Conditions for HCN synthesis and catalyst activation over Pt±Rh gauzes

Transcription

1 Applied Catalysis A: General 180 (1999) 287±298 Conditions for HCN synthesis and catalyst activation over Pt±Rh gauzes A.G. Dietz III, L.D. Schmidt * Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN , USA Received 2 April 1998; received in revised form 9 October 1998; accepted 9 October 1998 Abstract We have examined different processing conditions for the synthesis of HCN by the ammoxidation of methane over woven Pt±Rh gauzes in an autothermal bench scale reactor. Compared to the conventional air processes, the HCN yield can be improved 10±15% by preheating the reactant gases, and HCN throughput can be increased 140% by removing N 2 from the feed stream. We were able to attain operation in a high pressure bench scale reactor, and HCN yields were maintained above 0.60 up to 3.5 atm at 300% of the throughput achievable at 1 atm. We also investigated activation of the Pt±Rh gauze catalyst which occurs through facet and pit formation on the metal surface. A high temperature treatment reduced activation times from 30 to 3 h. Pits on the catalyst surface resulted from increased temperatures and NH 3 in the reactant gas, but HCN processing conditions were necessary for the catalyst to achieve best performance. # 1999 Elsevier Science B.V. All rights reserved. Keywords: HCN synthesis; Catalyst activation; Pt±Rh gauzes 1. Introduction Since the 1940s, the Andrussow process has been used industrially to synthesize HCN [1]. The process consists of reacting methane and ammonia in air over Pt±Rh gauze catalysts at high temperatures (11008C), at pressures just over 1 atm, and at very short contact times (0.1 ms). The process operates autothermally and nearly adiabatically in a tubular reactor in which gases pass through 20±50 gauze layers. The reaction is quenched downstream with a heat exchanger to collect HCN. *Corresponding author. Tel.: ; fax: The desired reaction for this process can be written as CH 4 NH O 2! HCN 3H 2 O; H ˆ 115 kcal=mol (1) Some of the other reactions that compete in parallel with methane ammoxidation (1) include ammonia oxidation (2), methane partial oxidation (3), ammonia decomposition (4), and methane combustion (5). NH O 2! 1 2 N H 2O; H ˆ 75 kcal=mol (2) CH O 2! 2H 2 CO; H ˆ 8:5kcal=mol (3) X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S X(98)

2 288 A.G. Dietz III, L.D. Schmidt / Applied Catalysis A: General 180 (1999) 287±298 NH 3! 1 2 N H 2; H ˆ 11 kcal=mol (4) CH 4 2O 2! 2H 2 O CO 2 ; H ˆ 191:8 kcal=mol (5) In addition to these competing parallel reactions, HCN can react in series in a hydrolysis reaction (6) to form ammonia and carbon monoxide. HCN H 2 O! NH 3 CO; H ˆ 12 kcal=mol (6) In this reaction, the products are thermodymically favored, and therefore, after reaction on the gauze, the gases are immediately cooled with downstream heat exchanger. This also helps to prevent the formation of HCN polymer lms inside the reactor. Although the Andrussow process for HCN synthesis has been used industrially for decades, optimization of HCN yields is still an active area for research. Some of the early experimental work [2±4] could not produce a systematic method to optimize the HCN yield, so plant operators tended to rely on trial and error. In a more systematic study using a 4 in. diamater tubular HCN pilot plant reactor [5], both the CH 4 /NH 3 ratio and the air/fuel ratio were varied to determine the optimal values for HCN selectivity and HCN production. In these experiments, they examined a wide range of operating conditions, in which they observed optimum HCN selectivities between 0.78 and 0.88, and the HCN yield between 0.57 and At typical plant conditions, the HCN selectivity was 0.80 and the HCN yield was Pan and Roth found that the HCN selectivity based on NH 3 was maximized at a higher CH 4 /NH 3 ratio than at the stoichiometric ratio for the ideal ratio for HCN yield, due to mostly increased NH 3 leakage beyond the gauze pack (i.e. a decrease in ammonia conversion at higher CH 4 /NH 3 ratio). They also found that the HCN selectivity and yield were maximized at feed gas ratios that corresponded to minima in the catalyst temperature. The work of Pan and Roth showed that feed conditions for methane ammoxidation in air at atmospheric pressure could be set to achieve an optimum in the HCN yield. Industrial molar yields to HCN are typically between 0.60 and 0.70 based on ammonia [1], and these values tend to vary between different gauze packs. Based on pilot plant data, kinetic and model studies, it is evident that improvement to the Andrussow process should still be possible. In this laboratory, the reaction kinetics for HCN synthesis over Pt and Rh polycrystalline foils have been examined at low pressures [6±8]. The reaction rates were t into Langmuir±Hinshelwood kinetics, and later used in an atmospheric pressure model with 13 simultaneous surface reactions to simulate methane ammoxidation [9]. The model agreed well with the experimental data and predicted that operation at higher pressures is possible due to the increased throughput available even though the HCN yield should decrease slightly. The results from all of these experiments were summarized in a review paper by Hickman and Schmidt [10]. Recently, alternative supports for HCN synthesis were examined in order to improve selectivity and/or reduce the catalyst cost [10,11], but the Pt±Rh gauze catalyst gave comparable HCN yields. However, the experiments involving HCN synthesis over monolithic supports provided interesting results which led to experiments involving millisecond reactors which are discussed elsewhere [12]. Low yields are observed when an Andrussow reactor is started up with an unused gauze pack. The Pt±Rh gauze catalysts activate over a period of 40±80 h, during which a change in HCN selectivity takes place. Investigation of new and used gauzes has shown that the surface rearranges leading to up to a 10-fold increase in surface area [13,14]. These changes include etching of the catalyst along grain boundaries causing faceting and pits which can be as deep as 1000 AÊ [15]. The increase in surface area has also been attributed to changes on a sub-micrometer level. Along with the surface rearrangement, Rh metal tends to segregate away from the surface during the rst 24 h. However, this process is mass transfer limited and the reaction goes to completion in that all oxygen is converted. Therefore, surface area should be relatively unimportant, since catalyst surface area should only effect the reaction rate when the process is reaction limited. The surface area of the catalyst will not effect the product distribution; it will only effect the conversion (neglecting temperature effects) if the reaction is the limiting step. In HCN synthesis, it is believed that the reaction is happening very quickly on the metal, and is limited by transport to and from the

3 A.G. Dietz III, L.D. Schmidt / Applied Catalysis A: General 180 (1999) 287± metal. That is why gauzes are used, and why pitting and faceting, rather than increased catalyst surface area, change the conversion and selectivities (due to increased boundary layer transport). It is not clear how restructuring affects selectivity. After the catalyst is activated, it is believed that carbon and/or Fe deposits on the catalyst block active metal sites causing deactivation over a period of months. No signi cant loss of Pt metal is seen from the gauzes operating at 11008C because the volatile PtO 2 species tends not to form in the reducing environment present during Andrussow HCN synthesis. This is in contrast to NH 3 oxidation to HNO 3 over Pt gauzes where signi cant metal loss occurs even at 8008C because the process operates in excess O 2, and so PtO 2 forms readily. Several different mechanisms for etching of Pt±Rh catalysts during HCN synthesis and NH 3 oxidation have been proposed. It has been suggested that surface diffusion and attack at surface defects may account for etching and pitting [16]. Another possibility is that volatile Pt oxides form on the surface, evaporate, diffuse, and then condense [17]. This can explain needles and ``wool'' structures seen during NH 3 oxidation, and can also account for Pt loss that occurs in this process. However, during HCN synthesis, no Pt loss is observed, and this must be accounted for in any mechanism that explains catalyst activation during HCN synthesis. In Section 3 of this paper, we discuss experiments in which we have extended operating conditions in the Andrussow process beyond methane ammoxidation in air at atmospheric pressure. We compare results for preheating the feed gases, operating in enriched air, decreasing the contact time, and raising the pressure. HCN synthesis at high pressure was discussed previously [18], although the reaction conditions were not those typically used industrially. In Section 4, we discuss activation of the Pt±Rh gauze catalysts. Since it normally takes 2±4 days before a catalyst becomes active, reducing this activation time, anddeterminingthe mechanism for activation would be quite important. 2. Experimental For the experiments described here, the catalyst consisted of ve 18 mm diameter circles of 80 mesh (80 wires per inch) woven Pt±Rh gauze, with a gauze pack volume of 0.2 cm 3, placed between two foam Al 2 O 3 monolith heat shields, which hold the gauze in place and act as radiation shields to maintain gauze temperatures close to adiabatic. For experiments at atmospheric pressure, the gauze pack and monoliths were surrounded in an alumina ber mat to insulate and mount the catalyst inside a 40 cm long, 19 mm inner diameter quartz reactor. Insulation was also placed outside this section of the reactor to ensure autothermal and nearly adiabatic operation like that expected in larger reactors. Upstream gases were preheated if necessary by wrapping the upstream section of the reactor with heating tape. At higher pressures, the gauze pack was placed inside a 40 cm long, 19 mm inner diameter stainless steel high pressure reactor for safety as described previously [18]. Reactant gases (CH 4,NH 3,O 2, and N 2 ) from high pressure gas cylinders were fed into the system and metered by mass ow controllers. After reaction, product gases were routed through stainless steel lines maintained at 1258C and into a gas chromatograph for analysis. Reactor and GC pressures were controlled independently to ensure a constant sample size for GC analysis. Temperatures at the catalyst surface were determined by welding a Pt±Rh thermocouple directly to one of the gauze layers in the gauze pack. Analysis of product gases was performed with an HP5890 gas chromatograph equipped with a 10 ft long 1/8 in. Hayesep C packed column. Since N 2 is a reactant in this system, it could not be used as a calibration gas. Therefore, the carbon-containing products were used for calibration (by closing the C atom balance with CH 4 fed). The H-atom balance was used as a check, and generally closed to within 5%. Flow rates through the mass ow controllers were accurate to within 0.05 slpm, and individual species concentrations were measured with a reproducibility of 1%. Overall, we regard compositions stated to be accurate to within 4%. The reaction was ignited by setting the gas ow rates to operating conditions and heating the gauze pack to 2108C. Since a fresh gauze pack must be activated under process conditions, steady state behavior was not recorded until after the reactor had been operated for 18±24 h or until no signi cant changes in

4 290 A.G. Dietz III, L.D. Schmidt / Applied Catalysis A: General 180 (1999) 287±298 product selectivities and conversions were detected. After this period, we could start up and shut down the reactor without observing any changes in catalyst activity. The reaction attained steady state operating conditions within 15 min of ignition, and no signi cant transients were observed during operation. Despite operating many catalysts for long periods of time (two weeks), the activity of the catalyst did not change signi cantly although deactivation typically occurs industrially after 2±3 months. Since the reactor was operated with fuel rich in ammable or explosive limits, the reaction was stopped by decreasing O 2 ow before decreasing the fuel ow. All experiments were performed in a fume hood, with manual gas ow shut off valves located outside the fume hood. 3. Results The results in this paper are divided into two sections. First, we will discuss the effects of various parameters on selectivity, conversion, and yield in the synthesis of HCN over Pt±Rh gauzes. These experiments were conducted on activated gauze catalysts and serve to elucidate which operating conditions give maxima in HCN selectivity, HCN yield and fuel conversion. We include the effects of fuel/o 2 ratio, CH 4 /NH 3 ratio, enriched air, preheat, pressure, and space velocity. Second, we show data during the activation stage of the HCN catalyst along with SEM micrographs of gauzes during various phases of the activation process and under different activation conditions. Since both CH 4 and NH 3 are fuels in this system, selectivity to HCN can be based on either feed. We calculate molar C and N selectivities as described previously [11], and we report HCN selectivities and yields on both bases. Unless otherwise indicated, the reactant gases were fed at a ow rate of 5 slpm which corresponds to a gas hourly space velocity of h 1 and an average residence time of s(pˆ1.2 atm, Tˆ11008C). Measured reaction temperatures on the catalyst surface were within 508C of the calculated adiabatic reaction temperatures, based on product compositions and selectivities arrived at experimentally HCN synthesis Fig. 1(a) shows HCN selectivity and yield, NH 3 and CH 4 conversion, and catalyst temperature for methane ammoxidation in air as a function of (CH 4 NH 3 )/O 2 ratio in the feed. As the fuel/o 2 ratio was raised, the conversion of both NH 3 and CH 4 decreased along with reaction temperature. The selectivity to HCN in both cases is at a maximum at slightly higher fuel/o 2 ratios than the maxima in HCN yield. At a fuel/o 2 ratio of 1.75 and at a reaction temperature of 11008C, the yield to HCN based on NH 3 reaches 0.65, and it reaches 0.60 based on CH 4. This fuel/o 2 ratio corresponds to an air/fuel ratio of 2.72 Fig. 1(b) shows variation of selectivity, conversion, and yield with the CH 4 /NH 3 ratio in the feed gases. Methane conversion remained above 0.90 over the entire range of conditions while ammonia conversion increased from 0.55 to 0.93 as the CH 4 /NH 3 ratio was increased. When either fuel was in large excess (either end of the x-axis), its conversion decreased. The HCN yield based on CH 4 reached its maximum at a CH 4 / NH 3 ratio of 1, but the HCN yield based on NH 3 continued to increase as the CH 4 /NH 3 ratio increased. The temperature of the catalyst near the regions of maximum yield was again 11008C. Based on these two control experiments, we chose to run further experiments at feed conditions of (CH 4 NH 3 )/ O 2ˆ1.75 and CH 4 /NH 3ˆ1.1. These values gave maxima in HCN yields and are near industrial operating conditions. We report results showing the effect of N 2 diluent in the feed gases in Fig. 2(a) and (b). Fig. 2(a) shows selectivity, conversion, and yield versus the volume percent of N 2 in the feed stream. The stoichiometry for ammoxidation in air corresponds to 59% N 2 diluent. As the amount of diluent decreased, the reaction temperature and the fuel conversions rose while the HCN selectivities dropped sharply leading to a substantial fall in the yield of HCN. However, as seen in Fig. 2(b), when the fuel/o 2 ratio was increased at 35% N 2 dilution, the HCN yield recovered to 0.60 at a fuel/ O 2 ratio of 2.05 and at a catalyst temperature near 11008C. The effect of preheating the feed gases to 3758C is shown in Fig. 3. The results are qualitatively similar to decreasing the diluent in that to achieve maximum selectivity, higher fuel/o 2 ratio is required. Though in

5 A.G. Dietz III, L.D. Schmidt / Applied Catalysis A: General 180 (1999) 287± Fig. 1. (a) Typical steady state selectivity, conversion, and yield for HCN synthesis in air versus (CH 4 NH 3 )/O 3 over a Pt-10% Rh gauze pack at 5 slpm with a fuel feed ratio of CH 4 /NH 3ˆ1.1 based on NH 3 (upper panel) and on CH 4 (lower panel). The O 2 conversion was >99% in all experiments. (b) Typical steady state selectivty, conversion, and yield for HCN synthesis in air versus CH 4 /NH 3 over a Pt-10% Rh gauze pack at 5 slpm and (CH 4 NH 3 )/O 2ˆ1.75. this case, the yield to HCN on both bases surpasses that in the control case and can be attributed to a slight increase in the conversion of both fuel species. The HCN yield based on NH 3 approached 0.70 and was over 0.60 based on CH 4. For these experiments, the catalyst temperature of 10508C at the maximum yield was slightly lower than that in the control runs. Fig. 4 shows selectivity, conversion, and yield as a function of space velocity. As space velocity increased, the selectivity to HCN remained constant, but the conversions of all of the fuel species, including O 2, decreased. The breakthrough of O 2 emphasizes the importance of contact time in the HCN synthesis process. Fig. 5(a) and (b) show the effect of increasing pressure on the HCN selectivity and yield. In Fig. 5(a), the pressure was increased while holding the mass ow rate of the fuel species constant. Since the gas velocity is inversely proportional to pressure, the contact time over the catalyst increased as pressure increased. In Fig. 5(b), the contact time over the catalyst was held constant by increasing the ow rate of the reactants as pressure was raised. When the mass ow rate was xed, the fuel conversions did not change, but the selectivity to HCN decreased as pressure increased. When the velocity across the catalyst was held constant, both of the fuel conversions decreased with increasing pressure. The selectivity to HCN based on CH 4 decreased slightly, but

6 292 A.G. Dietz III, L.D. Schmidt / Applied Catalysis A: General 180 (1999) 287±298 Fig. 2. (a) Typical steady state selectivity, conversion, and yield for HCN synthesis versus vol% N 2 in the feed at 5 slpm, CH 4 /NH 3ˆ1.1, and (CH 4 NH 3 )/O 2ˆ1.75. (b) Typical steady state selectivity, conversion, and yield for HCN synthesis in enriched air (35% N 2 ) versus (CH 4 NH 3 )/O 2 over a Pt-10% Rh gauze pack at 5 slpm with a fuel feed ratio of CH 4 /NH 3ˆ1.1. most of the decrease in HCN yield can be accounted for by the change in fuel conversion Catalyst activation Unused Pt±Rh gauzes need to be activated for HCN synthesis before maximum yields can be achieved because initial HCN selectivities are typically 0.20 on a fresh gauze. Note that the O 2 conversion remains high even on a fresh gauze. Typically, the activation process consists of operating the catalyst at feed conditions that previously gave maximum yields. As Fig. 6(a) and (b) show, this process can take on the order 15±30 h using normal activation conditions. It is important to note that a signi cant change in product selectivity takes place along with a smaller change in reactant conversion before maximum HCN yields occur. In addition, the temperature of the gauze decreases during the activation period even though fuel conversion increases, because the activated gauze favors less exothermic reactions. Fig. 7 shows data for the activation of a Pt±Rh gauze packs when the feed conditions were made more fuel lean (fuel/o 2 ratioˆ1.46) and the feed gases are preheated. Both of these changes served to increase the temperature on the catalyst surface to 14008C during the activation. The data in Fig. 7 were taken at a fuel/o 2 ratio of 1.75 without preheat, and the horizontal lines indicate the time period when the high temperature treatments were applied. The

7 A.G. Dietz III, L.D. Schmidt / Applied Catalysis A: General 180 (1999) 287± Fig. 3. Typical steady state selectivity, conversion, and yield for HCN synthesis in air versus (CH 4 NH 3 )/O 2 over a Pt-10% Rh gauze pack at 5 slpm with a fuel feed ratio of CH 4 /NH 3ˆ1.1. The reactant gases were preheated to 3758C. activation time decreased from 15 to 30 h to under 4 h including time needed to change feed conditions while the selectivity to HCN and NH 3 conversion increased after the heat treatment. The gauzes activated with the high temperature treatment were run for days afterward without signs of deactivation. SEM micrographs were taken of the upper layer of the Pt±Rh gauze catalysts after being exposed to different activation environments. These pictures are shown in Figs. 8 and 9, and the ammonia based HCN selectivity, HCN yield, and NH 3 conversion for each case are shown in Table 1. Fig. 8(a) shows a fresh Pt±Rh gauze. In Fig. 8(a)± (d), we show micrographs of gauzes that have been exposed to HCN activation environment for 8 h each. Fig. 4. Typical steady state selectivity, conversion, and yield for HCN synthesis in air versus space velocity at CH 4 /NH 3ˆ1.1, and (CH 4 NH 3 )/O 2ˆ1.75. The temperature of the catalyst during the activation was varied by changing operating conditions and/or the degree of preheat. Catalyst pitting became much more pronounced at higher temperatures. In addition, the HCN yield increased with increased activation temperature. Fig. 9(a) and (b) show micrographs of gauzes that were exposed to activation conditions with methane and oxygen only and ammonia and oxygen only. Both gauzes were exposed for a total of 4 h, and the catalyst temperature was 12008C. The gauze after methane oxidation showed faceting on the surface, but no signi cant pitting while the gauze exposed to ammonia oxidation had a large number of pits. However, in both cases, the HCN yield is substantially lower than that for normal HCN activation after 20 h or activation through heat treatment.

8 294 A.G. Dietz III, L.D. Schmidt / Applied Catalysis A: General 180 (1999) 287±298 Fig. 5. (a) Typical steady state selectivity, conversion, and yield in HCN synthesis in air versus pressure at 5 slpm, CH 4 /NH 3ˆ1.1, and (CH 4 NH 3 )/O 2ˆ1.75. (b) Typical steady state selectivity, conversion, and yield in HCN synthesis in air versus pressure at t resˆ0.6 ms, CH 4 / NH 3ˆ1.1, and (CH 4 NH 3 )/O 2ˆ1.75. Table 1 Summary of SEM gauzes Figure Temperature (8C) Time (min) S NH3 Y NH3 C NH3 Fig. 8(a) Fresh gauze Fig. 8(b) Temperature variation Fig. 8(c) Temperature variation Fig. 8(d) Temperature variation Fig. 9(a) Methane oxidation Fig. 9(b) Ammonia oxidation Discussion Since the HCN yield in the industrial Andrussow process ranges between 0.60 and 0.70, small improvements in yield or changes in process conditions that increase throughput make a signi cant impact on the pro tability of the HCN process. However, changing operating conditions in a large scale process to conduct experiments is usually not feasible. Therefore, we have designed reactors and conducted experiments at the bench scale in operating regimes slightly different from those seen industrially in order to examine any

9 A.G. Dietz III, L.D. Schmidt / Applied Catalysis A: General 180 (1999) 287± Fig. 6. Selectivity, conversion, and yield versus time during normal catalyst activation in HCN synthesis at 5 slpm, CH 4 /NH 3ˆ1.1, and (CH 4 NH 3 )/O 2ˆ1.75. possible bene ts. In addition to process conditions, by treating the catalyst we may be able to improve yields and/or prolong the life of the catalyst. Fig. 7. Selectivity, conversion, and yield versus time after heat treatments (indicated by bars) during catalyst activation in HCN synthesis at 5 slpm. In Table 2, we summarize the experimental results for the bench scale Andrussow reactor. Included in the table are data from Pan at similar operating conditions. Table 2 HCN synthesis results Pressure Temperature (8C) Fuel/O 2 S HCN Y HCN Comments Industrial conditions (bench scale) Industrial conditions (Pan) Enriched air (35% N 2 diluent) Reactant gases preheated (3758C) High throughput (4base case) Yield decreases from 0.58 to 0.45 High pressure Selectivity decreases from 0.78 to 0.65 High pressure, high throughput Conversion and selectivity decrease, but yield can be maintained up to 3 atm

10 296 A.G. Dietz III, L.D. Schmidt / Applied Catalysis A: General 180 (1999) 287±298 Fig. 8. SEM images of gauze with various temperature variations (as detailed in Table 1). Wire width is 90 m. Fig. 9. SEM images of gauzes used in methane and ammonia oxidation. Wire width is 90 m. Compared to the pilot plant, the bench scale reactor provides similar operating temperature, HCN selectivity, and yield. Since the results are comparable, changes seen on the bench scale should occur in the pilot plant or larger scale. Exact duplication of the HCN yields and selectivities is unlikely, since gauzes activate to different degrees, but the qualitative results from each experiment should be comparable. When the air fed to the reactor is enriched with O 2 and when the reactant gases are preheated, the maxima in selectivity and yield moves to higher fuel/o 2 ratios, but the catalyst temperature is still 11008C. With enriched air, the HCN yield does not improve over the control case, but the HCN production rate would increase by 160% since more fuel can be fed to the reactor. When air is used and the reactants are preheated, the HCN throughput increase by 20%, and the yield improves. Here the N 2 may be acting as a third body to quench radicals in the gas phase thus preventing homogeneous HCN hydrolysis to NH 3 and CO. Conversion of fuel and O 2 decreases substantially as the space velocity is raised by raising reactant ow rate causing a sharp decrease in HCN yield. Increasing the pressure in the reactor adversely affects the selec-

11 A.G. Dietz III, L.D. Schmidt / Applied Catalysis A: General 180 (1999) 287± tivity to HCN but not the fuel conversion. When the pressure is increased along with ow rate to maintain a constant residence time across the catalyst, both selectivity and conversion decrease at higher pressures. However, the HCN yield does not change substantially for pressures up to 3.5 atm. The HCN throughput at these conditions was 300% of that in the control experiments. The decrease in conversion with increasing ow rate at constant pressure is most likely a contact time effect. Mass transfer rates should not decrease, but the increased amount of fuel needs a longer reactor to completely react. When pressure is increased, the mass transfer coef cient decreases as 1/P [19], thereby decreasing mass transfer rates to and from the surface negatively affecting the HCN selectivity. The decrease in mass transfer rates would also explain the drop in conversion at higher pressures at constant velocity since contact time is preserved. As discussed previously [18], homogeneous reactions are very sensitive to pressure. The drop in the HCN selectivity may be due to higher concentrations of gas free radical species which serve to increase reaction rates of competing reactions such as HCN hydrolysis. However, since Fig. 5(a) and (b) show that the selectivity to HCN only decreases slightly with increasing pressure, this is the evidence that homogeneous reactions are not very signi cant in the HCN synthesis process even at reaction temperatures 11008C. Several attempts have been made to explain the activation of Pt±Rh gauze catalysts in HCN synthesis [13±15]. The SEM micrographs and the activation experiments presented earlier show that higher operating temperatures increase the rate of surface rearrangement. Higher temperatures increase surface diffusion coef cients which would lead to decreased activation time. Higher temperatures could also increase the volatility of the metal, but no signi cant Pt loss was detected upon completion of the experiments. With boundary layers over the gauzes being of the wire diameters, the region near the surface of the gauze should be relatively stagnant [16,20]). During NH 3 oxidation experiments over Pt±Rh gauzes, Lyubovsky et al. estimated Pt vapor pressures in this boundary layer at 10 3 Torr which is many orders of magnitude higher than the equilibrium value at this temperature. It is likely that the sublimation of Pt also occurs to some extent during HCN synthesis, since the boundary layer characteristics and temperatures are similar. The vaporized Pt could then recondense, removing Pt from pits and depositing it elsewhere. This can be seen as the catalyst becomes pitted, and the diameter of the gauze wires increases 10±15% during the course of the activation period as can be seen in Fig. 8(a) and (d). The increased rates for restructuring caused by the higher temperature treatment are better explained by higher vapor pressures of Pt above the surface rather than an increase in surface diffusion rates. Although NH 3 seems to be required to cause pits on the surface, increase roughness and surface area, complete catalyst activity is achieved only under HCN process conditions. Less Pt loss is detected during HCN synthesis since this process has lower partial pressures of O 2 and operates in a reducing environment. If this rearrangement serves to increase surface sites available to force the HCN synthesis reaction, then this competing process would begin to take over allowing adsorbed C and O species to react, causing an increase in the selectivity to HCN should be seen. The rearrangement could also serve to increase roughness and disturb the boundary layer at a sub-micron scale, affecting chemistry very close to the catalytic surface. X-ray photoelectron spectroscopy was performed on some of the gauze samples to determine the metal composition during the activation process. We found that an unused gauze had a Pt/Rh ratio of 14.9 on the surface. After 1 h the Pt/Rh ratio was 10.8, and after 30 h (activated), the Pt/Rh ratio was The data for the unused and activated gauzes agree with those of Cowans et al., but it is interesting to note that Rh segregation from the surface happens very early in the activation process, while signi cant changes to HCN selectivity occur during the entire course of the activation process. 5. Summary We investigated the synthesis of HCN in a bench scale Andrussow reactor and made comparisons to larger scale processes. Operation with enriched O 2 in the feed gases or preheating the feed gases is desirable

12 298 A.G. Dietz III, L.D. Schmidt / Applied Catalysis A: General 180 (1999) 287±298 in that HCN throughput can be increased since operation at higher fuel/o 2 ratios is possible. Preheating the feed gases also serves to increase the yield of HCN. Operation at higher pressures is possible without a seriously negative effect on HCN yield up to 3.5 atm at ow rates 300% of those at atmospheric pressure. Activation of the Pt±Rh gauze catalyst can be completed in 3 h compared to 30 h for the typical process by subjecting the gauze pack to higher temperatures under the HCN synthesis environment. Increased surface mobility at higher temperatures can explain the decrease in activation time. References [1] C.N. Satterfield, Heterogeneous Catalysis in Industrial Practice, 2nd ed., McGraw-Hill, New York, [2] A. Chretien, A. Thomas, Bull. Soc. Chim. 15(5) (1948) 354. [3] U. Maffezzoni, Chim. Ind. 69 (1953) 842. [4] R. Mihail, Chem. Tech. 9 (1957) 9±344. [5] B.Y.K. Pan, R.G. Roth, Optimization of yield through feed composition: HCN process, Ind. Eng. Chem. Proc. Des. Dev. 7(1) (1968) 53±61. [6] D. Hasenberg, L.D. Schmidt, HCN synthesis from CH 4 and NH 3 on clean Rh, J. Catal. 91(1) (1985) 116±131. [7] D. Hasenberg, L.D. Schmidt, HCN synthesis from CH 4 and NH 3 on platinum, J. Catal. 97(1) (1986) 156±168. [8] D. Hasenberg, L.D. Schmidt, HCN synthesis from CH 4,NH 3, and O 2 on clean Pt, J. Catal. 104(2) (1987) 441±453. [9] N. Waletzko, L.D. Schmidt, Modeling catalytic gauze reactors: HCN synthesis, AIChE J. 34(7) (1987) 1146±1156. [10] L.D. Schmidt, D.A. Hickman, Surface chemistry and engineering of HCN synthesis, in: K.A. Johnson (Ed.), Catalysis of Organic Reactions, 1993, pp. 195±212. [11] S.S. Bharadwaj, L.D. Schmidt, HCN Synthesis by Ammoxidation of Methane and Ethane on Pt Monoliths, J. Molecular Catalysis A; Chem. 105 (1996) 145±148. [12] D.A. Hickman, L.D. Schmidt, Production of syngas by direct catalytic oxidation of methane, Science 259 (1993) 343±346. [13] D.R. Anderson, Catalytic etching of platinum alloy gauzes, J. Catal. 113 (1998) 475±489. [14] B.Y.K. Pan, Characteristics of Pt±Rh gauze catalyst and kinetics of the HCN synthesis, J. Catal. 21(1) (1971) 27±38. [15] B.A. Cowens, K.A. Jurman, W.N. Delgass, Y.Z. Li, R. Reifenberger, T.A. Koch, Scanning tunnelling microscopy od platinum±rhodium gauze HCN catalysts, J. Catal. 125(2) (1990) 501±513. [16] L.D. Schmidt, D. Luss, Physical and chemical characterization of platinum±rhodium gauze catalysts, J. Catal. 22(2) (1971) 269±279. [17] M. Flytzani-Stephanopoulos, Chem. Eng. Sci. 34 (1979) 365± 378. [18] A.G. Dietz III, L.D. Schmidt, Effect of pressure on three catalytic partial oxidation reactions at millisecond contact times, Catal. Lett. 33(1)(2) (1995) 15±30. [19] C.L. Cussler, Diffusion: Mass Transfer in Fluid Systems, Cambridge University Press, New York, 1984, pp. 230±231. [20] M.R. Lyubovsky, V.V. Barelko, Formation of metal wool structures and dynamics of catalytic etching of platinum surfaces during ammonia oxidation, J. Catal. 149 (1994) 23±35.