Zirconium‐Assisted Activation of Palladium To Boost Syngas Production by Methane Dry Reforming

Abstract C‐saturated Pd0 nanoparticles with an extended phase boundary to ZrO2 evolve from a Pd0Zr0 precatalyst under CH4 dry reforming conditions. This highly active catalyst state fosters bifunctional action: CO2 is efficiently activated at oxidic phase boundary sites and PdxC provides fast supply of C‐atoms toward the latter.


Supplemental Results
S1: Schematic representation of the two types of employed model catalysts and their distinct chemical and structural development under dry reforming conditions. The simplified schematic representation of the "inverse" pre-catalyst is based on a fcc (100) lattice plane containig Zr 0 atoms in the subsurface region. The subsurface intermetallic state has been verified in setup 1 by a combination of XPS and LEIS, as well as the "ZrO 2 islands on top" state after oxidative segregation under reaction conditions. The schematic lattice of the bulk intermetallic pre-catalyst does not reflect the polycrystalline Pd 2 Zr + Pd 3 Zr phase mixture used in reality. Under DRM conditions, crystalline Pd 0 is formed, as well as t-ZrO 2 , which is also not represented with its actual 3D crystal structure. Both newly formed phases -Pd and t-ZrO 2form a nanocrystalline conglomerate on top of the polycrystalline Pd x Zr y substrate. The particle size and the lattice constant of Pd 0 were estimated to ~7.5 nm/ 3.914 Å in-situ during DRM at 800°C and to 10.3 nm/ 3.898 Å after DRM and subsequent O 2 oxidation. S2: Pre-DRM state of the bulk-intermetallic PdZr precursor. Despite the dominance of Zr +4 species in the Zr3d region, the intermetallic Zr 0 component is clearly visible at a BE of 179.6 eV [2,3] . Already the short contact to the ambient during transfer to the AP-XPS chamber caused oxidative segregation of the bimetallic toward a top layer of ZrO 2 and metallic Pd. The Pd3d 5/2 region is superimposed by the second harmonic contribution of the O1s signal at a nominal BE of 333.5 eV, otherwise only metallic Pd is observed at a BE of 335.0 eV. In the C1s region a certain amount of graphite-type C is already present before DRM, with minor contributions of C x H y species and C-oxygenates [11,13,17] .

S3:
Changes of Pd lattice parameter and particle size with time observed by in-situ XRD during reoxidation of the post-DRM sample in a flow of 2 ml/min clean oxygen at ambient pressure and 600°C. Between 25 and 35 min exposure to O 2 , a lowering of the lattice constant by ~0.016 Å takes place. We assume that the partial oxidation of Pd metal toward PdO is accompanied by simultaneous carbon depletion of the Pd bulk, thus leading to a smaller lattice parameter. S4: Temperature programmed DRM reaction rate profiles obtained on the subsurface Zr 0 -Pd foil precatalyst versus single phase ZrO 2 film and clean Pd foil. The reaction conditions were identical to those of Figure 2 in the main paper. Only a slight promotion of DRM activity was observed on the subsurface Zr 0 -Pd foil precatalyst relative to phase-pure ZrO 2 . Pure Pd is hardly active, anyway.
The CVD preparation of sub-monolayer ZrO x H y islands, and in due course, of our "inverse" subsurface-Zr 0 -intermetallic precatalysts on bulk Pd foil, was originally motivated by a potentially scalable promotional role of variable phase boundary dimensions on the DRM activity. What actually came out was initially surprising, namely that all these "inverse" systems were only slightly more active than the sum of their individual oxidic and metallic surface fractions. The explanation for strong DRM promotion given in the main paper for the bulk-intermetallic precursor requires sufficiently high concentrations of reactive dissolved C arriving through the metallic phase toward the PB, which is obviously not the case on the infinite bulk "inverse" models on Pd foil. Thus, also changes of the initial bimetallic Zr 0 amount between 5% and 90% led to only minor differences in the observed activities. In practice, these tiny differences were not scalable with the in-situ DRM induced PB dimensions for several reasons: -poor signal to noise ratio due to low overall activity, together with too small activity differences. Given our experimental error bars at these rather low total rates, we cannot quantify the relative contributions of the two surface regions relative to the PB sites.
-non-scalable amounts of reactive dissolved C in the Pd foil bulk (which are both time-, temperature and oxide-coveragedependent) -non-scalable island density of re-segregated ZrO x H y from the bimetallic precursor state, thus the exact amount of phaseboundary sites cannot be quantified. The exact nanoscopic size-distribution of the in-situ segregated islands is not known, and thus, a normalization of these tiny rate differences to well-understood and -quantified phase-boundary dimensions is actually impossible in our view. One would have to employ in-situ microscopic techniques at the atomic scale under realistic DRM conditions, which is experimentally out of reach.
-even if the latter experiment was possible, the very small activity differences would not allow for a clear scaling relation between PB sites and activity differences, for the above mentioned reasons.   S5: Relative intensity variations of the C bulk , C graphite and C oxygenate C1s intensities with XPS probe depth. We note that also the oxygenate region exhibits an apparent intensity increase at higher photon energies. It is safe to say that a volume-dissolved species such as C bulk should exhibit reduced signal attenuation with increasing h/probe depth/photoelectron kinetic energy, in particular relative to surface-located graphene/graphite layers. In analogy, the oxygenate C1s intensity appears to be less attenuated than C graphite . According to scheme S1, the corrosion of the bulk intermetallic leads to a "conglomerate" of nanosized Pd 0 and t-ZrO 2 domains, which inherently leads to a large amont of "buried" phase boundary sites. An enhanced concentration of oxygenate species at these sites would result in a "quasi-3D" depth distribution perpendicular to the outer (geometrical) surface, and thus to a similar response to photon energy variation as for C bulk . We emphasize that this should be definitly seen as a speculation, as the poor data quality and the ambiguity of the background subtraction in the oxygenate C1s region does not allow for reliable assignments of intensity trends.

S6:
Comparison of the initial/pre-DRM and in-situ clean-off carbon concentrations in pure CO 2 on the initially bulkintermetallic catalyst. After exposure to clean CH 4 , which led to the carbon level depicted in the left spectrum of Figure 3 A), the exposure to 0.3 mbar clean CO 2 was kept for 150 min at a temperature of ~740 °C. We note that the C clean-off became very slow -slower than for the CH 4 -(or also DRM-) induced C graphite fraction -at a concentration level slightly below the precatalyst C-level, indicating that the remaining "ambient-induced" C-fraction is kinetically less accessible. After 150 min, the experiment had to be cancelled because of limited beamtime. From the peak areas of Fig. S6, values for 11.3 atom % C on the initial and 8.5 atom % C on the in-situ-CO 2 states were derived via a standard quantification routine assuming homogeneously mixed elements. We can hardly speculate on the chemical nature, location and diffusional mobility of these ambient exposure-induced C-species, and only roughly estimate their remaining life time in clean CO 2 . Based on an intensity decrease of ~2-3% within roughly 2-3 hours, it would take at least 6 hours to remove the largest fraction. A possible explanation for the low reactivity could involve their location in more remote zones with respect to the active PB region and/or otherwise impaired diffusional properties. 740°C. Both sets of spectra were recorded at the same total pressure in order to establish comparable gas-phase absorption conditions for the photoelectrons.

S7:
In-situ AP-XPS spectra on the initial subsurface Zr 0 -Pd foil catalyst under DRM conditions. Intensity trends associated with changing gas phase conditions are depicted in the bar viewgraph at the right side. Complete carbon clean-off is observed in pure CO 2 ; the carbon present after growth in clean CH 4 appears to be rather located on top of the ZrO 2 islands, which are formed in-situ under DRM conditions. A lower and time-independent steady-state carbon concentration than that growing in pure CH 4 is established in the 1:1 DRM reactant mixture. response of C bulk appears to be generally stronger. Up to ~700°C this may be caused by the increase of C solubility and the simultaneously accelerated CH 4 decomposition rate with increasing temperature, meaning that the Pd particles exhibit enhanced (net) carbon dissolution. Beyond ~700°C, thermodynamics generally favour the C depletion, since ΔG of CH 4  2H 2 + C s decreases with increasing T, but at a lower rate than ΔG of the inverse Boudouard reaction CO 2 + C s -> 2CO. The temperature in which the rate of C s generation exceeds that of C oxidation by CO 2 is around 725 ºC, as already reported in [18] . A complementary kinetic model accounting for increasingly fast C bulk depletion toward CO via the PB at higher temperatures is proposed to explain this trend. A detailed numerical balancing of PB clean-off rates of C bulk , C bulk supply both via graphite redissolution and continuous carbon supply via CH 4 from the gas phase, has not yet been attempted.  Trockenreformierung zu sehen, sowie untergeordnete Beiträgen von C x H y Spezies und C-Oxygenaten [11,13,17] . während der Trockenreformierung) gebildeten Phasengrenze skalierbar.