Compact V2O5WO3/Tio2 Monolith Catalyst for Vessel DeNox Application

Abstract

Using a corrugated paper as the substrate, a novel compact monolith catalyst assigned as V2O5-WO3/TiO2 for vessel De-NOx application was prepared by wash coating method. The prepared corrugated paper catalyst exhibited comparable NO conversion but lower SO2/SO3 conversion than that of commercial catalyst at low temperature of 200 and 250°C.

Introduction

In order to meet the requirement of International Maritime Organization (IMO) announced NOx Tier III regulations on vessels, the selective catalytic reduction (SCR) with NH₃ at low temperature <250°C has been widely studied over various catalysts [1-2]. Among those catalysts, the V₂O₅–WO₃/TiO₂ is the most representative catalyst [3].

For mobile vessel NH₃–SCR system, it is obvious that, not only the good reactivity but also compact property like low bulk density is required for the catalyst. However, surprisingly, to our knowledge, most literatures focused only the catalysts reactivity performance, but neglecting the compact issue of the catalysts system. In light of these, a commercial corrugated paper is applied here as the substrate to prepare V₂O₅–WO₃/TiO₂ catalyst by wash-coating method. The catalyst performance was evaluated at both 200 and 250°C to investigate its application possibility for the vessel NH₃–SCR process.

Experimental

A commercial corrugated paper (Dongguan Chunhing paper Co., Ltd) was used as a substrate in Figure 1(a). This corrugated paper has a 790 m²/m³ contact area per volume, which is comparable to commercial honeycomb catalyst, whereas the bulk density is only 60% of the commercial honeycomb catalyst value. The composition of this material is SiO₂, Al₂O₃, MgO and CaO. A wash–coating method with the slurry containing commercial WO₃-TiO₂ and Ammonium vanadium oxide was employed to prepare catalysts in this study. The V₂O₅ loading was controlled at 3.8 wt. %. The prepared sample was calcined at 450°C for 5 h.

Figure 1: Image and size profile (a) Paper corrugated support; (b) Honeycomb catalysts.

As a control sample, a commercial V₂O₅–WO₃/TiO₂ honeycomb catalyst with 3.8 wt. % V₂O₅ loading shown in Figure 1(b) was provided by Jiangsu Longyuan Catalyst Co., Ltd.

The elemental composition was determined by the AES–ICP in a Thermo–Electron model 3580 instrument. The XRD was recorded on a Bruker D8 Advanced A25 diffractometer. SEM images were taken by the FEI Quanta 200. The XPS analyses were conducted on a MultiLab ESCA 2000 X-ray photoelectron spectrometer with Mg–Kα radiation at 300 W.

The catalysts evaluation was performed over a fixed bed reactor. A 57.6 mL prepared corrugated paper catalyst or commercial honeycomb catalyst was loaded into a reactor with O.D.60.5 mm and I.D. 52.9 mm. The catalyst performance is evaluated with both NO conversion and SO₂/SO₃ conversion. For the De-NOx activity test, a stimulated gas composed of 180 ppm NH₃, 180 ppm NO, 7% O₂, 10% H₂O with N₂ balance was applied at SV 28125 h^-1. For the SO₂/SO₃ oxidation test, 500 ppm SO₂, 4% O₂, 10% H₂O with N₂ balance gas was applied at SV 7734 h^-1.

The rate constant K_NO is used here to compare prepared corrugated paper catalyst and commercial honeycomb catalyst [4]:

Where, AP is the contact area per volume, η is the NO conversion, SV is the space velocity.

Results and Discussion

The prepared catalyst in Figure 2(a) showed the similar XRD pattern as that of commercial WO₃–TiO₂ powder in Figure 2(b), while negligible V₂O₅ peak could be detected. This implied that the vanadium oxides are well dispersed. The WO₃ refraction peak was only shown after calcining the commercial WO₃–TiO₂ powder at 1100°C to transform the anatase TiO₂ to rutile TiO₂ in Figure 2(c).

Figure 2: XRD over prepared catalyst (a) prepared catalyst in this study (b) commercial TiO₂-WO₃ (c) calcine b at 1100°C.

XPS Results in Figure 3 further explained the existence of anatase TiO₂ and WO₃ over prepared catalyst. The existence of 45% V⁵⁺, 45% V⁴⁺ and 10% V³⁺ were detected also. According to Youn et al.[5], the existence of V³⁺ and V⁵⁺ in XPS most likely form the polymerized species, while the V⁴⁺ may result in both isolated and polymerized surface vanadium species. Due to the higher liability of lattice oxygen atoms, the polymeric vanadium species were reported to be 10 times more active than the monomeric species at low temperature 200–250°C. The influence of the V state over the De-NOx activity at lower temperature will be discussed in detail in our other study.

Figure 3: XPS over prepared catalyst in this study.

The influence of coated catalyst layer thickness on the De–NOx activity was investigated in Figure 4. At both 200 and 250°C, the De-NOx activity increased significantly with increasing the catalyst layer thickness until 0.2 mm. Continuing increase the catalyst layer thickness from 0.2 to 0.4 mm, negligible improvement of De-NOx activity was evidenced. This suggested that the De- NOx reaction was confined into a narrow surface layer. Yang et al. reported that over a honeycomb monolith support wash coated SCR catalyst, the activity increased with increasing the coating thickness and reached at 0.11 mm thickness as comparable as that of the 100% active components-molded catalyst [6].

Figure 4: Influence of catalyst layer thickness on the De-NOx activity and reactivity comparison between prepared and commercial catalysts.

Figure 5 showed the performance comparison Results in terms of K_NO and SO₂/SO₃ conversion over corrugated paper coated catalyst (catalysts layer thickness 0.2 × 2 mm) and commercial extruded honeycomb catalyst (catalysts layer thickness 0.8 mm). The prepared corrugated paper catalyst exhibited comparable NO conversion but lower SO₂/SO₃ conversion than that of commercial catalyst. It is known that, the reduction of NO depends mainly on the external geometric surface area and therefore is little influenced by the change of the catalyst layer thickness. But, the conversion of SO₂ to SO₃ occurs in the whole catalytic volume and thus can be reduced by decreasing the thickness of the active phase layer [7]. However, we must notice that the V₂O₅- WO₃/TiO₂ catalyst is weak in the presence of SO₂ in the temperature range of 200 and 250°C. And further research to improve its resistance against SO₂ is under progress by modifying the catalysts with Mo.

Figure 5: The De-NOx performance comparison between prepared and commercial catalysts.

ConclusionS

The compact issue of the De–NOx catalyst for vessel is always neglected by many researchers, a novel catalyst V₂O₅–WO₃/ TiO₂ based on the corrugated paper support with low bulk density but good reactivity was proposed in this study. With increasing the coated catalyst layer until 0.2 mm, the De-NOx activity increases significantly. Comparing with a commercial extruded honeycomb catalyst with 0.8 mm catalyst layer thickness, the prepared catalyst in this study with 0.2 × 2 mm catalyst layer thickness showed better performance in terms of K_NO and SO₂/SO₃ conversion.

References

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