Catalytic Processes Involving Dihydrogen Complexes and Other Sigma-bond Complexes

The discovery of dihydrogen complexes, L_nM(H₂), pointed to direct transfer of hydrogen from coordinated H₂ ligands to substrates as an operable pathway in catalysis both in homogeneous and heterogeneous systems. Sigma complexes, L_nM(η²-H–X) (X=H, Si, C, etc), are indeed relevant in hydrogenation as well as silane alcoholysis and methane conversion.     

Comparison of basic gas cycles under the restriction of constant heat addition

Theoretical thermal efficiencies and theoretical effectivenesses of six kinds of basic gas cycles, including the Carnot, Otto, Diesel, Takemura, Atkinson and Joule cycles, were compared with one another under the restriction of constant heat-addition. Relations between the available work content of heat added and the exergy change in several thermodynamic processes were also examined. It is concluded that the Atkinson cycle has the highest thermal efficiency and the second best effectiveness among the gas cycles examined. The exergy changes in an isochoric heating-process in closed systems and in an isobaric heating-process in open systems agree with the available work contents of heat added in these processes. Thus, the exergy increment in a heating process should not be generally adopted as the denominator for the theoretical effectiveness.     

Effect of the addition of alkali metals on the metallic phase of Pt/Al2O3 catalysts

The effect of the addition of alkali metals to Pt/Al₂O₃ catalysts for dehydrogenation of paraffins was studied. Results can be interpreted in terms of different effects. Thus, the addition of alkali metals produces a poisoning of the acid sites of the alumina support and a modification of the characteristics of the metallic phase. The latter effect involves not only an increase of the Pt particles size, but also an electronic modification of the metallic phase. These two effects are more marked with alkali metals with higher ionic radius, such as K.     

Adsorption and reaction of acrolein on titanium oxide single crystal surfaces: coupling versus condensation

The reactions of acrolein have been investigated on TiO₂(0 0 1) single crystal surfaces by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS). Two carbon–carbon bond-forming reactions were observed. The first, on defect-containing surfaces, is reductive coupling to form olefins. The high reaction yield of ca. 80% shows the high activity of such surfaces for carbon–oxygen bond dissociation (needed for surface oxygen restoration) and carbon–carbon bond formation to make olefins. The second reaction, observed on the stoichiometric surface, is condensation of two acrolein molecules to give a C₆H₈O product tentatively identified as 2-methyl-2,4-pentadienal. Condensation reactions of carbonyls are characteristic of TiO₂ surfaces; for acrolein, this reaction is proposed to involve initial hydrogen addition followed by nucleophilic attack on a second molecule of acrolein. This results in an aldol condensation followed by dehydration.

NEXAFS analyses were conducted in order to differentiate the states of molecularly adsorbed acrolein on the two distinctly different surfaces. The C=O bond of adsorbed acrolein is maintained in the case of the stoichiometric surface (evidenced by a π_C=O^* transition at 286.6 eV), while it is absent on the reduced surface. The absence of this NEXAFS transition on the reduced surface suggests that the O atom of the C=O bond has reacted with the oxygen-deficient lattice. The restoration of these oxygen deficiencies is concomitant with the formation of the reductive coupling products (as observed by TPD and XPS experiments).     

加铬铸铁的生产

介绍了加铬铸铁生产中加铬的作用、合适的加入量、加入方式和注意事项等。     

关于数论中一个猜想的证明

在平面几何中 ,我们都知道 ,一个三角形的任意两边之和必大于第三边 ,这就是三角不等式。在数论中有不少这类不等式。关于π(x) (π(x)表示x内所含素数的个数 ) ,也有这样的猜想[1 ] ,即对于自然数x >1 ,y >1总有π(x) +π(y)≥π(x +y)     

商品苄丙酮含量的测定

采用氯化溴加成反应测定苄叉丙酮含量，该法简单快速，准确度高。     

环丙烷类化合物在有机合成中的应用

环丙烷类化合物能与多种化学试剂作用并生成许多类型的化合物。在有机合成上具有广泛的用途。     

Patterns of knowledge in children's addition.

Patterns of conceptual and procedural knowledge of addition were examined in 5- to 8-year-olds (N = 80). Conceptual knowledge was measured by assessing children's responses to problems in which addends were reordered or decomposed and recombined. Problems were presented using abstract symbols, numbers, and physical objects. Children were more successful in noticing that addends had been reordered rather than decomposed and in noticing the decomposition of addends presented with objects rather than with symbols. Distinct profiles of procedural competence were derived from an analysis of children's problem-solving accuracy and strategies. Profiles were associated with patterns of conceptual knowledge and with age, although age and conceptual knowledge were not related. Findings highlight the usefulness of identifying profiles of procedural and conceptual knowledge for understanding the development of children's knowledge of addition. (PsycINFO Database Record (c) 2010 APA, all rights reserved)