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排序方式: 共有5123条查询结果,搜索用时 31 毫秒
61.
Patrick McGrath 《Electrochimica acta》2007,53(3):1365-1371
We use cyclic voltammetry (CV) on fuel cell electrodes to elucidate the important differences between adsorbates resulting from carbon monoxide adsorption and methanol adsorption onto commercial Pt/C electrocatalysts in a sulfuric acid electrolyte. Under open circuit conditions, methanol was found to adsorb preferentially onto the Pt sites associated with “strongly bound” hydrogen. The sites associated with “weakly bound” hydrogen adsorbed methanol more slowly. In the case of CO adsorption, which requires no adsorbate dehydrogenation, all adsorption sites showed similar affinity towards the adsorbate. Electrochemical oxidation of the adsorbates derived from both methanol and CO exposure exhibit slower oxidation when the adsorbate is associated with cubic-packed-like sites than from close-packed-steps and other sites. NMR of a 13CO-adlayer prepared by electrochemical adsorption from low concentration 13CH3OH shows a lower NMR shift and smaller linewidth than the previously reported values for electrochemically adsorbed 13CO gas. These results are interpreted in terms of adsorbate motion on the electrocatalyst surface. 相似文献
62.
63.
Stakheev A. Yu. Gabrielsson P. Gekas I. Teleguina N. S. Bragina G. O. Tolkachev N. N. Baeva G. N. 《Topics in Catalysis》2007,42(1-4):143-147
Pt/Al2O3 and Pt/BaO/Al2O3 catalysts (1 wt% Pt, 10 wt%BaO) were sulfated under conditions simulating a real NSR catalyst operation. Comparative TPR
and XPS studies of sulfur removal from Pt/Al2O3 and Pt/BaO/Al2O3 catalysts indicate that the sulfur removal from Al2O3 surface precedes reductive decomposition of BaSO4 (250–400 °C). Barium sulfate decomposition started with further increase in desulfation temperature at the point of surface
atomic ratio Ba:S = 1 (~450o). Simultaneously, an intensive formation of sulfide species on the catalyst surface was observed. Thermodynamic analysis
of the desulfation process allows us to hypothesize that barium sulfide formation may hinder sulfur removal under reducing
conditions. 相似文献
64.
Nobuhiro Iwasa Tomoyuki Mayanagi Noriaki Ogawa Kentaro Sakata Nobutsune Takezawa 《Catalysis Letters》1998,54(3):119-123
Pd and Pt supported on ZnO, Ga2O3 and In2O3 exhibit high catalytic performance for the steam reforming of methanol, CH3OH+H2OCO2+3HH2, and the dehydrogenation of methanol to HCOOCH3, 2CH3OHHCOOCH3+2HH2. Combined results with temperature-programmed reduction (TPR) and XRD method revealed that Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys were produced upon reduction. Over the catalysts having the alloy phase, the reactions proceeded selectively, whereas the catalysts having metallic phase exhibited poor selectivities. 相似文献
65.
Borgna A. Le Normand F. Garetto T. Apesteguia C. R. Moraweck B. 《Catalysis Letters》1992,13(3):175-188
The effect of an oxidative atmosphere (300 °C) is studied on fresh and sintered unchlorinated naphtha reforming catalysts containing 0.6–1% Pt. The TPR profiles show that only one species is formed using our experimental conditions, regardless of the mean crystallite size of the metal particles. The structural information supplied by EXAFS compared with cuboctahedral particle modeling, implies that such species is a surface platinum oxide, the structure of which is close to that of PtO2, but largely distorted. This is true whether the catalyst is sintered or not. 相似文献
66.
Electropolymerization of phenol and mono-, di-, tri-, pentachlorophenols was studied using EQCM on a Pt electrode at 0.78 V (SHE) in 1 M NaOH solution containing 0.1 M of the corresponding phenol. The highest electropolymerization rate was found for ortho-substituted chlorophenols indicating a weak fouling of the electrode. Low electropolymerization rates for para-substituted chlorophenols suggest a low permeability of the polymer film, resulting in rapid electrode fouling. The EQCM data suggest that electropolymerization of chlorophenols occurs without Cl-elimination for the monomers with unsubstituted ortho and para positions. Dechlorination is most pronounced for electropolymerization of para-substituted isomers. The mechanism of electropolymerization of chlorophenols is discussed. 相似文献
67.
Z. Vít 《Catalysis Letters》1992,13(1-2):131-136
Activity and selectivity of carbon supported Mo catalyst was tested in parallel hydrodenitrogenation (HDN) of pyridine and hydrodesulfurization (HDS) of thiophene in the temperature range 260–350 °C at 2 MPa of hydrogen pressure and compared with that of commercial NiMo-alumina catalyst Shell 324. The main advantages of carbon supported Mo sulfide over commercial NiMo catalyst can be summarized as follows: the markedly higher HDN and better HDS activities normalized to moles of active metals, the lower content of piperidine in the reaction products and the distinctly better selectivity towards HDN reaction. 相似文献
68.
Xiaobao Li Richard P. K. Wells Peter B. Wells Graham J. Hutchings 《Catalysis Letters》2003,89(3-4):163-167
The premodification of a 5 wt% Pt/-Al2O3 catalyst with cinchonidine (0.01 and 0.2 g g-1
catalyst) is described and discussed. Premodification is carried out by treating the catalyst with a solution of cinchonidine followed by solvent removal. Catalysts premodified in this way give the same ee and initial rate of reaction for the enantioselective hydrogenation of ethyl pyruvate as those using the standard in situ modification procedure. Investigations of different solvents for premodification and reaction (dichloromethane, ethanol) show that it is the solvent used for the reaction that controls the observed enantioselection. Premodified catalysts also display the initial transient behavior typically observed with in situ modified catalysts in which the ee increases with conversion in the early part of the reaction. Premodified catalysts show an enhanced rate of reaction when ethanol is used as the reaction solvent compared with in situ modified catalysts under the same conditions. Premodification using aerobic conditions gives the best results and premodified catalysts can be stored prior to use for up to a week without loss of catalytic performance. 相似文献
69.
70.
S. Bernal M. A. Cauqui G. A. Cifredo J. M. Gatica C. Larese J. A. Prez Omil 《Catalysis Today》1996,29(1-4):77-81
The combined application of chemical techniques and HREM shows that platinum deactivation in Pt/CeO2 catalysts steadily increases with the reduction temperature (Tr: 473–1173 K). Though no suppression of the H2 adsorption was observed, the TPD-H2 traces indicate the occurrence of significant chemical changes with Tr. As revealed by HREM, the metal decoration starts at Tr: 973 K, a temperature well above those at which the chemical effects can be observed. 相似文献