Elsevier

Applied Catalysis B: Environmental

Volume 237, 5 December 2018, Pages 1021-1032
Applied Catalysis B: Environmental

Three-way catalysis with supported gold catalysts: Poisoning effects of hydrocarbons

https://doi.org/10.1016/j.apcatb.2018.06.063Get rights and content

Highlights

  • Two poisoning routes by co-reactants identified for CO oxidation over supported Au.

  • Route 1 – competitive adsorption combined with electron transfer towards Au.

  • Route 2 specific for propene – formation of deposits.

  • Deposits do not block CO adsorption sites (!) but prevent supply of active support oxygen.

  • At higher temperatures, active oxygen burns deposits, → normal CO oxidation rates.

Abstract

Poisoning phenomena recently observed during the conversion of three-way catalysis model feed (CO, NO propene, O2, without or with water) over supported Au catalysts [V. Ulrich et al., Applied Catalysis B 203 (2017) 572] were investigated by reaction studies (replacing propene by propane in model feed, study of CO oxidation without and with relevant additional components), by temperature-programmed oxidation of deposits (TG, TPO), by studying self-regeneration phenomena and by DRIFTS of CO adsorption and co-adsorption of CO and propene under different conditions. During CO oxidation in the presence of other components, two different poisoning phenomena were observed. In the presence of propane, propene, and/or NO, CO oxidation was significantly inhibited below 370 K over Au on Al2O3 or on La-stabilized Al2O3, but not on Au/CeZrOx where activity was relatively low anywhere. DRIFTS data on (co-)adsorption of CO and propene suggest that this poisoning is not merely a result of site blocking by the coadsorbate, but that electron transfer from the co-adsorbates towards the active sites, which are assumed to be at the perimeter of positively charged Au clusters with the support, might have additionally quenched the reaction. At higher temperatures, a different poisoning mechanism operates in propene-containing mixtures. From studies by TPO, TG, and DRIFTS, we conclude the formation of carbonaceous residues, which do not block, however, CO adsorption sites but the support surface around Au clusters instead. While this inhibits CO oxidation involving active support oxygen, the same oxygen species might combust the deposits at higher temperatures. This results in self-regeneration phenomena depending on the redox activity of the support, the temperature and the propene content in the feed. In transient experiments, it was observed that NO is most favorably reduced by intermediates of coke formation from propene. Under stationary conditions, however, NO reduction, mostly by CO, remains insufficient.

Introduction

Since the 1970s the three-way catalyst (TWC) has become an important component of all gasoline-driven cars aiming at the attenuation of their environmental impact [1]. It converts the harmful exhaust components CO, unburned hydrocarbons (HC), and NOx (NO and NO2) into the harmless compounds CO2, N2, and H2O. To catalyze these reactions, the TWC contains Pt, Pd, and Rh in different combinations supported on refractive oxides with high surface area (e.g. γ-Al2O3, ZrO2, stabilized against sintering by La or Ba additives), or on oxygen storage compounds (usually Ce-Zr mixed oxides) [2]. Operation at low temperatures, e.g. during the cold start phase or under idling conditions, is a sore point with present-day TWCs, which has been addressed by a number of secondary approaches (traps, close-coupled converters) [3]. However, further improvement of the TWC itself is also desirable to solve this problem more directly. Including gold into TWC formulations is an interesting approach along this line. The idea has been part of critical discussions [[4], [5], [6]].

In 2002, application of a supported Au catalyst in three-way catalysis was first reported in the open literature by Mellor et al. [7]. The authors deposited gold on a complex support comprising ZrO2-stabilized CeO2, ZrO2, and TiO2 and promoted with Co oxides, ZnOx, MgO, BaO, and Rh. Under reducing conditions, this catalyst fully oxidized CO, propane, and propene and reduced NO, though at rather high temperature: light-off temperatures T50 (Temperature of 50% conversion) were reported to be ≈573 K with Rh added, and 653 K without Rh. Under oxidizing conditions, NO reduction did not reach even 50% conversion [7]. We have recently examined monometallic Au supported on γ-Al2O3, La-stabilized Al2O3, TiO2, and Ce-ZrOx for its behavior in three-way catalysis [8]. The choice of these supports was motivated by the composition of real TWCs (see above) and by the tremendous role of Au/TiO2 in recent research on low-temperature CO oxidation. CO conversion was fast on these catalysts, whereas NO reduction remained insufficient over all of them. In the model feed containing CO together with propene, NO, and oxygen, CO oxidation was strongly attenuated, with light-off temperatures far above those observed in a binary CO/O2 feed. Propene was proposed to poison CO oxidation in this feed. Its poisoning efficiency strongly depended on the water content in the feed and on the nature of the support, being most pronounced under dry conditions and on non-reducible supports. In the present paper we report more evidence on the nature of the poisoning of CO oxidation during three-way catalysis over supported Au catalysts, in particular on its origin and on opportunities to alleviate the effect.

Section snippets

Catalyst preparation

Details of catalyst preparation have been reported recently [8]. Al2O3, La-stabilized Al2O3, and a ceria-zirconia mixed oxide, which will be denoted as “CeZrOx” were used as supports for gold deposition. CeZrOx was donated by Umicore & Co. KG Hanau (Germany). Both alumina-based supports were supplied by Sasol Germany GmbH (Puralox SCFa 140 and SCFa 140-L3), supplier information about composition is given in the supporting information. Gold (1.6–1.8 wt-%) was deposited on these supports by a

Textural and Au dispersion in spent catalysts

Data on texture and Au particle size after one sequence of the feed variation experiment with final catalyst reoxidation is summarized in Table 1. The most severe part of the procedure experienced by the catalysts is exposure to the dry feed for about 50 min followed by oxidation in 2% O2/He for 30 min, all at 673 K (Fig. S1). This caused the number-weighted average Au particle size to increase by a factor of ≈1.5. There is also a slight decrease of BET surface area and pore volume, for the BET

Conclusions

During CO oxidation over supported Au catalysts in the presence of other components typical of three-way catalysis, two different poisoning phenomena were observed. In the presence of propane, propene, and/or NO, CO oxidation was significantly inhibited below 370 K over Au supported on Al2O3 or on La-stabilized Al2O3, but not on Au/CeZrOx where activity was relatively low anywhere. Based on DRIFTS data on (co)adsorption of CO and propene we propose that this poisoning results from competitive

Acknowledgements

We gratefully acknowledge financial support by the German Science Foundation (DFG, Grant No. Gr 1447/24-1), as well as by the Russian Academy of Sciences and Federal Agency of Scientific Organizations (Project No. AAAA-A17-117041710086-6). We also thank Ms. N. Arshadi and Mr. K. Ollegott for their help in performing this study, and Sasol Germany GmbH and Umicore & Co. KG Hanau (Germany) for donations of supports.

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