Hydrogen production over Co-promoted Mo-S water gas shift catalysts supported on ZrO2

doi:10.1016/j.apcata.2019.117361

Applied Catalysis A: General

Volume 590, 25 January 2020, 117361

https://doi.org/10.1016/j.apcata.2019.117361 Get rights and content

Highlights

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Co addition enhanced the WGS activity of Mo-S/ZrO₂ catalysts.
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The formation of CoMo-S species increased with Co content up to Co/Mo = 0.3.
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CO conversion was stable during 4 weeks of WGS reaction in 7000 ppm H₂S containing feed at 35,000 h⁻¹ GHSV and 350 °C.
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The optimal atomic Co/Mo ratio at monolayer coverage on ZrO₂ resulted in enhanced WGS activity.
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The active metal loading in the best CoMo-S catalyst was lower than in pure Mo-S/ZrO₂ and commercial catalysts.

Abstract

A series of CoMo-S/ZrO₂ catalysts were prepared by modifying Co and Mo content using incipient wetness impregnation method, investigated in the water gas shift (WGS) reaction in the presence of H₂S and characterized by TEM, XRD, Raman, TPR, and XPS. The supported CoMo-S catalyst corresponding to the CoMo-O monolayer coverage displayed the presence of highly dispersed CoMo-S species that were characterized by the highest extent of sulfidation. The presence of cobalt facilitated the formation of active CoMo-S species that were more H₂S-dependent as compared to Mo-S entities. The Co/Mo = 0.3 catalyst at monolayer CoMo-O coverage was the most active and thermally stable, as well as the least H₂S-dependent during 4 weeks of WGS reaction among all catalysts investigated.

Graphical abstract

Introduction

Hydrogen is an environmentally friendly and high energy density (120 MJ/kg) molecule that can be used as a fuel for stationary electric power plants and mobile fuel cells with zero CO₂ emissions [[1], [2], [3], [4]]. Most hydrogen (∼96%) is currently produced from fossil fuel feedstocks: 48% from high-temperature steam reforming of natural gas, 30% from partial oxidation of refinery oil, and 18% from coal gasification [5,6]. The major challenges of using fossil fuels for hydrogen production are high CO₂ emissions, the uneven distribution, and shortage of fossil fuel reserves [7]. However, hydrogen production from biomass-derived syngas could overcome these challenges, since biomass is a carbon-neutral and abundant sustainable energy resource [[7], [8], [9]].

The water gas shift (WGS) reaction is a key operation in maximizing the production of hydrogen from biomass-derived syngas [7,8]. As the WGS reaction is reversible and mildly exothermic, the WGS activity depends on reaction temperature ( $C O (g) + H_{2} O (g) \leftrightarrow C O_{2} (g) + H_{2} (g) [Δ H^{0} = - 41.1 k J / m o l]$ ) [10,11]. Typical WGS reactors consist of a high-temperature shift (HTS, 350∼450 °C) and a low-temperature shift (LTS, ∼250 °C) stages due to the thermodynamic limitations of this reaction [[10], [11], [12]]. However, it is quite challenging to apply typical WGS catalysts in hydrogen production using biomass-derived syngas, since biomass feedstocks contain a considerable amount of sulfur [13].

The main challenge of using current Fe₂O₃-Cr₂O₃ HTS catalysts is the easy deactivation of these catalysts when the H₂S concentration in the syngas exceeds 500 ppm [14], while the commercially available Cu–ZnO–Al₂O₃ (LTS) catalysts deactivate completely in the presence of trace levels of H₂S (∼10 ppm) [15]. On the other hand, the molybdenum-based catalysts have gained significant interest as a sulfur-tolerant WGS catalyst in recent years. However, Mo-based catalysts are sulfur-dependent, requiring at least 100 ppm of H₂S to display WGS activity [16]. Currently, no catalyst can maintain WGS activity over the entire range of sulfur content encountered in biomass (0 ∼ 15,000 ppm on dry basis) [13]. Moreover, the WGS activity of Mo-based catalysts is relatively low as compared to conventional HTS and LTS WGS catalysts, Fe- and Cu-based [11,17] and needs to be further improved for this application.

As compared to traditional supports, such as alumina, ZrO₂ is a promising support for Mo-based WGS catalysts because it can improve their catalytic activity due to weak MoO₃-support interactions. For example, alumina-supported Mo catalysts pre-activated above 500 °C interacted strongly with Al₂O₃ and displayed lower activity in hydrodesulfurization (HDS) of thiophene than these catalysts pre-activated below 400 °C [18]. On the other hand, Shetty et al. reported that the ZrO₂-supported Mo catalysts characterized by weak interactions between ZrO₂ and Mo species showed high catalytic activity in the hydrodeoxygenation of m-cresol to toluene [19]. The Mo/ZrO₂ catalysts showed the greatest extent of MoO₃ reduction to MoO₂ among the ZrO₂-, TiO₂-, Al₂O₃-, SiO₂-, and MgO-supported Mo catalysts [20], suggesting that MoO₃ interacted weakly with ZrO₂ support.

The high density of surface Mo-O sites is traditionally achieved by enhancing their dispersion as a function of MoO₃ surface coverage on oxide supports. High hydrodeoxygenation (HDO) of anisole activity was observed over the Mo-O/ZrO₂ catalysts containing a monolayer MoO₃ surface coverage due to the presence of highly dispersed Mo oxide species [21]. Al₂O₃-supported Mo-based catalysts at monolayer MoO₃ surface coverage showed the highest WGS activity [22,23]. The modified dispersion of Mo-O species on ZrO₂ at different MoO₃ loadings has been reported by Chary et al. [24]. However, the effects of CoMo-O surface coverage on ZrO₂ support have not been widely studied in the WGS reaction.

Several additives were proposed to promote the WGS activity of Mo-based catalysts. Alkali metal promoters have been suggested to improve the WGS activity by enhancing reducibility and sulfidation of Mo [25], while the high mobility of alkali promoters at high reaction temperature had a negative impact on the stability of Mo-based catalysts [26]. Nickel (Ni)-promoted Mo-S catalysts have been reported to show enhanced WGS activity and bimetallic synergistic effect [[27], [28], [29], [30]], whereas the methanation side-reaction and increased carbon deposition are typical disadvantages of Ni-Mo catalysts for hydrogen production. Cobalt was reported as the optimal additive to improve the WGS activity of Mo-based catalysts by increasing the density of active sites [[31], [32], [33], [34], [35]]. CoMo-S/TiO₂-Al₂O₃ catalysts showed higher WGS activity than commercial sour shift catalysts (CoMo-MgO-Al₂O₃) at H₂O/CO = 1 [36,37].

The WGS reaction pathways for the Al₂O₃-supported Mo-S and Co-promoted Mo-S species have been investigated by DFT calculations and in experimental studies [38,39]. The CoMo-S and Mo-S sites in the CoMo-S/Al₂O₃ catalysts were distinguished and quantified using CO adsorption followed by in-situ IR spectroscopy (IR/CO) at low temperature [38,40]. Chen et al. reported that the intrinsic activity of CoMo-S sites in the WGS reaction at low temperature (< 300 °C) was higher than that of unpromoted Mo-S sites [38]. Recent DFT calculations over CoMo-S WGS catalysts reported that the overall reaction barrier for the COOH-associated mechanism proposed for the WGS reaction on the S edge of Co-MoS₂ was lower than that of the redox mechanism on the S edge of MoS₂ [39]. The Co addition can reduce the height of the H₂O dissociation energy (H₂O → OH + H) on the sulfur edge sites of Co-MoS₂ as compared to that of unpromoted Mo-S [39,41], which is one of the rate-determining steps in the WGS reaction.

We investigated two series of ZrO₂ supported CoMo-S catalysts. In the first series, we investigated the influence of CoMo-O surface coverage ranging from 0.5 to 4 layers on ZrO₂ on the WGS activity. In the second series, the effect of Co/Mo ratios at monolayer surface coverage were investigated. The long-term stability and H₂S-dependence of CoMo-S/ZrO₂ catalysts as a function of CoMo-O surface coverage and Co/Mo atomic ratios were also investigated employing an H₂S containing feed, while both series of CoMo-S/ZrO₂ catalysts were characterized by TEM, XRD, Raman, TPR, and XPS.

Section snippets

Catalyst synthesis

Supported CoMo catalysts were prepared by incipient wetness impregnation of the ZrO₂ support with a solution of ammonium molybdate tetrahydrate (Fisher Scientific) and cobalt nitrate (Alfa Aesar). Crushed and sieved 100 ∼ 500 μm particles of ZrO₂ were employed, which were supplied by Saint-Gobain. After incipient impregnation with aqueous solutions of the Co and Mo precursors, the supported catalysts were dried overnight at 80 °C. The dried catalysts were then calcined at 500 °C in air for 5 h.

Characterization of n ML CoMo/ZrO₂ catalysts

Fig. 1 shows the TEM images of n ML CoMo-S catalysts. The inset of Fig. 1a illustrates the presence of 2.8 Å and 3.7 Å lattice fringes in the 0.5 ML catalyst, which correspond to the (-111) and (110) planes of ZrO₂. The small dark dots (white arrow) suggested the presence of CoMo-S, indicating that CoMo-S species partially covered the ZrO₂ support since the Co and Mo content in the 0.5 ML catalyst was insufficient to coat entire ZrO₂ surface. The inset of Fig. 1b demonstrates less distinct

Conclusions

A series of CoMo catalysts supported on ZrO₂ were synthesized at different CoMo-O surface coverage ranging from 0.5 to 4 ML. Highly dispersed CoMo-S species at monolayer coverage and increased ordering of CoMo-S layers at multilayer coverage were observed by TEM, XRD, and Raman spectroscopy in sulfided n ML CoMo-S catalysts. The WGS activity of these n ML catalysts increased with the surface coverage up to theoretical monolayer and then decreased for the 4 ML catalyst. The atomic S/Mo ratios

Acknowledgment

The authors gratefully acknowledge funding support from the Ohio Coal Development Office (OCDO) through the grants OCRC-R-14-11 and OCRC-R-16-11.

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Hydrogen production over Co-promoted Mo-S water gas shift catalysts supported on ZrO2

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Catalyst synthesis

Characterization of n ML CoMo/ZrO2 catalysts

Conclusions

Acknowledgment

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Int. J. Hydrogen Energy

Fuel

Int. J. Hydrogen Energy

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J. Power Sources

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J. Catal.

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Fuel

Appl. Catal. A Gen.

Appl. Catal. A Gen.

Appl. Catal. A Gen.

Ranliao Huaxue Xuebao J. Fuel Chem. Technol.

Catal. Today

Catal. Today

Int. J. Hydrogen Energy

Fuel

J. Catal.

J. Energy Chem.

Chem. Eng. Sci.

Catal. Today

Appl. Catal. A Gen.

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

Appl. Catal. A Gen.

Appl. Catal. A Gen.

Appl. Catal. A Gen.

J. Catal.

Hydrogen production over Co-promoted Mo-S water gas shift catalysts supported on ZrO₂

Characterization of n ML CoMo/ZrO₂ catalysts