Enhanced Adhesion Strength of Pt/γ-Al₂O₃ Catalysts on STS-444 Substrate via γ-Al₂O₃ Intermediate Layer Formation: Application for CO and C₃H₆ Oxidation

Abstract

Pt/γ-Al₂O₃ catalysts coated on honeycomb-shaped stainless STS-444 steel substrates with a γ-Al₂O₃ intermediate layer were prepared using a conventional washcoating method. The intermediate layer was formed on the substrate surface through oxidation using pack cementation. The monolithic catalysts with the intermediate layer were fabricated for potential applications to pre-turbocharger catalysts, which suffer from severe conditions such as vibrations of the engine and high flow rates of exhaust gas. Adhesive strength tests and simultaneous oxidation reactions of CO and C₃H₆ were carried out for the Pt/γ-Al₂O₃ monolithic catalysts with and without the intermediate layer. The catalysts with an intermediate layer showed much stronger adhesion than the catalysts without an intermediate layer. Thus, the formation of a γ-Al₂O₃ intermediate layer by surface oxidation through pack cementation facilitated a significant enhancement of the catalyst adhesion strength without catalytic performance degradation.

1. Introduction

Hazardous gas pollutants, such as carbon monoxide (CO), hydrocarbons (HCs), and nitrogen oxides (NOx), emitted from gasoline and diesel engines have caused severe environmental problems [1]. Several types of exhaust gas after treatment technology to reduce the pollution have been developed, including three-way catalysts, diesel oxidation catalysts, diesel particulate filters, and selective catalytic reduction catalysts [2]. One efficient method to reduce the CO and HCs emissions is to place a small catalyst upstream of the engine’s turbocharger as a pre-turbocharger catalyst, PTC [2,3]. CO and HCs could be more effectively reduced by an oxidizing reaction and additional heat from the oxidation on the PTC, especially during cold start.

Metallic substrates have many advantages, such as higher structural rigidity, higher mechanical durability, and lower back pressure, so they are more appropriate as PTC than ceramic substrates [4,5,6]. The adhesive strength of the catalyst to the metallic substrates on PTC, however, might not be strong enough to be durable under very fast flow of the exhaust gas and vibration between engine and turbocharger, resulting in catalyst detachment problems. In particular, the coating layer of metal oxide-based catalysts on metallic substrates can be easily detached because the bonding property between metallic and ceramic materials is fundamentally poor [7,8]. To solve the problem, an intermediate layer with strong adhesion to both the metallic substrate and catalyst layer is adapted to reduce detachment of the catalyst on PTC. There have been several reports on the effect of intermediate layers, such as Al₂O₃ [9,10], mordenite framework inverted zeolite [11], carbon pencil [12], TiO₂ [13], and gel coating [14], to enhance adhesion.

A pack cementation method was employed to form an Al₂O₃ intermediate layer suitable for the alumina-based catalyst layer. The gas diffusion method is a very simple and cost-effective process using STS-444 substrates and embedded powders, which undergo sublimation during heating under argon atmosphere. Thus, pack cementation has low equipment dependency and confers high bond strength to the coated layer [15]. When aluminum powder as a precursor was selected, the substrate surface was aluminized due to thermal reactions between the sublimated aluminum gas and substrates [16]. It was reported that an Fe₂Al₅ layer is formed on a stainless steel substrate when the substrate is coated with aluminum. Furthermore, the coated Fe₂Al₅ layer can act as a precursor for the Al₂O₃ formation when the coated substrate is oxidized at a high temperature in air [16].

In this study, we developed Pt/γ-Al₂O₃ catalyst with the γ-Al₂O₃ intermediate layer with strong adhesion through the oxidation of Fe₂Al₅ on the STS-444 substrate. The microstructure of the Pt/γ-Al₂O₃ catalyst coating layer was characterized through field emission-scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) analyses. The adhesion strengths of Pt/γ-Al₂O₃ catalysts on the STS-444 substrate were compared with and without an intermediate layer. CO and C₃H₆ oxidation reaction tests were performed using Pt/γ-Al₂O₃ catalysts on STS-444 substrates before and after adhesion tests to evaluate their catalytic activities and the effects of the intermediate layer. The reactions were tested under very high gas hourly space velocity (GHSV) conditions mimicking the PTC circumstances.

2. Results and Discussion

2.1. Characterization of γ-Al₂O₃ Intermediate Layer and Pt/γ-Al₂O₃ Catalyst

The XRD patterns of the γ-Al₂O₃ and Pt/γ-Al₂O₃ powders are shown in Figure 1. γ-Al₂O₃ showed diffraction peaks located at 2θ = 19.7°, 32.5°, 37.1°, 39.4°, 45.9°, 61.1°, and 66.8°, which correspond to the (111), (220), (311), (222), (400), (511), and (440) lattice planes of γ-Al₂O₃ (ICSD-66559), respectively [17,18]. However, no significant difference was observed in the XRD patterns of γ-Al₂O₃ and Pt/γ-Al₂O₃. In the XRD pattern of Pt/γ-Al₂O₃, the Pt peak could not be confirmed because the Pt content was as low as 2 wt% in the mixture, and nano-sized Pt particles were well dispersed on γ-Al₂O₃ [19,20]. However, the Pt content of 1.93 wt% was confirmed through inductively coupled plasma-optical emission spectrometry (ICP-OES) of Pt/γ-Al₂O₃. These results show that γ-Al₂O₃ and Pt are homogeneously dispersed.

2.2. Morphology of γ-Al₂O₃ Intermediate Layer on the STS-444 Substrate

Cross-section images of the STS-444 substrate, γ-Al₂O₃ intermediate layer, and Pt/γ-Al₂O₃ catalyst was observed using FE-SEM and EDS analyses. Figure 2a,d show smooth surfaces without any cracks on the STS-444 substrate. When the γ-Al₂O₃ intermediate layer was coated onto the STS-444 surface, the substrate surface becomes rough, as shown in Figure 2b. The thickness of the γ-Al₂O₃ intermediate layer was around 1.4 μm (Figure 2e). Figure 2c,f shows 80 μm thick Pt/γ-Al₂O₃ catalyst coating adhered well to the intermediate layer.

To investigate the elemental composition of the γ-Al₂O₃ intermediate layer on the STS-444 substrate, elemental mapping was conducted; the mapping profiles are shown in Figure 3a–f. Cr and Fe were uniformly found on the STS-444 surface, as shown in Figure 3e,f. In addition, Al and O show a high density on the surface and a low density on the substrate, demonstrating the formation of the γ-Al₂O₃ intermediate layer on the STS-444 substrate (Figure 3c,d). Therefore, the mapping result of γ-Al₂O₃/STS-444 indicates that the γ-Al₂O₃ intermediate layer was successfully formed on the STS-444 substrate. The EDS spectra shown in Figure 3g confirm the elemental composition of γ-Al₂O₃/STS-444, wherein the contents of C, O, Al, Cr, and Fe were 39.98, 4.69, 3.80, 10.02, and 41.51 wt%, respectively.

2.3. Adhesion Strength of Washcoated Pt/γ-Al₂O₃ Catalyst Layer

Figure 4 shows the weight loss of the Pt/γ-Al₂O₃ catalyst coating layer on STS-444 with and without the γ-Al₂O₃ intermediate layer during the adhesion tests to evaluate the effects of the intermediate layer on the adhesive strength of the catalyst layers coated on metallic substrates. The adhesion strength of the γ-Al₂O₃/STS-444 sample was much higher than that of the conventional STS-444 monolithic sample. After the 60-min adhesion test, only 13% of the catalyst coating layer was lost for the sample with the γ-Al₂O₃ intermediate layer, whereas 33% of the coating layer was detached for the sample without the intermediate layer. This result indicates that the γ-Al₂O₃ intermediate layer formed by surface oxidation through pack cementation drastically enhances the adhesion between the Pt/γ-Al₂O₃ catalyst coating layer and the metallic substrate.

2.4. CO and C₃H₆ Oxidation over Pt/γ-Al₂O₃ Monolithic Catalysts with and without γ-Al₂O₃ Intermediate Layer

Figure 5 shows the light-off activities of Pt/γ-Al₂O₃ with and without an intermediate layer for CO and C₃H₆ oxidations under very high GHSV conditions of 100,000 and 300,000 h⁻¹. The catalytic activities of Pt/γ-Al₂O₃ samples were compared before and after the adhesive strength tests. Figure 5a shows CO conversion over the catalysts with and without the intermediate layer under the GHSV of 100,000 h⁻¹. First, both samples (STS-444 metallic substrate in the presence and absence of the γ-Al₂O₃ intermediate layer) showed a stable conversion graph with 80% conversion at approximately 160 °C and 90% conversion at around 170 °C. However, after the adhesion test, the intermediate layer sample showed a more stable conversion ratio than that without the intermediate layer. The conversion ratios of 80% and 90% were obtained at approximately 170 °C and 200 °C for the catalyst with the intermediate layer, respectively, but were 80% and 90% at 210 °C and 250 °C for the catalyst without the intermediate layer, respectively. As shown in Figure 5b, the CO conversion test was performed at 300,000 h⁻¹ for both samples. Both catalyst samples presented similar CO conversion rates of 80% and 90% at 230 and 240 °C, respectively, before the adhesion test. In contrast, after the adhesion test, 80% conversion was observed for the catalyst with the intermediate layer, but the sample without an intermediate layer did not even reach 70% conversion up to 250 °C. The absence of an intermediate layer induces a very weak catalytic effect toward CO conversion under these conditions. Furthermore, Figure 5c shows the C₃H₆ conversion for both samples under the GHSV condition of 100,000 h⁻¹. The conversion temperatures of the samples with and without the intermediate layer were similar at 165 °C (80%) and 175 °C (90%) before the adhesion test. However, the catalyst with the intermediate layer exhibited conversion temperatures of approximately 175 °C (80%) and 200 °C (90%) after the adhesive strength test, whereas the sample without an intermediate layer showed conversion temperatures of 200 °C (80%) and 250 °C (85%). Figure 5d shows the C₃H₆ conversion at 300,000 h⁻¹ for both samples. Both presented the same C₃H₆ conversion temperature at 220 °C (80%) and 250 °C (90%) before the adhesion test. However, after the adhesion test, the conversion temperature of the sample with the intermediate layer was at 250 °C (80%). The samples without an intermediate layer did not even reach 70% conversion up to 350 °C.

As a result of these tests, both the gas conversion performance of the Pt/γ-Al₂O₃ catalysts without the γ-Al₂O₃ intermediate layer was severely degraded after the adhesive strength test because a significant amount of the catalyst coating layer was detached from the STS-444 substrate. However, the Pt/γ-Al₂O₃ catalysts with the γ-Al₂O₃ intermediate layer showed excellent CO and C₃H₆ oxidation activity even after the adhesive strength test. Thus, the intermediate layer formation on the metallic substrate greatly enhances the adhesive strength and CO/C₃H₆ oxidation performance of the metallic monolithic catalysts, which, in turn, can considerably strengthen the long-term mechanical durability of the catalysts.

3. Materials and Methods

3.1. Formation of γ-Al₂O₃ Intermediate Layer on the STS-444 Substrate

The honeycomb-shaped stainless STS-444 steel substrates used in this study are cylindrical with a diameter of 2.54 cm and a height of 2.54 cm. To coat the substrate with aluminum, mixed powders of Al (coating material), AlCl₃ (evaporator), and Al₂O₃ (anti-sintering material) were placed with the substrate in an alumina crucible. The alumina crucible and alumina lid were tightly bonded with a ceramic bond (Cerabond^®, Cera-Chem, Chennai, India) for the thermal treatment. The tightly sealed crucibles were placed in a tube furnace under Ar atmosphere and maintained at 500 °C for 6 h. Further details of the process can be found in our previous report [16]. The oxidation treatments were carried out at 500 °C for 12 h to form an in situ γ-Al₂O₃ layer on top of the Fe₂Al₅ layer formed on the surface of the STS-444 substrate. The oxidation treatments can form a well adhered γ-Al₂O₃ layer on top of the aluminide-coated surface.

3.2. Preparation of Pt-Based Monolithic Catalysts Coated on γ-Al₂O₃/STS-444 Substrate

Pt/γ-Al₂O₃ catalyst was synthesized by incipient wetness impregnation. The γ-alumina (MI386, Solvay, Brussels, Belgium) support, which has a large surface area (~200 m²/g), and a platinum ethanolamine solution (16 wt% Pt, Strong Industry, Incheon, Korea) were mixed together in the deionized water solvent. The mixture was mixed at room temperature for 30 min using a vacuum evaporator and then mixed at 90 °C to evaporate the moisture. After drying for 2 h in a circulation dryer to remove residual moisture from the synthetic catalyst, the catalyst particle size was adjusted with a 300 μm sieve and calcined at 120 °C for 4 h and 500 °C for 4 h. A monolithic catalyst was prepared by coating the prepared Pt/γ-Al₂O₃ catalyst on a metallic STS-444 substrate as follows. In order to coat the catalyst on the manufactured metal monolithic structure, the synthesized catalyst and distilled water were mixed to prepare a slurry, followed by ball-milling for 24 h to uniformly produce an average particle size of 1–5 μm. Next, a commercial boehmite binder (Disperal P2, Sasol, Sandton, South Africa) was added to increase the adhesion strength, and washcoating was performed on the metal monolithic structure. It was coated with the desired catalyst whose weight was relative to that of the metal monolithic structure, followed by drying at 120 °C for 4 h and at 500 °C for 2 h.

3.3. Characterization of γ-Al₂O₃ Intermediate Layer and Pt/γ-Al₂O₃ Catalysts

The crystalline phases of the γ-Al₂O₃ and Pt/γ-Al₂O₃ powders were identified through XRD analysis (D-MAX 2500, Rigaku, Tokyo, Japan) with Cu-Kα radiation (40 kV, 100 mA). The thickness and components of the γ-Al₂O₃ and Pt/γ-Al₂O₃ layers were observed by scanning electron microscopy (JSM-6610LV, JEOL, Tokyo, Japan) at 15 kV coupled with EDS.

3.4. Adhesive Strength Test of Washcoated Pt/γ-Al₂O₃ Catalysts

The adhesive strength tests of the proposed catalyst layer were carried out using an ultrasonic vibration cleaner (SD-D400H, 40 kHz, 400 W). The monolithic catalysts with and without the γ-Al₂O₃ intermediate layer were placed in a cleaner filled with deionized water. The adhesive strength tests were performed for periods of 30–60 min. Tested samples were perfectly dried at 120 °C for 30 min prior to measuring the sample weight.

The weight loss of the catalyst coating layer was calculated by the following equation:

$$\mathsf{\Delta }\mathrm{W}=\left[\frac{\left(W_1-W_2\right)}{W_1-W_0}\right]\times 100\%$$

(1)

where W₀, W₁, and W₂ mean the weights of the bare substrate and washcoated monolithic catalysts before and after the adhesive strength test, respectively [2].

3.5. Simultaneous CO and C₃H₆ Oxidation Reactions

The performance of the monolithic catalysts toward the oxidation of CO and C₃H₆ gases was investigated using an in-house lab bench reactor. A total of 10 and 30 L/min of feed gas was introduced to the catalysts for GHSV (volumetric gas flow rate)/(volume of monolithic catalyst) of 100,000 and 300,000 h⁻¹, respectively. The reactant gas is composed of 500 ppm of CO, 500 ppm of C₃H₆, 5% of CO₂ in a dry air balance. The reactor was heated from 50 to 350 °C at a ramping rate of 1.7 °C/min. The product gases after catalysis were analyzed using a gas chromatography (GC) equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID) (Younglin, Anyang, Korea).

The conversions of CO and C₃H₆ were calculated by the following equations:

$$\mathrm{Conversion} \mathrm{of} \mathrm{CO}=\left[\frac{\left({\mathrm{Mol}}_{\mathrm{CO},\mathrm{inlet}}-{\mathrm{Mol}}_{\mathrm{CO},\mathrm{outlet}}\right)}{{\mathrm{Mol}}_{\mathrm{CO},\mathrm{inlet}}}\right]\times 100\%$$

(2)

$${\mathrm{Conversion} \mathrm{of} \mathrm{C}}_3{\mathrm{H}}_6=\left[\frac{\left({\mathrm{Mol}}_{\mathrm{C}3\mathrm{H}6,\mathrm{inlet}}-{\mathrm{Mol}}_{\mathrm{C}3\mathrm{H}6,\mathrm{outlet}}\right)}{{\mathrm{Mol}}_{\mathrm{C}3\mathrm{H}6,\mathrm{inlet}}}\right]\times 100\%$$

(3)

where Mol_inlet and Mol_outlet represent the mole number of CO or C₃H₆ before and after catalytic reactions, respectively.

4. Conclusions

Monolithic Pt/γ-Al₂O₃ catalysts coated on honeycomb-shaped stainless STS-444 steel substrates with a γ-Al₂O₃ intermediate layer were successfully prepared for potential pre-turbocharger catalyst applications. The intermediate layer was formed through aluminum pack cementation and oxidation process and acted as a buffer layer to enhance the catalyst adhesion property between the substrate and catalyst coating layer. The adhesion of the Pt/γ-Al₂O₃ catalysts with the intermediate layer was much stronger than those without an intermediate layer. This is because the γ-Al₂O₃ intermediate layer was well adhered to both the substrate and catalyst layers. CO and C₃H₆ oxidation reaction results after the adhesion tests showed that the catalysts with the intermediate layer have better reaction performance than those without the layer under very harsh conditions at GHSV of 100,000 and 300,000 h⁻¹. The CO and C₃H₆ oxidation performance of the conventional monolithic catalysts without the intermediate layer was degraded significantly after the adhesion test. The results show that the enhancement of catalyst adhesion by well-adhered intermediate layer formation could be a critical factor for catalytic applications, such as a small catalyst located near engines exposed to very high space velocity.