Stability of bimetallic Pd–Zn catalysts for the steam reforming of methanol

Abstract

ZnO-supported palladium-based catalysts have been shown in recent years to be both active and selective towards the steam reforming of methanol, although they are still considered to be less active than traditional copper-based catalysts. The activity of PdZn catalysts can be significantly improved by supporting them on alumina. Here we show that the Pd/ZnO/Al 2 O 3 catalysts have better long-term stability when compared with commercial Cu/ZnO/Al 2 O 3 catalysts, and that they are also stable under redox cycling. The Pd/ZnO/Al 2 O 3 catalysts can be easily regenerated by oxidation in air at 420 ºC followed by re-exposure to reaction conditions at 250 ºC, while the Cu/ZnO based catalysts do not recover their activity after oxidation. Reduction at high temperatures (>420 ºC) leads to Zn loss from the alloy nanoparticle surface resulting in a reduced catalyst activity. However, even after such extreme treatment, the catalyst activity is regained with time on stream under reaction conditions alone, leading to highly stable catalysts. These findings illustrate that the nanoparticle surface is dynamic and changes drastically depending on the environment, and that elevated reduction temperatures are not necessary to achieve high CO 2 selectivity.

Steam reforming of methanol on PdZn near-surface alloys on Pd(1 1 1) and Pd foil studied by in-situ XPS, LEIS and PM-IRAS

“…10). This vibrational frequency agrees well with the on-top CO peak reported for PdZn-ZnO-Al 2 O 3 powder catalysts, after hydrogen reduction/activation of the catalyst [37]. After annealing in 10 -6 mbar CO to 473 K, which in the range of the alloy stability window, the PdZn alloy was clearly established.…”

“…As some form of surface-bonded formaldehyde may either become quickly oxidized by interaction with the observed Zn-O(H) species or is simply thermally desorbed to the gas phase, the relative probabilities of thermal desorption as competing "chemical reaction" to total oxidation will determine the product partial pressure ratio. If the overall rate-limiting step is located within the dehydrogenation steps prior to formaldehyde, both the subsequent formaldehyde desorption and total oxidation pathways should be much faster, which can explain the observed simultaneous formation of CO 2 and CH 2 O up to 573 K. This scenario is supported by Ranganathan et al [37], who reported a faster conversion of formaldehyde than methanol toward CO 2 under otherwise identical reaction conditions. Moreover, the reaction probability toward CO 2 appears to be much higher on the supported PdZn/ZnO systems, which may be attributed to the much higher abundance of ZnO surface or ZnO/PdZn interface sites, i.e.…”

“…Also a comparison of "benchmark" turnover rates from the literature on different catalyst systems with the values of Figure 8 [37]. These values are somewhat higher than those reported in this work: we estimate a TOF of 0.01*2*2 = 0.04 at ~540 K and a maximum TOF of 0.031*2*2 = 0.144 at 570 K (assumptions for factor 4: all formaldehyde would become quickly converted toward CO 2 on supported catalysts, due to much stronger contribution of ZnO surface or metalsupport interface sites, and 50% of the original Pd surface atoms (on the clean foil) are available for reaction).…”

High-temperature steam reforming of methanol over ZnO–Al2O3 catalysts

“…Low temperature steam reforming mainly focuses on Cu-based [3][4][5][6][7][8][9][10][11][12][13][14][15] and Pd-based [16][17][18][19][20][21][22] catalysts. For Cu-based catalysts, the most extensively studied catalysts are Cu/ZnO and Cu/ZnO/Al 2 O 3 [7,9,10], etc.…”

“…CeO 2 can hinder the Cu sintering and enhance the thermal stability of catalysts [14]. Apart from Cu-based catalyst, Pd-based catalyst is another promising low temperature steam reforming catalyst [16][17][18][19][20][21][22]. Iwasa et al [20] first found Pd/ZnO had good activity, CO 2 selectivity and thermal stability for low temperature steam reforming of methanol, and all these were attributed to the formation of PdZn alloy.…”