This Minireview summarizes the fundamental results of a comparative inverse-model versus real-model catalyst approach toward methanol steam reforming (MSR) on the highly CO2-selective H2-reduced states of supported Pd/ZnO, Pd/Ga2O3, and Pd/In2O3 catalysts. Our model approach was extended to the related Pd/GeO2 and Pd/SnO2 systems, which showed previously unknown MSR performance. This approach allowed us to determine salient CO2-selectivity-guiding structural and electronic effects on the molecular level, to establish a knowledge-based approach for the optimization of CO2 selectivity. Regarding the inverse-model catalysts, in situ X-ray photoelectron spectroscopy (in situ XPS) studies on near-surface intermetallic PdZn, PdGa, and PdIn phases (NSIP), as well as bulk Pd2Ga, under realistic MSR conditions were performed alongside catalytic testing. To highlight the importance of a specifically prepared bulk intermetallic[BOND]oxide interface, unsupported bulk intermetallic compounds of PdxGay were chosen as additional MSR model compounds, which allowed us to clearly deduce, for example, the water-activating role of the special Pd2Ga-β-Ga2O3 intermetallic[BOND]oxide interaction. The inverse-model studies were complemented by their related “real-model” experiments. Structure–activity and structure–selectivity correlations were performed on epitaxially ordered PdZn, Pd5Ga2, PdIn, Pd3Snv, and Pd2Ge nanoparticles that were embedded in thin crystalline films of their respective oxides. The reductively activated “thin-film model catalysts” that were prepared by sequential Pd and oxide deposition onto NaCl(001) exhibited the required large bimetal[BOND]oxide interface and the highly epitaxial ordering that was required for (HR)TEM studies and for identification of the structural and catalytic (bi)metal[BOND]support interactions. To fully understand the bimetal[BOND]support interactions in the supported systems, our studies were extended to the MeOH- and formaldehyde-reforming properties of the clean supporting oxides. From a direct comparison of the “isolated” MSR performance of the purely bimetallic surfaces to that of the “isolated” oxide surfaces and of the “bimetal[BOND]oxide contact” systems, a pronounced “bimetal[BOND]oxide synergy” toward optimum CO2 activity/selectivity was most evident. Moreover, the system-specific mechanisms that led to undesired CO formation and to spoiling of the CO2 selectivity could be extracted
Graphical abstractPdIn intermetallic phases can be switched in methanol steam reforming between a CO2-selective multilayer and an In-diluted phase by annealing at 453 K or 623 K.Highlights► A multilayer Pd1In1 phase in MSR is highly CO2-selective between 493 and 623 K. ► An In-diluted PdIn intermetallic phase yields CO formation via full methanol dehydrogenation. ► Higher reaction temperatures are needed for comparable CO2-TOF values as on supported PdIn/In2O3. ► A bimetal-oxide synergism for efficient CO2 formation is operative.
Ab imetallic Cu/Cu 51 Zr 14 precatalyst,a ctivated in situ, for hydrogen generation from methanol and water providesv ery high CO 2 selectivity (> 99.9 %) and high H 2 yields. Referenced to the geometrics urface area of our model surface, highera ctivity of at least one order of magnitude was observed in comparison to supportedC u/ZrO 2 and Cu/ZnO/ZrO 2 catalysts. Evolution of structurala ctivation monitored by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron microscopy indicatest ransformation of the bimetallic Cu/Cu 51 Zr 14 precatalyst into an active, selective, and self-stabilizing state with coexistence of dispersed Cu and partially hydroxylated tetragonal ZrO 2 .The outstanding performance is assigned to the presence of ah igh interface-site concentration following in situ decompositiono ft he intermetallic compound. These actives ites result from the cooperation of Cu, responsible for methanol activation,a nd tetragonal ZrO 2 ,w hich activates the water by surface hydroxylation.Copper-based catalysts are widely used for technical applications in methanolc hemistry,w ell-known examples include methanol synthesis from syngasw ith an optimized CO/CO 2 ratio, hydrogenation/photoreduction of CO 2 to produce "renewable" methanol, and methanol steam reforming (MSR) as the reversal of the synthesis reactionf rom CO 2.[1] The ability to control product selectivity is ak ey criterion for technical usage. Therefore, to realize the efficient on-boardp roduction of clean hydrogen in, for example, automotive applications, the key targets for MSR are high CO 2 selectivity,l ow CO content,a nd maximum H 2 yield in the reformate.[2] With respect to the catalytic functiono fZ rO 2 in MSR, the simple addition of ZrO 2 to the conventionally used Cu/ZnO catalysts overcomes the inherent drawback of purely ZnO-based catalysts,t hat is, the poor sinterings tability.[2] Beneficial synergistic effectsf or methanol synthesis have also been described for Cu/Zn and the ternary Cu/Zn/Al system prepared by ac o-precipitation technique.[3]Synergistic Cu-ZrO 2 interactions have also been reported for Cu/ZrO 2 catalysts without ZnO, involving CuÀOÀZr bonds at the phase boundary. The Cu-ZrO 2 interactions are believed to play ac rucial role in steering the methanol reforming reaction to maximum CO 2 selectivity.[4] Specifically,ananocrystalline Cu/ tetragonal ZrO 2 catalyst synthesized by ap olymer templating technique [5] was reported to be more active, selective, and stable in MSR than the technical Cu/ZnO/Al 2 O 3 methanol synthesis catalyst.[6] Although the beneficial effects of the redox chemistry of Cu and the Cu 0 /Cu oxidized ratio at the interface has been suggesteda sa ni mportants electivity descriptor,a longside disorder and strain phenomena within the metallicC u phase, [2] ac ontradictingi nfluence hasa lso been reported. Both beneficial [4a, b] and adverse [4d] effectso ft he reducibility of Cu can be found in the literature. Nevertheless, any influence appears strongly connected to the quantity...
Atomic layer deposition (ALD) of alumina using trimethylaluminum (TMA) has technological importance in microelectronics. This process has demonstrated a high potential in applications of protective coatings on Cu surfaces for control of diffusion of Cu in Cu2S films in photovoltaic devices and sintering of Cu-based nanoparticles in liquid phase hydrogenation reactions. With this motivation in mind, the reaction between TMA and oxygen was investigated on Cu(111) and Cu2O/Cu(111) surfaces. TMA did not adsorb on the Cu(111) surface, a result consistent with density functional theory (DFT) calculations predicting that TMA adsorption and decomposition are thermodynamically unfavorable on pure Cu(111). On the other hand, TMA readily adsorbed on the Cu2O/Cu(111) surface at 473 K resulting in the reduction of some surface Cu1+ to metallic copper (Cu0) and the formation of a copper aluminate, most likely CuAlO2. The reaction is limited by the amount of surface oxygen. After the first TMA half-cycle on Cu2O/Cu(111), two-dimensional (2D) islands of the aluminate were observed on the surface by scanning tunneling microscopy (STM). According to DFT calculations, TMA decomposed completely on Cu2O/Cu(111). High-resolution electron energy loss spectroscopy (HREELS) was used to distinguish between tetrahedrally (Altet) and octahedrally (Aloct) coordinated Al3+ in surface adlayers. TMA dosing produced an aluminum oxide film, which contained more octahedrally coordinated Al3+ (Altet/Aloct HREELS peak area ratio ≈ 0.3) than did dosing O2 (Altet/Aloct HREELS peak area ratio ≈ 0.5). After the first ALD cycle, TMA reacted with both Cu2O and aluminum oxide surfaces in the absence of hydroxyl groups until film closure by the fourth ALD cycle. Then, TMA continued to react with surface Al–O, forming stoichiometric Al2O3. O2 half-cycles at 623 K were more effective for carbon removal than O2 half-cycles at 473 K or water half-cycles at 623 K. The growth rate was approximately 3–4 Å/cycle for TMA+O2 ALD (O2 half-cycles at 623 K). No preferential growth of Al2O3 on the steps of Cu(111) was observed. According to STM, Al2O3 grows homogeneously on Cu(111) terraces.
Abstract:The activation and catalytic performance of two representative Zr-containing intermetallic systems, namely Cu-Zr and Pd-Zr, have been comparatively studied operando using methanol steam reforming (MSR) as test reaction. Using an inverse surface science and bulk model catalyst approach, we monitored the transition of the initial metal/intermetallic compound structures into the eventual active and CO 2 -selective states upon contact to the methanol steam reforming mixture. For Cu-Zr, selected nominal stoichiometries ranging from Cu:Zr = 9:2 over 2:1 to 1:2 have been prepared by mixing the respective amounts of metallic Cu and Zr to yield different Cu-Zr bulk phases as initial catalyst structures. In addition, the methanol steam reforming performance of two Pd-Zr systems, that is, a bulk system with a nominal Pd:Zr = 2:1 stoichiometry and an inverse model system consisting of CVD-grown ZrO x H y layers on a polycrystalline Pd foil, has been comparatively assessed. While the CO 2 -selectivity and the overall catalytic performance of the Cu-Zr system is promising due to operando formation of a catalytically beneficial Cu-ZrO 2 interface, the case for Pd-Zr is different. For both Pd-Zr systems, the low-temperature coking tendency, the high water-activation temperature and the CO 2 -selectivity spoiling inverse WGS reaction limit the use of the Pd-Zr systems for selective MSR applications, although alloying of Pd with Zr opens water activation channels to increase the CO 2 selectivity.
We explored the surface chemistry of methane on Cu-promoted Ni–ZrO2 catalysts and observed a limited stability of the CuNi alloy under relevant reaction conditions.
The microstructure of the CO 2 -selective self-activating and self-stabilizing Cu-Zr bimetallic compound Cu 51 Zr 14 has been studied by a combination of high-resolution electron microscopy and energy-dispersive X-ray spectroscopy both before and after entering the CO 2 selective state in methanol steam reforming. Prior to catalysis, the phase composition of the catalyst is characterized by a microstructural mixture of Cu 51 Zr 14 and metallic Cu. The structure appears in a distinct needle-like morphology with a characteristic microstucture of small Cu particles embedded in the intermetallic matrix. In contrast, entering the CO 2 -selective state goes along with oxidative decompositioninvestigated by differential thermal analysis (DTA), thermogravimetry (TG) and mass spectrometry (MS) -and therefore massive structural and compositional changes of the Cu 51 Zr 14 compound both in the near-surface and bulk regions.The final state is then composed of a structurally very heterogeneous sample with Zr-rich and Cu-rich regions within the material bulk with a characteristic lamellar structure. Most importantly, the catalytically relevant surface regions are drastically corroded and depleted in Zr and are characterized by a majority of Cu in intimate contact with oxidized ZrO 2 exhibiting a well-ordered, predominantly tetragonal structure. This newly created Cu-ZrO 2 interface is believed to be the most significant descriptor steering the CO 2 selectivity. In due course, the new way of self-adjustment of the microstructure starting from well-defined intermetallic compounds in the catalytic reaction mixture might pave the way for a more systematic approach of controlled oxidative decomposition of intermetallic compounds acting as promising catalyst precursors.
The reaction between adsorbed trimethylaluminum (TMA) and water was studied on Pt(111) and Pd(111) surfaces. Upon exposure to TMA at approximately 10 −5 mbar, C-and Al-containing species appeared on both surfaces, as observed by X-ray photoelectron spectroscopy (XPS). On both surfaces, the adsorbed Al oxidation state observed by XPS was closest to metallic. Density functional theory (DFT) calculations suggest that decomposition to methyl aluminum (Al-CH 3 ; "MMA") or atomic Al is thermodynamically favorable. The formation of a Pd−Al alloy was observed on Pd (111), but Pt−Al alloy formation was not observed on Pt(111). Following TMA adsorption, each surface was exposed to water vapor at 400°C either at a pressure of 7 × 10 −6 mbar (UHV-XPS) or at 0.1 mbar (in situ XPS). The substrate and water dosing conditions determined the ability of each surface to remove residual carbon: on Pt(111), carbon from the TMA precursor was removed from Pt(111) during 0.1 mbar water exposure at 400°C, whereas carbon was not removed after the 7 × 10 −6 mbar water exposure. On Pd(111), however, carbon-containing fragments of TMA were removed at both water pressures. XPS also revealed another effect of water dosing conditions: the as-deposited Al was only fully oxidized to Al 2 O 3 during water exposure at 0.1 mbar, whereas mixed hydroxidecontaining and metallic Al species persisted after exposure to water at 7 × 10 −6 mbar on both surfaces.
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