Catalytic decomposition of ammonia on tungsten (100), (110), and (111) crystal faces

Abstract

The rates of catalytic decomposition of ammonia on (100), (110), and (111) single crystal faces of tungsten were measured over the temperature range 800–970 °K for ammonia pressures ranging from (0.5–100)× 10−3torr and for nitrogen and hydrogen pressures varying from (0–50) × 10−3torr. In all cases the rate of decomposition was of the form, rate=A+BPNH3(2/3), and was independent of nitrogen and hydrogen partial pressures. The constants A and B varied substantially with crystal face; the values of B for the (111), (100), and (110) faces were in approximate ratio 8.4:1.55:1 and these substantially established the decomposition rate for PNH3>5 × 10−3torr. Rate forms for NH3 and ND3 were compared on the (111) face at 860 °K, with results ANH3=AND3,BNH3≈ 1.47 BND3. The observed rate form is derived from a model involving nearly complete surface coverage by the species WN and small surface coverages by species W2N, W2N3H2, and WNH. The A term in the rate law is generated by the reaction, 2WN→W2N+(1/2)N2. This process was advocated as rate limiting in ammonia decomposition by Matsushita and Hansen and their rate law for this process obtained on polycrystalline tungsten furnishes an estimate of the A parameter in the same order of magnitude as that observed. The BPNH32/3 term results from a steady state balance of W2N3H2 and WNH decomposition reactions. Peng and Dawson have advocated decomposition of W2N3H as rate limiting in the decomposition of ammonia, and have also suggested WN dissociation may be rate limiting at low pressures, W2N3H dissociation at higher pressures. The present model therefore resembles that of Peng and Dawson in concept but not in detail.

Photoinduced and redox-induced transmembrane processes with vesicle-stabilized colloidal cadmium sulfide and multicharged viologen derivatives

“…To our knowledge, the synthesis of the following triply and quadruply charged viologens has not been described. The preparation of several analogous viologen compounds has been mentioned, but without synthetic procedure.15 (1) Synthesis of a Triply Charged Viologen. The synthetic route is given in Scheme I.…”

“…Preparation of C^N*-. 1,32.2 g, 19.0 mL) was mixed with 50 mL of toluene in a 250-mL bottle at room temperature. Trimethylamine (35 mmol, 2070 mg) was dissolved in 10 mL of precooled toluene at 0 °C and rapidly added to the 1,5-dibromopentane solution.…”

Crystal-plane-specific effect of promoters on active sites of vanadium pentoxide catalyst

“…In this study, we prepared model catalysts based on supported V205 catalysts for two purposes: (1) control of exposure of a specific crystal plane of V205, and (2) fixation of the structure of V205 after the addition of promoter oxides. Ti02 was chosen as a support because of good crystallographic fit with the (010) plane of V205 as proposed by Vejux and Courtine.18 It is a fact that V2Oj layers spread well on Ti02 supports, and V205/Ti02 catalysts are widely used in industry and scientific investigations, such as monolayer cataysts for oxidation of o-xylene.19 By use of a model catalyst having high exposure of the (010) plane of V205, the effect of promoters on the number of V=0 species on (010) plane has been clarified.…”

Ammonia-pretreatment-induced restructuring of iron single-crystal surfaces: Its effects on ammonia synthesis and on coadsorbed aluminum oxide and potassium

“…The presence of these highly ,.J coordinated sites might also explain the increased dissociation rate of ammonia over the (lII) plane of bcc tungsten relative to W(IIO) and W(I00) (20). It seems then that surfaces which expose C7 sites are stabilized by the chemisorption of atomic nitrogen and it would be ther-11 modynamically favorable for the Fe(111) surface, with a surface free energy (y) of 1460 ergs/cm 2 , to restructure to the more densely packed Fe(211) plane (Y211 = 1350 ergs/cm 2 )(21) (which contains C7 sites) in the presence of ammonia.…”