untreated AD(16). However, the promotion effect of HTT temperature on SBET is non-linear, and the slope of SBET gradually decreases with increasing the HTT temperature. For example, in the case of AD(16)-HTT(b/90), when the HTT temperature is elevated from 50 °C to 70 °C, the SBET is enlarged from 55.3 m2·g-1 to 110.2 m2·g-1. However, when the HTT temperature is further raised from 70 °C to 90 °C, a temperature increase of 20 °C yields only a SBET increment of 16.5 m2·g-1. Similar result is observed in the case of AD(8)-HTT(b/90). The effect of HTT time on SBET is shown in Fig. 4(b). The prolonged HTT time brings about an increase in SBET for all samples. However, similar to Fig. 4(a), the promotion effect of HTT time on SBET is also non-linear, with a rapid initial-increase followed by a slow climb.

Journal Pre-proof That is, as the HTT temperature or time further increases, the HTT effect is limited. It is also found that the promotion effect of HTT on SBET becomes significant with increasing the thickness of PAA film, which seems to imply that the HTT treatment is more effective for the samples having a thicker film. For example, when anodization is prolonged from 8 h to 16 h, the film thickness is increased from 54.5 μm of AD(8) to 89.9 μm of AD(16). When the same HTT treatment (70 °C, 90 min) is carried out, the SBET of AD(8)-HTT(70/90) is 68.9 m2·g-1, which is much smaller than that of AD(16)-HTT(70/90) (110.2 m2·g-1). The

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relationship between film thickness (or anodization time) and HTT will be discussed in detail in the next section. The combination of crystal structure and textural properties associated with the HTT-modified PAA sample

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brings some insights into the HTT mechanism. The HTT temperature higher than 50 °C results in the formation

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of α'-AlOOH by the hydrothermal reaction of amorphous alumina (Fig. 1). α'-AlOOH may exist in a sol state and

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adhere to the main pore skeleton. A larger volume of α'-AlOOH than amorphous alumina makes the pore diameter become small (as seen in Figs. 2 and 3). The HTT-modified sample (not calcined at 500 °C) is used in

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Fig. 5 to compare the PAA film morphology before and after calcination (500 °C, 180 min). Before calcination, slurry-like something, rather than the fine particles, adheres to the main pore walls. After calcination, a large

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number of particles appear at the same location, and the pore walls become rougher (Fig. 5(b), also shown in Fig. 2 or Fig. 3). Slurry-like something is guessed to be the α'-AlOOH sol, and the particles should correspond to γ-alumina (Fig. 1). A huge number of tiny slits among the γ-alumina particles is considered to contribute significantly to a great increase in SBET (Fig. 4).

3.3. Pore sealing limitation and PWT treatment As mentioned in Fig. 4, it is observed that the HTT treatment is more effective for the samples with thicker film prepared by prolonging anodization time. However, if only considering the influence of film thickness, the promotion effect of HTT on SBET should be independent of the film thickness (or anodization time). Therefore, it

Journal Pre-proof is necessary to study further the HTT mechanism to explain this contradiction. Table 1 list the morphology parameters of PAA samples anodized for different time. As the anodization time is prolonged (other conditions are consistent), the film thickness and pore size gradually increase, and the film density slowly decreases. It is accepted that anodization is a competitive process between film growth and film (including pore size) dissolution [33]. Under the same conditions of electrolyte, voltage and temperature, the enlarged pore size with increasing the anodization time is associated with the corrosion of alumina in the pore

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walls by acidic electrolyte [34]. A comparison of AD(8) with AD(16) reveals that there is almost one-fold difference in the pore diameter. In order to eliminate the pore size difference, a new sample having the same pore

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size as AD(16) (but with different film thickness) was prepared to investigate the influence of film thickness.

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AD(8) was immersed in 4 wt% oxalic acid solution at 30 °C for 300 min (PWT1), and then washed, dried

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and calcined at 350 °C for 1 h. The resulting sample is denoted as AD(8)-PWT1. Pore diameter is 70.5 nm for AD(8)-PWT1 with film thickness of 52.0 μm, 72.0 nm for AD(16) with film thickness of 89.9 μm, respectively.

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AD(8)-PWT1 and AD(16) were then treated by the same HTT treatment (70 °C, 90 min) and calcination

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treatment (500 °C, 3 h). SBET is 128.3 m2·g-1 for the former, 110.2 m2·g-1 for the latter, respectively. That is, if the

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samples have the same pore size, contrary to the conclusion of Fig. 4, increased film thickness will depress the promotion effect of HTT on SBET. Accordingly, it is believed that the small pore size, but not the low film thickness, should be responsible for the lower SBET of AD(8)-HTT(70/90) than that of AD(16)-HTT(70/90) (shown in Fig. 4). If only using the HTT mechanism aforementioned in section 3.2, it is hard to find the reason for the higher SBET of AD(8)-PWT1-HTT(70/90) than AD(16)-HTT (70/90). It is also difficult to explain why the HTT process is non-uniform (Fig. 2(b4)), and why the increase rate of SBET is gradually depressed with increasing HTT temperature or time (Fig. 4). Therefore, the HTT mechanism requires the following amendments. Pseudo-boehmite that has a larger volume than amorphous alumina will block the main pore channels and

Journal Pre-proof suspend further hydrothermal reactions. Strictly speaking, the shrunk main pore diameter increases the internal diffusion resistance of hydrothermal reaction. As shown in Fig. 4 and Table 1, the smaller the initial pore diameter (or the longer the pore depth), the more severe the limitation. Therefore, unfavorable pore sealing limitation, which occurs simultaneously in hydrothermal reaction, is believed to be responsible for the non-uniform effect of HTT along the film thickness. On the other hand, in order to break this pore sealing limitation, it was found to be effective in immersing in an acid solution under mild conditions, as observed in the

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above AD(8)-PWT1-HTT(70/90). It is understandable that the larger the pore diameter produced by acid solution corrosion, the lower the internal diffusion resistance of hydrothermal reaction, and more favorable the

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hydrothermal reaction.

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As shown in Fig. 1, when the calcination temperature exceeds 300 °C, pseudo-boehmite will dehydrate into

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γ-alumina. Therefore, the hydrothermal reaction degree can be determined by measuring the weight loss in the calcination step. The weight loss of the HTT-treated samples (without calcination) by TG analysis is presented in

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Fig. 6 to validate further the revised HTT mechanism. As the HTT temperature rises, the weight loss of all samples gradually increases, whether or not treated by PWT, indicating that higher HTT temperature produces

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more hydrated alumina. On the other hand, the PWT-treated sample gives a greater weight loss and a higher slope of weight loss than the untreated samples. This result demonstrates that the PWT treatment promotes the hydrothermal reaction, which is thought to support the above HTT mechanism. Fig. 7 compares SEM images of the PAA samples with or without PWT. The PWT treatment at 25 °C for 180 min (PWT2) significantly enlarges the pore diameter of AD(8) from 46.4 nm to 61.3 nm (Figs. 7(a1) and 7(b1)). After the subsequent HTT and calcination, although the morphology of the sample with PWT was roughly similar to the sample without PWT, there are still some differences in detail. More specifically, on the surface (Figs. 7(a2) and 7(b2)), the wrinkle size of AD(8)-PWT2-HTT(70/90) becomes smaller than that of AD(8)-HTT(70/90). On the cross-section of AD(8)-PWT2-HTT(70/90), the ordered skeleton structure is clearly

Journal Pre-proof visible, and the γ-alumina particles adhering to the pore walls become finer. The phenomenon that the main pore skeleton is fully packed with particles is not observed over AD(8)-PWT2-HTT(70/90). Fig. 4 gives SBET change of the samples with or without PWT2 (25 °C, 180 min) as a function of HTT temperature (Fig. 4(a)) or HTT time (Fig. 4(b)). All samples with PWT exhibit much higher SBET than the samples without PWT throughout the tested range of HTT temperature or time. For example, the SBET of AD(16)-PWT2-HTT(70/90) (146.0 m2·g-1) is about 1.3 times that of AD(16)-HTT(70/90) (110.2 m2·g-1).

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Furthermore, in comparison with the samples without PWT, the SBET of the sample with PWT rises faster with increasing HTT temperature (or HTT time). These results indicate that the PWT treatment intensifies the

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promotion effect of HTT on SBET, and the synergistic combination of PWT and HTT produces a PAA support

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with a large SBET comparable to commercial γ-alumina. The limited promotion effect of HTT on SBET, as

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observed over the samples without PWT, should be attributed to the changed pore structure, rather than phase-change saturation of hydrated alumina to γ-alumina reported in the literature (reported in ref.[21]).

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In addition, as shown in Table 1, it is also found that the SBET of the sample treated with PWT alone (without

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corrosion.

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HTT) increases from about 10 m2·g-1 to 30–40 m2·g-1, which is associated with the rough surface caused by acid

3.4 Catalytic activity of Pt-based PAA catalyst In this work, AD(8) samples treated by different modifications were used as catalyst support to synthesize several 0.25 wt% Pt-based PAA catalysts. Complete oxidation of toluene was selected as a probe reaction to examine the synergistic modification effect of PWT and HTT. For Pt-based catalysts, Pt dispersion is commonly regarded as an important factor affecting the catalytic activity. Generally, the texture properties of catalyst support, such as SBET, pore size and pore structure, will affect Pt dispersion. When the PAA support is subjected to different modifications, a significant difference in Pt

Journal Pre-proof dispersion is observed in Table 1. The unmodified AD(8) support gives the smallest SBET and the lowest Pt dispersion, while the AD(8)-PWT1-HTT (70/90) support with the largest SBET achieves the highest Pt dispersion. Fig. 8 shows the toluene conversion and CO2 yield as a function of temperature. The Pt-based AD(8)-PWT1-HTT (70/90) catalyst with the largest Pt dispersion also exhibits the highest catalytic activity. In Fig. 8, additionally, it is found that the CO2 yield is almost in agreement with the toluene conversion, indicating no observable byproduct formed during the toluene oxidation. This conclusion is consistent with our previous

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research [35]. Besides the largest SBET of AD(8)-PWT1-HTT(70/90), favorable pore structure is also considered to be one

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of the main reasons for achieving the highest Pt dispersion and the best activity. It is generally accepted that the

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role of support is not limited to disperse active metal, and may affect catalytic performance due to pore

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morphology, confinement and chemical effect [36]. A large active surface area can be obtained by using the catalyst support having a high SBET. However, the catalyst support with a large SBET usually consists of

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micropores or fine mesopores, which will affect the diffusion of reactants and products. This issue is critical for the VOCs complete oxidation, because the VOCs conversion of not less than 95% is required by the

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environmental regulations. In high conversion regions, it is possible that the catalyst activity is controlled by internal diffusion limitation, particularly in the case of large reactant molecules such as toluene. As described above, the main pore skeleton of the PAA support modified by HTT alone is almost fully packed with γ-alumina particles, as shown in Fig. 7(a3). In contrast, on the PWT and HTT co-treated samples, a great number of finer γ-alumina particles formed on the main pore walls contribute higher SBET, while maintaining the cylindrical main pore structure originated from anodization (e.g. AD(8)-PWT2-HTT(70/90) shown in Fig. 7(b3)). If the macropores (or large mesopores) formed during anodization are the 1st-dimension pore structure, the micropores (or fine mesopores) generated by the fine particles on the main pore walls can be thought as the 2nd-dimension pore structure. Although there is currently insufficient evidence to support further,

Journal Pre-proof it is understandable that the bimodal pore structure may be a decisive factor in achieving high SBET, Pt dispersion and activity over the PWT and HTT co-processed samples. It is believed that the large pore diameter of the 1st-dimension pore structure facilitates internal diffusion (Pt ions during impregnation, toluene molecules during the reaction), while the high SBET of the 2nd-dimension pore structure brings out large active surface area and favorable Pt dispersion. Fig. 8 also compares the catalytic performance of the Pt-based modified PAA catalysts and a commercial Pt-based cordierite honeycomb catalyst. The cordierite catalyst has a more favorable

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low-temperature activity than that of Pt-based AD(8)-PWT1-HTT (70/90), which should be associated with the higher Pt loading in the cordierite catalyst. However, note that the T95 (a reaction temperature at which 95%

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toluene conversion can be achieved) of the two catalysts is approximately the same, about 230 °C. It is generally

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believed that catalyst activity is likely to be controlled by internal diffusion limitations, especially in high

Pr

conversion regions (or high temperature regions). The bimodal pore structure of the AD(8)-PWT1-HTT (70/90) catalyst is beneficial to break the internal diffusion limitation in the high temperature regions. This is considered

al

to be one of the reasons why two catalysts have similar T95. Additionally, in relevant literature [8], Hong et al.

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used a PAA material shaped into θ-ring to prepare Pd-based catalyst for ethylanthraquinone hydrogenation. They

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reported that Pd-based PAA catalyst exhibited higher productivity than spherical Pd/Al2O3 catalyst due to the advantages in mass and heat transfer and confinement effect of cylindrical pores. Therefore, research into the bimodal pore structure of modified PAA catalysts is believed to be a promising field, especially in terms of promoting mass transfer and confinement effect. Further research is in process.

4. 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

Journal Pre-proof 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 (α'-AlOOH), rather than boehmite (α-AlOOH). The slurry-like α'-AlOOH sol adheres to the main pore walls. After the subsequent calcination, a large number of fine particles appear at the same location, which is considered to correspond to γ-alumina species. A huge number of tiny slits among the fine γ-alumina particles contribute significantly to a great increase in SBET.

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However, SEM images showed the HTT treatment is a non-uniform process along the thickness of PAA film. The promotion effect of HTT on SBET is non-linear, and the slope of SBET gradually decreases with increasing the

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HTT temperature or time. SEM images reveal that pseudo-boehmite having a larger volume than amorphous

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alumina will block the main pore channels and suspend further hydrothermal reactions. The shrunk main pore

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diameter increases the internal diffusion resistance of hydrothermal reaction. The smaller the initial pore diameter (or the longer the pore depth), the more severe the limitation. That is, unfavorable pore sealing

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limitation, which occurs simultaneously in hydrothermal reaction, is responsible for the non-uniform effect of HTT along the film thickness and the limited promotion effect of HTT on SBET. PWT treatment (immersing the

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unmodified PAA support in oxalic acid solution) ahead of HTT can break the pore sealing limitation. It is believed that the larger the pore diameter produced by acid solution corrosion, the lower the internal diffusion resistance of hydrothermal reaction, and more favorable the hydrothermal reaction. 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 activity than that with HTT modification alone. The presence of a bimodal pore structure was suggested to be a decisive reason.

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Table 1 Film thickness, pore size, film density, SBET, and Pt dispersion of different PAA samples.

Journal Pre-proof Sample

Thickness a /μm

Diameter a /nm

Density a /g·cm-3

SBET b /m2·g-1

Pt dispersion b c /%

AD(8)

54.5

46.4

2.3

68.9 (8.9)

29.8 (15.2)

AD(8)-PWT1

52.0

70.5

1.9

128.3 (36.5)

46.7 (21.4)

AD(8)-PWT2

53.7

61.3

1.9

94.4 (30.2)

40.1 (ND d)

AD(16)

89.9

72.0

1.9

110.2 (10.5)

ND (ND)

AD(16)-PWT2

86.6

85.1

1.5

146.0 (34.4)

ND (ND)

Data is associated with the samples without HTT modification;

b.

Data outside the brackets is associated with the samples treated by HTT (70°C, 90 min) and calcination (500°C, 3 h).

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a.

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Data in the brackets is associated with the samples without HTT modification;

Pt dispersion is associated with the 0.25 wt% Pt based PAA samples prepared by using different catalyst support;

d.

ND: not determined.

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c.

Fig. 1 XRD patterns of different samples: (a) AD(16); (b–f) AD(16)-HTT(b/90) without calcination (in which b is 40 °C for (b), 50 °C for (c), 70 °C for (d), 80 °C for (e), and 90 °C for (f), respectively); (h–n) AD(16)-HTT(70/90) calcined at different temperatures for 180 min (in which the calcination temperature is 200 °C for (h), 300 °C for (i), 325 °C for (j), 350 °C for (k), 375 °C for (l), 400 °C for (m), and 500 °C for (n), respectively); (g) commercial α'-AlOOH reference sample; (o) commercial γ-alumina reference sample.

Journal Pre-proof

▼

▼ ▼

g-Al2O3 ■ a'-AlOOH

▼

▼

▼

Intensity

▼

■

o n m l k j i

■

■

■

30

40

50

60

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20

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■

70

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Pr

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2θ /(o)

Fig. 2 SEM images of PAA sample treated at different HTT temperatures (after calcination).

h g f e d c b a

80

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Pr

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Fig. 3 SEM images of PAA samples treated for different HTT time (after calcination).

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Fig. 4 SBET of different PAA samples as a function of (a) HTT temperature and (b) HTT time (after calcination).

Journal Pre-proof

160 140 120 100 80 60 40 20

AD(8)-PWT2-HTT(b/90) AD(16)-PWT2-HTT(b/90)

50

80

90

HTT temperature /℃

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AD(8)-HTT(70/c) AD(16)-HTT(70/c)

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Pr

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160 140 120 100 80 60 40 20

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.

70

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SBET /m2 g-1

(b) 180

60

f

.

AD(8)-HTT(b/90) AD(16)-HTT(b/90)

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SBET /m2 g-1

(a) 180

30

60

AD(8)-PWT2-HTT(70/c) AD(16)-PWT2-HTT(70/c)

90

120

150

180

HTT time /min

Fig. 5 SEM images of AD(16)-HTT(50/90) before (a) and after (b) calcination (500 °C, 180 min)

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Journal Pre-proof

Fig. 6 Weight loss of PAA samples with and without PWT treatment

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20

ePr

12

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8 4

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Weight loss /wt%

16

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0

50

55

AD(16)-HTT(b/90) AD(16)-PWT2-HTT(b/90)

60

65

70

75

80

HTT temperature /oC

Fig. 7 SEM images of AD(8)-HTT(70/90) and AD(8)-PWT2-HTT(70/90) samples (after calcination).

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Journal Pre-proof

Fig. 8 Toluene conversion (a) and CO2 yield (b) over Pt-based PAA catalysts prepared by using different PAA supports.

100 80

(a)

60 40 AD(8)-HTT(70/90) AD(8)-PWT1-HTT(70/90) AD(8)-PWT2-HTT(70/90)

20

0 100 80

AD(8) AD(8)-PWT1

(b)

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60 40

AD(8)-HTT(70/90) AD(8)-PWT1-HTT(70/90) AD(8)-PWT2-HTT(70/90)

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20 0 175

200

225

250

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Pr

Temperature (oC)

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CO2 yield /%

Commercial honeycomb catalyst AD(8) AD(8)-PWT1

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Toluene conversion /%

Journal Pre-proof

275

Journal Pre-proof Graphical abstract

HTT

SBET is enlarged 1.3⁓2.0 times

PWT

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Pr

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HTT

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8