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Issue 7, 2012
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Low absorption vitreous carbon reactors for operandoXAS: a case study on Cu/Zeolites for selective catalytic reduction of NOx by NH3

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Abstract

We describe the use of vitreous carbon as an improved reactor material for an operandoX-ray absorption spectroscopy (XAS) plug-flow reactor. These tubes significantly broaden the operating range for operando experiments. Using selective catalytic reduction (SCR) of NOx by NH3 on Cu/Zeolites (SSZ-13, SAPO-34 and ZSM-5) as an example reaction, we illustrate the high-quality XAS data achievable with these reactors. The operando experiments showed that in Standard SCR conditions of 300 ppm NO, 300 ppm NH3, 5% O2, 5% H2O, 5% CO2 and balance He at 200 °C, the Cu was a mixture of Cu(I) and Cu(II) oxidation states. XANES and EXAFS fitting found the percent of Cu(I) to be 15%, 45% and 65% for SSZ-13, SAPO-34 and ZSM-5, respectively. For Standard SCR, the catalytic rates per mole of Cu for Cu/SSZ-13 and Cu/SAPO-34 were about one third of the rate per mole of Cu on Cu/ZSM-5. Based on the apparent lack of correlation of rate with the presence of Cu(I), we propose that the reaction occurs via a redox cycle of Cu(I) and Cu(II). Cu(I) was not found in in situSCR experiments on Cu/Zeolites under the same conditions, demonstrating a possible pitfall of in situ measurements. A Cu/SiO2 catalyst, reduced in H2 at 300 °C, was also used to demonstrate the reactor's operando capabilities using a bending magnet beamline. Analysis of the EXAFS data showed the Cu/SiO2 catalyst to be in a partially reduced Cu metal–Cu(I) state. In addition to improvements in data quality, the reactors are superior in temperature, stability, strength and ease of use compared to previously proposed borosilicate glass, polyimide tubing, beryllium and capillary reactors. The solid carbon tubes are non-porous, machinable, can be operated at high pressure (tested at 25 bar), are inert, have high material purity and high X-ray transmittance.

Graphical abstract: Low absorption vitreous carbon reactors for operandoXAS: a case study on Cu/Zeolites for selective catalytic reduction of NOx by NH3

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Publication details

The article was received on 20 Sep 2011, accepted on 17 Nov 2011 and first published on 09 Dec 2011


Article type: Paper
DOI: 10.1039/C1CP22992C
Citation: Phys. Chem. Chem. Phys., 2012,14, 2229-2238
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    Low absorption vitreous carbon reactors for operandoXAS: a case study on Cu/Zeolites for selective catalytic reduction of NOx by NH3

    V. F. Kispersky, A. J. Kropf, F. H. Ribeiro and J. T. Miller, Phys. Chem. Chem. Phys., 2012, 14, 2229
    DOI: 10.1039/C1CP22992C

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