Galvanic replacement, co-impregnation and sequential impregnation have been employed to prepare Pd-Cu bimetallic catalysts with less than 1 wt-% Cu and ca. 0.03 wt-% Pd for selective hydrogenation of ...acetylene in excess ethylene. High angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and H
2
chemisorption results confirmed that Pd-Cu singleatom alloy structures were constructed in all three bimetallic catalysts. Catalytic tests indicated that when the conversion of acetylene was above 99%, the selectivity of ethylene of these three single atom alloy catalysts was still more than 73%. Furthermore, the single atom alloy catalyst prepared by sequential incipient wetness impregnation was found to have the best stability among the three procedures used.
Ceria (CeO2) and zirconia (ZrO2) supported Pt and Co3O4-based nanocatalysts were synthesized and characterized by different instrumental techniques. The catalysts redox properties and active surface ...areas were evaluated using temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO) and H2-pulse chemisorption, respectively. The catalysts were tested for the thermal oxidation of carbon monoxide (CO), reduction of nitrous oxide (N2O) and conversion of N2O/CO mixture (1:1 vol%) . In catalytic tests, Pt–Co3O4/CeO2 (10:10%) oxidized CO up to 100% at 25 °C and Co3O4/CeO2 (20%) reduced N2O up to 90% at 320 °C. Moreover, Pt–Co3O4/CeO2 (10:10%) converted N2O/CO mixture to N2/CO2 up to 90% at about 210 °C. The low-temperature catalytic activity of Pt–Co3O4/CeO2 (10:10%) for CO oxidation and N2O/CO mixture redox conversion were attributed to uniform particle size, metals and support proper combination and electron interaction.
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•Ceria and zirconia supported Pt/Co3O4-based catalysts were synthesized and characterized by different instrumental techniques.•TPR, TPO and H2-pulse chemisorption techniques were used for the catalyst's redox properties and active surface areas.•Pt–Co3O4/CeO2 (10:10%) oxidized CO up to 100% at 25 °C and Co3O4/CeO2 (20%) reduced N2O up to 90% at 320 °C.•Pt–Co3O4/CeO2 (10:10%) converted N2O/CO mixture to N2/CO2 up to 90% at about 210 °C.
The physicochemical characteristics of ZnO-ZnCr2O4 (Zn-Cr-O) mixed oxides were determined by adsorption and spectroscopic methods. The catalytic activities of Zn-Cr-O was investigated for ...dehydrocyclization of ethylenediamine and aqueous glycerol to synthesize 2-methylpyrazine (2MP). Reductive pre-treatment was required for better 2MP yields and rate of 2MP was dependent upon strong basicity of Zn-Cr-O. Product distribution revealed that cyclocondensation of glycerol and ethylenediamine followed by homo-coupling of 2-pyrazinylmethanol occurred on moderate acid and strong base sites. On the contrary weak to moderate acidic and base sites were inefficient for homo-coupling of 2-pyrazinylmethanol. Role of acid-base properties was examined by pyridine and 2,6-dimethylpyridine poisoning studies.
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▶ Glycerol etherification is characterized by slow equilibration of higher ethers. ▶ Product distribution does not follow simplified kinetic model. ▶ Ionic liquid with Brønsted acid ...sites and heteropolyacid suppress tri-ether formation. ▶ Partial neutralization of Amberlyst 15 suppresses isobutene oligomerization.
Etherification of glycerol by isobutylene was conducted in a batch mode using acidic and partially neutralized Amberlyst-15 ionic resin,
p-toluenesulfonic acid, silicotungstic acid, cesium salt of silicotungstic acid, and ionic liquid containing sulfonic acid groups. All the catalysts are comparable in terms of the initial rate of glycerol conversion into mono-ether (except cesium salt of heteropolyacid), but differ substantially with respect to the yields of higher ethers of glycerol.
Ionic liquid and heteropolyacid are immiscible/insoluble in higher ethers of glycerol. As a result they have unique capability of suppressing the formation of tri-ether during the initial stage of glycerol conversion.
Acidic Amberlyst-15 in the form of fine powder has the highest activity per unit weight for glycerol etherification and relatively high activity in isobutene oligomerization. Partial exchange of acidic protons with cations Na
+, Ag
+, Mg
2+, and Al
3+ decreases the rates of all the processes, but isobutylene oligomerization is suppressed more efficiently. Ag
+- and Al
3+-modified Amberlyst demonstrates higher yields of tri-ether and has specific pattern of interaction with gaseous isobutene distinctive to other metal-substituted Amberlysts.
Quantitative measurement of the number of active surface sites on two-dimensional (2D) catalysts is one of the most crucial points in heterogeneous catalysis because it is used to determine the ...turnover frequency (TOF), which refers to the catalytic activity of model catalysts. However, because of the difficulty in identifying the effective active surface area on 2D heterogeneous catalysts, there is still the assumption that each metal atom is an active site. To shed light on these issues and to bridge the activity gaps between 2D and three-dimensional (3D) heterogeneous catalysts, we present an experimental approach that uses Pt nanoparticle (NP) arrays on a thin silicon wafer probed with CO pulse chemisorption, a widely used surface-sensitive technique, to determine the number of active sites and the area of the effective active surface. A Pt thin film and Pt NP arrays with two different NP sizes (i.e., 2.1 and 4.5 nm) were prepared as model systems for 2D catalysts. The effective active metal surface area determined using CO pulse chemisorption for these 2D catalysts is 53–79% of the apparent metal surface area that was obtained by measuring the surface area based on scanning electron microscopy images. This discrepancy between the active and apparent surface area is attributed to the presence of hydrocarbon contamination and organic capping layers on the catalysts. The results indicate that estimating the active sites of 2D catalysts by apparent surface area is reasonably in agreement with the number measured by chemisorption that is used to characterize 3D nanocatalysts. This experimental technique on 2D catalysts can be expected to provide information for extracting the true TOF of product molecules on 2D catalysts in gas-phase catalytic reactions.
Graphical Abstract
N
-Aryl acetamides synthesis from the corresponding nitro compounds via a reductive
N
-acetylation was achieved over Pd supported on Sn modified Al
2
O
3
catalyst using H
2
, acetic anhydride as ...acetylating agent in aqueous media at ambient temperature. The pyridine-IR data demonstrated a majority of Lewis acid sites compared to Brønsted acid sites on the catalyst surface. Pyridine-IR and CO pulse chemisorption results emphasized a combination of surface palladium in conjunction with Lewis acid sites were responsible for the high activity of 2wt%Pd/5wt%Sn–Al
2
O
3
over 2wt%Pd/Al
2
O
3
catalyst with consistent activity and selectivity for four recycles. The bulk and surface properties of the catalysts were characterized by BET-SA, XRD, XPS, TPD of NH
3
, H
2
-TPR, CO pulse chemisorption, TEM and the promotional effect of surface Lewis acid sites are rationalized by in situ pyridine adsorbed DRIFT spectroscopy.
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► Decalin dehydrogenation over Pt supported catalyst. ► Superior activity of “Pt/C” over “Pt/Al2O3” catalyst due to hydrogen spillover. ► Phase transformation of alumina boehmite ...phase of alumina to γ or δ phase.
One weight percent Pt impregnated on activated carbon (Pt/AC) and alumina supports (Pt/Al2O3) have been tested for decalin dehydrogenation for the production of clean H2. CO pulse chemisorption results indicate nearly same Pt dispersion in both Pt/AC and Pt/Al2O3 catalysts. The comparison of activity between Pt/AC and Pt/Al2O3 catalysts revealed superior decalin dehydrogenation activity over Pt/AC both in terms of turn over frequency as well as the yield towards H2 production which is due to the spill over of H2 on Pt/AC. The Pt dispersion in Pt/Al2O3 is lost due to the phase transformation of support (boehmite phase of alumina to γ phase) during reduction treatment and this is the main reason for the loss of decalin dehydrogenation activity. It is concluded that spill over hydrogen is also an important aspect in addition to Pt dispersion for getting higher H2 yield from decalin dehydrogenation.
Masking of either acidic or basic sites of aluminum oxide, by co-feeding of NH
3 or CO
2, respectively, along with isophorone, indicate the involvement of weak/moderate basic sites to abstract a ...proton to form an enolate ion followed by involvement of weak/moderate acid sites for dienone–phenol rearrangement to yield aromatization products and involvement of strong acidic sites for yielding decomposition products.
In the armatization of isophorone, acidic and basic sites play a crucial role. Masking of either acidic or basic sites of aluminum oxide, by co-feeding of NH
3 or CO
2, respectively, along with isophorone, indicate the involvement of weak/moderate basic sites to abstract a proton to form an enolate ion followed by involvement of weak/moderate acid sites for dienone-phenol rearrangement to yield aromatization products and involvement of strong acidic sites for yielding decomposition products.
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► A reliable procedure was defined to measure the dispersion of Pt nanoparticles supported on Yttria-Stabilised Zirconia. ► A pre-reduction step in hydrogen at 500
°C and a H
2 pulse ...chemisorption at −28
°C are required. ► Surface oxygen vacancies on the YSZ support are involved in H
2 and CO chemisorption.
Platinum nanoparticles were dispersed by wet impregnation on YSZ (8
mol% Y
2O
3) an ionically conducting support. The Pt dispersion of a series of catalysts was determined by using H
2 pulse chemisorption. Temperature Programmed Reduction (TPR) experiments were performed to characterize the pre-reduction step before dispersion measurements. Finally, a fast and reliable procedure was defined for the Pt dispersion measurements consisting in a pre-reduction step in hydrogen at 500
°C before the H
2 pulse chemisorption performed at −28
°C. Reproducible Pt dispersion values were obtained in good agreement with those measured by pulse CO chemisorptions and static volumetric titration. In addition, the combination of Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy and TPR has evidenced the role of surface oxygen vacancies on YSZ support for both H
2 and CO chemisorptions.
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