Identification of a Catalytically Highly Active Surface Phase for CO Oxidation over PtRh Nanoparticles under Operando Reaction Conditions

Pt-Rh alloy nanoparticles on oxide supports are widely employed in heterogeneous catalysis with applications ranging from automotive exhaust control to energy conversion. To improve catalyst performance, an atomic-scale correlation of the nanoparticle surface structure with its catalytic activity under industrially relevant operando conditions is essential. Here, we present x-ray diffraction data sensitive to the nanoparticle surface structure combined with in situ mass spectrometry during near ambient pressure CO oxidation. We identify the formation of ultrathin surface oxides by detecting x-ray diffraction signals from particular nanoparticle facets and correlate their evolution with the sample’s enhanced catalytic activity. Our approach opens the door for an in-depth characterization of well-defined, oxide-supported nanoparticle based catalysts under operando conditions with unprecedented atomic-scale resolution.

Figure 1

(a) High-resolution reciprocal space map in the bulk fcc ($H,K=2-H$, $L$) plane under reaction conditions, evidencing a truncated octahedral nanoparticle shape in (b) (see [30] for explanation of the additional diffraction features). (c) (001) type facet with surface unit cell indicated in green, the Rh $c(2\times 8)$ surface oxide and reciprocal space in surface coordinates of the (001) facet. (d) (111) type facet with surface unit cell indicated in orange, the Rh $p(9\times 9)$ surface oxide and reciprocal space in surface coordinates. Filled and open squares: particle Bragg peaks and CTRs, respectively; red arrows: performed reference line scans.

Figure 2

MS data of (a) CO, (b) ${\mathrm{O}}_2$, and (c) ${\mathrm{CO}}_2$ as a function of time and the set mass flows for conditions (i)–(iv). Left $y$ axis (black): MS signals measured by gas leaking into the chamber’s UHV part. Right $y$ axis (red) (a) and (b): set mass flows.

Figure 3

Reference scans for surface oxide formation: (a), (b) on the (001) top facets ($H_{001}=0.875$), (c) on the (111) side facets ($K_{111}=0$). Open circles: pure CO (i). Upward triangles: understoichiometric in ${\mathrm{O}}_2$ (ii). Squares: stoichiometric conditions (iii). Downward triangles: overstoichiometric in ${\mathrm{O}}_2$ (iv). Scans are rescaled for clarity. Solid (red) lines represent fits to the data (see text). (d) O-Rh-O trilayer surface oxide present on the (001) and (111) facets. (e) Average particle shape as determined from fitting the asymmetrically broadened Bragg peak line scans for condition iv: $\overline{D}=11.6\mathrm{nm}$, $\overline{H}=4.0\mathrm{nm}$ [30].

Figure 4

Time-resolved MS data and x-ray diffraction signals at (a): $H_{001}=0.875$, $K_{001}=0.51$, at (b): $H_{111}=0.91$, $K_{111}=0$. Upper panels, left $y$ axis: measured CO MS signal (black solid lines), right $y$ axis: set CO flow (red dashed line); middle panels: left $y$ axis: measured ${\mathrm{O}}_2$ MS signal (black solid line), right $y$ axis: set ${\mathrm{O}}_2$ flow (red dashed line); bottom panels: left $y$ axis: measured ${\mathrm{CO}}_2$ MS signal (black solid line), right $y$ axis (green solid line): diffracted intensity.