Diffuse Reflectance Infrared Spectroscopic Studies of Adsorbed Carbon Monoxide on Supported Rhodium Catalysts PDF Download
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Author: Kenneth Warren Van Every
Publisher:
ISBN:
Category : Carbon monoxide
Languages : en
Pages : 458
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Author: Kenneth Warren Van Every
Publisher:
ISBN:
Category : Carbon monoxide
Languages : en
Pages : 458
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Author: James R. DiAndreth
Publisher:
ISBN:
Category : Catalysts
Languages : en
Pages : 230
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Author: C. H. Dai
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ISBN:
Category :
Languages : en
Pages : 19
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The reaction of carbon monoxide and oxygen over supported rhodium films has been studied using infrared spectroscopy. The focus of work was the reactivity of the various CO/Rh/X (X=Al2O3, SiO2, TiO2) surface states for supported catalysts having high and low Rh loading. Under the reaction conditions the 'linear CO' species was the most stable toward oxidation, but this could have been a result of an oxidized Rh surface. A new CO/Rh surface species has been proposed which exhibits an infrared band at 2000/cm for a 0.5% Rh/TiO2 film. This species is believed to be a bridged carbonyl between Rh+1 and the TiO2 support. Originator-supplied keywords include: Infrared spectroscopy, and Carbon dioxide.
Author: Lawrence Nwankwo Orji
Publisher:
ISBN:
Category : Chemisorption
Languages : en
Pages : 616
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Author: Richard Paul Underwood
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ISBN:
Category :
Languages : en
Pages : 320
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Author: Samuel M. Fehr
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Category :
Languages : de
Pages : 0
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Abstract: Using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy to study surface species of reaction intermediates on heterogeneous catalysts requires particular precautions, when assigning IR bands to intermediates apparently resulting from the reaction of H2, CO, and CO2. In accordance with earlier work, it is shown that in the investigation of the heterogeneous methanol synthesis, several gas phase bands of CO2 were misassigned in previous studies as adsorbates on the catalyst surface. Thus, several combination bands and overtones of CO2 in the 2200–750 cm−1 range – notably those at 2130, 2112, 2107, 2094, 2077, ∼2056, 1933, ∼1920, ∼1079, ∼1050, ∼973, and ∼948 cm−1 – were already misinterpreted as adsorbates/intermediates. Some of these bands exhibit similar (low) intensities as surface species and are in the range of typical adsorbed CO or methoxy/methanol vibrations. Higher pressures and temperatures, which are necessary to study industrial catalysts by in situ IR spectroscopy, even amplify this effect. In addition, due to a Fermi resonance at a CO2 partial pressure above ∼10 bar, two further bands appear at 1388 and 1285 cm−1. This is also within the range typically associated with surface adsorbates. In order to avoid misassignments of IR bands for in situ or operando DRIFT spectroscopy, those occurring at CO2 pressures up to 30 bar in the widely used Praying Mantis TM High Temperature Reaction Chamber are presented here and assigned to their origin as combination bands and overtones of gaseous CO2
Author:
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Category : Metallurgy
Languages : en
Pages : 1076
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Author:
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Category :
Languages : en
Pages : 29
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Time-resolved FT-IR spectra of carbon monoxide hydrogenation over alumina-supported ruthenium particles were recorded on themillisecond time scale at 700 K using pulsed release of CO and a continuous flow of H2/N2 (ratio 0.067 or 0.15, 1 atm total pressure). Adsorbed carbon monoxide was detected along with gas phase products methane (3016 and 1306 cm-1), water (1900 +- 1300 cm-1), and carbon dioxide (2348 cm-1). Aside from adsorbed CO, no other surface species were observed. The rate of formation of methane is 2.5 +- 0.4 s-1 and coincides with the rate of carbon dioxide growth (3.4 +- 0.6 s-1), thus indicating that CH4 and CO2 originate from a common intermediate. The broad band of adsorbed carbon monoxide has a maximum at 2010 cm-1 at early times (36 ms) that shifts gradually to 1960 cm-1 over a period of 3 s as a result of the decreasing surface concentration of CO. Kinetic analysis of the adsorbed carbon monoxide reveals that surface sites absorbing at the high frequency end of the infrared band are temporally linked to gas phase product growth. Specifically, a (linear) CO site at 2026 cm-1 decays with a rate constant of 2.9 +- 0.1 s-1, which coincides with the rise constant of CH4. This demonstrates that the linear CO site at 2026 cm-1 is the kinetically most relevant one for the rate-determining CO dissociation step under reaction conditions at 700 K.
Author: Shao-Chun Wang (Ph.D.)
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Category :
Languages : en
Pages : 0
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The molecular-level understanding of surface sites and reaction mechanisms is key for the development of the field of heterogeneous catalysis. Infrared spectroscopy, studying molecular vibrations, is widely used to investigate the structures of surface-bound species in heterogeneous catalysts. Heterogeneous catalytic reactions proceed via adsorption of the reagent(s), surface reaction, and desorption of the product(s), leading to complex in-situ or operando IR spectra. However, the information of active species is usually concealed by spectator species which do not participate in the catalytic cycle. Modulation excitation spectroscopy serves as a tool to increase the signal-to-noise level and to differentiate between the active species and spectator species during a catalytic reaction. In Chapter 2, we report on the use of diffuse reflectance FT-IR spectroscopy (DRIFTS) with a modulation excitation (ME) approach followed by mass spectrometry (MS) to investigate the reaction of ethanol to n-butanol over hydroxyapatite (HAP). The approach allows for a vibrational characterization of the active surface species and the formulation of a consistent mechanism. Based on our experimental observations, Ca2+/OH- can be put forward as the main active site for the aldol condensation. POH/OH- acid-base pair is proposed as the active site for the Meerwein-Ponndorf-Verley (MPV) direct hydrogen transfer for the n-butanol formation.In Chapter 3, we describe the use of NO as a probe molecule in low-temperature IR spectroscopy to identify and quantify copper species in the state-of-the-art commercial NOx abatement catalyst, Cu ion-exchanged chabazite zeolite. While bulk analysis can reveal the total concentration of copper in the catalyst, the amount of ion-exchanged copper is more difficult to determine due to the appearance of non-exchanged Cu species, CuOx. Molecules such as carbon monoxide (CO) and nitric oxide (NO) are routinely used as a probe to investigate the copper speciation in order to draw structure-activity correlations. However, NO is easy to decompose and reacts with copper species at ambient temperature, causing complexity in IR spectra. Here, we develop NO adsorption IR spectroscopy at cryogenic conditions to avoid the undesired reactions. The observed IR peaks for Cu+(NO)2 and Cu2+(NO) species can be used to quantify the amount of exchanged copper species in a broad range of samples, including a commercial wash-coated honeycomb. Calibration curves for Cu+(NO)2 and Cu2+(NO) are determined for copper loadings up to 3.99 wt% with silica to alumina ratio of 16-22 and quantitative agreement with complementary hydrogen temperature-programmed reduction (H2-TPR) results is established. Our methodology allows us to identify different Cu species in Cu-CHA, such as Z2Cu(II), Z1Cu(II)OH and Cu dimers, based on their distinct IR signatures. In addition, the perturbed T-O-T framework vibration - characterized at 400 oC - can also be used as a complementary method to quantify Z2Cu(II) species. This work demonstrates that cryogenic NO-IR is a facile technique to identify and quantify the exchanged copper species in Cu-CHA to accelerate catalyst development. In chapter 4, we extend the NO-IR method to characterize the ion-exchanged species in Cu-ZSM-5 and Fe-CHA. The adsorbed NO shows distinct IR characteristics for exchanged copper species at 1914 cm-1 and the calibration curve for Cu2+ species for estimating high Cu-loading Cu-ZSM-5 is determined. In the case of Fe-CHA, the NO-IR is not efficient due to the formation of NO monomer and trimer on copper species, causing complexity in the IR spectra. Carbon monoxide (CO) is then applied as an alternative probe molecule. The results of CO-IR for Fe-CHA showed more defined IR features for Fe2+(CO) than in NO-IR. With this toolbox in hand, the calibration curve for the concentration of Fe2+ in Fe-CHA by CO-IR is then established. Perspectives for future research are outlined in Chapter 5, the preliminary results for a Cu-Ga binary catalyst for methanol production from CO2 hydrogenation was tested by using the DRIFTS cell with ambient pressure. However, due to thermodynamic limitation, the reaction favors reverse water gas shift under ambient pressure. The newly designed operando DRIFTS cell allows to minimize the exchange time of concentration modulation and have the capability of holding at high pressure (> 25 bar) and high temperature (> 250 oC) which is suitable for investigating the heterogeneous catalysts for CO2 hydrogenation. With the well-establish tool, we will be able to investigate surface-bound species and reaction mechanisms under working conditions by IR spectroscopy. Not only can gas phase catalytic reactions be studied by DRIFTS, but heterogeneous catalytic reactions in liquid phase can also be investigated by attenuation total reflection IR spectroscopy (ATR-IR). Competitive adsorption with reactants and solvents is especially important in the case of (micro-) porous catalysts where the composition inside the pores can be very different from the bulk due to size exclusion and confinement effects. ATR-IR with the ME approach can shed light on mechanistic insights for liquid phase reactions. Lastly, with the success of low temperature NO and CO-IR method development, characterizing different metal ion-exchanged zeolites for various applications such as methane to methanol, syngas to dimethyl ether, and NOx abatement is crucial for structure-reactivity correlation. Ultimately, low temperature NO and CO-IR can be established as facile techniques to identify and quantify metal ion species located in different types of zeolites.
Author: Wayne Allen Bollinger
Publisher:
ISBN:
Category :
Languages : en
Pages : 188
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