Origin of different deactivation of Pd/SnO2 and Pd/GeO2 catalysts in methanol dehydrogenation and reforming: A comparative study

Abstract

Pd particles supported on SnO2 and GeO2 have been structurally investigated by X-ray diffraction, (High-Resolution) transmission and scanning electron microscopy after different reductive treatments to monitor the eventual formation of bimetallic phases and catalytically tested in methanol dehydrogenation/reforming. For both oxides this included a thin film sample with well-defined Pd particles and a powder catalyst prepared by incipient wetness impregnation. The hexagonal and the tetragonal polymorph were studied for powder GeO2. Pd2Ge formation was observed on all GeO2-supported catalysts, strongly depending on the specific sample used. Reduction of the thin film at 573 K resulted in full transformation into the bimetallic state. The partial solubility of hexagonal GeO2 in water and its thermal structural instability yielded Pd2Ge formation at 473 K, at the cost of a structurally inhomogeneous support and Ge metal formation at higher reduction temperatures. Pd on tetragonal GeO2 entered a state of strong metal-support interaction after reduction at 573-673 K, resulting in coalescing Pd2Ge particles on a sintered and re-crystallized support, apparently partially covering the bimetallic particles and decreasing the catalytic activity. Pd2Ge on amorphous thin film and hexagonal GeO2 converted methanol primarily via dehydrogenation to CO and H2. At 573 K, formation of Pd2Sn and also PdSn occurred on the Pd/SnO2 thin film. Pd3Sn2 (and to some extent Pd2Sn) were predominantly obtained on the respective powder catalyst. Strong deactivation with increasing reduction temperature was observed, likely not based on the classical strong metal-support interaction effect, but rather on a combination of missing active structural ensembles on Sn-enriched bimetallic phases and the formation of metallic β-Sn. Correlations to Pd and its bimetallics supported on ZnO, Ga2O3 and In2O3 were also discussed.

From Oxide‐Supported Palladium to Intermetallic Palladium Phases: Consequences for Methanol Steam Reforming

“…PdZn bulk [52] 99 (673) --120 open [b] no PdGa NSIP [66] unselective, CO strongly predominant < 1. 3 [90] 86 (623) 0.1 (590) 7.3 10 À3 61 yes no bcc-In 2 O 3 bulk [73] > 95 (673) 3.5 10 À3 (590) < 1 10 À6 100 [a] no no PdIn/bcc-In 2 O 3 [76] > 95 (673) 4 10 À2 (590) 1.5 10 À3 88 no no deactivated relative to NSIP SnO 2 [77] 86 (673) 6 10 À3 (590) 1.5 10 À4 113 no no Pd 2 Sn/SnO 2 [79] reductively deactivated for both CO and CO 2 -----Pd 3 Sn 2 /SnO 2 [79] reductively deactivated for both CO and CO 2 -----PdSn/SnO 2 [79] reductively deactivated for both CO and CO 2 -----GeO 2 (tetr) [77] very low activity, 50 % CO 2 [a] (673) no no GeO 2 (hex) [77] inactive -----Pd 2 Ge/GeO 2 (amorph) [79] unselective, CO strongly predominant no no Pd 2 Ge/GeO 2 (tetr) [79] deactivated by SMSI -----Pd 2 Ge/GeO 2 (hex) [79] deactivated by encapsulation ----- The extension of these experiments to small Ga 2 O 3 -supported PdÀGa particles revealed the formation of structurally stable Pd 2 Ga and Pd 5 Ga 2 particles after reduction with H 2 at temperatures between 523 K (thin films) and 673 K (powder samples, only Pd 2 Ga); [67,68] PdGa particles were only formed at higher temperatures (about 973 K). In general, the intermetallic PdÀ Ga particles are much less sinter-and structurally stable than their PdZn counterpart.…”

DFT study of CO adsorption on Pd-SnO2(1 1 0) surfaces

“…X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SSIM) were employed to study the gas sensing properties of Pd ultra-thin films deposited on pyrolytically prepared SnO x substrates and on SnO 2 powders in real sensing condition, i.e., during heating in air and in hydrogenenriched air [12,13]. In order to investigate the possible formation of bimetallic phases, Pd particles supported on SnO 2 were studied by X-ray diffraction and transmission and scanning electron microscopy after different reductive treatments by Lorenz et al [14]. Pd particles supported on SnO 2 oxidize easier and are more difficult to reduce than a Pd(1 1 1) single crystal due to the interaction between the Pd particles and the SnO 2 substrate.…”

Nanostructured copper, chromium, and tin oxide multicomponent materials as catalysts for methanol decomposition: 11C-radiolabeling study

“…5). At the same time, reflections at about 30.6°, 32.0°, 43.8°and 44.9°2h which corresponds to (2 0 0), (1 0 1), (2 2 0) and (2 1 1) lattice spacing, of metallic Sn [40] were registered in the XRD patterns of the sample after the cata- lytic test (Fig. 1c).…”