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. 2018 Sep 7;8(9):8255-8262.
doi: 10.1021/acscatal.8b01509. Epub 2018 Jul 26.

Unraveling the H2 Promotional Effect on Palladium-Catalyzed CO Oxidation Using a Combination of Temporally and Spatially Resolved Investigations

Caomhán Stewart  1 , Emma K Gibson  2   3 , Kevin Morgan  1 , Giannantonio Cibin  4 , Andrew J Dent  4 , Christopher Hardacre  5 , Evgenii V Kondratenko  6 , Vita A Kondratenko  6 , Colin McManus  1 , Scott Rogers  3   7 , Cristina E Stere  5 , Sarayute Chansai  5 , Yi-Chi Wang  8 , Sarah J Haigh  8 , Peter P Wells  3   4   9 , Alexandre Goguet  1
Affiliations

Unraveling the H2 Promotional Effect on Palladium-Catalyzed CO Oxidation Using a Combination of Temporally and Spatially Resolved Investigations

Caomhán Stewart et al. ACS Catal. .

Abstract

The promotional effect of H2 on the oxidation of CO is of topical interest, and there is debate over whether this promotion is due to either thermal or chemical effects. As yet there is no definitive consensus in the literature. Combining spatially resolved mass spectrometry and X-ray absorption spectroscopy (XAS), we observe a specific environment of the active catalyst during CO oxidation, having the same specific local coordination of the Pd in both the absence and presence of H2. In combination with Temporal Analysis of Products (TAP), performed under isothermal conditions, a mechanistic insight into the promotional effect of H2 was found, providing clear evidence of nonthermal effects in the hydrogen-promoted oxidation of carbon monoxide. We have identified that H2 promotes the Langmuir-Hinshelwood mechanism, and we propose this is linked to the increased interaction of O with the Pd surface in the presence of H2. This combination of spatially resolved MS and XAS and TAP studies has provided previously unobserved insights into the nature of this promotional effect.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Variation of coordination to Pd and O and the conversion of CO, O2, and H2 along the length of the catalyst bed for (a) CO oxidation and (b) CO oxidation in the presence of H2. Coordination numbers are plotted with the temperature profile for (c) CO oxidation and (d) CO oxidation in the presence of H2.
Figure 2
Figure 2
Magnitude component of the k3-weighted non-phase-corrected Fourier transform of the EXAFS data at the front, middle, and end of the catalyst bed of the (a) fresh catalyst, (b) the catalyst under CO oxidation, and (c) the catalyst under CO oxidation with H2. Features consistent with scattering from O, Pd (of Pd0), and Pd (of PdO) are highlighted in blue, green, and yellow boxes, respectively.
Figure 3
Figure 3
Relative contributions of a Pd metal standard and PdO as a function of position in the catalyst bed during (a) CO oxidation and (b) CO oxidation with H2.
Figure 4
Figure 4
CO conversion (a) upon pulsing an 18O2:CO:Ar = 1:1:2 mixture over pre-oxidized (white) and pre-reduced (red) Pd/Al2O3 at 200 °C. (b) Product distributions of C16O2, C16O18O, and C18O2 for CO oxidation upon pulsing an 18O2:CO:Ar = 1:1:2 mixture (white) and CO oxidation upon sequential pulsing of H2:Ne = 1:1 and 18O2:CO:Ar = 1:1:2 mixtures with a time delay of 0.1 s (red) over the preoxidized catalyst.
Figure 5
Figure 5
Schematic of the SPACI-FB equipment.
Figure 6
Figure 6
SPACI-FB equipment on beamline B18 at the Diamond Light Source.
See this image and copyright information in PMC

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