On the enhanced sulfur and coking tolerance of Ni-Co-rare earth oxide catalysts for the dry reforming of methane

doi:10.1016/j.jcat.2020.11.028

Journal of Catalysis

Volume 393, January 2021, Pages 215-229

https://doi.org/10.1016/j.jcat.2020.11.028 Get rights and content

Highlights

•
Sulfur tolerance of Ni-based dry reforming of methane catalysts examined.
•
Both Ce/Zr and Ce/La oxides with Ni are stable at low sulfur concentrations.
•
Only Ni-Co supported on Ce/Zr oxide not rapidly deactivated at >20 ppm sulfur.
•
Ni-Co catalysts also show much lower coking rates in dry reforming.
•
Improved performance linked to strong Co-Ni and strong metal-oxide interactions.

Abstract

Sulfur and coking tolerance of Ni-based dry reforming catalysts were examined. Catalysts utilizing both Ce/Zr and Ce/La oxide supports, some with additional Co, were tested. Long-term reaction runs were conducted with and without sulfur in the feed. Catalysts were also characterized by STEM, XPS, XAFS and XANES and CO chemisorption. Only catalysts where Co was also present, and supported on the Ce-Zr oxide, were capable of extended sulfur tolerance at >20 ppm sulfur. This tolerance, along with a greatly reduced coking rate, is linked to Co in intimate contact with Ni, the mixture existing as clusters anchored and influenced electronically by the oxide support. The activation of methane takes place on these sites. Larger metal aggregates formed by ripening during reaction appear to be spectators. The measured activation energies for dry reforming suggest that CO₂ activation takes place at the oxide interface, and is a kinetically significant step.

Graphical abstract

Introduction

Carbon dioxide or “dry” reforming of methane (DRM) is potentially a better way to utilize greenhouse gases (GHGs) compared to steam reforming, since two GHGs are consumed and made into a useful industrial gas stream of theoretically 1:1 H₂/CO, CH₄ + CO₂ → 2H₂ + 2CO.

The rationale and catalyst technology behind DRM have been reviewed extensively over the years [1], [2], [3], [4], [5]. However, several challenges remain in order to enable large scale implementation of DRM. The high temperatures required (typically >700 °C) is one, and relatively fast deactivation of non-noble metal (mainly Ni-based) catalysts the other. The former challenge can be addressed by application of hybrid technologies such as oxy-steam CO₂ reforming (aka “tri-reforming”) or CH₄ bireforming (dry plus steam reforming). The latter challenge has been addressed by the alloying or mixing of Ni with certain other metals (Co, Mn, Mo), and/or the application of supports that slow down or control crystallite ripening and deactivation [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. As the deactivation can be due to coking, active metal sintering, or poisoning by sulfur, chlorine etc., or some combination of these, no support is at present considered optimal for all feeds. The two most important methane sources, natural gas and biogas, can both contain significant amounts of sulfur compounds (up to 100 ppm) in the form of H₂S, mercaptans, sulfides and tetrahydrothiophene [32], all of which can eventually lead to catalyst poisoning. Tuning the electronic surroundings of the active sites, adding adsorbent beds and choosing supports that provide more oxygen vancancies are all considered possible ways of designing anti-sulfur poisoning catalysts [33], [34], [35], [36].

We have discussed previously how to design and evaluate catalysts that can minimize coke formation and improve stability in DRM [37], [38]. It was found that mixed rare-earth oxide supports such as CeO₂-ZrO₂ and CeO₂-La₂O₃ can both lead to low-coking rates in DRM, with Ni-based catalysts. Addition of Co leads to further gains in stability for largely unknown reasons, because although Co(0) can activate CH₄ at <430 °C [39], Co/CeO₂ is itself a not too active metal at more typical DRM conditions [37]. This study will focus on a comparison of long-term behavior of both supports, how sulfur-tolerant the resulting catalysts are, the Ni-Co interactions, and how the final catalyst states for the two supports actually differ greatly. This last result has implications toward the mechanism of DRM that will be exploited further in subsequent work. Fresh and spent catalysts of both types have been characterized by XAFS, CO chemisorption, STEM, and XPS.

Section snippets

Catalyst preparation

Ce/La mixed oxides were prepared by a templated sol-gel method adapted from previous work [37], [40], [41], [42], [43]. The precursors for Ce and La were (NH₄)₂Ce(NO₃)₆ (98+%, Alfa) and La(NO₃)·6H₂O (99.9%, Alfa), which were dissolved in 96% water/3% methanol/1% TMAOH surfactant (25% in methanol). The molar ratio Ce/La was 3:1, with a 1/100 wt ratio of the oxides in the sol-gel synthesis mixture. Ammonia solution was used to gradually adjust the pH to 10.3 during aging (90 °C, 2 d). After

Long-term catalyst evaluations

Catalysts from previous work that were found to exhibit low rates of coking and high initial activities were tested over longer periods in a conventional fixed-bed reactor at 775 °C [37]. No Co-only catalysts are included because these were found to be less active than any Ni-based ones. The activities can be quantified as the product of the GHSV (mL/(g·h), typically 44000), and the CH₄ conversion, on a molar basis. For a zero-order reaction, this is the same as the average rate of reaction,

.1 Origin of sulfur tolerance

At low sulfur concentrations in the feed, all catalysts tested here are fairly stable. The Ni-only catalysts show some deactivation, but they do so even when sulfur is not present (Fig. 1), so this is not due to sulfur poisoning. But in a high sulfur concentration environment (20–30 ppm), only catalysts where Ni is paired with Co, and Ce with Zr, and the Ce/Zr ratio is low (1:1) can survive over the long term (Fig. 2). The addition of Co in particular is crucial to the sulfur tolerance. Simpler

Conclusions

1)
Extended sulfur tolerance in Ni-based DRM catalysts at the 20–30 ppm level can be attained by the addition of Co, and supporting the metals on mixed rare earth oxides such as Ce-Zr oxide, that exhibit high concentrations of oxygen defect sites. Extended sulfur tolerance at <1 ppm is possible with both Ce-Zr and Ce-La oxides, and without Co.
2)
In catalysts containing both transition metals, at least some of the Co is interacting with the Ni. The combination reduces the rate of coking even further,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Science Foundation (grant number CBET 1510435). The authors would like to thank Drs. Amitava Roy and Orhan Kizilkaya of the LSU Center for Advanced Structures and Microdevices for help with XAS and XPS. Aberration-corrected STEM imaging was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

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On the enhanced sulfur and coking tolerance of Ni-Co-rare earth oxide catalysts for the dry reforming of methane

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Catalyst preparation

Long-term catalyst evaluations

.1 Origin of sulfur tolerance

Conclusions

Declaration of Competing Interest

Acknowledgements

Adv. Catal.

J. CO2 Utilization

Appl. Catal. A: Gen

Catal. Today

J. Catal.

Appl. Catal. B

Appl. Catal. B, Env.

Appl. Catal. A

Appl. Catal. A, Gen.

Appl. Catal. A

Appl. Catal., A

Appl. Catal. A

Catal. Today

Appl. Catal. A

J. Mol. Catal. A: Chem.

Appl. Catal. B, Env.

Catal. Today

Prog. Energy Combust. Sci.

Chem. Eng. Sci.: X

J. Catal.

J. Catal.

Catal. Today

Chem. Eng. Sci.

J. Catal.

Appl. Catal. A: Gen.

Surf. Sci.

Thin Solid Films

Appl. Surf. Sci.

Appl. Surf. Sci.

Carbon

J. Catal.

J. Catal.

Appl. Catal. A

Appl. Catal., A

Chem. Eng. J.

Appl. Catal. A

Appl. Catal. B

Catal. Today

J. Catal.

J. Catal.

Appl. Catal. B: Environ.

Appl. Catal. B

Intl. J. Hydrogen Energy

New trends in reforming technologies: from hydrogen industrial plants to multifuel microreformers

Catal. Rev.

Fundamentals and catalytic applications of CeO2 based materials

Chem. Rev.

Application of ceria in CO2 conversion catalysis

ACS Catal.

Progress in synthesis of highly active and stable nickel-based catalysts for carbon dioxide reforming of methane

ChemSusChem

Inverse NiAl2O4 on LaAlO3–Al2O3: unique catalytic structure for stable CO2 reforming of methane

J. Phys. Chem. C

Progress in the synthesis of catalyst supports: synergistic effects of nanocomposites for attaining long-term stable activity in CH4−CO2 dry reforming

Ind. Eng. Chem. Res.

J. CO₂ Utilization

Fundamentals and catalytic applications of CeO₂ based materials

Application of ceria in CO₂ conversion catalysis

Inverse NiAl₂O₄ on LaAlO₃–Al₂O₃: unique catalytic structure for stable CO₂ reforming of methane

Progress in the synthesis of catalyst supports: synergistic effects of nanocomposites for attaining long-term stable activity in CH₄−CO₂ dry reforming