Alcohol selective oxidation over modified foam-silver catalysts

Applied Catalysis A: General, 2004

Scanning electron microscopy (SEM), specific surface area measurements, in situ Raman spectroscopy and steady-state microreactor experiments were used to explore the relationship between silver catalyst morphology and performance in the titled reaction. Two electrolytic silver catalysts, obtained from different commercial suppliers, were examined in relation to their physico-chemical and catalytic properties. The work focused on the activation behaviour of the catalysts under conditions of industrial CH 2 O synthesis (ca. 923-953 K, 1 atm). The results show that the silver catalysts, denoted A and B, differed enormously in their surface morphology, which in turn impacted on their oxygen chemisorption behaviour and initial catalytic properties. Silver catalyst A possessed a high grain boundary density in its surface, and exhibited enhanced initial activity and selectivity to CH 2 O during CH 3 OH oxidation at 598-973 K compared to catalyst B, which possessed significantly lower surface area and roughness. Performance differences correlated strongly with the speciation of oxygen on the catalysts during CH 3 OH oxidation, which is discussed in terms of a framework comprising three chemically distinct atomic oxygen species: two surface species (denoted O ␣ and O ␥) and a bulk-dissolved species (denoted O ␤). The population of each state was found to be both temperature and structure-sensitive. The weakly bound O ␣ species was dominant at low temperatures and opened reaction pathways towards CH 2 O, HCOOCH 3 , CO 2 and HCOOH. At temperatures of industrial CH 2 O manufacture (ca. 923 K), the strongly bound O ␥ species was the dominant oxygen state and selectively activated the catalysts for the oxidative-dehydrogenation of CH 3 OH to CH 2 O + H 2 O. The dissociative chemisorption of O 2 and the surface segregation of O ␤ served to replenish the O ␣ and O ␥ consumed by reaction. The CH 2 O yield on both catalysts increased with temperature up to 923 K, reflecting accompanying increases in the surface O ␥ /O ␣ ratio. Grain boundaries facilitated the formation of O ␥ and O ␤ , which explained why catalyst A exhibited the higher initial activity and selectivity to CH 2 O. The performance of both catalysts, in particular catalyst B, improved with time-on-stream at 923 K before stabilising after approximately 48 h of operation. These improvements were due to reaction-induced changes in catalyst morphology, which created structures that promoted O ␤ and O ␥ formation. Results were used to develop reaction schemes for CH 3 OH oxidation on fresh and aged silver catalysts under industrial conditions. The critical role of defects in the silver catalyst structure, such as grain boundaries in the efficient 'start up' operation of electrolytic silver catalysts for CH 2 O production is demonstrated.

Applied Catalysis A: …, 2004

Introduction 1.1 Historical perspective 1.2 Physical properties of formaldehyde 1.3 Uses of formaldehyde 1.4 Industrial formaldehyde manufacture 1.5 Silver catalyst manufacture 1.6 Silver catalyst deactivation and lifetime 1.7 Project objectives and strategy 1.8 References 1

Topics in Catalysis, 2019

Electrolytic silver particles were studied in relation to its morphology changes under different reactive and non-reactive atmospheres, and its catalytic activity in oxidation of methanol to formaldehyde (MTF), carbon monoxide to carbon dioxide, and hydrogen to water. Scanning electron microscopy and X-ray diffraction (XRD) were applied to analyze structural changes in the silver catalyst after exposure or interaction with nitrogen, oxygen, methanol/water, carbon monoxide and hydrogen, applied either individually or in selected combinations, at temperatures approaching 700 °C. The as-received Ag catalyst consists of agglomerated, faceted, polycrystalline particles. These undergo massive morphological changes during MTF reaction conditions. It was found that Ag catalysts exposed to oxygen-free atmospheres (N 2 , H 2 /N 2 and CH 3 OH/H 2 O/N 2) at 650 °C exhibit minimal changes in surface morphology compared to the fresh catalyst, while severe restructuring occurs on the mesoscopic scale under oxygen containing atmospheres (O 2 /N 2 , H 2 /O 2 /N 2 and CO/O 2 /N 2) at elevated temperature. This restructuring renders a smoothened surface with refacetted areas and many pinholes, while a small primary crystallite size (~ 40 nm, XRD) is maintained. Such pinholes are commonly described as a result of sub-surface oxygen/hydrogen/ hydroxyl interactions. Here, they are present in all samples exposed to oxygen, indicating that presence of hydrogen is not prerequisite. For the CO and H 2 oxidation subsystems , the initial activity was comparable. But, while the conversion of H 2 is preserved during 70 h time on stream, the CO conversion gradually reduces from 70 to 10%. This suggests that the restructuring associated with dissolution of O at high temperature inhibits the CO to CO 2 pathway.

Journal of Molecular Catalysis A: Chemical, 2007

Reticulated ceramic foams, prepared from steel making wastes using an earlier described multilayer replication method, have been used as support for preparing Au/Al 2 O 3 and Au/CeO 2 catalytic devices, by depositing a coating of Al 2 O 3 or CeO 2 and loading with gold by ion exchange. The obtained catalytic systems are active to the 2-propanol oxidation reaction, presenting the Au/CeO 2-coated monolith the best catalytic results because of the combination of the oxidation capability of gold atoms with the redox properties of the ceria phase.

2022

Through the impregnation method, Ag catalysts with different support (such as TiO 2 and γ-Al 2 O 3) were prepared and then tested for catalytic oxidation of formaldehyde (HCHO) at low temperatures. The Ag/TiO 2 catalyst exhibited strong catalytic performance, converting HCHO to CO 2 and H 2 O at around 95˚C. However, the Ag/Al 2 O 3 catalysts showed much lower activity and reached 100% conversion at 125˚C. The Ag-based catalysts were next characterized by several methods (XRD, TEM, FT-R, BET and H 2-TPR). Results of characterization revealed that support dramatically impacts the size and dispersion of Ag particles. The XRD analysis showed the existence of different peaks of the silver on the surface of Al 2 O 3 in the contrast with TiO 2 no specific peaks exist. Therefore, the size of the Ag particles and their dispersion are the most important factors that affect their catalytic performance for formaldehyde oxidation. In terms of catalytic performance for HCHO oxidation, the Ag/TiO 2 catalyst possesses the best Ag dispersion, as well as the smallest Ag particle size.

The properties of silver in the oxidative dehydrogenation of methanol were studied in a flow reactor under near industrial conditions. The influences of temperature, concentration of both reactants, gas velocity, space velocity, the form of the silver catalyst and surface composition of the catalyst were studied. A model for the reaction is proposed which is based on the experimental observations and on the nature'of suggested that different oxygen in the reactions to CO, Cop and conversion to CO.

Applied Catalysis A-general, 1994

The results of investigations of the electronic and catalytic properties of silver catalysts by IR spectroscopy of adsorbed carbon monoxide, diffuse reflectance electron spectroscopy, electron microscopy and catalytic testing have been combined and examined. Of those metals of which the configuration of the valence electronic shell is dns 1 (Cu, Ag, Au, Pt, Pd, Rh, Ru) , silver has the highest effective charge as M+ cation due to the most resistant d-electron shell. Supports and modifying additions of metal oxides have a pronounced effect on the electronic state and red-ox properties of silver as a result of donor-accepter interaction of the metal oxide. The factors that stabilize the ionic state of silver and increase its effective charge seem to improve the catalytic properties of silver in methanol oxidation. A hypothesis on the active electronic state of silver in this process-Ag + cations with maximum effective charge-is suggested based on comparison of spectroscopic and catalytic data.

Catalysis Communications, 2007

Catalysts of new structural type-foam metals-have been tested in the process of complete oxidation of aromatic compounds and neutralization of gas emissions of furnaces for wire enameling. Foam catalysts have better hydrodynamic and mechanical characteristics as compared with commercial catalysts, high catalytic activity and life time in the processes of neutralization of waste gases exceeding the indices of the traditional granular and honeycomb catalysts.

Nanometric particles (10−30 nm) of thermally activated Congo-red (CR) entrapped within silver deposited on TiO 2 nanofibers form a new triple hybridorganic, metallic, and ceramiccatalyst (CR@Ag/TiO 2-nf). It is shown that the downscaling and the use of nanofibers as supports cause CR@Ag/TiO 2-nf to considerably outperform the unsupported powder CR@Ag as well as the supported undoped Ag/TiO 2-nf catalysts in all of the studied parameters of the industrially important oxidation of methanol to formaldehyde. These parameters include the weight hourly space velocity (WHSV), the space time yield (STY), the reaction conversion, and the reaction selectivity values. Typically, the WHSV shows a huge jumpapproximately 18 times highercompared to unsupported powder; the STY shows a 10-fold increase out of which 25% is achieved due to the CR doping; and the fully activated catalyst (second run) provided a conversion of 98% and selectivity of 85%. The high efficiency of this novel type of triple hybrid catalyst is explained on the basis of structural−morphological characteristics such as higher porosity, larger surface area, and relatively low sinterability of the separated doped silver particles which are provided by the TiO 2 nanofibers support.

Chemistry - A European Journal, 2012