| 1. |
- Bakhtiary-Davijany, Hamidreza, et al.
(author)
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Analysis of External and Internal Mass Transfer at Low Reynolds Numbers in a Multiple-Slit Packed Bed Microstructured Reactor for Synthesis of Methanol from Syngas
- 2012
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In: Industrial & Engineering Chemistry Research. - : American Chemical Society (ACS). - 0888-5885 .- 1520-5045. ; 51:42, s. 13574-13579
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Journal article (peer-reviewed)abstract
- The possibility of mass transfer limitations in an integrated micro packed bed reactor-heat exchanger (IMPBRHE) for methanol synthesis was experimentally investigated. Experiments were performed with three different particle size distributions (50-200 mu m) of a Cu-based catalyst at 80 bar and 215-270 degrees C. Negligible effects of pore diffusion limitations on the performance of the reactor under methanol synthesis conditions for catalyst particle diameters up to 125 mu m were found. Due to a very low Reynolds numbers (similar to 1) and dominance of molecular diffusion, variation of the total pressure was applied as a suitable technique to alter the diffusivities of reactants in the gas mixture by dilution, while keeping the reactant flow and partial pressure constant. No significant change in the CO conversion was observed in the temperature range 235-255 degrees C, pressure range 50-90 bar, and for reactant contact times of 105-308 ms.g/mL. The same procedure was applied to a laboratory. fixed bed reactor with similar results. Possible heat transfer effects associated with the dilution were shown to be negligible. We therefore conclude that both reactor systems operate in the absence of external mass transfer limitations.
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| 2. |
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| 3. |
- Strømsheim, Marie D., et al.
(author)
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Near Ambient Pressure XPS Investigation of CO Oxidation Over Pd3Au(100)
- 2017
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In: Topics in Catalysis. - : Springer Science and Business Media LLC. - 1022-5528 .- 1572-9028. ; 60:17-18, s. 1439-1448
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Journal article (peer-reviewed)abstract
- The CO oxidation behavior under excess oxygen and near stoichiometric conditions over the surface of Pd3Au(100) has been studied by combining near-ambient pressure X-ray photoelectron spectroscopy and quadrupole mass spectrometry and compared to Pd(100). During heating and cooling cycles, normal hysteresis in the CO2 production, i.e. with the light-off temperature being higher than the extinction temperature, is observed for both surfaces. On both Pd3Au(100) and Pd(100) the (√5 × √5)R27° surface oxide structure is present during CO2 production under excess oxygen conditions (O2:CO = 10:1), while at near stoichiometric conditions (O2:CO = 1:1) the surfaces are covered with atomic oxygen. Au as alloying element hence induces only minor differences in the observed hysteresis and the active phase compared to pure Pd. Alloying with Au thus yields a different behavior compared to Ag, where reversed hysteresis is observed for CO2 production over Pd75Ag25(100) at similar conditions [Fernandes et al., ACS Catal. (2016) 4154].
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| 4. |
- Strømsheim, Marie D., et al.
(author)
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Segregation dynamics of a Pd-Ag surface during CO oxidation investigated by NAP-XPS
- 2022
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In: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 384-386, s. 265-273
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Journal article (peer-reviewed)abstract
- The dynamic changes in composition in the near-surface region of a Pd75%Ag25%(100) single crystal were monitored using near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) during CO oxidation under oxygen rich conditions at a total pressure of 1.1 mbar. Six CO oxidation temperature cycles were investigated at different heating rates and maximum temperatures of 450 °C or 600 °C. It was found that the history of the bimetallic sample plays an important role, as the CO2 formation profile varies depending on initial conditions, and previous heating rates and maximum temperatures. In terms of CO coverage effects, normal, reversed and no hysteresis behaviour were all observed. In agreement with previous modelling predictions, the NAP-XPS data confirm a dynamic segregation behaviour upon heating/cooling where the amount of Pd in the surface region decreases with increasing temperature. Nevertheless, the Pd 3d5/2 core level relative area assessment is not fully capable of capturing all the surface dynamics inferred from the temperature dependent CO2 formation profiles, due to the probing depth. While residing at ambient temperature in the reaction mixture, however, there is a build-up of adsorbed CO at the surface showing that CO induces segregation of Pd to the topmost surface layer under these conditions. In total, this suggests that the segregation is kinetically relatively facile during temperature cycling, and that adsorbate coverage is the main controlling factor for the surface termination.
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| 5. |
- Svenum, Ingeborg Helene, et al.
(author)
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Activity and segregation behavior of Pd75%Ag25%(1 1 1) during CO oxidation – An in situ NAP-XPS investigation
- 2023
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In: Journal of Catalysis. - : Elsevier BV. - 0021-9517. ; 417, s. 194-201
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Journal article (peer-reviewed)abstract
- The surface reactivity during CO oxidation over Pd(1 1 1) and Pd75%Ag25%(1 1 1) single crystals has been investigated using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) under excess oxygen conditions (p ≈ 2 mbar, O2:CO=10). The (√6×√6) Pd5O4 surface oxide is present on Pd(1 1 1) when the surface is highly active under the applied conditions. The CO2 formation profile follows a hysteresis upon temperature cycling between 150 and 450 °C that depends on the temperature ramp rate. The behavior is different for Pd75%Ag25%(1 1 1), with the CO2 formation rate considerably lower, no surface oxide and only chemisorbed oxygen present at the surface at high temperature, reversible scrambling of the (near) surface Pd/(Pd+Ag) ratio, and no hysteresis behavior observed. The results suggest that the reactant activation is affected by both surface composition and surface termination, i.e. mechanistically different on Pd75%Ag25%(1 1 1) than Pd(1 1 1), and less efficient on (1 1 1) as compared to the (1 0 0) alloy counterpart.
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