TiO2 SCR Catalysts Research Articles

Insights into P-doping effect of the activity and anti-SO2/H2O poisoning of V2O5-MoO3/TiO2 catalysts for NH3-SCR

With the increasingly stringent flue gas emission standards, water-sulfur poisoning has become one of the most pressing problems in the SCR process of coal-fired power plants.SO2 not only deactivates the catalyst, but also reacts with water and NH3 to form NH4HSO4, which then corrodes the equipment. Catalysts with water-sulfur tolerance urgently need to be developed. NH4H2PO4-modified V2O5-MoO3/TiO2 SCR catalysts is prepared by an improved wet impregnation method. The effect of P element modification on the denitration performance of the V2O5-MoO3/TiO2 SCR catalyst was studied, and the catalyst was characterized by XRD, BET, SEM, TEM, XPS, TPR, TPD and Raman. When the phosphorus doping amount was 0.6 % (wt%), the P0.6-SCR catalyst could achieve a NOx conversion rate of 95 %, and its optimal reaction temperature was 350 ℃. The characterization results show that the P element made the active components on the catalyst surface further dispersed, delayed the sintering of the catalyst at the calcination stage, and promoted the generation of B-acidic sites, enhances the surface acidity of the catalyst, and promotes the transition of VOx from a monomer state to a polymer state. Meanwhile, the P0.6-SCR catalyst exhibits outstanding sulfur resistance while ensuring high denitration efficiency.

Revisiting the Impact of Tungsten on the Catalytic Properties of Ammonia-SCR V2O5-WO3/TiO2 Catalysts: Geometric vs. Electronic Effects

The SCR performance of V2O5-WO3/TiO2 SCR-catalysts characterized by different surface W density (2.1W/nm2 and 9.5W/nm2) and different surface V density varying in the range 1–8V/nm2 has been investigated in order to clarify existing controversies on the preferential involvement of electronic and geometric effects in the catalytic properties. It was found that tungsten has a weak effect on the VOx cluster size distribution through contraction of dilution effect. In contrast, the optimal interaction between W and V, when both reach their highest composition, appears to be a relevant parameter that can enhance their acidic properties and improve the catalytic efficiency in dry conditions. On the other hand, an absence of significant interaction leads to discontinuity due to deactivation. In the presence of steam, acidic properties are averaged, lowering the impact of the V to W ratio. Finally, the critical importance of acidic properties which outperform redox properties in the definition of active site is pointed out in the light of this study.

Improved Sulfur Resistance of COMMERCIAl V2O5-WO3/TiO2 SCR Catalyst Modified by Ce and Cu

The accumulation of NH4HSO4 leads to the deactivation of commercial V2O5-WO3/TiO2 catalyst (VWTi) in practical application. The commercial catalyst is modified with 0.3 wt. % Ce and 0.05 wt. % Cu (donated as VWCeCuTi), and its sulfur resistance is noticeably improved. After loading 20 wt. % NH4HSO4, the NOx conversion of VWCeCuTi-S remains 40% at 250 °C, higher than that of VWTi-S (25%). Through a series of characterization analyses, it was found that the damaged surface areas and acid sites are the key factors for the deactivation of S-poisoned samples. However, surface-active oxygen and NO adsorption are increased by NH4HSO4 deposition, and the L–H mechanism is promoted over S-poisoned samples. Due to the interaction between V, Ce and Cu, the surface-active oxygen over VWCeCuTi-S is increased, and then NO adsorption is promoted. In addition, VWCeCuTi-S obtains a higher V5+ ratio and a better redox property than VWTi-S, which in turn accelerates the NH3-SCR reaction. More NO adsorption and encouraged reaction contribute to the better sulfur resistance of VWCeCuTi.

Deactivation Influence of HF on the V2O5–WO3–TiO2 SCR Catalyst

The electrolytic aluminum waste contains harmful components, such as fluorides. Sending it to coal-fired boilers seems to be a prospective scheme to achieve the harmless treatment of waste. However, the negative influence of F on the SCR catalyst should be examined. In this study, a fixed-bed reactor system is built up to study the deactivation of the V2O5–WO3–TiO2 SCR catalyst by HF when the electrolytic aluminum waste is co-fired with coal in boilers. The activity of the catalyst is evaluated by the NH3-SCR reaction. The deactivation mechanism is investigated through tests of NH3 temperature-programmed desorption, H2 temperature-programmed reduction, X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller (BET), in situ diffuse reflectance infrared Fourier transform spectroscopy, and Raman spectroscopy. The results show that for deactivated catalyst samples, the SCR catalytic activity drops obviously, especially when they work at lower temperatures. Although HF could enhance Brønsted acid sites, increase the Oα (adsorbed oxygen) concentration, and facilitate the Lewis acid site formation, it would not compensate for decreases of V5+ on the catalyst surface and BET surface area, which results in an activity reduction of the catalyst sample.

상용 차바사이트 SCR 촉매의 열적 열화와 코킹의 내구성 향상을 위한 조촉매 연구

This study is to investigate the promoter to improve the durability of thermal aging and coking of commercial chabasite SCR catalysts. Cu-SCR+WO₃ SCR catalyst has improved NOx conversion at low and medium temperature, and WO₃ increased the strength of acid point to speed up the reaction rate. When C₃HSUB6/SUB coexisted (C/N = 9), the de-NOx/CO performance of the Cu-SCR+TiO₂ SCR catalyst was improved by adding TiO₂ to poisoning. An appropriate amount of TiO₂ strengthened the acidity of the SCR catalyst, which improved durability against carbon poisoning. The Cu-SCR+TiO₂ SCR catalyst had a 4.7% higher NOx conversion under C₃HSUB6/SUB coexistence conditions in de-NOx performance according to 54 g/L coking and C3H6 coexistence (C/N = 9) poisoning. This deteriorated de-NOx performance by clogging the active site of the SCR catalyst when coking excessively at 54 g/L. Cu-SCR+TiO₂ SCR catalyst on which TiO₂ was loaded has improved durability against catalyst coking when C₃HSUB6/SUB coexists. It is necessary to derive an optimal loading amount in the future.

Effect of biofuel- and lube oil-originated sulfur and phosphorus on the performance of Cu-SSZ-13 and V2O5-WO3/TiO2 SCR catalysts

Two different SCR catalysts, V2O5-WO3/TiO2 and Cu-SSZ-13, were exposed to biodiesel exhausts generated by a diesel burner. The effect of phosphorus and sulfur on the SCR performance of these catalysts was investigated by doping the fuel with P-, S-, or P + S-containing compounds. Elemental analyses showed that both catalysts captured phosphorus while only Cu-SSZ-13 captured sulfur. High molar P/V ratios, up to almost 3, were observed for V2O5-WO3/TiO2, while the highest P/Cu ratios observed were slightly above 1 for the Cu-SSZ-13 catalyst. Although the V2O5-WO3/TiO2 catalyst captured more P than did the Cu-SSZ-13 catalyst, a higher degree of deactivation was observed for the latter, especially at low temperatures. For both catalysts, phosphorus exposure resulted in suppression of the SCR performance over the entire temperature range. Sulfur exposure, on the other hand, resulted in deactivation of the Cu-SSZ-13 catalyst mainly at temperatures below 300-350 °C. The use of an oxidation catalyst upstream of the SCR catalyst during the exhaust-exposure protects the SCR catalyst from phosphorus poisoning by capturing phosphorus. The results in this work will improve the understanding of chemical deactivation of SCR catalysts and aid in developing durable aftertreatment systems.

K+ deactivation of V2O5-WO3/TiO2 catalyst during selective catalytic reduction of NO with NH3: Effect of vanadium content

The effect of vanadium content on the resistance to K+ deactivation of V2O5-WO3/TiO2 SCR catalyst in biomass-fired flue gas was investigated. Catalytic activity and ammonia oxidation were measured, and the properties of fresh and K+-deactivated catalysts were characterized by XRD, N2 physisorption, H2-TPR, NH3-TPD and NH3-DRIFT. The BET surface area decreases with increased vanadium content for both fresh and K+-poisoned samples, but it is not responsible for catalyst deactivation. Ammonia oxidation starts from 300 °C and becomes more important with increasing vanadium content and at higher temperature. K+ can inhibit ammonia oxidation, but inactivates the newly generated NO2 to be reduced. The increase of vanadium content reduces mainly the Lewis acid sites, while the amount of BrØnsted acid sites increases. Monomeric and polymeric vanadium are the dominant species on the TiO2 support, and the amount of isolated vanadyl (V = O) species decreases with V2O5 content while the amount of V-OH species in polymeric vanadia increases. Isolated vanadyl species are advantageous to high-temperature catalytic activity while polymeric vanadia species increase ammonia and K+ adsorption. The (3 wt% V2O5)-WO3/TiO2 catalyst shows the best performance for both NO reduction and K+ resistance due to it containing both monomeric and polymeric vanadia species (or V = O and V-OH). Catalysts with 3 wt% V2O5 are preferable for flue gases with high alkali metal contents. Finally, the mechanism of reaction for different vanadium contents and corresponding K+-poisoning are also discussed.

Leaching behavior of vanadium from spent SCR catalyst and its immobilization in cement-based solidification/stabilization with sulfurizing agent

To understand the potential leaching toxicity of V in spent SCR catalysts for human and environment, the leaching behavior of V in spent V2O5-WO3/TiO2 SCR catalysts before and after cement stabilization/solidification (S/S) process was investigated. The results showed that the cement mixing ratio, curing time and solidifying agents (Na2S and sodium diethyl dithiocarbamate (DDTCNa)) obviously affect the leaching ability of V in spent SCR catalysts. The leaching amount, leaching concentration and leaching rate of V in original spent SCR catalyst are 137.98 mg/kg, 13.79 mg/L and 1.82%, respectively. After the spent SCR catalyst was solidified with cement and cured for 28 days, the leaching concentrations of V in catalyst-cement solidified waste are 2.27 mg/L for 30% of cement addition and 0.14 mg/L for 70% of cement addition, respectively. Increasing cement mixing ratio and curing time could decrease the leaching concentrations of V in the cement solidified waste by affecting the generation of V5+-bearing species, which was identified by XRD, BET, SEM and FTIR. The heavy metal V could be immobilized through the formation of VO43--bearing compounds in hydrating cement matrix. Evaluation on the leaching concentrations of V after adding Na2S and DDTCNa in cement solidified waste revealed that Na2S and DDTCNa are capable of reducing leaching concentrations of V from 6.25 mg/L to 5.74 mg/L and 4.65 mg/L, respectively. DDTCNa showed a better solidification performance for V compared to Na2S.

Simulation of diesel exhaust aftertreatment system DOC—pipe—SCR: The effects of Pt loading, PtOx formation and pipe configuration on the deNOx performance

A combined exhaust aftertreatment system consisting of a diesel oxidation catalyst (DOC), pipe and selective catalytic reduction of nitrogen oxides (SCR) is studied in this paper by the means of mathematical modeling and simulations. Pt/γ-Al2O3 DOC and V2O5-WO3/TiO2 SCR catalysts for heavy-duty Diesel engines are examined. First, spatially 1D models of the DOC, SCR and pipe are introduced and calibrated using the measured engine test data. The models are then employed in a simulation study of the effects of Pt loading, PtOx formation and pipe configuration on the NO2 yield in DOC and the resulting deNOx performance of the SCR in the driving cycles ETC and WHTC. It is shown that there exists optimum washcoat loading in the DOC with respect to NOx conversion in SCR and that the minimization of heat losses in the connecting pipe can further improve the NOx conversion. Finally, the decrease of NO oxidation activity in DOC due to PtOx formation and its impact on NOx conversion in SCR is quantified over the repeated driving cycles, showing 2–10% difference between the deNOx performance with the pre-reduced DOC and after few hours of operation under oxidizing conditions. Based on the simulation results, an improved configuration of the system is proposed that achieves up to 6% higher overall NOx conversion.

Effect of different potassium species on the deactivation of V2O5-WO3/TiO2 SCR catalyst: Comparison of K2SO4, KCl and K2O

The effect of different potassium species, including K2O, KCl and K2SO4, on the deactivation of V2O5-WO3/TiO2 catalysts were measured in the research. Potassium species were supplied by impregnating catalyst with corresponding KNO3, KCl and K2SO4 aqueous solution. Catalytic activity was also measured and the physical and chemical properties of fresh and poisoned catalysts were characterized by XRD, N2 physisorption, NH3-TPD, H2-TPR, XPS and NH3-DRIFTS. Deactivation could be observed in the potassium-containing catalysts and the deactivation rate follows KCl > K2O > K2SO4. The surface chemisorbed oxygen was also reduced and the downward trend was in good accordance with the SCR activity. In addition, both the amount and stability of the Brønsted and Lewis acid sites dropped after potassium introduction, and the reducibility of surface active species decreased. The introduction of SO42− created new Brønsted acid sites and performed advantages to ammonia adsorption and NO reduction. Cl− could react with vanadia active sites to form -O-V-Cl bond, which provided some new ammonia adsorption sites, but these newly generated sites could not revitalize the adsorbed ammonia and were helpless to catalytic activity promotion. Besides, KCl presented the maximum impact on catalyst reducibility was another contribution to its largest activity decline. Finally, the probable poisoning mechanisms of different potassium species over V2O5-WO3/TiO2 catalysts were proposed.

Effect of Ce doping on the resistance of Na over V2O5-WO3/TiO2 SCR catalysts

Enhanced performance of Ce doping on the catalytic activity of V2O5-WO3/TiO2 catalysts and its resistance properties to Na poisoning have been investigated in the research. Different Na species (NaCl and Na2O) were deposited on CeO2-V2O5-WO3/TiO2 catalysts by impregnation method. The results presented that CeO2-V2O5-WO3/TiO2 catalysts exhibited a higher NH3-SCR activity and better Na resistance than the V2O5-WO3/TiO2 catalyst, and its poisoning resistance to Na2O is more superior than to NaCl, caused by more acid sites over the 10%CeO2-V2O5-WO3/TiO2 (Ce10VWTi) catalyst, which indicated that the poisoning resistance of Na2O on Ce10VWTi catalysts is higher than that of NaCl. Combined with the characterization results, the dopant of Ce could enhance the surface chemisorbed oxygen on the Na poisoning V2O5-WO3/TiO2 catalysts, facilitate the redox cycle, and increase the intensity of acid sites due to the newly formed Brønsted acid sites stemmed from Ce3+−NH4+, thereby promoting catalytic activity and Na poisoning resistance.

Amorphous saturated cerium–tungsten–titanium oxide nanofiber catalysts for NOx selective catalytic reaction

A nano-fibrous, amorphous supersaturated CeO2/W–TiO2 SCR catalyst endowed with well-connected and open porosity, high reactivity, and tunable chemistry is herein proposed.

A neutral and coordination regeneration method of Ca-poisoned V2O5-WO3/TiO2 SCR catalyst

Polyether surfactant was employed for the regeneration of a poisoned by calcium V2O5-WO3/TiO2 catalyst. The OP-10 (alkylphenol polyoxyethylene (10) ether) surfactant exhibited better CaO removal performance than EL-60 (cremophor EL 60) and AEO9 (polyethyleneglycol (9) mono-dodecyl ether). The Ca2+-aromatic interactions developed in OP-10 increased the solubility of CaO in water and Ca2+ could be removed by washing. The high Ca2+ removal rate by OP-10 surfactant led to the reversible improvement of surface acid sites and active oxygen species. Compared with the traditional method of dilute sulfuric acid solution, the OP-10 presented higher Ca2+ removal rate with lower V2O5 loss, which is expected to be a facile and environmental benign method for the regeneration of Ca2+-poisoned NH3-SCR industrial catalysts.

A study on deactivation of V2O5[sbnd]WO3[sbnd]TiO2 SCR catalyst by alkali metals during entrained-flow combustion

To study the deactivation mechanism of V2O5WO3TiO2 SCR catalyst by alkali metals, an entrained-flow combustion system is built, and the deactivation process is accelerated by exposing the catalyst to aerosols which are generated from combustion of the Zhundong coal sample doped with potassium and sodium salts. As the exposure time and load rate of alkali salt increase, the contamination of catalyst becomes more serious and the activity of catalyst decreases. The poisoning effects of different alkali salts on catalyst follow a sequence of Na2CO3 < K2CO3 < Na2SO4 < K2SO4 < NaCl < KCl. The combined poisoning effect of chlorides of potassium and sodium lies between those of the individual effects. Further investigations are performed on catalyst through NH3-chemisorption test and characterizations of BET, SEM and EDX, XRF, FTIR and XPS. The results show that after exposing to aerosols, the surface of catalyst is contaminated by particles of alkali salts with different morphologies. The decrease of the specific surface area and the masking of V2O5 can cause the mechanical deactivation of catalyst. The chemical deactivation also occurs because the deposited particles of alkali salts can coordinate to the hydroxyl groups on the surface and interact with the lattice oxygen in the vanadium species. The chemical deactivation makes the dominant contribution to the activity reduction of the catalyst.

Multivariate analysis of the effect of biodiesel-derived contaminants on V2O5‑WO3/TiO2 SCR catalysts

This study investigates the effect of biodiesel-derived contaminants on vanadia-based NH3-SCR catalysts in heavy-duty exhaust aftertreatment. The aim was to study, not only the effect of single contaminants on the catalyst performance, but also of possible interaction effects between poisons.The effect of six potential catalyst poisons (Na, K, Mg, P, S and Zn) was evaluated using an experimental design and multivariate data analysis. Monolithic V2O5-WO3/TiO2 catalysts were subjected to accelerated laboratory-scale aging, where the six contaminants were fed simultaneously using a wet impregnation method. In addition to NOx conversion tests, the catalysts were characterized by means of ICP-OES, SEM-EDX, XPS, N2 physisorption and NH3-TPD. The lab-aged samples were compared to fresh and vehicle-aged catalysts.The accelerated aging method showed good reproducibility and gave rise to surface compounds similar to those found in vehicle-aged catalysts. Despite plausible differences regarding penetration depth of the contaminants into the walls of the catalyst, the aging method appears to be an efficient way to point out significant chemical poisons.The model obtained from the experimental design was found to correlate well with the experimental data and can therefore be used to predict effects of the various poisons and poison interactions. Significant effects on the NOx conversion were found for P, S, Na, Mg and K as well as for the interactions P×Na, P×K and S×Na. A poisoning effect was found for Mg, Na, K, P×K, and P×Na, where Na and K exhibited the strongest poisoning effect. The deactivating effect of alkali was lowered in the presence of phosphorus and sulfur, which is explained by the formation of phosphates and sulfates, preventing the interaction of the alkali metals with the vanadia active sites.

Chemical deactivation of V2O5-WO3/TiO2 SCR catalyst by combined effect of potassium and chloride

V2O5-WO3/TiO2 catalyst was poisoned by impregnation with NH4Cl, KOH and KCl solution, respectively. The catalysts were characterized by X-ray diffraction (XRD), inductively coupled plasma (ICP), N2 physisorption, Raman, UV-vis, NH3 adsorption, temperature-programmed reduction of hydrogen (H2-TPR), temperature-programmed oxidation of ammonia (NH3-TPO) and selective catalytic reduction of NOx with ammonia (NH3-SCR). The deactivation effects of poisoning agents follow the sequence of KCl>KOH≫NH4Cl. The addition of ammonia chloride enlarges the pore size of the titania support, and promotes the formation of highly dispersed V = O vanadyl which improves the oxidation of ammonia and the high-temperature SCR activity. K+ ions are suggested to interact with vanadium and tungsten species chemically, resulting in a poor redox property of catalyst. More importantly, potassium can reduce the Bronsted acidity of catalysts and decrease the stability of Bronsted acid sites significantly. The more severe deactivation of the KCl-treated catalyst can be mainly ascribed to the higher amount of potassium resided on catalyst.

Redox behaviour of vanadium during hydrogen–oxygen exposure of the V2O5-WO3/TiO2 SCR catalyst at 250 °C

A series of experiments was conducted to reveal the impact on the catalyst and NH3-SCR activity of an industrial deNOx catalyst from Haldor Topsøe A/S (V2O5-WO3/TiO2) by exposure to H2 and H2/O2. Treatment of the SCR catalyst was done by exposure for 60min with three different concentrations of H2 (0–2%) in an 8% O2/N2 mixture, where the SCR activity was measured before and after the hydrogen treatment. The results show that the activity of the SCR catalyst is only negligibly affected during exposure to the H2/O2 gas and in all cases it returned reversibly to the initial NOx conversion rate after reexposure to the standard NH3-SCR gas. In situ electron paramagnetic resonance (EPR) and UV–vis spectroscopy at 250°C suggest that a significant part of V4+ present during SCR conditions was oxidized to V5+ during exposure to 2% H2+8% O2. However, the distribution of vanadium in oxidation states V4+ and V5+ returned to the initial redox equilibrium in less than an hour after the hydrogen–oxygen treatment, emphasizing the reversibility of the V4+-V5+ system.

Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils and urea solution: Part II. Characterization study of the effect of alkali and alkaline earth metals

A characterization study on a practice-oriented V2O5/WO3–TiO2 SCR catalyst deactivated by Ca and K, respectively, was carried out using NH3-TPD, DRIFT spectroscopy, and XPS as well as theoretical DFT calculations. It was found from NH3-TPD experiments that strongly basic elements like K or Ca drastically affect the acidity of the catalysts. Detailed DRIFT spectroscopy experiments revealed that these poisoning agents mostly interact with the Brønsted acid sites of the V2O5 active phase, thus affecting the NH3 adsorption. Moreover, these experiments also indicated that the V5+=O sites are much less reactive on the poisoned catalysts. XPS investigations of the O 1s binding energies showed that the oxygen atoms of the V5+=O sites are affected by the presence of the poisoning agents. Based on these results and on DFT calculations with model clusters of the vanadia surface, the poisoning mechanism is explained by the stabilization of the non atomic holes of the (010) V2O5 phase as a result of the deactivation element. Consequently, V–OH Brønsted acid sites and V5+=O sites are inhibited, which are both of crucial importance in the SCR process. The deactivation model also gives an explanation to the very low concentrations of potassium needed to deactivate the SCR catalyst, since one metal atom sitting on such a non-atomic hole site deactivates up to four active vanadium centers.

An IR study of thermally stable V2O5-WO3 -TiO2 SCR catalysts modified with silica and rare-earths (Ce, Tb, Er)

The catalytic activity of silica-free and silica-modified rare earth (Ce, Tb, Er) containing V2O5-WO3 -TiO2 catalysts in the selective catalytic reduction of NO by ammonia has been investigated as a function of ageing temperatures. The adsorption of ammonia on the catalysts and the behavior of their surface hydroxy groups and of bulk vibrations has also been studied by IR spectroscopy. Rare earths slightly decrease the catalytic activity of catalysts in a fresh state, and this has been attributed to the perturbation, observed by IR, of the vanadyl groups with a likely lowering of their Lewis acidity. However, rare earths (in particular Tb and Er) increase strongly the catalytic activity of catalysts after ageing. Silica only does not seem to have a positive effect on thermal stability and activity when vanadium is present. It has been concluded that rare earths strongly increase the thermal resistance of the catalysts and inhibit rutilization and surface area loss because they do not penetrate the anatase bulk while tend to cover the external surface. In addition the negative action of free vanadium on phase stability is decreased due to formation of rare earth vanadates.

Chemical deactivation of V2O5/WO3–TiO2 SCR catalysts by additives and impurities from fuels, lubrication oils, and urea solution: I. Catalytic studies

The influence of the combustion products of different lubrication oil additives (Ca, Mg, Zn, P, B, Mo) and impurities in Diesel fuel (K from raps methyl ester) or urea solution (Ca, K) on the activity and selectivity of vanadia-based SCR catalysts were investigated. Standard V2O5/WO3–TiO2 catalysts coated on metal substrates (400cpsi) were impregnated with water soluble compounds of these elements and calcined at 400 and 550°C, in order to investigate the chemical deactivation potential of different elements and combinations of them.It was found that potassium strongly reduced the adsorption equilibrium constant KNH3of ammonia. At small ammonia concentrations in the feed, only part of the active sites were covered with ammonia resulting in a reduced SCR reaction rate. At high ammonia concentrations, the surface coverage and SCR reaction rate increased, but high SCR activity at concurrent low ammonia emissions was impossible. Calcium caused less deactivation than potassium and did not affect the ammonia adsorption to the same extent, but it lowered the intrinsic SCR reaction rate. Moreover, deactivation by calcium was much reduced if counter-ions of inorganic acids were present (order of improvement: SO42−>PO43−>BO33−). Zinc was again less deactivating than calcium, but the positive effect of the counter-ions was weaker than in case of calcium. The degree of N2O production at T>500°C, which is typical for V2O5/WO3–TiO2 catalysts, was not influenced by the different compounds, except for molybdenum, which induced a small increase in N2O formation.