用IR-电子天平重量吸附法快速测定催化剂表面酸度

建立了用IR 电子天平重量吸附法测定固体催化剂及载体表面酸度的分析方法,解决了实际工作中对酸度较低的催化剂及载体的表面酸度测定不准确的问题。结果表明:该方法与原有的IR 石英弹簧重量法相比,分析结果的相对标准偏差有较明显的提高,单个样品的分析时间由8～10h减少到4h左右。     

Correlation between the surface acidity and activity of bismuth tungstate catalysts

The surface acid strength distribution in bismuth tungstate catalysts with Bi/W ratios of 2/3, 1/1, 2/1 and 6/1 has been investigated. The results are interpreted attributing higher acidities to W–O–W bonds and lower acidities to W=O bonds. An apparent correlation is observed between the surface acidities of the catalysts and their activities in the oxidation and ammoxidation of propylene and 1-butene.

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Bi/W, 2/3, 1/1, 2/1 6/1. , W–O–W W=O. 1-.

     

Determination of the acidity of supports and metal catalysts by the pulse technique

This paper reports the use of the pulse technique for the determination of the acidity of various solids and supported metal systems, using cyclohexylamine (pK_a=10.6) and pyridine (pK_a=5.25) as titrants. The results obtained show that the deposition of Pt results in increased acid properties in the solids investigated.

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, (pK_a=10,6) (pK_a5,25) . , Pt .

     

Determination of the acidity of catalysts by thermoadsorption with a cyclic temperature program

A novel method is described, consisting in the determination of admolecules which are the most loosely bonded on the surface. This permits a better characterization of active centers on catalysts.

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, , , , .

     

Influence of calcination temperature on surface acidity of the solid superacid of sulfated alumina

The acidity of sulfated alumina catalysts was studied by analyzing DTG and heat of Ar adsorption together with the isomerization of pentane. The initial acid sites were Brönsted-type, and converted into Lewis-type upon increasing the pretreatment temperature. The heat of Ar adsorption of the most active sulfated alumina was 18.9 kJ mol^–1, this value being a little smaller than that of sulfated zirconia (23.6 kJ mol^–1).This revised version was published online in December 2005 with corrections to the Cover Date.     

Investigation of surface phenomena on solid catalysts by thermogravimetry

NH₃ and CO₂ adsorptions were measured for seven silica-alumina catalysts, containing various concentrations of sodium ion. The measurements were carried out in two different ways: with the classical volumetric method, and a dynamic one, taking DTA, TG and DTG curves simultaneously. Great similarities are observed between the results obtained with these methods. Adsorption measurement by thermal analysis may be useful for semiquantitative studies.

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Zusammenfassung Die Adsorption von NH₃ und CO₂ wurde an sieben Silica-Aluminium-oxid-Katalysatoren gemessen, welche verschiedene Konzentrationen von Na-Ionen enthielten. Die Messungen wurden nach der klassischen volumetrischen Methode und mit der dynamischen Methode, bei welcher die DTA, TG und DTG-Kurven simultan aufgenommen wurden, durchgeführt. Eine große Ähnlichkeit der mit den zwei Methoden erhaltenen Resultate kann beobachtet werden. Die Adsorptionsmessung mit der Thermoanalyse kann für semiquantitative Untersuchungen von Nutzen sein.

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Résumé On a mesuré l'adsorption de NH₃ et de CO₂ sur sept catalyseurs à base de silice et d'oxyde d'aluminium avec différentes teneurs en ions sodium. Les mesures ont été effectuées suivant deux procédés: par la méthode volumétrique classique et par ATD, TG et TGD simultanées. On peut observer une grande analogie entre les résultats obtenus par les deux méthodes. Les mesures d'adsorption par analyse thermique peuvent être utiles dans le cas d'études semiquantitatives.

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NH₃ CO₂ - , . : , , . , . , , .

     

Investigation of surface phenomena on solid catalysts by thermogravimetry

Thermogravimetry was applied to the investigation of the phenomena of adsorption, chemisorption, desorption and surface reactions on solid catalysts. The measurements were carried out with 13 NaX and NaY type zeolites and with silica gels treated with sodium hydroxide, in the presence of cumene and methanol. On the basis of the quantitative results it is possible to give an interpretation of surface phenomena and to establish the existence of various active centres on the surface of the catalysts.

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Zusammenfassung Adsorptions-, Chemisorptions- und Desorptionserscheinungen, sowie Oberflächenreaktionen an festen Katalysatoren wurden thermogravimetrisch untersucht. Die Messungen wurden an Zeoliten der Typen 13 NaX und NaY und an mit Natriumhydroxid behandelten Silikagelen in Gegenwart von Cumol und Methanol durchgeführt. Aufgrund der quantitativen Ergebnisse war es möglich, die Oberflächenerscheinungen zu deuten und das Vorhandensein von verschiedenen aktiven Zentren an der Katalysatoroberfläche nachzuweisen.

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Résumé On a appliqué la méthode thermogravimétrique à l'étude des phénomènes d'adsorption, de chemisorption et de désorption ainsi qu'aux réactions de surface sur des catalyseurs solides. Les mesures ont été effectuées sur zéolites de type 13 NaX et NaY et sur des silicagels traités par la soude en présence de cumène et de méthanol. Des résultats quantitatifs permettent de donner une interprétation des phénomènes de surface et d'établir l'existence de divers centres actifs à la surface des catalyseurs.

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, , , . , , , , . 13 NaX NaY , . .

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The authors express their gratitude to Prof. S. Malinowski for help and discussion and to Mr. W. Dziklinski for his technical help.     

31P chemical shift of adsorbed trialkylphosphine oxides for acidity characterization of solid acids catalysts

A comprehensive study has been made to predict the adsorption structures and (31)P NMR chemical shifts of various trialkylphosphine oxides (R3PO) probe molecules, viz., trimethylphosphine oxide (TMPO), triethylphosphine oxide (TEPO), tributylphosphine oxide (TBPO), and trioctylphosphine oxide (TOPO), by density functional theory (DFT) calculations based on 8T zeolite cluster models with varied Si-H bond lengths. A linear correlation between the (31)P chemical shifts and proton affinity (PA) was observed for each of the homologous R3PO probe molecules examined. It is found that the differences in (31)P chemical shifts of the R3POH(+) adsorption complexes, when referring to the corresponding chemical shifts in their crystalline phase, may be used not only in identifying Br?nsted acid sites with varied acid strengths but also in correlating the (31)P NMR data obtained from various R3PO probes. Such a chemical shift difference therefore can serve as a quantitative measure during acidity characterization of solid acid catalysts when utilizing (31)P NMR of various adsorbed R3PO, as proposed in our earlier report (Zhao; et al. J. Phys. Chem. B 2002, 106, 4462) and also illustrated herein by using a mesoporous H-MCM-41 aluminosilicate (Si/Al = 25) test adsorbent. It is indicative that, with the exception of (TMPO), variations in the alkyl chain length of the R3PO (R = C(n)H(2n+1); n > or = 2) probe molecules have only negligible effect on the (31)P chemical shifts (within experimental error of ca. 1-2 ppm) either in their crystalline bulk or in their corresponding R3POH(+) adsorption complexes. Consequently, an average offset of 8 +/- 2 ppm was observed for (31)P chemical shifts of adsorbed R3PO with n > or = 2 relative to TMPO (n = 1). Moreover, by taking the value of 86 ppm predicted for TMPO adsorbed on 8T cluster models as a threshold for superacidity (Zheng; et al. J. Phys. Chem. B 2008, 112, 4496), a similar threshold (31)P chemical shift of ca. 92-94 ppm was deduced for TEPO, TBPO, and TOPO.     

Surface acidity of supported vanadia catalysts

The surface acidity of SiO₂, γ-Al₂O₃ and TiO₂ supported vanadia catalysts has been studied by the microcalorimetry and infrared spectroscopy using ammonia as the probe molecule. The acidity in terms of nature, number and strength was correlated with surface structures of vanadia species in the catalysts, characterized by X-ray diffraction and UV-Vis spectroscopy. It was found that the dispersion and surface structure of vanadia species depend on the nature of supports and loading and affect strongly the surface acidity. On SiO₂, vanadium species is usually in the form of polycrystalline V₂O₅ even for the catalyst with low loading (3%) and these V₂O₅ crystallites exhibit similar amount of Brönsted and Lewis acid sites. The 25%V₂O₅/SiO₂ catalyst possesses substantial amount of V₂O₅ crystallites on the surface with the initial heat of 105 kJ mol^-1 and coverage of about 600 mmol g^-1 for ammonia adsorption. Vanadia can be well dispersed on g-Al₂O₃and TiO₂ to form isolated tetrahedral species and polymeric two-dimensional network. Addition of vanadia on γ-Al₂O₃ results in the change of acidity from that associated with g-Al₂O₃ (mainly Lewis sites) to that associated with vanadia (mainly Brönsted sites) and leads to the decreased acid strength. The 3%V₂O₅/TiO₂ catalyst may have the vanadia structure of incomplete polymeric two-dimensional network that possesses the Ti-O-V-OH groups at edges showing strong Brönsted acidity with the initial heat of about 140 kJ mol^-1 for ammonia adsorption. On the other hand, the 10%V₂O₅/TiO₂ catalyst may have well defined polymeric two-dimensional vanadia network, possessing V-O-V-OH groups that exhibit rather weak Brönsted acidity with the heat of 90 kJ mol^-1 for NH₃ adsorption. V₂O₅ crystallites are formed on the 25%V₂O₅/TiO₂ catalyst, which exhibit the acid properties similar to those for 25%V₂O₅ on SiO₂ and γ-Al₂O₃.     

Determination of the surface acidity of a free-base corrole in a self-assembled monolayer

Mixed self-assembled monolayers (SAMs) containing a corrole moiety have been prepared to examine their electrochemical properties and surface acidity, with the eventual goal of biosensor development. Mixed SAMs consisting of 6-mercapto-hexanol (6-MHO) and 8-amino-1-octanethiol (8-AOT) in varying ratios were modified with a free-base corrole and characterized via Osteryoung square-wave voltammetry and contact angle measurements. The surface acidity of the free-base corrole was determined using an electrochemical titration method, with pK _a values established at 6.4 when using Fe(CN) ₆ ^4? /Fe(CN) ₆ ^3? as a redox probe and at 6.7 when using iodide, and assigned to the CorH₄ ⁺ ? CorH₃ + H⁺ equilibrium.     

The reactions of benzofuroxan with carbonyl compounds on the surface of solid catalysts

The cyclocondensations of benzofuroxan 1a with carbonyl compounds were smoothly and efficiently carried out by the adsorption of the components on the surface of silica gel or a molecular sieve to form a 2,3-disubstituted quinoxaline 1,4-dioxide. When the reactions using a molecular sieve 3A (powder) were carried out at 90°, the actual reaction times were reduced to 0.5-2 hours. Although Duerckheimer has reported the isolation of only the 7-substituted quinoxaline 1,4-dioxide when 5-methoxybenzofuroxan 1e was allowed to react with ethyl acetoacetate 2j , it produced only the 6-methoxy isomer as the reaction product by our method. 5-Carboxybenzofuroxan 1f did not react with the carbonyl compound.     

Determination and perturbation of the electronic potentials of solid catalysts for innovative catalysis

Concerns about energy and the environment are motivating a reexamination of catalytic processes, aiming to achieve more efficient and improved catalysis compatible with sustainability. Designing an active site for such heterogeneous catalytic processes remains a challenge leading to a next level breakthrough. Herein, we discuss a fundamental aspect of heterogeneous catalysis: the chemical potential of electrons in solid catalysts during thermal catalysis, which directly reflects the consequent catalytic reaction rate. The use of electrochemical tools during thermal catalysis allows for the quantitative determination of the ill-defined chemical potentials of solids in operando, whereby the potential–rate relationship can be established. Furthermore, the electrochemical means can also introduce the direct perturbation of catalyst potentials, in turn, perturbing the coverage of adsorbates functioning as poison, promoters, or reactants. We collect selected publications on these aspects, and provide a viewpoint bridging the fields of thermal- and electro-catalysis.

Concerns about energy and the environment are motivating a reexamination of catalytic processes, aiming to achieve more efficient and improved catalysis compatible with sustainability.     

Infrared spectroscopic studies of the acidity of chromium molybdate catalysts

NH₃ adsorption on chromium molybdate at 25 and 350°C has been studied. Ammonia chemisorption has been found on electron-acceptor and Brønsted centers. The acceptor capacity of cations is shown to change as compared to individual oxides and upon high-temperature interaction with ammonia.

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NH₃ 25° 350°. . .

     

Investigation of surface phenomena on solid catalysts by simultaneous TG and DTA

Simultaneous thermogravimetry and DTA were used to investigate the phenomena of adsorption, desorption and surface reactions of water vapour on NaX and NaYtype and NaX-exchanged zeolites, and silica and alumina gels treated with sodium hydroxide. On the basis of the quantitative results, it was possible to give an interpretation of the surface phenomena and to establish the existence of various active centres on the surface of the catalysts. Water in zeolites is probably bound by oxygen-alkali metal bonds.

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Zusammenfassung Thermogravimetrie und DTA wurden simultan zur Untersuchung von Adsorptions-, Desorptions- und Oberflächenreaktionserscheinungen an Zeolit-Typen von NaX und NaY und NaX Ionenaustausch sowie an mit Natriumhydroxid in Gegenwart von Wasser behandelten Silika- und Aluminiumoxid-hydroxid-gelen [eingesetzt. Aufgrund der quantitativen Ergebnisse war es möglich eine Deutung der Oberflächenerscheinungen zu geben und die Existenz verschiedener aktiver Zentren an der Oberfläche der Katalysatoren festzustellen. In Zeoliten ist Wasser wahrscheinlich durch Sauerstoff-Alkalimetallbindungen fixiert.

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Résumé La thermogravimétrie et l'analyse thermique différentielle ont été appliquées simultanément à l'étude des phénomènes d'adsorption, de désorption et de réactions en surface sur des zéolites échangeuses d'ions de type NaX et NaY et NaX ainsi que sur des gels de silice et d'alumine traités par la soude en présence d'eau. Les résultats quantitatifs permettent de proposer une interprétation des phénomènes de surface et d'établir l'existence de plusieurs centres actifs à la surface des catalyseurs. Dans les zéolites, l'eau est probablement liée par des liaisons oxygène-métal alcalin.

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DTA , NaX, NaY NaX , , . , . , - .

     

Investigation of surface phenomena on solid catalysts by simultaneous thermogravimetry and DTA

Thermogravimetry and simultaneous DTA were used to investigate the phenomena of adsorption, desorption and surface reactions on solid catalysts. The measurements were carried out with NaX and X-type ion-exchanged zeolites and silica-alumina gels (Ketjen) treated with sodium hydroxide in the presence of carbon dioxide. On the basis of the quantitative results it is possible to give an interpretation of the surface phenomena and to establish the existence of various active centres on the surfaces of the catalysts. In the case of the adsorption of carbon dioxide on X-type zeolites, the quantity of carbon dioxide adsorbed is much smaller on ion-exchanged zeolites than on NaX.

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Zusammenfassung Thermogravimetrie und simultane DTA wurden zur Untersuchung der Adsorptions-, Desorptions-Erscheinungen und Oberflächenreaktionen an festen Katalysatoren eingesetzt. Die Messungen wurden and NaX und Zeolithen des Typs X sowie an Silika-Aluminiumoxidgelen Ketjen durchgeführt, welche mit Natriumhydroxid in Gegenwart von Kohlendioxid behandelt worden waren. Aufgrund der quantitativen Ergebnisse ist es möglich eine Deutung der Oberfiächenerscheinungen zu geben und die Existenz verschiedener aktiver Zentren an der Katalysatorenoberfläche nachzuweisen. Im Falle der Adsorption von Kohlendioxid an Zeolithen des Typs X ist die Menge des adsorbierten Kohlendioxids am Ionenaustauscher viel niedriger als am NaX.

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Résumé Application de la TG et de l'ATD simultanées à l'étude des phénomènes d'adsorption, de désorption et de réactions de surface sur des catalyseurs solides. Les mesures ont été effectuées avec des zéolites échangeuses d'ions de type NaX et X ainsi qu'avec des gels Ketjen silice-alumine traités par NaOH en présence de CO₂. Les résultats quantitatifs permettent de donner une interprétation des phénomènes de surface et d'établir l'existence de divers centres actifs à la surface des catalyseurs. Dans le cas de l'adsorption de CO₂ sur les zéolites de type X, la quantité de CO₂ adsorbé est beaucoup plus faible que sur les zéolites de type NaX.

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DTA , . - NaX X - « », . . X, - , NaX.

     

Catalytic properties,surface composition,and acidity of Ga-ZSM-5 catalysts for aromatization of lower paraffins

A study has been made of the catalytic and acidic properties together with the surface composition of the zeolite HZSM-5 modified by gallium by hydrothermal treatment with a sodium gallate solution and also by impregnation with gallium nitrate solution. It has been established that the first method of introducing gallium produces a more selective catalyst for aromatization of n-butane. It has been shown that under the conditions of the catalytic reaction several types of active centers incorporating Ga are formed: Ga₂O₃ on the zeolite surface, gallium ions inside the zeolite canals, and gallium atoms at the surface of and in the lattice that are bonded to OH groups.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2726–2732, December, 1991.